Far-ultraviolet energy distributions of the metal-poor A stars HD 109995 and HD 161817

NASA Technical Reports Server (NTRS)

Boehm-Vitense, E.

1981-01-01

Low-resolution IUE spectra at wavelengths between 1300 and 3400 A of the metal-poor stars HD 109995 (A1p) and HD 161817 (A4p) have been compared with model-atmosphere energy distributions computed by Kurucz (1979). Good overall agreement is found. Effective temperatures, metal abundances, and angular diameters could be determined. Assuming an absolute visual magnitude of 0.7, the previously determined gravity log = 3 yields masses of 0.5 solar masses for both stars. It is found that the theoretical UBV colors calculated earlier agree reaonably well with the ones observed for these stars.

Chromospheric activity of cool giant stars

NASA Technical Reports Server (NTRS)

Steiman-Cameron, T. Y.

1986-01-01

During the seventh year of IUE twenty-six spectra of seventeen cool giant stars ranging in spectral type from K3 thru M6 were obtained. Together with spectra of fifteen stars observed during the sixth year of IUE, these low-resolution spectra have been used to: (1) examine chromospheric activity in the program stars and late type giants in general, and (2) evaluate the extent to which nonradiative heating affects the upper levels of cool giant photospheres. The stars observed in this study all have well determined TiO band strengths, angular diameters (determined from lunar occulations), bolometric fluxes, and effective temperatures. Chromospheric activity can therefore be related to effective temperatures providing a clearer picture of activity among cool giant stars than previously available. The stars observed are listed.

Stellar integrated fluxes for 216 stars in the wavelength range 380 nm-900 NM

NASA Astrophysics Data System (ADS)

Petford, A. D.; Blackwell, D. E.; Booth, A. J.; Haddock, D. J.; Leggett, S. K.; Mountain, C. M.; Selby, M. J.; Arribas, S.

1988-09-01

The paper reports measurements of the integrated fluxes over the wavelength range 380 nm - 900 nm for 216 stars using a Reticon spectrometer in conjunction with the 1 m Kapteyn telescope of the Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. Methods are proposed for deriving visible integrated fluxes from 13-colour photometry, UBVRI and BV photometry. Such fluxes are useful for deriving stellar effective temperatures and angular diameters.

The Biggest Star in the Sky

NASA Astrophysics Data System (ADS)

1997-03-01

An international team of astronomers has used large telescopes in Chile and Australia to measure the biggest star in the sky. The star, designated R Doradus , is of the so-called red giant type and is located in the southern constellation of Dorado. Its apparent diameter (i.e., the size which the star appears to have when seen from the Earth) is larger than any other so far observed, except for the Sun. In particular, it exceeds by more than 30 % that of Betelgeuse , which for the past 75 years has held the title of star with the largest apparent size. Measuring sizes of stars Measuring the sizes of stars is very difficult due to their enormous distances. For example, if our Sun were placed at the distance of the next closest star (four light-years away), it would have about the same apparent size as a DM 1 (or US quarter-dollar) coin placed at a distance of 500 km (about 0.01 arcsec). Even for the most powerful astronomical telescopes, it is a very challenging task to measure such small angles. Ideally, the angular resolution of a telescope (its capability to resolve fine details in celestial sources) increases with its diameter. In practice, although ground-based optical telescopes now have diameters up to 10 metres, their actual resolution of visual light is that of a telescope of only about 20 centimetres aperture. This is because of the constant turbulence in the Earth's atmosphere. This turbulence causes the stars to twinkle in a way which delights the poets but frustrates the astronomers, since it blurs the fine details of the images. The first, and largest, star apart from the Sun to have its diameter measured was Betelgeuse, the brightest star in the constellation of Orion. Its angular diameter was found to be 0.044 arcsec by Albert Michelson and his team who used the Hooker telescope on Mt. Wilson in California in the early 1920s, pioneering interferometry techniques. Betelgeuse kept its title as the star with the largest apparent size for the next 75 years. This title has now been taken by R Doradus. R Doradus is a variable star in the constellation of Dorado (the Swordfish), located in the far southern sky. At a distance of about 200 light years it is relatively nearby. R Doradus is a variable star with a period of about 338 days, changing its magnitude from approximately 4.8 at maximum (when it is visible with the unaided eye) to 6.6 at minimum (when it requires a small telescope). Interferometry at the NTT In August 1993, the team of astronomers [1] pointed the ESO 3.5-metre New Technology Telescope (NTT) towards R Doradus. For these observations, the NTT was covered with an opaque mask with seven holes arranged on a 3.3-metre diameter circle. Each of these holes had a diameter of 25 cm, which was smaller than the cells of turbulence in the atmosphere above. The main motivation for using the mask was to suppress the effects of the turbulence and in this way restore the full resolution capability of the NTT [2]. Caption to ESO PR Photo 07/97 [JPG, 63k] The seven light beams from a star were brought to interfere with each other at the telescope's focus. Each pair of holes in the mask produced a fringe pattern in the image of the star, so at any moment there were 21 distinct fringe patterns (see ESO Press Photo 07/97 [63k] ). A camera in the focal plane recorded these fringes, their contrast being determined during subsequent computer analysis. A star which is very far away will appear too small for its disk to be resolved by the telescope. All of the 21 fringes will then have approximately the same contrast. On the other hand, if the star is closer by and has a perceptible size, the contrast of the fringe patterns will be reduced for widely separate mask holes. By comparing the fringe contrast of the target star with that of a more distant, unresolved star, it is then possible to estimate the size of the target. The present NTT observations were made at infrared wavelengths (1.25 microns) with the SHARP camera, developed by the Max-Planck Institut for Extraterrestrial Physics (Garching, Germany). Several hundred very short exposures of R Doradus were made, each lasting 0.1 second (this is short enough to freeze the 21 fringe patterns in each exposure). Immediately thereafter, a similar series of observations was made of an unresolved calibrator ' star (Gamma Reticuli). This procedure was repeated several times, producing thousands of images to be analysed. Additional observations were made in 1995 with the NTT as well as the 3.9-m Anglo-Australian Telescope at Siding Spring (Australia). These observations, and the application of different interferometric data analysis techniques to similar data sets, confirmed the results of the earlier ones. The results The results clearly showed that R Doradus is extended, having an angular diameter of 0.057 +- 0.005 arcsec (assuming that the star appears as a uniform disk). This apparent size is 30% larger than Betelgeuse! The bigger a star's apparent diameter, the more easily it can be resolved. The surprise is therefore not only the large diameter of R Doradus, but also the fact that this was not discovered earlier. Many of the larger stars were already measured by Albert Michelson and his team. The reason for the late discovery is most likely the southern latitude of R~Doradus, which makes it inaccessible to the stellar interferometers predominantly located on the northern hemisphere. R Doradus is an inconspicuous star at visible wavelength but is one of the brightest in the sky in the infrared. This led Robert Wing (Ohio State University) to predict in 1971 that R Doradus should have a large angular size. Only now has this prediction been confirmed. The NTT observations were made in the infrared. At first sight it may seem more sensible to observe at shorter, visible wavelengths because this would result in better angular resolution. However, measurements in the infrared - although more difficult to perform - result in a better estimate of the diameter of the underlying atmosphere ( photosphere ) of a star. The combination of a high-quality telescope and a high-quality infrared camera made this result possible. R Doradus is approximately 200 light years away. The measured size implies that it has a physical diameter of 370 +- 50 times that of the Sun, or well over 250 million km! If R Doradus would be placed at the centre of the Solar System, its surface would be outside of the orbit of Mars. Although even bigger stars are known - Betelgeuse for one - none appears as large in the sky because they are all at greater distances. The very large apparent size of R Doradus is due to the combination of its relative proximity and large physical size. R Doradus has about the same mass as the Sun, but it is 6500 times brighter [3]. Interferometry with the VLT Although much more difficult, interferometry can also be done combining light from different telescopes. This has been successfully demonstrated by teams in France, UK and USA. As the telescopes can be some distance apart, the separation of the collecting apertures can be much increased, simulating a telescope with a diameter of a hundred metres, and the angular resolution can reach the level of the milli-arcsec. This will be the case when the ESO Very Large Telescope Interferometer (VLTI) becomes operational some years from now. The VLTI is able to combine the light of four telescopes with 8.2-m diameter, and also from several smaller, movable auxiliary 1.8-m telescopes, at separations of up to 200 metres. The VLTI will be a very powerful tool for studying small details in many astronomical objects. The team has already made observations of R~Doradus with the Anglo-Australian Telescope which show the star to have structure on its surface, analogous to (but many times larger than) Sun spots. The VLTI would provide forty times more resolution, allowing such structures to be studied in incredible detail. More information about this research project An article describing the results will appear in the April 21, 1997 issue of the British scientific journal Monthly Notices of the Royal Astronomical Society . Notes: [1] Tim R. Bedding, J. Gordon Robertson and Ralph G. Marson (School of Physics, University of Sydney, Australia), Albert A. Zijlstra and Oskar von der Lühe (ESO), John R. Barton (Anglo-Australian Observatory, Epping, Australia) and Brian S. Carter (South African Astronomical Observatory, Observatory, South Africa). [2] For more details, see the article First light from the NTT Interferometer on page 2 of the December 1993 issue of the ESO house journal The Messenger. [3] This number (6500) was incorrected given (as 180) on the printed version of this Press Release. Sorry for the inconvenience!. How to obtain ESO Press Information ESO Press Information is made available on the World-Wide Web (URL: http://www.eso.org../). ESO Press Photos may be reproduced, if credit is given to the European Southern Observatory.

RadioAstron Science Program Five Years after Launch: Main Science Results

NASA Astrophysics Data System (ADS)

Kardashev, N. S.; Alakoz, A. V.; Andrianov, A. S.; Artyukhov, M. I.; Baan, W.; Babyshkin, V. E.; Bartel, N.; Bayandina, O. S.; Val'tts, I. E.; Voitsik, P. A.; Vorobyov, A. Z.; Gwinn, C.; Gomez, J. L.; Giovannini, G.; Jauncey, D.; Johnson, M.; Imai, H.; Kovalev, Y. Y.; Kurtz, S. E.; Lisakov, M. M.; Lobanov, A. P.; Molodtsov, V. A.; Novikov, B. S.; Pogodin, A. V.; Popov, M. V.; Privesenzev, A. S.; Rudnitski, A. G.; Rudnitski, G. M.; Savolainen, T.; Smirnova, T. V.; Sobolev, A. M.; Soglasnov, V. A.; Sokolovsky, K. V.; Filippova, E. N.; Khartov, V. V.; Churikova, M. E.; Shirshakov, A. E.; Shishov, V. I.; Edwards, P.

2017-12-01

The RadioAstron ground-space interferometer provides the highest angular resolution achieved now in astronomy. The detection of interferometric fringes from quasars with this angular resolution on baselines of 100-200 thousand km suggests the brightness temperatures which exceed the Compton limit by two orders of magnitude. Polarimetric measurements on ground-space baselines have revealed fine structure testifying to recollimation shocks on scales of 100-250 μas and a helical magnetic field near the base of radio emission in BL Lacertae. Substructure within a the scattering disk of pulsar emission on interferometer baselines (from 60000 to 250000 km) was discovered. This substructure is produced by action of the interstellar interferometer with an effective baseline of about 1 AU and the effective angular resolution of better than 1 μas. Diameters of scattering disks were measured for several pulsars, and distances to diffusing screens were evaluated. The ground-space observations of sources of the maser radiation in lines of water and hydroxyl have shown that the maser sources in star-forming regions remain unresolved on baselines, which considerably exceed the Earth diameter. These very compact and bright features with angular sizes of about 20-60 μas correspond to linear sizes of about 5-10 million km (several solar diameters).

Direct printing of miniscule aluminum alloy droplets and 3D structures by StarJet technology

NASA Astrophysics Data System (ADS)

Gerdes, B.; Zengerle, R.; Koltay, P.; Riegger, L.

2018-07-01

Drop-on demand printing of molten metal droplets could be used for prototyping 3D objects as a promising alternative to laser melting technologies. However, to date, only few printheads have been investigated for this purpose, and they used only a limited range of materials. The pneumatically actuated StarJet technology enables the direct and non-contact printing of molten metal microdroplets from metal melts at high temperatures. StarJet printheads utilize nozzle chips featuring a star-shaped orifice geometry that leads to formation of droplets inside the nozzle with high precision. In this paper, we present a novel StarJet printhead for printing aluminum (Al) alloys featuring a hybrid design with a ceramic reservoir for the molten metal and an outer shell fabricated from stainless steel. The micro machined nozzle chip is made from silicon carbide (SiC). This printhead can be operated at up to 950 °C, and is capable of printing high melting point metals like Al alloys in standard laboratory conditions. In this work, an aluminum–silicon alloy that features 12% silicon (AlSi12) is printed. The printhead, nozzle, and peripheral actuation system are optimized for stable generation of AlSi12 droplets with high monodispersity, low angular deviation, and miniaturized droplet diameters. As a result, a stable drop-on-demand printing of droplets exhibiting diameters of d droplet  =  702 µm  ±  1% is demonstrated at 5 Hz with a low angular deviation of 0.3°, when a nozzle chip with 500 µm orifice diameter is used. Furthermore, AlSi12 droplets featuring d droplet  =  176 µm  ±  7% are printed when using a nozzle chip with an orifice diameter of 130 µm. Moreover, we present directly printed objects from molten Al alloy droplets, such as high aspect ratio, free-standing walls (aspect ratio 12:1), and directly printed, flexible springs, to demonstrate the principle of 3D printing with molten metal droplets.

Optical Multi-Channel Intensity Interferometry - Or: How to Resolve O-Stars in the Magellanic Clouds

NASA Astrophysics Data System (ADS)

Trippe, Sascha; Kim, Jae-Young; Lee, Bangwon; Choi, Changsu; Oh, Junghwan; Lee, Taeseok; Yoon, Sung-Chul; Im, Myungshin; Park, Yong-Sun

2014-12-01

Intensity interferometry, based on the Hanbury Brown--Twiss effect, is a simple and inexpensive method for optical interferometry at microarcsecond angular resolutions; its use in astronomy was abandoned in the 1970s because of low sensitivity. Motivated by recent technical developments, we argue that the sensitivity of large modern intensity interferometers can be improved by factors up to approximately 25,000, corresponding to 11 photometric magnitudes, compared to the pioneering Narrabri Stellar Interferometer. This is made possible by (i) using avalanche photodiodes (APD) as light detectors, (ii) distributing the light received from the source over multiple independent spectral channels, and (iii) use of arrays composed of multiple large light collectors. Our approach permits the construction of large (with baselines ranging from few kilometers to intercontinental distances) optical interferometers at the cost of (very) long-baseline radio interferometers. Realistic intensity interferometer designs are able to achieve limiting R-band magnitudes as good as m_R≈14, sufficient for spatially resolved observations of main-sequence O-type stars in the Magellanic Clouds. Multi-channel intensity interferometers can address a wide variety of science cases: (i) linear radii, effective temperatures, and luminosities of stars, via direct measurements of stellar angular sizes; (ii) mass--radius relationships of compact stellar remnants, via direct measurements of the angular sizes of white dwarfs; (iii) stellar rotation, via observations of rotation flattening and surface gravity darkening; (iv) stellar convection and the interaction of stellar photospheres and magnetic fields, via observations of dark and bright starspots; (v) the structure and evolution of multiple stars, via mapping of the companion stars and of accretion flows in interacting binaries; (vi) direct measurements of interstellar distances, derived from angular diameters of stars or via the interferometric Baade--Wesselink method; (vii) the physics of gas accretion onto supermassive black holes, via resolved observations of the central engines of luminous active galactic nuclei; and (viii) calibration of amplitude interferometers by providing a sample of calibrator stars.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Baines, Ellyn K.; Armstrong, J. Thomas, E-mail: ellyn.baines@nrl.navy.mil, E-mail: tarmstr@crater.nrl.navy.mil

We measured the angular diameter of the exoplanet host star {epsilon} Eridani using the Navy Optical Interferometer. We determined its physical radius, effective temperature, and mass by combining our measurement with the star's parallax, photometry from the literature, and the Yonsei-Yale isochrones, respectively. We used the resulting stellar mass of 0.82 {+-} 0.05 M{sub Sun} plus the mass function from Benedict et al. to calculate the planet's mass, which is 1.53 {+-} 0.22 M{sub Jupiter}. Using our new effective temperature, we also estimated the extent of the habitable zone for the system.

Ring structure in the HII region of NGC 5930

NASA Astrophysics Data System (ADS)

Su, Bu-Mei; Mutel, R. L.; Zhang, Fu-Jing; Li, Yong-Sheng

1992-03-01

Radio continuous observations of the barred spiral galaxy NGC5930 at 2- and 3.6-cm wavelengths have been carried out with the VLA. It has been found that at 2 cm the HII region appears to be a ring structure on which hot spots are distributed. The outer angular diameter of the ring is 2.2 arcsec, and the inner angular diameter - 0.3 arcsec. The center is a hole from which no radio emission has been detected. The electron density in the HII region is 80 - 90 cu cm, and its mass is 10 exp 7 solar mass units. In NGC 5930 there is very strong infrared radiation. The infrared luminosity is 10 exp 6 times larger than the radio luminosity. There is a steep Balmer attenuation. This is a region where a star is being formed violently.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Baines, Ellyn K.; Armstrong, J. Thomas; Van Belle, Gerard T., E-mail: ellyn.baines@nrl.navy.mil

We used the Navy Precision Optical Interferometer to measure the limb-darkened angular diameter of the exoplanet host star {kappa} CrB and obtained a value of 1.543 {+-} 0.009 mas. We calculated its physical radius (5.06 {+-} 0.04 R{sub Sun }) and used photometric measurements from the literature with our diameter to determine {kappa} CrB's effective temperature (4788 {+-} 17 K) and luminosity (12.13 {+-} 0.09 L{sub Sun }). We then placed the star on an Hertzsprung-Russell diagram to ascertain the star's age (3.42{sup +0.32}{sub -0.25} Gyr) and mass (1.47 {+-} 0.04 M{sub Sun }) using a metallicity of [Fe/H] =more » +0.15. With this mass, we calculated the system's mass function with the orbital elements from a variety of sources, which produced a range of planetary masses: m{sub p}sin i = 1.61-1.88 M{sub Jup}. We also updated the extent of the habitable zone for the system using our new temperature.« less

Study of exoplanets host stars with VEGA/CHARA

NASA Astrophysics Data System (ADS)

Ligi, R.; Mourard, D.; Lagrange, Anne-Marie; Perraut, Karine; Tallon-Bosc, I.

2012-07-01

In the framework of the understanding of extrasolar systems, the study of host stars is a fundamental point. We need to understand the link between them and the presence of companions, i.e. what makes a star becoming a host star. In this perspective, we used the instrument called VEGA, situated at Mount Wilson (California) on the CHARA array to perform optical interferometric measurements. Interferometry at visible wavelengths allows reaching very high spatial frequencies well adapted for very small (less than 1 millisecond of arc) angular diameters. Therefore, we can access limb darkening measurements which is one of the very few directly measurable constraints on the structure of the atmosphere of a star. From this we can derive stars fundamental parameters. A precise measurement within spectral lines is also a very powerful tool to study the temperature and density structure of the atmosphere of distant stars. Besides, the detection of exoplanets is also related to this method. Combined with the radial velocity method and the transit method, one can study the atmosphere of exoplanets and learn more about their internal structure. We started a large program of observations made of 40 stars hosting exoplanets and observable by VEGA/CHARA. We will measure their limb darkened diameters and derive their parameters. We also aim at better understanding stellar noise sources like spots, and study surface brightness relationships.

ARTIST'S CONCEPT -- 'HOT JUPITER' AROUND THE STAR HD 209458

NASA Technical Reports Server (NTRS)

2002-01-01

This is an artist's impression of the gas-giant planet orbiting the yellow, Sun-like star HD 209458, 150 light-years from Earth. Astronomers used NASA's Hubble Space Telescope to look at this world and make the first direct detection of an atmosphere around an extrasolar planet. The planet was not directly seen by Hubble. Instead, the presence of sodium was detected in light filtered through the planet's atmosphere when it passed in front of its star as seen from Earth (an event called a transit). The planet was discovered in 1999 by its subtle gravitational pull on the star. The planet is 70 percent the mass of Jupiter, the largest planet in our solar system. Its orbit is tilted nearly edge-on to Earth, which allows repeated transit observations. The planet is merely 4 million miles from the star. The distance between the pair is so close that the yellow star looms in the sky, with an angular diameter 23 times larger than the full Moon's diameter as seen from Earth, and glows 500 times brighter than our Sun. At this precarious distance the planet's atmosphere is heated to 2000 degrees Fahrenheit (1100 degrees Celsius). But the planet is big enough to hold onto its seething atmosphere. Illustration Credit: NASA and Greg Bacon (STScI/AVL)

Blurred Star Image Processing for Star Sensors under Dynamic Conditions

PubMed Central

Zhang, Weina; Quan, Wei; Guo, Lei

2012-01-01

The precision of star point location is significant to identify the star map and to acquire the aircraft attitude for star sensors. Under dynamic conditions, star images are not only corrupted by various noises, but also blurred due to the angular rate of the star sensor. According to different angular rates under dynamic conditions, a novel method is proposed in this article, which includes a denoising method based on adaptive wavelet threshold and a restoration method based on the large angular rate. The adaptive threshold is adopted for denoising the star image when the angular rate is in the dynamic range. Then, the mathematical model of motion blur is deduced so as to restore the blurred star map due to large angular rate. Simulation results validate the effectiveness of the proposed method, which is suitable for blurred star image processing and practical for attitude determination of satellites under dynamic conditions. PMID:22778666

Massive star formation by accretion. II. Rotation: how to circumvent the angular momentum barrier?

NASA Astrophysics Data System (ADS)

Haemmerlé, L.; Eggenberger, P.; Meynet, G.; Maeder, A.; Charbonnel, C.; Klessen, R. S.

2017-06-01

Context. Rotation plays a key role in the star-formation process, from pre-stellar cores to pre-main-sequence (PMS) objects. Understanding the formation of massive stars requires taking into account the accretion of angular momentum during their PMS phase. Aims: We study the PMS evolution of objects destined to become massive stars by accretion, focusing on the links between the physical conditions of the environment and the rotational properties of young stars. In particular, we look at the physical conditions that allow the production of massive stars by accretion. Methods: We present PMS models computed with a new version of the Geneva Stellar Evolution code self-consistently including accretion and rotation according to various accretion scenarios for mass and angular momentum. We describe the internal distribution of angular momentum in PMS stars accreting at high rates and we show how the various physical conditions impact their internal structures, evolutionary tracks, and rotation velocities during the PMS and the early main sequence. Results: We find that the smooth angular momentum accretion considered in previous studies leads to an angular momentum barrier and does not allow the formation of massive stars by accretion. A braking mechanism is needed in order to circumvent this angular momentum barrier. This mechanism has to be efficient enough to remove more than two thirds of the angular momentum from the inner accretion disc. Due to the weak efficiency of angular momentum transport by shear instability and meridional circulation during the accretion phase, the internal rotation profiles of accreting stars reflect essentially the angular momentum accretion history. As a consequence, careful choice of the angular momentum accretion history allows circumvention of any limitation in mass and velocity, and production of stars of any mass and velocity compatible with structure equations.

Sizes, Shapes, and Satellites of Asteroids from Occultations

NASA Astrophysics Data System (ADS)

Dunham, David W.; Herald, David; Preston, Steve; Timerson, Brad; Maley, Paul; Frappa, Eric; Hayamizu, Tsutomu; Talbot, John; Poro, Atila

2016-01-01

For 40 years, the sizes and shapes of many dozens of asteroids have been determined from observations of asteroidal occultations, and over a thousand high-precision positions of the asteroids relative to stars have been measured. Some of the first evidence for satellites of asteroids was obtained from the early efforts; now, the orbits and sizes of some satellites discovered by other means have been refined from occultation observations. Also, several close binary stars have been discovered, and the angular diameters of some stars have been measured from analysis of these observations. The International Occultation Timing Association (IOTA) coordinates this activity worldwide, from predicting and publicizing the events, to accurately timing the occultations from as many stations as possible, and publishing and archiving the observations.

Review and latest news from the VEGA/CHARA facility

NASA Astrophysics Data System (ADS)

Nardetto, N.; Mourard, D.; Perraut, K.; Tallon-Bosc, I.; Meilland, A.; Stee, P.; Ligi, R.; Challouf, M.; Clausse, J.-M.; Berio, P.; Spang, A.

2014-12-01

The VEGA instrument located at the focus of the Center for High Angular Resolution Astronomy (CHARA) array in California is a collaborating project between the Lagrange laboratory in Nice, where it has been developed (Mourard et al. 2009, 2011), the IPAG (Grenoble) and CRAL (Lyon) laboratories, and the CHARA group at Mount Wilson Observatory. The outcome from this international collaboration is to provide to the community a visible spectro-interferometer with an unprecedented angular resolution of 0.3 milli-second of arc (mas) together with a spectral resolution of 5000 or 30000. With such an instrument it becomes possible to determine simultaneously the size and the kinematic of the photosphere and/or of the circumstellar environment of the star as a function of the wavelength, which basically means for each spectral channel in the continuum and/or within spectral lines (in Hα for instance). The only limitation is to get enough signal to noise ratio in each spectral channel. We can currently reach a limiting magnitude of 8 in visible in medium spectral resolution (5000) and 4.5 in high resolution (30000). In this proceeding, we illustrate the two main subjects studied with the VEGA instrument, namely (1) how angular diameters are useful to accurately derive the fundamental parameters of stars, (2) how the spectral resolution can allow to study the kinematical structure of stars or even to derive chromatic images of stellar objects.

Accurate effective temperatures of the metal-poor benchmark stars HD 140283, HD 122563, and HD 103095 from CHARA interferometry

NASA Astrophysics Data System (ADS)

Karovicova, I.; White, T. R.; Nordlander, T.; Lind, K.; Casagrande, L.; Ireland, M. J.; Huber, D.; Creevey, O.; Mourard, D.; Schaefer, G. H.; Gilmore, G.; Chiavassa, A.; Wittkowski, M.; Jofré, P.; Heiter, U.; Thévenin, F.; Asplund, M.

2018-03-01

Large stellar surveys of the Milky Way require validation with reference to a set of `benchmark' stars whose fundamental properties are well determined. For metal-poor benchmark stars, disagreement between spectroscopic and interferometric effective temperatures has called the reliability of the temperature scale into question. We present new interferometric measurements of three metal-poor benchmark stars, HD 140283, HD 122563, and HD 103095, from which we determine their effective temperatures. The angular sizes of all the stars were determined from observations with the PAVO beam combiner at visible wavelengths at the CHARA array, with additional observations of HD 103095 made with the VEGA instrument, also at the CHARA array. Together with photometrically derived bolometric fluxes, the angular diameters give a direct measurement of the effective temperature. For HD 140283, we find θLD = 0.324 ± 0.005 mas, Teff = 5787 ± 48 K; for HD 122563, θLD = 0.926 ± 0.011 mas, Teff = 4636 ± 37 K; and for HD 103095, θLD = 0.595 ± 0.007 mas, Teff = 5140 ± 49 K. Our temperatures for HD 140283 and HD 103095 are hotter than the previous interferometric measurements by 253 and 322 K, respectively. We find good agreement between our temperatures and recent spectroscopic and photometric estimates. We conclude some previous interferometric measurements have been affected by systematic uncertainties larger than their quoted errors.

Spacecraft angular velocity estimation algorithm for star tracker based on optical flow techniques

NASA Astrophysics Data System (ADS)

Tang, Yujie; Li, Jian; Wang, Gangyi

2018-02-01

An integrated navigation system often uses the traditional gyro and star tracker for high precision navigation with the shortcomings of large volume, heavy weight and high-cost. With the development of autonomous navigation for deep space and small spacecraft, star tracker has been gradually used for attitude calculation and angular velocity measurement directly. At the same time, with the dynamic imaging requirements of remote sensing satellites and other imaging satellites, how to measure the angular velocity in the dynamic situation to improve the accuracy of the star tracker is the hotspot of future research. We propose the approach to measure angular rate with a nongyro and improve the dynamic performance of the star tracker. First, the star extraction algorithm based on morphology is used to extract the star region, and the stars in the two images are matched according to the method of angular distance voting. The calculation of the displacement of the star image is measured by the improved optical flow method. Finally, the triaxial angular velocity of the star tracker is calculated by the star vector using the least squares method. The method has the advantages of fast matching speed, strong antinoise ability, and good dynamic performance. The triaxial angular velocity of star tracker can be obtained accurately with these methods. So, the star tracker can achieve better tracking performance and dynamic attitude positioning accuracy to lay a good foundation for the wide application of various satellites and complex space missions.

Angular Diameters of Stars from the Mark III Optical Interferometer

DTIC Science & Technology

2003-11-01

0.105 8775 .............. Peg 16.528 0.165 16.326 0.229 16.464 0.230 15.970 0.319 17.982 0.180 8796 .............. 56 Peg 2.190 0.048 2.031...1.000 1.000 1.000 8796 .............. 56 Peg 1.001 0.005 0.987 0.012 . . . . . . Notes.—If an entry for a star does not have an error estimate, that...3.92 M2.5 III 3639 47 8775 .............. Peg 2.42 1.50 4.63 15.22 M2.5 II 3448 42 8796 .............. 56 Peg 4.77 0.97 . . . 0.54 G8.0 I 4152

AB Dor Moving Group Stars Resolved with the CHARA Array

NASA Astrophysics Data System (ADS)

Schaefer, G. H.; White, R. J.; Baines, E. K.; Boyajian, T. S.; ten Brummelaar, T. A.; Farrington, C. D.; Sturmann, J.; Sturmann, L.; Turner, N. H.

2018-05-01

We present interferometric measurements obtained with the CHARA Array of 13 adolescent-age stars in nearby moving groups. The motivation was to spatially resolve the largest stars and to search for binary companions. Nine stars have diameters smaller than the resolution limit and no evidence for companions within 0.5–50 mas and ΔH < 2.0 mag. The diameters of three stars were spatially resolved: GJ 159 (0.582 ± 0.016 mas) and GJ 393 (0.564 ± 0.021 mas) in the AB Dor moving group, and former member HD 89744 (0.556 ± 0.032 mas). Combining the angular diameters with their distances and bolometric fluxes, we measured radii and effective temperatures. The temperatures of GJ 159 (6286 ± 123 K) and GJ 393 (3515 ± 68 K) are consistent with spectroscopic measurements. Comparisons with evolutionary models show that HD 89744 has evolved off the main sequence. GJ 159 and GJ 393 lie within 1.5σ of the zero-age main sequence, complicating their age estimates because it is unclear whether the stars are contracting or expanding. GJ 159 has a mass of 1.2 ± 0.1 {M}ȯ with an age spanning 0.021–3.0 Gyr. Its debris disk and lithium abundance favor a young age. GJ 393 has a mass of 0.42 ± 0.03 {M}ȯ and a lower limit on its age 0.06 Gyr. This overlaps with the age of the moving group; however, an older age would be more consistent with its slow rotation, low activity, and luminosity, suggesting that GJ 393 is a kinematic interloper.

LSS 2018: A double-lined spectroscopic binary central star with an extremely large reflection effect

NASA Technical Reports Server (NTRS)

Drilling, J. S.

1985-01-01

LSS 2018, the central star of the planetry nebulae DS1, was found to be a double-lined spectroscopic binary with a period of 8.571 hours. Light variations with the same period were observed in U, B, and V; in the wavelength regions defined by the two IUE cameras; and in the strength of the CIII 4647 emission line. The light variations can be accurately predicted by a simple reflection effect, and an analysis of the light curves yields the angular diameter and effective temperature of the primary, the radii of the two stars in terms of their separation, and the inclination of the system. Analysis of the radial velocities then yields the masses of the two stars, their separation, the distance of the system, the absolute magnitude of the primary, and the size of the nebula.

INTERNAL GRAVITY WAVES IN MASSIVE STARS: ANGULAR MOMENTUM TRANSPORT

DOE Office of Scientific and Technical Information (OSTI.GOV)

Rogers, T. M.; Lin, D. N. C.; McElwaine, J. N.

2013-07-20

We present numerical simulations of internal gravity waves (IGW) in a star with a convective core and extended radiative envelope. We report on amplitudes, spectra, dissipation, and consequent angular momentum transport by such waves. We find that these waves are generated efficiently and transport angular momentum on short timescales over large distances. We show that, as in Earth's atmosphere, IGW drive equatorial flows which change magnitude and direction on short timescales. These results have profound consequences for the observational inferences of massive stars, as well as their long term angular momentum evolution. We suggest IGW angular momentum transport may explainmore » many observational mysteries, such as: the misalignment of hot Jupiters around hot stars, the Be class of stars, Ni enrichment anomalies in massive stars, and the non-synchronous orbits of interacting binaries.« less

A hybrid method for accurate star tracking using star sensor and gyros.

PubMed

Lu, Jiazhen; Yang, Lie; Zhang, Hao

2017-10-01

Star tracking is the primary operating mode of star sensors. To improve tracking accuracy and efficiency, a hybrid method using a star sensor and gyroscopes is proposed in this study. In this method, the dynamic conditions of an aircraft are determined first by the estimated angular acceleration. Under low dynamic conditions, the star sensor is used to measure the star vector and the vector difference method is adopted to estimate the current angular velocity. Under high dynamic conditions, the angular velocity is obtained by the calibrated gyros. The star position is predicted based on the estimated angular velocity and calibrated gyros using the star vector measurements. The results of the semi-physical experiment show that this hybrid method is accurate and feasible. In contrast with the star vector difference and gyro-assisted methods, the star position prediction result of the hybrid method is verified to be more accurate in two different cases under the given random noise of the star centroid.

Bringing the Visible Universe into Focus with Robo-AO

PubMed Central

Baranec, Christoph; Riddle, Reed; Law, Nicholas M.; Ramaprakash, A.N.; Tendulkar, Shriharsh P.; Bui, Khanh; Burse, Mahesh P.; Chordia, Pravin; Das, Hillol K.; Davis, Jack T.C.; Dekany, Richard G.; Kasliwal, Mansi M.; Kulkarni, Shrinivas R.; Morton, Timothy D.; Ofek, Eran O.; Punnadi, Sujit

2013-01-01

The angular resolution of ground-based optical telescopes is limited by the degrading effects of the turbulent atmosphere. In the absence of an atmosphere, the angular resolution of a typical telescope is limited only by diffraction, i.e., the wavelength of interest, λ, divided by the size of its primary mirror's aperture, D. For example, the Hubble Space Telescope (HST), with a 2.4-m primary mirror, has an angular resolution at visible wavelengths of ~0.04 arc seconds. The atmosphere is composed of air at slightly different temperatures, and therefore different indices of refraction, constantly mixing. Light waves are bent as they pass through the inhomogeneous atmosphere. When a telescope on the ground focuses these light waves, instantaneous images appear fragmented, changing as a function of time. As a result, long-exposure images acquired using ground-based telescopes - even telescopes with four times the diameter of HST - appear blurry and have an angular resolution of roughly 0.5 to 1.5 arc seconds at best. Astronomical adaptive-optics systems compensate for the effects of atmospheric turbulence. First, the shape of the incoming non-planar wave is determined using measurements of a nearby bright star by a wavefront sensor. Next, an element in the optical system, such as a deformable mirror, is commanded to correct the shape of the incoming light wave. Additional corrections are made at a rate sufficient to keep up with the dynamically changing atmosphere through which the telescope looks, ultimately producing diffraction-limited images. The fidelity of the wavefront sensor measurement is based upon how well the incoming light is spatially and temporally sampled1. Finer sampling requires brighter reference objects. While the brightest stars can serve as reference objects for imaging targets from several to tens of arc seconds away in the best conditions, most interesting astronomical targets do not have sufficiently bright stars nearby. One solution is to focus a high-power laser beam in the direction of the astronomical target to create an artificial reference of known shape, also known as a 'laser guide star'. The Robo-AO laser adaptive optics system2,3 employs a 10-W ultraviolet laser focused at a distance of 10 km to generate a laser guide star. Wavefront sensor measurements of the laser guide star drive the adaptive optics correction resulting in diffraction-limited images that have an angular resolution of ~0.1 arc seconds on a 1.5-m telescope. PMID:23426078

Bringing the visible universe into focus with Robo-AO.

PubMed

Baranec, Christoph; Riddle, Reed; Law, Nicholas M; Ramaprakash, A N; Tendulkar, Shriharsh P; Bui, Khanh; Burse, Mahesh P; Chordia, Pravin; Das, Hillol K; Davis, Jack T C; Dekany, Richard G; Kasliwal, Mansi M; Kulkarni, Shrinivas R; Morton, Timothy D; Ofek, Eran O; Punnadi, Sujit

2013-02-12

The angular resolution of ground-based optical telescopes is limited by the degrading effects of the turbulent atmosphere. In the absence of an atmosphere, the angular resolution of a typical telescope is limited only by diffraction, i.e., the wavelength of interest, λ, divided by the size of its primary mirror's aperture, D. For example, the Hubble Space Telescope (HST), with a 2.4-m primary mirror, has an angular resolution at visible wavelengths of ~0.04 arc seconds. The atmosphere is composed of air at slightly different temperatures, and therefore different indices of refraction, constantly mixing. Light waves are bent as they pass through the inhomogeneous atmosphere. When a telescope on the ground focuses these light waves, instantaneous images appear fragmented, changing as a function of time. As a result, long-exposure images acquired using ground-based telescopes--even telescopes with four times the diameter of HST--appear blurry and have an angular resolution of roughly 0.5 to 1.5 arc seconds at best. Astronomical adaptive-optics systems compensate for the effects of atmospheric turbulence. First, the shape of the incoming non-planar wave is determined using measurements of a nearby bright star by a wavefront sensor. Next, an element in the optical system, such as a deformable mirror, is commanded to correct the shape of the incoming light wave. Additional corrections are made at a rate sufficient to keep up with the dynamically changing atmosphere through which the telescope looks, ultimately producing diffraction-limited images. The fidelity of the wavefront sensor measurement is based upon how well the incoming light is spatially and temporally sampled. Finer sampling requires brighter reference objects. While the brightest stars can serve as reference objects for imaging targets from several to tens of arc seconds away in the best conditions, most interesting astronomical targets do not have sufficiently bright stars nearby. One solution is to focus a high-power laser beam in the direction of the astronomical target to create an artificial reference of known shape, also known as a 'laser guide star'. The Robo-AO laser adaptive optics system, employs a 10-W ultraviolet laser focused at a distance of 10 km to generate a laser guide star. Wavefront sensor measurements of the laser guide star drive the adaptive optics correction resulting in diffraction-limited images that have an angular resolution of ~0.1 arc seconds on a 1.5-m telescope.

Generation-X: An X-ray observatory designed to observe first light objects

NASA Astrophysics Data System (ADS)

Windhorst, Rogier A.; Cameron, R. A.; Brissenden, R. J.; Elvis, M. S.; Fabbiano, G.; Gorenstein, P.; Reid, P. B.; Schwartz, D. A.; Bautz, M. W.; Figueroa-Feliciano, E.; Petre, R.; White, N. E.; Zhang, W. W.

2006-03-01

The new cosmological frontier will be the study of the very first stars, galaxies and black holes in the early Universe. These objects are invisible to the current generation of X-ray telescopes, such as Chandra. In response, the Generation-X ("Gen-X") Vision Mission has been proposed as a future X-ray observatory which will be capable of detecting the earliest objects. X-ray imaging and spectroscopy of such faint objects demands a large collecting area and high angular resolution. The Gen-X mission plans 100 m 2 collecting area at 1 keV (1000× that of Chandra), and with an angular resolution of 0.1″. The Gen-X mission will operate at Sun-Earth L2, and might involve four 8 m diameter telescopes or even a single 20 m diameter telescope. To achieve the required effective area with reasonable mass, very lightweight grazing incidence X-ray optics must be developed, having an areal density 100× lower than in Chandra, with mirrors as thin as 0.1 mm requiring active on-orbit figure control. The suite of available detectors for Gen-X should include a large-area high resolution imager, a cryogenic imaging spectrometer, and a grating spectrometer. We discuss use of Gen-X to observe the birth of the first black holes, stars and galaxies, and trace their cosmic evolution.

Detection of a late B star companion of the bright cluster giant C PUP equals HD 63032

NASA Astrophysics Data System (ADS)

Groote, D.; Reimers, D.

1983-03-01

IUE observations show that c Pup, the central bright K giant in the open cluster NGC 2451, has a blue companion. A fit of theoretical line blanketed model atmosphere fluxes to the observed energy distribution yields reddening E(B-V) = 0.15 (from λ2200 Å feature), an effective temperature Te = 10,200K, and an angular diameter θ = 0.060. If the companion is a main-sequence star, c Pup and its companion are located at a distance of 310 ± 50 pc which lends additional support to membership of c Pup in NGC 2451. The evolutionary status of c Pup is briefly discussed.

Interferometric diameters of five evolved intermediate-mass planet-hosting stars measured with PAVO at the CHARA Array

NASA Astrophysics Data System (ADS)

White, T. R.; Huber, D.; Mann, A. W.; Casagrande, L.; Grunblatt, S. K.; Justesen, A. B.; Silva Aguirre, V.; Bedding, T. R.; Ireland, M. J.; Schaefer, G. H.; Tuthill, P. G.

2018-04-01

Debate over the planet occurrence rates around intermediate-mass stars has hinged on the accurate determination of masses of evolved stars, and has been exacerbated by a paucity of reliable, directly-measured fundamental properties for these stars. We present long-baseline optical interferometry of five evolved intermediate-mass (˜ 1.5 M⊙) planet-hosting stars using the PAVO beam combiner at the CHARA Array, which we combine with bolometric flux measurements and parallaxes to determine their radii and effective temperatures. We measured the radii and effective temperatures of 6 Lyncis (5.12±0.16 R⊙, 4949±58 K), 24 Sextantis (5.49±0.18 R⊙, 4908±65 K), κ Coronae Borealis (4.77±0.07 R⊙, 4870±47 K), HR 6817 (4.45±0.08 R⊙, 5013±59 K), and HR 8641 (4.91±0.12 R⊙, 4950±68 K). We find disagreements of typically 15 % in angular diameter and ˜ 200 K in temperature compared to interferometric measurements in the literature, yet good agreement with spectroscopic and photometric temperatures, concluding that the previous interferometric measurements may have been affected by systematic errors exceeding their formal uncertainties. Modelling based on BaSTI isochrones using various sets of asteroseismic, spectroscopic, and interferometric constraints tends to favour slightly (˜ 15 %) lower masses than generally reported in the literature.

Interferometric diameters of five evolved intermediate-mass planet-hosting stars measured with PAVO at the CHARA Array

NASA Astrophysics Data System (ADS)

White, T. R.; Huber, D.; Mann, A. W.; Casagrande, L.; Grunblatt, S. K.; Justesen, A. B.; Silva Aguirre, V.; Bedding, T. R.; Ireland, M. J.; Schaefer, G. H.; Tuthill, P. G.

2018-07-01

Debate over the planet occurrence rates around intermediate-mass stars has hinged on the accurate determination of masses of evolved stars, and has been exacerbated by a paucity of reliable, directly measured fundamental properties for these stars. We present long-baseline optical interferometry of five evolved intermediate-mass (˜ 1.5 M⊙) planet-hosting stars using the PAVO beam combiner at the CHARA Array, which we combine with bolometric flux measurements and parallaxes to determine their radii and effective temperatures. We measured the radii and effective temperatures of 6 Lyncis (5.12 ± 0.16 R⊙, 4949 ± 58 K), 24 Sextantis (5.49 ± 0.18 R⊙, 4908 ± 65 K), κ Coronae Borealis (4.77 ± 0.07 R⊙, 4870 ± 47 K), HR 6817 (4.45 ± 0.08 R⊙, 5013 ± 59 K), and HR 8461 (4.91 ± 0.12 R⊙, 4950 ± 68 K). We find disagreements of typically 15 per cent in angular diameter and ˜200 K in temperature compared to interferometric measurements in the literature, yet good agreement with spectroscopic and photometric temperatures, concluding that the previous interferometric measurements may have been affected by systematic errors exceeding their formal uncertainties. Modelling based on BaSTI isochrones using various sets of asteroseismic, spectroscopic, and interferometric constraints tends to favour slightly (˜15 per cent) lower masses than generally reported in the literature.

The evolution of rotating stars. III - Predicted surface rotation velocities for stars which conserve total angular momentum

NASA Technical Reports Server (NTRS)

Endal, A. S.; Sofia, S.

1979-01-01

Predicted surface rotation velocities for Population I stars at 10, 7, 5, 3, and 1.5 solar masses are presented. The surface velocities were computed for angular momentum with no radial redistribution, complete redistribution, and partial redistribution as predicted by consideration of circulation currents in rotating stars. Near the main sequence, rotational effects can reduce the moment of inertia of a star, so nonrotating models underestimate the expected velocities for evolving stars. On the red giant branch, angular momentum redistribution reduces the surface velocity by a factor of 2 or more, relative to the velocity expected for no radial redistribution. This removes the discrepancy between predicted and observed rotation rates for the K giants and makes it unlikely that these stars lose significant amounts of angular momentum by stellar winds. Calculations indicate that improved observations of the red giants in the Hyades cluster can be used to determine how angular momentum is redistributed by convection

Spin-up of a rapidly rotating star by angular momentum loss - Effects of general relativity

NASA Technical Reports Server (NTRS)

Cook, Gregory B.; Shapiro, Stuart L.; Teukolsky, Saul A.

1992-01-01

It has recently been shown that a rapidly rotating Newtonian star can spin up by radiating angular momentum. Extremely fast pulsars losing energy and angular momentum by magnetic dipole radiation or gravitational radiation may exhibit this behavior. Here, we show that this phenomenon is more widespread for rapidly rotating stars in general relativity. We construct and tabulate polytropic sequences of fully relativistic rotating stars of constant rest mass and entropy. We find that the range of adiabatic indices allowing spin-up extends somewhat above 4/3 because of the nonlinear effects of relativistic gravity. In addition, there is a new class of 'supramassive' stars which will inevitably spin up by losing angular momentum regardless of their equation of state. A supramassive star, spinning up via angular momentum loss, will ultimately evolve until it becomes unstable to catastrophic collapse to a black hole. Spin-up in a rapidly rotating star may thus be an observational precursor to such collapse.

Revolution evolution: tracing angular momentum during star and planetary system formation

NASA Astrophysics Data System (ADS)

Davies, Claire Louise

2015-04-01

Stars form via the gravitational collapse of molecular clouds during which time the protostellar object contracts by over seven orders of magnitude. If all the angular momentum present in the natal cloud was conserved during collapse, stars would approach rotational velocities rapid enough to tear themselves apart within just a few Myr. In contrast to this, observations of pre-main sequence rotation rates are relatively slow (∼ 1 - 15 days) indicating that significant quantities of angular momentum must be removed from the star. I use observations of fully convective pre-main sequence stars in two well-studied, nearby regions of star formation (namely the Orion Nebula Cluster and Taurus-Auriga) to determine the removal rate of stellar angular momentum. I find the accretion disc-hosting stars to be rotating at a slower rate and contain less specific angular momentum than the disc-less stars. I interpret this as indicating a period of accretion disc-regulated angular momentum evolution followed by near-constant rotational evolution following disc dispersal. Furthermore, assuming that the age spread inferred from the Hertzsprung-Russell diagram constructed for the star forming region is real, I find that the removal rate of angular momentum during the accretion-disc hosting phase to be more rapid than that expected from simple disc-locking theory whereby contraction occurs at a fixed rotation period. This indicates a more efficient process of angular momentum removal must operate, most likely in the form of an accretion-driven stellar wind or outflow emanating from the star-disc interaction. The initial circumstellar envelope that surrounds a protostellar object during the earliest stages of star formation is rotationally flattened into a disc as the star contracts. An effective viscosity, present within the disc, enables the disc to evolve: mass accretes inwards through the disc and onto the star while momentum migrates outwards, forcing the outer regions of the disc to expand. I used spatially resolved submillimetre detections of the dust and gas components of protoplanetary discs, gathered from the literature, to measure the radial extent of discs around low-mass pre-main sequence stars of ∼ 1-10 Myr and probe their viscous evolution. I find no clear observational evidence for the radial expansion of the dust component. However, I find tentative evidence for the expansion ofthe gas component. This suggests that the evolution of the gas and dust components of protoplanetary discs are likely governed by different astrophysical processes. Observations of jets and outflows emanating from protostars and pre-main sequence stars highlight that it may also be possible to remove angular momentum from the circumstellar material. Using the sample of spatially resolved protoplanetary discs, I find no evidence for angular momentum removal during disc evolution. I also use the spatially resolved debris discs from the Submillimetre Common-User Bolometer Array-2 Observations of Nearby Stars survey to constrain the amount of angular momentum retained within planetary systems. This sample is compared to the protoplanetary disc angular momenta and to the angular momentum contained within pre-stellar cores. I find that significant quantities of angular momentum must be removed during disc formation and disc dispersal. This likely occurs via magnetic braking during the formation of the disc, via the launching of a disc or photo-evaporative wind, and/or via ejection of planetary material following dynamical interactions.

STELLAR PARAMETERS FOR HD 69830, A NEARBY STAR WITH THREE NEPTUNE MASS PLANETS AND AN ASTEROID BELT

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tanner, Angelle; Boyajian, Tabetha S.; Brewer, John M.

We used the CHARA Array to directly measure the angular diameter of HD 69830, home to three Neptune mass planets and an asteroid belt. Our measurement of 0.674 ± 0.014 mas for the limb-darkened angular diameter of this star leads to a physical radius of R {sub *} = 0.9058 ± 0.0190 R {sub ☉} and luminosity of L {sub *} = 0.622 ± 0.014 L {sub ☉} when combined with a fit to the spectral energy distribution of the star. Placing these observed values on an Hertzsprung-Russel diagram along with stellar evolution isochrones produces an age of 10.6 ± 4 Gyr and mass of 0.863 ± 0.043 M {sub ☉}. We usemore » archival optical echelle spectra of HD 69830 along with an iterative spectral fitting technique to measure the iron abundance ([Fe/H] = –0.04 ± 0.03), effective temperature (5385 ± 44 K), and surface gravity (log g = 4.49 ± 0.06). We use these new values for the temperature and luminosity to calculate a more precise age of 7.5 ± 3 Gyr. Applying the values of stellar luminosity and radius to recent models on the optimistic location of the habitable zone produces a range of 0.61-1.44 AU; partially outside the orbit of the furthest known planet (d) around HD 69830. Finally, we estimate the snow line at a distance of 1.95 ± 0.19 AU, which is outside the orbit of all three planets and its asteroid belt.« less

On-Sky Tests of a High-Power Pulsed Laser for Sodium Laser Guide Star Adaptive Optics

NASA Astrophysics Data System (ADS)

Otarola, Angel; Hickson, Paul; Gagné, Ronald; Bo, Yong; Zuo, Junwei; Xie, Shiyong; Feng, Lu; Rochester, Simon; Budker, Dmitry; Shen, Shixia; Xue, Suijian; Min, Li; Wei, Kai; Boyer, Corinne; Ellerbroek, Brent; Hu, Jingyao; Peng, Qinjun; Xu, Zuyan

2016-03-01

We present results of on-sky tests performed in the summer of 2013 to characterize the performance of a prototype high-power pulsed laser for adaptive optics. The laser operates at a pulse repetition rate (PRR) of 600-800Hz, with a 6% duty cycle. Its coupling efficiency was found to be, in the best test case (using 18W of transmitted power), 231±14 photons s-1 sr-1 atom-1 W-1 m2 when circular polarization was employed and 167±17 photons s-1 sr-1 atom-1 W-1 m2 with linear polarization. No improvement was seen when D2b repumping was used, but this is likely due to the relatively large laser guide star (LGS) diameter, typically 10 arcsec or more, which resulted in low irradiance levels. Strong relaxation oscillations were present in the laser output, which have the effect of reducing the coupling efficiency. To better understand the results, a physical modeling was performed using the measured pulse profiles and parameters specific to these tests. The model results, for a 10 arcsec angular size LGS spot, agree well with the observations. When extrapolating the physical model for a sub-arcsecond angular size LGS (typical of what is needed for a successful astronomical guide star), the model predicts that this laser would have a coupling efficiency of 130 photons s-1 sr-1 atom-1 W-1 m2, using circular polarization and D2b repumping, for a LGS diameter of 0.6 arcsec Full Width at Half Maximum (FWHM), and free of relaxation oscillations in the 589 nm laser light.

The evolution of angular momentum among zero-age main-sequence solar-type stars

NASA Technical Reports Server (NTRS)

Soderblom, David R.; Stauffer, John R.; Macgregor, Keith B.; Jones, Burton F.

1993-01-01

We consider a survey of rotation among F, G, and K dwarfs of the Pleiades in the context of other young clusters (Alpha Persei and the Hyades) and pre-main-sequence (PMS) stars (in Taurus-Auriga and Orion) in order to examine how the angular momentum of a star like the sun evolves during its early life on the main sequence. The rotation of PMS stars can be evolved into distributions like those seen in the young clusters if there is only modest, rotation-independent angular momentum loss prior to the ZAMS. Even then, the ultrafast rotators (UFRs, or ZAMS G and K dwarfs with v sin i equal to or greater than 30 km/s) must owe their extra angular momentum to their conditions of formation and to different angular momentum loss rates above a threshold velocity, for it is unlikely that these stars had angular momentum added as they neared the ZAMS, nor can a spread in ages within a cluster account for the range of rotation seen. Only a fraction of solar-type stars are thus capable of becoming UFRs, and it is not a phase that all stars experience. Simple scaling relations (like the Skumanich relation) applied to the observed surface rotation rates of young solar-type stars cannot reproduce the way in which the Pleiades evolve into the Hyades. We argue that invoking internal differential rotation in these ZAMS stars can explain several aspects of the observations and thus can provide a consistent picture of ZAMS angular momentum evolution.

Developing an Optical Lunar Occultation Measurement Reduction System for Observations at Kaau Observatory

NASA Astrophysics Data System (ADS)

Malawi, Abdulrahman A.

2013-06-01

We present here a detailed explanation of the reduction method that we use to determine the angular diameters of the stars occulted by the dark limb of the moon. This is a main part of the lunar occultation observation program running at King Abdul Aziz University observatory since late 1993. The process is based on the least square model fitting method of analyzing occultation data, first introduced by Nather et al. (Astron. J. 75:963, 1970).

Satellite angular velocity estimation based on star images and optical flow techniques.

PubMed

Fasano, Giancarmine; Rufino, Giancarlo; Accardo, Domenico; Grassi, Michele

2013-09-25

An optical flow-based technique is proposed to estimate spacecraft angular velocity based on sequences of star-field images. It does not require star identification and can be thus used to also deliver angular rate information when attitude determination is not possible, as during platform de tumbling or slewing. Region-based optical flow calculation is carried out on successive star images preprocessed to remove background. Sensor calibration parameters, Poisson equation, and a least-squares method are then used to estimate the angular velocity vector components in the sensor rotating frame. A theoretical error budget is developed to estimate the expected angular rate accuracy as a function of camera parameters and star distribution in the field of view. The effectiveness of the proposed technique is tested by using star field scenes generated by a hardware-in-the-loop testing facility and acquired by a commercial-off-the shelf camera sensor. Simulated cases comprise rotations at different rates. Experimental results are presented which are consistent with theoretical estimates. In particular, very accurate angular velocity estimates are generated at lower slew rates, while in all cases the achievable accuracy in the estimation of the angular velocity component along boresight is about one order of magnitude worse than the other two components.

Satellite Angular Velocity Estimation Based on Star Images and Optical Flow Techniques

PubMed Central

Fasano, Giancarmine; Rufino, Giancarlo; Accardo, Domenico; Grassi, Michele

2013-01-01

An optical flow-based technique is proposed to estimate spacecraft angular velocity based on sequences of star-field images. It does not require star identification and can be thus used to also deliver angular rate information when attitude determination is not possible, as during platform de tumbling or slewing. Region-based optical flow calculation is carried out on successive star images preprocessed to remove background. Sensor calibration parameters, Poisson equation, and a least-squares method are then used to estimate the angular velocity vector components in the sensor rotating frame. A theoretical error budget is developed to estimate the expected angular rate accuracy as a function of camera parameters and star distribution in the field of view. The effectiveness of the proposed technique is tested by using star field scenes generated by a hardware-in-the-loop testing facility and acquired by a commercial-off-the shelf camera sensor. Simulated cases comprise rotations at different rates. Experimental results are presented which are consistent with theoretical estimates. In particular, very accurate angular velocity estimates are generated at lower slew rates, while in all cases the achievable accuracy in the estimation of the angular velocity component along boresight is about one order of magnitude worse than the other two components. PMID:24072023

Binary neutron stars with arbitrary spins in numerical relativity

NASA Astrophysics Data System (ADS)

Tacik, Nick; Foucart, Francois; Pfeiffer, Harald P.; Haas, Roland; Ossokine, Serguei; Kaplan, Jeff; Muhlberger, Curran; Duez, Matt D.; Kidder, Lawrence E.; Scheel, Mark A.; Szilágyi, Béla

2015-12-01

We present a code to construct initial data for binary neutron star systems in which the stars are rotating. Our code, based on a formalism developed by Tichy, allows for arbitrary rotation axes of the neutron stars and is able to achieve rotation rates near rotational breakup. We compute the neutron star angular momentum through quasilocal angular momentum integrals. When constructing irrotational binary neutron stars, we find a very small residual dimensionless spin of ˜2 ×10-4 . Evolutions of rotating neutron star binaries show that the magnitude of the stars' angular momentum is conserved, and that the spin and orbit precession of the stars is well described by post-Newtonian approximation. We demonstrate that orbital eccentricity of the binary neutron stars can be controlled to ˜0.1 % . The neutron stars show quasinormal mode oscillations at an amplitude which increases with the rotation rate of the stars.

A Herschel-Resolved Debris Disk Around the Nearby G Star HIP 32480

NASA Technical Reports Server (NTRS)

Stapelfeldt, K.

2011-01-01

The Herschel Space Observatory is providing unprecedented sensitivity and angular resolution in the far-infrared. The DUNES Key Project (DUst around NEarby Stars, PI Carlos Eiroa) has finished its survey of 133 FGK stars within 25 pc of the Sun using the PACS photometer at 100 and 160 microns. We report the detection of a resolved debris ring around HIP 32480, a G0 star 16.5 parsecs distant. The ring is almost 300 AU in diameter and inclined 30 degrees from edge-on. We present a thermal emission model for the system that fits the Spitzer spectroscopy and Herschel images of the system. We find a minimum grainsize of approximately 4 microns in the main ring and a distinct warm dust population interior to it. Faint detached emission features just outside the ring may trace a separate, more distant ring in the system. The non-detection of the ring in archival HST/ACS coronagraphic images limits the dust grain albedo in the ring to be no more than 10%.

A Resolved Debris Disk Around the Nearby G Star HIP 32480

NASA Technical Reports Server (NTRS)

Stapelfeldt, K. R.; Bryden, G. C.; Marshall, J.; Eiroa, C.; Absil, O.; Mora, A.; Krist, J. E.; Su, K. Y. L.

2012-01-01

The Herschel Space Observatory is providing unprecedented sensitivity and angular resolution in the far-infrared. The DUNES Key Project (DUst around NEarby Stars, PI Carlos Eiroa) has finished its survey of 133 FGK stars within 25 pc of the Sun using the PACS photometer at 100 and 160 microns. We report the detection of a resolved debris ring around HIP 32480, a GO star 16.5 parsecs distant. The ring is almost 300 AU in diameter and inclined 30 degrees from edge-on. We present a thermal emission model for the system that fits the Spitzer spectroscopy and Herschel images of the system. We find a minimum grain-size of 4 microns in the main ring and a distinct warm dust population interior to it. Faint detached emission features just outside the ring may trace a separate, more distant ring in the system. The non-detection of the ring in archival HST/ACS coronagraphic images limits the dust grain albedo in the ring to be no more than 10%.

Interferometric observations of main-sequence stars: fundamental stellar astrophysics, circumstellar matter, and kinematics

NASA Astrophysics Data System (ADS)

Bakker, Eric J.; Eiroa, Carlos

2003-10-01

With our minds focussed on the direct detection of planets using the space interferometry mission DARWIN/TPF, we have made an attempt to identify how the set of ESO Very Large Telescope Interferometer instruments available now, and in the near future (VINCI, MIDI, AMBER, GENIE, FINITO and PRIMA) could contribute to the DARWIN/TPF precursory science program. In particular related to the identification of a short list of science stars to be observed with DARWIN/TPF. We have identified two research projects which can be viewed as DARWIN/TPF precursory science and can be embarked upon shortly using the available VLTI instruments: (1) the direct measurement of stellar angular diameters of a statistically meaningful sample of main-sequence stars with AMBER; (2) an interferometric study of those main-sequence stars that exhibit an infrared excess with either AMBER or MIDI. On the longer run, VLTI can obviously make a significant impact through the exploitation of the infrared nuller GENIE and the astrometric facility PRIMA.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Jatmiko, A. T. P.; Puannandra, G. P.; Hapsari, R. D.

Lunar Occultation (LO) is an event where limb of the Moon passing over a particular heavenly bodies such as stars, asteroids, or planets. In other words, during the event, stars, asteroids and planets are occulted by the Moon. When occulted objects contact the lunar limb, there will be a diffraction fringe(s) which can be measured photometrically, until the signal vanishes into noise. This event will give us a valuable information about binarities (of stars) and/or angular diameters estimation (of stars, planets, asteroids) in milliarcsecond resolution, by fitting with theoretical LO pattern. CCDs are common for LO observation because of itsmore » fast read out, and recently are developed for sub-meter class telescope. In this paper, our LO observation attempt of μ Sgr and its progress report are presented. The observation was conducted on July 30{sup th}, 2012 at Bosscha Observatory, Indonesia, using 45cm f/12 GOTO telescope combined with ST-9 XE CCD camera and Bessel B filter. We used drift-scan method to obtain light curve of the star as it was disappearing behind Moon's dark limb. Our goal is to detect binarity (or multiplicity) of this particular object.« less

Distance and absolute magnitudes of the brightest stars in the dwarf galaxy Sextans A

NASA Technical Reports Server (NTRS)

Sandage, A.; Carlson, G.

1982-01-01

In an attempt to improve present bright star calibration, data were gathered for the brightest red and blue stars and the Cepheids in the Im V dwarf galaxy, Sextans A. On the basis of a magnitude sequence measured to V and B values of about 22 and 23, respectively, the mean magnitudes of the three brightest blue stars are V=17.98 and B=17.88. The three brightest red supergiants have V=18.09 and B=20.14. The periods and magnitudes measured for five Cepheids yield an apparent blue distance modulus of 25.67 + or - 0.2, via the P-L relation, and the mean absolute magnitudes of V=-7.56 and B=-5.53 for the red supergiants provide additional calibration of the brightest red stars as distance indicators. If Sextans A were placed at the distance of the Virgo cluster, it would appear to have a surface brightness of 23.5 mag/sq arcec. This, together with the large angular diameter, would make such a galaxy easily discoverable in the Virgo cluster by means of ground-based surveys.

Fundamental Stellar Properties of M-Dwarfs from the CHARA Array

NASA Astrophysics Data System (ADS)

Berger, D. H.; Gies, D. R.; McAlister, H. A.; ten Brummelaar, T. A.; Henry, T. J.; Sturmann, J.; Sturmann, L.; Turner, N. H.; Ridgway, S. T.; Aufdenberg, J. P.; Mérand, A. M.

2005-12-01

We report the angular diameters of six M dwarfs ranging in spectral type from M1.0 V to M3.0 V measured with Georgia State University's CHARA Array, a long-baseline optical interferometer located at Mount Wilson Observatory. Observations were made with the longest baselines in the near infrared K'-band and yielded angular diameters less than one milliarcsecond. Using an iterative process combining parallaxes from the NStars program and photometrically-derived bolometric luminosities and masses, we calculated effective temperatures, surface gravities, and stellar radii. Our results are consistent with other empirical measurements of M-dwarf radii, but found that current models underestimate the true stellar radii by up to 15-20%. We suggest that theoretical models for low mass stars may be lacking an opacity source that alters the computed stellar radii. Science operations at the Array are supported by the National Science Foundation through NSF Grant AST--0307562 and by Georgia State University through the College of Arts and Sciences and the Office of the Vice President for Research. Financial support for DHB was provided by the National Science Foundation through grant AST--0205297.

Evidence for the distribution of angular velocity inside the sun and stars

NASA Technical Reports Server (NTRS)

1972-01-01

A round table discussion of problems of solar and stellar spindown and theory is presented. Observational evidence of the angular momentum of the solar wind is included, emphasizing the distribution of angular velocity inside the sun and stars.

Vortex coronagraphs for the Habitable Exoplanet Imaging Mission concept: theoretical performance and telescope requirements

NASA Astrophysics Data System (ADS)

Ruane, Garreth; Mawet, Dimitri; Mennesson, Bertrand; Jewell, Jeffrey; Shaklan, Stuart

2018-01-01

The Habitable Exoplanet Imaging Mission concept requires an optical coronagraph that provides deep starlight suppression over a broad spectral bandwidth, high throughput for point sources at small angular separation, and insensitivity to temporally varying, low-order aberrations. Vortex coronagraphs are a promising solution that performs optimally on off-axis, monolithic telescopes and may also be designed for segmented telescopes with minor losses in performance. We describe the key advantages of vortex coronagraphs on off-axis telescopes such as (1) unwanted diffraction due to aberrations is passively rejected in several low-order Zernike modes relaxing the wavefront stability requirements for imaging Earth-like planets from <10 to >100 pm rms, (2) stars with angular diameters >0.1 λ / D may be sufficiently suppressed, (3) the absolute planet throughput is >10 % , even for unfavorable telescope architectures, and (4) broadband solutions (Δλ / λ > 0.1) are readily available for both monolithic and segmented apertures. The latter make use of grayscale apodizers in an upstream pupil plane to provide suppression of diffracted light from amplitude discontinuities in the telescope pupil without inducing additional stroke on the deformable mirrors. We set wavefront stability requirements on the telescope, based on a stellar irradiance threshold set at an angular separation of 3 ± 0.5λ / D from the star, and discuss how some requirements may be relaxed by trading robustness to aberrations for planet throughput.

The Chara Array Angular Diameter of HR 8799 Favors Planetary Masses for Its Imaged Companions

DTIC Science & Technology

2012-12-10

because of the shallow convective zones in A-type stars ( Charbonneau 1991). Accretion at these rates will quickly establish abundance anomalies within a...few Myr, but these anomalies will likewise disappear in as little as 1 Myr once the accretion has terminated (Turcotte & Charbonneau 1993); the... Charbonneau , P. 1991, ApJ, 372, L33 Chen, C. H., Sargent, B. A., Bohac, C., et al. 2006, ApJS, 166, 351 Claret, A., & Bloemen, S. 2011, A&A, 529, A75 Cox, A. N

An investigation of 11 previously unstudied open star clusters

NASA Astrophysics Data System (ADS)

Tadross, A. L.

2009-02-01

The main astrophysical properties of 11 previously unstudied open star clusters are probed with JHK Near-IR (2MASS) photometry of Cutri et al. [Cutri, R., et al., 2003. The IRSA 2MASS All-sky Point Source Catolog, NASA/IPAC Infrared Science Archive] and proper motions (NOMAD) astrometry of Zacharias et al. [Zacharias, N., Monet, D., Levine, S., Urban, S., Gaume, R., Wycoff, G., 2004. American Astro. Soc. Meeting 36, 1418]. The fundamental parameters have been derived for IC (1434, 2156); King (17, 18, 20, 23, 26); and Dias (2, 3, 4, 7, 8), for which no prior parameters are available in the literature. The clusters' centers coordinates and angular diameters are re-determined, while ages, distances, and color excesses for these clusters are estimated here for the first time.

Evidence for changes in the angular velocity of the surface regions of the sun and stars

NASA Technical Reports Server (NTRS)

1972-01-01

A round table discussion of problems of solar and stellar spindown and theory is presented. Observational evidence of the angular momentum of the solar wind is included, emphasizing changes in the angular velocity of the surface regions of the sun and stars.

Ultracompact Blue Dwarf Galaxies: Hubble Space Telescope Imaging and Stellar Population Analysis

NASA Astrophysics Data System (ADS)

Corbin, Michael R.; Vacca, William D.; Cid Fernandes, Roberto; Hibbard, John E.; Somerville, Rachel S.; Windhorst, Rogier A.

2006-11-01

We present deep Hubble Space Telescope (HST) Advanced Camera for Surveys/High Resolution Channel U-, narrow-V-, and I-band images of nine ``ultracompact'' blue dwarf galaxies (UCBDs) selected from the Sloan Digital Sky Survey (SDSS). We define UCBDs as local (z<0.01) star-forming galaxies having angular diameters less than 6" and physical diameters <1 kpc. They are also among the most metal-poor galaxies known, including objects having 12+log(O/H)<7.65, and are found to reside within voids. Both the HST images and the objects' SDSS optical spectra reveal that they are composites of young (~1-10 Myr) populations that dominate their light and older (~10 Gyr) populations that dominate their stellar masses, which we estimate to be ~107-108 Msolar. An intermediate-age (~107-109 yr) population is also indicated in most objects. The objects do not appear to be as dynamically disturbed as the prototype UCBD, POX 186, but the structure of several of them suggests that their current star formation has been triggered by the collisions/mergers of smaller clumps of stars. In one case, HS 0822+3542, the images resolve what may be two small (~100 pc) components that have recently collided, supporting this interpretation. In six of the objects much of the star formation is concentrated in young massive clusters, contributing to their compactness in ground-based images. The evidence that the galaxies consist mainly of ~10 Gyr old stars establishes that they are not protogalaxies, forming their first generation of stars. Their low metallicities are more likely to be the result of the escape of supernova ejecta, rather than youth.

Interstellar Scattering Towards the Galactic Center as Probed by OH/IR Stars

NASA Technical Reports Server (NTRS)

Vanlangevelde, Huib Jan; Frail, Dale A.; Cordes, James M.; Diamond, Philip J.

1992-01-01

Angular broadening measurements are reported of 20 OH/IR stars near the galactic center. This class of sources is known to have bright, intrinsically compact (less than or equal to 20 mas) maser components within their circumstellar shells. VLBA antennas and the VLA were used to perform a MKII spectral line VLBI experiment. The rapid drop in correlated flux with increasing baseline, especially for sources closest to the galactic center, is attributed to interstellar scattering. Angular diameters were measured for 13 of our sources. Lower limits were obtained for the remaining seven. With the data, together with additional data taken from the literature, the distribution was determined of interstellar scattering toward the galactic center. A region was found of pronounced scattering nearly centered on SgrA*. Two interpretations are considered for the enhanced scattering. One hypothesis is that the scattering is due to a clump of enhanced turbulence, such as those that lie along lines of sight to other known objects, that has no physical relationship to the galactic center. The other model considers the location of the enhanced scattering to arise in the galactic center itself. The physical implications of the models yield information on the nature of interstellar scattering.

RADIUS-DEPENDENT ANGULAR MOMENTUM EVOLUTION IN LOW-MASS STARS. I

DOE Office of Scientific and Technical Information (OSTI.GOV)

Reiners, Ansgar; Mohanty, Subhanjoy, E-mail: Ansgar.Reiners@phys.uni-goettingen.de

2012-02-10

Angular momentum evolution in low-mass stars is determined by initial conditions during star formation, stellar structure evolution, and the behavior of stellar magnetic fields. Here we show that the empirical picture of angular momentum evolution arises naturally if rotation is related to magnetic field strength instead of to magnetic flux and formulate a corrected braking law based on this. Angular momentum evolution then becomes a strong function of stellar radius, explaining the main trends observed in open clusters and field stars at a few Gyr: the steep transition in rotation at the boundary to full convection arises primarily from themore » large change in radius across this boundary and does not require changes in dynamo mode or field topology. Additionally, the data suggest transient core-envelope decoupling among solar-type stars and field saturation at longer periods in very low mass stars. For solar-type stars, our model is also in good agreement with the empirical Skumanich law. Finally, in further support of the theory, we show that the predicted age at which low-mass stars spin down from the saturated to unsaturated field regimes in our model corresponds remarkably well to the observed lifetime of magnetic activity in these stars.« less

First Surface-resolved Results with the Infrared Optical Telescope Array Imaging Interferometer: Detection of Asymmetries in Asymptotic Giant Branch Stars

NASA Astrophysics Data System (ADS)

Ragland, S.; Traub, W. A.; Berger, J.-P.; Danchi, W. C.; Monnier, J. D.; Willson, L. A.; Carleton, N. P.; Lacasse, M. G.; Millan-Gabet, R.; Pedretti, E.; Schloerb, F. P.; Cotton, W. D.; Townes, C. H.; Brewer, M.; Haguenauer, P.; Kern, P.; Labeye, P.; Malbet, F.; Malin, D.; Pearlman, M.; Perraut, K.; Souccar, K.; Wallace, G.

2006-11-01

We have measured nonzero closure phases for about 29% of our sample of 56 nearby asymptotic giant branch (AGB) stars, using the three-telescope Infrared Optical Telescope Array (IOTA) interferometer at near-infrared wavelengths (H band) and with angular resolutions in the range 5-10 mas. These nonzero closure phases can only be generated by asymmetric brightness distributions of the target stars or their surroundings. We discuss how these results were obtained and how they might be interpreted in terms of structures on or near the target stars. We also report measured angular sizes and hypothesize that most Mira stars would show detectable asymmetry if observed with adequate angular resolution.

Beyond the diffraction limit of optical/IR interferometers. II. Stellar parameters of rotating stars from differential phases

NASA Astrophysics Data System (ADS)

Hadjara, M.; Domiciano de Souza, A.; Vakili, F.; Jankov, S.; Millour, F.; Meilland, A.; Khorrami, Z.; Chelli, A.; Baffa, C.; Hofmann, K.-H.; Lagarde, S.; Robbe-Dubois, S.

2014-09-01

Context. As previously demonstrated on Achernar, one can derive the angular radius, rotational velocity, axis tilt, and orientation of a fast-rotating star from the differential phases obtained by spectrally resolved long baseline interferometry using earth-rotation synthesis. Aims: We applied this method on a small sample of stars for different spectral types and classes, in order to generalize the technique to other rotating stars across the H-R diagram and determine their fundamental parameters. Methods: We used differential phase data from the AMBER/VLTI instrument obtained prior to refurbishing its spectrometer in 2010. With the exception of Fomalhaut, which has been observed in the medium-resolution mode of AMBER (λ/δλ ≈ 1500), our three other targets, Achernar, Altair, and δ Aquilae offered high-resolution (λ/δλ ≈ 12 000) spectro-interferometric data around the Brγ absorption line in K band. These data were used to constrain the input parameters of an analytical, still realistic model to interpret the observations with a systematic approach for the error budget analysis in order to robustly conclude on the physics of our 4 targets. We applied the super resolution provided by differential phases φdiff to measure the size (equatorial radius Req and angular diameter ⌀eq), the equatorial rotation velocity (Veq), the inclination angle (i), and the rotation axis position angle (PArot) of 4 fast-rotating stars: Achernar, Altair, δ Aquilae, and Fomalhaut. The stellar parameters of the targets were constrained using a semi-analytical algorithm dedicated to fast rotators SCIROCCO. Results: The derived parameters for each star were Req = 11.2 ± 0.5 R⊙, Veqsini = 290 ± 17 km s-1, PArot = 35.4° ± 1.4°, for Achernar; Req = 2.0 ± 0.2 R⊙, Veqsini = 226 ± 34 km s-1, PArot = -65.5° ± 5.5°, for Altair; Req = 2.2 ± 0.3 R⊙, Veqsini = 74 ± 35 km s-1, PArot = -101.2° ± 14°, for δ Aquilae; and Req = 1.8 ± 0.2 R⊙, Veqsini = 93 ± 16 km s-1, PArot = 65.6° ± 5°, for Fomalhaut. They were found to be compatible with previously published values from differential phase and visibility measurements, while we were able to determine, for the first time, the inclination angle i of Fomalhaut (i = 90° ± 9°) and δ Aquilae (i = 81° ± 13°), and the rotation-axis position angle PArot of δ Aquilae. Conclusions: Beyond the theoretical diffraction limit of an interferometer (ratio of the wavelength to the baseline), spatial super resolution is well suited to systematically estimating the angular diameters of rotating stars and their fundamental parameters with a few sets of baselines and the Earth-rotation synthesis provided a high enough spectral resolution. Based on observations performed at the European Southern Observatory, Chile, under ESO AMBER-consortium GTO program IDs 084.D-0456 081.D-0293 and 082.C-0376.Figure 5 is available in electronic form at http://www.aanda.org

A New Stellar Atmosphere Grid and Comparisons with HST /STIS CALSPEC Flux Distributions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bohlin, Ralph C.; Fleming, Scott W.; Gordon, Karl D.

The Space Telescope Imaging Spectrograph has measured the spectral energy distributions for several stars of types O, B, A, F, and G. These absolute fluxes from the CALSPEC database are fit with a new spectral grid computed from the ATLAS-APOGEE ATLAS9 model atmosphere database using a chi-square minimization technique in four parameters. The quality of the fits are compared for complete LTE grids by Castelli and Kurucz (CK04) and our new comprehensive LTE grid (BOSZ). For the cooler stars, the fits with the MARCS LTE grid are also evaluated, while the hottest stars are also fit with the NLTE Lanzmore » and Hubeny OB star grids. Unfortunately, these NLTE models do not transition smoothly in the infrared to agree with our new BOSZ LTE grid at the NLTE lower limit of T {sub eff} = 15,000 K. The new BOSZ grid is available via the Space Telescope Institute MAST archive and has a much finer sampled IR wavelength scale than CK04, which will facilitate the modeling of stars observed by the James Webb Space Telescope . Our result for the angular diameter of Sirius agrees with the ground-based interferometric value.« less

A New Stellar Atmosphere Grid and Comparisons with HST/STIS CALSPEC Flux Distributions

NASA Astrophysics Data System (ADS)

Bohlin, Ralph C.; Mészáros, Szabolcs; Fleming, Scott W.; Gordon, Karl D.; Koekemoer, Anton M.; Kovács, József

2017-05-01

The Space Telescope Imaging Spectrograph has measured the spectral energy distributions for several stars of types O, B, A, F, and G. These absolute fluxes from the CALSPEC database are fit with a new spectral grid computed from the ATLAS-APOGEE ATLAS9 model atmosphere database using a chi-square minimization technique in four parameters. The quality of the fits are compared for complete LTE grids by Castelli & Kurucz (CK04) and our new comprehensive LTE grid (BOSZ). For the cooler stars, the fits with the MARCS LTE grid are also evaluated, while the hottest stars are also fit with the NLTE Lanz & Hubeny OB star grids. Unfortunately, these NLTE models do not transition smoothly in the infrared to agree with our new BOSZ LTE grid at the NLTE lower limit of T eff = 15,000 K. The new BOSZ grid is available via the Space Telescope Institute MAST archive and has a much finer sampled IR wavelength scale than CK04, which will facilitate the modeling of stars observed by the James Webb Space Telescope. Our result for the angular diameter of Sirius agrees with the ground-based interferometric value.

Star tracking method based on multiexposure imaging for intensified star trackers.

PubMed

Yu, Wenbo; Jiang, Jie; Zhang, Guangjun

2017-07-20

The requirements for the dynamic performance of star trackers are rapidly increasing with the development of space exploration technologies. However, insufficient knowledge of the angular acceleration has largely decreased the performance of the existing star tracking methods, and star trackers may even fail to track under highly dynamic conditions. This study proposes a star tracking method based on multiexposure imaging for intensified star trackers. The accurate estimation model of the complete motion parameters, including the angular velocity and angular acceleration, is established according to the working characteristic of multiexposure imaging. The estimation of the complete motion parameters is utilized to generate the predictive star image accurately. Therefore, the correct matching and tracking between stars in the real and predictive star images can be reliably accomplished under highly dynamic conditions. Simulations with specific dynamic conditions are conducted to verify the feasibility and effectiveness of the proposed method. Experiments with real starry night sky observation are also conducted for further verification. Simulations and experiments demonstrate that the proposed method is effective and shows excellent performance under highly dynamic conditions.

Observational calibration of the projection factor of Cepheids. I. The type II Cepheid κ Pavonis

NASA Astrophysics Data System (ADS)

Breitfelder, J.; Kervella, P.; Mérand, A.; Gallenne, A.; Szabados, L.; Anderson, R. I.; Willson, M.; Le Bouquin, J.-B.

2015-04-01

Context. The distance of pulsating stars, in particular Cepheids, are commonly measured using the parallax of pulsation technique. The different versions of this technique combine measurements of the linear diameter variation (from spectroscopy) and the angular diameter variation (from photometry or interferometry) amplitudes, to retrieve the distance in a quasi-geometrical way. However, the linear diameter amplitude is directly proportional to the projection factor (hereafter p-factor), which is used to convert spectroscopic radial velocities (i.e., disk integrated) into pulsating (i.e., photospheric) velocities. The value of the p-factor and its possible dependence on the pulsation period are still widely debated. Aims: Our goal is to measure an observational value of the p-factor of the type-II Cepheid κ Pavonis. Methods: The parallax of the type-II Cepheid κ Pav was measured with an accuracy of 5% using HST/FGS. We used this parallax as a starting point to derive the p-factor of κ Pav, using the SPIPS technique (Spectro-Photo-Interferometry of Pulsating Stars), which is a robust version of the parallax-of-pulsation method that employs radial velocity, interferometric and photometric data. We applied this technique to a combination of new VLTI/PIONIER optical interferometric angular diameters, new CORALIE and HARPS radial velocities, as well as multi-colour photometry and radial velocities from the literature. Results: We obtain a value of p = 1.26 ± 0.07 for the p-factor of κ Pav. This result agrees with several of the recently derived Period-p-factor relationships from the literature, as well as previous observational determinations for Cepheids. Conclusions: Individual estimates of the p-factor are fundamental to calibrating the parallax of pulsation distances of Cepheids. Together with previous observational estimates, the projection factor we obtain points to a weak dependence of the p-factor on period. Based on observations realized with ESO facilities at Paranal Observatory under program IDs 091.D-0020 and 093.D-0316.Based on observations collected at ESO La Silla Observatory using the Coralie spectrograph mounted to the Swiss 1.2 m Euler telescope, under program CNTAC2014A-5.

Tests of stellar model atmospheres by optical interferometry. VLTI/VINCI limb-darkening measurements of the M4 giant ψ Phe

NASA Astrophysics Data System (ADS)

Wittkowski, M.; Aufdenberg, J. P.; Kervella, P.

2004-01-01

We present K-band interferometric measurements of the limb-darkened (LD) intensity profile of the M 4 giant star ψ Phoenicis obtained with the Very Large Telescope Interferometer (VLTI) and its commissioning instrument VINCI. High-precision squared visibility amplitudes in the second lobe of the visibility function were obtained employing two 8.2 m Unit Telescopes (UTs). This took place one month after light from UTs was first combined for interferometric fringes. In addition, we sampled the visibility function at small spatial frequencies using the 40 cm test siderostats. Our measurement constrains the diameter of the star as well as its center-to-limb intensity variation (CLV). We construct a spherical hydrostatic PHOENIX model atmosphere based on spectrophotometric data from the literature and compare its CLV prediction with our interferometric measurement. We compare as well CLV predictions by plane-parallel hydrostatic PHOENIX, ATLAS 9, and ATLAS 12 models. We find that the Rosseland angular diameter as predicted by comparison of the spherical PHOENIX model with spectrophotometry is in good agreement with our interferometric diameter measurement. The shape of our measured visibility function in the second lobe is consistent with all considered PHOENIX and ATLAS model predictions, and is significantly different to uniform disk (UD) and fully darkened disk (FDD) models. We derive high-precision fundamental parameters for ψ Phe, namely a Rosseland angular diameter of 8.13 ± 0.2 mas, with the Hipparcos parallax corresponding to a Rosseland linear radius R of 86 ± 3 R⊙, and an effective temperature of 3550 ± 50 K, with R corresponding to a luminosity of \\log L/L⊙=3.02 ± 0.06. Together with evolutionary models, these values are consistent with a mass of 1.3 ± 0.2 M⊙, and a surface gravity of \\log g = 0.68 ± 0.11. Based on public data released from the European Southern Observatory VLTI obtained from the ESO/ST-ECF Science Archive Facility. The VLTI was operated with the commissioning instrument VINCI and the MONA beam combiner.

Pox 186: A Nearby Protogalaxy?

NASA Astrophysics Data System (ADS)

Corbin, Michael

1999-07-01

Blue Compact Dwarf Galaxies {BCDGs} typically consist of clusters of early-type stars embedded in older, evolved stellar populations similar in size and shape to normal dwarf ellipticals. However, deep ground-based CCD images of one faint BCDG, Pox 186, reveal a very compact { 5" diameter} structure with no evidence of an underlying older population. Optical spectroscopy of this object also indicates that a large number of Wolf-Rayet stars are present, which implies that a burst of star formation must have occurred very recently {<=sssim 10^7 years ago}. It has thus been suggested that Pox 186 is a protogalaxy, forming its very first generation of stars. Further investigation of this possibility requires the high angular resolution and ultraviolet spectral coverage that only HST can provide. Using WFPC2, we propose to image the galaxy in the U, V, and I bands, in order to better test for the presence of an underlying evolved population and to reveal any substructure in its star-forming regions. Using STIS, we will obtain low-resolution ultraviolet spectra of the galaxy for combination with ground-based spectra covering the optical through near infrared. This will allow us to determine its spectral energy distribution, metallicity, and dust content, which will in turn constrain its age and star formation history.

Observations of H II regions around Zeta OPH and other O-B stars

NASA Astrophysics Data System (ADS)

Shestakova, L. I.; Kutirev, A. S.; Ataev, A. Sh.

1988-01-01

A Fabry-Perot spectrometer was used to measure the emission intensities in H-beta near Zeta Oph, Alpha Vir, Alpha Cam, and HD 188209. The spectrometer sensitivity is 0.2 rayleighs, the intensity measurement accuracy is 20 percent. Ionization zone boundaries are determined for Zeta Oph and Alpha Vir; the angular diameters of both regions are about 15 deg. The contour of the H II region near Zeta Oph on the level of the double background in the southwest does not close; instead, it expands again and incorporates the region associated with the B-association II Sco.

Mid-Infrared Interferometric Monitoring of Evolved Stars: The Dust Shell Around the Mira Variable RR Aquilae at 13 Epochs

DTIC Science & Technology

2011-01-01

photometric and interferometric data. 15. SUBJECT TERMS 16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF ABSTRACT Same as Report (SAR) 18. NUMBER OF...λ = 2.2 μm, Δλ = 0.4 μm) angular size with the Infrared Optical Telescope Array ( IOTA ). The uniform disk diameter (UD) of θUD = 10.73 ± 0.66 mas at...with IOTA in the H-band, and classified RR Aql as a target with no detectable asymmetries. The IRAS flux at 12 μm is 332 Jy. The light curve in the V

Angular momentum transport by heat-driven g-modes in slowly pulsating B stars

NASA Astrophysics Data System (ADS)

Townsend, R. H. D.; Goldstein, J.; Zweibel, E. G.

2018-03-01

Motivated by recent interest in the phenomenon of waves transport in massive stars, we examine whether the heat-driven gravity (g) modes excited in slowly pulsating B (SPB) stars can significantly modify the stars' internal rotation. We develop a formalism for the differential torque exerted by g modes, and implement this formalism using the GYRE oscillation code and the MESASTAR stellar evolution code. Focusing first on a 4.21M⊙ model, we simulate 1 000 yr of stellar evolution under the combined effects of the torque due to a single unstable prograde g mode (with an amplitude chosen on the basis of observational constraints), and diffusive angular momentum transport due to convection, overshooting, and rotational instabilities. We find that the g mode rapidly extracts angular momentum from the surface layers, depositing it deeper in the stellar interior. The angular momentum transport is so efficient that by the end of the simulation, the initially non-rotating surface layers are spun in the retrograde direction to ≈ 30 per cent of the critical rate. However, the additional inclusion of magnetic stresses in our simulations almost completely inhibits this spin-up. Expanding our simulations to cover the whole instability strip, we show that the same general behaviour is seen in all SPB stars. After providing some caveats to contextualize our results, we hypothesize that the observed slower surface rotation of SPB stars (as compared to other B-type stars) may be the direct consequence of the angular momentum transport that our simulations demonstrate.

Smearing model and restoration of star image under conditions of variable angular velocity and long exposure time.

PubMed

Sun, Ting; Xing, Fei; You, Zheng; Wang, Xiaochu; Li, Bin

2014-03-10

The star tracker is one of the most promising attitude measurement devices widely used in spacecraft for its high accuracy. High dynamic performance is becoming its major restriction, and requires immediate focus and promotion. A star image restoration approach based on the motion degradation model of variable angular velocity is proposed in this paper. This method can overcome the problem of energy dispersion and signal to noise ratio (SNR) decrease resulting from the smearing of the star spot, thus preventing failed extraction and decreased star centroid accuracy. Simulations and laboratory experiments are conducted to verify the proposed methods. The restoration results demonstrate that the described method can recover the star spot from a long motion trail to the shape of Gaussian distribution under the conditions of variable angular velocity and long exposure time. The energy of the star spot can be concentrated to ensure high SNR and high position accuracy. These features are crucial to the subsequent star extraction and the whole performance of the star tracker.

Wind-driven angular momentum loss in binary systems. I - Ballistic case

NASA Technical Reports Server (NTRS)

Brookshaw, Leigh; Tavani, Marco

1993-01-01

We study numerically the average loss of specific angular momentum from binary systems due to mass outflow from one of the two stars for a variety of initial injection geometries and wind velocities. We present results of ballistic calculations in three dimensions for initial mass ratios q of the mass-losing star to primary star in the range q between 10 exp -5 and 10. We consider injection surfaces close to the Roche lobe equipotential surface of the mass-losing star, and also cases with the mass-losing star underfilling its Roche lobe. We obtain that the orbital period is expected to have a negative time derivative for wind-driven secular evolution of binaries with q greater than about 3 and with the mass-losing star near filling its Roche lobe. We also study the effect of the presence of an absorbing surface approximating an accretion disk on the average final value of the specific angular momentum loss. We find that the effect of an accretion disk is to increase the wind-driven angular momentum loss. Our results are relevant for evolutionary models of high-mass binaries and low-mass X-ray binaries.

High angular resolution N-band observation of the silicate carbon star IRAS08002-3803 with the VLTI/MIDI instrument . Dusty environment spatially resolved

NASA Astrophysics Data System (ADS)

Ohnaka, K.; Driebe, T.; Hofmann, K.-H.; Leinert, Ch.; Morel, S.; Paresce, F.; Preibisch, Th.; Richichi, A.; Schertl, D.; Schöller, M.; Waters, L. B. F. M.; Weigelt, G.; Wittkowski, M.

2006-01-01

We present the results of N-band spectro-interferometric observations of the silicate carbon star IRAS08002-3803 with the MID-infrared Interferometric instrument (MIDI) at the Very Large Telescope Interferometer (VLTI) of the European Southern Observatory (ESO). The observations were carried out using two unit telescopes (UT2 and UT3) with projected baseline lengths ranging from 39 to 47 m. Our observations of IRAS08002-3803 have spatially resolved the dusty environment of a silicate carbon star for the first time and revealed an unexpected wavelength dependence of the angular size in the N band: the uniform-disk diameter is found to be constant and 36 mas (72 Rstar) between 8 and 10 μm, while it steeply increases longward of 10 μm to reach 53 mas (106 Rstar) at 13 μm. Model calculations with our Monte Carlo radiative transfer code show that neither spherical shell models nor axisymmetric disk models consisting of silicate grains alone can simultaneously explain the observed wavelength dependence of the visibility and the spectral energy distribution (SED). We propose that the circumstellar environment of IRAS08002-3803 may consist of two grain species coexisting in the disk: silicate and a second grain species, for which we consider amorphous carbon, large silicate grains, and metallic iron grains. Comparison of the observed visibilities and SED with our models shows that such disk models can fairly - though not entirely satisfactorily - reproduce the observed SED and N-band visibilities. Our MIDI observations and the radiative transfer calculations lend support to the picture where oxygen-rich material around IRAS08002-3803 is stored in a circumbinary disk surrounding the carbon-rich primary star and its putative low-luminosity companion.

Effective star tracking method based on optical flow analysis for star trackers.

PubMed

Sun, Ting; Xing, Fei; Wang, Xiaochu; Li, Jin; Wei, Minsong; You, Zheng

2016-12-20

Benefiting from rapid development of imaging sensor technology, modern optical technology, and a high-speed computing chip, the star tracker's accuracy, dynamic performance, and update rate have been greatly improved with low power consumption and miniature size. The star tracker is currently one of the most competitive attitude measurement sensors. However, due to restrictions of the optical imaging system, difficulties still exist in moving star spot detection and star tracking when in special motion conditions. An effective star tracking method based on optical flow analysis for star trackers is proposed in this paper. Spot-based optical flow, based on a gray gradient between two adjacent star images, is analyzed to distinguish the star spot region and obtain an accurate star spot position so that the star tracking can keep continuous under high dynamic conditions. The obtained star vectors and extended Kalman filter (EKF) are then combined to conduct an angular velocity estimation to ensure region prediction of the star spot; this can be combined with the optical flow analysis result. Experiment results show that the method proposed in this paper has advantages in conditions of large angular velocity and large angular acceleration, despite the presence of noise. Higher functional density and better performance can be achieved; thus, the star tracker can be more widely applied in small satellites, remote sensing, and other complex space missions.

A star recognition method based on the Adaptive Ant Colony algorithm for star sensors.

PubMed

Quan, Wei; Fang, Jiancheng

2010-01-01

A new star recognition method based on the Adaptive Ant Colony (AAC) algorithm has been developed to increase the star recognition speed and success rate for star sensors. This method draws circles, with the center of each one being a bright star point and the radius being a special angular distance, and uses the parallel processing ability of the AAC algorithm to calculate the angular distance of any pair of star points in the circle. The angular distance of two star points in the circle is solved as the path of the AAC algorithm, and the path optimization feature of the AAC is employed to search for the optimal (shortest) path in the circle. This optimal path is used to recognize the stellar map and enhance the recognition success rate and speed. The experimental results show that when the position error is about 50″, the identification success rate of this method is 98% while the Delaunay identification method is only 94%. The identification time of this method is up to 50 ms.

THE IMPACT OF STELLAR FEEDBACK ON THE STRUCTURE, SIZE, AND MORPHOLOGY OF GALAXIES IN MILKY-WAY-SIZED DARK MATTER HALOS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Agertz, Oscar; Kravtsov, Andrey V., E-mail: o.agertz@surrey.ac.uk

We use cosmological zoom-in simulations of galaxy formation in a Milky-Way-sized halo started from identical initial conditions to investigate the evolution of galaxy sizes, baryon fractions, morphologies, and angular momenta in runs with different parameters of the star formation–feedback cycle. Our fiducial model with a high local star formation efficiency, which results in efficient feedback, produces a realistic late-type galaxy that matches the evolution of basic properties of late-type galaxies: stellar mass, disk size, morphology dominated by a kinematically cold disk, stellar and gas surface density profiles, and specific angular momentum. We argue that feedback’s role in this success ismore » twofold: (1) removal of low angular momentum gas, and (2) maintaining a low disk-to-halo mass fraction, which suppresses disk instabilities that lead to angular momentum redistribution and a central concentration of baryons. However, our model with a low local star formation efficiency, but large energy input per supernova, chosen to produce a galaxy with a similar star formation history as our fiducial model, leads to a highly irregular galaxy with no kinematically cold component, overly extended stellar distribution, and low angular momentum. This indicates that only when feedback is allowed to become vigorous via locally efficient star formation in dense cold gas do resulting galaxy sizes, gas/stellar surface density profiles, and stellar disk angular momenta agree with observed z = 0 galaxies.« less

Close binary evolution. II. Impact of tides, wind magnetic braking, and internal angular momentum transport

NASA Astrophysics Data System (ADS)

Song, H. F.; Meynet, G.; Maeder, A.; Ekström, S.; Eggenberger, P.; Georgy, C.; Qin, Y.; Fragos, T.; Soerensen, M.; Barblan, F.; Wade, G. A.

2018-01-01

Context. Massive stars with solar metallicity lose important amounts of rotational angular momentum through their winds. When a magnetic field is present at the surface of a star, efficient angular momentum losses can still be achieved even when the mass-loss rate is very modest, at lower metallicities, or for lower-initial-mass stars. In a close binary system, the effect of wind magnetic braking also interacts with the influence of tides, resulting in a complex evolution of rotation. Aims: We study the interactions between the process of wind magnetic braking and tides in close binary systems. Methods: We discuss the evolution of a 10 M⊙ star in a close binary system with a 7 M⊙ companion using the Geneva stellar evolution code. The initial orbital period is 1.2 days. The 10 M⊙ star has a surface magnetic field of 1 kG. Various initial rotations are considered. We use two different approaches for the internal angular momentum transport. In one of them, angular momentum is transported by shear and meridional currents. In the other, a strong internal magnetic field imposes nearly perfect solid-body rotation. The evolution of the primary is computed until the first mass-transfer episode occurs. The cases of different values for the magnetic fields and for various orbital periods and mass ratios are briefly discussed. Results: We show that, independently of the initial rotation rate of the primary and the efficiency of the internal angular momentum transport, the surface rotation of the primary will converge, in a time that is short with respect to the main-sequence lifetime, towards a slowly evolving velocity that is different from the synchronization velocity. This "equilibrium angular velocity" is always inferior to the angular orbital velocity. In a given close binary system at this equilibrium stage, the difference between the spin and the orbital angular velocities becomes larger when the mass losses and/or the surface magnetic field increase. The treatment of the internal angular momentum transport has a strong impact on the evolutionary tracks in the Hertzsprung-Russell Diagram as well as on the changes of the surface abundances resulting from rotational mixing. Our modelling suggests that the presence of an undetected close companion might explain rapidly rotating stars with strong surface magnetic fields, having ages well above the magnetic braking timescale. Our models predict that the rotation of most stars of this type increases as a function of time, except for a first initial phase in spin-down systems. The measure of their surface abundances, together, when possible, with their mass-luminosity ratio, provide interesting constraints on the transport efficiencies of angular momentum and chemical species. Conclusions: Close binaries, when studied at phases predating any mass transfer, are key objects to probe the physics of rotation and magnetic fields in stars.

From Head to Sword: The Clustering Properties of Stars in Orion

NASA Astrophysics Data System (ADS)

Gomez, Mercedes; Lada, Charles J.

1998-04-01

We investigate the structure in the spatial distributions of optically selected samples of young stars in the Head (lambda Orionis) and in the Sword (Orion A) regions of the constellation of Orion with the aid of stellar surface density maps and the two-point angular correlation function. The distributions of young stars in both regions are found to be nonrandom and highly clustered. Stellar surface density maps reveal three distinct clusters in the lambda Ori region. The two-point correlation function displays significant features at angular scales that correspond to the radii and separations of the three clusters identified in the surface density maps. Most young stars in the lambda Ori region (~80%) are presently found within these three clusters, consistent with the idea that the majority of young stars in this region were formed in dense protostellar clusters that have significantly expanded since their formation. Over a scale of ~0.05d-0.5d the correlation function is well described by a single power law that increases smoothly with decreasing angular scale. This suggests that, within the clusters, the stars either are themselves hierarchically clustered or have a volume density distribution that falls steeply with radius. The relative lack of Hα emission-line stars in the one cluster in this region that contains OB stars suggests a timescale for emission-line activity of less than 4 Myr around late-type stars in the cluster and may indicate that the lifetimes of protoplanetary disks around young stellar objects are reduced in clusters containing O stars. The spatial distribution of young stars in the Orion A region is considerably more complex. The angular correlation function of the OB stars (which are mostly foreground to the Orion A molecular cloud) is very similar to that of the Hα stars (which are located mostly within the molecular cloud) and significantly different from that of the young stars in the lambda Ori region. This suggests that, although spatially separated, both populations in the Orion A region may have originated from a similar fragmentation process. Stellar surface density maps and modeling of the angular correlation function suggest that somewhat less than half of the OB and Hα stars in the Orion A cloud are presently within well-defined stellar clusters. Although all the OB stars could have originated in rich clusters, a significant fraction of the Hα stars appear to have formed outside such clusters in a more spatially dispersed manner. The close similarity of the angular correlation functions of the OB and Hα stars toward the molecular cloud, in conjunction with the earlier indications of a relatively high star formation rate and high gas pressure in this cloud, is consistent with the idea that older, foreground OB stars triggered the current episode of star formation in the Orion A cloud. One of the OB clusters (Upper Sword) that is foreground to the cloud does not appear to be associated with any of the clusterings of emission-line stars, again suggesting a timescale (<4 Myr) for emission-line activity and disk lifetimes around late-type stars born in OB clusters.

MESA models of the evolutionary state of the interacting binary epsilon Aurigae

NASA Astrophysics Data System (ADS)

Gibson, Justus L.; Stencel, Robert E.

2018-06-01

Using MESA code (Modules for Experiments in Stellar Astrophysics, version 9575), an evaluation was made of the evolutionary state of the epsilon Aurigae binary system (HD 31964, F0Iap + disc). We sought to satisfy several observational constraints: (1) requiring evolutionary tracks to pass close to the current temperature and luminosity of the primary star; (2) obtaining a period near the observed value of 27.1 years; (3) matching a mass function of 3.0; (4) concurrent Roche lobe overflow and mass transfer; (5) an isotopic ratio 12C/13C = 5 and, (6) matching the interferometrically determined angular diameter. A MESA model starting with binary masses of 9.85 + 4.5 M⊙, with a 100 d initial period, produces a 1.2 + 10.6 M⊙ result having a 547 d period, and a single digit 12C/13C ratio. These values were reached near an age of 20 Myr, when the donor star comes close to the observed luminosity and temperature for epsilon Aurigae A, as a post-RGB/pre-AGB star. Contemporaneously, the accretor then appears as an upper main-sequence, early B-type star. This benchmark model can provide a basis for further exploration of this interacting binary, and other long-period binary stars.

Lunar occultation observations at the SAO RAS 6-m telescope

NASA Astrophysics Data System (ADS)

Dyachenko, V.; Richichi, A.; Balega, Yu; Beskakotov, A.; Maksimov, A.; Mitrofanova, A.; Rastegaev, D.

2018-06-01

We have initiated a program to systematically observe lunar occultations from the SAO RAS 6-m telescope (BTA). So far, twenty-five events have been recorded with some of them leading to accurate measurements of angular diameters in late-type stars and of binary stars. One interesting aspect is that the observations are carried out by the same group and with the same equipment dedicated also to speckle interferometry, so that many of the target objects are being investigated by two independent and complementary techniques almost simultaneously. This represents a novel approach with a potential to provide more complete and extended results than possible until now. In this paper we focus on a general description of the scientific aim and methods, and we provide an overview of the results including an assessment of the quantitative performance, showing that milliarcsecond resolution is achieved on sources as faint as ≈12 mag. Among the sources we discuss in detail are the binary stars SAO 98270 and μ Cet, and the resolved late-type stars 74 Gem, DE Psc and IRC+00213. Other stars with positive results are the subject of dedicated papers. We foresee to continue routine observations with this facility in the immediate future, and to coordinate with other observatories equipped with similar instrumentation.

Gravitational-Wave Luminosity of Binary Neutron Stars Mergers

NASA Astrophysics Data System (ADS)

Zappa, Francesco; Bernuzzi, Sebastiano; Radice, David; Perego, Albino; Dietrich, Tim

2018-03-01

We study the gravitational-wave peak luminosity and radiated energy of quasicircular neutron star mergers using a large sample of numerical relativity simulations with different binary parameters and input physics. The peak luminosity for all the binaries can be described in terms of the mass ratio and of the leading-order post-Newtonian tidal parameter solely. The mergers resulting in a prompt collapse to black hole have the largest peak luminosities. However, the largest amount of energy per unit mass is radiated by mergers that produce a hypermassive neutron star or a massive neutron star remnant. We quantify the gravitational-wave luminosity of binary neutron star merger events, and set upper limits on the radiated energy and the remnant angular momentum from these events. We find that there is an empirical universal relation connecting the total gravitational radiation and the angular momentum of the remnant. Our results constrain the final spin of the remnant black hole and also indicate that stable neutron star remnant forms with super-Keplerian angular momentum.

Gravitational-Wave Luminosity of Binary Neutron Stars Mergers.

PubMed

Zappa, Francesco; Bernuzzi, Sebastiano; Radice, David; Perego, Albino; Dietrich, Tim

2018-03-16

We study the gravitational-wave peak luminosity and radiated energy of quasicircular neutron star mergers using a large sample of numerical relativity simulations with different binary parameters and input physics. The peak luminosity for all the binaries can be described in terms of the mass ratio and of the leading-order post-Newtonian tidal parameter solely. The mergers resulting in a prompt collapse to black hole have the largest peak luminosities. However, the largest amount of energy per unit mass is radiated by mergers that produce a hypermassive neutron star or a massive neutron star remnant. We quantify the gravitational-wave luminosity of binary neutron star merger events, and set upper limits on the radiated energy and the remnant angular momentum from these events. We find that there is an empirical universal relation connecting the total gravitational radiation and the angular momentum of the remnant. Our results constrain the final spin of the remnant black hole and also indicate that stable neutron star remnant forms with super-Keplerian angular momentum.

Sizing up the stars

NASA Astrophysics Data System (ADS)

Boyajian, Tabetha S.

For the main part of this dissertation, I have executed a survey of nearby, main sequence A, F, and G-type stars with the CHARA Array, successfully measuring the angular diameters of forty-four stars to better than 4% accuracy. The results of these observations also yield empirical determinations of stellar linear radii and effective temperatures for the stars observed. In addition, these CHARA-determined temperatures, radii, and luminosities are fit to Yonsei-Yale isochrones to constrain the masses and ages of the stars. These quantities are compared to the results found in Allende Prieto & Lambert (1999), Holmberg et al. (2007), and Takeda (2007), who indirectly determine these same properties by fitting models to observed photometry. I find that for most cases, the models underestimate the radius of the star by ~ 12%, while in turn they overestimate the effective temperature by ~ 1.5-4%, when compared to my directly measured values, with no apparent correlation to the star's metallicity or color index. These overestimated temperatures and underestimated radii in these works appear to cause an additional offset in the star's surface gravity measurements, which consequently yield higher masses and younger ages, in particular for stars with masses greater than ~ 1.3 [Special characters omitted.] . Alternatively, these quantities I measure are also compared to direct measurements from a large sample of eclipsing binary stars in Andersen (1991), and excellent agreement is seen within both data sets. Finally, a multi-parameter solution is found to fit color-temperature-metallicity values of the stars in this sample to provide a new calibration of the effective temperature scale for these types of stars. Published work in the field of stellar interferometry and optical spectroscopy of early-type stars are presented in Appendix D and E, respectively. INDEX WORDS: Interferometry, Infrared, Stellar Astronomy, Fundamental Properties, Effective Temperatures, Stellar Radii

Rotation of Giant Stars

NASA Astrophysics Data System (ADS)

Kissin, Yevgeni; Thompson, Christopher

2015-07-01

The internal rotation of post-main sequence stars is investigated, in response to the convective pumping of angular momentum toward the stellar core, combined with a tight magnetic coupling between core and envelope. The spin evolution is calculated using model stars of initial mass 1, 1.5, and 5 {M}⊙ , taking into account mass loss on the giant branches. We also include the deposition of orbital angular momentum from a sub-stellar companion, as influenced by tidal drag along with the excitation of orbital eccentricity by a fluctuating gravitational quadrupole moment. A range of angular velocity profiles {{Ω }}(r) is considered in the envelope, extending from solid rotation to constant specific angular momentum. We focus on the backreaction of the Coriolis force, and the threshold for dynamo action in the inner envelope. Quantitative agreement with measurements of core rotation in subgiants and post-He core flash stars by Kepler is obtained with a two-layer angular velocity profile: uniform specific angular momentum where the Coriolis parameter {Co}\\equiv {{Ω }}{τ }{con}≲ 1 (here {τ }{con} is the convective time), and {{Ω }}(r)\\propto {r}-1 where {Co}≳ 1. The inner profile is interpreted in terms of a balance between the Coriolis force and angular pressure gradients driven by radially extended convective plumes. Inward angular momentum pumping reduces the surface rotation of subgiants, and the need for a rejuvenated magnetic wind torque. The co-evolution of internal magnetic fields and rotation is considered in Kissin & Thompson, along with the breaking of the rotational coupling between core and envelope due to heavy mass loss.

Mission Study for Generation-X: A Large Area and High Angular Observatory to Study the Early Universe

NASA Technical Reports Server (NTRS)

Brissenden, Roger

2005-01-01

In this report we provide a summary of the technical progress achieved during the last year Generation-X Vision Mission Study. In addition, we provide a brief programmatic status. The Generation-X (Gen-X) Vision Mission Study investigates the science requirements, mission concepts and technology drivers for an X-ray telescope designed to study the new frontier of astrophysics: the birth and evolution of the first stars, galaxies and black holes in the early Universe. X-ray astronomy offers an opportunity to detect these via the activity of the black holes, and the supernova explosions and gamma-ray burst afterglows of the massive stars. However, such objects are beyond the grasp of current missions which are operating or even under development. Our team has conceived a Gen-X Vision Mission based on an X-ray observatory with 100 m2 collecting area at 1 keV (1000 times larger than Chandra) and 0.1 arcsecond angular resolution (several times better than Chandra and 50 times better than the Constellation-X resolution goal). Such a high energy observatory will be capable of detecting the earliest black holes and galaxies in the Universe, and will also study extremes of density, gravity, magnetic fields, and kinetic energy which cannot be created in laboratories. In our study we develop the mission concept and define candidate technologies and performance requirements for Gen-X. The baseline Gen-X mission involves four 8 m diameter X-ray telescopes operating at Sun-Earth L2. We trade against an alternate concept of a single 26 m diameter telescope with focal plane instruments on a separate spacecraft. A telescope of this size will require either robotic or human-assisted in-flight assembly. The required effective area implies that extremely lightweight grazing incidence X-ray optics must be developed. To achieve the required areal density of at least 100 times lower than for Chandra, we study 0.2 mm thick mirrors which have active on-orbit figure control. We also study the suite of required detectors, including a large FOV high angular resolution imager, a cryogenic imaging spectrometer and a reflection grating spectrometer.

The Generation-X X-ray Observatory Vision Mission and Technology Study

NASA Technical Reports Server (NTRS)

Figueroa-Feliciano, Enectali

2004-01-01

The new frontier in astrophysics is the study of the birth and evolution of the first stars, galaxies and black holes in the early Universe. X-ray astronomy opens a window into these objects by studying the emission from black holes, supernova explosions and the gamma-ray burst afterglows of massive stars. However, such objects are beyond the grasp of current or near-future observatories. X-ray imaging and spectroscopy of such distant objects will require an X-ray telescope with large collecting area and high angular resolution. Our team has conceived the Generation-X Vision Mission based on an X-ray observatory with 100 sq m collecting area at 1 keV (1000 times larger than Chandra) and 0.1 arcsecond angular resolution (several times better than Chandra and 50 times better than the Constellation-X resolution goal). Such an observatory would be capable of detecting the earliest black holes and galaxies in the Universe, and will also study extremes of density, gravity, magnetic fields, and kinetic energy which cannot be created in laboratories. NASA has selected the Generation-X mission for study under its Vision Mission Program. We describe the studies being performed to develop the mission concept and define candidate technologies and performance requirements for Generation-X. The baseline Generation-X mission involves four 8m diameter X-ray telescopes operating at Sun-Earth L2. We trade against an alternate concept of a single 26m diameter telescope with focal plane instruments on a separate spacecraft. A telescope of this size will require either robotic or human-assisted in-flight assembly. The required effective area implies that extremely lightweight grazing incidence X-ray optics must be developed. To achieve the required aerial density of at least 100 times lower than in Chandra, we will study 0.1mm thick mirrors which have active on-orbit figure control. We discuss the suite of required detectors, including a large FOV high angular resolution imager, a cryogenic imaging spectrometer and a grating spectrometer. We outline the development roadmap to confront the many technological challenges far implementing the Generation-X mission.

The ultraviolet variability of early-type supergiants

NASA Technical Reports Server (NTRS)

Underhill, A. B.

1984-01-01

Four early-type supergiants - HD 79186 (B5 Ia), HD 96919 (B9 Ia), HD 105056 (ON9.7 Iae), and HD 148379 (B2 Iae) - have been observed with the low-resolution spectrographs of IUE in the large aperture on 14 days. The behavior of the ultraviolet fluxes with time is studied. The light from all four stars seems to vary. Typically the dispersion about the mean magnitude at any wavelength corresponds to + or - 0.085, + or - 0.080, + or - 0.101, and + or - 0.106 mag, respectively. These amplitudes exceed the typical uncertainty in an IUE measurement of flux by about a factor of 3; they are somewhat larger than the variations known in the visible wavelength range. There are insufficient data to investigate periodicity in the observed light changes. The effective temperatures and angular diameters of the stars have been estimated using the present ultraviolet photometry, published UBV and uvby photometry, and the model-atmosphere fluxes reported by Kurucz in 1979. The program stars have dimensions typical for their spectral types. A brief discussion is given of possible causes of the variability of hot supergiants.

An ATP System for Deep-Space Optical Communication

NASA Technical Reports Server (NTRS)

Lee, Shinhak; Irtuzm Gerardi; Alexander, James

2008-01-01

An acquisition, tracking, and pointing (ATP) system is proposed for aiming an optical-communications downlink laser beam from deep space. In providing for a direction reference, the concept exploits the mature technology of star trackers to eliminate the need for a costly and potentially hazardous laser beacon. The system would include one optical and two inertial sensors, each contributing primarily to a different portion of the frequency spectrum of the pointing signal: a star tracker (<10 Hz), a gyroscope (<50 Hz), and a precise fluid-rotor inertial angular-displacement sensor (sometimes called, simply, "angle sensor") for the frequency range >50 Hz. The outputs of these sensors would be combined in an iterative averaging process to obtain high-bandwidth, high-accuracy pointing knowledge. The accuracy of pointing knowledge obtainable by use of the system was estimated on the basis of an 8-cm-diameter telescope and known parameters of commercially available star trackers and inertial sensors: The single-axis pointing-knowledge error was found to be characterized by a standard deviation of 150 nanoradians - below the maximum value (between 200 and 300 nanoradians) likely to be tolerable in deep-space optical communications.

Lunar occultations of Aldebaran and other late-type stars observed from Devasthal

NASA Astrophysics Data System (ADS)

Richichi, A.; Sharma, S.; Pandey, A. K.; Pandey, R.; Sinha, T.; Norharizan, M. D.

2018-02-01

We report on lunar occultations of Aldebaran (α Tau) and other ten, mostly late-type, stars observed with the Devasthal 1.3-m telescope. We derive a detailed brightness profile for Aldebaran, confirming the presence of asymmetries already recently described in a related work. We test the origin of such asymmetries by means of simulations of the effect of scintillation on the reconstructed profiles. We also derive angular diameters for two M giants, Z Cnc and SAO 161635, which we discuss in the context of previous determinations. We find first-time companions around two other stars, SAO 161665 and WZ Psc, and we detect one more previously known binary, SAO 94060. This is the first systematic effort to observe lunar occultations events at this facility, and demonstrates the capability to carry out milliarcsecond-level investigations on sources down to ≈ 9 mag. We plan to continue this routine program in the coming years, eventually utilizing also the 3.6 m DOT telescope recently erected at Devasthal for deeper sensitivity and higher accuracy.

IMPLICATIONS OF RAPID CORE ROTATION IN RED GIANTS FOR INTERNAL ANGULAR MOMENTUM TRANSPORT IN STARS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tayar, Jamie; Pinsonneault, Marc H., E-mail: tayar.1@osu.edu

2013-09-20

Core rotation rates have been measured for red giant stars using asteroseismology. These data, along with helioseismic measurements and open cluster spin-down studies, provide powerful clues about the nature and timescale for internal angular momentum transport in stars. We focus on two cases: the metal-poor red giant KIC 7341231 ({sup O}tto{sup )} and intermediate-mass core helium burning stars. For both, we examine limiting case studies for angular momentum coupling between cores and envelopes under the assumption of rigid rotation on the main sequence. We discuss the expected pattern of core rotation as a function of mass and radius. In themore » case of Otto, strong post-main-sequence coupling is ruled out and the measured core rotation rate is in the range of 23-33 times the surface value expected from standard spin-down models. The minimum coupling timescale (0.17-0.45 Gyr) is significantly longer than that inferred for young open cluster stars. This implies ineffective internal angular momentum transport in early first ascent giants. By contrast, the core rotation rates of evolved secondary clump stars are found to be consistent with strong coupling given their rapid main-sequence rotation. An extrapolation to the white dwarf regime predicts rotation periods between 330 and 0.0052 days, depending on mass and decoupling time. We identify two key ingredients that explain these features: the presence of a convective core and inefficient angular momentum transport in the presence of larger mean molecular weight gradients. Observational tests that can disentangle these effects are discussed.« less

The Baade-Wesselink projection factor of the δ-Scuti stars AI Vel and β Cas

NASA Astrophysics Data System (ADS)

Guiglion, G.; Nardetto, N.; Domiciano de Souza, A.; Mathias, P.; Mourard, D.; Poretti, E.

2012-12-01

The Baade-Wesselink method of distance determination is based on the oscillations of pulsating stars. After determining the angular diameter and the linear radius variations, the distance is derived by a simple ratio. The linear radius variation is measured by integrating the pulsation velocity (hereafter V_{puls}) over one pulsating cycle. However, from observations we have only access to the radial velocity (V_{rad}) because of the projection along the line-of-sight. The projection factor, used to convert the radial velocity into the pulsation velocity, is defined by: p = V_{puls} / V_{rad}. We aim to derive the projection factor for two δ-Scuti stars, the high amplitude pulsator AI Vel and the fast rotator β Cas. The geometric component of the projection factor is derived using a limb-darkening model of the intensity distribution of AI Vel, and a fast rotator model for β Cas. Then, by comparing the radial velocity curves of several spectral lines forming at different levels in the atmosphere, we derive directly the velocity gradient (in a part of the atmosphere of the star) using SOPHIE/OHP data for β Cas and HARPS/ESO data for AI Vel, which is used to derive a dynamical projection factor for both stars. We find p = 1.44 ± 0.05 for AI Vel and p = 1.41 ± 0.25 for β Cas. By comparing Cepheids and δ-Scuti stars, these results bring valuable insights into the dynamical structure of pulsating star atmospheres.

1-D Imaging of the Dynamical Atmosphere of the Red Supergiant Betelgeuse in the CO First Overtone Lines with VLTI/AMBER

NASA Astrophysics Data System (ADS)

Ohnaka, K.

2014-09-01

We present high-spatial and high-spectral resolution observations of the red supergiant Betelgeuse in the CO first overtone lines near 2.3μm with the AMBER instrument at the Very Large Telescope Interferometer (VLTI). Our AMBER observations in 2008 spatially resolved the gas motions in a stellar atmosphere (photosphere and extended molecular outer atmosphere) for the first time other than the Sun. From our second observations one year later, we have reconstructed 1-D images in the individual CO lines with an angular resolution of 9.8 mas and a spectral resolution of 6000 by applying the self-calibration technique to restore the Fourier phase from the differential phase measurements. The reconstructed 1-D images reveal that the star appears different in the blue and red wing of the individual CO lines. In the blue wing, the star shows a pronounced, asymmetrically extended component at least up to 1.3 R⋆, while such a component does not appear in the red wing 1-D image. This can be explained by a model in which the CO gas patch (or clump) more than half as large as the star is moving slightly outward with 0-5 km s-1, while the gas in the remaining region is infalling fast with 20-30 km s-1. Comparison between the CO line data taken in 2008 and 2009 shows a significant time variation in the dynamics of the photosphere and outer atmosphere. However, the 1-D images in the continuum show only a slight deviation from a limb-darkened disk with an angular diameter of 42.49±0.06 mas, which leads to an effective temperature of 3690± 54 K. Moreover, the continuum data taken in 2008 and 2009 reveal no or only marginal time variations, much smaller than the maximum variation predicted by the current 3-D convection simulation. The derived continuum diameter also shows that the near-IR size of Betelgeuse has been nearly constant over the last 18 years, in marked contrast to the recently reported noticeable decrease in the mid-IR size.

An edge-on translucent dust disk around the nearest AGB star, L2 Puppis. VLT/NACO spectro-imaging from 1.04 to 4.05 μm and VLTI interferometry

NASA Astrophysics Data System (ADS)

Kervella, P.; Montargès, M.; Ridgway, S. T.; Perrin, G.; Chesneau, O.; Lacour, S.; Chiavassa, A.; Haubois, X.; Gallenne, A.

2014-04-01

As the nearest known AGB star (d = 64 pc) and one of the brightest (mK ≈ -2), L2 Pup is a particularly interesting benchmark object to monitor the final stages of stellar evolution. We report new serendipitous imaging observations of this star with the VLT/NACO adaptive optics system in twelve narrow-band filters covering the 1.0-4.0 μm wavelength range. These diffraction-limited images reveal an extended circumstellar dust lane in front of the star that exhibits a high opacity in the J band and becomes translucent in the H and K bands. In the L band, extended thermal emission from the dust is detected. We reproduced these observations using Monte Carlo radiative transfer modeling of a dust disk with the RADMC-3D code. We also present new interferometric observations with the VLTI/VINCI and MIDI instruments. We measured in the K band an upper limit to the limb-darkened angular diameter of θLD = 17.9 ± 1.6 mas, converting to a maximum linear radius of R = 123 ± 14 R⊙. Considering the geometry of the extended K band emission in the NACO images, this upper limit is probably close to the actual angular diameter of the star. The position of L2 Pup in the Hertzsprung-Russell diagram indicates that this star has a mass of about 2 M⊙ and is probably experiencing an early stage of the asymptotic giant branch. We did not detect any stellar companion of L2 Pup in our adaptive optics and interferometric observations, and we attribute its apparent astrometric wobble in the Hipparcos data to variable lighting effects on its circumstellar material. However, we do not exclude the presence of a binary companion, because the large loop structure extending to more than 10 AU to the northeast of the disk in our L-band images may be the result of interaction between the stellar wind of L2 Pup and a hidden secondary object. The geometric configuration that we propose, with a large dust disk seen almost edge-on, appears particularly favorable to test and develop our understanding of the formation of bipolar nebulae. Based on observations made with ESO telescopes at Paranal Observatory, under ESO programs 090.D-0144(A), 074.D-0198(C) and an unreferenced VLTI/VINCI program.FITS files are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/564/A88

Stability of Rigidly Rotating Relativistic Stars with Soft Equations of State against Gravitational Collapse

NASA Astrophysics Data System (ADS)

Shibata, Masaru

2004-04-01

We study secular stability against a quasi-radial oscillation for rigidly rotating stars with soft equations of state in general relativity. The polytropic equations of state with polytropic index n between 3 and 3.05 are adopted for modeling the rotating stars. The stability is determined in terms of the turning-point method. It is found that (1) for n>~3.04, all the rigidly rotating stars are unstable against the quasi-radial oscillation and (2) for n>~3.01, the nondimensional angular momentum parameter q≡cJ/GM2 (where J, M, G, and c denote the angular momentum, the gravitational mass, the gravitational constant, and the speed of light, respectively) for all marginally stable rotating stars is larger than unity. A semianalytic calculation is also performed, and good agreement with the numerical results is confirmed. The final outcome after axisymmetric gravitational collapse of rigidly rotating and marginally stable massive stars with q>1 is predicted, assuming that the rest-mass distribution as a function of the specific angular momentum is preserved and that the pressure never halt the collapse. It is found that even for 1~2.5, the significant angular momentum will prevent the direct formation of a black hole.

Collapse and Nonlinear Instability of AdS Space with Angular Momentum

NASA Astrophysics Data System (ADS)

Choptuik, Matthew W.; Dias, Óscar J. C.; Santos, Jorge E.; Way, Benson

2017-11-01

We present a numerical study of rotational dynamics in AdS5 with equal angular momenta in the presence of a complex doublet scalar field. We determine that the endpoint of gravitational collapse is a Myers-Perry black hole for high energies and a hairy black hole for low energies. We investigate the time scale for collapse at low energies E , keeping the angular momenta J ∝E in anti-de Sitter (AdS) length units. We find that the inclusion of angular momenta delays the collapse time, but retains a t ˜1 /E scaling. We perturb and evolve rotating boson stars, and find that boson stars near AdS space appear stable, but those sufficiently far from AdS space are unstable. We find that the dynamics of the boson star instability depend on the perturbation, resulting either in collapse to a Myers-Perry black hole, or development towards a stable oscillating solution.

Magnetic braking in young late-type stars. The effect of polar spots

NASA Astrophysics Data System (ADS)

Aibéo, A.; Ferreira, J. M.; Lima, J. J. G.

2007-10-01

Context: The existence of rapidly rotating cool stars in young clusters implies a reduction of angular momentum loss rate for a certain period of the star's early life. Recently, the concentration of magnetic flux near the poles of these stars has been proposed as an alternative mechanism to dynamo saturation in order to explain the saturation of angular momentum loss. Aims: In this work we study the effect of magnetic surface flux distribution on the coronal field topology and angular momentum loss rate. We investigate if magnetic flux concentration towards the pole is a reasonable alternative to dynamo saturation. Methods: We construct a 1D wind model and also apply a 2-D self-similar analytical model, to evaluate how the surface field distribution affects the angular momentum loss of the rotating star. Results: From the 1D model we find that, in a magnetically dominated low corona, the concentrated polar surface field rapidly expands to regions of low magnetic pressure resulting in a coronal field with small latitudinal variation. We also find that the angular momentum loss rate due to a uniform field or a concentrated field with equal total magnetic flux is very similar. From the 2D wind model we show that there are several relevant factors to take into account when studying the angular momentum loss from a star. In particular, we show that the inclusion of force balance across the field in a wind model is fundamental if realistic conclusions are to be drawn from the effect of non-uniform surface field distribution on magnetic braking. This model predicts that a magnetic field concentrated at high latitudes leads to larger Alfvén radii and larger braking rates than a smoother field distribution. Conclusions: From the results obtained, we argue that the magnetic surface field distribution towards the pole does not directly limit the braking efficiency of the wind.

Neutron tori around Kerr black holes

NASA Technical Reports Server (NTRS)

Witt, H. J.; Jaroszynski, M.; Haensel, P.; Paczynski, B.; Wambsganss, J.

1994-01-01

Models of stationary, axisymmetric, non-self-gravitating tori around stellar mass Kerr black holes are calculated. Such objects may form as a result of a merger between two neutron stars, a neutron star and a stellar mass black hole, or a 'failed supernova' collapse of a single rapidly rotating star. We explore a large range of parameters: the black hole mass and angular momentum, the torus mass, angular momentum and entropy. Physical conditions within the tori are similar to those in young and hot neutron stars, but their topology is different, and the range of masses and energies is much larger.

Propeller-driven outflows from an MRI disc

NASA Astrophysics Data System (ADS)

Lii, Patrick S.; Romanova, Marina M.; Ustyugova, Galina V.; Koldoba, Alexander V.; Lovelace, Richard V. E.

2014-06-01

Accreting magnetized stars may be in the propeller regime of disc accretion in which the angular velocity of the stellar magnetosphere exceeds that of the inner disc. In these systems, the stellar magnetosphere acts as a centrifugal barrier and plays a dominant role in the inner disc dynamics by inhibiting matter accretion on to the star. In this work, we investigate the dynamics of the propeller regime using axisymmetric MHD simulations of MRI-driven accretion on to a rapidly rotating magnetized star. The disc matter is inhibited from accreting on to the star and instead accumulates at the disc-magnetosphere boundary, slowly building up a reservoir of matter. Some of this matter diffuses into the outer magnetosphere where it picks up angular momentum and is ejected as an outflow which gradually collimates at larger distances from the star. If the ejection rate is smaller than the disc's accretion rate, then the matter accumulates at the disc-magnetosphere boundary faster than it can be ejected. In this situation, accretion on to the propelling star proceeds through the episodic accretion cycle in which episodes of matter accumulation are followed by a brief episode of simultaneous ejection and accretion on to the star. In addition to the matter-dominated wind component, the propeller also drives a well-collimated, magnetically dominated Poynting jet which transports energy and angular momentum away from the star. The propelling stars undergo strong spin-down due to the outflow of angular momentum in the wind and jet. We measure spin-down time-scales of ˜1.2 Myr for a cTTs in the strong propeller regime of accretion. The propeller mechanism may explain some of the jets and winds observed around some T Tauri stars as well as the nature of their ejections. It may also explain some of the quasi-periodic variability observed in cataclysmic variables, millisecond pulsars and other magnetized stars.

Observing the Sun with micro-interferometric devices: a didactic experiment

NASA Astrophysics Data System (ADS)

Defrère, D.; Absil, O.; Hanot, C.; Riaud, P.; Magette, A.; Marion, L.; Wertz, O.; Finet, F.; Steenackers, M.; Habraken, S.; Surdej, A.; Surdej, J.

2014-04-01

Measuring the angular diameter of celestial bodies has long been the main purpose of stellar interferometry and was its historical motivation. Nowadays, stellar interferometry is widely used for various other scientific purposes that require very high angular resolution measurements. In terms of angular spatial scales probed, observing distant stars located 10 to 100~pc away with a large hectometric interferometer is equivalent to observing our Sun with a micrometric baseline. Based on this idea, we have manufactured a set of micro-interferometric devices and tested them on the sky. The micro-interferometers consist of a chrome layer deposited on a glass plate that has been drilled by laser lithography to produce micron-sized holes with configurations corresponding to proposed interferometer projects such as CARLINA, ELSA, KEOPS, and OVLA. In this paper, we describe these interferometric devices and present interferometric observations of the Sun made in the framework of Astrophysics lectures being taught at the Liège University. By means of a simple photographic camera placed behind a micro-interferometric device, we observed the Sun and derived its angular size. This experiment provides a very didactic way to easily obtain fringe patterns similar to those that will be obtained with future large imaging arrays. A program written in C also allows to reproduce the various point spread functions and fringe patterns observed with the micro-interferometric devices for different types of sources, including the Sun.

MHD Simulations of Magnetized Stars in the Propeller Regime of Accretion

NASA Astrophysics Data System (ADS)

Lii, Patrick; Romanova, Marina; Lovelace, Richard

2014-01-01

Accreting magnetized stars may be in the propeller regime of disc accretion in which the angular velocity of the stellar magnetosphere exceeds that of the inner disc. In these systems, the stellar magnetosphere acts as a centrifugal barrier and inhibits matter accretion onto the rapidly rotating star. Instead, the matter accreting through the disc accumulates at the disc-magnetosphere interface where it picks up angular momentum and is ejected from the system as a wide-angled outflow which gradually collimates at larger distances from the star. If the ejection rate is lower than the accretion rate, the matter will accumulate at the boundary faster than it can be ejected; in this case, accretion onto the star proceeds through an episodic accretion instability in which the episodes of matter accumulation are followed by a brief episode of simultaneous ejection and accretion of matter onto the star. In addition to the matter dominated wind component, the propeller outflow also exhibits a well-collimated, magnetically-dominated Poynting jet which transports energy and angular momentum away from the star. The propeller mechanism may explain some of the weakly-collimated jets and winds observed around some T Tauri stars as well as the episodic variability present in their light curves. It may also explain some of the quasi-periodic variability observed in cataclysmic variables, millisecond pulsars and other magnetized stars.

Angular Momentum Evolution in Young Low Mass Stars

NASA Astrophysics Data System (ADS)

Pinzón, G.; de La Reza, R.

2006-06-01

During the last decades, the study of rotation in young low mass stars has been one of the more active areas in the field of stellar evolution. Many theoretical efforts have been made to understand the angular momentum evolution and our picture now, reveals the main role of the stellar magnetic field in all pre-main sequence stage (Ghosh & Lamb 1979, ApJ, 234, 296; Cameron & Campbell 1993, A&A, 274, 309; Cameron & Campbell 1995, A&A, 298, 133; Kúker, Henning, & Rúdiger 2003, ApJ, 589, 397; Matt & Pudritz 2005, MNRAS, 356, 167). The mean rotation of most of the cool low mass stars remains roughly constant during the T Tauri stage. This can be explained by the disc locking scenario. This paradigm suggest that star start out as CTTS with periods of 4-14 days, perhaps locked to their disc, and that this disc is eventually lost mainly by accretion. At the current time, it is not clear that this is true for all low mass stars. Some authors have questioned its validity for stars less massive than 0.5 solar masses. Although the reality may eventually turn out to be considerably more complex, a simple consideration of the effects of and limits on disc locking of young low mass stars seems necessary.We have investigated the exchange of angular momentum between a low mass star and an accretion disc during the Hayashi Track (Pinzón, Kúker, & de la Reza 2005, in preparation) and also along the first 100Myr of stellar evolution. The model incorporates changes in the star's moment of inertia, magnetic field strength (Elstner & Rúdiger 2000, A&A, 358, 612), angular momentum loss by a magnetic wind and an exponential decrease of the accretion rate. The lifetime of the accretion disc is a free parameter in our model. The resulting rotation rates are in agreement with observed vsin and photometric periods for young stars belonging to co-moving groups and open young clusters.

Presupernova Evolution of Differentially Rotating Massive Stars Including Magnetic Fields

NASA Astrophysics Data System (ADS)

Heger, A.; Woosley, S. E.; Spruit, H. C.

2005-06-01

As a massive star evolves through multiple stages of nuclear burning on its way to becoming a supernova, a complex, differentially rotating structure is set up. Angular momentum is transported by a variety of classic instabilities and also by magnetic torques from fields generated by the differential rotation. We present the first stellar evolution calculations to follow the evolution of rotating massive stars including, at least approximately, all these effects, magnetic and nonmagnetic, from the zero-age main sequence until the onset of iron-core collapse. The evolution and action of the magnetic fields is as described by Spruit in 2002, and a range of uncertain parameters is explored. In general, we find that magnetic torques decrease the final rotation rate of the collapsing iron core by about a factor of 30-50 when compared with the nonmagnetic counterparts. Angular momentum in that part of the presupernova star destined to become a neutron star is an increasing function of main-sequence mass. That is, pulsars derived from more massive stars rotate faster and rotation plays a more important role in the star's explosion. The final angular momentum of the core has been determined-to within a factor of 2-by the time the star ignites carbon burning. For the lighter stars studied, around 15 Msolar, we predict pulsar periods at birth near 15 ms, though a factor of 2 range is easily tolerated by the uncertainties. Several mechanisms for additional braking in a young neutron star, especially by fallback, are explored.

A theory of ring formation around Be stars

NASA Technical Reports Server (NTRS)

Huang, S.-S.

1976-01-01

A theory for the formation of gaseous rings around Be stars is developed which involves the combined effect of stellar rotation and radiation pressure. A qualitative scenario of ring formation is outlined in which the envelope formed about a star from ejected material is in the form of a disk in the equatorial plane, collisions between ejected gas blobs are inevitable, and particles with high angular momenta form a rotating ring around the star. A quantitative description of this process is then formulated by considering the angular momentum and dynamical energy of the ejected matter as well as those of the ring alone, without introducing any other assumptions.

Supranova Events from Spun-up Neutron Stars: An Explosion in Search of an Observation

NASA Astrophysics Data System (ADS)

Vietri, Mario; Stella, Luigi

1999-12-01

We consider a formation scenario for supramassive neutron stars (SMNSs) that takes place through mass and angular momentum transfer from a close companion during a low-mass X-ray binary phase, with the ensuing suppression of the magnetic field. After the end of the mass transfer phase, SMNSs will lose, through magnetic dipole radiation, most of their angular momentum, triggering the star's collapse to a black hole. We discuss the rate of occurrence of these collapses and propose that these stars, because of the baryon-clear environment in which the implosion/explosion takes place, are the originators of gamma-ray bursts.

Circumstellar Disks Around Rapidly Rotating Be-type Stars

NASA Astrophysics Data System (ADS)

Touhami, Yamina

2012-01-01

Be stars are rapidly rotating B-type stars that eject large amounts of gaseous material into a circumstellar equatorial disk. The existence of this disk has been confirmed through the presence of several observational signatures such as the strong hydrogen emission lines, the IR flux excess, and the linear polarization detected from these systems. Here we report simultaneous near-IR interferometric and spectroscopic observations of circumstellar disks around Be stars obtained with the CHARA Array long baseline interferometer and the Mimir spectrograph at Lowell observatory. The goal of this project was to measure precise angular sizes and to characterize the fundamental geometrical and physical properties of the circumstellar disks. We were able to determine spatial extensions, inclinations, and position angles, as well as the gas density profile of the circumstellar disks using an elliptical Gaussian model and a physical thick disk model, and we show that the K-band interferometric angular sizes of the circumstellar disks are correlated with the H-alpha angular sizes. By combining the projected rotational velocity of the Be star with the disk inclination derived from interferometry, we provide estimates of the equatorial rotational velocities of these rapidly rotating Be stars.

High-resolution observations of IRAS 08544-4431. Detection of a disk orbiting a post-AGB star and of a slow disk wind

NASA Astrophysics Data System (ADS)

Bujarrabal, V.; Castro-Carrizo, A.; Winckel, H. Van; Alcolea, J.; Contreras, C. Sánchez; Santander-García, M.; Hillen, M.

2018-06-01

Context. Aims: In order to study the effects of rotating disks in the post-asymptotic giant branch (post-AGB) evolution, we observe a class of binary post-AGB stars that seem to be systematically surrounded by equatorial disks and slow outflows. Although the rotating dynamics had only been well identified in three cases, the study of such structures is thought to be fundamental to the understanding of the formation of disks in various phases of the late evolution of binary stars and the ejection of planetary nebulae from evolved stars. Methods: We present ALMA maps of 12CO and 13CO J = 3-2 lines in the source IRAS 08544-4431, which belongs to the above mentioned class of objects. We analyzed the data by means of nebula models, which account for the expectedly composite source and can reproduce the data. From our modeling, we estimated the main nebula parameters, including the structure and dynamics and the density and temperature distributions. We discuss the uncertainties of the derived values and, in particular, their dependence on the distance. Results: Our observations reveal the presence of an equatorial disk in rotation; a low-velocity outflow is also found, probably formed of gas expelled from the disk. The main characteristics of our observations and modeling of IRAS 08544-4431 are similar to those of better studied objects, confirming our interpretation. The disk rotation indicates a total central mass of about 1.8 M⊙, for a distance of 1100 pc. The disk is found to be relatively extended and has a typical diameter of 4 × 1016 cm. The total nebular mass is 2 × 10-2 M⊙, of which 90% corresponds to the disk. Assuming that the outflow is due to mass loss from the disk, we derive a disk lifetime of 10 000 yr. The disk angular momentum is found to be comparable to that of the binary system at present. Assuming that the disk angular momentum was transferred from the binary system, as expected, the high values of the disk angular momentum in this and other similar disks suggest that the size of the stellar orbits has significantly decreased as a consequence of disk formation.

The rotation of discs around neutron stars: dependence on the Hall diffusion

NASA Astrophysics Data System (ADS)

Faghei, Kazem; Salehi, Fatemeh

2018-01-01

In this paper, we study the dynamics of a geometrically thin, steady and axisymmetric accretion disc surrounding a rotating and magnetized star. The magnetic field lines of star penetrate inside the accretion disc and are twisted due to the differential rotation between the magnetized star and the disc. We apply the Hall diffusion effect in the accreting plasma, because of the Hall diffusion plays an important role in both fully ionized plasma and weakly ionized medium. In the current research, we show that the Hall diffusion is also an important mechanism in accreting plasma around neutron stars. For the typical system parameter values associated with the accreting X-ray binary pulsar, the angular velocity of the inner regions of disc departs outstandingly from Keplerian angular velocity, due to coupling between the magnetic field of neutron star and the rotating plasma of disc. We found that the Hall diffusion is very important in inner disc and increases the coupling between the magnetic field of neutron star and accreting plasma. On the other word, the rotational velocity of inner disc significantly decreases in the presence of the Hall diffusion. Moreover, the solutions imply that the fastness parameter decreases and the angular velocity transition zone becomes broad for the accreting plasma including the Hall diffusion.

Forming disc galaxies in major mergers - III. The effect of angular momentum on the radial density profiles of disc galaxies

NASA Astrophysics Data System (ADS)

Peschken, N.; Athanassoula, E.; Rodionov, S. A.

2017-06-01

We study the effect of angular momentum on the surface density profiles of disc galaxies, using high-resolution simulations of major mergers whose remnants have downbending radial density profiles (type II). As described in the previous papers of this series, in this scenario, most of the disc mass is acquired after the collision via accretion from a hot gaseous halo. We find that the inner and outer disc scalelengths, as well as the break radius, correlate with the total angular momentum of the initial merging system, and are larger for high-angular momentum systems. We follow the angular momentum redistribution in our simulated galaxies, and find that like the mass, the disc angular momentum is acquired via accretion, I.e. to the detriment of the gaseous halo. Furthermore, high-angular momentum systems give more angular momentum to their discs, which directly affects their radial density profile. Adding simulations of isolated galaxies to our sample, we find that the correlations are valid also for disc galaxies evolved in isolation. We show that the outer part of the disc at the end of the simulation is populated mainly by inside-out stellar migration, and that in galaxies with higher angular momentum, stars travel radially further out. This, however, does not mean that outer disc stars (in type II discs) were mostly born in the inner disc. Indeed, generally the break radius increases over time, and not taking this into account leads to overestimating the number of stars born in the inner disc.

REVIEWS OF TOPICAL PROBLEMS: The differential rotation of stars

NASA Astrophysics Data System (ADS)

Kitchatinov, Leonid L.

2005-05-01

Astronomical observations of recent years have substantially extended our knowledge of the rotation of stars. Helioseismology has found out that the equator-to-pole decline in the angular velocity observed on the solar surface traces down to the deep interior of the Sun. New information has been gained regarding the dependence of the rotational nonuniformities on the angular velocity and mass of the star. These achievements have prompted the development of the theory of differential rotation, which is the focal point of this review. Nonuniform rotation results from the interaction of turbulent convection with rotation. The investigation into the turbulent mechanisms of angular-momentum transport has reached a level at which the obtained results can serve as the basis for developing quantitative models of stellar rotation. Such models contain virtually no free parameters but closely reproduce the helioseismological data on the internal rotation of the Sun. The theoretical predictions on the differential rotation of the stars agree with observations. A brief discussion is held here on the relation between the magnetic activity of stars and the nonuniformity of their rotation and on prospects for further development of the theory.

The Polarization of Achernar

NASA Astrophysics Data System (ADS)

McDavid, D.

2005-11-01

Recent near-infrared measurements of the angular diameter of Achernar (the bright Be star alpha Eridani) with the ESO VLT interferometer have been interpreted as the detection of an extremely oblate photosphere, with a ratio of equatorial to polar radius of at least 1.56 ± 0.05 and a minor axis orientation of 39° ± 1° (from North to East). The optical linear polarization of this star during an emission phase in 1995 September was 0.12 ± 0.02% at position angle 37° ± 8° (in equatorial coordinates), which is the direction of the projection of the rotation axis on the plane of the sky according to the theory of polarization by electron scattering in an equatorially flattened circumstellar disk. These two independent determinations of the orientation of the rotation axis are therefore in agreement. The observational history of correlations between Hα emission and polarization as found in the literature is that of a typical Be star, with the exception of an interesting question raised by the contrast between Schröder's measurement of a small polarization perpendicular to the projected rotation axis in 1969--70 and Tinbergen's measurement of zero polarization in 1974.5, both at times when emission was reportedly absent.

Spectral photometry of extreme helium stars: Ultraviolet fluxes and effective temperature

NASA Technical Reports Server (NTRS)

Drilling, J. S.; Schoenberner, D.; Heber, U.; Lynas-Gray, A. E.

1982-01-01

Ultraviolet flux distributions are presented for the extremely helium rich stars BD +10 deg 2179, HD 124448, LSS 3378, BD -9 deg 4395, LSE 78, HD 160641, LSIV -1 deg 2, BD 1 deg 3438, HD 168476, MV Sgr, LS IV-14 deg 109 (CD -35 deg 11760), LSII +33 deg 5 and BD +1 deg 4381 (LSIV +2 deg 13) obtained with the International Ultraviolet Explorer (IUE). Broad band photometry and a newly computed grid of line blanketed model atmospheres were used to determine accurate angular diameters and total stellar fluxes. The resultant effective temperatures are in most cases in satisfactory agreement with those based on broad band photometry and/or high resolution spectroscopy in the visible. For two objects, LSII +33 deg 5 and LSE 78, disagreement was found between the IUE observations and broadband photometry: the colors predict temperatures around 20,000 K, whereas the UV spectra indicate much lower photospheric temperatures of 14,000 to 15,000 K. The new temperature scale for extreme helium stars extends to lower effective temperatures than that of Heber and Schoenberner (1981) and covers the range from 8,500 K to 32,000 K.

Fast rotating neutron stars with realistic nuclear matter equation of state

NASA Astrophysics Data System (ADS)

Cipolletta, F.; Cherubini, C.; Filippi, S.; Rueda, J. A.; Ruffini, R.

2015-07-01

We construct equilibrium configurations of uniformly rotating neutron stars for selected relativistic mean-field nuclear matter equations of state (EOS). We compute, in particular, the gravitational mass (M ), equatorial (Req) and polar (Rpol) radii, eccentricity, angular momentum (J ), moment of inertia (I ) and quadrupole moment (M2) of neutron stars stable against mass shedding and secular axisymmetric instability. By constructing the constant frequency sequence f =716 Hz of the fastest observed pulsar, PSR J1748-2446ad, and constraining it to be within the stability region, we obtain a lower mass bound for the pulsar, Mmin=[1.2 - 1.4 ]M⊙ , for the EOS employed. Moreover, we give a fitting formula relating the baryonic mass (Mb) and gravitational mass of nonrotating neutron stars, Mb/M⊙=M /M⊙+(13 /200 )(M /M⊙)2 [or M /M⊙=Mb/M⊙-(1 /20 )(Mb/M⊙)2], which is independent of the EOS. We also obtain a fitting formula, although not EOS independent, relating the gravitational mass and the angular momentum of neutron stars along the secular axisymmetric instability line for each EOS. We compute the maximum value of the dimensionless angular momentum, a /M ≡c J /(G M2) (or "Kerr parameter"), (a /M )max≈0.7 , found to be also independent of the EOS. We then compare and contrast the quadrupole moment of rotating neutron stars with the one predicted by the Kerr exterior solution for the same values of mass and angular momentum. Finally, we show that, although the mass quadrupole moment of realistic neutron stars never reaches the Kerr value, the latter is closely approached from above at the maximum mass value, as physically expected from the no-hair theorem. In particular, the stiffer the EOS, the closer the mass quadrupole moment approaches the value of the Kerr solution.

The structure of motion in a 4-component galaxy mass model

NASA Astrophysics Data System (ADS)

Caranicolas, N. D.

1996-03-01

We use a composite galaxy model consisting of a disk-halo, bulge, nucleus and dark-halo components in order to investigate the motion of stars in ther-z plane. It is observed that high angular momentum stars move in regular orbits. The majority of orbits are box orbits. There are also banana-like orbits. For a given value of energy, only a fraction of the low angular momentum stars — those going near the nucleus — show chaotic motion while the rest move in regular orbits. Again one observes the above two kinds of orbits. In addition to the above one can also see orbits with the characteristics of the 2/3 and 3/4 resonance. It is also shown that, in the absence of the bulge component, the area of chaotic motion in the surface of section increases, significantly. This suggests that a larger number of low angular momentum stars are in chaotic orbits in galaxies with massive nuclei and no bulge components.

Observational calibration of the projection factor of Cepheids. II. Application to nine Cepheids with HST/FGS parallax measurements

NASA Astrophysics Data System (ADS)

Breitfelder, J.; Mérand, A.; Kervella, P.; Gallenne, A.; Szabados, L.; Anderson, R. I.; Le Bouquin, J.-B.

2016-03-01

Context. The distance to pulsating stars is classically estimated using the parallax-of-pulsation (PoP) method, which combines spectroscopic radial velocity (RV) measurements and angular diameter (AD) estimates to derive the distance of the star. A particularly important application of this method is the determination of Cepheid distances in view of the calibration of their distance scale. However, the conversion of radial to pulsational velocities in the PoP method relies on a poorly calibrated parameter, the projection factor (p-factor). Aims: We aim to measure empirically the value of the p-factors of a homogeneous sample of nine bright Galactic Cepheids for which trigonometric parallaxes were measured with the Hubble Space Telescope (HST) Fine Guidance Sensor. Methods: We use the SPIPS algorithm, a robust implementation of the PoP method that combines photometry, interferometry, and radial velocity measurements in a global modeling of the pulsation of the star. We obtained new interferometric angular diameter measurements using the PIONIER instrument at the Very Large Telescope Interferometer (VLTI), completed by data from the literature. Using the known distance as an input, we derive the value of the p-factor of the nine stars of our sample and study its dependence with the pulsation period. Results: We find the following p-factors: p = 1.20 ± 0.12 for RT Aur, p = 1.48 ± 0.18 for T Vul, p = 1.14 ± 0.10 for FF Aql, p = 1.31 ± 0.19 for Y Sgr, p = 1.39 ± 0.09 for X Sgr, p = 1.35 ± 0.13 for W Sgr, p = 1.36 ± 0.08 for β Dor, p = 1.41 ± 0.10 for ζ Gem, and p = 1.23 ± 0.12 for ℓ Car. Conclusions: The values of the p-factors that we obtain are consistently close to p = 1.324 ± 0.024. We observe some dispersion around this average value, but the observed distribution is statistically consistent with a constant value of the p-factor as a function of the pulsation period (χ2 = 0.669). The error budget of our determination of the p-factor values is presently dominated by the uncertainty on the parallax, a limitation that will soon be waived by Gaia. Based on observations carried out with ESO facilities at Paranal Observatory under program 093.D-0316, 094.D-0773 and 094.D-0584.

Evidence of asymmetries in the Aldebaran photosphere from multiwavelength lunar occultations

NASA Astrophysics Data System (ADS)

Richichi, A.; Dyachenko, V.; Pandey, A. K.; Sharma, S.; Tasuya, O.; Balega, Y.; Beskakotov, A.; Rastegaev, D.; Dhillon, V. S.

2017-01-01

We have recorded three lunar occultations of Aldebaran (α Tau) at different telescopes and using various band-passes, from the ultraviolet to the far red. The data have been analysed using both model-dependent and model-independent methods. The derived uniform-disc angular diameter values have been converted to limb-darkened values using model atmosphere relations and are found in broad agreement among themselves and with previous literature values. The limb-darkened diameter is about 20.3 mas on average. However, we have found indications that the photospheric brightness profile of Aldebaran may have not been symmetric, a finding already reported by other authors for this and for similar late-type stars. At the sampling scale of our brightness profile, between 1 and 2 mas, the uniform and limb-darkened disc models may not be a good description for Aldebaran. The asymmetries appear to differ with wavelength and over the 137-d time span of our measurements. Surface spots appear as a likely explanation for the differences between observations and the models.

Seismic evidence for the loss of stellar angular momentum before the white-dwarf stage.

PubMed

Charpinet, S; Fontaine, G; Brassard, P

2009-09-24

White-dwarf stars represent the final products of the evolution of some 95% of all stars. If stars were to keep their angular momentum throughout their evolution, their white-dwarf descendants, owing to their compact nature, should all rotate relatively rapidly, with typical periods of the order of a few seconds. Observations of their photospheres show, in contrast, that they rotate much more slowly, with periods ranging from hours to tens of years. It is not known, however, whether a white dwarf could 'hide' some of its original angular momentum below the superficial layers, perhaps spinning much more rapidly inside than at its surface. Here we report a determination of the internal rotation profile of a white dwarf using a method based on asteroseismology. We show that the pulsating white dwarf PG 1159-035 rotates as a solid body (encompassing more than 97.5% of its mass) with the relatively long period of 33.61 +/- 0.59 h. This implies that it has lost essentially all of its angular momentum, thus favouring theories which suggest important angular momentum transfer and loss in evolutionary phases before the white-dwarf stage.

The multiplicity of massive stars: A high angular resolution survey with the HST fine guidance sensor

DOE Office of Scientific and Technical Information (OSTI.GOV)

Aldoretta, E. J.; Gies, D. R.; Henry, T. J.

2015-01-01

We present the results of an all-sky survey made with the Fine Guidance Sensor on the Hubble Space Telescope to search for angularly resolved binary systems among massive stars. The sample of 224 stars is comprised mainly of Galactic O- and B-type stars and luminous blue variables, plus a few luminous stars in the Large Magellanic Cloud. The FGS TRANS mode observations are sensitive to the detection of companions with an angular separation between 0.″01 and 1.″0 and brighter than △m=5. The FGS observations resolved 52 binary and 6 triple star systems and detected partially resolved binaries in 7 additionalmore » targets (43 of these are new detections). These numbers yield a companion detection frequency of 29% for the FGS survey. We also gathered literature results on the numbers of close spectroscopic binaries and wider astrometric binaries among the sample, and we present estimates of the frequency of multiple systems and the companion frequency for subsets of stars residing in clusters and associations, field stars, and runaway stars. These results confirm the high multiplicity fraction, especially among massive stars in clusters and associations. We show that the period distribution is approximately flat in increments of logP. We identify a number of systems of potential interest for long-term orbital determinations, and we note the importance of some of these companions for the interpretation of the radial velocities and light curves of close binaries that have third companions.« less

Astrometric Measurements of Triple Star System 15379+3006 STF 1963AB, STF 1963AC

NASA Astrophysics Data System (ADS)

Russell, Harker; Miller, Lindsey; Beltzer-Sweeney, Alexander; Shilts, Trey; Stojimirovic, Irena

2018-04-01

Research team PRSM reports astrometric measurements of the double star system WDS 15379+3006 (STF 1963AB, STF 1963AC) obtained using the iTelescope Network. By performing CCD astrometry, the team determined a position angle of 298.4° ± 0.1° with an angular separation of 05. 28" ± 0.1" for STF 1963AB, and a position angle of 116.1° ± 0.1° with an angular separation of 32.35" ± 0.1" for STF 1963AC. The angular separation and position angle have changed from previous measurements.

Olivier Chesneau's Work on Low Mass Stars

NASA Astrophysics Data System (ADS)

Lagadec, E.

2015-12-01

During his too short career, Olivier Chesneau pioneered the study of the circumstellar environments of low mass evolved stars using very high angular resolution techniques. He applied state of the art high angular resolution techniques, such as optical interferometry and adaptive optics imaging, to the the study of a variety of objects, from AGB stars to Planetary Nebulae, via e.g. Born Again stars, RCB stars and Novae. I present here an overview of this work and most important results by focusing on the paths he followed and key encounters he made to reach these results. Olivier liked to work in teams and was very strong at linking people with complementary expertises to whom he would communicate his enthusiasm and sharp ideas. His legacy will live on through the many people he inspired.

VLTI-GRAVITY measurements of cool evolved stars

NASA Astrophysics Data System (ADS)

Wittkowski, M.; Rau, G.; Chiavassa, A.; Höfner, S.; Scholz, M.; Wood, P. R.; de Wit, W. J.; Eisenhauer, F.; Haubois, X.; Paumard, T.

2018-06-01

Context. Dynamic model atmospheres of Mira stars predict variabilities in the photospheric radius and in atmospheric molecular layers which are not yet strongly constrained by observations. Aims: Here we measure the variability of the oxygen-rich Mira star R Peg in near-continuum and molecular bands. Methods: We used near-infrared K-band spectro-interferometry with a spectral resolution of about 4000 obtained at four epochs between post-maximum and minimum visual phases employing the newly available GRAVITY beam combiner at the Very Large Telescope Interferometer (VLTI). Results: Our observations show a continuum radius that is anti-correlated with the visual lightcurve. Uniform disc (UD) angular diameters at a near-continuum wavelength of 2.25 μm are steadily increasing with values of 8.7 ± 0.1 mas, 9.4 ± 0.1 mas, 9.8 ± 0.1 mas, and 9.9 ± 0.1 mas at visual phases of 0.15, 0.36, 0,45, 0.53, respectively. UD diameters at a bandpass around 2.05 μm, dominated by water vapour, follow the near-continuum variability at larger UD diameters between 10.7 mas and 11.7 mas. UD diameters at the CO 2-0 bandhead, instead, are correlated with the visual lightcurve and anti-correlated with the near-continuum UD diameters, with values between 12.3 mas and 11.7 mas. Conclusions: The observed anti-correlation between continuum radius and visual lightcurve is consistent with an earlier study of the oxygen-rich Mira S Lac, and with recent 1D CODEX dynamic model atmosphere predictions. The amplitude of the variation is comparable to the earlier observations of S Lac, and smaller than predicted by CODEX models. The wavelength-dependent visibility variations at our epochs can be reproduced by a set of CODEX models at model phases between 0.3 and 0.6. The anti-correlation of water vapour and CO contributions at our epochs suggests that these molecules undergo different processes in the extended atmosphere along the stellar cycle. The newly available GRAVITY instrument is suited to conducting longer time series observations, which are needed to provide strong constraints on the model-predicted intra- and inter-cycle variability. Based on observations made with the VLT Interferometer at Paranal Observatory under programme IDs 60.A-9176 and 098.D-0647.

Stellar Rotation on the Main Sequence

NASA Astrophysics Data System (ADS)

Soderblom, D.; Murdin, P.

2000-11-01

The conservation of ANGULAR MOMENTUM is the one effective counterbalance to the inexorable pull of gravity in the universe, and so everything rotates. Stars acquire their angular momentum when they form, and, indeed, the manner in which nearly all this initial angular momentum is dissipated remains poorly understood, but without substantial angular momentum loss an interstellar cloud could never ...

The Relation between Cosmological Redshift and Scale Factor for Photons

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tian, Shuxun, E-mail: tshuxun@mail.bnu.edu.cn; Department of Physics, Wuhan University, Wuhan 430072

The cosmological constant problem has become one of the most important ones in modern cosmology. In this paper, we try to construct a model that can avoid the cosmological constant problem and have the potential to explain the apparent late-time accelerating expansion of the universe in both luminosity distance and angular diameter distance measurement channels. In our model, the core is to modify the relation between cosmological redshift and scale factor for photons. We point out three ways to test our hypothesis: the supernova time dilation; the gravitational waves and its electromagnetic counterparts emitted by the binary neutron star systems;more » and the Sandage–Loeb effect. All of this method is feasible now or in the near future.« less

Rotation of the asymptotic giant branch star R Doradus

NASA Astrophysics Data System (ADS)

Vlemmings, W. H. T.; Khouri, T.; Beck, E. De; Olofsson, H.; García-Segura, G.; Villaver, E.; Baudry, A.; Humphreys, E. M. L.; Maercker, M.; Ramstedt, S.

2018-05-01

High-resolution observations of the extended atmospheres of asymptotic giant branch (AGB) stars can now directly be compared to the theories that describe stellar mass loss. Using Atacama Large Millimeter/submillimeter Array (ALMA) high angular resolution (30 × 42 mas) observations, we have for the first time resolved stellar rotation of an AGB star, R Dor. We measure an angular rotation velocity of ωR sin i = (3.5 ± 0.3) × 10-9 rad s-1, which indicates a rotational velocity of |υrot sin i| = 1.0 ± 0.1 km s-1 at the stellar surface (R* = 31.2 mas at 214 GHz). The rotation axis projected on the plane of the sky has a position angle Φ = 7 ± 6°. We find that the rotation of R Dor is two orders of magnitude faster than expected for a solitary AGB star that will have lost most of its angular momentum. Its rotational velocity is consistent with angular momentum transfer from a close companion. As a companion has not been directly detected, we suggest R Dor has a low-mass, close-in companion. The rotational velocity approaches the critical velocity, set by the local sound speed in the extended envelope, and is thus expected to affect the mass-loss characteristics of R Dor.

Rotational velocities of A-type stars. IV. Evolution of rotational velocities

NASA Astrophysics Data System (ADS)

Zorec, J.; Royer, F.

2012-01-01

Context. In previous works of this series, we have shown that late B- and early A-type stars have genuine bimodal distributions of rotational velocities and that late A-type stars lack slow rotators. The distributions of the surface angular velocity ratio Ω/Ωcrit (Ωcrit is the critical angular velocity) have peculiar shapes according to spectral type groups, which can be caused by evolutionary properties. Aims: We aim to review the properties of these rotational velocity distributions in some detail as a function of stellar mass and age. Methods: We have gathered vsini for a sample of 2014 B6- to F2-type stars. We have determined the masses and ages for these objects with stellar evolution models. The (Teff,log L/L⊙)-parameters were determined from the uvby-β photometry and the HIPPARCOS parallaxes. Results: The velocity distributions show two regimes that depend on the stellar mass. Stars less massive than 2.5 M⊙ have a unimodal equatorial velocity distribution and show a monotonical acceleration with age on the main sequence (MS). Stars more massive have a bimodal equatorial velocity distribution. Contrarily to theoretical predictions, the equatorial velocities of stars from about 1.7 M⊙ to 3.2 M⊙ undergo a strong acceleration in the first third of the MS evolutionary phase, while in the last third of the MS they evolve roughly as if there were no angular momentum redistribution in the external stellar layers. The studied stars might start in the ZAMS not necessarily as rigid rotators, but with a total angular momentum lower than the critical one of rigid rotators. The stars seem to evolve as differential rotators all the way of their MS life span and the variation of the observed rotational velocities proceeds with characteristic time scales δt ≈ 0.2 tMS, where tMS is the time spent by a star in the MS. Full Table 1 is only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/537/A120Appendices are available in electronic form at http://www.aanda.org

The High Angular Resolution Multiplicity of Massive Stars

DTIC Science & Technology

2009-02-01

binaries: visual – stars: early-type – stars: individual ( iota Ori, delta Ori, delta Sco) – techniques: interferometric Online-only material...STATEMENT Approved for public release; distribution unlimited 13. SUPPLEMENTARY NOTES 14. ABSTRACT 15. SUBJECT TERMS 16. SECURITY

Angular velocity estimation based on star vector with improved current statistical model Kalman filter.

PubMed

Zhang, Hao; Niu, Yanxiong; Lu, Jiazhen; Zhang, He

2016-11-20

Angular velocity information is a requisite for a spacecraft guidance, navigation, and control system. In this paper, an approach for angular velocity estimation based merely on star vector measurement with an improved current statistical model Kalman filter is proposed. High-precision angular velocity estimation can be achieved under dynamic conditions. The amount of calculation is also reduced compared to a Kalman filter. Different trajectories are simulated to test this approach, and experiments with real starry sky observation are implemented for further confirmation. The estimation accuracy is proved to be better than 10^-4  rad/s under various conditions. Both the simulation and the experiment demonstrate that the described approach is effective and shows an excellent performance under both static and dynamic conditions.

Techniques for High-contrast Imaging in Multi-star Systems. II. Multi-star Wavefront Control

NASA Astrophysics Data System (ADS)

Sirbu, D.; Thomas, S.; Belikov, R.; Bendek, E.

2017-11-01

Direct imaging of exoplanets represents a challenge for astronomical instrumentation due to the high-contrast ratio and small angular separation between the host star and the faint planet. Multi-star systems pose additional challenges for coronagraphic instruments due to the diffraction and aberration leakage caused by companion stars. Consequently, many scientifically valuable multi-star systems are excluded from direct imaging target lists for exoplanet surveys and characterization missions. Multi-star Wavefront Control (MSWC) is a technique that uses a coronagraphic instrument’s deformable mirror (DM) to create high-contrast regions in the focal plane in the presence of multiple stars. MSWC uses “non-redundant” modes on the DM to independently control speckles from each star in the dark zone. Our previous paper also introduced the Super-Nyquist wavefront control technique, which uses a diffraction grating to generate high-contrast regions beyond the Nyquist limit (nominal region correctable by the DM). These two techniques can be combined as MSWC-s to generate high-contrast regions for multi-star systems at wide (Super-Nyquist) angular separations, while MSWC-0 refers to close (Sub-Nyquist) angular separations. As a case study, a high-contrast wavefront control simulation that applies these techniques shows that the habitable region of the Alpha Centauri system can be imaged with a small aperture at 8× {10}-9 mean raw contrast in 10% broadband light in one-sided dark holes from 1.6-5.5 λ/D. Another case study using a larger 2.4 m aperture telescope such as the Wide-Field Infrared Survey Telescope uses these techniques to image the habitable zone of Alpha Centauri at 3.2× {10}-9 mean raw contrast in monochromatic light.

Self-similar Hot Accretion Flow onto a Neutron Star

NASA Astrophysics Data System (ADS)

Medvedev, Mikhail V.; Narayan, Ramesh

2001-06-01

We consider hot, two-temperature, viscous accretion onto a rotating, unmagnetized neutron star. We assume Coulomb coupling between the protons and electrons, as well as free-free cooling from the electrons. We show that the accretion flow has an extended settling region that can be described by means of two analytical self-similar solutions: a two-temperature solution that is valid in an inner zone, r<~102.5, where r is the radius in Schwarzschild units; and a one-temperature solution that is valid in an outer zone, r>~102.5. In both zones the density varies as ρ~r-2 and the angular velocity as Ω~r-3/2. We solve the flow equations numerically and confirm that the analytical solutions are accurate. Except for the radial velocity, all gas properties in the self-similar settling zone, such as density, angular velocity, temperature, luminosity, and angular momentum flux, are independent of the mass accretion rate; these quantities do depend sensitively on the spin of the neutron star. The angular momentum flux is outward under most conditions; therefore, the central star is nearly always spun down. The luminosity of the settling zone arises from the rotational energy that is released as the star is braked by viscosity, and the contribution from gravity is small; hence, the radiative efficiency, η=Lacc/Mc2, is arbitrarily large at low M. For reasonable values of the gas adiabatic index γ, the Bernoulli parameter is negative; therefore, in the absence of dynamically important magnetic fields, a strong outflow or wind is not expected. The flow is also convectively stable but may be thermally unstable. The described solution is not advection dominated; however, when the spin of the star is small enough, the flow transforms smoothly to an advection-dominated branch of solution.

Constraints on the Efficiency of Radial Migration in Spiral Galaxies

NASA Astrophysics Data System (ADS)

Daniel, Kathryne J.; Wyse, Rosemary F. G.

2015-01-01

A transient spiral arm can permanently rearrange the orbital angular momentum of the stellar disk without inducing kinematic heating. This phenomenon is called radial migration because a star's orbital angular momentum determines its mean orbital radius. Should radial migration be an efficient process it could cause a large fraction of disk stars to experience significant changes in their individual orbital angular momenta on dynamically short timescales. Such scenarios have strong implications for the chemical, structural and kinematic evolution of disk galaxies. We have undertaken an investigation into the physical dependencies of the efficiency of radial migration on stellar kinematics and spiral structure. In order for a disk star to migrate radially, it must first be 'trapped' in a particular family of orbits, called horseshoe orbits, that occur near the radius of corotation with a spiral pattern. Thus far, the only analytic criterion for horseshoe orbits has been for stars with zero random orbital energy. We present our analytically derived 'capture criterion' for stars with some finite random orbital energy in a disk with a given rotation curve. Our capture criterion predict that trapping in a horseshoe orbit is primarily determined by whether or not the position of a star's mean orbital radius (determined by its orbital angular momentum) is within the 'capture region', the location and shape of which can be derived from the capture criterion. We visualize and confirm this prediction via numerically integrated orbits. We then apply our capture criterion to snap shot models of disk galaxies to determine (1) the radial distribution of the fraction of stars initially trapped in horseshoe orbits, and (2) the dependence of the total fraction of captured stars in the disk on the radial component of the stellar velocity dispersion (σR) and the amplitude of the spiral perturbation to the underlying potential at corotation. We here present a model of an exponential disk with a flat rotation curve where the initial fraction of stars trapped in horseshoe orbits falls with increasing velocity dispersion as exp[-σR^2].

The Green Bank Telescope: First Full Winter of Operation at 3mm

NASA Astrophysics Data System (ADS)

Lockman, Felix J.

2017-06-01

The winter of 2016-2017 marks the first season for the Green Bank Telescope (GBT) with full instrumentation in the 3mm band. ARGUS, a 16-pixel array, provides spectroscopic capabilities over 80-116 GHz. MUSTANG-2, a 223 pixel bolometer array, provides extremely sensitive continuum mapping capabilities over a 30 GHz band centered on 90 GHz at an angular resolution of 9”. In addition, there is a 2-pixel receiver that covers the lower part of the 3mm band, 67-93 GHz, for spectroscopy, continuum measurements, and VLBI.In March, under good night-time conditions, the GBT angular resolution at 109 GHz was measured to be 6.5”. This corresponds to 1.16 lambda/Diameter, exactly as expected from theoretical considerations and identical to the wavelength/Diameter ratio measured at much lower frequencies. Near sidelobe levels are below -20 dB.This poster will review some results that highlight the GBT’s new capabilities in the 3mm band, including new insights into the origin of the anomalous microwave emission, 13CO measurements of a cloud in the Milky Way halo that is in the process making the transition between atomic and molecular gas, HCO+ measurements of infall in a star-forming region, and measurements of dust emission and its spectrum in Orion.The Green Bank Observatory is a facility of the National Science Foundation, operated under a cooperative agreement by Associated Universities, Inc.

49 CFR Appendix B to Part 564 - Information To Be Submitted for Long Life Replaceable Light Sources of Limited Definition

Code of Federal Regulations, 2011 CFR

2011-10-01

.... A. Angular locations, diameters, key/keyway sizes, and any other interchangeability dimensions for... base in the bulb holder such as tabs, keys, keyways, surface, etc. III. Bulb Holder Interchangeability Dimensions and Tolerances. A. Mating angular locations, diameters, key/keyway sizes, any other...

49 CFR Appendix B to Part 564 - Information To Be Submitted for Long Life Replaceable Light Sources of Limited Definition

Code of Federal Regulations, 2012 CFR

2012-10-01

.... A. Angular locations, diameters, key/keyway sizes, and any other interchangeability dimensions for... base in the bulb holder such as tabs, keys, keyways, surface, etc. III. Bulb Holder Interchangeability Dimensions and Tolerances. A. Mating angular locations, diameters, key/keyway sizes, any other...

Rotational mixing in carbon-enhanced metal-poor stars with s-process enrichment

NASA Astrophysics Data System (ADS)

Matrozis, E.; Stancliffe, R. J.

2017-10-01

Carbon-enhanced metal-poor (CEMP) stars with s-process enrichment (CEMP-s) are believed to be the products of mass transfer from an asymptotic giant branch (AGB) companion, which has long since become a white dwarf. The surface abundances of CEMP-s stars are thus commonly assumed to reflect the nucleosynthesis output of the first AGB stars. We have previously shown that, for this to be the case, some physical mechanism must counter atomic diffusion (gravitational settling and radiative levitation) in these nearly fully radiative stars, which otherwise leads to surface abundance anomalies clearly inconsistent with observations. Here we take into account angular momentum accretion by these stars. We compute in detail the evolution of typical CEMP-s stars from the zero-age main sequence, through the mass accretion, and up the red giant branch for a wide range of specific angular momentum ja of the accreted material, corresponding to surface rotation velocities, vrot, between about 0.3 and 300 kms-1. We find that only for ja ≳ 1017 cm2s-1 (vrot > 20 kms-1, depending on mass accreted) angular momentum accretion directly causes chemical dilution of the accreted material. This could nevertheless be relevant to CEMP-s stars, which are observed to rotate more slowly, if they undergo continuous angular momentum loss akin to solar-like stars. In models with rotation velocities characteristic of CEMP-s stars, rotational mixing primarily serves to inhibit atomic diffusion, such that the maximal surface abundance variations (with respect to the composition of the accreted material) prior to first dredge-up remain within about 0.4 dex without thermohaline mixing or about 0.5-1.5 dex with thermohaline mixing. Even in models with the lowest rotation velocities (vrot ≲ 1 kms-1), rotational mixing is able to severely inhibit atomic diffusion, compared to non-rotating models. We thus conclude that it offers a natural solution to the problem posed by atomic diffusion and cannot be neglected in models of CEMP-s stars. A quantitative summary of the models presented in this paper (mainly the stellar properties and surface abundances at key points of the evolution) is only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/606/A55

Evidence for Cluster to Cluster Variations in Low-mass Stellar Rotational Evolution

NASA Astrophysics Data System (ADS)

Coker, Carl T.; Pinsonneault, Marc; Terndrup, Donald M.

2016-12-01

The concordance model for angular momentum evolution postulates that star-forming regions and clusters are an evolutionary sequence that can be modeled with assumptions about protostar-disk coupling, angular momentum loss from magnetized winds that saturates in a mass-dependent fashion at high rotation rates, and core-envelope decoupling for solar analogs. We test this approach by combining established data with the large h Per data set from the MONITOR project and new low-mass Pleiades data. We confirm prior results that young low-mass stars can be used to test star-disk coupling and angular momentum loss independent of the treatment of internal angular momentum transport. For slow rotators, we confirm the need for star-disk interactions to evolve the ONC to older systems, using h Per (age 13 Myr) as our natural post-disk case. There is no evidence for extremely long-lived disks as an alternative to core-envelope decoupling. However, our wind models cannot evolve rapid rotators from h Per to older systems consistently, and we find that this result is robust with respect to the choice of angular momentum loss prescription. We outline two possible solutions: either there is cosmic variance in the distribution of stellar rotation rates in different clusters or there are substantially enhanced torques in low-mass rapid rotators. We favor the former explanation and discuss observational tests that could be used to distinguish them. If the distribution of initial conditions depends on environment, models that test parameters by assuming a universal underlying distribution of initial conditions will need to be re-evaluated.

2-dimensional models of rapidly rotating stars I. Uniformly rotating zero age main sequence stars

NASA Astrophysics Data System (ADS)

Roxburgh, I. W.

2004-12-01

We present results for 2-dimensional models of rapidly rotating main sequence stars for the case where the angular velocity Ω is constant throughout the star. The algorithm used solves for the structure on equipotential surfaces and iteratively updates the total potential, solving Poisson's equation by Legendre polynomial decomposition; the algorithm can readily be extended to include rotation constant on cylinders. We show that this only requires a small number of Legendre polynomials to accurately represent the solution. We present results for models of homogeneous zero age main sequence stars of mass 1, 2, 5, 10 M⊙ with a range of angular velocities up to break up. The models have a composition X=0.70, Z=0.02 and were computed using the OPAL equation of state and OPAL/Alexander opacities, and a mixing length model of convection modified to include the effect of rotation. The models all show a decrease in luminosity L and polar radius Rp with increasing angular velocity, the magnitude of the decrease varying with mass but of the order of a few percent for rapid rotation, and an increase in equatorial radius Re. Due to the contribution of the gravitational multipole moments the parameter Ω2 Re3/GM can exceed unity in very rapidly rotating stars and Re/Rp can exceed 1.5.

Distribution and Kinematics of the HCN (3-2) Emission Down to the Innermost Region in the Envelope of the O-rich Star W Hydrae

NASA Astrophysics Data System (ADS)

Muller, Sébastien; Dinh-V-Trung; He, Jin-Hua; Lim, Jeremy

2008-09-01

We report high angular resolution observations of the HCN (3-2) line emission in the circumstellar envelope of the O-rich star W Hya with the Submillimeter Array. The proximity of this star allows us to image its molecular envelope with a spatial resolution of just ~40 AU, corresponding to about 10 times the stellar diameter. We resolve the HCN (3-2) emission and find that it is centrally peaked and has a roughly spherically symmetrical distribution. This shows that HCN is formed in the innermost region of the envelope (within ~10 stellar radii), which is consistent with predictions from pulsation-driven shock chemistry models, and rules out the scenario in which HCN forms through photochemical reactions in the outer envelope. Our model suggests that the envelope decreases steeply in temperature and increases smoothly in velocity with radius, inconsistent with the standard model for mass-loss driven by radiative pressure on dust grains. We detect a velocity gradient of ~5 km s-1 in the northwest-southeast direction over the central 40 AU. This velocity gradient is reminiscent of that seen in OH maser lines, and could be caused by the rotation of the envelope or by a weak bipolar outflow.

Bread and Butter Astro-kinetics with GEST

NASA Astrophysics Data System (ADS)

Rhie, S. H.

2001-12-01

The Galactic Exoplanet Survey Telescope (GEST) is a proposed MIDEX mission to survey microlensing extrasolar planets of Mars mass and upward. GEST/MIDEX telescope has a relatively small diameter (1m) and a large focal plane ( 2.2 square degrees) filled with 0.6 billion red-sensitive 10 micron pixels with large well-depths (> 100,000 electrons). The polar orbit allows continuous view of the target field near the Galactic center and stable PSFs necessary for efficient dithering and undersampling. The mission is designed to be able to monitor about 100 million stars continuously 8 months per year for three consecutive years, and this will produce 50,000 close-in transit giant planets as well as terrestrial microlensing planets and free-floating planets. An impressive database of variable stars will be one of the biproducts of such a massive survey as is well proved from ground-based microlensing surveys. Here we analyse the effects and utilities of astrometric and parallactic measurements that are more or less uniquely allowed by the fine angular resolutions and high precision photometries of the survey from space.

Stellar occultation spikes as probes of atmospheric structure and composition. [for Jupiter

NASA Technical Reports Server (NTRS)

Elliot, J. L.; Veverka, J.

1976-01-01

The characteristics of spikes observed in occultation light curves of Beta Scorpii by Jupiter are discussed in terms of the gravity-gradient model. The occultation of Beta Sco by Jupiter on May 13, 1971, is reviewed, and the gravity-gradient model is defined as an isothermal atmosphere of constant composition in which the refractivity is a function only of the radial coordinate from the center of refraction, which is assumed to lie parallel to the local gravity gradient. The derivation of the occultation light curve in terms of the atmosphere, the angular diameter of the occulted star, and the occultation geometry is outlined. It is shown that analysis of the light-curve spikes can yield the He/H2 concentration ratio in a well-mixed atmosphere, information on fine-scale atmospheric structure, high-resolution images of the occulted star, and information on ray crossing. Observational limits are placed on the magnitude of horizontal refractivity gradients, and it is concluded that the spikes are the result of local atmospheric density variations: atmospheric layers, density waves, or turbulence.

Revealing Large-Scale Asymetries in the Winds of Hot, Luminous Stars Using Spectroscopy and Polarimetry

NASA Astrophysics Data System (ADS)

St-Louis, Nicole

2015-08-01

The winds of hot, luminous stars are known to show small but also large scale density structures. Ultimately, these departures from spherical symmetry are important for the understanding of the loss of angular momentum from the star and are crucial in determining its rotation rate. There are many observational signatures of these departures from a uniform and spherically symmetric outflow. This poster will present results from spectroscopic and polarimetric observations of Wolf-Rayet stars, the descendants of massive O stars, that reveal large-scale asymmetries in their winds and discuss what can be learned about the structure of these winds and about the the physical mechanism responsible for generating them. Very little is known about the rotation rates of these small, He-burning stars which are the direct progenitors of at least some supernova explosions. If enough angular momentum is retained in the core, some may also very well be the progenitors of long gamma-ray bursts.

Evidence for Different Disk Mass Distributions between Early- and Late-type Be Stars in the BeSOS Survey

DOE Office of Scientific and Technical Information (OSTI.GOV)

Arcos, C.; Kanaan, S.; Curé, M.

The circumstellar disk density distributions for a sample of 63 Be southern stars from the BeSOS survey were found by modeling their H α emission line profiles. These disk densities were used to compute disk masses and disk angular momenta for the sample. Average values for the disk mass are 3.4 × 10{sup −9} and 9.5 × 10{sup −10} M {sub ⋆} for early (B0–B3) and late (B4–B9) spectral types, respectively. We also find that the range of disk angular momentum relative to the star is (150–200) J {sub ⋆}/ M {sub ⋆} and (100–150) J {sub ⋆}/ M {submore » ⋆}, again for early- and late-type Be stars, respectively. The distributions of the disk mass and disk angular momentum are different between early- and late-type Be stars at a 1% level of significance. Finally, we construct the disk mass distribution for the BeSOS sample as a function of spectral type and compare it to the predictions of stellar evolutionary models with rapid rotation. The observed disk masses are typically larger than the theoretical predictions, although the observed spread in disk masses is typically large.« less

Angular momentum transfer in primordial discs and the rotation of the first stars

NASA Astrophysics Data System (ADS)

Hirano, Shingo; Bromm, Volker

2018-05-01

We investigate the rotation velocity of the first stars by modelling the angular momentum transfer in the primordial accretion disc. Assessing the impact of magnetic braking, we consider the transition in angular momentum transport mode at the Alfvén radius, from the dynamically dominated free-fall accretion to the magnetically dominated solid-body one. The accreting protostar at the centre of the primordial star-forming cloud rotates with close to breakup speed in the case without magnetic fields. Considering a physically motivated model for small-scale turbulent dynamo amplification, we find that stellar rotation speed quickly declines if a large fraction of the initial turbulent energy is converted to magnetic energy (≳ 0.14). Alternatively, if the dynamo process were inefficient, for amplification due to flux freezing, stars would become slow rotators if the pre-galactic magnetic field strength is above a critical value, ≃10-8.2 G, evaluated at a scale of nH = 1 cm-3, which is significantly higher than plausible cosmological seed values (˜10-15 G). Because of the rapid decline of the stellar rotational speed over a narrow range in model parameters, the first stars encounter a bimodal fate: rapid rotation at almost the breakup level, or the near absence of any rotation.

Angular velocity estimation from measurement vectors of star tracker.

PubMed

Liu, Hai-bo; Yang, Jun-cai; Yi, Wen-jun; Wang, Jiong-qi; Yang, Jian-kun; Li, Xiu-jian; Tan, Ji-chun

2012-06-01

In most spacecraft, there is a need to know the craft's angular rate. Approaches with least squares and an adaptive Kalman filter are proposed for estimating the angular rate directly from the star tracker measurements. In these approaches, only knowledge of the vector measurements and sampling interval is required. The designed adaptive Kalman filter can filter out noise without information of the dynamic model and inertia dyadic. To verify the proposed estimation approaches, simulations based on the orbit data of the challenging minisatellite payload (CHAMP) satellite and experimental tests with night-sky observation are performed. Both the simulations and experimental testing results have demonstrated that the proposed approach performs well in terms of accuracy, robustness, and performance.

Gas kinematics, morphology and angular momentum in the FIRE simulations

NASA Astrophysics Data System (ADS)

El-Badry, Kareem; Quataert, Eliot; Wetzel, Andrew; Hopkins, Philip F.; Weisz, Daniel R.; Chan, T. K.; Fitts, Alex; Boylan-Kolchin, Michael; Kereš, Dušan; Faucher-Giguère, Claude-André; Garrison-Kimmel, Shea

2018-01-01

We study the z = 0 gas kinematics, morphology and angular momentum content of isolated galaxies in a suite of cosmological zoom-in simulations from the FIRE project spanning Mstar = 106-11 M⊙. Gas becomes increasingly rotationally supported with increasing galaxy mass. In the lowest mass galaxies (Mstar < 108 M⊙), gas fails to form a morphological disc and is primarily dispersion and pressure supported. At intermediate masses (Mstar = 108-10 M⊙), galaxies display a wide range of gas kinematics and morphologies, from thin, rotating discs to irregular spheroids with negligible net rotation. All the high-mass (Mstar = 1010-11 M⊙) galaxies form rotationally supported gas discs. Many of the haloes whose galaxies fail to form discs harbour high angular momentum gas in their circumgalactic medium. The ratio of the specific angular momentum of gas in the central galaxy to that of the dark matter halo increases significantly with galaxy mass, from 〈jgas〉/〈jDM〉 ∼ 0.1 at M_star=10^{6-7} M_{⊙} to 〈jgas〉/〈jDM〉 ∼ 2 at Mstar = 1010-11 M⊙. The reduced rotational support in the lowest mass galaxies owes to (a) stellar feedback and the UV background suppressing the accretion of high angular momentum gas at late times, and (b) stellar feedback driving large non-circular gas motions. We broadly reproduce the observed scaling relations between galaxy mass, gas rotation velocity, size and angular momentum, but may somewhat underpredict the incidence of disky, high angular momentum galaxies at the lowest observed masses (Mstar = (106-2 × 107) M⊙). Stars form preferentially from low angular momentum gas near the galactic centre and are less rotationally supported than gas. The common assumption that stars follow the same rotation curve as gas thus substantially overestimates the simulated galaxies' stellar angular momentum, particularly at low masses.

The mid-infrared diameter of W Hydrae

NASA Astrophysics Data System (ADS)

Zhao-Geisler, R.; Quirrenbach, A.; Köhler, R.; Lopez, B.; Leinert, C.

2011-06-01

Aims: Asymptotic giant branch (AGB) stars are among the largest distributors of dust into the interstellar medium, and it is therefore important to understand the dust formation process and sequence in their strongly pulsating extended atmosphere. By monitoring the AGB star W Hya interferometrically over a few pulsations cycles, the upper atmospheric layers can be studied to obtain information on their chemical gas and dust composition and their intracycle and cycle-to-cycle behavior. Methods: Mid-infrared (8-13 μm) interferometric data of W Hya were obtained with MIDI/VLTI between April 2007 and September 2009, covering nearly three pulsation cycles. The spectrally dispersed visibility data of all 75 observations were analyzed by fitting a circular fully limb-darkened disk (FDD) model to all data and individual pulsation phases. Asymmetries were studied with an elliptical FDD. Results: Modeling results in an apparent angular FDD diameter of W Hya of about (80 ± 1.2) mas (7.8 AU) between 8 and 10 μm, which corresponds to an about 1.9 times larger diameter than the photospheric one. The diameter gradually increases up to (105 ± 1.2) mas (10.3 AU) at 12 μm. In contrast, the FDD relative flux fraction decreases from (0.85 ± 0.02) to (0.77 ± 0.02), reflecting the increased flux contribution from a fully resolved surrounding silicate dust shell. The asymmetric character of the extended structure could be confirmed. An elliptical FDD yields a position angle of (11 ± 20)° and an axis ratio of (0.87 ± 0.07). A weak pulsation dependency is revealed with a diameter increase of (5.4 ± 1.8) mas between visual minimum and maximum, while detected cycle-to-cycle variations are smaller. Conclusions: W Hya's diameter shows a behavior that is very similar to the Mira stars RR Sco and S Ori and can be described by an analogous model. The constant diameter part results from a partially resolved stellar disk, including a close molecular layer of H2O, while the increase beyond 10 μm can most likely be attributed to the contribution of a spatially resolved nearby Al2O3 dust shell. Probably due to the low mass-loss rate, close Fe-free silicate dust could not be detected. The results suggest that the formation of amorphous Al2O3 occurs mainly at visual minimum. A possible close Al2O3 dust shell has now been revealed in a few objects calling for self-consistent dynamic atmospheric models including dust formation close to the star. The asymmetry might be explained by an enhanced dust concentration along an N-S axis. Based on observations made with the Very Large Telescope Interferometer (VLTI) at the Paranal Observatory under program IDs 079.D-0140, 080.D-0005, 081.D-0198, 082.D-0641 and 083.D-0294.FITS files of the calibrated visibilities are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/530/A120

Optical interferometry and Gaia parallaxes for a robust calibration of the Cepheid distance scale

NASA Astrophysics Data System (ADS)

Kervella, Pierre; Mérand, Antoine; Gallenne, Alexandre; Trahin, Boris; Borgniet, Simon; Pietrzynski, Grzegorz; Nardetto, Nicolas; Gieren, Wolfgang

2018-04-01

We present the modeling tool we developed to incorporate multi-technique observations of Cepheids in a single pulsation model: the Spectro-Photo-Interferometry of Pulsating Stars (SPIPS). The combination of angular diameters from optical interferometry, radial velocities and photometry with the coming Gaia DR2 parallaxes of nearby Galactic Cepheids will soon enable us to calibrate the projection factor of the classical Parallax-of-Pulsation method. This will extend its applicability to Cepheids too distant for accurate Gaia parallax measurements, and allow us to precisely calibrate the Leavitt law's zero point. As an example application, we present the SPIPS model of the long-period Cepheid RS Pup that provides a measurement of its projection factor, using the independent distance estimated from its light echoes.

MESA models for the evolutionary status of the epsilon Aurigae disk-eclipsed binary system

NASA Astrophysics Data System (ADS)

Stencel, Robert E.; Gibson, Justus

2018-06-01

The brightest member of the class of disk-eclipsed binary stars is the Algol-like long-period binary, epsilon Aurigae (HD 31964, F0Iap + disk, http://adsabs.harvard.edu/abs/2016SPIE.9907E..17S ). Using MESA (Modules for Experiments in Stellar Astrophysics, version 9575), we have made an evaluation of its evolutionary state. We sought to satisfy several observational constraints, including: (1) requiring evolutionary tracks to pass close to the current temperature and luminosity of the primary star; (2) obtaining a period near the observed value of 27.1 years; (3) matching a mass function of 3.0; (4) concurrent Roche lobe overflow and mass transfer; (5) an isotopic ratio 12C / 13C = 5 and, (6) matching the interferometrically determined angular diameter. A MESA model starting with binary masses of 9.85 + 4.5 solar masses, with a 100 day initial period, produces a 1.2 + 10.6 solar masses result having a 547 day period, plus a single digit 12C / 13C ratio. These values were reached near an age of 20 Myr, when the donor star comes close to the observed luminosity and temperature for epsilon Aurigae A, as a post-RGB/pre-AGB star. Contemporaneously, the accretor then appears as an upper main sequence, early B-type star. This benchmark model can provide a basis for further exploration of this interacting binary, and other long period binary stars. This report has been submitted to MNRAS, along with a parallel investigation of mass transfer stream and disk sub-structure. The authors are grateful to the estate of William Herschel Womble for the support of astronomy at the University of Denver.

A High Angular Resolution Survey of Massive Stars in Cygnus OB2: Results from the Hubble Space Telescope Fine Guidance Sensors

DTIC Science & Technology

2014-02-01

F. J. Moffat9, and N. R. Walborn2 1 Center for High Angular Resolution Astronomy, Department of Physics and Astronomy, Georgia State University, P.O...Martin Drive, Baltimore, MD 21218, USA; nelan@stsci.edu, walborn@stsci.edu 3 Department of Natural Sciences, University of South Carolina Beaufort, 1 ...Online-only material: figure set 1 . INTRODUCTION Massive stars (10M) play a fundamental role in the evo- lution of the universe, from influencing

Enabling Super-Nyquist Wavefront Control on WFIRST

NASA Astrophysics Data System (ADS)

Bendek, Eduardo; Belikov, Ruslan; Sirbu, Dan; Shaklan, Stuart B.; Eldorado Riggs, A. J.

2018-01-01

A large fraction of sun-like stars is contained in Binary systems. Within 10pc there are 70 FGK stars from which, 43 belong to a multi-star system, and 28 of them have companion leak that is greater than 1e-9 contrast assuming typical Hubble-quality space optics. Currently, those binary stars are not included in the WFIRST-CGI target list, but they could be observed if high-contrast imaging around binary star systems using WFIRST is possible, increasing by 70% the number of possible FGK targets for the mission. The Multi-Star Wavefront Control (MSWC) algorithm can be used to suppress the companion star leakage. If the targets have angular separations larger than the Nyquist controllable region of the Deformable Mirror the MSWC must operate in its Super-Nyquist (SN) mode. This mode requires a target star replica within the SN region in order to provide the energy, and coherent light necessary to null speckles at SN angular separations. For the case of WFIRST, about half of the targets that can be observed using MSWC have angular separations larger than the Nyquist controllable region of the 48x48 actuator Deformable Mirror (DM) to be used. Here, we discuss multiple alternatives to generate those PSF replicas with minimal or no impact to the WFIRST Coronagraph instrument such as 1) the addition of a movable diffractive pupil mounted of the Shape Pupil wheel. 2) Design of a modified Shape Pupil design able to create a dark zone and at the same time diffract a small fraction of the starlight on the SN region. 3) Predict the minimum residual quilting on Xinetics DM that would allow observing a given target.

High resolution of fast-rotating stars across the H-R diagram: photosphere and circumstellar environment

NASA Astrophysics Data System (ADS)

Domiciano de Souza, Armando

2014-12-01

Rotation is a fundamental parameter that governs the physical structure and evolution of stars, for example by generating internal circulations of matter and angular momentum, which in turn change the stellar lifetime. Massive stars (spectral types OBA) are those presenting the highest rotation velocities and thus those for which the consequences of rotation are the strongest. On the external layers of the star, fast-rotation induces in particular (1) a flattening (equatorial radius higher than the polar radius) and (2) a gravity darkening (non-uniform distribution of flux, and thus effective temperature, between the poles and the equator). This important modification in the photospheric physical structure can also drive an anisotropic (axisymmetric) mass and angular momentum loss, originating for example the complex circumstellar environments around Be and supergiant B[e] stars. The techniques of high angular and high spectral resolution allow a detailed study of the effects of rotation on the stellar photosphere and circumstellar environment across the H-R diagram. Thanks to these techniques, and in particular to the optical/infrared long-baseline interferometry, our knowledge on the impact of rotation in stellar physics was highly deepened since the beginning of the XXI century. The results described in this Habilitation Thesis are placed in this context and are the fruit a double approach combining both (1) observation, mainly with the ESO-VLT(I) instruments (e.g. NACO, VISIR, MIDI, AMBER, PIONIER) and (2) astrophysical modeling with different codes, including also radiation transfer (CHARRON, HDUST, FRACS). I present, in particular, the results obtained on three fast-rotating stars: Altair (A7V; delta Scuti), Achernar (B6Ve; Be star), and CPD-57° 2874 (supergiant B[e] star).

DEATH-STAR: Silicon and photovoltaic fission fragment detector arrays for light-ion induced fission correlation studies

DOE Office of Scientific and Technical Information (OSTI.GOV)

Koglin, J. D.; Burke, J. T.; Fisher, S. E.

Here, the Direct Excitation Angular Tracking pHotovoltaic-Silicon Telescope ARray (DEATH-STAR) combines a series of 12 silicon detectors in a ΔE–E configuration for charged particle identification with a large-area array of 56 photovoltaic (solar) cells for detection of fission fragments. The combination of many scattering angles and fission fragment detectors allows for an angular-resolved tool to study reaction cross sections using the surrogate method, anisotropic fission distributions, and angular momentum transfers through stripping, transfer, inelastic scattering, and other direct nuclear reactions. The unique photovoltaic detectors efficiently detect fission fragments while being insensitive to light ions and have a timing resolution ofmore » 15.63±0.37 ns. Alpha particles are detected with a resolution of 35.5 keV 1σ at 7.9 MeV. Measured fission fragment angular distributions are also presented.« less

DEATH-STAR: Silicon and Photovoltaic Fission Fragment Detector Arrays for Light-Ion Induced Fission Correlation Studies

NASA Astrophysics Data System (ADS)

Koglin, J. D.; Burke, J. T.; Fisher, S. E.; Jovanovic, I.

2017-05-01

The Direct Excitation Angular Tracking pHotovoltaic-Silicon Telescope ARray (DEATH-STAR) combines a series of 12 silicon detectors in a ΔE - E configuration for charged particle identification with a large-area array of 56 photovoltaic (solar) cells for detection of fission fragments. The combination of many scattering angles and fission fragment detectors allows for an angular-resolved tool to study reaction cross sections using the surrogate method, anisotropic fission distributions, and angular momentum transfers through stripping, transfer, inelastic scattering, and other direct nuclear reactions. The unique photovoltaic detectors efficiently detect fission fragments while being insensitive to light ions and have a timing resolution of 15.63±0.37 ns. Alpha particles are detected with a resolution of 35.5 keV 1σ at 7.9 MeV. Measured fission fragment angular distributions are also presented.

DEATH-STAR: Silicon and photovoltaic fission fragment detector arrays for light-ion induced fission correlation studies

DOE PAGES

Koglin, J. D.; Burke, J. T.; Fisher, S. E.; ...

2017-02-20

Here, the Direct Excitation Angular Tracking pHotovoltaic-Silicon Telescope ARray (DEATH-STAR) combines a series of 12 silicon detectors in a ΔE–E configuration for charged particle identification with a large-area array of 56 photovoltaic (solar) cells for detection of fission fragments. The combination of many scattering angles and fission fragment detectors allows for an angular-resolved tool to study reaction cross sections using the surrogate method, anisotropic fission distributions, and angular momentum transfers through stripping, transfer, inelastic scattering, and other direct nuclear reactions. The unique photovoltaic detectors efficiently detect fission fragments while being insensitive to light ions and have a timing resolution ofmore » 15.63±0.37 ns. Alpha particles are detected with a resolution of 35.5 keV 1σ at 7.9 MeV. Measured fission fragment angular distributions are also presented.« less

Flattening and surface-brightness of the fast-rotating star δ Persei with the visible VEGA/CHARA interferometer

NASA Astrophysics Data System (ADS)

Challouf, M.; Nardetto, N.; Domiciano de Souza, A.; Mourard, D.; Tallon-Bosc, I.; Aroui, H.; Farrington, C.; Ligi, R.; Meilland, A.; Mouelhi, M.

2017-08-01

Context. Rapid rotation is a common feature for massive stars, with important consequences on their physical structure, flux distribution and evolution. Fast-rotating stars are flattened and show gravity darkening (non-uniform surface intensity distribution). Another important and less studied impact of fast-rotation in early-type stars is its influence on the surface brightness colour relation (hereafter SBCR), which could be used to derive the distance of eclipsing binaries. Aims: The purpose of this paper is to determine the flattening of the fast-rotating B-type star δ Per using visible long-baseline interferometry. A second goal is to evaluate the impact of rotation and gravity darkening on the V - K colour and surface brightness of the star. Methods: The B-type star δ Per was observed with the VEGA/CHARA interferometer, which can measure spatial resolutions down to 0.3 mas and spectral resolving power of 5000 in the visible. We first used a toy model to derive the position angle of the rotation axis of the star in the plane of the sky. Then we used a code of stellar rotation, CHARRON, in order to derive the physical parameters of the star. Finally, by considering two cases, a static reference star and our best model of δ Per, we can quantify the impact of fast rotation on the surface brightness colour relation (SBCR). Results: We find a position angle of 23 ± 6 degrees. The polar axis angular diameter of δ Per is θp = 0.544 ± 0.007 mas, and the derived flatness is r = 1.121 ± 0.013. We derive an inclination angle for the star of I = 85+ 5-20 degrees and a projected rotation velocity Vsini = 175+ 8-11 km s-1 (or 57% of the critical velocity). We find also that the rotation and inclination angle of δ Per keeps the V - K colour unchanged while it decreasing its surface-brightness by about 0.05 mag. Conclusions: Correcting the impact of rotation on the SBCR of early-type stars appears feasible using visible interferometry and dedicated models.

Techniques for High Contrast Imaging in Multi-Star Systems II: Multi-Star Wavefront Control

NASA Technical Reports Server (NTRS)

Sirbu, D.; Thomas, S.; Belikov, R.

2017-01-01

Direct imaging of exoplanets represents a challenge for astronomical instrumentation due to the high-contrast ratio and small angular separation between the host star and the faint planet. Multi-star systems pose additional challenges for coronagraphic instruments because of the diffraction and aberration leakage introduced by the additional stars, and as a result are not planned to be on direct imaging target lists. Multi-star wavefront control (MSWC) is a technique that uses a coronagraphic instrument's deformable mirror (DM) to create high-contrast regions in the focal plane in the presence of multiple stars. Our previous paper introduced the Super-Nyquist Wavefront Control (SNWC) technique that uses a diffraction grating to enable the DM to generate high-contrast regions beyond the nominal controllable region. These two techniques can be combined to generate high-contrast regions for multi-star systems at any angular separations. As a case study, a high-contrast wavefront control (WC) simulation that applies these techniques shows that the habitable region of the Alpha Centauri system can be imaged reaching 8 times 10(exp -9) mean contrast in 10 percent broadband light in one-sided dark holes from 1.6-5.5 lambda (wavelength) divided by D (distance).

Rapidly rotating neutron stars in general relativity: Realistic equations of state

NASA Technical Reports Server (NTRS)

Cook, Gregory B.; Shapiro, Stuart L.; Teukolsky, Saul A.

1994-01-01

We construct equilibrium sequences of rotating neutron stars in general relativity. We compare results for 14 nuclear matter equations of state. We determine a number of important physical parameters for such stars, including the maximum mass and maximum spin rate. The stability of the configurations to quasi-radial perturbations is assessed. We employ a numerical scheme particularly well suited to handle rapid rotation and large departures from spherical symmetry. We provide an extensive tabulation of models for future reference. Two classes of evolutionary sequences of fixed baryon rest mass and entropy are explored: normal sequences, which behave very much like Newtonian sequences, and supramassive sequences, which exist for neutron stars solely because of general relativistic effects. Adiabatic dissipation of energy and angular momentum causes a star to evolve in quasi-stationary fashion along an evolutionary sequence. Supramassive sequences have masses exceeding the maximum mass of a nonrotating neutron star. A supramassive star evolves toward eventual catastrophic collapse to a black hole. Prior to collapse, the star actually spins up as it loses angular momentum, an effect that may provide an observable precursor to gravitational collapse to a black hole.

CCD Measurements of Double and Multiple Stars at NAO Rozhen and ASV in 2015

NASA Astrophysics Data System (ADS)

Cvetković, Z.; Pavlović, R.; Boeva, S.

2017-04-01

Results of CCD observations of 154 double or multiple stars, made with the 2 m telescope of the Bulgarian National Astronomical Observatory at Rozhen over five nights in 2015, are presented. This is the ninth series of measurements of CCD frames obtained at Rozhen. We also present results of CCD observations of 323 double or multiple stars made with the 0.6 m telescope of the Serbian Astronomical Station on the mountain of Vidojevica over 23 nights in 2015. This is the fourth series of measurements of CCD frames obtained at this station. This paper contains the results for the position angle and angular separation for 801 pairs and residuals for 127 pairs with published orbital elements or linear solutions. The angular separations are in the range from 1.″52 to 201.″56, with a median angular separation of 8.″26. We also present eight pairs that are measured for the first time and linear elements for five pairs.

Detection of Solar-Like Oscillations, Observational Constraints, and Stellar Models for θ Cyg, the Brightest Star Observed by the Kepler Mission

DOE PAGES

Guzik, Joyce Ann; Houdek, G.; Chaplin, W. J.; ...

2016-10-21

θ Cygni is an F3 spectral type magnitude V = 4.48 main-sequence star that was the brightest star observed by the original Kepler spacecraft mission. Short-cadence (58.8 s) photometric data using a custom aperture were first obtained during Quarter 6 (2010 June–September) and subsequently in Quarters 8 and 12–17. We present analyses of solar-like oscillations based on Q6 and Q8 data, identifying angular degree l = 0, 1, and 2 modes with frequencies of 1000–2700 μHz, a large frequency separation of 83.9 ± 0.4 μHz, and maximum oscillation amplitude at frequency ν max = 1829 ± 54 μHz. We alsomore » present analyses of new ground-based spectroscopic observations, which, combined with interferometric angular diameter measurements, give T eff = 6697 ± 78 K, radius 1.49 ± 0.03 R ⊙, [Fe/H] = $-$0.02 ± 0.06 dex, and log g = 4.23 ± 0.03. We calculate stellar models matching these constraints using the Yale Rotating Evolution Code and the Asteroseismic Modeling Portal. The best-fit models have masses of 1.35–1.39 M ⊙ and ages of 1.0–1.6 Gyr. θ Cyg's T eff and log g place it cooler than the red edge of the γ Doradus instability region established from pre-Kepler ground-based observations, but just at the red edge derived from pulsation modeling. Lastly, the pulsation models show γ Dor gravity modes driven by the convective blocking mechanism, with frequencies of 1–3 cycles per day (11 to 33 μHz). However, gravity modes were not seen in Kepler data; one signal at 1.776 cycles per day (20.56 μHz) may be attributable to a faint, possibly background, binary.« less

Angular Momentum in Disk Wind Revealed in the Young Star MWC 349A

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhang, Qizhou; Claus, Brian; Watson, Linda

Disk winds are thought to play a critical role in star birth. As winds extract excess angular momentum from accretion disks, matter in the disk can be transported inward to the star to fuel mass growth. However, observational evidence of wind carrying angular momentum has been very limited. We present Submillimeter Array (SMA) observations of the young star MWC 349A in the H26 α and H30 α recombination lines. The high signal-to-noise ratios made possible by the maser emission process allow us to constrain the relative astrometry of the maser spots to milli-arcsecond precision. Previous observations of the H30 αmore » line with the SMA and the Plateau de Bure interferometer (PdBI) showed that masers are distributed in the disk and wind. Our new high-resolution observations of the H26 α line reveal differences in spatial distribution from that of the H30 α line. H26 α line masers in the disk are excited in a thin annulus with a radius of about 25 au, while the H30 α line masers are formed in a slightly larger annulus with a radius of 30 au. This is consistent with expectations for maser excitation in the presence of an electron density variation of approximately R {sup −4}. In addition, the H30 α and H26 α line masers arise from different parts in the wind. This difference is also expected from maser theory. The wind component of both masers exhibits line-of-sight velocities that closely follow a Keplerian law. This result provides strong evidence that the disk wind extracts significant angular momentum, thereby facilitating mass accretion in the young star.« less

Can Coulomb repulsion for charged particle beams be overcome?

NASA Astrophysics Data System (ADS)

Retsky, Michael W.

2004-01-01

Mutual repulsion of discrete charged particles or Coulomb repulsion is widely considered to be an ultimate hard limit in charged particle optics. It prevents the ability to finely focus high current beams into a small spots at large distances from the defining apertures. A classic example is the 1970s era "Star Wars" study of an electron beam directed energy weapon as an orbiting antiballistic missile device. After much analysis, it was considered physically impossible to focus a 1000-amp 1-GeV beam into a 1-cm diameter spot 1000-km from the beam generator. The main reason was that a 1-cm diameter beam would spread to 5-m diameter at 1000-km due to Coulomb repulsion. Since this could not be overcome, the idea was abandoned. But is this true? What if the rays were reversed? That is, start with a 5-m beam converging slightly with the same nonuniform angular and energy distribution as the electrons from the original problem were spreading at 1000-km distance. Could Coulomb repulsion be overcome? Looking at the terms in computational studies, some are reversible while others are not. Since the nonreversible terms should be small, it might be possible to construct an electron beam directed energy weapon.

A tidal theory for the origin of the solar nebula

NASA Technical Reports Server (NTRS)

Kobrick, M.; Kaula, W. M.

1979-01-01

A model for the origin of the solar nebula is developed with attention to the significance of angular momentum considerations. Evidence that stars are born in groups rather than singly is examined. It is shown that protostars which are members of typical galactic clusters have some probability of undergoing a gravitational encounter with another star while they are collapsing. According to the model, these encounters impart disproportionate amounts of angular momentum to the later material to fall in toward already centrally condensed fragments. The amount of central condensation of a fragment is the overriding factor in determining its stability against destruction by tidal forces. The encounter also imparts angular momentum to matter that is still accreting onto the protosun.

Temperature profiles of accretion discs around rapidly rotating strange stars in general relativity: A comparison with neutron stars

NASA Astrophysics Data System (ADS)

Bhattacharyya, S.; Thampan, A. V.; Bombaci, I.

2001-06-01

We compute the temperature profiles of accretion discs around rapidly rotating strange stars, using constant gravitational mass equilibrium sequences of these objects, considering the full effect of general relativity. Beyond a certain critical value of stellar angular momentum (J), we observe the radius (r_orb) of the innermost stable circular orbit (ISCO) to increase with J (a property seen neither in rotating black holes nor in rotating neutron stars). The reason for this is traced to the crucial dependence of dr_orb/dJ on the rate of change of the radial gradient of the Keplerian angular velocity at r_orb with respect to J. The structure parameters and temperature profiles obtained are compared with those of neutron stars, as an attempt to provide signatures for distinguishing between the two. We show that when the full gamut of strange star equation of state models, with varying degrees of stiffness are considered, there exists a substantial overlap in properties of both neutron stars and strange stars. However, applying accretion disc model constraints to rule out stiff strange star equation of state models, we notice that neutron stars and strange stars exclusively occupy certain parameter spaces. This result implies the possibility of distinguishing these objects from each other by sensitive observations through future X-ray detectors.

Fast core rotation in red-giant stars as revealed by gravity-dominated mixed modes.

PubMed

Beck, Paul G; Montalban, Josefina; Kallinger, Thomas; De Ridder, Joris; Aerts, Conny; García, Rafael A; Hekker, Saskia; Dupret, Marc-Antoine; Mosser, Benoit; Eggenberger, Patrick; Stello, Dennis; Elsworth, Yvonne; Frandsen, Søren; Carrier, Fabien; Hillen, Michel; Gruberbauer, Michael; Christensen-Dalsgaard, Jørgen; Miglio, Andrea; Valentini, Marica; Bedding, Timothy R; Kjeldsen, Hans; Girouard, Forrest R; Hall, Jennifer R; Ibrahim, Khadeejah A

2011-12-07

When the core hydrogen is exhausted during stellar evolution, the central region of a star contracts and the outer envelope expands and cools, giving rise to a red giant. Convection takes place over much of the star's radius. Conservation of angular momentum requires that the cores of these stars rotate faster than their envelopes; indirect evidence supports this. Information about the angular-momentum distribution is inaccessible to direct observations, but it can be extracted from the effect of rotation on oscillation modes that probe the stellar interior. Here we report an increasing rotation rate from the surface of the star to the stellar core in the interiors of red giants, obtained using the rotational frequency splitting of recently detected 'mixed modes'. By comparison with theoretical stellar models, we conclude that the core must rotate at least ten times faster than the surface. This observational result confirms the theoretical prediction of a steep gradient in the rotation profile towards the deep stellar interior.

VizieR Online Data Catalog: Main-sequence A, F, G, and K stars photometry (Boyajian+, 2013)

NASA Astrophysics Data System (ADS)

Boyajian, T. S.; von Braun, K.; van Belle, G.; Farrington, C.; Schaefer, G.; Jones, J.; White, R.; McAlister, H. A.; Ten Brummelaar, T. A.; Ridgway, S.; Gies, D.; Sturmann, L.; Sturmann, J.; Turner, N. H.; Goldfinger, P. J.; Vargas, N.

2016-07-01

Akin to the observing outlined in DT1 and DT2, observations for this project were made with the CHARA Array, a long-baseline optical/infrared interferometer located on Mount Wilson Observatory in southern California. The target stars were selected based on their approximate angular size (a function of their intrinsic linear size and distance to the observer). We limit the selection to stars with angular sizes >0.45mas, in order to adequately resolve their sizes to a few percent precision with the selected instrument setup. Note that all stars that meet this requirement are brighter than the instrumental limits of our detector by several magnitudes. The stars also have no known stellar companion within 3-arcsec to avoid contamination of incoherent light in the interferometers' field of view. From 2008 to 2012, we used the CHARA Classic beam combiner operating in the H band (λH=1.67um) and the K' band (λK'=2.14um) to collect observations of 23 stars using CHARA's longest baseline combinations. (5 data files).

Constraints on the Energy Density Content of the Universe Using Only Clusters of Galaxies

NASA Technical Reports Server (NTRS)

Molnar, Sandor M.; Haiman, Zoltan; Birkinshaw, Mark

2003-01-01

We demonstrate that it is possible to constrain the energy content of the Universe with high accuracy using observations of clusters of galaxies only. The degeneracies in the cosmological parameters are lifted by combining constraints from different observables of galaxy clusters. We show that constraints on cosmological parameters from galaxy cluster number counts as a function of redshift and accurate angular diameter distance measurements to clusters are complementary to each other and their combination can constrain the energy density content of the Universe well. The number counts can be obtained from X-ray and/or SZ (Sunyaev-Zeldovich effect) surveys, the angular diameter distances can be determined from deep observations of the intra-cluster gas using their thermal bremsstrahlung X-ray emission and the SZ effect (X-SZ method). In this letter we combine constraints from simulated cluster number counts expected from a 12 deg2 SZ cluster survey and constraints from simulated angular diameter distance measurements based on using the X-SZ method assuming an expected accuracy of 7% in the angular diameter distance determination of 70 clusters with redshifts less than 1.5. We find that R, can be determined within about 25%, A within 20%, and w within 16%. Any cluster survey can be used to select clusters for high accuracy distance measurements, but we assumed accurate angular diameter distance measurements for only 70 clusters since long observations are necessary to achieve high accuracy in distance measurements. Thus the question naturally arises: How to select clusters of galaxies for accurate diameter distance determinations? In this letter, as an example, we demonstrate that it is possible to optimize this selection changing the number of clusters observed, and the upper cut off of their redshift range. We show that constraints on cosmological parameters from combining cluster number counts and angular diameter distance measurements, as opposed to general expectations, will not improve substantially selecting clusters with redshifts higher than one. This important conclusion allow us to restrict our cluster sample to clusters closer than one, in a range where the observational time for accurate distance measurements are more manageable. Subject headings: cosmological parameters - cosmology: theory - galaxies: clusters: general - X-rays: galaxies: clusters

Combinations of 148 navigation stars and the star tracker

NASA Technical Reports Server (NTRS)

Duncan, R.

1980-01-01

The angular separation of all star combinations for 148 nav star on the onboard software for space transportation system-3 flight and following missions is presented as well as the separation of each pair that satisfies the viewing constraints of using both star trackers simultaneously. Tables show (1) shuttle star catalog 1980 star position in M 1950 coordinates; (2) two star combination of 148 nav stars; and (3) summary of two star-combinations of the star tracker 5 deg filter. These 148 stars present 10,875 combinations. For the star tracker filters of plus or minus 5 deg, there are 875 combinations. Formalhaut (nav star 26) has the best number of combinations, which is 33.

Boundary between stable and unstable regimes of accretion. Ordered and chaotic unstable regimes

NASA Astrophysics Data System (ADS)

Blinova, A. A.; Romanova, M. M.; Lovelace, R. V. E.

2016-07-01

We present a new study of the Rayleigh-Taylor unstable regime of accretion on to rotating magnetized stars in a set of high grid resolution three-dimensional magnetohydrodynamic simulations performed in low-viscosity discs. We find that the boundary between the stable and unstable regimes is determined almost entirely by the fastness parameter ωs = Ω⋆/ΩK(rm), where Ω⋆ is the angular velocity of the star and ΩK(rm) is the angular velocity of the Keplerian disc at the disc-magnetosphere boundary r = rm. We found that accretion is unstable if ωs ≲ 0.6. Accretion through instabilities is present in stars with different magnetospheric sizes. However, only in stars with relatively small magnetospheres, rm/R⋆ ≲ 7, do the unstable tongues produce chaotic hotspots on the stellar surface and irregular light curves. At even smaller values of the fastness parameter, ωs ≲ 0.45, multiple irregular tongues merge, forming one or two ordered unstable tongues that rotate with the angular frequency of the inner disc. This transition occurs in stars with even smaller magnetospheres, rm/R⋆ ≲ 4.2. Most of our simulations were performed at a small tilt of the dipole magnetosphere, Θ = 5°, and a small viscosity parameter α = 0.02. Test simulations at higher α values show that many more cases become unstable, and the light curves become even more irregular. Test simulations at larger tilts of the dipole Θ show that instability is present, however, accretion in two funnel streams dominates if Θ ≳ 15°. The results of these simulations can be applied to accreting magnetized stars with relatively small magnetospheres: Classical T Tauri stars, accreting millisecond X-ray pulsars, and cataclysmic variables.

Abundances of disk and bulge giants from high-resolution optical spectra. I. O, Mg, Ca, and Ti in the solar neighborhood and Kepler field samples

NASA Astrophysics Data System (ADS)

Jönsson, H.; Ryde, N.; Nordlander, T.; Pehlivan Rhodin, A.; Hartman, H.; Jönsson, P.; Eriksson, K.

2017-02-01

Context. The Galactic bulge is an intriguing and significant part of our Galaxy, but it is hard to observe because it is both distant and covered by dust in the disk. Therefore, there are not many high-resolution optical spectra of bulge stars with large wavelength coverage, whose determined abundances can be compared with nearby, similarly analyzed stellar samples. Aims: We aim to determine the diagnostically important alpha elements of a sample of bulge giants using high-resolution optical spectra with large wavelength coverage. The abundances found are compared to similarly derived abundances from similar spectra of similar stars in the local thin and thick disks. In this first paper we focus on the solar neighborhood reference sample. Methods: We used spectral synthesis to derive the stellar parameters as well as the elemental abundances of both the local and bulge samples of giants. We took special care to benchmark our method of determining stellar parameters against independent measurements of effective temperatures from angular diameter measurements and surface gravities from asteroseismology. Results: In this first paper we present the method used to determine the stellar parameters and elemental abundances, evaluate them, and present the results for our local disk sample of 291 giants. Conclusions: When comparing our determined spectroscopic temperatures to those derived from angular diameter measurements, we reproduce these with a systematic difference of +10 K and a standard deviation of 53 K. The spectroscopic gravities reproduce those determined from asteroseismology with a systematic offset of +0.10 dex and a standard deviation of 0.12 dex. When it comes to the abundance trends, our sample of local disk giants closely follows trends found in other works analyzing solar neighborhood dwarfs, showing that the much brighter giant stars are as good abundance probes as the often used dwarfs. Based on observations made with the Nordic Optical Telescope (programs 51-018 and 53-002), operated by the Nordic Optical Telescope Scientific Association at the Observatorio del Roque de los Muchachos, La Palma, Spain, of the Instituto de Astrofisica de Canarias, and on spectral data retrieved from PolarBase at Observatoire Midi Pyrénées.Full Tables A.1 and A.3 are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/598/A100

Constraints on radial migration in spiral galaxies - II. Angular momentum distribution and preferential migration

NASA Astrophysics Data System (ADS)

Daniel, Kathryne J.; Wyse, Rosemary F. G.

2018-05-01

The orbital angular momentum of individual stars in galactic discs can be permanently changed through torques from transient spiral patterns. Interactions at the corotation resonance dominate these changes and have the further property of conserving orbital circularity. We derived in an earlier paper an analytic criterion that an unperturbed stellar orbit must satisfy in order for such an interaction to occur, i.e. for it to be in a trapped orbit around corotation. We here use this criterion in an investigation of how the efficiency of induced radial migration for a population of disc stars varies with the angular momentum distribution of that population. We frame our results in terms of the velocity dispersion of the population, this being an easier observable than is the angular momentum distribution. Specifically, we investigate how the fraction of stars in trapped orbits at corotation varies with the velocity dispersion of the population, for a system with an assumed flat rotation curve. Our analytic results agree with the finding from simulations that radial migration is less effective in populations with `hotter' kinematics. We further quantify the dependence of this trapped fraction on the strength of the spiral pattern, finding a higher trapped fraction for higher amplitude perturbations.

The formation of high-mass binary star systems

NASA Astrophysics Data System (ADS)

Lund, Kristin; Bonnell, Ian A.

2018-06-01

We develop a semi-analytic model to investigate how accretion onto wide low-mass binary stars can result in a close high-mass binary system. The key ingredient is to allow mass accretion while limiting the gain in angular momentum. We envision this process as being regulated by an external magnetic field during infall. Molecular clouds are made to collapse spherically with material either accreting onto the stars or settling in a disk. Our aim is to determine what initial conditions are needed for the resulting binary to be both massive and close. Whether material accretes, and what happens to the binary separation as a result, depends on the relative size of its specific angular momentum, compared to the specific angular momentum of the binary. When we add a magnetic field we are introducing a torque to the system which is capable of stripping the molecular cloud of some of its angular momentum, and consequently easing the formation of high-mass binaries. Our results suggest that clouds in excess of 1000 M⊙ and radii of 0.5 pc or larger, can easily form binary systems with masses in excess of 25 M⊙ and separations of order 10 R⊙ with magnetic fields of order 100 μG (mass-to-flux ratios of order 5).

Angular and linear fields of view of Galilean telescopes and telemicroscopes.

PubMed

Katz, Milton

2007-06-01

The calculation of the angular fields of view (FOVs) of Galilean telescopes generally necessitates the calculation of the pupils and ports. This, in turn, requires knowledge of the optical design of the telescope, in particular, the focal lengths or powers of the objective and ocular lenses. Equations for finding the FOV that obviate the need to calculate pupils and ports, or even to know the lens powers of the telescope, are presented in this article. The equations can be used to find the FOVs in image space of real Galilean telescopes of known magnification, merely by measuring the distance between the objective and ocular lenses and the diameter of the objective lens. The equations include the effects of eye pupil diameter and eye relief. Linear FOVs (LFOVs) of Galilean telemicroscopes are similarly determined. Two image space angular FOV equations were derived: (1) an equation to determine the angular FOVs of a telescope with various amounts of vignetting and eye relief; and (2) an equivalent equation for the LFOVs of telescopes fitted with lens caps for near vision. The FOV increases linearly with increasing vignetting. Increasing the eye relief results in a nonlinear decrease in the FOV, shown as a fraction of the normalized value for zero eye relief. Decrements in the FOVs with increasing eye relief as a fraction of the normalized field angle when the eye relief = 0 are shown to be constant regardless of the vignetting level. A transition of the objective lens from field stop to aperture stop occurs when the eye pupil diameter exceeds the diameter of the objective lens divided by the magnification. Equations have been derived for Galilean telescopes and telemicroscopes that make it unnecessary to find pupils and ports, or to know the powers of the lenses. They provide a direct and simple evaluation of angular and LFOVs as functions of magnification, objective lens diameter, eye pupil diameter, eye relief, and vignetting, and enable comparisons of actual telescopes.

Angular momentum of dwarf galaxies

NASA Astrophysics Data System (ADS)

Kurapati, Sushma; Chengalur, Jayaram N.; Pustilnik, Simon; Kamphuis, Peter

2018-05-01

Mass and specific angular momentum are two fundamental physical parameters of galaxies. We present measurements of the baryonic mass and specific angular momentum of 11 void dwarf galaxies derived from neutral hydrogen (HI) synthesis data. Rotation curves were measured using 3D and 2D tilted ring fitting routines, and the derived curves generally overlap within the error bars, except in the central regions where, as expected, the 3D routines give steeper curves. The specific angular momentum of void dwarfs is found to be high compared to an extrapolation of the trends seen for higher mass bulge-less spirals, but comparable to that of other dwarf irregular galaxies that lie outside of voids. As such, our data show no evidence for a dependence of the specific angular momentum on the large scale environment. Combining our data with the data from the literature, we find a baryonic threshold of ˜109.1 M⊙ for this increase in specific angular momentum. Interestingly, this threshold is very similar to the mass threshold below which the galaxy discs start to become systematically thicker. This provides qualitative support to the suggestion that the thickening of the discs, as well as the increase in specific angular momentum, are both results of a common physical mechanism, such as feedback from star formation. Quantitatively, however, the amount of star formation observed in our dwarfs appears insufficient to produce the observed increase in specific angular momentum. It is hence likely that other processes, such as cold accretion of high angular momentum gas, also play a role in increasing the specific angular momentum.

Determining Our Motion Through the Galaxy

NASA Astrophysics Data System (ADS)

Kohler, Susanna

2016-12-01

Though we dont notice it from our point of view, were hurtling through space at breakneck speed and one of the contributors to our overall motion through the universe is the Suns revolutionaround the center of our galaxy. A recent study uses an unusual approach to measure the speed of this rotation.Moving While Sitting StillWe know that the Sun zips rapidly around the center of the Milky Way our orbitalspeed is somewhere around250 km/s, or 560,000 mph! Getting a precise measurement of this velocity is useful because we can combine it with the observed proper motion of Sgr A*, the black hole at the center of our galaxy, to determine the distance from us to the center of the Milky Way. This is an important baseline for lots of other measurements.Example particle orbits modeled within the galactic potential. The top panel represents a starwith zero angular momentum, which is scattered into a chaotic orbit after interacting with the galactic nucleus. [Hunt et al. 2016]But how can we measure the Suns revolutionspeed accurately? A team of scientists led by Jason Hunt (Dunlap Institute at University of Toronto, Canada) have suggested a unique approach to pin down this value: look for missing stars in the solar neighborhood.Missing StarsThe stars around us should exhibit a distribution of velocities describing their orbits about the galactic center but those stars with zero angular momentum should have plunged directly into the galactic center long ago. These stars would have been scattered onto chaotic halo orbits after their plunge, resulting in a dearth of stars with zero angular momentum around us today.By looking at the relative speeds of the stars moving around us, then, we should see a dip in the velocity distribution corresponding to the missing zero-angular-momentum stars. By noting the relative velocity at which that dip occurs, we cleverly reveal the negative of our motion around the galactic center.Velocity distribution for stars within 700 pc of the Sun. A dip in the distribution (marked with an arrow) is noticeable between 210 and 270 km/s. [Hunt et al. 2016]Where Are We and How Fast Are We Going?Hunt and collaborators use a combination of the first data release from ESAs Gaia mission and a star catalog from the Radial Velocity Experiment to examine the motions of a total of over 200,000 stars in the solar neighborhood. They find that there is indeed a lack of disk stars with velocities close to zero angular momentum. They then compare modeled stellar orbits to the data to estimate the relative speed at which the dip in the velocity distribution occurs.From this information, the authors obtain a measurement of 2399 km/s for the Suns revolutionvelocity around the galactic center. They combine this value with a proper motion measurement of Sgr A* to calculate the distance to the galactic center: 7.90.3 kpc (or about 26,000 light-years).Both of these measurements can be improved with future Gaia data releases, which will contain many orders of magnitude more stars. This clever technique, therefore, proves a useful way of better constraining our position and motion through the Milky Way.CitationJason A. S. Hunt et al 2016 ApJL 832 L25. doi:10.3847/2041-8205/832/2/L25

Orion Optical Navigation for Loss of Communication Lunar Return Contingencies

NASA Technical Reports Server (NTRS)

Getchius, Joel; Hanak, Chad; Kubitschek, Daniel G.

2010-01-01

The Orion Crew Exploration Vehicle (CEV) will replace the Space Shuttle and serve as the next-generation spaceship to carry humans back to the Moon for the first time since the Apollo program. For nominal lunar mission operations, the Mission Control Navigation team will utilize radiometric measurements to determine the position and velocity of Orion and uplink state information to support Lunar return. However, in the loss of communications contingency return scenario, Orion must safely return the crew to the Earth's surface. The navigation design solution for this loss of communications scenario is optical navigation consisting of lunar landmark tracking in low lunar orbit and star- horizon angular measurements coupled with apparent planetary diameter for Earth return trajectories. This paper describes the optical measurement errors and the navigation filter that will process those measurements to support navigation for safe crew return.

POET: Planetary Orbital Evolution due to Tides

NASA Astrophysics Data System (ADS)

Penev, Kaloyan

2014-08-01

POET (Planetary Orbital Evolution due to Tides) calculates the orbital evolution of a system consisting of a single star with a single planet in orbit under the influence of tides. The following effects are The evolutions of the semimajor axis of the orbit due to the tidal dissipation in the star and the angular momentum of the stellar convective envelope by the tidal coupling are taken into account. In addition, the evolution includes the transfer of angular momentum between the stellar convective and radiative zones, effect of the stellar evolution on the tidal dissipation efficiency, and stellar core and envelope spins and loss of stellar convective zone angular momentum to a magnetically launched wind. POET can be used out of the box, and can also be extended and modified.

Giant Eyes for the VLT Interferometer

NASA Astrophysics Data System (ADS)

2001-11-01

First Scientific Results with Combined Light Beams from Two 8.2-m Unit Telescopes Summary It started as a preparatory technical experiment and it soon developed into a spectacular success. Those astronomers and engineers who were present in the control room that night now think of it as the scientific dawn of the Very Large Telescope Interferometer (VLTI) . On October 29, 2001, ANTU and MELIPAL , two of the four VLT 8.2-m Unit Telescopes at the ESO Paranal Observatory, were linked for the first time. Light from the southern star Achernar (Alpha Eridani) was captured by the two telescopes and sent to a common focus in the observatory's Interferometric Laboratory. Following careful adjustments of the optical paths, interferometric fringes were soon recorded there, proving that the beams from the two telescopes had been successfully combined "in phase" . From an analysis of the observed pattern (the "fringe contrast"), the angular diameter of Achernar was determined to be 1.9 milli-arcsec. At the star's distance (145 light-years), this corresponds to a size of 13 million km. The observation is equivalent to measuring the size of a 4-metre long car on the surface of the Moon. This result marks the exciting starting point for operations with the Very Large Telescope Interferometer (VLTI) and it was immediately followed up by other scientific observations. Among these were the first measurements of the diameters of three red dwarf stars ("Kapteyn's star" - HD 33793, HD 217987 and HD 36395), a precise determination of the variable diameters of the pulsating Cepheid stars Beta Doradus and Zeta Geminorum (of great importance for the calibration of the universal distance scale), as well as a first interferometric measurement of the core of Eta Carinae , an intriguing, massive southern object that may possibly become the next supernova in our galaxy. This milestone is another important step towards the ultimate goal of the VLT project - to combine all four 8.2-m telescopes into the most powerful optical/infrared telescope system on Earth. When ready, it will be able to reveal at least 15 times finer details in astronomical objects than what is possible with any existing, single ground-based telescope. PR Photo 30a/01 : Overview of the VLT Interferometer . PR Photo 30b/01 : "Joint" stellar light-spot produced via ANTU and MELIPAL at the VLTI focus. PR Photo 30c/01 : Interferometric fringes from the star Achernar . PR Photo 30d/01 : Time sequence of fringes from Achernar. PR Photo 30e/01 : "Visibility curve" of the star Psi Phoenicis . Scientific Appendix First VLTI observations with two 8.2-m telescopes ESO PR Photo 30a/01 ESO PR Photo 30a/01 [Preview - JPEG: 357 x 400 pix - 82k] [Normal - JPEG: 713 x 800 pix - 208k] [Hi-Res - JPEG: 2673 x 3000 pix - 1.4M] ESO PR Photo 30b/01 ESO PR Photo 30b/01 [Preview - JPEG: 400 x 350 pix - 57k] [Normal - JPEG: 800 x 700 pix - 176k] Caption : PR Photo 30a/01 : Overview of the VLT Interferometer as it was operated when the light beams from two of the 8.2-m telescopes were combined. The VINCI instrument that was used for the present test, is located at the common focus in the Interferometric Laboratory. PR Photo 30b/01 shows one of the first "joint" light-spots from a star as seen at this VLTI focus and resulting from the superposition of light collected with the 8.2-m VLT ANTU and MELIPAL telescopes. Despite the long optical paths (about 200 m), the quality is excellent (FWHM = 0.45 arcsec). Note that this is not (yet) an image of the stellar surface. At 1 o'clock in the morning of October 30, 2001, ESO astronomers and engineers working in the VLTI Control Room successfully combined the light from ANTU and MELIPAL , two of the four 8.2-m VLT Unit Telescopes at the Paranal Observatory. The same night, a series of high-resolution test observations with the VINCI instrument [1] at the focus of the VLT Interferometer (VLTI) proved that this complex system was functioning extremely well, and within the technical specifications . Following about seven months after the moment of "VLTI first light" during which the light beams from two small test telescopes were combined - as described in detail in ESO Press Release 06/01 - this accomplishment above all serves as a demonstration of the possibilities and potential of interferometric observations with the four giant VLT telescopes. The two large telescopes used for the present test are separated by 102 metres. In order to properly combine the starlight received by them, a train of 25 mirrors is needed . All of them must be adjusted with a precision of one thousandth of a millimetre or better. As can be seen on PR Photo 30a/01 , the light from the observed star is first directed towards the Nasmyth focus by three mirrors in the telescope tube. From here, it continues towards the intermediate Coudé focus below the telescope and then onwards through a subterranean light duct to the VLTI Delay Lines that are installed in the Interferometric Tunnel . At the end of this long chain of mirrors and after traveling a distance of approximately 200 metres, the light finally reaches the VINCI instrument in which the two beams interact coherently (in phase) to produce "interferometric fringes". The tests have shown that the starlight arrives at the VINCI instrument with a pointing accuracy of about 1 arcsecond and, even more important, with a long-term tracking stability of the order of 0.2 arcseconds per hour. In fact, the image quality measured at the focus of VINCI is essentially identical to that of the individual telescopes at the Nasmyth (and Cassegrain) foci. Stellar images as sharp as 0.4 arcsec (note that this is the size of the "seeing disk" FWHM, not yet a real image of the stellar surface; the VLTI will start producing two-dimensional images of stars and other objects at a later stage) have been obtained at the interferometric focus, cf. PR Photo 30b/01 . The installation of an Adaptive Optics system (see below) will later reduce the image size to the theoretical limit of 0.057 arcsec (for observations with an 8.2-m telescope in the infrared K-band at wavelength 2.2 µm (or 0.032 arcsec in the J-band at 1.2 µm). First scientific results already during the test observations ESO PR Photo 30c/01 ESO PR Photo 30c/01 [Preview - JPEG: 400 x 368 pix - 50k] [Normal - JPEG: 800 x 736 pix - 136k] ESO PR Photo 30d/01 ESO PR Photo 30d/01 [Preview - JPEG: 400 x 332 pix - 168k] [Normal - JPEG: 800 x 663 pix - 440k] Caption : PR Photo 30c/01 shows the interferometric fringes of the star Achernar , as observed on the computer screen in the VLTI Control Room, at the moment of "First Light" with two 8.2-m VLT telescopes. PR Photo 30d/01 displays the time evolution of the interferometric fringes obtained on Achernar . Each horizontal scan represents a recorded fringe pattern, with time running vertically from bottom to top. PR Photo 30c/01 was extracted from one of these scans. The technical demonstration being so successful, the ESO astronomers and engineers involved in the development of the VLTI immediately decided to go one step further. And indeed, the interferometric fringes recorded with the light beams from two 8.2-m VLT telescopes during these initial technical tests have already led to some very valuable scientific results. The first star to be observed - the brightest star in the southern constellation Eridanus (The River) and known as Alpha Eridani or Achernar - is quite different from our Sun. It is estimated to be several times more massive and, with a surface temperature of about 20000 degrees, it is about three times hotter than our local star. The distance to Achernar has been measured by the ESA HIPPARCOS satellite as about 145 light-years, and from its apparent brightness, it is found to be almost 1000 times more luminous than the Sun. Consequently, it depletes its energy resources much faster and has a much shorter life expectancy (about 100 million years) than the Sun (about 10,000 million years). The new measurement with the VLTI found the angular diameter of Achernar to be 0.00192 ± 0.00005 arcsec . This is equivalent to the angle subtended by a 1 Euro coin (diameter 23.25 mm) as seen from a distance of 2500 km, or by a car (4 metres long) on the surface of the Moon. At the indicated distance, this angle also shows that the real size of Achernar is about 13 million kilometres, and that it is therefore nearly ten times larger than our Sun. Following that first observation, and in spite of the many technical tests scheduled at this moment of the VLTI commissioning work, the astronomers were able to carry out several other scientific observations. During this exciting first period of operation, among others, measurements were made of three red dwarf stars, three stars surrounded by disks, one red giant star, two Cepheid stars and one luminous blue variable star. Preliminary results from some of these observations are described in the Appendix. Angular measurements with the VLTI like the present ones will soon become routine and will allow astronomers to measure accurately the physical characteristics of many different types of stars. For instance, the precise measurement of the angular diameter of Achernar will make it possible to deduce directly and accurately its surface temperature, an important information for our understanding of the formation and evolution of such hot and massive stars. From 40-cm to 8.2-m The present event follows after half a year of much hard work by ESO astronomers and engineers. Earlier this year, the VLTI achieved "first fringes" by combining two small 40-cm siderostat telescopes ( ESO PR 06/01 ). Since then, ESO astronomers and engineers have upgraded the VLTI and are preparing it for regular observations that will start next year. The present results obtained with the combination of two giant telescopes constitute one important milestone along this road. Between March and October 2001, about 1000 individual measurements were carried out on celestial objects with the light beams from the small test telescopes. This process is on-going, as part of the commissioning of the VLTI, and is aimed at a detailed technical characterization of the interferometer and thorough knowledge of its performance. Such observations mainly serve to obtain technical data. Nevertheless, some of them also provide interesting scientific results . For example, during the week just prior to the first fringes now achieved with two large telescopes, nearly 150 measurements were obtained over 4 nights. Among them, five Mira stars (a type of large and cool, pulsating stars) and two close binary stellar systems were observed - some of them had never before been studied interferometrically. Moreover, a large number of objects were observed for calibration. These data are now being evaluated, and will help astronomers to refine their understanding of the capabilities of the VLTI - they will soon become available to the astronomical community via the VLT archive. In the same period, substantial additions were made to the system, e.g., a third Delay Line was installed in the Interferometric Tunnel. This allows the use of the telescopes on the east side of the beam combination laboratory (including MELIPAL) and also to combine the light beams from up to three telescopes at a later moment. The additional mirrors needed in order to permit the combination of the light from the two 8.2-m telescopes were installed. The extensive software that controls the telescopes and the instruments has undergone several revisions to accommodate the increased needs required by the more complex system of Unit Telescopes, delay lines and test instruments. At the same time, the overall reliability of the facility has been constantly improved. The path that the light travels from the two 8.2-m telescopes to the VINCI instrument must be kept constant to within a fraction of a micron , or better than one thousandth of a millimetre! Although it is therefore extremely sensitive to even very small disturbances, the VLT Interferometer has proven to be remarkably reliable and robust. For instance, an earthquake of magnitude 4+ on the Richter scale happened in August 2001 in the middle of a series of interferometric measurements. However, thanks to the many safeguards and compensatory measures built into the system, the VLTI continued to function all through the tremor. The observations were barely affected by the ground vibrations. It should also be noted that, unlike the 40-cm siderostat telescopes, the 8.2-m telescopes are so large that the images they produce are significantly affected by atmospheric turbulence. In order to overcome this problem, ESO is now developing a system of "Adaptive Optics" correctors ( MACAO ) which will "remove" the distortions introduced by the atmospheres by means of small, rapidly reacting computer-controlled deformable mirrors. From 2003, this system will increase the sensitivity of the VLTI by a factor of about 100 (5 magnitudes) compared to the present observations without adaptive optics. VLT Instrumentation The next steps in the VLTI project will be the integration of a new instrument working at a wavelength of 10 µm (the Mid-Infrared interferometric instrument for the VLTI (MIDI) ) in the middle of 2002, the addition of a fringe tracker ( FINITO ) and then of a 3-way, 3-photometric bands instrument (the near-infrared/red VLTI focal instrument (AMBER) ) at the beginning of 2003. Following closely will be the addition of three 1.8-m movable telescopes dedicated to interferometry, and of the Adaptive Optics system. With all these components in place, the VLTI will represent the most powerful interferometer available in the southern hemisphere, and will enable scientific investigations on a wide range of topics ranging from the direct detection of planets around other stars, to the formation and early evolution of stars, to the study of extragalactic objects. A dedication to Ariela Rijo On behalf of the staff, the Director of the Paranal Observatory adds this message: "The Paranal Observatory, while very pleased at the present success of the first fringes from two of the 8.2-m telescopes, at the same time is greatly saddened by the loss of our colleague Ariela Rijo who passed away on October 31" . "She was a wonderful person and an excellent colleague who contributed greatly to the implementation of the VLTI on Paranal. The Paranal Observatory dedicates this result to her memory". Note [1]: The VINCI instrument was built under ESO contract at the Observatoire de Paris (France) and the camera in this instrument was delivered by the MPI for Extraterrestrial Physics (Garching, Germany). The detector and the detector electronics was supplied by ESO. Scientific Appendix: First VLTI stellar measurements with two UTs ESO PR Photo 30e/01 ESO PR Photo 30e/01 [Preview - JPEG: 343 x 400 pix - 39k] [Normal - JPEG: 686 x 800 pix - 82k] Caption : PR Photo 30d/01 shows the "visibility curve" for the red giant star Psi Phoenicis as measured on two nights (16 data sets; three points to the right) with two VLT UTs (ANTU + MELIPAL) for three different positions in the sky and on four nights with the 40-cm test siderostats on a shorter 16-m baseline (8 data sets; one point to the left); see the text below. From the fitted curve, a preliminary value of the angular diameter is 8.21 ± 0.02 milli-arcsec (mas). This appendix presents some technical details of the measurements, obtained with the VLTI and two UTs during the first three test nights. While it must be emphasized that the stated results are still provisional, they clearly indicate the excellent performance of the VLTI already at this early stage and, not least, the great potential for important fundamental observations with this facility. Note in particular, that the quoted errors reflect the statistical uncertainty in the data only and that additional calibration errors must later be taken into account. The observational data were taken on a variety of astronomical objects, including three red dwarfs, three stars surrounded by disks, one red giant, two Cepheids and one luminous blue variable. All of these measurements were calibrated by observing a reference star of known angular size. Each data set required about ten minutes of continuous observations. Fringes were found on all pointed objects within a few minutes of time and kept for up to several hours. All data were deemed to be of high quality and will be analyzed in detail within the next weeks. A preliminary data reduction was possible for part of these objects and it gave the results listed below (all quoted values are uniform disk diameters): * For the blue dwarf Alpha Eridani , on which first fringes were found, 11 data sets were taken within three nights and an angular diameter of 1.92 ± 0.05 milli-arcsec (mas) could be estimated, which is precisely in line with previous measurements. * The nearby red dwarf HD 217987 was measured to have a diameter of 0.92 ± 0.05 mas, resulting from two data sets. This is the first measurement of the angular diameter of a star as small as a type M0 dwarf , and one of the very few available for cool main sequence stars in general. * The giant star HD 36167 was found from four data sets to have a diameter of 3.32 ± 0.02 mas. This measurement constitutes a significant refinement of the earlier, indirect estimate of 3.55 ± 0.06 mas (Cohen M. et al. 1999, Astronomical Journal 117, 1864). * For the three stars which are known to be surrounded by a disk, the following results were obtained: Epsilon Eridani 2.20 ± 0.02 mas (8 data sets in two nights); Fomalhaut (Alpha Piscis Austrini) 2.31 ± 0.02 mas (4 data sets); Beta Pictoris unresolved (4 data sets). Further analysis is expected to put a significant lower limit on the visibility for the latter star. * The two Cepheids Zeta Geminorum and Beta Doradus showed diameters of 1.78 ± 0.02 mas (7 data sets) and 2.00 ± 0.04 mas (6 data sets), respectively. The diameter of Zeta Geminorum has been measured before by three different interferometers. Its diameter is expected to vary between about 1.5 mas and 1.8 mas within ten days. On the date the VLTI data was taken, its phase was close to the foreseen maximum diameter. Beta Doradus has never been measured before. * The red giant Psi Phoenicis was measured on two nights (16 data sets) with the UTs for three different positions in the sky, hence with three different projected baselines. Some weeks earlier it had been measured on four nights with the 40-cm test siderostats (8 data sets) on a shorter 16-m baseline. The star was well resolved already in the previous measurements, but the addition of the data recently obtained with the UTs is of fundamental importance because with their longer baseline and larger light-gathering power, it now becomes possible to obtain visibility measurements beyond the first null, cf. PR Photo 30e/01 . Such measurements in the future will enable astronomers to measure fine details such as limb-darkening and deviations from spherical symmetry. The preliminary diameter value for this star is 8.21 ± 0.02 mas. * The enigmatic object Eta Carinae is a luminous blue variable, a supermassive star, which underwent a massive outburst in the 1840's. This outburst was responsible for the creation of the surrounding Homunculus Nebula . The central object is not well understood, but is likely to have a complex structure and therefore the first interferometric measurement with the VLTI is of great importance. Fringes with a low contrast (amplitude of about 20%) were detected, indicating that the central object is resolved on a scale of a few milliarcseconds. More observations will be obtained to further investigate this peculiar object.

Accretion Disks around Young Stars: An Observational Perspective

NASA Astrophysics Data System (ADS)

Ménard, F.; Bertout, C.

Accretion disks are pivotal elements in the formation and early evolution of solar-like stars. On top of supplying the raw material, their internal conditions also regulate the formation of planets. Their study therefore holds the key to solve this long standing mystery: how did our Solar System form? This chapter focuses on observational studies of the circumstellar environment, and in particular of circumstellar disks, associated with pre-main sequence solar-like stars. The direct measurement of disk parameters poses an obvious challenge: at the distance of the typical star forming regions ( e.g. 140 pc for Taurus), a planetary system like ours (with diameter simeq50 AU out to Pluto, but excluding the Kuiper belt which could extend much farther out) subtends only 0.35''. Yet its surface brightness is low in comparison to the bright central star and high angular and high contrast imaging techniques are required if one hopes to resolve and measure these protoplanetary disks. Fortunately, capable instruments providing 0.1'' resolution or better and high contrast have been available for just about 10 years now. They are covering a large part of the electromagnetic spectrum, from the UV/Optical with HST and the near-infrared from ground-based adaptive optics systems, to the millimetric range with long-baseline radio interferometers. It is therefore not surprising that our knowledge of the structure of the disks surrounding low-mass stars has made a gigantic leap forward in the last decade. In the following pages we will attempt to describe, in a historical perpective, the road that led to the idea that most solar-like stars are surrounded by an accretion disk at one point in their early life and how, nowadays, their structural and physical parameters can be estimated from direct observations. We will follow by a short discussion of a few of the constraints available regarding the evolution and dissipation of these disks. This last topic is particularly relevant today to understand the mechanism leading to the formation of planets.

Design of the STAR-X Telescope

NASA Technical Reports Server (NTRS)

Saha, Timo T.; Zhang, William W.; McClelland, Ryan S.

2017-01-01

Top-level science goals of the Survey and Time-domain Astrophysical Research eXplorer (STAR-X) include: investigations of most violent explosions in the universe, study of growth of black holes across cosmic time and mass scale, and measure how structure formation heats majority of baryons in the universe. To meet these goals, the field-of-view of the telescope should be about 1 square-degree, the angular resolution should be 5 arc-seconds or below across large part of the field-of-view. The on-axis effective area at 1 KeV should be about 2,000 sq cm. Payload cost and launch considerations limit the outer diameter, focal length, and mass to 1.3 meters, 5 meters, and 250 kilograms, respectively. Telescope design is based on a segmented meta-shell approach we have developed at Goddard Space Flight Center for the STAR-X telescope. The telescope shells are divided into 30-degree segments. Individual telescopes and meta-shells are nested inside each other to meet the effective area requirements in 0.5 - 6.0 KeV range. We consider Wolter-Schwarzschild, and Modified-Wolter-Schwarzschild telescope designs as basic building blocks of the nested STAR-X telescope. These designs offer an excellent resolution over a large field of views. Nested telescopes are vulnerable to stray light problems. We have designed a multi-component baffle system to eliminate direct and single-reflection light paths inside the telescopes. Large number of internal and external baffle vane structures are required to prevent stray rays from reaching the focal plane. We have developed a simple ray-trace based tool to determine the dimensions and locations of the baffles. In this paper, we present the results of our trade studies, baffle design studies, and optical performance analyses of the STAR-X telescope.

Optical Design of the STAR-X Telescope

NASA Technical Reports Server (NTRS)

Saha, Timo T.; Zhang, William W.; McClelland, Ryan S.

2017-01-01

Top-level science goals of the Survey and Time-domain Astrophysical Research eXplorer (STAR-X) include: investigations of most violent explosions in the universe, study of growth of black holes across cosmic time and mass scale, and measure how structure formation heats majority of baryons in the universe. To meet these goals, the field-of-view of the telescope should be about 1 square-degree, the angular resolution should be 5 arc-seconds or below across large part of the field-of-view. The on-axis effective area at 1 KeV should be about 2,000 sq cm. Payload cost and launch considerations limit the outer diameter, focal length, and mass to 1.3 meters, 5 meters, and 250 kilograms, respectively. Telescope design is based on a segmented meta-shell approach we have developed at Goddard Space Flight Center for the STAR-X telescope. The telescope shells are divided into 30-degree segments. Individual telescopes and meta-shells are nested inside each other to meet the effective area requirements in 0.5 - 6.0 KeV range. We consider Wolter-Schwarzschild, and Modified-Wolter-Schwarzschild telescope designs as basic building blocks of the nested STAR-X telescope. These designs offer an excellent resolution over a large field of views. Nested telescopes are vulnerable to stray light problems. We have designed a multi-component baffle system to eliminate direct and single-reflection light paths inside the telescopes. Large number of internal and external baffle vane structures are required to prevent stray rays from reaching the focal plane. We have developed a simple ray-trace based tool to determine the dimensions and locations of the baffles. In this paper, we present the results of our trade studies, baffle design studies, and optical performance analyses of the STAR-X telescope.

On the formation of SMC X-1: The effect of mass and orbital angular momentum loss

DOE Office of Scientific and Technical Information (OSTI.GOV)

Li, Tao; Li, X.-D., E-mail: litao@nju.edu.cn, E-mail: lixd@nju.edu.cn; The Key Laboratory of Modern Astronomy and Astrophysics, Ministry of Education, Nanjing 210093

SMC X-1 is a high-mass X-ray binary with an orbital period of 3.9 days. The mass of the neutron star is as low as ∼1M {sub ☉}, suggesting that it was likely formed through an electron-capture supernova rather than an iron-core collapse supernova. From the present system configurations, we argue that the orbital period at the supernova was ≲ 10 days. Since the mass transfer process between the neutron star's progenitor and the companion star before the supernova should have increased the orbital period to tens of days, a mechanism with efficient orbit angular momentum loss and relatively small massmore » loss is required to account for its current orbital period. We have calculated the evolution of the progenitor binary systems from zero-age main sequence to the pre-supernova stage with different initial parameters and various mass and angular momentum loss mechanisms. Our results show that the outflow from the outer Lagrangian point or a circumbinary disk formed during the mass transfer phase may be qualified for this purpose. We point out that these mechanisms may be popular in binary evolution and significantly affect the formation of compact star binaries.« less

Rotational broadening and conservation of angular momentum in post-extreme horizontal branch stars

NASA Astrophysics Data System (ADS)

Fontaine, G.; Latour, M.

2018-06-01

We show that the recent realization that isolated post-extreme horizontal branch (post-EHB) stars are generally characterized by rotational broadening with values of V rot sini between 25 and 30 km s-1 can be explained as a natural consequence of the conservation of angular momentum from the previous He-core burning phase on the EHB. The progenitors of these evolved objects, the EHB stars, are known to be slow rotators with an average value of V rot sini of 7.7 km s-1. This implies significant spin-up between the EHB and post-EHB phases. Using representative evolutionary models of hot subdwarf stars, we demonstrate that angular momentum conservation in uniformly rotating structures (rigid-body rotation) boosts that value of the projected equatorial rotation speed by a factor 3.6 by the time the model has reached the region of the surface gravity-effective temperature plane where the newly-studied post-EHB objects are found. This is exactly what is needed to account for their observed atmospheric broadening. We note that the decrease of the moment of inertia causing the spin-up is mostly due to the redistribution of matter that produces more centrally-condensed structures in the post-EHB phase of evolution, not to the decrease of the radius per se.

Non-Contact EDDY Current Hole Eccentricity and Diameter Measurement

NASA Technical Reports Server (NTRS)

Chern, E. James

1998-01-01

Precision holes are among the most critical features of a mechanical component. Deviations from permissible tolerances can impede operation and result in unexpected failure. We have developed an automated non-contact eddy current hole diameter and eccentricity measuring system. The operating principle is based on the eddy current lift-off effect, which is the coil impedance as a function of the distance between the coil and the test object. An absolute eddy current probe rotates in the hole. The impedance of each angular position is acquired and input to the computer for integration and analysis. The eccentricity of the hole is the profile of the impedance as a function of angular position as compared to a straight line, an ideal hole. The diameter of the hole is the sum of the diameter of the probe and twice the distance-calibrated impedance. An eddy current image is generated by integrating angular scans for a plurality of depths between the top and bottom to display the eccentricity profile. This system can also detect and image defects in the hole. The method for non-contact eddy current hole diameter and eccentricity measurement has been granted a patent by the U.S. Patent and Trademark Office.

"First Light" for the VLT Interferometer

NASA Astrophysics Data System (ADS)

2001-03-01

Excellent Fringes From Bright Stars Prove VLTI Concept Summary Following the "First Light" for the fourth of the 8.2-m telescopes of the VLT Observatory on Paranal in September 2000, ESO scientists and engineers have just successfully accomplished the next major step of this large project. On March 17, 2001, "First Fringes" were obtained with the VLT Interferometer (VLTI) - this important event corresponds to the "First Light" for an astronomical telescope. At the VLTI, it occurred when the infrared light from the bright star Sirius was captured by two small telescopes and the two beams were successfully combined in the subterranean Interferometric Laboratory to form the typical pattern of dark and bright lines known as " interferometric fringes ". This proves the success of the robust VLTI concept, in particular of the "Delay Line". On the next night, the VLTI was used to perform a scientific measurement of the angular diameter of another comparatively bright star, Alpha Hydrae ( Alphard ); it was found to be 0.00929±0.00017 arcsec . This corresponds to the angular distance between the two headlights of a car as seen from a distance of approx. 35,000 kilometres. The excellent result was obtained during a series of observations, each lasting 2 minutes, and fully confirming the impressive predicted abilities of the VLTI . This first observation with the VLTI is a monumental technological achievement, especially in terms of accuracy and stability . It crucially depends on the proper combination and functioning of a large number of individual opto-mechnical and electronic elements. This includes the test telescopes that capture the starlight, continuous and extremely precise adjustment of the various mirrors that deflect the light beams as well as the automatic positioning and motion of the Delay Line carriages and, not least, the optimal tuning of the VLT INterferometer Commissionning Instrument (VINCI). These initial observations prove the overall concept for the VLTI . It was first envisaged in the early 1980's and has been continuously updated, as new technologies and materials became available during the intervening period. The present series of functional tests will go on for some time and involve many different configurations of the small telescopes and the instrument. It is then expected that the first combination of light beams from two of the VLT 8.2-m telescopes will take place in late 2001 . According to current plans, regular science observations will start from 2002, when the European and international astronomical community will have access to the full interferometric facility and the specially developed VLTI instrumentation now under construction. A wide range of scientific investigations will then become possible, from the search for planets around nearby stars, to the study of energetic processes at the cores of distant galaxies. With its superior angular resolution (image sharpness), the VLT is now beginning to open a new era in observational optical and infrared astronomy. The ambition of ESO is to make this type of observations available to all astronomers, not just the interferometry specialists. Video Clip 03/01 : Various video scenes related to the VLTI and the "First Fringes". PR Photo 10a/01 : "First Fringes" from the VLTI on the computer screen. PR Photo 10b/01 : Celebrating the VLTI "First Fringes" . PR Photo 10c/01 : Overview of the VLT Interferometer . PR Photo 10d/01 : Interferometric observations: Fringes from two stars of different angular size . PR Photo 10e/01 : Interferometric observations: Change of fringes with increasing baseline . PR Photo 10f/01 : Aerial view of the installations for the VLTI on the Paranal platform. PR Photo 10g/01 : Stations for the VLTI Auxiliary Telescopes. PR Photo 10h/01 : A test siderostat in place for observations. PR Photo 10i/01 : A test siderostat ( close-up ). PR Photo 10j/01 : One of the Delay Line carriages in the Interferometric Tunnel. PR Photo 10k/01 : The VINCI instrument in the Interferometric Laboratory. PR Photo 10l/01 : The VLTI Control Room . "First Fringes at the VLTI": A great moment! First light of the VLT Interferometer - PR Video Clip 03/01 [MPEG - x.xMb] ESO PR Video Clip 03/01 "First Light of the VLT Interferometer" (March 2001) (5025 frames/3:21x min) [MPEG Video+Audio; 144x112 pix; 6.9Mb] [MPEG Video+Audio; 320x240 pix; 13.7Mb] [RealMedia; streaming; 34kps] [RealMedia; streaming; 200kps] ESO Video Clip 03/01 provides a quick overview of the various elements of the VLT Interferometer and the important achievement of "First Fringes". The sequence is: General view of the Paranal observing platform. The "stations" for the VLTI Auxiliary Telescopes. Statement by the Manager of the VLT project, Massimo Tarenghi . One of the VLTI test telescopes ("siderostats") is being readied for observations. The Delay Line carriages in the Interferometric Tunnel move. The VINCI instrument in the Interferometric Laboratory is adjusted. Platform at sunset, before the observations. Astronomers and engineers prepare for the first observations in the VLTI Control Room in the Interferometric Building. "Interferometric Fringes" on the computer screen. Concluding statements by Andreas Glindemann , VLTI Project Leader, and Massimo Tarenghi . Distant view of the installations at Paranal at sunset (on March 1, 2001). The moment of "First Fringes" at the VLTI occurred in the evening of March 17, 2001 . The bright star Sirius was observed with two small telescopes ("siderostats"), specially constructed for this purpose during the early VLTI test phases. ESO PR Video Clip 03/01 includes related scenes and is based on a more comprehensive documentation, now available as ESO Video News Reel No. 12. The star was tracked by the two telescopes and the light beams were guided via the Delay Lines in the Interferometric Tunnel to the VINCI instrument [1] at the Interferometric Laboratory. The path lengths were continuously adjusted and it was possible to keep them stable to within 1 wavelength (2.2 µm, or 0.0022 mm) over a period of at least 2 min. Next night, several other stars were observed, enabling the ESO astronomers and engineers in the Control Room to obtain stable fringe patterns more routinely. With the special software developed, they also obtained 'on-line' an accurate measurement of the angular diameter of a star. This means that the VLTI delivered its first valid scientific result, already during this first test . First observation with the VLTI ESO PR Photo 10a/01 ESO PR Photo 10a/01 [Preview - JPEG: 400 x 315 pix - 96k] [Normal - JPEG: 800 x 630 pix - 256k] [Hi-Res - JPEG: 3000 x 2400 pix - 1.7k] ESO PR Photo 10b/01 ESO PR Photo 10b/01 [Preview - JPEG: 400 x 218 pix - 80k] [Normal - JPEG: 800 x 436 pix - 204k] Caption : PR Photo 10a/01 The "first fringes" obtained with the VLTI, as seen on the computer screen during the observation (upper right window). The fringe pattern arises when the light beams from two small telescopes are brought together in the VINCI instrument. The pattern itself contains information about the angular extension of the observed object, here the bright star Sirius . More details about the interpretation of this pattern is given in Appendix A. PR Photo 10b/01 : Celebrating the moment of "First Fringes" at the VLTI. At the VLTI control console (left to right): Pierre Kervella , Vincent Coudé du Foresto , Philippe Gitton , Andreas Glindemann , Massimo Tarenghi , Anders Wallander , Roberto Gilmozzi , Markus Schoeller and Bill Cotton . Bertrand Koehler was also present and took the photo. Technical information about PR Photo 10a/01 is available below. Following careful adjustment of all of the various components of the VLTI, the first attempt to perform a real observation was initiated during the night of March 16-17, 2001. "Fringes" were actually acquired during several seconds, leading to further optimization of the Delay Line optics. The next night, March 17-18, stable fringes were obtained on the bright stars Sirius and Lambda Velorum . The following night, the first scientifically valid results were obtained during a series of observations of six stars. One of these, Alpha Hydrae , was measured twice, with an interval of 15 minutes between the 2-min integrations. The measured diameters were highly consistent, with a mean of 0.00929±0.00017 arcsec. This new VLTI measurement is in full agreement with indirect (photometric) estimates of about 0.009 arcsec. The overall performance of the VLTI was excellent already in this early stage. For example, the interferometric efficiency ('contrast' on a stellar point source) was measured to be 87% and stable to within 1.3% over several days. This performance will be further improved following additional tuning. The entire operation of the VLTI was performed remotely from the Control Room, as this will also be the case in the future. Another great advantage of the VLTI concept is the possibility to analyse the data at the control console. This is one of the key features of the VLTI that contributes to make it a very user-friendly facility. Overview of the VLT Interferometer ESO PR Photo 10c/01 ESO PR Photo 10c/01 [Preview - JPEG: 400 x 410 pix - 60k] [Normal - JPEG: 800 x 820 pix - 124k] [Hi-Res - JPEG: 3000 x 3074 pix - 680k] Caption : PR Photo 10c/01 Overview of the VLT Interferometer, with the various elements indicated. In this case, the light beams from two of the 8.2-m telescopes are combined. The VINCI instrument that was used for the present test, is located at the common focus in the Interferometric Laboratory. The interferometric principle is based on the phase-stable combination of light beams from two or more telescopes at a common interferometric focus , cf. PR Photo 10c/01 . The light from a celestial object is captured simultaneously by two or more telescopes. For the first tests, two "siderostats" with 40-cm aperture are used; later on, two or more 8.2-m Unit Telescopes will be used, as well as several moving 1.8-m Auxiliary Telescopes (ATs), now under construction at the AMOS factory in Belgium. Via several mirrors and through the Delay Line, that continuously compensates for changes in the path length introduced by the Earth's rotation as well as by other effects (e.g., atmospheric turbulence), the light beams are guided towards the interferometric instrument VINCI at the common interferometric focus. It is located in the subterranean Interferometric Laboratory , at the centre of the observing platform on the top of the Paranal mountain. Photos of some of the VLTI elements are shown in Appendix B. The interferometric technique allows achieving images, as sharp as those of a telescope with a diameter equivalent to the largest distance between the telescopes in the interferometer. For the VLTI, this distance is about 200 metres, resulting in a resolution of 0.001 arcsec in the near-infrared spectral region (at 1 µm wavelength), or 0.0005 arcsec in visual light (500 nm). The latter measure corresponds to about 2 metres on the surface of the Moon. The VLTI instruments The installation and putting into operation of the VLTI at Paranal is a gradual process that will take several years. While the present "First Fringe" event is of crucial importance, the full potential of the VLTI will only be reached some years from now. This will happen with the successive installation of a number of highly specialised instruments, like the near-infrared/red VLTI focal instrument (AMBER) , the Mid-Infrared interferometric instrument for the VLTI (MIDI) and the instrument for Phase-Referenced Imaging and Microarcsecond Astrometry (PRIMA). Already next year, the three 1.8-m Auxiliary Telescopes that will be fully devoted to interferometric observations, will arrive at Paranal. Ultimately, it will be possible to combine the light beams from all the large and small telescopes. Great research promises Together, they will be able to achieve an unprecedented image sharpness (angular resolution) in the optical/infrared wavelength region, and thanks to the great light-collecting ability of the VLT Unit Telescopes, also for observations of quite faint objects. This will make it possible to carry out many different front-line scientific studies, beyond the reach of other instruments. There are many promising research fields that will profit from VLTI observations, of which the following serve as particularly interesting examples: * The structure and composition of the outer solar system, by studies of individual moons, Trans-Neptunian Objects and comets. * The direct detection and imaging of exoplanets in orbit around other stars. * The formation of star clusters and their evolution, from images and spectra of very young objects. * Direct views of the surface structures of stars other than the Sun. * Measuring accurate distances to the most prominent "stepping stones" in the extragalactic distance scale, e.g., galactic Cepheid stars, the Large Magellanic Cloud and globular clusters. * Direct investigations of the physical mechanisms responsible for stellar pulsation, mass loss and dust formation in stellar envelopes and evolution to the Planetary Nebula and White Dwarf stages. * Close-up studies of interacting binary stars to better understand their mass transfer mechanisms and evolution. * Studies of the structure of the circum-stellar environment of stellar black holes and neutron stars. * The evolution of the expanding shells of unstable stars like novae and supernovae and their interaction with the interstellar medium. * Studying the structure and evolution of stellar and galactic nuclear accretion disks and the associated features, e.g., jets and dust tori. * With images and spectra of the innermost regions of the Milky Way galaxy, to investigate the nature of the nucleus surrounding the central black hole. Clearly, there will be no lack of opportunities for trailblazing research with the VLTI. The "First Fringes" constitute a very important milestone in this direction. Appendix A: How does it work? ESO PR Photo 10d/01 ESO PR Photo 10d/01 [Preview - JPEG: 400 x 290 pix - 24k] [Normal - JPEG: 800 x 579 pix - 68k] [Hi-Res - JPEG: 3000 x 2170 pix - 412k] ESO PR Photo 10e/01 ESO PR Photo 10e/01 [Preview - JPEG: 400 x 219 pix - 32k] [Normal - JPEG: 800 x 438 pix - 64k] [Hi-Res - JPEG: 3000 x 1644 pix - 336k] Caption : PR Photo 10d/01 demonstrates in a schematic way, how the images of two stars of different angular size (left) will look like, with a single telescope (middle) and with an interferometer like the VLTI (right). Whereas there is little difference with one telescope, the fringe patterns at the interferometer are quite different. Conversely, the appearance of this pattern provides a measure of the star's angular diameter. In PR Photo 10e/01 , interferometric observations of a single star are shown, as the distance between the two telescopes is gradually increased. The observed pattern at the focal plane clearly changes, and the "fringes" disappear completely. See the text for more details. The principle behind interferometry is the "coherent optical interference" of light beams from two or more telescopes, due to the wave nature of light. The above illustrations serve to explain what the astronomers observe in the simplest case, that of a single star with a certain angular size, and how this can be translated into a measurement of this size. In PR Photo 10d/01 , the difference between two stars of different diameter is illustrated. While the image of the smaller star displays strong interference effects (i.e., a well visible fringe pattern), those of the larger star are much less prominent. The "visibility" of the fringes is therefore a direct measure of the size; the stronger they appear (the "larger the contrast"), the smaller is the star. If the distance between the two telescopes is increased when a particular star is observed ( PR Photo 10e/01 ), then the fringes become less and less prominent. At a certain distance, the fringe pattern disppears completely. This distance is directly related to the angular size of the star. Appendix B: Elements of the VLT Interferometer Contrary to other large astronomical telescopes, the VLT was designed from the beginning with the use of interferometry as a major goal . For this reason, the four 8.2-m Unit Telescopes were positioned in a quasi-trapezoidal configuration and several moving 1.8-m telescopes were included into the overall VLT concept, cf. PR Photo 10f/01 . The photos below show some of the key elements of the VLT Interferometer during the present observations. They include the siderostats , 40-cm telescopes that serve to capture the light from a comparatively bright star ( Photos 10g-i/01 ), the Delay Lines ( Photo 10j/01 ), and the VINCI instrument ( Photo 10k/01) Earlier information about the development and construction of the individual elements of the VLTI is available as ESO PR 04/98 , ESO PR 14/00 and ESO PR Photos 26a-e/00.

CONNECTING ANGULAR MOMENTUM AND GALACTIC DYNAMICS: THE COMPLEX INTERPLAY BETWEEN SPIN, MASS, AND MORPHOLOGY

DOE Office of Scientific and Technical Information (OSTI.GOV)

Teklu, Adelheid F.; Remus, Rhea-Silvia; Dolag, Klaus

The evolution and distribution of the angular momentum of dark matter (DM) halos have been discussed in several studies over the past decades. In particular, the idea arose that angular momentum conservation should allow us to infer the total angular momentum of the entire DM halo from measuring the angular momentum of the baryonic component, which is populating the center of the halo, especially for disk galaxies. To test this idea and to understand the connection between the angular momentum of the DM halo and its galaxy, we use a state-of-the-art, hydrodynamical cosmological simulation taken from the set of Magneticummore » Pathfinder simulations. Thanks to the inclusion of the relevant physical processes, the improved underlying numerical methods, and high spatial resolution, we successfully produce populations of spheroidal and disk galaxies self-consistently. Thus, we are able to study the dependence of galactic properties on their morphology. We find that (1) the specific angular momentum of stars in disk and spheroidal galaxies as a function of their stellar mass compares well with observational results; (2) the specific angular momentum of the stars in disk galaxies is slightly smaller compared to the specific angular momentum of the cold gas, in good agreement with observations; (3) simulations including the baryonic component show a dichotomy in the specific stellar angular momentum distribution when splitting the galaxies according to their morphological type (this dichotomy can also be seen in the spin parameter, where disk galaxies populate halos with slightly larger spin compared to spheroidal galaxies); (4) disk galaxies preferentially populate halos in which the angular momentum vector of the DM component in the central part shows a better alignment to the angular momentum vector of the entire halo; and (5) the specific angular momentum of the cold gas in disk galaxies is approximately 40% smaller than the specific angular momentum of the total DM halo and shows a significant scatter.« less

ANGULAR MOMENTUM TRANSPORT BY ACOUSTIC MODES GENERATED IN THE BOUNDARY LAYER. I. HYDRODYNAMICAL THEORY AND SIMULATIONS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Belyaev, Mikhail A.; Rafikov, Roman R.; Stone, James M., E-mail: rrr@astro.princeton.edu

The nature of angular momentum transport in the boundary layers of accretion disks has been one of the central and long-standing issues of accretion disk theory. In this work we demonstrate that acoustic waves excited by supersonic shear in the boundary layer serve as an efficient mechanism of mass, momentum, and energy transport at the interface between the disk and the accreting object. We develop the theory of angular momentum transport by acoustic modes in the boundary layer, and support our findings with three-dimensional hydrodynamical simulations, using an isothermal equation of state. Our first major result is the identification ofmore » three types of global modes in the boundary layer. We derive dispersion relations for each of these modes that accurately capture the pattern speeds observed in simulations to within a few percent. Second, we show that angular momentum transport in the boundary layer is intrinsically nonlocal, and is driven by radiation of angular momentum away from the boundary layer into both the star and the disk. The picture of angular momentum transport in the boundary layer by waves that can travel large distances before dissipating and redistributing angular momentum and energy to the disk and star is incompatible with the conventional notion of local transport by turbulent stresses. Our results have important implications for semianalytical models that describe the spectral emission from boundary layers.« less

Mechanisms of the self-organization of star-shaped polymers with a varied structure of branching center based on fullerene C{sub 60} in solutions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lebedev, V. T., E-mail: vlebedev@pnpi.spb.ru; Toeroek, Gy.; Vinogradova, L. V.

The self-organization of star-shaped polymers in toluene has been studied by small-angle neutron scattering. Polystyrene stars with a mono-C{sub 60} branching center are ordered into globular clusters ({approx}1700 nm in diameter), whereas stars with a double (C{sub 60}-C{sub 60}) center are ordered into anisotropic structures (superchains), which are linked (depending on the concentration) into triads (chain clusters {approx}2500 nm in diameter). On the contrary, heteroarm polystyrene and poly-2-vinylpyridine stars with a C{sub 60} center are weakly associated into dimers. Moderately polar stars with arms composed of polystyrene and diblock copolymer (poly-2-vinylpyridine-poly-tret-butyl methacrylate) form short chains composed of four macromolecules, whilemore » stars of higher polarity based on polystyrene and poly-tret-butyl methacrylate form clusters containing {approx}12 macromolecules {approx}50 nm in diameter. Thus, by varying the structure of the center and the arm polarity, one can control the modes of star structuring.« less

Stars get dizzy after lunch

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhang, Michael; Penev, Kaloyan

2014-06-01

Exoplanet searches have discovered a large number of {sup h}ot Jupiters{sup —}high-mass planets orbiting very close to their parent stars in nearly circular orbits. A number of these planets are sufficiently massive and close-in to be significantly affected by tidal dissipation in the parent star, to a degree parameterized by the tidal quality factor Q {sub *}. This process speeds up their star's rotation rate while reducing the planet's semimajor axis. In this paper, we investigate the tidal destruction of hot Jupiters. Because the orbital angular momenta of these planets are a significant fraction of their star's rotational angular momenta,more » they spin up their stars significantly while spiraling to their deaths. Using the Monte Carlo simulation, we predict that for Q {sub *} = 10{sup 6}, 3.9 × 10{sup –6} of stars with the Kepler Target Catalog's mass distribution should have a rotation period shorter than 1/3 day (8 hr) due to accreting a planet. Exoplanet surveys such as SuperWASP, HATnet, HATsouth, and KELT have already produced light curves of millions of stars. These two facts suggest that it may be possible to search for tidally destroyed planets by looking for stars with extremely short rotational periods, then looking for remnant planet cores around those candidates, anomalies in the metal distribution, or other signatures of the recent accretion of the planet.« less

Spectral Irradiance Calibration in the Infrared. Part 6; 3-35 microns Spectra of Three Southern Standard Stars

NASA Technical Reports Server (NTRS)

Cohen, Martin; Witteborn, Fred C.; Bregman, Jesse D.; Wooden, Diane H.; Salama, Alberto; Metcalfe, Leo

1996-01-01

We present three new absolutely calibrated continuous stellar spectra from 3 to 35 microns, constructed as far as possible from actual observed spectral fragments taken from the Kuiper Airborne Observatory (KAO), and the IRAS Low Resolution Spectrometer (LRS). These stars- alpha(sup 1) Cen, alpha TrA, and epsilon Car-augment our previous archive of complete absolutely calibrated spectra for northern K and M giants. All these spectra have a common calibration pedigree. The wavelength coverage is ideal for calibration of many existing and proposed ground-based, airborne, and satellite sensors. KAO and IRAS data in the 15-30 micron range suggest that the spectra of cool giants are close to Rayleigh-Jeans slopes. Our observations of alpha(sup 1) Cen, absolutely calibrated via our adopted Sirius model, indicate an angular diameter in very good agreement with values in the literature, demonstrating 'closure' of the set of spectra within our absolute framework. We compare our observed alpha(sup 1) Cen spectrum with a published grid of theoretical models from Kurucz, and adopt a plausible theoretical shape, that fits our spectrum, as a secondary reference spectrum in the southern sky.

Rapidly rotating polytropes in general relativity

NASA Technical Reports Server (NTRS)

Cook, Gregory B.; Shapiro, Stuart L.; Teukolsky, Saul A.

1994-01-01

We construct an extensive set of equilibrium sequences of rotating polytropes in general relativity. We determine a number of important physical parameters of such stars, including maximum mass and maximum spin rate. The stability of the configurations against quasi-radial perturbations is diagnosed. Two classes of evolutionary sequences of fixed rest mass and entropy are explored: normal sequences which behave very much like Newtonian evolutionary sequences, and supramassive sequences which exist solely because of relativistic effects. Dissipation leading to loss of angular momentum causes a star to evolve in a quasi-stationary fashion along an evolutionary sequence. Supramassive sequences evolve towards eventual catastrophic collapse to a black hole. Prior to collapse, the star must spin up as it loses angular momentum, an effect which may provide an observational precursor to gravitational collapse to a black hole.

Black-hole binaries as relics of gamma-ray burst/hypernova explosions

NASA Astrophysics Data System (ADS)

Moreno Mendez, Enrique

The Collapsar model, in which a fast-spinning massive star collapses into a Kerr black hole, has become the standard model to explain long-soft gamma-ray bursts and hypernova explosions (GRB/HN). However, stars massive enough (those with ZAMS mass ≳ (18--20) M⊙ ) to produce these events evolve through a path that loses too much angular momentum to produce a central engine capable of delivering the necessary energy. In this work I suggest that the soft X-ray transient sources are the remnants of GRBs/HNe. Binaries in which the massive primary star evolves a carbon-oxygen burning core, then start to transfer material to the secondary star (Case C mass transfer), causing the orbit to decay until a common-envelope phase sets in. The secondary spirals in, further narrowing the orbit of the binary and removing the hydrogen envelope of the primary star. Eventually the primary star becomes tidally locked and spins up, acquiring enough rotational energy to power up a GRB/HN explosion. The central engine producing the GRB/HN event is the Kerr black hole acting through the Blandford-Znajek mechanism. This model can explain not only the long-soft GRBs, but also the subluminous bursts (which comprise ˜ 97% of the total), the long-soft bursts and the short-hard bursts (in a neutron star, black hole merger). Because of our binary evolution through Case C mass transfer, it turns out that for the subluminous and cosmological bursts, the angular momentum O is proportional to m3/2D , where mD is the mass of the donor (secondary star). This binary evolution model has a great advantage over the Woosley Collapsar model; one can "dial" the donor mass in order to obtain whatever angular momentum is needed to drive the explosion. Population syntheses show that there are enough binaries to account for the progenitors of all known classes of GRBs.

Mass loss from solar-type stars

NASA Technical Reports Server (NTRS)

Hartmann, L.

1985-01-01

The present picture of mass loss from solar-type (low-mass) stars is described, with special emphasis on winds from pre-main-sequence stars. Attention is given to winds from T Tauri stars and to angular momentum loss. Prospects are good for further advances in our understanding of the powerful mass loss observed from young stars; ultraviolet spectra obtainable with the Space Telescope should provide better estimates of mass loss rates and a clearer picture of physical conditions in the envelopes of these stars. To understand the mass ejection from old, slowly rotating main-sequence stars, we will have to study the sun.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Guzik, J. A.; Houdek, G.; Chaplin, W. J.

θ Cygni is an F3 spectral type magnitude V = 4.48 main-sequence star that was the brightest star observed by the original Kepler spacecraft mission. Short-cadence (58.8 s) photometric data using a custom aperture were first obtained during Quarter 6 (2010 June–September) and subsequently in Quarters 8 and 12–17. We present analyses of solar-like oscillations based on Q6 and Q8 data, identifying angular degree l = 0, 1, and 2 modes with frequencies of 1000–2700 μ Hz, a large frequency separation of 83.9 ± 0.4 μ Hz, and maximum oscillation amplitude at frequency ν {sub max} = 1829 ± 54more » μ Hz. We also present analyses of new ground-based spectroscopic observations, which, combined with interferometric angular diameter measurements, give T {sub eff} = 6697 ± 78 K, radius 1.49 ± 0.03 R {sub ⊙}, [Fe/H] = -0.02 ± 0.06 dex, and log g = 4.23 ± 0.03. We calculate stellar models matching these constraints using the Yale Rotating Evolution Code and the Asteroseismic Modeling Portal. The best-fit models have masses of 1.35–1.39 M {sub ⊙} and ages of 1.0–1.6 Gyr. θ Cyg’s T {sub eff} and log g place it cooler than the red edge of the γ Doradus instability region established from pre- Kepler ground-based observations, but just at the red edge derived from pulsation modeling. The pulsation models show γ Dor gravity modes driven by the convective blocking mechanism, with frequencies of 1–3 cycles per day (11 to 33 μ Hz). However, gravity modes were not seen in Kepler data; one signal at 1.776 cycles per day (20.56 μ Hz) may be attributable to a faint, possibly background, binary.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Guzik, Joyce Ann; Houdek, G.; Chaplin, W. J.

θ Cygni is an F3 spectral type magnitude V = 4.48 main-sequence star that was the brightest star observed by the original Kepler spacecraft mission. Short-cadence (58.8 s) photometric data using a custom aperture were first obtained during Quarter 6 (2010 June–September) and subsequently in Quarters 8 and 12–17. We present analyses of solar-like oscillations based on Q6 and Q8 data, identifying angular degree l = 0, 1, and 2 modes with frequencies of 1000–2700 μHz, a large frequency separation of 83.9 ± 0.4 μHz, and maximum oscillation amplitude at frequency ν max = 1829 ± 54 μHz. We alsomore » present analyses of new ground-based spectroscopic observations, which, combined with interferometric angular diameter measurements, give T eff = 6697 ± 78 K, radius 1.49 ± 0.03 R ⊙, [Fe/H] = $-$0.02 ± 0.06 dex, and log g = 4.23 ± 0.03. We calculate stellar models matching these constraints using the Yale Rotating Evolution Code and the Asteroseismic Modeling Portal. The best-fit models have masses of 1.35–1.39 M ⊙ and ages of 1.0–1.6 Gyr. θ Cyg's T eff and log g place it cooler than the red edge of the γ Doradus instability region established from pre-Kepler ground-based observations, but just at the red edge derived from pulsation modeling. Lastly, the pulsation models show γ Dor gravity modes driven by the convective blocking mechanism, with frequencies of 1–3 cycles per day (11 to 33 μHz). However, gravity modes were not seen in Kepler data; one signal at 1.776 cycles per day (20.56 μHz) may be attributable to a faint, possibly background, binary.« less

SONS: The JCMT legacy survey of debris discs in the submillimetre

NASA Astrophysics Data System (ADS)

Holland, Wayne S.; Matthews, Brenda C.; Kennedy, Grant M.; Greaves, Jane S.; Wyatt, Mark C.; Booth, Mark; Bastien, Pierre; Bryden, Geoff; Butner, Harold; Chen, Christine H.; Chrysostomou, Antonio; Davies, Claire L.; Dent, William R. F.; Di Francesco, James; Duchêne, Gaspard; Gibb, Andy G.; Friberg, Per; Ivison, Rob J.; Jenness, Tim; Kavelaars, JJ; Lawler, Samantha; Lestrade, Jean-François; Marshall, Jonathan P.; Moro-Martin, Amaya; Panić, Olja; Phillips, Neil; Serjeant, Stephen; Schieven, Gerald H.; Sibthorpe, Bruce; Vican, Laura; Ward-Thompson, Derek; van der Werf, Paul; White, Glenn J.; Wilner, David; Zuckerman, Ben

2017-09-01

Debris discs are evidence of the ongoing destructive collisions between planetesimals, and their presence around stars also suggests that planets exist in these systems. In this paper, we present submillimetre images of the thermal emission from debris discs that formed the SCUBA-2 Observations of Nearby Stars (SONS) survey, one of seven legacy surveys undertaken on the James Clerk Maxwell Telescope between 2012 and 2015. The overall results of the survey are presented in the form of 850 μm (and 450 μm, where possible) images and fluxes for the observed fields. Excess thermal emission, over that expected from the stellar photosphere, is detected around 49 stars out of the 100 observed fields. The discs are characterized in terms of their flux density, size (radial distribution of the dust) and derived dust properties from their spectral energy distributions. The results show discs over a range of sizes, typically 1-10 times the diameter of the Edgeworth-Kuiper Belt in our Solar system. The mass of a disc, for particles up to a few millimetres in size, is uniquely obtainable with submillimetre observations and this quantity is presented as a function of the host stars' age, showing a tentative decline in mass with age. Having doubled the number of imaged discs at submillimetre wavelengths from ground-based, single-dish telescope observations, one of the key legacy products from the SONS survey is to provide a comprehensive target list to observe at high angular resolution using submillimetre/millimetre interferometers (e.g. Atacama Large Millimeter Array, Smithsonian Millimeter Array).

Stellar Angular Momentum Distributions and Preferential Radial Migration

NASA Astrophysics Data System (ADS)

Wyse, Rosemary; Daniel, Kathryne J.

2018-04-01

I will present some results from our recent investigations into the efficiency of radial migration in stellar disks of differing angular momentum distributions, within a given adopted 2D spiral disk potential. We apply to our models an analytic criterion that determines whether or not individual stars are in orbits that could lead to radial migration around the corotation resonance. We couch our results in terms of the local stellar velocity dispersion and find that the fraction of stars that could migrate radially decreases as the velocity dispersion increases. I will discuss implications and comparisons with the results of other approaches.

Constraints on Grain Formation around Carbon Stars from Laboratory Studies of Presolar Graphite

NASA Astrophysics Data System (ADS)

Bernatowicz, Thomas J.; Akande, Onaolapo Wali; Croat, Thomas K.; Cowsik, Ramanath

2005-10-01

We report the results of an investigation into the physical conditions in the mass outflows of asymptotic giant branch (AGB) carbon stars that are required for the formation of micron-sized presolar graphite grains, with and without previously formed internal crystals of titanium carbide (TiC). A lower mass limit of 1.1 Msolar for stars capable of contributing grains to the solar nebula is derived. This mass limit, in conjunction with a mass-luminosity relation for carbon stars, identifies the region of the H-R diagram relevant to the production of presolar graphite. Detailed dynamical models of AGB outflows, along with constraints provided by kinetics and equilibrium thermodynamics, indicate that grain formation occurs at radii from 2.3 to 3.7 AU for AGB carbon stars in the 1.1-5 Msolar range. This analysis also yields time intervals available for graphite growth that are on the order of a few years. By considering the luminosity variations of carbon stars, we show that grains formed during minima in the luminosity are likely to be evaporated subsequently, while those formed at luminosity maxima will survive. We calculate strict upper limits on grain sizes for graphite and TiC in spherically symmetric AGB outflows. Graphite grains can reach diameters in the observed micron size range (1-2 μm) only under ideal growth conditions (perfect sticking efficiency, no evaporation, no depletion of gas species contributing to grain growth), and then only in outflows from carbon stars with masses <~2.5 Msolar. The same is true for TiC grains that are found within presolar graphite, which have mean diameters of 24+/-14 nm. In general, the mass-loss rates that would be required to produce the observed grain sizes in spherically symmetric outflows are at least an order of magnitude larger than the maximum observed AGB carbon star mass-loss rates. These results, as well as pressure constraints derived from equilibrium thermodynamics, force us to conclude that presolar graphite and TiC must form in regions of enhanced density (clumps, jets) in AGB outflows having small angular scales. As shown in the companion paper by Croat et al., the enrichment of 12C in many AGB graphites, and the overabundances of the s-process elements Mo, Zr, and Ru in the carbides found within them, often greatly exceed the values observed astronomically in AGB outflows. These observations not only lend further support to the idea that the outflows are clumpy, but also imply that the outflowing matter is not well mixed in the circumstellar envelope out to the radii where grain condensation takes place.

Spectral Irradiance Calibration in the Infrared. Part 7; New Composite Spectra, Comparison with Model Atmospheres, and Far-Infrared Extrapolations

NASA Technical Reports Server (NTRS)

Cohen, Martin; Witteborn, Fred C.; Carbon, Duane F.; Davies, John K.; Wooden, Diane H.; Bregman, Jesse D.

1996-01-01

We present five new absolutely calibrated continuous stellar spectra constructed as far as possible from spectral fragments observed from the ground, the Kuiper Airborne Observatory (KAO), and the IRAS Low Resolution Spectrometer. These stars-alpha Boo, gamma Dra, alpha Cet, gamma Cru, and mu UMa-augment our six, published, absolutely calibrated spectra of K and early-M giants. All spectra have a common calibration pedigree. A revised composite for alpha Boo has been constructed from higher quality spectral fragments than our previously published one. The spectrum of gamma Dra was created in direct response to the needs of instruments aboard the Infrared Space Observatory (ISO); this star's location near the north ecliptic pole renders it highly visible throughout the mission. We compare all our low-resolution composite spectra with Kurucz model atmospheres and find good agreement in shape, with the obvious exception of the SiO fundamental, still lacking in current grids of model atmospheres. The CO fundamental seems slightly too deep in these models, but this could reflect our use of generic models with solar metal abundances rather than models specific to the metallicities of the individual stars. Angular diameters derived from these spectra and models are in excellent agreement with the best observed diameters. The ratio of our adopted Sirius and Vega models is vindicated by spectral observations. We compare IRAS fluxes predicted from our cool stellar spectra with those observed and conclude that, at 12 and 25 microns, flux densities measured by IRAS should be revised downwards by about 4.1% and 5.7%, respectively, for consistency with our absolute calibration. We have provided extrapolated continuum versions of these spectra to 300 microns, in direct support of ISO (PHT and LWS instruments). These spectra are consistent with IRAS flux densities at 60 and 100 microns.

The STIS CCD Spectroscopic Line Spread Functions

NASA Technical Reports Server (NTRS)

Gull, T.; Lindler, D.; Tennant, D.; Bowers, C.; Grady, C.; Hill, R. S.; Malumuth, E.

2002-01-01

We characterize the spectroscopic line spread functions of the spectroscopic CCD modes for high contrast objects. Our long range goal is to develop tools that accurately extract spectroscopic information of faint, point or extended sources in the vicinity of bright, point sources at separations approaching the realizable angular limits of HST with STIS. Diffracted and scattered light due to the HST optics, and scattered light effects within the STIS are addressed. Filter fringing, CCD fringing, window reflections, and scattering within the detector and other effects are noted. We have obtained spectra of several reference stars, used for flux calibration or for coronagraphic standards, that have spectral distributions ranging from very red to very blue. Spectra of each star were recorded with the star in the aperture and with the star blocked by either the F1 or F2 fiducial. Plots of the detected starlight along the spatial axis of the aperture are provided for four stars. With the star in the aperture, the line spread function is quite noticeable. Placing the star behind one of the fiducials cuts the scattered light and the diffracted light, is detectable even out to 1OOOOA. When the star is placed behind either fiducial, the scattered and diffracted light components, at three arcseconds displacement from the star, are below lop6 the peak of the star at wavelengths below 6000A; at the same angular distance, scattered light does contaminate the background longward of 6000A up to a level of 10(exp -5).

High-resolution multi-band imaging for validation and characterization of small Kepler planets

DOE Office of Scientific and Technical Information (OSTI.GOV)

Everett, Mark E.; Silva, David R.; Barclay, Thomas

2015-02-01

High-resolution ground-based optical speckle and near-infrared adaptive optics images are taken to search for stars in close angular proximity to host stars of candidate planets identified by the NASA Kepler Mission. Neighboring stars are a potential source of false positive signals. These stars also blend into Kepler light curves, affecting estimated planet properties, and are important for an understanding of planets in multiple star systems. Deep images with high angular resolution help to validate candidate planets by excluding potential background eclipsing binaries as the source of the transit signals. A study of 18 Kepler Object of Interest stars hosting amore » total of 28 candidate and validated planets is presented. Validation levels are determined for 18 planets against the likelihood of a false positive from a background eclipsing binary. Most of these are validated at the 99% level or higher, including five newly validated planets in two systems: Kepler-430 and Kepler-431. The stellar properties of the candidate host stars are determined by supplementing existing literature values with new spectroscopic characterizations. Close neighbors of seven of these stars are examined using multi-wavelength photometry to determine their nature and influence on the candidate planet properties. Most of the close neighbors appear to be gravitationally bound secondaries, while a few are best explained as closely co-aligned field stars. Revised planet properties are derived for each candidate and validated planet, including cases where the close neighbors are the potential host stars.« less

Coulomb repulsion and the electron beam directed energy weapon

NASA Astrophysics Data System (ADS)

Retsky, Michael W.

2004-09-01

Mutual repulsion of discrete charged particles or Coulomb repulsion is widely considered to be an ultimate hard limit in charged particle optics. It prevents the ability to finely focus high current beams into small spots at large distances from defining apertures. A classic example is the 1970s era "Star Wars" study of an electron beam directed energy weapon as an orbiting antiballistic missile device. After much analysis, it was considered physically impossible to focus a 1000-amp 1-GeV beam into a 1-cm diameter spot 1000-km from the beam generator. The main reason was that a 1-cm diameter beam would spread to 5-m diameter at 1000-km due to Coulomb repulsion. Since this could not be overcome, the idea was abandoned. But is this true? What if the rays were reversed? That is, start with a 5-m beam converging slightly with the same nonuniform angular and energy distribution as the electrons from the original problem were spreading at 1000-km distance. Could Coulomb repulsion be overcome? Looking at the terms in computational studies, some are reversible while others are not. Based on estimates, the nonreversible terms should be small - of the order of 0.1 mm. If this is true, it is possible to design a practical electron beam directed weapon not limited by Coulomb repulsion.

Highly-evolved stars

NASA Technical Reports Server (NTRS)

Heap, S. R.

1981-01-01

The ways in which the IUE has proved useful in studying highly evolved stars are reviewed. The importance of high dispersion spectra for abundance analyses of the sd0 stars and for studies of the wind from the central star of NGC 6543 and the wind from the 0 type component of Vela X-1 is shown. Low dispersion spectra are used for absolute spectrophotometry of the dwarf nova, Ex Hya. Angular resolution is important for detecting and locating UV sources in globular clusters.

Spatially-Resolved Observations of Giant Stars with SPHERE

NASA Astrophysics Data System (ADS)

Khouri, Theo

2018-04-01

SPHERE on the VLT is an extreme adaptive optics instrument that produces images with unprecedented angular resolution at visible and near-infrared wavelengths. Its primary goal is imaging, low-resolution spectroscopic, and polarimetric characterization of extra-solar planetary systems. Nonetheless, the high spatial resolution and the instrument design optimized for observations in a narrow field of view around bright targets make SPHERE the perfect instrument for obtaining spatially-resolved images of close-by giant, evolved stars. This is particularly true at the shortest wavelengths available with SPHERE, where the angular resolution is best (> 20 mas) and these stars appear larger (< 70 mas). In this talk, I will review how SPHERE has been used to study the surfaces and extended atmospheres of evolved stars and how these observations advance our understanding of the stellar pulsations and convective motions that shape these stars. Moreover, I will present recent results from a monitoring campaign of the star R Doradus using SPHERE with observations taken at twelve epochs over eight months that reveal features on the stellar disc varying on timescales of a few weeks. Finally, I will present quasi-simultaneous observations with SPHERE and ALMA that spatially resolve the stellar discs of two asymptotic giant branch stars, Mira and R Doradus, and discuss what such multi-wavelength observation campaigns can teach us about the processes that shape evolved stars.

Natural guide-star processing for wide-field laser-assisted AO systems

NASA Astrophysics Data System (ADS)

Correia, Carlos M.; Neichel, Benoit; Conan, Jean-Marc; Petit, Cyril; Sauvage, Jean-Francois; Fusco, Thierry; Vernet, Joel D. R.; Thatte, Niranjan

2016-07-01

Sky-coverage in laser-assisted AO observations largely depends on the system's capability to guide on the faintest natural guide-stars possible. Here we give an up-to-date status of our natural guide-star processing tailored to the European-ELT's visible and near-infrared (0.47 to 2.45 μm) integral field spectrograph - Harmoni. We tour the processing of both the isoplanatic and anisoplanatic tilt modes using the spatio-angular approach whereby the wavefront is estimated directly in the pupil plane avoiding a cumbersome explicit layered estimation on the 35-layer profiles we're currently using. Taking the case of Harmoni, we cover the choice of wave-front sensors, the number and field location of guide-stars, the optimised algorithms to beat down angular anisoplanatism and the performance obtained with different temporal controllers under split high-order/low-order tomography or joint tomography. We consider both atmospheric and far greater telescope wind buffeting disturbances. In addition we provide the sky-coverage estimates thus obtained.

A laboratory demonstration of the capability to image an Earth-like extrasolar planet.

PubMed

Trauger, John T; Traub, Wesley A

2007-04-12

The detection and characterization of an Earth-like planet orbiting a nearby star requires a telescope with an extraordinarily large contrast at small angular separations. At visible wavelengths, an Earth-like planet would be 1 x 10(-10) times fainter than the star at angular separations of typically 0.1 arcsecond or less. There are several proposed space telescope systems that could, in principle, achieve this. Here we report a laboratory experiment that reaches these limits. We have suppressed the diffracted and scattered light near a star-like source to a level of 6 x 10(-10) times the peak intensity in individual coronagraph images. In a series of such images, together with simple image processing, we have effectively reduced this to a residual noise level of about 0.1 x 10(-10). This demonstrates that a coronagraphic telescope in space could detect and spectroscopically characterize nearby exoplanetary systems, with the sensitivity to image an 'Earth-twin' orbiting a nearby star.

f-Mode Secular Instabilities in Deleptonizing Fizzlers

NASA Astrophysics Data System (ADS)

Imamura, James N.; Durisen, Richard H.

2004-12-01

Fizzlers are intermediate states that may form between white dwarf and neutron star densities during the collapse of massive rotating stars. This paper studies the gravitational radiation reaction (GRR) driven f-mode secular instabilities of fizzlers with angular momentum distributions h(mc) appropriate to the core collapse of massive rotating stars, where h is the specific angular momentum and mc is the cylindrical mass fraction. For core collapses that maintain axial symmetry, the h(mc) of the remnant reflects the conditions in the precollapse stellar core, and, thus, the h(mc) will resemble that of a uniformly rotating star supported by the pressure of relativistically degenerate electrons. Such an h(mc) concentrates most angular momentum toward the equatorial region of the object. The onset of f-mode secular instabilities in such fizzlers is affected strongly by the h(mc), whereas instability depends only weakly on compressibility. For a broad range of fizzler equations of state and the core h(mc), the f-mode secular instability thresholds drop to T/W~0.034-0.042, 0.019-0.021, and 0.012-0.0135, for m=2, 3, and 4, respectively. These same thresholds with the Maclaurin spheroid h(mc) are T/W=0.13-0.15, 0.10-0.11, and 0.08-0.09, respectively. The growth times τgw for GRR-driven m=2 modes are long. For fizzlers with specific angular momentum J/M~1.5×1016 cm2 s-1 and T/W<~0.24 (ρc<~1014 g cm-3), τgw>400 s. For these fizzlers, τgw>>τde, the deleptonization timescale, and GRR-driven secular instabilities will not grow along a deleptonizing fizzler sequence except, possibly, at T/W near the dynamic bar mode instability threshold, T/W~0.27.

Gravitational waves from neutron star excitations in a binary inspiral

NASA Astrophysics Data System (ADS)

Parisi, Alessandro; Sturani, Riccardo

2018-02-01

In the context of a binary inspiral of mixed neutron star-black hole systems, we investigate the excitation of the neutron star oscillation modes by the orbital motion. We study generic eccentric orbits and show that tidal interaction can excite the f -mode oscillations of the star by computing the amount of energy and angular momentum deposited into the star by the orbital motion tidal forces via closed form analytic expressions. We study the f -mode oscillations of cold neutron stars using recent microscopic nuclear equations of state, and we compute their imprint into the emitted gravitational waves.

The origin of ultra-compact binaries

NASA Technical Reports Server (NTRS)

Hachisu, Izumi; Miyaji, Shigeki; Saio, Hideyuki

1987-01-01

The origin of ultra-compact binaries composed of a neutron star and a low-mass (about 0.06 solar mass) white dwarf is considered. Taking account of the systemic losses of mass and angular momentum, it was found that a serious difficulty exists in the scenarios which involve tidal captures of a normal star (a main sequence star or a red giant) by a neutron star. This difficulty can be avoided if a red giant star is captured by a massive white dwarf (M is approx. greater than 1.2 solar masses), which becomes a neutron star through the accretion induced collapse.

A near-infrared interferometric survey of debris-disc stars. VI. Extending the exozodiacal light survey with CHARA/JouFLU

NASA Astrophysics Data System (ADS)

Nuñez, P. D.; Scott, N. J.; Mennesson, B.; Absil, O.; Augereau, J.-C.; Bryden, G.; ten Brummelaar, T.; Ertel, S.; Coudé du Foresto, V.; Ridgway, S. T.; Sturmann, J.; Sturmann, L.; Turner, N. J.; Turner, N. H.

2017-12-01

We report the results of high-angular-resolution observations that search for exozodiacal light in a sample of main sequence stars and sub-giants. Using the "jouvence" of the fiber linked unit for optical recombination (JouFLU) at the center for high angular resolution astronomy (CHARA) telescope array, we have observed a total of 44 stars. Out of the 44 stars, 33 are new stars added to the initial, previously published survey of 42 stars performed at CHARA with the fiber linked unit for optical recombination (FLUOR). Since the start of the survey extension, we have detected a K-band circumstellar excess for six new stars at the 1% level or higher, four of which are known or candidate binaries, and two for which the excess could be attributed to exozodiacal dust. We have also performed follow-up observations of 11 of the stars observed in the previously published survey and found generally consistent results. We do however detect a significantly larger excess on three of these follow-up targets: Altair, υ And and κ CrB. Interestingly, the last two are known exoplanet host stars. We perform a statistical analysis of the JouFLU and FLUOR samples combined, which yields an overall exozodi detection rate of . We also find that the K-band excess in FGK-type stars correlates with the existence of an outer reservoir of cold (≲100 K) dust at the 99% confidence level, while the same cannot be said for A-type stars.

Planet Formation in Disks with Inclined Binary Companions: Can Primordial Spin-Orbit Misalignment be Produced?

NASA Astrophysics Data System (ADS)

Zanazzi, J. J.; Lai, Dong

2018-04-01

Many hot Jupiter (HJ) systems have been observed to have their stellar spin axis misaligned with the planet's orbital angular momentum axis. The origin of this spin-orbit misalignment and the formation mechanism of HJs remain poorly understood. A number of recent works have suggested that gravitational interactions between host stars, protoplanetary disks, and inclined binary companions may tilt the stellar spin axis with respect to the disk's angular angular momentum axis, producing planetary systems with misaligned orbits. These previous works considered idealized disk evolution models and neglected the gravitational influence of newly formed planets. In this paper, we explore how disk photoevaporation and planet formation and migration affect the inclination evolution of planet-star-disk-binary systems. We take into account planet-disk interactions and the gravitational spin-orbit coupling between the host star and the planet. We find that the rapid depletion of the inner disk via photoevaporation reduces the excitation of stellar obliquities. Depending on the formation and migration history of HJs, the spin-orbit coupling between the star and the planet may reduces and even completely suppress the excitation of stellar obliquities. Our work constrains the formation/migration history of HJs. On the other hand, planetary systems with "cold" Jupiters or close-in super-earths may experience excitation of stellar obliquities in the presence of distant inclined companions.

Planet formation in discs with inclined binary companions: can primordial spin-orbit misalignment be produced?

NASA Astrophysics Data System (ADS)

Zanazzi, J. J.; Lai, Dong

2018-07-01

Many hot Jupiter (HJ) systems have been observed to have their stellar spin axis misaligned with the planet's orbital angular momentum axis. The origin of this spin-orbit misalignment and the formation mechanism of HJs remain poorly understood. A number of recent works have suggested that gravitational interactions between host stars, protoplanetary discs, and inclined binary companions may tilt the stellar spin axis with respect to the disc's angular angular momentum axis, producing planetary systems with misaligned orbits. These previous works considered idealized disc evolution models and neglected the gravitational influence of newly formed planets. In this paper, we explore how disc photoevaporation and planet formation and migration affect the inclination evolution of planet-star-disc-binary systems. We take into account planet-disc interactions and the gravitational spin-orbit coupling between the host star and the planet. We find that the rapid depletion of the inner disc via photoevaporation reduces the excitation of stellar obliquities. Depending on the formation and migration history of HJs, the spin-orbit coupling between the star and the planet may reduces and even completely suppress the excitation of stellar obliquities. Our work constrains the formation/migration history of HJs. On the other hand, planetary systems with `cold' Jupiters or close-in super-earths may experience excitation of stellar obliquities in the presence of distant inclined companions.

Magnetocentrifugally driven flows from young stars and disks. 1: A generalized model

NASA Technical Reports Server (NTRS)

Shu, Frank; Najita, Joan; Ostriker, Eve; Wilkin, Frank; Ruden, Steven; Lizano, Susana

1994-01-01

We propose a generalized model for stellar spin-down, disk accretion, and truncation, and the origin of winds, jets, and bipolar outflows from young stellar objects. We consider the steady state dynamics of accretion of matter from a viscous and imperfectly conducting disk onto a young star with a strong magnetic field. For an aligned stellar magnetosphere, shielding currents in the surface layers of the disk prevent stellar field lines from penetrating the disk everywhere except for a range of radii about pi = R(sub x), where the Keplerian angular speed of rotation Omega(sub x) equals the angular speed of the star Omega(sub *). For the low disk accretion rates and high magnetic fields associated with typical T Tauri stars, R(sub x) exceeds the radius of the star R(sub *) by a factor of a few, and the inner disk is effectively truncated at a radius R(sub t) somewhat smaller than R(sub x). Where the closed field lines between R(sub t) and R(sub x) bow sufficiently inward, the accreting gas attaches itself to the field and is funneled dynamically down the effective potential (gravitational plus centrifugal) onto the star. Contrary to common belief, the accompanying magnetic torques associated with this accreting gas may transfer angular momentum mostly to the disk rather than to the star. Thus, the star can spin slowly as long as R(sub x) remains significantly greater than R(sub *). Exterior to R(sub x) field lines threading the disk bow outward, which makes the gas off the mid-plane rotate at super-Keplerian velocities. This combination drives a magnetocentrifugal wind with a mass-loss rate M(sub w) equal to a definite fraction f of the disk accretion rate M(sub D). For high disk accretion rates, R(sub x) is forced down to the stellar surface, the star is spun to breakup, and the wind is generated in a manner identical to that proposed by Shu, Lizano, Ruden, & Najita in a previous communication to this journal. In two companion papers (II and III), we develop a detailed but idealized theory of the magnetocentrifugal acceleration process.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Dam, Dick van, E-mail: a.d.v.dam@tue.nl; Haverkort, Jos E. M.; Abujetas, Diego R.

The emission from nanowires can couple to waveguide modes supported by the nanowire geometry, thus governing the far-field angular pattern. To investigate the geometry-induced coupling of the emission to waveguide modes, we acquire Fourier microscopy images of the photoluminescence of nanowires with diameters ranging from 143 to 208 nm. From the investigated diameter range, we conclude that a few nanometers difference in diameter can abruptly change the coupling of the emission to a specific mode. Moreover, we observe a diameter-dependent width of the Gaussian-shaped angular pattern in the far-field emission. This dependence is understood in terms of interference of the guidedmore » modes, which emit at the end facets of the nanowire. Our results are important for the design of quantum emitters, solid state lighting, and photovoltaic devices based on nanowires.« less

Magnetic Inclination E Ects In Star-Planet Magnetic Interactions

NASA Astrophysics Data System (ADS)

Strugarek, Antoine

2017-10-01

A large fraction of the exoplanets discovered today are in a close-in orbit around their host star. This proximity allows them to be magnetically connected to their host, which lead to e cient energy and angular momentum exchanges between the star and the planet. We carry out three-dimensional magneto-hydrodynamic simulations of close-in star-planet systems to characterize the e ect of the inclination of the planetary magnetic eld on the star-planet magnetic interaction. We parametrize this e ect in scaling laws depending on the star, planet, and stellar wind properties that can be applied to any exoplanetary systems around cool stars.

Scalar Resonant Relaxation of Stars around a Massive Black Hole

NASA Astrophysics Data System (ADS)

Bar-Or, Ben; Fouvry, Jean-Baptiste

2018-06-01

In nuclear star clusters, the potential is governed by the central massive black hole (MBH), so that stars move on nearly Keplerian orbits and the total potential is almost stationary in time. Yet, the deviations of the potential from the Keplerian one, due to the enclosed stellar mass and general relativity, will cause the stellar orbits to precess. Moreover, as a result of the finite number of stars, small deviations of the potential from spherical symmetry induce residual torques that can change the stars’ angular momentum faster than the standard two-body relaxation. The combination of these two effects drives a stochastic evolution of orbital angular momentum, a process named “resonant relaxation” (RR). Owing to recent developments in the description of the relaxation of self-gravitating systems, we can now fully describe scalar resonant relaxation (relaxation of the magnitude of the angular momentum) as a diffusion process. In this framework, the potential fluctuations due to the complex orbital motion of the stars are described by a random correlated noise with statistical properties that are fully characterized by the stars’ mean field motion. On long timescales, the cluster can be regarded as a diffusive system with diffusion coefficients that depend explicitly on the mean field stellar distribution through the properties of the noise. We show here, for the first time, how the diffusion coefficients of scalar RR, for a spherically symmetric system, can be fully calculated from first principles, without any free parameters. We also provide an open source code that evaluates these diffusion coefficients numerically.

Cyclic and secular variation in the temperatures and radii of extreme helium stars

NASA Astrophysics Data System (ADS)

Jeffery, C. Simon; Starling, Rhaana L. C.; Hill, Philip W.; Pollacco, Don

2001-02-01

The ultraviolet properties of 17 extreme helium stars have been examined using 150 IUE spectra. Combining short-wave and long-wave image pairs and using a grid of hydrogen-deficient model atmospheres and a χ2 minimization procedure, 70 measurements of effective temperature (Teff), angular diameters (θ) and interstellar extinction (EB_V) were obtained. In most cases, these were in good agreement with previous measurements, but there are some ambiguities in the case of the hotter stars, where the solutions for Teff and EB_V become degenerate, and in the case of the cooler stars with large EB_V, where the total flux is no longer dominated by the ultraviolet. The behaviour of 12 helium stars was examined over an interval exceeding 10yr. The surfaces of four stars (HD 168476, HD 160641, BD -9°4395 and BD -1°3438) were found to be heating at rates between 20 and 120Kyr-1, in remarkable agreement with theoretical predictions. This result provides the first direct evidence that extreme helium stars are helium shell-burning stars of up to ~0.9Msolar contracting towards the white dwarf sequence. Low-luminosity helium stars do not show a detectable contraction, also in agreement with theory, although one, BD +10°2179, may be expanding. The short-term behaviour of three variable helium stars (PV Tel variables: HD 168476, BD +1°4381, LSIV -1°2) was examined over a short interval in 1995. All three showed changes in Teff and θ on periods consistent with previous observations. Near-simultaneous radial velocity (v) measurements were used to establish the total change in radius, with some reservations concerning the adopted periods. Subsequently, measurements of the stellar radii and distances could be derived. With Teff and surface gravities established previously, stellar luminosities and masses were thus obtained directly from observation. In the case of HD 168476, the mass is 0.94 ± 0.68 M\\odot. Assuming a similar gravity for LSIV -1°2 based on its neutral helium line profiles, its mass becomes 0.79 ± 0.46 M\\odot. The θ amplitude for BD +1°4381 appears to be overestimated by the IUE measurements and leads to a nonsensical result. These first direct measurements of luminous extreme helium star masses agree well with previous estimates from stellar structure and pulsation theory.

Inefficient Angular Momentum Transport in Accretion Disk Boundary Layers: Angular Momentum Belt in the Boundary Layer

NASA Astrophysics Data System (ADS)

Belyaev, Mikhail A.; Quataert, Eliot

2018-04-01

We present unstratified 3D MHD simulations of an accretion disk with a boundary layer (BL) that have a duration ˜1000 orbital periods at the inner radius of the accretion disk. We find the surprising result that angular momentum piles up in the boundary layer, which results in a rapidly rotating belt of accreted material at the surface of the star. The angular momentum stored in this belt increases monotonically in time, which implies that angular momentum transport mechanisms in the BL are inefficient and do not couple the accretion disk to the star. This is in spite of the fact that magnetic fields are advected into the BL from the disk and supersonic shear instabilities in the BL excite acoustic waves. In our simulations, these waves only carry a small fraction (˜10%) of the angular momentum required for steady state accretion. Using analytical theory and 2D viscous simulations in the R - ϕ plane, we derive an analytical criterion for belt formation to occur in the BL in terms of the ratio of the viscosity in the accretion disk to the viscosity in the BL. Our MHD simulations have a dimensionless viscosity (α) in the BL that is at least a factor of ˜100 smaller than that in the disk. We discuss the implications of these results for BL dynamics and emission.

Protomagnetar and black hole formation in high-mass stars

NASA Astrophysics Data System (ADS)

Obergaulinger, M.; Aloy, M. Á.

2017-07-01

Using axisymmetric simulations coupling special relativistic magnetohydrodynamics (MHD), an approximate post-Newtonian gravitational potential and two-moment neutrino transport, we show different paths for the formation of either protomagnetars or stellar mass black holes. The fraction of prototypical stellar cores which should result in collapsars depends on a combination of several factors, among which the structure of the progenitor star and the profile of specific angular momentum are probably the foremost. Along with the implosion of the stellar core, we also obtain supernova-like explosions driven by neutrino heating and hydrodynamic instabilities or by magneto-rotational effects in cores of high-mass stars. In the latter case, highly collimated, mildly relativistic outflows are generated. We find that after a rather long post-collapse phase (lasting ≳1 s) black holes may form in cases both of successful and failed supernova-like explosions. A basic trend is that cores with a specific angular momentum smaller than that obtained by standard, one-dimensional stellar evolution calculations form black holes (and eventually collapsars). Complementary, protomagnetars result from stellar cores with the standard distribution of specific angular momentum obtained from prototypical stellar evolution calculations including magnetic torques and moderate to large mass-loss rates.

Particle flow within a transonic compressor rotor passage with application to laser-Doppler velocimetry

NASA Technical Reports Server (NTRS)

Maxwell, B. R.

1975-01-01

A theoretical analysis was conducted of the dynamic behavior of micron size particles moving in the three-dimensional flow field of a rotating transonic axial-flow air compressor rotor. The particle velocity lag and angular deviation relative to the gas were determined as functions of particle diameter, mass density and radial position. Particle size and density were varied over ranges selected to correspond to typical laser-Doppler velocimeter (LDV) flow field mapping applications. It was found that the particles move essentially on gas stream surfaces and that particle tracking is relatively insensitive to the rotor radial coordinate. Velocity lag and angular deviation increased whenever particle size or mass density increased, and particle tracking was more sensitive to a change in particle diameter than to a corresponding change in mass density. Results indicated that velocity and angular deviations generally less than 1 percent and 1 degree could be achieved with 1 gm/cc tracer particles with diameters of 1 micron or less.

Strömgren survey for asteroseismology and galactic archaeology: Let the saga begin

DOE Office of Scientific and Technical Information (OSTI.GOV)

Casagrande, L.; Dotter, A.; Milone, A. P.

2014-06-01

Asteroseismology has the capability of precisely determining stellar properties that would otherwise be inaccessible, such as radii, masses, and thus ages of stars. When coupling this information with classical determinations of stellar parameters, such as metallicities, effective temperatures, and angular diameters, powerful new diagnostics for Galactic studies can be obtained. The ongoing Strömgren survey for Asteroseismology and Galactic Archaeology has the goal of transforming the Kepler field into a new benchmark for Galactic studies, similar to the solar neighborhood. Here we present the first results from a stripe centered at a Galactic longitude of 74° and covering latitude from aboutmore » 8° to 20°, which includes almost 1000 K giants with seismic information and the benchmark open cluster NGC 6819. We describe the coupling of classical and seismic parameters, the accuracy as well as the caveats of the derived effective temperatures, metallicities, distances, surface gravities, masses, and radii. Confidence in the achieved precision is corroborated by the detection of the first and secondary clumps in a population of field stars with a ratio of 2 to 1 and by the negligible scatter in the seismic distances among NGC 6819 member stars. An assessment of the reliability of stellar parameters in the Kepler Input Catalog is also performed, and the impact of our results for population studies in the Milky Way is discussed, along with the importance of an all-sky Strömgren survey.« less

Cosmic infrared background measurements and star formation history from Planck

NASA Astrophysics Data System (ADS)

Serra, Paolo; Serra

2014-05-01

We present new measurements of Cosmic Infrared Background (CIB) anisotropies using Planck. Combining HFI data with IRAS, the angular auto- and cross-frequency power spectrum is measured from 143 to 3000 GHz. After careful removal of the contaminants (cosmic microwave background anisotropies, Galactic dust and Sunyaev-Zeldovich emission), and a complete study of systematics, the CIB power spectrum is measured with unprecedented signal to noise ratio from angular multipoles l ~ 150 to 2500. The interpretation based on the halo model is able to associate star-forming galaxies with dark matter halos and their subhalos, using a parametrized relation between the dust-processed infrared luminosity and (sub-)halo mass, and it allows to simultaneously fit all auto- and cross- power spectra very well. We find that the star formation history is well constrained up to redshifts around 2, and agrees with recent estimates of the obscured star-formation density using Spitzer and Herschel. However, at higher redshift, the accuracy of the star formation history measurement is strongly degraded by the uncertainty in the spectral energy distribution of CIB galaxies. We also find that the mean halo mass which is most efficient at hosting star formation is log(M eff/M ⊙) = 12.6 and that CIB galaxies have warmer temperatures as redshift increases.

Detecting binary neutron star systems with spin in advanced gravitational-wave detectors

NASA Astrophysics Data System (ADS)

Brown, Duncan A.; Harry, Ian; Lundgren, Andrew; Nitz, Alexander H.

2012-10-01

The detection of gravitational waves from binary neutron stars is a major goal of the gravitational-wave observatories Advanced LIGO and Advanced Virgo. Previous searches for binary neutron stars with LIGO and Virgo neglected the component stars’ angular momentum (spin). We demonstrate that neglecting spin in matched-filter searches causes advanced detectors to lose more than 3% of the possible signal-to-noise ratio for 59% (6%) of sources, assuming that neutron star dimensionless spins, cJ/GM2, are uniformly distributed with magnitudes between 0 and 0.4 (0.05) and that the neutron stars have isotropically distributed spin orientations. We present a new method for constructing template banks for gravitational-wave searches for systems with spin. We present a new metric in a parameter space in which the template placement metric is globally flat. This new method can create template banks of signals with nonzero spins that are (anti-)aligned with the orbital angular momentum. We show that this search loses more than 3% of the maximum signal-to-noise for only 9% (0.2%) of binary neutron star sources with dimensionless spins between 0 and 0.4 (0.05) and isotropic spin orientations. Use of this template bank will prevent selection bias in gravitational-wave searches and allow a more accurate exploration of the distribution of spins in binary neutron stars.

Constraints on the Energy Content of the Universe from a Combination of Galaxy Cluster Observables

NASA Technical Reports Server (NTRS)

Molnar, Sandor M.; Haiman, Zoltan; Birkinshaw, Mark; Mushotzky, Richard F.

2003-01-01

We demonstrate that constraints on cosmological parameters from the distribution of clusters as a function of redshift (dN/dz) are complementary to accurate angular diameter distance (D(sub A)) measurements to clusters, and their combination significantly tightens constraints on the energy density content of the Universe. The number counts can be obtained from X-ray and/or SZ (Sunyaev-Ze'dovich effect) surveys, and the angular diameter distances can be determined from deep observations of the intra-cluster gas using their thermal bremsstrahlung X-ray emission and the SZ effect. We combine constraints from simulated cluster number counts expected from a 12 deg(sup 2) SZ cluster survey and constraints from simulated angular diameter distance measurements based on the X-ray/SZ method assuming a statistical accuracy of 10% in the angular diameter distance determination of 100 clusters with redshifts less than 1.5. We find that Omega(sub m), can be determined within about 25%, Omega(sub lambda) within 20% and w within 16%. We show that combined dN/dz+(sub lambda) constraints can be used to constrain the different energy densities in the Universe even in the presence of a few percent redshift dependent systematic error in D(sub lambda). We also address the question of how best to select clusters of galaxies for accurate diameter distance determinations. We show that the joint dN/dz+ D(lambda) constraints on cosmological parameters for a fixed target accuracy in the energy density parameters are optimized by selecting clusters with redshift upper cut-offs in the range 0.55 approx. less than 1. Subject headings: cosmological parameters - cosmology: theory - galaxies:clusters: general

Multi-epoch VLTI-PIONIER imaging of the supergiant V766 Cen

NASA Astrophysics Data System (ADS)

Wittkowski, M.; Abellán, F. J.; Arroyo-Torres, B.; Chiavassa, A.; Guirado, J. C.; Marcaide, J. M.; Alberdi, A.; de Wit, W. J.; Hofmann, K.-H.; Meilland, A.; Millour, F.; Mohamed, S.; Sanchez-Bermudez, J.

2017-09-01

Context. The star V766 Cen (=HR 5171A) was originally classified as a yellow hypergiant but lately found to more likely be a 27-36 M⊙ red supergiant (RSG). Recent observations indicated a close eclipsing companion in the contact or common-envelope phase. Aims: Here, we aim at imaging observations of V766 Cen to confirm the presence of the close companion. Methods: We used near-infrared H-band aperture synthesis imaging at three epochs in 2014, 2016, and 2017, employing the PIONIER instrument at the Very Large Telescope Interferometer (VLTI). Results: The visibility data indicate a mean Rosseland angular diameter of 4.1 ± 0.8 mas, corresponding to a radius of 1575 ± 400 R⊙. The data show an extended shell (MOLsphere) of about 2.5 times the Rosseland diameter, which contributes about 30% of the H-band flux. The reconstructed images at the 2014 epoch show a complex elongated structure within the photospheric disk with a contrast of about 10%. The second and third epochs show qualitatively and quantitatively different structures with a single very bright and narrow feature and high contrasts of 20-30%. This feature is located toward the south-western limb of the photospheric stellar disk. We estimate an angular size of the feature of 1.7 ± 0.3 mas, corresponding to a radius of 650 ± 150 R⊙, and giving a radius ratio of 0.42+0.35-0.10 compared to the primary stellar disk. Conclusions: We interpret the images at the 2016 and 2017 epochs as showing the close companion, or a common envelope toward the companion, in front of the primary. At the 2014 epoch, the close companion is behind the primary and not visible. Instead, the structure and contrast at the 2014 epoch are typical of a single RSG harboring giant photospheric convection cells. The companion is most likely a cool giant or supergiant star with a mass of 5+15-3 M⊙. Based on observations made with the VLT Interferometer at Paranal Observatory under programme IDs 092.D-0096, 092.C-0312, and 097.D-0286.Olivier Chesneau was PI of the program 092.D-0096. He unfortunately passed away before seeing the results coming out of it. This Letter may serve as a posthumous tribute to his inspiring work on this source.

Incompressible Modes Excited by Supersonic Shear in Boundary Layers: Acoustic CFS Instability

NASA Astrophysics Data System (ADS)

Belyaev, Mikhail A.

2017-02-01

We present an instability for exciting incompressible modes (e.g., gravity or Rossby modes) at the surface of a star accreting through a boundary layer. The instability excites a stellar mode by sourcing an acoustic wave in the disk at the boundary layer, which carries a flux of energy and angular momentum with the opposite sign as the energy and angular momentum density of the stellar mode. We call this instability the acoustic Chandrasekhar-Friedman-Schutz (CFS) instability, because of the direct analogy to the CFS instability for exciting modes on a rotating star by emission of energy in the form of gravitational waves. However, the acoustic CFS instability differs from its gravitational wave counterpart in that the fluid medium in which the acoustic wave propagates (I.e., the accretion disk) typically rotates faster than the star in which the incompressible mode is sourced. For this reason, the instability can operate even for a non-rotating star in the presence of an accretion disk. We discuss applications of our results to high-frequency quasi-periodic oscillations in accreting black hole and neutron star systems and dwarf nova oscillations in cataclysmic variables.

Incompressible Modes Excited by Supersonic Shear in Boundary Layers: Acoustic CFS Instability

DOE Office of Scientific and Technical Information (OSTI.GOV)

Belyaev, Mikhail A., E-mail: mbelyaev@berkeley.edu

We present an instability for exciting incompressible modes (e.g., gravity or Rossby modes) at the surface of a star accreting through a boundary layer. The instability excites a stellar mode by sourcing an acoustic wave in the disk at the boundary layer, which carries a flux of energy and angular momentum with the opposite sign as the energy and angular momentum density of the stellar mode. We call this instability the acoustic Chandrasekhar–Friedman–Schutz (CFS) instability, because of the direct analogy to the CFS instability for exciting modes on a rotating star by emission of energy in the form of gravitationalmore » waves. However, the acoustic CFS instability differs from its gravitational wave counterpart in that the fluid medium in which the acoustic wave propagates (i.e., the accretion disk) typically rotates faster than the star in which the incompressible mode is sourced. For this reason, the instability can operate even for a non-rotating star in the presence of an accretion disk. We discuss applications of our results to high-frequency quasi-periodic oscillations in accreting black hole and neutron star systems and dwarf nova oscillations in cataclysmic variables.« less

The Death Spiral of the Hot Jupiter Exoplanet HD 189733b

NASA Astrophysics Data System (ADS)

Dowling Jones, Liam; Marchioni, Lucas; Guinan, Edward; Engle, Scott

2018-01-01

HD 189733 is a quintessential example of hot Jupiter-type exoplanet systems in which a gas giant planet with a mass similar to Jupiter is orbiting extremely close to its host star. HD 189733 is the nearest and brightest hot Jupiter system discovered so far and undergoes transit eclipses. Because of this, HD 189733 is well studied across the electromagnetic spectrum. It consists of a 7.7 mag K1.5 V host star and a Jupiter-size planet orbiting with a period of P =2.22 days, only located only 0.030 AU from its host star.About ten years ago HD 189733 system was discovered to be accompanied by gravitationally-bound red dwarf M4 V star companion (HD 189733 B). It was found previously by Guinan et al. (2017) that the age measurement (~0.7 Gyr) of the K-type star indicated by its 11.95 day rotation period and corresponding moderately high levels of coronal X-ray and chromospheric emissions do not agree with the much older age of ~6 - 9 Gyr indicated from the low X-ray activity of the dM companion star. This age discrepancy is can be resolved by assuming an increase in angular momentum or “spin-up” of the HD 189733A by its hosted planet. It is probable is that this extra angular momentum was acquired from the orbiting exoplanet from the tidal and magnetic interactions of the planet and host star.Photometric observations of the planetary transit eclipses of HD 189733b have been carried out for over 11 years. Using new transit timings that we have obtained with the 1.3-m Robotically Controlled Telescope (RCT) when combined with numerous timings available in the literature, we have discovered a very small decrease in the orbital period of the HD 189733b. The change in period is dP/dt = 0.87 sec/100 yrs. This finding support the transfer of orbital angular momentum of the planet to the host star - thus spinning-up the host star and shrinking the orbit of the planet. At this rate of period decrease, the planet will be tidally disrupted in less than 40 million years. However, this planetary disruption will likely occur much sooner because the decrease in the planet’s orbital period is expected to speed-up as the planet gets closer to the star.

Rotation of low-mass stars - A new probe of stellar evolution

NASA Technical Reports Server (NTRS)

Pinsonneault, M. H.; Kawaler, Steven D.; Demarque, P.

1990-01-01

Models of stars of various masses and rotational parameters were developed and compared with observations of stars in open clusters of various ages in order to analyze the evolution of rotating stars from the early premain sequence to an age of 1.7 x 10 to the 9th yrs. It is shown that, for stars older than 10 to the 8th yrs and less massive than 1.1 solar mass, the surface rotation rates depend most strongly on the properties of the angular momentum loss. The trends of the currently available observations suggest that the rotation periods are a good indicator of the field-star ages.

Hubble Space Telescope Fine Guidance Sensor interferometric observations of the core of 30 doradus

NASA Technical Reports Server (NTRS)

Lattanzi, M. G.; Hershey, J. L.; Burg, R.; Taff, L. G.; Holfeltz, S. T.; Bucciarelli, B.; Evans, I. N.; Gilmozzi, R.; Pringle, J.; Walborn, N. R.

1994-01-01

We present the results of the first high angular resolution observations taken with a Fine Guidance Sensor (FGS) aboard the Hubble Space Telescope (HST) of a star cluster embedded in very bright background. The strong and complex background around the R136 cluster in the 30 Dor nebula does not prevent the FGS from achieving performance close to its angular resolution limit of approximately 0.015 sec per axis with reliable photometry. These FGS observations establish that the central object in R136a is a triple star with the third component delta V = 1.1 mag fainter than the primary star al approximately 0.08 sec way. We estimate from the grid of models of Maeder (1990) that the present mass of al is between 30 and 80 solar masses, with the main-sequence progenitor between 60 and 120 solar masses.

MASSIM, the Milli-Arc-Second Structure Imager

NASA Technical Reports Server (NTRS)

Skinner, Gerry

2008-01-01

The MASSIM (Milli-Arc-Second Structure Imager) mission will use a set of achromatic diffractive-refractive Fresnel lenses to achieve imaging in the X-ray band with unprecedented angular resolution. It has been proposed for study within the context of NASA's "Astrophysics Strategic Mission Concept Studies" program. Lenses on an optics spacecraft will focus 5-11 keV X-rays onto detectors on a second spacecraft flying in formation 1000 km away. It will have a point-source sensitivity comparable with that of the current generation of major X-ray observatories (Chandra, XMM-Newton) but an angular resolution some three orders of magnitude better. MASSIM is optimized for the study of jets and other phenomena that occur in the immediate vicinity of black holes and neutron stars. It can also be used for studying other phenomena on the milli-arc-second scale, such as those involving proto-stars, the surfaces and surroundings of nearby active stars and interacting winds.

Spatially resolving the outer atmosphere of the M giant BK Virginis in the CO first overtone lines with VLTI/AMBER

NASA Astrophysics Data System (ADS)

Ohnaka, K.; Hofmann, K.-H.; Schertl, D.; Weigelt, G.; Malbet, F.; Massi, F.; Meilland, A.; Stee, Ph.

2012-01-01

Context. The mass-loss mechanism in normal K-M giant stars with small variability amplitudes is not yet understood, although the majority among red giant stars are precisely of this type. Aims: We present high-spatial and high-spectral resolution observations of the M7 giant BK Vir with AMBER at the Very Large Telescope Interferometer (VLTI). Our aim is to probe the physical properties of the outer atmosphere by spatially resolving the star in the individual CO first overtone lines. Methods: BK Vir was observed between 2.26 and 2.31 μm using the 16-32-48 m telescope configuration with an angular resolution of 9.8 mas and a spectral resolution of 12 000. Results: The uniform-disk diameters observed in the CO first overtone lines are 12 - 31% larger than those measured in the continuum. We also detected asymmetry in the CO line-forming region, which manifests itself as non-zero/non-π differential and closure phases. The data taken 1.5 months apart show possible time variation on a spatial scale of 30 mas (corresponding to 3 × stellar diameter) at the CO band head. Comparison of the observed data with the MARCS photospheric model shows that whereas the observed CO line spectrum can be satisfactorily reproduced by the model, the angular sizes observed in the CO lines are much larger than predicted by the model. Our model with two additional CO layers above the MARCS photosphere reproduces the observed spectrum and interferometric data in the CO lines simultaneously. This model suggests that the inner CO layer at ~1.2 R⋆ is very dense and warm with a CO column density of ~1022 cm-2 and temperatures of 1900 - 2100 K, while the outer CO layer at 2.5-3.0 R⋆ is characterized by column densities of 1019-1020 cm-2 and temperatures of 1500 - 2100 K. Conclusions: Our AMBER observations of BK Vir have spatially resolved the extended molecular outer atmosphere of a normal M giant in the individual CO lines for the first time. The temperatures derived for the CO layers are higher than, or equal to, the uppermost layer of the MARCS photospheric model, implying the operation of some heating mechanism in the outer atmosphere. This study also illustrates that testing photospheric models only with the spectra of strong molecular or atomic features can be misleading. Based on AMBER observations made with the Very Large Telescope Interferometer of the European Southern Observatory. Program ID: 081.D-0233(A) (AMBER Guaranteed Time Observation).

Crustal entrainment and pulsar glitches.

PubMed

Chamel, N

2013-01-04

Large pulsar frequency glitches are generally interpreted as sudden transfers of angular momentum between the neutron superfluid permeating the inner crust and the rest of the star. Despite the absence of viscous drag, the neutron superfluid is strongly coupled to the crust due to nondissipative entrainment effects. These effects are shown to severely limit the maximum amount of angular momentum that can possibly be transferred during glitches. In particular, it is found that the glitches observed in the Vela pulsar require an additional reservoir of angular momentum.

Optical design of the STAR-X telescope

NASA Astrophysics Data System (ADS)

Saha, Timo T.; Zhang, William W.; McClelland, Ryan S.

2017-08-01

Top-level science objectives of the Survey and Time-domain Astrophysical Research eXplorer (STAR-X) include: investigations of most violent explosions in the universe, study of growth of black holes across cosmic time and mass scale, and measure how structure formation heats majority of baryons in the universe. To meet these objectives, the STAR-X telescope requires a field of view of about 1 square-degree, an angular resolution of 5 arc-seconds or better across large part of the field of view. The on-axis effective area at 1 keV should be about 2,000 cm2 . Payload cost and launch considerations limit the outer diameter, focal length, and mass to 1.3 meters, 5 meters, and 250 kilograms, respectively. Telescope design is based on a segmented meta-shell approach we have developed at Goddard Space Flight Center. The telescope mirror shells are divided into segments. Individual shells are nested inside each other to meet the effective area requirements in 0.5 - 6.0 keV range. We consider Wolter-Schwarzschild, and Modified-WolterSchwarzschild telescopes. These designs offer an excellent PSF over a large field of view. Nested shells are vulnerable to stray light problems. We have designed a multi-component baffle system to eliminate direct and single-reflection light paths inside the mirror assembly. Large numbers of internal and external baffles are required to prevent stray rays from reaching the focal plane. We have developed a simple ray-trace tool to determine the dimensions and locations of the baffles. In this paper, we present the results of our trade studies, baffle design studies, and optical performance analyses of the STAR-X telescope.

Dynamo magnetic field-induced angular momentum transport in protostellar nebulae - The 'minimum mass' protosolar nebula

NASA Technical Reports Server (NTRS)

Stepinski, T. F.; Levy, E. H.

1990-01-01

Magnetic torques can produce angular momentum redistribution in protostellar nebulas. Dynamo magnetic fields can be generated in differentially rotating and turbulent nebulas and can be the source of magnetic torques that transfer angular momentum from a protostar to a disk, as well as redistribute angular momentum within a disk. A magnetic field strength of 100-1000 G is needed to transport the major part of a protostar's angular momentum into a surrounding disk in a time characteristic of star formation, thus allowing formation of a solar-system size protoplanetary nebula in the usual 'minimum-mass' model of the protosolar nebula. This paper examines the possibility that a dynamo magnetic field could have induced the needed angular momentum transport from the proto-Sun to the protoplanetary nebula.

The Angular Momentum Distribution and Baryon Content of Star-forming Galaxies at z ˜ 1-3

NASA Astrophysics Data System (ADS)

Burkert, A.; Förster Schreiber, N. M.; Genzel, R.; Lang, P.; Tacconi, L. J.; Wisnioski, E.; Wuyts, S.; Bandara, K.; Beifiori, A.; Bender, R.; Brammer, G.; Chan, J.; Davies, R.; Dekel, A.; Fabricius, M.; Fossati, M.; Kulkarni, S.; Lutz, D.; Mendel, J. T.; Momcheva, I.; Nelson, E. J.; Naab, T.; Renzini, A.; Saglia, R.; Sharples, R. M.; Sternberg, A.; Wilman, D.; Wuyts, E.

2016-08-01

We analyze the angular momenta of massive star-forming galaxies (SFGs) at the peak of the cosmic star formation epoch (z ˜ 0.8-2.6). Our sample of ˜360 log(M */M ⊙) ˜ 9.3-11.8 SFGs is mainly based on the KMOS3D and SINS/zC-SINF surveys of Hα kinematics, and collectively provides a representative subset of the massive star-forming population. The inferred halo scale angular momentum distribution is broadly consistent with that theoretically predicted for their dark matter halos, in terms of mean spin parameter < λ > ˜ 0.037 and its dispersion (σ logλ ˜ 0.2). Spin parameters correlate with the disk radial scale and with their stellar surface density, but do not depend significantly on halo mass, stellar mass, or redshift. Our data thus support the long-standing assumption that on average, even at high redshifts, the specific angular momentum of disk galaxies reflects that of their dark matter halos (j d = j DM). The lack of correlation between λ × (j d /j DM) and the nuclear stellar density Σ*(1 kpc) favors a scenario where disk-internal angular momentum redistribution leads to “compaction” inside massive high-redshift disks. For our sample, the inferred average stellar to dark matter mass ratio is ˜2%, consistent with abundance matching results. Including the molecular gas, the total baryonic disk to dark matter mass ratio is ˜5% for halos near 1012 M ⊙, which corresponds to 31% of the cosmologically available baryons, implying that high-redshift disks are strongly baryon dominated. Based on observations obtained at the Very Large Telescope of the European Southern Observatory, Paranal, Chile (ESO Programme IDs 075.A-0466, 076.A-0527, 079.A-0341, 080.A-0330, 080.A-0339, 080.A-0635, 081.B-0568, 081.A-0672, 082.A-0396, 183.A-0781, 087.A-0081, 088.A-0202, 088.A-0209, 091.A-0126, 092.A-0091, 093.A-0079, 094.A-0217, 095.A-0047, 096.A-0025).

DYNAMICS OF TIDALLY CAPTURED PLANETS IN THE GALACTIC CENTER

DOE Office of Scientific and Technical Information (OSTI.GOV)

Trani, Alessandro A.; Bressan, Alessandro; Mapelli, Michela

2016-11-01

Recent observations suggest ongoing planet formation in the innermost parsec of the Galactic center. The supermassive black hole (SMBH) might strip planets or planetary embryos from their parent star, bringing them close enough to be tidally disrupted. Photoevaporation by the ultraviolet field of young stars, combined with ongoing tidal disruption, could enhance the near-infrared luminosity of such starless planets, making their detection possible even with current facilities. In this paper, we investigate the chance of planet tidal captures by means of high-accuracy N -body simulations exploiting Mikkola's algorithmic regularization. We consider both planets lying in the clockwise (CW) disk andmore » planets initially bound to the S-stars. We show that tidally captured planets remain on orbits close to those of their parent star. Moreover, the semimajor axis of the planetary orbit can be predicted by simple analytic assumptions in the case of prograde orbits. We find that starless planets that were initially bound to CW disk stars have mild eccentricities and tend to remain in the CW disk. However, we speculate that angular momentum diffusion and scattering by other young stars in the CW disk might bring starless planets into orbits with low angular momentum. In contrast, planets initially bound to S-stars are captured by the SMBH on highly eccentric orbits, matching the orbital properties of the clouds G1 and G2. Our predictions apply not only to planets but also to low-mass stars initially bound to the S-stars and tidally captured by the SMBH.« less

Generation of doubly charged vortex beam by concentrated loading of glass disks along their diameter.

PubMed

Skab, Ihor; Vasylkiv, Yuriy; Krupych, Oleh; Savaryn, Viktoriya; Vlokh, Rostyslav

2012-04-10

We show that a system of glass disks compressed along their diameters enables one to induce a doubly charged vortex beam in the emergent light when the incident light is circularly polarized. Using such a disk system, one can control the efficiency of conversion of the spin angular momentum to the orbital angular momentum by a loading force. The consideration presented here can be extended for the case of crystalline materials with high optical damage thresholds in order to induce high-power vortex beams.

Star formation with disc accretion and rotation. I. Stars between 2 and 22 M⊙ at solar metallicity

NASA Astrophysics Data System (ADS)

Haemmerlé, L.; Eggenberger, P.; Meynet, G.; Maeder, A.; Charbonnel, C.

2013-09-01

Context. The way angular momentum is built up in stars during their formation process may have an impact on their further evolution. Aims: In the framework of the cold disc accretion scenario, we study how angular momentum builds up inside the star during its formation for the first time and what the consequences are for its evolution on the main sequence (MS). Methods: Computation begins from a hydrostatic core on the Hayashi line of 0.7 M⊙ at solar metallicity (Z = 0.014) rotating as a solid body. Accretion rates depending on the luminosity of the accreting object are considered, which vary between 1.5 × 10-5 and 1.7 × 10-3 M⊙ yr-1. The accreted matter is assumed to have an angular velocity equal to that of the outer layer of the accreting star. Models are computed for a mass-range on the zero-age main sequence (ZAMS) between 2 and 22 M⊙. Results: We study how the internal and surface velocities vary as a function of time during the accretion phase and the evolution towards the ZAMS. Stellar models, whose evolution has been followed along the pre-MS phase, are found to exhibit a shallow gradient of angular velocity on the ZAMS. Typically, the 6 M⊙ model has a core that rotates 50% faster than the surface on the ZAMS. The degree of differential rotation on the ZAMS decreases when the mass increases (for a fixed value of vZAMS/vcrit). The MS evolution of our models with a pre-MS accreting phase show no significant differences with respect to those of corresponding models computed from the ZAMS with an initial solid-body rotation. Interestingly, there exists a maximum surface velocity that can be reached through the present scenario of formation for masses on the ZAMS larger than 8 M⊙. Typically, only stars with surface velocities on the ZAMS lower than about 45% of the critical velocity can be formed for 14 M⊙ models. Reaching higher velocities would require starting from cores that rotate above the critical limit. We find that this upper velocity limit is smaller for higher masses. In contrast, there is no restriction below 8 M⊙, and the whole domain of velocities to the critical point can be reached.

The GALAH Survey: Stellar streams and how stellar velocity distributions vary with Galactic longitude, hemisphere and metallicity

NASA Astrophysics Data System (ADS)

Quillen, Alice C.; De Silva, Gayandhi; Sharma, Sanjib; Hayden, Michael; Freeman, Ken; Bland-Hawthorn, Joss; Žerjal, Maruša; Asplund, Martin; Buder, Sven; D'Orazi, Valentina; Duong, Ly; Kos, Janez; Lin, Jane; Lind, Karin; Martell, Sarah; Schlesinger, Katharine; Simpson, Jeffrey D.; Zucker, Daniel B.; Zwitter, Tomaz; Anguiano, Borja; Carollo, Daniela; Casagrande, Luca; Cotar, Klemen; Cottrell, Peter L.; Ireland, Michael; Kafle, Prajwal R.; Horner, Jonathan; Lewis, Geraint F.; Nataf, David M.; Ting, Yuan-Sen; Watson, Fred; Wittenmyer, Rob; Wyse, Rosemary

2018-04-01

Using GALAH survey data of nearby stars, we look at how structure in the planar (u, v) velocity distribution depends on metallicity and on viewing direction within the Galaxy. In nearby stars with distance d ≲ 1 kpc, the Hercules stream is most strongly seen in higher metallicity stars [Fe/H]>0.2. The Hercules stream peak v value depends on viewed galactic longitude, which we interpret as due to the gap between the stellar stream and more circular orbits being associated with a specific angular momentum value of about 1640 km s-1 kpc. The association of the gap with a particular angular momentum value supports a bar resonant model for the Hercules stream. Moving groups previously identified in Hipparcos observations are easiest to see in stars nearer than 250 pc, and their visibility and peak velocities in the velocity distributions depends on both viewing direction (galactic longitude and hemisphere) and metallicity. We infer that there is fine structure in local velocity distributions that varies over distances of a few hundred pc in the Galaxy.

Model atmospheres and radiation of magnetic neutron stars: Anisotropic thermal emission

NASA Technical Reports Server (NTRS)

Pavlov, G. G.; Shibanov, Yu. A.; Ventura, J.; Zavlin, V. E.

1994-01-01

We investigate the anisotropy of the thermal radiation emitted by a surface element of a neutron star atmosphere (e.g., by a polar cap of a radio pulsar). Angular dependences of the partial fluxes at various photon energies, and spectra at various angles are obtained for different values of the effective temperature T(sub eff) and magnetic field strength B, and for different directions of the magnetic field. It is shown that the local radiation of the magnetized neutron star atmospheres is highly anisotropic, with the maximum flux emitted in the magnetic field direction. At high B the angular dependences in the soft X-ray range have two maxima, a high narrow peak along B and a lower and broader maximum at intermediate angles. The radiation is strongly polarized, the modulation of the degree of polarization due to the rotation of the neurtron star may be much higher than that for the radiative flux. The results obtained are compared with recent ROSAT observations of the thermal-like radiation from the radio pulsars PSR 1929+10 and PSR J0437-4715.

Interferometry from Space

NASA Technical Reports Server (NTRS)

Carpenter, Kenneth

2007-01-01

Space-based interferometric observatories will be challenging projects, equal at least to that of building the Great Observatories (the Hubble Space Telescope (HST), Spitzer Space Telescope (SST), Chandra X-ray Observatory, and the Gamma Ray Observatory), if not the Pyramids of Eygpt - but they represent the next logical step in examining our Universe at substantially higher angular resolution. Increasing our resolving power by factors of 100 or more (as is needed to make meaningful improvements in this observational arena) over existing facilities such as HST and SST requires mirror diameters (100's to 1000's of meters) much larger than can be supported by single or segmented mirrors - and thus the design and construction of sparse aperture, inteferometric arrays such as those described herein will be required. But just imagine the rewards of being able to see, for the first time, the surfaces of other stars, the location and type of extrasolar planets and even pictures of those same planets, the inner workings of Active Galactic Nuclei, the close-in details of supernovae explosions, black hole event horizons, and the infrared universe at the same resolution of the UV-optical Hubble Deep Fields. As a slight variation on the "Star Trek: Enterprise" theme song might say, it'll be a "long road, getting from here to there", but it will one well-worth taking.

Advanced speckle sensing for internal coronagraphs

NASA Astrophysics Data System (ADS)

Noecker, Charley; Shaklan, Stuart; Wallace, James K.; Kern, Brian; Give'on, Amir; Kasdin, Jeremy; Belikov, Ruslan; Kendrick, Steve

2011-10-01

A 4-8m diameter telescope carrying a coronagraph instrument is a leading candidate for an anticipated flagship mission to detect and characterize Earth-size exoplanets in the 2020s.1 Many candidate coronagraph instruments have been proposed, and one is close to meeting some of the principal requirements for that mission. But the telescope and instrument will need exquisite stability and precise control of the incoming wavefront to enable detection of faint companions (10-10of the star) at an angular separation of 2-4 Airy radii. In particular, wavefront errors cause speckles in the image, and variations in those speckles can confound the exoplanet detection. This challenge is compounded by the background light from zodiacal dust around our Sun and the target star, which limits the speed with which we can estimate and correct the speckles. We are working on developing coherent speckle detection techniques that will allow rapid calibration of speckles on the science detector, allowing subtraction in post-processing or correction with deformable mirrors. The expected speed improvement allows a much quicker timeline for measurement & calibration, which reduces the required telescope stability requirement and eases both the flight system design and the challenge of ground testing. We will describe the experiments and summarize progress to date.

Dust and molecular shells in asymptotic giant branch stars

NASA Astrophysics Data System (ADS)

Zhao-Geisler, R.; Quirrenbach, A.; Köhler, R.; Lopez, B.

2012-09-01

Context. Asymptotic giant branch (AGB) stars are one of the largest distributors of dust into the interstellar medium. However, the wind formation mechanism and dust condensation sequence leading to the observed high mass-loss rates have not yet been constrained well observationally, in particular for oxygen-rich AGB stars. Aims: The immediate objective in this work is to identify molecules and dust species which are present in the layers above the photosphere, and which have emission and absorption features in the mid-infrared (IR), causing the diameter to vary across the N-band, and are potentially relevant for the wind formation. Methods: Mid-IR (8-13 μm) interferometric data of four oxygen-rich AGB stars (R Aql, R Aqr, R Hya, and W Hya) and one carbon-rich AGB star (V Hya) were obtained with MIDI/VLTI between April 2007 and September 2009. The spectrally dispersed visibility data are analyzed by fitting a circular fully limb-darkened disk (FDD). Results: The FDD diameter as function of wavelength is similar for all oxygen-rich stars. The apparent size is almost constant between 8 and 10 μm and gradually increases at wavelengths longer than 10 μm. The apparent FDD diameter in the carbon-rich star V Hya essentially decreases from 8 to 12 μm. The FDD diameters are about 2.2 times larger than the photospheric diameters estimated from K-band observations found in the literature. The silicate dust shells of R Aql, R Hya and W Hya are located fairly far away from the star, while the silicate dust shell of R Aqr and the amorphous carbon (AMC) and SiC dust shell of V Hya are found to be closer to the star at around 8 photospheric radii. Phase-to-phase variations of the diameters of the oxygen-rich stars could be measured and are on the order of 15% but with large uncertainties. Conclusions: From a comparison of the diameter trend with the trends in RR Sco and S Ori it can be concluded that in oxygen-rich stars the overall larger diameter originates from a warm molecular layer of H2O, and the gradual increase longward of 10 μm can be most likely attributed to the contribution of a close Al2O3 dust shell. The chromatic trend of the Gaussian FWHM in V Hya can be explained with the presence of AMC and SiC dust. The observations suggest that the formation of amorphous Al2O3 in oxygen-rich stars occurs mainly around or after visual minimum. However, no firm conclusions can be drawn concerning the mass-loss mechanism. Future modeling with hydrostatic and self-consistent dynamical stellar atmospheric models will be required for a more certain understanding. Based on observations made with the Very Large Telescope Interferometer (VLTI) at the Paranal Observatory under program IDs 079.D-0140, 080.D-0005, 081.D-0198, 082.D-0641 and 083.D-0294.Color versions of the figures and Appendices A-C are available in electronic form at http://www.aanda.orgFITS files of the calibrated visibilities are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/545/A56

Living With A Red Dwarf: Rotation, Starspots, Activity Cycles, Coronal X-ray Activity And X-uv Irradiances Of Proxima Centauri

NASA Astrophysics Data System (ADS)

Jason, Merritt; Guinan, E.; Engle, S.; Pojmanski, G.

2007-12-01

As part of our Living with a Red Dwarf Program, we have carried out a detailed study of the radiative and plasma properties of the nearby dM5.5e star Proxima Centauri. Proxima Cen is noteworthy as the nearest star to the Sun. Because of its proximity ( 4.3 L.Y.) and membership in the α Cen system, Proxima Cen is an important star to use as a surrogate for solar-aged mid-dM stars. It is relatively bright (V = 11-mag) and has well determined observational and physical properties (MV, Teff, [Fe/H], angular diameter, mass and age). Importantly for our purposes, Proxima Cen has a reliable age of 5.5-6.0 Gyr from its association with the α Cen system in which α Cen A (G2 V) has a reliable isochronal age determination. We have analyzed 5 years of ASAS-3, V-band photometry to search for evidence of short- and long-term variations in brightness that could arise from magnetically related phenomenon (star spots, faculae, and possible UV flares). We also examine its coronal X-ray emission and variations as well as the stars chromospheric and transition regions in the UV from IUE and FUSE observations. The X-UV/optical data are combined and irradiances are calculated for use in extrasolar planet studies. From the photometry we find a rotational modulation of Prot = 83.5 days, in excellent agreement with the earlier HST/FGS study of Benedict et al. (1998). The character of its light variations indicates possible differential rotation as well as a probable long-term activity cycle of 6.9 +/- 0.5 yrs. Although Proxima Cen should be a fully convective star with a different magnetic dynamo (α2) than our Sun (αΩ), its overall magnetic behavior appears to be solar-like. This research is supported by grants from NSF/RUI AST-507536 and NASA Grants NNX06AD386 and NNG04G038G. We are grateful for this support.

Speckle interferometry of Hipparcos link stars. III

NASA Technical Reports Server (NTRS)

White, Graeme L.; Jauncey, David L.; Reynolds, John E.; Blackmore, David R.; Matcher, Steven J.

1991-01-01

A third list of stars is presented which have been tested by speckle interferometry for use in the Hubble Space Telescope link between the Hipparcos astrometric reference frame and the extragalactic VLBI reference frame. Structural information on angular scales of 0.15-1.2 arcsec for 34 Southern Hemisphere stars is reported from observations made with the Imperial College Speckle Interferometer mounted on the Mount Stromlo 1.9-m telescope. Twenty-four percent of the stars (8 out of the 34) show evidence of multiplicity, in agreement with previous observations in this program.

Galaxy spin as a formation probe: the stellar-to-halo specific angular momentum relation

NASA Astrophysics Data System (ADS)

Posti, Lorenzo; Pezzulli, Gabriele; Fraternali, Filippo; Di Teodoro, Enrico M.

2018-03-01

We derive the stellar-to-halo specific angular momentum relation (SHSAMR) of galaxies at z = 0 by combining (i) the standard Λcold dark matter tidal torque theory, (ii) the observed relation between stellar mass and specific angular momentum (the Fall relation), and (iii) various determinations of the stellar-to-halo mass relation (SHMR). We find that the ratio fj = j*/jh of the specific angular momentum of stars to that of the dark matter (i) varies with mass as a double power law, (ii) always has a peak in the mass range explored and iii) is three to five times larger for spirals than for ellipticals. The results have some dependence on the adopted SHMR and we provide fitting formulae in each case. For any choice of the SHMR, the peak of fj occurs at the same mass where the stellar-to-halo mass ratio f* = M*/Mh has a maximum. This is mostly driven by the straightness and tightness of the Fall relation, which requires fj and f* to be correlated with each other roughly as f_j∝ f_\\ast ^{2/3}, as expected if the outer and more angular momentum rich parts of a halo failed to accrete on to the central galaxy and form stars (biased collapse). We also confirm that the difference in the angular momentum of spirals and ellipticals at a given mass is too large to be ascribed only to different spins of the parent dark-matter haloes (spin bias).

How good a clock is rotation? The stellar rotation-mass-age relationship for old field stars

DOE Office of Scientific and Technical Information (OSTI.GOV)

Epstein, Courtney R.; Pinsonneault, Marc H., E-mail: epstein@astronomy.ohio-state.edu, E-mail: pinsono@astronomy.ohio-state.edu

2014-01-10

The rotation-mass-age relationship offers a promising avenue for measuring the ages of field stars, assuming the attendant uncertainties to this technique can be well characterized. We model stellar angular momentum evolution starting with a rotation distribution from open cluster M37. Our predicted rotation-mass-age relationship shows significant zero-point offsets compared to an alternative angular momentum loss law and published gyrochronology relations. Systematic errors at the 30% level are permitted by current data, highlighting the need for empirical guidance. We identify two fundamental sources of uncertainty that limit the precision of rotation-based ages and quantify their impact. Stars are born with amore » range of rotation rates, which leads to an age range at fixed rotation period. We find that the inherent ambiguity from the initial conditions is important for all young stars, and remains large for old stars below 0.6 M {sub ☉}. Latitudinal surface differential rotation also introduces a minimum uncertainty into rotation period measurements and, by extension, rotation-based ages. Both models and the data from binary star systems 61 Cyg and α Cen demonstrate that latitudinal differential rotation is the limiting factor for rotation-based age precision among old field stars, inducing uncertainties at the ∼2 Gyr level. We also examine the relationship between variability amplitude, rotation period, and age. Existing ground-based surveys can detect field populations with ages as old as 1-2 Gyr, while space missions can detect stars as old as the Galactic disk. In comparison with other techniques for measuring the ages of lower main sequence stars, including geometric parallax and asteroseismology, rotation-based ages have the potential to be the most precise chronometer for 0.6-1.0 M {sub ☉} stars.« less

Fundamental parameters of He-weak and He-strong stars

NASA Astrophysics Data System (ADS)

Cidale, L. S.; Arias, M. L.; Torres, A. F.; Zorec, J.; Frémat, Y.; Cruzado, A.

2007-06-01

Context: He-weak and He-strong stars are chemically peculiar AB objects whose He lines are anomalously weak or strong for their MK spectral type. The determination of fundamental parameters for these stars is often more complex than for normal stars due to their abundance anomalies. Aims: We discuss the determination of fundamental parameters: effective temperature, surface gravity, and visual and bolometric absolute magnitudes of He-weak and He-strong stars. We compare our values with those derived independently from methods based on photometry and model fitting. Methods: We carried out low resolution spectroscopic observations in the wavelength range 3400-4700 Å of 20 He-weak and 8 He-strong stars to determine their fundamental parameters by means of the Divan-Chalonge-Barbier (BCD) spectrophotometric system. This system is based on the measurement of the continuum energy distribution around the Balmer discontinuity (BD). For a few He-weak stars we also estimate the effective temperatures and the angular diameters by integrating absolute fluxes observed over a wide spectral range. Non-LTE model calculations are carried out to study the influence of the He/H abundance ratio on the emergent radiation of He-strong stars and on their T_eff determination. Results: We find that the effective temperatures, surface gravities and bolometric absolute magnitudes of He-weak stars estimated with the BCD system and the integrated flux method are in good agreement between each other, and they also agree with previous determinations based on several different methods. The mean discrepancy between the visual absolute magnitudes derived using the hipparcos parallaxes and the BCD values is on average ±0.3 mag for He-weak stars, while it is ±0.5 mag for He-strong stars. For He-strong stars, we note that the BCD calibration, based on stars in the solar environment, leads to overestimated values of T_eff. By means of model atmosphere calculations with enhanced He/H abundance ratios we show that larger He/H ratios produce smaller BD which naturally explains the T_eff overestimation. We take advantage of these calculations to introduce a method to estimate the He/H abundance ratio in He-strong stars. The BD of HD 37479 suggests that the T_eff of this star remains fairly constant as the star spectrum undergoes changes in the intensity of H and He absorption lines. Data for the He-strong star HD 66765 are reported for the first time. Observations taken at CASLEO, operating under agreement of CONICET and the Universities of La Plata, Córdoba and San Juan, Argentina. Tables [see full text]-[see full text] and Appendix A are only available in electronic form at http://www.aanda.org

Hot-Jupiter Breakfasts Realign Stars

NASA Astrophysics Data System (ADS)

Kohler, Susanna

2015-08-01

Two researchers at the University of Chicago have recently developed a new theory to explain an apparent dichotomy in the orbits of planets around cool vs. hot stars. Their model proposes that the spins of cool stars are affected when they ingest hot Jupiters (HJs) early in their stellar lifetimes. A Puzzling Dichotomy: In exoplanet studies, there is a puzzling difference observed between planet orbits around cool and hot (those with Teff ≥ 6250 K) stars: the orbital planes of planets around cool stars are primarily aligned with the host star's spin, whereas the orbital planes of planets around hot stars seem to be randomly distributed. Previous attempts to explain this dichotomy have focused on tidal interactions between the host star and the planets observed in the system. Now Titos Matsakos and Arieh Königl have taken these models a step further — by including in their calculations not only the effects of observed planets, but also those of HJs that may have been swallowed by the star long before we observed the systems. Modeling Meals: Plots of the distribution of the obliquity λ for hot Jupiters around cool hosts (upper plot) and hot hosts (lower plot). The dashed line shows the initial distribution, the bins show the model prediction for the final distribution after the systems evolve, and the black dots show the current observational data. [Matsakos & Königl, 2015]" class="size-thumbnail wp-image-223" height="386" src="http://aasnova.org/wp-content/uploads/2015/08/fig22-260x386.png" width="260" /> Plots of the distribution of the obliquity λ for hot Jupiters around cool hosts (upper plot) and hot hosts (lower plot). The dashed line shows the initial distribution, the bins show the model prediction for the final distribution after the systems evolve, and the black dots show the current observational data. [Matsakos & Königl, 2015] The authors' model assumes that as HJs are formed and migrate inward through the protoplanetary disk, they stall out near the star (where they have periods of ~2 days) and get stranded as the gas disk evaporates around them. Tidal interactions can cause these planets to become ingested by the host star within 1 Gyr. Using Monte Carlo simulations, the authors model these star-planet tidal interactions and evolve a total of 10^6 systems: half with hot (Teff = 6400 K), main-sequence hosts, and half with cool (Teff = 5500 K), solar-type hosts. The initial obliquities — the angle between the stellar spin and the planets' orbital angular momentum vectors — are randomly distributed between 0° and 180°. The authors find that early stellar ingestion of planets might be very common: to match observations, roughly half of all stellar hosts must ingest an HJ early in their lifetimes! This scenario results in a good match with observational data: about 50% of cool hosts' spins become roughly aligned with the orbital plane of their planets after they absorb the orbital angular momentum of the HJ they ingest. Hot stars, on the other hand, generally retain their random distributions of obliquity, because their angular momentum is typically higher than the orbital angular momentum of the ingested planet. Citation: Titos Matsakos and Arieh Königl 2015, ApJ, 809, L20. doi: 10.1088/2041-8205/809/2/L20

Dynamical Model for Spindown of Solar-type Stars

NASA Astrophysics Data System (ADS)

Sood, Aditi; Kim, Eun-jin; Hollerbach, Rainer

2016-12-01

After their formation, stars slow down their rotation rates by the removal of angular momentum from their surfaces, e.g., via stellar winds. Explaining how this rotation of solar-type stars evolves in time is currently an interesting but difficult problem in astrophysics. Despite the complexity of the processes involved, a traditional model, where the removal of angular momentum by magnetic fields is prescribed, has provided a useful framework to understand observational relations between stellar rotation, age, and magnetic field strength. Here, for the first time, a spindown model is proposed where loss of angular momentum by magnetic fields evolves dynamically, instead of being prescibed kinematically. To this end, we evolve the stellar rotation and magnetic field simultaneously over stellar evolution time by extending our previous work on a dynamo model which incorporates nonlinear feedback mechanisms on rotation and magnetic fields. We show that our extended model reproduces key observations and is capable of explaining the presence of the two branches of (fast and slow rotating) stars which have different relations between rotation rate Ω versus time (age), magnetic field strength | B| versus rotation rate, and frequency of magnetic field {ω }{cyc} versus rotation rate. For fast rotating stars we find that: (I) there is an exponential spindown {{Ω }}\\propto {e}-1.35t, with t measured in Gyr; (II) magnetic activity saturates for higher rotation rate; (III) {ω }{cyc}\\propto {{{Ω }}}0.83. For slow rotating stars we find: (I) a power-law spindown {{Ω }}\\propto {t}-0.52; (II) that magnetic activity scales roughly linearly with rotation rate; (III) {ω }{cyc}\\propto {{{Ω }}}1.16. The results obtained from our investigations are in good agreement with observations. The Vaughan-Preston gap is consistently explained in our model by the shortest spindown timescale in this transition from fast to slow rotators. Our results highlight the importance of self-regulation of magnetic fields and rotation by direct and indirect interactions involving nonlinear feedback in stellar evolution.

Cosmic equation of state from combined angular diameter distances: Does the tension with luminosity distances exist?

NASA Astrophysics Data System (ADS)

Cao, Shuo; Zhu, Zong-Hong

2014-10-01

Using relatively complete observational data concerning four angular diameter distance (ADD) measurements and combined SN +GRB observations representing current luminosity distance (LD) data, this paper investigates the compatibility of these two cosmological distances considering three classes of dark energy equation of state (EoS) reconstruction. In particular, we use strongly gravitationally lensed systems from various large systematic gravitational lens surveys and galaxy clusters, which yield the Hubble constant independent ratio between two angular diameter distances Dl s/Ds data. Our results demonstrate that, with more general categories of standard ruler data, ADD and LD data are compatible at 1 σ level. Second, we note that consistency between ADD and LD data is maintained irrespective of the EoS parametrizations: there is a good match between the universally explored Chevalier-Polarski-Linder model and other formulations of cosmic equation of state. Especially for the truncated generalized equation of state (GEoS) model with β =-2 , the conclusions obtained with ADD and LD are almost the same. Finally, statistical analysis of generalized dark energy equation of state performed on four classes of ADD data provides stringent constraints on the EoS parameters w0 , wβ, and β , which suggest that dark energy was a subdominant component at early times. Moreover, the GEoS parametrization with β ≃1 seems to be a more favorable two-parameter model to characterize the cosmic equation of state, because the combined angular diameter distance data (SGL +CBF +BAO +WMAP 9 ) provide the best-fit value β =0.75 1-0.480+0.465 .

The convective photosphere of the red supergiant CE Tauri. I. VLTI/PIONIER H-band interferometric imaging

NASA Astrophysics Data System (ADS)

Montargès, M.; Norris, R.; Chiavassa, A.; Tessore, B.; Lèbre, A.; Baron, F.

2018-06-01

Context. Red supergiant stars are one of the latest stages in the evolution of massive stars. Their photospheric convection may play an important role in the launching mechanism of their mass loss; however, its characteristics and dynamics are still poorly constrained. Aims: By observing red supergiant stars with near infrared interferometry at different epochs, we expect to reveal the evolution of bright convective features on their stellar surface. Methods: We observed the M2Iab-Ib red supergiant star CE Tau with the VLTI/PIONIER instrument in the H band at two different epochs separated by one month. Results: We derive the angular diameter of the star and basic stellar parameters, and reconstruct two reliable images of its H-band photosphere. The contrast of the convective pattern of the reconstructed images is 5 ± 1% and 6 ± 1% for our two epochs of observation. Conclusions: The stellar photosphere shows few changes between the two epochs. The contrast of the convective pattern is below the average contrast variations obtained on 30 randomly chosen snapshots of the best matching 3D radiative hydrodynamics simulation: 23 ± 1% for the original simulation images and 16 ± 1% for the maps degraded to the reconstruction resolution. We offer two hypotheses to explain this observation. CE Tau may be experiencing a quiet convective activity episode or it could be a consequence of its warmer effective temperature (hence its smaller radius) compared to the simulation. Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programs 298.D-5005(A) and 298.D-5005(B).Reconstructed images as FITS files and basic stellar parameters are available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/614/A12Animated gif of the two epochs is available at http://https://www.aanda.org

A Three-dimensional Simulation of a Magnetized Accretion Disk: Fast Funnel Accretion onto a Weakly Magnetized Star

NASA Astrophysics Data System (ADS)

Takasao, Shinsuke; Tomida, Kengo; Iwasaki, Kazunari; Suzuki, Takeru K.

2018-04-01

We present the results of a global, three-dimensional magnetohydrodynamics simulation of an accretion disk with a rotating, weakly magnetized central star. The disk is threaded by a weak, large-scale poloidal magnetic field, and the central star has no strong stellar magnetosphere initially. Our simulation investigates the structure of the accretion flows from a turbulent accretion disk onto the star. The simulation reveals that fast accretion onto the star at high latitudes occurs even without a stellar magnetosphere. We find that the failed disk wind becomes the fast, high-latitude accretion as a result of angular momentum exchange mediated by magnetic fields well above the disk, where the Lorentz force that decelerates the rotational motion of gas can be comparable to the centrifugal force. Unlike the classical magnetospheric accretion scenario, fast accretion streams are not guided by magnetic fields of the stellar magnetosphere. Nevertheless, the accretion velocity reaches the free-fall velocity at the stellar surface due to the efficient angular momentum loss at a distant place from the star. This study provides a possible explanation why Herbig Ae/Be stars whose magnetic fields are generally not strong enough to form magnetospheres also show indications of fast accretion. A magnetically driven jet is not formed from the disk in our model. The differential rotation cannot generate sufficiently strong magnetic fields for the jet acceleration because the Parker instability interrupts the field amplification.

A primordial origin for misalignments between stellar spin axes and planetary orbits.

PubMed

Batygin, Konstantin

2012-11-15

The existence of gaseous giant planets whose orbits lie close to their host stars ('hot Jupiters') can largely be accounted for by planetary migration associated with viscous evolution of proto-planetary nebulae. Recently, observations of the Rossiter-McLaughlin effect during planetary transits have revealed that a considerable fraction of hot Jupiters are on orbits that are misaligned with respect to the spin axes of their host stars. This observation has cast doubt on the importance of disk-driven migration as a mechanism for producing hot Jupiters. Here I show that misaligned orbits can be a natural consequence of disk migration in binary systems whose orbital plane is uncorrelated with the spin axes of the individual stars. The gravitational torques arising from the dynamical evolution of idealized proto-planetary disks under perturbations from massive distant bodies act to misalign the orbital planes of the disks relative to the spin poles of their host stars. As a result, I suggest that in the absence of strong coupling between the angular momentum of the disk and that of the host star, or of sufficient dissipation that acts to realign the stellar spin axis and the planetary orbits, the fraction of planetary systems (including systems of 'hot Neptunes' and 'super-Earths') whose angular momentum vectors are misaligned with respect to their host stars will be commensurate with the rate of primordial stellar multiplicity.

Plan-B - Do All Planetary Nebulae Derive From Binaries?

NASA Astrophysics Data System (ADS)

De Marco, Orsola; PLAN-B working Group

2007-12-01

The planetary nebula (PN) field is facing a paradigm problem. For the last thirty years the role of binarity in the formation and shaping of PNe has been hotly debated. The majority of the active research community favored a scenario in which the majority of PNe are formed by single asymptotic giant stars that impart elliptical and bipolar shapes to their ejected envelopes by means of rotation and magnetic fields. However it has recently come to light that magnetic fields and rotation would not survive in a single star for long enough to be dynamically important. What is needed is an angular momentum source which can resupply the star of rotation at the right time. This angular momentum reservoir is most likely in the form of a binary companion. Today we know of only a handful of binary central stars of PN which are close enough to have interacted. Detecting binary central stars has therefore become paramount to provide an observational confirmation of the binary hypothesis. This task has however proven to be difficult, since most of the traditional techniques are difficult to apply to these bright, windy, and pulsating stars. In June 2007 an international working group has therefore been forged to aggressively tackle this observational challenge with a diverse range of observational approaches. This This work is funded in part by NSF grant AST-0607111 (PI: De Marco)

OBSERVATIONAL EVIDENCE AGAINST LONG-LIVED SPIRAL ARMS IN GALAXIES

DOE Office of Scientific and Technical Information (OSTI.GOV)

Foyle, K.; Rix, H.-W.; Walter, F.

2011-07-10

We test whether the spiral patterns apparent in many large disk galaxies should be thought of as dynamical features that are stationary in a corotating frame for {approx}> t{sub dyn}, as implied by the density wave approach for explaining spiral arms. If such spiral arms have enhanced star formation (SF), observational tracers for different stages of the SF sequence should show a spatial ordering, from upstream to downstream in the corotating frame: dense H I, CO, tracing molecular hydrogen gas, 24 {mu}m emission tracing enshrouded SF, and UV emission tracing unobscured young stars. We argue that such a spatial orderingmore » should be reflected in the angular cross-correlation (CC, in polar coordinates) using all azimuthal positions among pairs of these tracers; the peak of the CC should be offset from zero, in different directions inside and outside the corotation radius. Recent spiral SF simulations by Dobbs and Pringle show explicitly that for the case of a stationary spiral arm potential such angular offsets between gas and young stars of differing ages should be observable as cross-correlation offsets. We calculate the angular cross-correlations for different observational SF sequence tracers in 12 nearby spiral galaxies, drawing on a data set with high-quality maps of the neutral gas (H I, THINGS) and molecular gas (CO, HERACLES), along with 24 {mu}m emission (Spitzer, SINGS); we include FUV images (GALEX) and 3.6 {mu}m emission (Spitzer, IRAC) for some galaxies, tracing aging stars and longer timescales. In none of the resulting tracer cross-correlations for this sample do we find systematic angular offsets, which would be expected for a stationary dynamical spiral pattern of well-defined pattern speed. This result indicates that spiral density waves in their simplest form are not an important aspect of explaining spirals in large disk galaxies.« less

The statistical mechanics of relativistic orbits around a massive black hole

NASA Astrophysics Data System (ADS)

Bar-Or, Ben; Alexander, Tal

2014-12-01

Stars around a massive black hole (MBH) move on nearly fixed Keplerian orbits, in a centrally-dominated potential. The random fluctuations of the discrete stellar background cause small potential perturbations, which accelerate the evolution of orbital angular momentum by resonant relaxation. This drives many phenomena near MBHs, such as extreme mass-ratio gravitational wave inspirals, the warping of accretion disks, and the formation of exotic stellar populations. We present here a formal statistical mechanics framework to analyze such systems, where the background potential is described as a correlated Gaussian noise. We derive the leading order, phase-averaged 3D stochastic Hamiltonian equations of motion, for evolving the orbital elements of a test star, and obtain the effective Fokker-Planck equation for a general correlated Gaussian noise, for evolving the stellar distribution function. We show that the evolution of angular momentum depends critically on the temporal smoothness of the background potential fluctuations. Smooth noise has a maximal variability frequency {{ν }max }. We show that in the presence of such noise, the evolution of the normalized angular momentum j=\\sqrt{1-{{e}2}} of a relativistic test star, undergoing Schwarzschild (in-plane) general relativistic precession with frequency {{ν }GR}/{{j}2}, is exponentially suppressed for j\\lt {{j}b}, where {{ν }GR}/jb2˜ {{ν }max }, due to the adiabatic invariance of the precession against the slowly varying random background torques. This results in an effective Schwarzschild precession-induced barrier in angular momentum. When jb is large enough, this barrier can have significant dynamical implications for processes near the MBH.

The Interior Angular Momentum of Core Hydrogen Burning Stars from Gravity-mode Oscillations

NASA Astrophysics Data System (ADS)

Aerts, C.; Van Reeth, T.; Tkachenko, A.

2017-09-01

A major uncertainty in the theory of stellar evolution is the angular momentum distribution inside stars and its change during stellar life. We compose a sample of 67 stars in the core hydrogen burning phase with a {log} g value from high-resolution spectroscopy, as well as an asteroseismic estimate of the near-core rotation rate derived from gravity-mode oscillations detected in space photometry. This assembly includes 8 B-type stars and 59 AF-type stars, covering a mass range from 1.4 to 5 M ⊙, I.e., it concerns intermediate-mass stars born with a well-developed convective core. The sample covers projected surface rotation velocities v\\sin I\\in [9,242] km s-1 and core rotation rates up to 26 μHz, which corresponds to 50% of the critical rotation frequency. We find deviations from rigid rotation to be moderate in the single stars of this sample. We place the near-core rotation rates in an evolutionary context and find that the core rotation must drop drastically before or during the short phase between the end of the core hydrogen burning and the onset of core helium burning. We compute the spin parameter, which is the ratio of twice the rotation rate to the mode frequency (also known as the inverse Rossby number), for 1682 gravity modes and find the majority (95%) to occur in the sub-inertial regime. The 10 stars with Rossby modes have spin parameters between 14 and 30, while the gravito-inertial modes cover the range from 1 to 15.

Modelling a Set of Carbon-Rich AGB Stars at High-Angular Resolution

NASA Astrophysics Data System (ADS)

Rau, Gioia; Hron, Josef; Paladini, Claudia; Aringer, Bernard; Eriksson, Kjell; Marigo, Paola; Nowotny, Walter; Grellmann, Rebekka

2016-07-01

We compared spectro-photometric and interferometric observations of six carbon-rich AGB stars with a grid of self-consistentmodel atmospheres. The targets are: R Lep, R Vol, Y Pav, AQ Sgr, U Hya and X TrA. Please refer to the publication Rau et al. 2016(subm.) for further details on those findings.

77 FR 14517 - Southern Star Central Gas Pipeline, Inc.; Notice of Request Under Blanket Authorization

Federal Register 2010, 2011, 2012, 2013, 2014

2012-03-12

... DEPARTMENT OF ENERGY Federal Energy Regulatory Commission [Docket No. CP12-73-000] Southern Star..., 2012 Southern Star Central Gas Pipeline, Inc. (Southern Star), 4700 State Highway 56, Owensboro... Star proposes to replace 3 miles of 12-inch diameter XT pipeline by constructing approximately 3 miles...

75 FR 8053 - Southern Star Central Gas Pipeline, Inc.; Notice of Request Under Blanket Authorization

Federal Register 2010, 2011, 2012, 2013, 2014

2010-02-23

... DEPARTMENT OF ENERGY Federal Energy Regulatory Commission [Docket No. CP10-48-000] Southern Star... that on January 29, 2010, Southern Star Central Gas Pipeline, Inc. (Southern Star), 4700 State Highway... TTY, (202) 502-8659. Specifically, Southern Star proposes to replace two miles of 12- inch diameter XT...

SPIN EVOLUTION OF ACCRETING YOUNG STARS. I. EFFECT OF MAGNETIC STAR-DISK COUPLING

DOE Office of Scientific and Technical Information (OSTI.GOV)

Matt, Sean P.; Greene, Thomas P.; Pinzon, Giovanni

2010-05-10

We present a model for the rotational evolution of a young, solar mass star interacting with an accretion disk. The model incorporates a description of the angular momentum transfer between the star and the disk due to a magnetic connection, and includes changes in the star's mass and radius and a decreasing accretion rate. The model also includes, for the first time in a spin evolution model, the opening of the stellar magnetic field lines, as expected to arise from twisting via star-disk differential rotation. In order to isolate the effect that this has on the star-disk interaction torques, wemore » neglect the influence of torques that may arise from open field regions connected to the star or disk. For a range of magnetic field strengths, accretion rates, and initial spin rates, we compute the stellar spin rates of pre-main-sequence stars as they evolve on the Hayashi track to an age of 3 Myr. How much the field opening affects the spin depends on the strength of the coupling of the magnetic field to the disk. For the relatively strong coupling (i.e., high magnetic Reynolds number) expected in real systems, all models predict spin periods of less than {approx}3 days, in the age range of 1-3 Myr. Furthermore, these systems typically do not reach an equilibrium spin rate within 3 Myr, so that the spin at any given time depends upon the choice of initial spin rate. This corroborates earlier suggestions that, in order to explain the full range of observed rotation periods of approximately 1-10 days, additional processes, such as the angular momentum loss from powerful stellar winds, are necessary.« less

Probing the possibility of hotspots on the central neutron star in HESS J1731-347

NASA Astrophysics Data System (ADS)

Suleimanov, V. F.; Klochkov, D.; Poutanen, J.; Werner, K.

2017-04-01

The X-ray spectra of the neutron stars located in the centers of supernova remnants Cas A and HESS J1731-347 are well fit with carbon atmosphere models. These fits yield plausible neutron star sizes for the known or estimated distances to these supernova remnants. The evidence in favor of the presence of a pure carbon envelope at the neutron star surface is rather indirect and is based on the assumption that the emission is generated uniformly by the entire stellar surface. Although this assumption is supported by the absence of pulsations, the observational upper limit on the pulsed fraction is not very stringent. In an attempt to quantify this evidence, we investigate the possibility that the observed spectrum of the neutron star in HESS J1731-347 is a combination of the spectra produced in a hydrogen atmosphere of the hotspots and of the cooler remaining part of the neutron star surface. The lack of pulsations in this case has to be explained either by a sufficiently small angle between the neutron star spin axis and the line of sight, or by a sufficiently small angular distance between the hotspots and the neutron star rotation poles. As the observed flux from a non-uniformly emitting neutron star depends on the angular distribution of the radiation emerging from the atmosphere, we have computed two new grids of pure carbon and pure hydrogen atmosphere model spectra accounting for Compton scattering. Using new hydrogen models, we have evaluated the probability of a geometry that leads to a pulsed fraction below the observed upper limit to be about 8.2%. Such a geometry thus seems to be rather improbable but cannot be excluded at this stage.

THE DEPENDENCE OF STELLAR MASS AND ANGULAR MOMENTUM LOSSES ON LATITUDE AND THE INTERACTION OF ACTIVE REGION AND DIPOLAR MAGNETIC FIELDS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Garraffo, Cecilia; Drake, Jeremy J.; Cohen, Ofer

Rotation evolution of late-type stars is dominated by magnetic braking and the underlying factors that control this angular momentum loss are important for the study of stellar spin-down. In this work, we study angular momentum loss as a function of two different aspects of magnetic activity using a calibrated Alfvén wave-driven magnetohydrodynamic wind model: the strengths of magnetic spots and their distribution in latitude. By driving the model using solar and modified solar surface magnetograms, we show that the topology of the field arising from the net interaction of both small-scale and large-scale field is important for spin-down rates andmore » that angular momentum loss is not a simple function of large scale magnetic field strength. We find that changing the latitude of magnetic spots can modify mass and angular momentum loss rates by a factor of two. The general effect that causes these differences is the closing down of large-scale open field at mid- and high-latitudes by the addition of the small-scale field. These effects might give rise to modulation of mass and angular momentum loss through stellar cycles, and present a problem for ab initio attempts to predict stellar spin-down based on wind models. For all the magnetogram cases considered here, from dipoles to various spotted distributions, we find that angular momentum loss is dominated by the mass loss at mid-latitudes. The spin-down torque applied by magnetized winds therefore acts at specific latitudes and is not evenly distributed over the stellar surface, though this aspect is unlikely to be important for understanding spin-down and surface flows on stars.« less

Yet Another Model for the Gamma-Ray Bursts

NASA Astrophysics Data System (ADS)

Leonard, P. J. T.

2000-05-01

We consider whether a gamma-ray burst can result from a merger between a neutron star and a massive main-sequence star in a binary system following a supernova explosion. The scenario for how this can happen is outlined in Leonard, Hills & Dewey 1994, ApJ, 423, L19-L22. The initially more massive star in a massive binary system evolves and undergoes core collapse to produce a neutron star and supernova. Since the outer layers of the originally more massive star have been transferred to the other star, then the supernova may be hydrogen deficient. The newly-formed neutron star receives a random kick during the explosion. In a small fraction of the cases, the kick has the appropriate direction and amplitude to remove most of the orbital angular momentum of the post-supernova binary system. The result is an orbit with a pericenter smaller than the radius of the non-exploding star. The neutron star rather quickly becomes embedded in the other star, and sinks to its center, giving the envelope of the merged object a lot of rotational angular momentum in the process. Leonard, Hills & Dewey estimate the rate of this process in the Galaxy to be 0.06 per square kpc per Myr for secondaries more massive than 15 solar masses. The fate of the merged object has been the source of much speculation, and we shall assume that a collapsar-like scenario results. That is, the neutron star experiences runaway accretion, collapses into a black hole, which continues to accrete, and produces a pair of jets that bore their way out of the merged object. Observers who lie in the direction of either jet will see a gamma-ray burst. Roughly 1% of supernovae in massive binary systems result in neutron stars quickly becoming embedded in the secondaries, and of those which produce black holes, only 1% would be observable as gamma-ray bursts, if the jets are beamed into 1% of the sky.

A new interferometric study of four exoplanet host stars: θ Cygni, 14 Andromedae, υ Andromedae and 42 Draconis

NASA Astrophysics Data System (ADS)

Ligi, R.; Mourard, D.; Lagrange, A. M.; Perraut, K.; Boyajian, T.; Bério, Ph.; Nardetto, N.; Tallon-Bosc, I.; McAlister, H.; ten Brummelaar, T.; Ridgway, S.; Sturmann, J.; Sturmann, L.; Turner, N.; Farrington, C.; Goldfinger, P. J.

2012-09-01

Context. Since the discovery of the first exoplanet in 1995 around a solar-type star, the interest in exoplanetary systems has kept increasing. Studying exoplanet host stars is of the utmost importance to establish the link between the presence of exoplanets around various types of stars and to understand the respective evolution of stars and exoplanets. Aims: Using the limb-darkened diameter (LDD) obtained from interferometric data, we determine the fundamental parameters of four exoplanet host stars. We are particularly interested in the F4 main-sequence star, θ Cyg, for which Kepler has recently revealed solar-like oscillations that are unexpected for this type of star. Furthermore, recent photometric and spectroscopic measurements with SOPHIE and ELODIE (OHP) show evidence of a quasi-periodic radial velocity of ~150 days. Models of this periodic change in radial velocity predict either a complex planetary system orbiting the star, or a new and unidentified stellar pulsation mode. Methods: We performed interferometric observations of θ Cyg, 14 Andromedae, υ Andromedae and 42 Draconis for two years with VEGA/CHARA (Mount Wilson, California) in several three-telescope configurations. We measured accurate limb darkened diameters and derived their radius, mass and temperature using empirical laws. Results: We obtain new accurate fundamental parameters for stars 14 And, υ And and 42 Dra. We also obtained limb darkened diameters with a minimum precision of ~1.3%, leading to minimum planet masses of Msini = 5.33 ± 0.57, 0.62 ± 0.09 and 3.79 ± 0.29 MJup for 14 And b, υ And b and 42 Dra b, respectively. The interferometric measurements of θ Cyg show a significant diameter variability that remains unexplained up to now. We propose that the presence of these discrepancies in the interferometric data is caused either by an intrinsic variation of the star or an unknown close companion orbiting around it. Based on interferometric observations with the VEGA/CHARA instrument.Appendix A is available in electronic form at http://www.aanda.org

Theory of quasi-spherical accretion in X-ray pulsars

NASA Astrophysics Data System (ADS)

Shakura, N.; Postnov, K.; Kochetkova, A.; Hjalmarsdotter, L.

2012-02-01

A theoretical model for quasi-spherical subsonic accretion on to slowly rotating magnetized neutron stars is constructed. In this model, the accreting matter subsonically settles down on to the rotating magnetosphere forming an extended quasi-static shell. This shell mediates the angular momentum removal from the rotating neutron star magnetosphere during spin-down episodes by large-scale convective motions. The accretion rate through the shell is determined by the ability of the plasma to enter the magnetosphere. The settling regime of accretion can be realized for moderate accretion rates ? g s-1. At higher accretion rates, a free-fall gap above the neutron star magnetosphere appears due to rapid Compton cooling, and accretion becomes highly non-stationary. From observations of the spin-up/spin-down rates (the angular rotation frequency derivative ?, and ? near the torque reversal) of X-ray pulsars with known orbital periods, it is possible to determine the main dimensionless parameters of the model, as well as to estimate the magnetic field of the neutron star. We illustrate the model by determining these parameters for three wind-fed X-ray pulsars GX 301-2, Vela X-1 and GX 1+4. The model explains both the spin-up/spin-down of the pulsar frequency on large time-scales and the irregular short-term frequency fluctuations, which can correlate or anticorrelate with the X-ray flux fluctuations in different systems. It is shown that in real pulsars an almost iso-angular-momentum rotation law with ω˜ 1/R2, due to strongly anisotropic radial turbulent motions sustained by large-scale convection, is preferred.

Protostellar Jets: The Revolution with ALMA

NASA Astrophysics Data System (ADS)

Podio, Linda

2017-11-01

Fast and collimated molecular jets as well as slower wide-angle outflows are observed since the earliest stages of the formation of a new star, when the protostellar embryo accretes most of its final mass from the dense parental envelope. Early theoretical studies suggested that jets have a key role in this process as they can transport away angular momentum thus allowing the star to form without reaching its break-up speed. However, an observational validation of these theories is still challenging as it requires to investigate the interface between jets and disks on scales of fractions to tens of AUs. For this reason, many questions about the origin and feedback of protostellar jets remain unanswered, e.g. are jets ubiquitous at the earliest stages of star formation? Are they launched by a magneto-centrifugal mechanism as suggested by theoretical models? Are they able to remove (enough) angular momentum? What is the jet/outflow feedback on the forming star-disk system in terms of transported mass/momentum and shock-induced chemical alterations? The advent of millimetre interferometers such as NOEMA and ALMA with their unprecedented combination of angular resolution and sensitivity are now unraveling the core of pristine jet-disk systems. While NOEMA allows to obtain the first statistically relevant surveys of protostellar jet properties and ubiquity, recent ALMA observations provide the first solid signatures of jet rotation and new insight on the chemistry of the protostellar region. I will review the most recent and exciting results obtained in the field and show how millimetre interferometry is revolutionising our comprehension of protostellar jets.

Two-dimensional models of fast rotating early-type stars

NASA Astrophysics Data System (ADS)

Rieutord, Michel

2015-08-01

Rotation has now become an unavoidable parameter of stellar models, but for most massive or intermediate-mass stars rotation is fast, at least of a significant fraction of the critical angular velocity. Current spherically symmetric models try to cope with this feature of the stars using various approximations, like for instance the so-called shellular rotation usually accompanied with a diffusion that is meant to represent the mixing induced by rotationally generated flows. Such approximations may be justified in the limit of slow rotation where anisotropies and associated flows are weak. However, when rotation is fast, say larger than 50% of the critical velocities the use of a spherically symmetric 1D-model is doubtful. This is not only because of the centrifugal flattening of the star, but also because of the flows that are induced by the baroclinic torque that naturally appears in the radiative envelope of an early-type (rotating) star. These flows face the cylindrical symmetry of the Coriolis force and the spheroidal symmetry of the effective gravity.In this talk I shall present the latest results of the ESTER project that has taken up the challenge of making two-dimensional (axisymmetric) models of stars rotating at any rotation rate. In particular, I will focus on main sequence massive and intermediate-mass stars. I'll show what should be expected in such stars as far as the differential rotation and the associated meridional circulation are concerned, notably the emergence of a Stewartson layer along the tangential cylinder of the core. I'll also indicate what may be inferred about the evolution of an intermediate-mass star at constant angular momentum and how Be stars may form. I shall finally give some comparisons between models and observations of the gravity darkening on some nearby fast rotators as it has been derived from interferometric observations. In passing, I'll also discuss how 2D models can help to recover the fundamental parameters of a star.

The GALAH survey: stellar streams and how stellar velocity distributions vary with Galactic longitude, hemisphere, and metallicity

NASA Astrophysics Data System (ADS)

Quillen, Alice C.; De Silva, Gayandhi; Sharma, Sanjib; Hayden, Michael; Freeman, Ken; Bland-Hawthorn, Joss; Žerjal, Maruša; Asplund, Martin; Buder, Sven; D'Orazi, Valentina; Duong, Ly; Kos, Janez; Lin, Jane; Lind, Karin; Martell, Sarah; Schlesinger, Katharine; Simpson, Jeffrey D.; Zucker, Daniel B.; Zwitter, Tomaz; Anguiano, Borja; Carollo, Daniela; Casagrande, Luca; Cotar, Klemen; Cottrell, Peter L.; Ireland, Michael; Kafle, Prajwal R.; Horner, Jonathan; Lewis, Geraint F.; Nataf, David M.; Ting, Yuan-Sen; Watson, Fred; Wittenmyer, Rob; Wyse, Rosemary

2018-07-01

Using GALAH (GALactic Archaeology with HERMES) survey data of nearby stars, we look at how structure in the planar (u, v) velocity distribution depends on metallicity and on viewing direction within the Galaxy. In nearby stars with distance d ≲ 1 kpc, the Hercules stream is most strongly seen in higher metallicity stars [Fe/H] > 0.2. The Hercules stream peak v value depends on viewed galactic longitude, which we interpret as due to the gap between the stellar stream and more circular orbits being associated with a specific angular momentum value of about 1640 km s-1 kpc. The association of the gap with a particular angular momentum value supports a bar resonant model for the Hercules stream. Moving groups previously identified in Hipparcos(HIgh Precision Parallax COllecting Satellite) observations are easiest to see in stars nearer than 250 pc, and their visibility and peak velocities in the velocity distributions depends on both viewing direction (galactic longitude and hemisphere) and metallicity. We infer that there is fine structure in local velocity distributions that varies over distances of a few hundred pc in the Galaxy.

Imaging and Modeling Nearby Stellar Systems through Infrared Interferometers

NASA Astrophysics Data System (ADS)

Che, Xiao; Monnier, J. D.; Ten Brummelaar, T.; Sturmann, L.; Millan-Gabet, R.; Baron, F.; Kraus, S.; Zhao, M.; CHARA

2014-01-01

Long-baseline infrared interferometers with sub-milliarcsecond angular resolution can now resolve photospheric features and the circumstellar environments of nearby massive stars. Closure phase measurements have made model-independent imaging possible. During the thesis, I have expanded Michigan Infrared Combiner (MIRC) from a 4-beam combiner to a 6-beam combiner to improve the (u,v) coverage, and installed Photometric Channels system to reduce the RMS of data by a factor of 3. I am also in charge of the Wavefront Sensor of the CHARA Adaptive Optics project to increase the sensitivity of the telescope array to enlarge the observable Young Stellar Objects (YSOs). My scientific research has focused on using mainly MIRC at CHARA to model and image rapidly rotating stars. The results are crucial for testing the next generation of stellar models that incorporate evolution of internal angular momentum. Observations of Be stars with MIRC have resolved the innermost parts of the disks, allowing us to study the evolution of the disks and star-disk interactions. I have also adopted a semi-analytical disk model to constrain Mid-InfraRed (MIR) disks of YSOs using interferometric and spectroscopic data.

Kinematical evolution of tidally limited star clusters: rotational properties

NASA Astrophysics Data System (ADS)

Tiongco, Maria A.; Vesperini, Enrico; Varri, Anna Lisa

2017-07-01

We present the results of a set of N-body simulations following the long-term evolution of the rotational properties of star cluster models evolving in the external tidal field of their host galaxy, after an initial phase of violent relaxation. The effects of two-body relaxation and escape of stars lead to a redistribution of the ordered kinetic energy from the inner to the outer regions, ultimately determining a progressive general loss of angular momentum; these effects are reflected in the overall decline of the rotation curve as the cluster evolves and loses stars. We show that all of our models share the same dependence of the remaining fraction of the initial rotation on the fraction of the initial mass lost. As the cluster evolves and loses part of its initial angular momentum, it becomes increasingly dominated by random motions, but even after several tens of relaxation times, and losing a significant fraction of its initial mass, a cluster can still be characterized by a non-negligible ratio of the rotational velocity to the velocity dispersion. This result is in qualitative agreement with the recently observed kinematical complexity that characterizes several Galactic globular clusters.

AME - Asteroseismology Made Easy. Estimating stellar properties by using scaled models

NASA Astrophysics Data System (ADS)

Lundkvist, Mia; Kjeldsen, Hans; Silva Aguirre, Victor

2014-06-01

Context. Stellar properties and, in particular stellar radii of exoplanet host stars, are essential for measuring the properties of exoplanets, therefore it is becoming increasingly important to be able to supply reliable stellar radii fast. Grid-modelling is an obvious choice for this, but that only offers a low degree of transparency to non-specialists. Aims: Here we present a new, easy, fast, and transparent method of obtaining stellar properties for stars exhibiting solar-like oscillations. The method, called Asteroseismology Made Easy (AME), can determine stellar masses, mean densities, radii, and surface gravities, as well as estimate ages. We present AME as a visual and powerful tool that could be useful, in particular, in light of the large number of exoplanets being found. Methods: AME consists of a set of figures from which the stellar parameters can be deduced. These figures are made from a grid of stellar evolutionary models that cover masses ranging from 0.7 M⊙ to 1.6 M⊙ in steps of 0.1 M⊙ and metallicities in the interval -0.3 dex ≤ [Fe/H] ≤ +0.3 dex in increments of 0.1 dex. The stellar evolutionary models are computed using the Modules for Experiments in Stellar Astrophysics (MESA) code with simple input physics. Results: We have compared the results from AME with results for three groups of stars: stars with radii determined from interferometry (and measured parallaxes), stars with radii determined from measurements of their parallaxes (and calculated angular diameters), and stars with results based on modelling their individual oscillation frequencies. We find that a comparison of the radii from interferometry to those from AME yields a weighted mean of the fractional differences of just 2%. This is also the level of deviation that we find when we compare the parallax-based radii to the radii determined from AME. Conclusions: The comparison between independently determined stellar parameters and those found using AME show that our method can provide reliable stellar masses, radii, and ages, with median uncertainties in the order of 4%, 2%, and 25%, respectively. http://sac.au.dk/scientific-data/ame

Results of Observations of Occultations of Stars by Main-Belt and Trojan Asteroids, and the Promise of Gaia

NASA Astrophysics Data System (ADS)

Dunham, David W.; Herald, David Russell; Preston, Steven; Loader, Brian; Bixby Dunham, Joan

2016-10-01

For 40 years, the sizes and shapes of scores of asteroids have been determined from observations of asteroidal occultations, and many hundreds of high-precision positions of the asteroids relative to stars have been measured. Earlier this year, the 3000th observation of an asteroidal occultation was documented. Some of the first evidence for satellites of asteroids was obtained from the early efforts; now, the orbits and sizes of some satellites discovered by other means have been refined from occultation observations. Also, several close binary stars have been discovered, and the angular diameters of some stars have been measured from analysis of these observations. The International Occultation Timing Association (IOTA) coordinates this activity worldwide, from predicting and publicizing the events, to accurately timing the occultations from as many stations as possible, and publishing and archiving the observations. The first observations were timed visually, but now nearly all observations are either video-recorded, or recorded with CCD drift scans, allowing small magnitude-drop events to be recorded, and resulting in more consistent results. Techniques have been developed allowing one or two observers to set up multiple stations with small telescopes, video cameras, and timers, thereby recording many chords, even across a whole asteroid; some examples will be shown.Later this year, the first release of Gaia data will allow us to greatly improve the vast star catalog that we use for both predicting and analyzing these events. Although the first asteroidal data will wait until the 4th Gaia release, before that, we can greatly improve the orbits of asteroids that have occulted 3 or more stars in the past so that we can start computing the paths of future occultations by them to few km accuracy. In a couple of years, we'll be able to realistically predict one to two orders of magnitude more events than we can now, allowing efforts to be concentrated on smaller objects of the highest scientific interest, including some comets.

Angular momentum exchange in white dwarf binaries accreting through direct impact

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sepinsky, J. F.; Kalogera, V., E-mail: jeremy.sepinsky@scranton.edu, E-mail: vicky@northwestern.edu

We examine the exchange of angular momentum between the component spins and the orbit in semi-detached double white dwarf binaries undergoing mass transfer through direct impact of the transfer stream. We approximate the stream as a series of discrete massive particles ejected in the ballistic limit at the inner Lagrangian point of the donor toward the accretor. This work improves upon similar earlier studies in a number of ways. First, we self-consistently calculate the total angular momentum of the orbit at all times. This includes changes in the orbital angular momentum during the ballistic trajectory of the ejected mass, asmore » well as changes during the ejection/accretion due to the radial component of the particle's velocity. Second, we calculate the particle's ballistic trajectory for each system, which allows us to determine the precise position and velocity of the particle upon accretion. We can then include specific information about the radius of the accretor as well as the angle of impact. Finally, we ensure that the total angular momentum is conserved, which requires the donor star spin to vary self-consistently. With these improvements, we calculate the angular momentum change of the orbit and each binary component across the entire parameter space of direct impact double white dwarf binary systems. We find a significant decrease in the amount of angular momentum removed from the orbit during mass transfer, as well as cases where this process increases the angular momentum of the orbit at the expense of the spin angular momentum of the donor. We conclude that, unlike earlier claims in the literature, mass transfer through direct impact need not destabilize the binary and that the quantity and sign of the orbital angular momentum transfer depends on the binary properties, particularly the masses of the double white dwarf binary component stars. This stabilization may significantly impact the population synthesis calculations of the expected numbers of events/systems for which double white dwarfs may be a progenitor, e.g., Type Ia supernovae, Type.Ia supernovae, and AM CVn.« less

Occultation evidence for a satellite of the Trojan asteroid (911) Agamemnon

NASA Astrophysics Data System (ADS)

Timerson, Bradley; Brooks, John; Conard, Steven; Dunham, David W.; Herald, David; Tolea, Alin; Marchis, Franck

2013-10-01

On 2012 January 19, observers in the northeastern United States of America observed an occultation of 8.0-mag HIP 41337 star by the Jupiter-Trojan (911) Agamemnon, including one video recorded with a 36 cm telescope that shows a deep brief secondary occultation that is likely due to a satellite, of about 5 km (most likely 3-10 km) across, at 278±5 km (0.0931″) from the asteroid's center as projected in the plane of the sky. A satellite this small and this close to the asteroid could not be resolved in the available VLT adaptive optics observations of Agamemnon recorded in 2003. The outline of Agamemnon is fit well by an ellipse with dimensions 190.6±0.9 km by 143.8±1.5 km. The angular diameter of HIP 41337 was found to be 0.5±0.1 milli-arcsec. After (624) Hektor, this could be the second Jupiter Trojan asteroid known to possess a small satellite.

An Exoplanet Spinning Up Its Star

NASA Astrophysics Data System (ADS)

Kohler, Susanna

2016-11-01

We know that the large masses of stars govern the orbits of the planets that circle them but a large, close-in planet can also influence the rotation of its host star. A recently discovered, unusual hot Jupiter may be causing its star to spin faster than it should.Exotic PlanetsHot Jupiters are gas giants of roughly Jupiters size that orbit close in to their host stars. Though these planets are easy to detect their large sizes and frequent transits mean surveys have a good chance of catching them we havent found many of them, suggesting that planetary systems containing hot Jupiters are fairly unusual.The period-folded light curve of HATS-18, revealing the transit of the hot Jupiter HATS-18b. The period is P=0.8378 days. [Penev et al. 2016]Studying this exotic population of planets, however, can help us to better understand how gas giants form and evolve in planetary systems. New observations of hot Jupiters may also reveal how stars and close-in planets interact through radiation, gravity, and magnetic fields.The recent discovery of a transiting hot Jupiter a little over 2000 light-years away therefore presents an exciting opportunity!A Speeding GiantThe discovery of HATS-18b, a planet of roughly 2 times Jupiters mass and 1.3 times its radius, was announced in a study led by Kaloyan Penev (Princeton University). The planet was discovered using the HATSouth transit survey network, which includes instruments in Chile, Namibia, and Australia, and follow-up photometry and spectroscopy was conducted at a variety of ground-based observatories.HATS-18bs properties are particularly unusual: this hot Jupiter is zipping around its host star which is very similar to the Sun at the incredible pace of one orbit every 0.84 days. HATS-18bs orbit is more than 20 times closer to its host star than Mercurys is to the Sun, bringing it so close it nearly grazes the stars surface!Size of the planetary orbit relative to the stellar radius as a function of the stellar rotation period, for transiting planets with orbital periods shorter than 2 days and masses greater than 0.1 Jupiter masses. HATS-18b is denoted by the red star. [Penev et al. 2016]Tidal InteractionsWhat happens when a massive planet orbits this close to its star? Tidal interactions between the star and the planet cause tidal dissipation in the star, resulting in decay of the planets orbit. But there may be an additional effect of this interaction in the case of HATS-18b, the authors claim: the planet may be transferring some of its angular momentum to the star.As stars age, they should gradually spin slower as they lose angular momentum viastellar winds. But Penev and collaborators note that this exoplanets host star, HATS-18, spins roughly three times as fast asits inferred age suggests it should. The authors conclude that the angular momentum lost by the planet as its orbit shrinks is deposited in the star, causing the star to spin up.HATS-18 is an excellent laboratory for studying how very short-period planets interact with their stars in fact, Penev and collaborators have already used their observations of the system to constrain models of tidal dissipation from Sun-like stars. Additional observations of HATS-18 and other short-period systems should allow us to further test models of how planetary systems form and evolve.CitationK. Penev et al 2016 AJ 152 127. doi:10.3847/0004-6256/152/5/127

Rotation and magnetism in intermediate-mass stars

NASA Astrophysics Data System (ADS)

Quentin, Léo G.; Tout, Christopher A.

2018-06-01

Rotation and magnetism are increasingly recognized as important phenomena in stellar evolution. Surface magnetic fields from a few to 20 000 G have been observed and models have suggested that magnetohydrodynamic transport of angular momentum and chemical composition could explain the peculiar composition of some stars. Stellar remnants such as white dwarfs have been observed with fields from a few to more than 109 G. We investigate the origin of and the evolution, on thermal and nuclear rather than dynamical time-scales, of an averaged large-scale magnetic field throughout a star's life and its coupling to stellar rotation. Large-scale magnetic fields sustained until late stages of stellar evolution with conservation of magnetic flux could explain the very high fields observed in white dwarfs. We include these effects in the Cambridge stellar evolution code using three time-dependant advection-diffusion equations coupled to the structural and composition equations of stars to model the evolution of angular momentum and the two components of the magnetic field. We present the evolution in various cases for a 3 M_{⊙} star from the beginning to the late stages of its life. Our particular model assumes that turbulent motions, including convection, favour small-scale field at the expense of large-scale field. As a result, the large-scale field concentrates in radiative zones of the star and so is exchanged between the core and the envelope of the star as it evolves. The field is sustained until the end of the asymptotic giant branch, when it concentrates in the degenerate core.

UIT ultraviolet imaging of 30 Doradus

NASA Technical Reports Server (NTRS)

Hintzen, P.; Cheng, K.-P.; Michalitsianos, A.; Bohlin, R.; O'Connell, R.; Cornett, R.; Roberts, M.; Smith, A.; Smith, E.; Stecher, T.

1992-01-01

During the Astro-1 mission, near- and far-UV images of the 30 Doradus region were obtained using the Ultraviolet Imaging Telescope (UIT). These wide-field, 40 min in diameter, high spatial resolution, 2-3 sec, UIT UV images reveal a rich field of luminous UV-bright stars, clusters, and associations. There are 181 stars brighter than m(sub 2558A) = 16.5 and 197 stars brighter than m(sub 1615A) = 16.4 within 3 min diameter of the 30 Doradus central cluster. We have derived UV fluxes emitted from the 30 Doradus central cluster and from its UV bright core, R136. The region within 5 sec of R136 produces approximately 14% of the far-UV flux (lambda = 1892 A) and approximately 16% of the near-UV flux (lambda = 2558 A) emitted from the 3 min diameter central cluster. The derived UV luminosity of R136 at 1892 A is only 7.8 times that of the nearby O6-7 Iaf star, R139, and the m(sub 1892) - m(sub v) colors of R136 are similar to other O or Wolf-Rayet stars in the same region. These UIT data, combined with other published observations at longer wavelengths, indicate that there is no observational evidence for a supermassive star in R136.

UIT ultraviolet imaging of 30 Doradus

NASA Astrophysics Data System (ADS)

Hintzen, P.; Cheng, K.-P.; Michalitsianos, A.; Bohlin, R.; O'Connell, R.; Cornett, R.; Roberts, M.; Smith, A.; Smith, E.; Stecher, T.

During the Astro-1 mission, near- and far-UV images of the 30 Doradus region were obtained using the Ultraviolet Imaging Telescope (UIT). These wide-field, 40 min in diameter, high spatial resolution, 2-3 sec, UIT UV images reveal a rich field of luminous UV-bright stars, clusters, and associations. There are 181 stars brighter than m2558A = 16.5 and 197 stars brighter than m1615A = 16.4 within 3 min diameter of the 30 Doradus central cluster. We have derived UV fluxes emitted from the 30 Doradus central cluster and from its UV bright core, R136. The region within 5 sec of R136 produces approximately 14% of the far-UV flux (lambda = 1892 A) and approximately 16% of the near-UV flux (lambda = 2558 A) emitted from the 3 min diameter central cluster. The derived UV luminosity of R136 at 1892 A is only 7.8 times that of the nearby O6-7 Iaf star, R139, and the m1892 - mv colors of R136 are similar to other O or Wolf-Rayet stars in the same region. These UIT data, combined with other published observations at longer wavelengths, indicate that there is no observational evidence for a supermassive star in R136.

Disk-Anchored Magnetic Propellers - A Cure for the SW Sex Syndrome

NASA Astrophysics Data System (ADS)

Horne, Keith

In AE Aqr, magnetic fields transfer energy and angular momentum from a rapidly-spinning white dwarf to material in the gas stream from the companion star, with the effect of spinning down the white dwarf while flinging the gas stream material out of the binary system. This magnetic propeller produces a host of observable signatures, chief among which are broad, single-peaked, flaring emission lines with phase-shifted orbital kinematics. SW Sex stars have accretion disks, but also broad, single-peaked, phase-shifted emission lines similar to those seen in AE Aqr. We propose that a magnetic propeller similar to that which operates in AE Aqr is also at work in SW Sex stars - and to some extent in all nova-like systems. The propeller is anchored in the inner accretion disk, rather than, or in addition to, the white dwarf. Energy and angular momentum are thereby extracted from the inner disk and transferred to gas-stream material flowing above the disk, which is consequently pitched out of the system. This provides a non-local, dissipationless angular-momentum-extraction mechanism, which should result in cool inner disks with temperature profiles flatter than T propto R^{-3/4}, as observed in eclipse mapping studies of nova-like variables. The disk-anchored magnetic propeller model appears to explain qualitatively most if not all of the peculiar features of the SW Sex syndrome.

Collapse of differentially rotating neutron stars and cosmic censorship

NASA Astrophysics Data System (ADS)

Giacomazzo, Bruno; Rezzolla, Luciano; Stergioulas, Nikolaos

2011-07-01

We present new results on the dynamics and gravitational-wave emission from the collapse of differentially rotating neutron stars. We have considered a number of polytropic stellar models having different values of the dimensionless angular momentum J/M2, where J and M are the asymptotic angular momentum and mass of the star, respectively. For neutron stars with J/M2<1, i.e. “sub-Kerr” models, we were able to find models that are dynamically unstable and that collapse promptly to a rotating black hole. Both the dynamics of the collapse and the consequent emission of gravitational waves resemble those seen for uniformly rotating stars, although with an overall decrease in the efficiency of gravitational-wave emission. For stellar models with J/M2>1, i.e. “supra-Kerr” models, on the other hand, we were not able to find models that are dynamically unstable and all of the computed supra-Kerr models were found to be far from the stability threshold. For these models a gravitational collapse is possible only after a very severe and artificial reduction of the pressure, which then leads to a torus developing nonaxisymmetric instabilities and eventually contracting to a stable axisymmetric stellar configuration. While this does not exclude the possibility that a naked singularity can be produced by the collapse of a differentially rotating star, it also suggests that cosmic censorship is not violated and that generic conditions for a supra-Kerr progenitor do not lead to a naked singularity.

Requirements for Radial Migration: How does the migrating fraction depend on stellar velocity dispersion?

NASA Astrophysics Data System (ADS)

Tolfree, Kathryne; Wyse, R. F.

2014-01-01

Radial migration is a way to rearrange the orbital angular momentum of stars in an spiral disk without inducing kinematic heating. When radial migration is very efficient, a large fraction of disk stars experience significant changes in their orbital angular momenta in a short period of time. Such scenarios have strong implications for the chemical and kinematic evolution of disk galaxies. We have undertaken an investigation of the physical dependencies of the efficiency of radial migration on stellar kinematics and spiral structure by deriving the fraction of stars that can migrate radially given certain conditions. In order for a star in a spiral disk to migrate radially, it must first be “captured" in a family of resonant orbits near the radius of corotation with a spiral pattern. Thus far, the only analytic criterion for capture has been for stars in circular orbits. We present the capture criterion for stars on non-circular orbits in a disk galaxy. We then use our analytically derived capture criteria to model the radial distribution of the captured fraction in an exponential disk with a flat rotation curve as well as the dependence of the total captured fraction in the disk on the radial component of the stellar velocity dispersion (σR) and the amplitude of the spiral perturbation to the underlying potential at corotation (|Φs|CR). We find that the captured fraction goes as Exp[-σR2/|Φs|CR].

Requirements for Radial Migration: How Does the Migrating Fraction Depend on Stellar Velocity Dispersion?

NASA Astrophysics Data System (ADS)

Tolfree, K. J. D.; Wyse, R. F. G.

2014-03-01

Radial migration is a mechanism that can rearrange the orbital angular momentum of stars in a spiral disk without inducing kinematic heating. When radial migration is very efficient, a large fraction of disk stars experience significant changes in their orbital angular momenta over a short period of time. Such scenarios have strong implications for the chemical and kinematic evolution of disk galaxies. We have undertaken an investigation of the physical dependencies of the efficiency of radial migration on stellar kinematics and spiral structure by deriving the fraction of stars that can migrate radially. In order for a star in a spiral disk to migrate radially, it must first be “captured” in a family of resonant orbits near the radius of corotation with a transient spiral pattern. To date, the only analytic criterion for capture has been for stars in circular orbits. We present the capture criterion for disk stars on non-circular orbits. We then use our analytically derived capture criterion to model the radial distribution of the captured fraction in an exponential disk with a flat rotation curve. Further, we derive the dependence of the total captured fraction in the disk on the radial component of the stellar velocity dispersion (σR) and the amplitude of the spiral perturbation to the underlying potential evaluated at corotation (|Φs|CR). We find that within an annulus centered around corotation where σR is constant, the captured fraction goes as e-σR2/|Φs|CR.

Narrowly peaked forward light scattering on particulate media: II. Angular spreading of light scattered by polystyrene microspheres

NASA Astrophysics Data System (ADS)

Turcu, Ioan; Bratfalean, Radu; Neamtu, Silvia

2008-07-01

The adequacy of the effective phase function (EPF) used to describe the light scattered at small angles was tested on aqueous suspensions of polystyrene microspheres. Angular resolved light scattering measurements were performed on two types of latex suspension, which contained polystyrene spheres of 3 µm and 5 µm diameters, respectively. The experimental data were fitted with two EPF approximants. If the polystyrene spheres are at least 3 µm in diameter the quasi-ballistic light scattering process can be described relatively well by the EPF in a small angular range centered in the forward direction. The forward light scattering by macroscopic samples containing microspheres can be modeled relatively well if the true Mie single particle scattering phase function is replaced by a simpler Henyey-Greenstein dependence having the same width at half-height as the first scattering lobe.

Angular-domain scattering interferometry.

PubMed

Shipp, Dustin W; Qian, Ruobing; Berger, Andrew J

2013-11-15

We present an angular-scattering optical method that is capable of measuring the mean size of scatterers in static ensembles within a field of view less than 20 μm in diameter. Using interferometry, the method overcomes the inability of intensity-based models to tolerate the large speckle grains associated with such small illumination areas. By first estimating each scatterer's location, the method can model between-scatterer interference as well as traditional single-particle Mie scattering. Direct angular-domain measurements provide finer angular resolution than digitally transformed image-plane recordings. This increases sensitivity to size-dependent scattering features, enabling more robust size estimates. The sensitivity of these angular-scattering measurements to various sizes of polystyrene beads is demonstrated. Interferometry also allows recovery of the full complex scattered field, including a size-dependent phase profile in the angular-scattering pattern.

The {sup 18}O(d,p){sup 19}O reaction and the ANC method

DOE Office of Scientific and Technical Information (OSTI.GOV)

Burjan, V.; Hons, Z.; Kroha, V.

2014-05-09

The neutron capture rate {sup 18}O(n,γ){sup 19}O is important for analysis of nucleosynthesis in inhomogeneous Big Bang models and also for models of processes in massive red giant stars and AGB stars. Angular distributions of the {sup 18}O(d,p){sup 19}O reaction were measured at a deuteron energy of 16.3 MeV in NPI in Řež, Czech Republic, with the aim to determine Asymptotic Normalization Coefficients which can then be used for indirect determination of the direct contribution to the {sup 18}O(n,γ){sup 19}O process. In the experiment, the gas target with {sup 18}O isotope of high purity 99.9 % was used thus eliminatingmore » any contaminating reactions. Reaction products were measured by the set of 8 ΔE-E telescopes consisting of thin and thick silicon surface-barrier detectors. Angular distributions of proton transfers corresponding to 6 levels of {sup 19}O up to the 4.1093 MeV excitation energy were determined. The analysis of angular distributions in the angular range from 6 to 64 degree including also the angular distribution of elastically scattered deuterons was carried out by means of ECIS and DWUCK codes. From the determined ANCs the direct contribution to the radiative capture {sup 18}O(n,γ){sup 19}O was deduced and compared with existing direct measurements.« less

The angular momentum of cosmological coronae and the inside-out growth of spiral galaxies

NASA Astrophysics Data System (ADS)

Pezzulli, Gabriele; Fraternali, Filippo; Binney, James

2017-05-01

Massive and diffuse haloes of hot gas (coronae) are important intermediaries between cosmology and galaxy evolution, storing mass and angular momentum acquired from the cosmic web until eventual accretion on to star-forming discs. We introduce a method to reconstruct the rotation of a galactic corona, based on its angular momentum distribution (AMD). This allows us to investigate in what conditions the angular momentum acquired from tidal torques can be transferred to star-forming discs and explain observed galaxy-scale processes, such as inside-out growth and the build-up of abundance gradients. We find that a simple model of an isothermal corona with a temperature slightly smaller than virial and a cosmologically motivated AMD is in good agreement with galaxy evolution requirements, supporting hot-mode accretion as a viable driver for the evolution of spiral galaxies in a cosmological context. We predict moderately sub-centrifugal rotation close to the disc and slow rotation close to the virial radius. Motivated by the observation that the Milky Way has a relatively hot corona (T ≃ 2 × 106 K), we also explore models with a temperature larger than virial. To be able to drive inside-out growth, these models must be significantly affected by feedback, either mechanical (ejection of low angular momentum material) or thermal (heating of the central regions). However, the agreement with galaxy evolution constraints becomes, in these cases, only marginal, suggesting that our first and simpler model may apply to a larger fraction of galaxy evolution history.

Integrated spectral study of small angular diameter galactic open clusters

NASA Astrophysics Data System (ADS)

Clariá, J. J.; Ahumada, A. V.; Bica, E.; Pavani, D. B.; Parisi, M. C.

2017-10-01

This paper presents flux-calibrated integrated spectra obtained at Complejo Astronómico El Leoncito (CASLEO, Argentina) for a sample of 9 Galactic open clusters of small angular diameter. The spectra cover the optical range (3800-6800 Å), with a resolution of ˜14 Å. With one exception (Ruprecht 158), the selected clusters are projected into the fourth Galactic quadrant (282o < l < 345o) near the Galactic plane (∣b∣ ≤ 9o). We performed simultaneous estimates of foreground interstellar reddening and age by comparing the continuum distribution and line strenghts of the cluster spectra with those of template cluster spectra with known parameters. We thus provide spectroscopic information independent from that derived through color-magnitude diagram studies. We found three clusters (Collinder 249, NGC 4463 and Ruprecht 122) younger than ˜40 Myr, four moderately young ones (BH 92, Harvard 5, Hogg 14 and Pismis 23) with ages within 200-400 Myr, and two intermediate-age ones (Ruprecht 158 and ESO 065-SC07) with ages within 1.0-2.2 Gyr. The derived foreground E(B - V) color excesses vary from around 0.0 in Ruprecht 158 to ˜1.1 in Pismis 23. In general terms, the results obtained show good agreement with previous photometric results. In Ruprecht 158 and BH 92, however, some differences are found between the parameters here obtained and previous values in the literature. Individual spectra of some comparatively bright stars located in the fields of 5 out of the 9 clusters here studied, allowed us to evaluate their membership status. The current cluster sample complements that of 46 open clusters previously studied by our group in an effort to gather a spectral library with several clusters per age bin. The cluster spectral library that we have been building is an important tool to tie studies of resolved and unresolved stellar content.

Rapid Rotation of a Heavy White Dwarf

NASA Astrophysics Data System (ADS)

Kohler, Susanna

2017-05-01

New Kepler observations of a pulsating white dwarf have revealed clues about the rotation of intermediate-mass stars.Learning About ProgenitorsStars weighing in at under 8 solar masses generally end their lives as slowly cooling white dwarfs. By studying the rotation of white dwarfs, therefore, we are able to learn about the final stages of angular momentum evolution in these progenitor stars.Most isolated field white dwarfs cluster in mass around 0.62 solar masses, which corresponds to a progenitor mass of around 2.2 solar masses. This abundance means that weve already learned a good deal about the final rotation of low-mass (13 solar-mass) stars. Our knowledge about the angular momentum of intermediate-mass (38 solar-mass) stars, on the other hand, remains fairly limited.Fourier transform of the pulsations from SDSSJ0837+1856. The six frequencies of stellar variability, marked with red dots, reveal a rotation period of 1.13 hours. [Hermes et al. 2017]Record-Breaking FindA newly discovered white dwarf, SDSSJ0837+1856, is now helping to shed light on this mass range. SDSSJ0837+1856 appears to be unusually massive: its measured at 0.87 solar masses, which corresponds to a progenitor mass of roughly 4.0 solar masses. Determining the rotation of this white dwarf would therefore tell us about the final stages of angular momentum in an intermediate-mass star.In a new study led by J.J. Hermes (Hubble Fellow at University of North Carolina, Chapel Hill), a team of scientists presents a series of measurements of SDSSJ0837+1856 that suggest its the highest-mass and fastest-rotating isolated pulsating white dwarf known.Histogram of rotation rates determined from the asteroseismology of pulsating white dwarfs (marked in red). SDSSJ0837+1856 (indicated in black) is more massive and rotates faster than any other known pulsating white dwarf. [Hermes et al. 2017]Rotation from PulsationsWhy pulsating? In the absence of measurable spots and other surface features, the way we measure the rotation rate of a star is using asteroseismology. In this process, observations of a stars tiny oscillations can reveal information about its internal structure and rotation.Hermes and collaborators used Kepler K2 observations spanning nearly 75 days in addition to ground-based follow-up and spectroscopy to estimate the white dwarfs rotation period based on its observed internal pulsations. The resulting rotation rate, 1.13 0.02 hours, is the fastest rotation period ever measured for an isolated pulsating white dwarf.Placing SDSSJ0837+1856 in the context of other white dwarfs with measured rotation periods, the authors argue that there seems to be a connection between the highest-mass white dwarfs and the fastest rotators. More observations of this kind will help us to determine whether this is a general trend that tells us something significant about the angular momentum evolution of intermediate-mass stars.CitationJ. J. Hermes et al 2017 ApJL 841 L2. doi:10.3847/2041-8213/aa6ffc

An Extended Kalman Filter-Based Attitude Tracking Algorithm for Star Sensors

PubMed Central

Li, Jian; Wei, Xinguo; Zhang, Guangjun

2017-01-01

Efficiency and reliability are key issues when a star sensor operates in tracking mode. In the case of high attitude dynamics, the performance of existing attitude tracking algorithms degenerates rapidly. In this paper an extended Kalman filtering-based attitude tracking algorithm is presented. The star sensor is modeled as a nonlinear stochastic system with the state estimate providing the three degree-of-freedom attitude quaternion and angular velocity. The star positions in the star image are predicted and measured to estimate the optimal attitude. Furthermore, all the cataloged stars observed in the sensor field-of-view according the predicted image motion are accessed using a catalog partition table to speed up the tracking, called star mapping. Software simulation and night-sky experiment are performed to validate the efficiency and reliability of the proposed method. PMID:28825684

An Extended Kalman Filter-Based Attitude Tracking Algorithm for Star Sensors.

PubMed

Li, Jian; Wei, Xinguo; Zhang, Guangjun

2017-08-21

Efficiency and reliability are key issues when a star sensor operates in tracking mode. In the case of high attitude dynamics, the performance of existing attitude tracking algorithms degenerates rapidly. In this paper an extended Kalman filtering-based attitude tracking algorithm is presented. The star sensor is modeled as a nonlinear stochastic system with the state estimate providing the three degree-of-freedom attitude quaternion and angular velocity. The star positions in the star image are predicted and measured to estimate the optimal attitude. Furthermore, all the cataloged stars observed in the sensor field-of-view according the predicted image motion are accessed using a catalog partition table to speed up the tracking, called star mapping. Software simulation and night-sky experiment are performed to validate the efficiency and reliability of the proposed method.

Stellar Magnetism, Winds and their Effects on Planetary Environments

NASA Astrophysics Data System (ADS)

Vidotto, A. A.

2016-08-01

Here, I review some recent works on magnetism of cool, main-sequence stars, their winds and potential impact on surrounding exoplanets. The winds of these stars are very tenuous and persist during their lifetime. Although carrying just a small fraction of the stellar mass, these magnetic winds carry away angular momentum, thus regulating the rotation of the star. Since cool stars are likely to be surrounded by planets, understanding the host star winds and magnetism is a key step towards characterisation of exoplanetary environments. As rotation and activity are intimately related, the spin down of stars leads to a decrease in stellar activity with age. As a consequence, as stars age, a decrease in high-energy (X-ray, extreme ultraviolet) irradiation is observed, which can a ect the evaporation of exoplanetary atmospheres and, thus, also altering exoplanetary evolution.

RadioAstron Maser Observations: a Record in Angular Resolution

NASA Astrophysics Data System (ADS)

Sobolev, A. M.; Shakhvorostova, N. N.; Alakoz, A. V.; Baan, W. A.; RadioAstron Maser Team

2017-06-01

Extremely long baselines of the space-ground interferometer RadioAstron allow to achieve ultra-high angular resolutions. The possibility of detection of a maser emission with resolutions about tens of micro-arcseconds was arguable before successful experiments reported in this paper. We present the results of the maser survey obtained by RadioAstron during first 5 years of operation. Extremely high angular resolution of 11 microarcseconds have been achieved in observations of the megamaser galaxy NGC 4258. For the galaxy at the distance about 7 Mpc this corresponds to linear resolution around 80 AU. Very compact features with angular sizes about 20 micro-arcseconds have been detected in star-forming regions of our Galaxy. Corresponding linear sizes are about 5-10 millions of kilometers.

Orientations of AM Her stars from their polarization properties - The case of the missing AM Her stars

NASA Technical Reports Server (NTRS)

Brainerd, J. J.; Lamb, D. Q.

1985-01-01

Observation of the polarization properties of the AM Her stars is used to determine their orientations. It is found that the AM Her stars are randomly distributed with respect to their inclination angle i but not with respect to their magnetic colatitude delta. The stars are concentrated along the semicircle cos-squared delta + cos-squared i = 1, where delta is the angle between the magnetic moment and the spin axis of the degenerate dwarf. This result implies that the discovery of AM Her stars is strongly affected by observational selection effects. It is suggested that these effects are a result of the way in which the AM Her stars have been identified observationally, and of the angular properties of high harmonic cyclotron emission. It is estimated that there are about three times as many AM Her stars as have been found so far.

Chromospheric Activity in Cool Luminous Stars

NASA Astrophysics Data System (ADS)

Dupree, Andrea

2018-04-01

Spatially unresolved spectra of giant and supergiant stars demonstrate ubiquitous signatures of chromospheric activity, variable outflows, and winds. The advent of imaging techniques and spatially resolved spectra reveal complex structures in these extended stellar atmospheres that we do not understand. The presence and behavior of these atmospheres is wide ranging and impacts stellar activity, magnetic fields, angular momentum loss, abundance determinations, and the understanding of stellar cluster populations.

The Multiplicity of Massive Stars: A High Angular Resolution Survey With The HST Fine Guidance Sensor

DTIC Science & Technology

2015-01-01

al. (2014), and of the Large Magellanic Cloud (LMC) Tarantula Nebula region by Sana et al. (2013b), demonstrate that the binary frequency may be »70...Monte-Carlo method to fit spectroscopic results for a large sample of O-type stars in the Tarantula Nebula region of the LMC, and they find a best fit

Disks around stars and the growth of planetary systems.

PubMed

Greaves, Jane S

2005-01-07

Circumstellar disks play a vital evolutionary role, providing a way to move gas inward and onto a young star. The outward transfer of angular momentum allows the star to contract without breaking up, and the remnant disk of gas and particles is the reservoir for forming planets. High-resolution spectroscopy is uncovering planetary dynamics and motion within the remnant disk, and imaging at infrared to millimeter wavelengths resolves disk structure over billions of years of evolution. Most stars are born with a disk, and models of planet formation need to form such bodies from the disk material within the disk's 10-million-year life-span.

Eating a planet and spinning up

NASA Astrophysics Data System (ADS)

Qureshi, Ahmed; Naoz, Smadar; Shkolnik, Evgenya L.

2018-01-01

One of the predictions of high eccentricity planetary migration is that many planets will end up plunging into their host stars. We investigate the consequence of planetary mergers on their stellar hosts’ spin-period. Energy and angular momentum conservation yield that a planet consumption by a star will spin-up of the star. We find that our calculations align with the observed bifurcation in the stellar spin-period in young clusters. After a Sun-like star has eaten a planet, it will then, spin down due to magnetic braking, consistent with the observed lack of fast rotators in old clusters. The agreement between the calculations presented here and the observed spin-period of stars in young clusters provides circumstantial evidence that planetary accretion onto their host stars is a generic feature in planetary-system evolution.

SED16 autonomous star tracker night sky testing

NASA Astrophysics Data System (ADS)

Foisneau, Thierry; Piriou, Véronique; Perrimon, Nicolas; Jacob, Philippe; Blarre, Ludovic; Vilaire, Didier

2017-11-01

The SED16 is an autonomous multi-missions star tracker which delivers three axis satellite attitude in an inertial reference frame and the satellite angular velocity with no prior information. The qualification process of this star sensor includes five validation steps using optical star simulator, digitized image simulator and a night sky tests setup. The night sky testing was the final step of the qualification process during which all the functions of the star tracker were used in almost nominal conditions : Autonomous Acquisition of the attitude, Autonomous Tracking of ten stars. These tests were performed in Calern in the premises of the OCA (Observatoire de la Cote d'Azur). The test set-up and the test results are described after a brief review of the sensor main characteristics and qualification process.

Angular Momentum Transfer and Fractional Moment of Inertia in Pulsar Glitches

NASA Astrophysics Data System (ADS)

Eya, I. O.; Urama, J. O.; Chukwude, A. E.

2017-05-01

We use the Jodrell Bank Observatory glitch database containing 472 glitches from 165 pulsars to investigate the angular momentum transfer during rotational glitches in pulsars. Our emphasis is on pulsars with at least five glitches, of which there are 26 that exhibit 261 glitches in total. This paper identifies four pulsars in which the angular momentum transfer, after many glitches, is almost linear with time. The Lilliefore test on the cumulative distribution of glitch spin-up sizes in these glitching pulsars shows that glitch sizes in 12 pulsars are normally distributed, suggesting that their glitches originate from the same momentum reservoir. In addition, the distribution of the fractional moment of inertia (I.e., the ratio of the moment of inertia of neutron star components that are involved in the glitch process) have a single mode, unlike the distribution of fractional glitch size (Δν/ν), which is usually bimodal. The mean fractional moment of inertia in the glitching pulsars we sampled has a very weak correlation with the pulsar spin properties, thereby supporting a neutron star interior mechanism for the glitch phenomenon.

Angular Momentum Transfer and Fractional Moment of Inertia in Pulsar Glitches

DOE Office of Scientific and Technical Information (OSTI.GOV)

Eya, I. O.; Urama, J. O.; Chukwude, A. E., E-mail: innocent.eya@unn.edu.ng, E-mail: innocent.eya@gmail.com

We use the Jodrell Bank Observatory glitch database containing 472 glitches from 165 pulsars to investigate the angular momentum transfer during rotational glitches in pulsars. Our emphasis is on pulsars with at least five glitches, of which there are 26 that exhibit 261 glitches in total. This paper identifies four pulsars in which the angular momentum transfer, after many glitches, is almost linear with time. The Lilliefore test on the cumulative distribution of glitch spin-up sizes in these glitching pulsars shows that glitch sizes in 12 pulsars are normally distributed, suggesting that their glitches originate from the same momentum reservoir.more » In addition, the distribution of the fractional moment of inertia (i.e., the ratio of the moment of inertia of neutron star components that are involved in the glitch process) have a single mode, unlike the distribution of fractional glitch size (Δ ν / ν ), which is usually bimodal. The mean fractional moment of inertia in the glitching pulsars we sampled has a very weak correlation with the pulsar spin properties, thereby supporting a neutron star interior mechanism for the glitch phenomenon.« less

The Balloon Experimental Twin Telescope for Infrared Interferometry (BETTII): High Angular Resolution Astronomy at Far-Infrared Wavelengths

NASA Technical Reports Server (NTRS)

Rinehart, Stephen A.

2008-01-01

Astronomical studies at infrared wavelengths have dramatically improved our understanding of the universe, and observations with Spitzer, the upcoming Herschel mission. and SOFIA will continue to provide exciting new discoveries. The comparatively low spatial resolution of these missions, however. is insufficient to resolve the physical scales on which mid- to far-infrared emission arises, resulting in source and structure ambiguities that limit our ability to answer key science questions. Interferometry enables high angular resolution at these wavelengths. We have proposed a new high altitude balloon experiment, the Balloon Experimental Twin Telescope for Infrared Interferometry (BETTII). High altitude operation makes far-infrared (30- 300micron) observations possible, and BETTII's 8-meter baseline provides unprecedented angular resolution (-0.5 arcsec) in this band. BETTII will use a double- Fourier instrument to simultaneously obtain both spatial and spectral informatioT. he spatially resolved spectroscopy provided by BETTII will address key questions about the nature of disks in young cluster stars and active galactic nuclei and the envelopes of evolved stars. BETTII will also lay the groundwork for future space interferometers.

Star-Forming Regions in Orion as a Dust Evolution Laboratory

NASA Astrophysics Data System (ADS)

Wiebe, D.; Murga, M.; Sivkova, E.

2017-06-01

Star-forming regions (SFR) represent a convenient opportunity to study various processes related both to dust growth and to dust destruction. While extragalactic SFRs allow considering these processes in a wide range of metallicities, UV field intensities, etc., the Orion star-forming complex opens up a possibility to observe dust evolution with an unprecedented angular resolution. We review various observations related to dust evolution in some most prominent Orion regions, paying special attention to organic dust evolution, and introduce a new model of organic dust evolution.

Asteroseismology: Theory and phenomenology

NASA Technical Reports Server (NTRS)

Brown, Timothy M.

1994-01-01

Seismic studies of the Sun have succeeded in mapping the variation of sound speed with depth in the Sun, and variation of angular velocity with both depth and latitude. Many stars besides the Sun may also be amenable to asteroseismic analysis. Stars of roughly solar type should of course behave in ways similar to the sun, and stars of this sort form a large fraction of the potential targets for asteroseismology. But several other types of stars (delta scuti stars, roAP stars, and the pulsating white dwarfs) also have the desired pulsation characteristics. Pulsations in some of these stars are, for various reasons, much easier to observe than in the Sun-like stars. Virtually all unambiguous observations of multi-mode pulsators relate to these other categories of stars. Since oscillation mode frequencies are arguably the most precise measurement relating to a star that we can make, a few tens of such frequencies may still be of great importance to our understanding of the stellar structure and evolution.

The solar wind in time: a change in the behaviour of older winds?

NASA Astrophysics Data System (ADS)

O'Fionnagáin, D.; Vidotto, A. A.

2018-05-01

In this paper, we model the wind of solar analogues at different ages to investigate the evolution of the solar wind. Recently, it has been suggested that winds of solar type stars might undergo a change in properties at old ages, whereby stars older than the Sun would be less efficient in carrying away angular momentum than what was traditionally believed. Adding to this, recent observations suggest that old solar-type stars show a break in coronal properties, with a steeper decay in X-ray luminosities and temperatures at older ages. We use these X-ray observations to constrain the thermal acceleration of winds of solar analogues. Our sample is based on the stars from the `Sun in Time' project with ages between 120 and 7000 Myr. The break in X-ray properties leads to a break in wind mass-loss rates (\\dot{M}) at roughly 2 Gyr, with \\dot{M} (t < 2 Gyr) ∝ t-0.74 and \\dot{M} (t > 2 Gyr) ∝ t-3.9. This steep decay in \\dot{M} at older ages could be the reason why older stars are less efficient at carrying away angular momentum, which would explain the anomalously rapid rotation observed in older stars. We also show that none of the stars in our sample would have winds dense enough to produce thermal emission above 1-2 GHz, explaining why their radio emissions have not yet been detected. Combining our models with dynamo evolution models for the magnetic field of the Earth, we find that, at early ages (≈100 Myr), our Earth had a magnetosphere that was three or more times smaller than its current size.

M-dwarf stellar winds: the effects of realistic magnetic geometry on rotational evolution and planets

NASA Astrophysics Data System (ADS)

Vidotto, A. A.; Jardine, M.; Morin, J.; Donati, J. F.; Opher, M.; Gombosi, T. I.

2014-02-01

We perform three-dimensional numerical simulations of stellar winds of early-M-dwarf stars. Our simulations incorporate observationally reconstructed large-scale surface magnetic maps, suggesting that the complexity of the magnetic field can play an important role in the angular momentum evolution of the star, possibly explaining the large distribution of periods in field dM stars, as reported in recent works. In spite of the diversity of the magnetic field topologies among the stars in our sample, we find that stellar wind flowing near the (rotational) equatorial plane carries most of the stellar angular momentum, but there is no preferred colatitude contributing to mass-loss, as the mass flux is maximum at different colatitudes for different stars. We find that more non-axisymmetric magnetic fields result in more asymmetric mass fluxes and wind total pressures ptot (defined as the sum of thermal, magnetic and ram pressures). Because planetary magnetospheric sizes are set by pressure equilibrium between the planet's magnetic field and ptot, variations of up to a factor of 3 in ptot (as found in the case of a planet orbiting at several stellar radii away from the star) lead to variations in magnetospheric radii of about 20 per cent along the planetary orbital path. In analogy to the flux of cosmic rays that impact the Earth, which is inversely modulated with the non-axisymmetric component of the total open solar magnetic flux, we conclude that planets orbiting M-dwarf stars like DT Vir, DS Leo and GJ 182, which have significant non-axisymmetric field components, should be the more efficiently shielded from galactic cosmic rays, even if the planets lack a protective thick atmosphere/large magnetosphere of their own.

Progress and Challenges in SPH Simulations of Disk Galaxy Formation: The Combined Role of Resolution and the Star Formation Density Threshold

NASA Astrophysics Data System (ADS)

Mayer, L.

2012-07-01

We review progress in cosmological SPH simulations of disk galaxy formation. We discuss the role of numerical resolution and sub-grid recipes of star formation and feedback from supernovae, higlighting the important role of a high star formation density threshold comparable to that of star forming molecular gas phase. Two recent succesfull examples, in simulations of the formation of gas-rich bulgeless dwarf galaxies and in simulations of late-type spirals (the ERIS simulations), are presented and discussed. In the ERIS simulations, already in the progenitors at z = 3 the resolution is above the threshold indicated by previous idealized numerical experiments as necessary to minimize numerical angular momentum loss (Kaufmann et al. 2007). A high star formation density threshold maintains an inhomogeneous interstellar medium, where star formation is clustered, and thus the local effect of supernovae feedback is enhanced. As a result, outflows are naturally generated removing 2/3 of the baryons in galaxies with Vvir˜50 km/s and ˜ 30% of the baryons in galaxies with (Vvir ˜ 150 km/s). Low angular momentum baryons are preferentially removed since the strongest bursts of star formation occur predominantly near the center, especially after a merger event. This produces pure exponential disks or small bulges depending on galaxy mass, and, correspondingly, slowly rising or nearly flat rotation curves that match those of observed disk galaxies. In dwarfs the rapid mass removal by outflows generates a core-like distribution in the dark matter. Furthermore, contrary to the common picture, in the ERIS spiral galaxies a bar/pseudobulge forms rapidly, and not secularly, as a result of mergers and interactions at high-z.

Transit timing variations for planets co-orbiting in the horseshoe regime

DOE Office of Scientific and Technical Information (OSTI.GOV)

Vokrouhlický, David; Nesvorný, David, E-mail: vokrouhl@cesnet.cz, E-mail: davidn@boulder.swri.edu

2014-08-10

Although not yet detected, pairs of exoplanets in 1:1 mean motion resonance probably exist. Low eccentricity, near-planar orbits, which in the comoving frame follow horseshoe trajectories, are one of the possible stable configurations. Here we study transit timing variations (TTVs) produced by mutual gravitational interaction of planets in this orbital architecture, with the goal to develop methods that can be used to recognize this case in observational data. In particular, we use a semi-analytic model to derive parametric constraints that should facilitate data analysis. We show that characteristic traits of the TTVs can directly constrain the (1) ratio of planetarymore » masses and (2) their total mass (divided by that of the central star) as a function of the minimum angular separation as seen from the star. In an ideal case, when transits of both planets are observed and well characterized, the minimum angular separation can also be inferred from the data. As a result, parameters derived from the observed transit timing series alone can directly provide both planetary masses scaled to the central star mass.« less

Quantifying Stellar Mass Loss with High Angular Resolution Imaging

DTIC Science & Technology

2009-02-19

material – via massive winds, planetary nebulae and supernova explosions – seeding the interstellar medium with heavier elements. Subsequent...of Planetary Nebulae (Harpaz, ApJ, 498,293, (1998)), impacts the pre-explosion characteristic of SNII (Taylor, “The Stars”, Cambridge (1994)), and...A 464, 119) or may have an important role, such as Be Stars, W-R stars, and planetary nebulae . The Future of Interferometric O/IR Imaging. The

Stellar models simulating the disk-locking mechanism and the evolutionary history of the Orion Nebula cluster and NGC 2264

NASA Astrophysics Data System (ADS)

Landin, N. R.; Mendes, L. T. S.; Vaz, L. P. R.; Alencar, S. H. P.

2016-02-01

Context. Rotational evolution in young stars is described by pre-main sequence evolutionary tracks including non-gray boundary conditions, rotation, conservation of angular momentum, and simulations of disk-locking. Aims: By assuming that disk-locking is the regulation mechanism for the stellar angular velocity during the early stages of pre-main sequence evolution, we use our rotating models and observational data to constrain disk lifetimes (Tdisk) of a representative sample of low-mass stars in two young clusters, the Orion Nebula cluster (ONC) and NGC 2264, and to better understand their rotational evolution. Methods: The period distributions of the ONC and NGC 2264 are known to be bimodal and to depend on the stellar mass. To follow the rotational evolution of these two clusters' stars, we generated sets of evolutionary tracks from a fully convective configuration with low central temperatures (before D- and Li-burning). We assumed that the evolution of fast rotators can be represented by models considering conservation of angular momentum during all stages and of moderate rotators by models considering conservation of angular velocity during the first stages of evolution. With these models we estimate a mass and an age for all stars. Results: The resulting mass distribution for the bulk of the cluster population is in the ranges of 0.2-0.4 M⊙ and 0.1-0.6 M⊙ for the ONC and NGC 2264, respectively. For the ONC, we assume that the secondary peak in the period distribution is due to high-mass objects still locked in their disks, with a locking period (Plock) of ~8 days. For NGC 2264 we make two hypotheses: (1) the stars in the secondary peak are still locked with Plock = 5 days, and (2) NGC 2264 is in a later stage in the rotational evolution. Hypothesis 2 implies in a disk-locking scenario with Plock = 8 days, a disk lifetime of 1 Myr and, after that, constant angular momentum evolution. We then simulated the period distribution of NGC 2264 when the mean age of the cluster was 1 Myr. Dichotomy and bimodality appear in the simulated distribution, presenting one peak at 2 days and another one at 5-7 days, indicating that the assumption of Plock = 8 days is plausible. Our hypotheses are compared with observational disk diagnoses available in the literature for the ONC and NGC 2264, such as near-infrared excess, Hα emission, and spectral energy distribution slope in the mid-infrared. Conclusions: Disk-locking models with Plock = 8 days and 0.2 Myr ≤ Tdisk ≤ 3 Myr are consistent with observed periods of moderate rotators of the ONC. For NGC 2264, the more promising explanation for the observed period distribution is an evolution with disk-locking (with Plock near 8 days) during the first 1 Myr, approximately, but after this, the evolution continued with constant angular momentum. Full Table 1 is only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/586/A96

Testing tidal theory for evolved stars by using red-giant binaries observed by Kepler

NASA Astrophysics Data System (ADS)

Beck, P. G.; Mathis, S.; Gallet, F.; Charbonnel, C.; Benbakoura, M.; García, R. A.; do Nascimento, J.-D.

2018-06-01

Tidal interaction governs the redistribution of angular momentum in close binary stars and planetary systems and determines the systems evolution towards the possible equilibrium state. Turbulent friction acting on the equilibrium tide in the convective envelope of low-mass stars is known to have a strong impact on this exchange of angular momentum in binaries. Moreover, theoretical modelling in recent literature as well as presented in this paper suggests that the dissipation of the dynamical tide, constituted of tidal inertial waves propagating in the convective envelope, is weak compared to the dissipation of the equilibrium tide during the red-giant phase. This prediction is confirmed when we apply the equilibrium-tide formalism developed by Zahn (1977), Verbunt & Phinney (1995), and Remus, Mathis & Zahn (2012) onto the sample of all known red-giant binaries observed by the NASA Kepler mission. Moreover, the observations are adequately explained by only invoking the equilibrium tide dissipation. Such ensemble analysis also benefits from the seismic characterisation of the oscillating components and surface rotation rates. Through asteroseismology, previous claims of the eccentricity as an evolutionary state diagnostic are discarded. This result is important for our understanding of the evolution of multiple star and planetary systems during advanced stages of stellar evolution.

General relativistic satellite astrometry. II. Modeling parallax and proper motion

NASA Astrophysics Data System (ADS)

de Felice, F.; Bucciarelli, B.; Lattanzi, M. G.; Vecchiato, A.

2001-07-01

The non-perturbative general relativistic approach to global astrometry introduced by de Felice et al. (\\cite{defetal}) is here extended to account for the star motions on the Schwarzschild celestial sphere. A new expression of the observables, i.e. angular distances among stars, is provided, which takes into account the effects of parallax and proper motions. This dynamical model is then tested on an end-to-end simulation of the global astrometry mission GAIA. The results confirm the findings of our earlier work, which applied to the case of a static (angular coordinates only) sphere. In particular, measurements of large arcs among stars (each measurement good to ~ 100 mu arcsec, as expected for V ~ 17 mag stars) repeated over an observing period comparable to the mission lifetime foreseen for GAIA, can be modeled to yield estimates of positions, parallaxes, and annual proper motions good to ~ 15 mu arcsec. This second round of experiments confirms, within the limitations of the simulation and the assumptions of the current relativistic model, that the space-born global astrometry initiated with Hipparcos can be pushed down to the 10-5 arcsec accuracy level proposed with the GAIA mission. Finally, the simplified case we have solved can be used as reference for testing the limiting behavior of more realistic models as they become available.

Two-dimensional models of early-type fast rotating stars: the ESTER project

NASA Astrophysics Data System (ADS)

Rieutord, Michel

In this talk I present the latest results of the ESTER project that has taken up the challenge of building two dimensional (axisymmetric) models of stars rotating at any rotation rate. In particular, I focus on main sequence massive and intermediate mass stars. I show what should be expected in such stars as far as the differential rotation and the associated meridional circulation are concerned, notably the emergence of a Stewartson layer along the tangent cylinder of the core. I also indicate what may be inferred about the evolution of an intermediate-mass star at constant angular momentum and how Be stars may form. I finally give some comparisons between models and observations of the gravity darkening on some nearby fast rotators as it has been derived from interferometric observations. In passing, I also discuss how 2D models can help to recover the fundamental parameters of a star.

Sizes, Shapes, and Satellites of Asteroids from Occultations

NASA Astrophysics Data System (ADS)

Waring Dunham, David; Herald, David Russell; Preston, Steve; Timerson, Bradley; Maley, Paul; Frappa, Eric; Hayamizu, Tsutomu; Talbot, John; Poro, Atila

2015-08-01

For 40 years, the sizes and shapes of dozens of asteroids have been determined from observations of asteroidal occultations. Some of the first evidence for satellites of asteroids was obtained from the early efforts; now, the orbits and sizes of some satellites discovered by other means have been refined from occultation observations. Also, several close binary stars have been discovered, and the angular diameters of some stars have been measured from analysis of these observations. The International Occultation Timing Association (IOTA) coordinates this activity worldwide, from predicting and publicizing the events, to accurately timing the occultations from as many stations as possible, and publishing and archiving the observations.The release of the Hipparcos and Tycho catalogs in 1997, from ESA’s Hipparcos space mission, revolutionized asteroidal occultation work, increasing the routine accuracy of the predictions and the annual number of observations by an order of magnitude. IOTA developed an efficient procedure for predicting the occultations using a combination of new star catalogs, based on Hipparcos and new star catalogs, generated mainly at the U. S. Naval Observatory (USNO), and new observations of asteroids relative to the improved astrometric nets mainly from USNO’s Flagstaff Astrometric Scanning Transit Telescope and JPL’s Table Mountain Observatory. In addition, many IOTA observers now use inexpensive low-light-level video cameras and specially built GPS video time inserters to accurately time the events. This automation has also allowed some observers to deploy multiple remote video stations across occultation paths. Then, one observer can record several “chords” across the asteroid. The cameras are sensitive enough that easily-hidden telescopes, many of which can be packed in standard air travel suitcases, can be used for many of the predicted occultations. IOTA’s network of regional coordinators collect and reduce the observations, which are deposited annually in Asteroid Occultations V12.0. EAR-A-3-RDR-OCCULTATIONS-V12.0. NASA Planetary Data System, 2014, where they are available to researchers worldwide.

On the Lack of Circumbinary Planets Orbiting Isolated Binary Stars

NASA Astrophysics Data System (ADS)

Fleming, David P.; Barnes, Rory; Graham, David E.; Luger, Rodrigo; Quinn, Thomas R.

2018-05-01

We outline a mechanism that explains the observed lack of circumbinary planets (CBPs) via coupled stellar–tidal evolution of isolated binary stars. Tidal forces between low-mass, short-period binary stars on the pre-main sequence slow the stellar rotations transferring rotational angular momentum to the orbit as the stars approach the tidally locked state. This transfer increases the binary orbital period, expanding the region of dynamical instability around the binary, and destabilizing CBPs that tend to preferentially orbit just beyond the initial dynamical stability limit. After the stars tidally lock, we find that angular momentum loss due to magnetic braking can significantly shrink the binary orbit, and hence the region of dynamical stability, over time, impacting where surviving CBPs are observed relative to the boundary. We perform simulations over a wide range of parameter space and find that the expansion of the instability region occurs for most plausible initial conditions and that, in some cases, the stability semimajor axis doubles from its initial value. We examine the dynamical and observable consequences of a CBP falling within the dynamical instability limit by running N-body simulations of circumbinary planetary systems and find that, typically, at least one planet is ejected from the system. We apply our theory to the shortest-period Kepler binary that possesses a CBP, Kepler-47, and find that its existence is consistent with our model. Under conservative assumptions, we find that coupled stellar–tidal evolution of pre-main sequence binary stars removes at least one close-in CBP in 87% of multi-planet circumbinary systems.

Discarded candidate companions to low-mass members of Chamaeleon I

NASA Astrophysics Data System (ADS)

Comerón, F.

2012-01-01

Context. Direct detections of brown dwarfs and planetary-mass companions to members of nearby star-forming regions provide important clues about the process of star formation, core fragmentation, and protoplanetary disk evolution. Aims: We study two faint objects at a very small angular distance from the low-mass star ESO-Hα-558 and the possible massive brown dwarf ESO-Hα-566, both of which are members of the Chamaeleon I star-forming region, to establish whether they are physical companions to those sources. If they are, their low luminosities should imply L or T spectral types, which have clearly detectable spectral features. Methods: Adaptive optics-assisted imaging and spectroscopy of both faint candidate companions has been obtained with the NACO instrument at the Very Large Telescope (VLT). Results: Photometry shows that the colors of both objects are compatible with them being moderately reddened, normal stars in the background of the Chamaeleon I clouds. This interpretation is confirmed spectroscopically, as the spectrum between 1.4 and 2.4 μm of both objects has a featureless, monotonic slope lacking the strong H2O absorption features that dominate cool stellar and substellar spectra in that domain. Conclusions: We demonstrate that the two faint sources seen very close to ESO-Hα-558 and ESO-Hα-566 are unrelated background stars, instead of giant planetary-mass companions as might be expected based on their faintness and angular proximity. Based on observations collected with the Very Large Telescope (VLT) at the European Southern Observatory, Paranal, Chile, under observing programmes 075.C-0809(B) and 078.C-0429(C).

Discovery of the Rotating Molecular Outflow and Disk in the CLASS-0/I Protostar [BHB2007]#11 in Pipe

NASA Astrophysics Data System (ADS)

Chihomi, Hara; Ryohei, Kawabe; Yoshito, Shimajiri; Junko, Ueda; Takashi, Tsukagoshi; Yasutaka, Kurono; Kazuya, Saigo; Fumitaka, Nakamura; Masao, Saito; Wilner, David

2013-07-01

The loss of angular momentum is inevitable in star formation processes, and the transportation of angular momentum by a molecular flow is widely thought to be one of the important processes. We present the results of our 2'h resolution Submillimeter Array (SMA) observations in CO, 13CO, and C18O(2-1) emissions toward a low-mass Class-0/I protostar, [BHB2007]#11 (hereafter B59#11) at the nearby star forming region, Barnard 59 in the Pipe Nebula (d=130 pc). B59#11 ejects a molecular outflow whose axis lies almost on the plane of the sky, and one of the best targets to investigate the envelope/disk rotation and the velocity structure of the molecular outflow. The 13CO and C18O observations have revealed that a compact (r ˜ 800 AU) and elongated structure of dense gas is associated with B59#11, which orients perpendicular to the outflow axis. Their distributions show the velocity gradients along their major axes, which are considered to arise from the envelope/disk rotation. The specific angular momentum is estimated to be (1.6+/-0.6)e-3 km/s pc. The power-law index of the radial profile of the rotation velocity changes from steeper one, i.e., ˜ -1 to -1/2 at a radius of 140 AU, suggesting the Keplerian disk is formed inside the radius. The central stellar mass is estimated to be ˜1.3 Msun. A collimated molecular outflow is detected from the CO observations. We found in the outflow a velocity gradient which direction is the same as that seen in the dense gas. This is interpreted to be due to the outflow rotation. The specific angular momentum of the outflow is comparable to that of the envelope, suggesting that this outflow play an important role to the ejection of the angular momentum from the envelope/disk system. This is the first case where both the Keplerian disk and the rotation of the molecular outflow were found in the Class-0 or I protostar, and provides one of good targets for ALMA to address the angular momentum ejection in course of star formation.

Modeling the early evolution of massive OB stars with an experimental wind routine. The first bi-stability jump and the angular momentum loss problem

NASA Astrophysics Data System (ADS)

Keszthelyi, Z.; Puls, J.; Wade, G. A.

2017-02-01

Context. Stellar evolution models of massive stars are very sensitive to the adopted mass-loss scheme. The magnitude and evolution of mass-loss rates significantly affect the main sequence evolution, and the properties of post-main sequence objects, including their rotational velocities. Aims: Driven by potential discrepancies between theoretically predicted and observationally derived mass-loss rates in the OB star range, we aim in particular to investigate the response to mass-loss rates that are lower than currently adopted, in parallel with the mass-loss behavior at the "first" bi-stability jump. Methods: We performed 1D hydrodynamical model calculations of single 20-60 M⊙ Galactic (Z = 0.014) stars where the effects of stellar winds are already significant in the main sequence phase. We have developed an experimental wind routine to examine the behavior and response of the models under the influence of different mass-loss rates. This observationally guided, simple and flexible wind routine is not a new mass-loss description but a useful tool based on the wind-momentum luminosity relation and other scaling relations, and provides a meaningful base for various tests and comparisons. Results: The main result of this study indicates a dichotomy between solutions of currently debated problems regarding mass-loss rates of hot massive stars. In a fully diffusive approach, and for commonly adopted initial rotational velocities, lower mass-loss rates than theoretically predicted require to invoke an additional source of angular momentum loss (either due to bi-stability braking, or yet unidentified) to brake down surface rotational velocities. On the other hand, a large jump in the mass-loss rates due to the bi-stability mechanism (a factor of 5-7 predicted by Vink et al. (2000, A&A, 362, 295), but a factor of 10-20 in modern models of massive stars) is challenged by observational results, and might be avoided if the early mass-loss rates agreed with the theoretically predicted values. Conclusions: We conclude that simultaneously adopting lower mass-loss rates and a significantly smaller jump in the mass-loss rates over the bi-stability region (both compared to presently used prescriptions) would require an additional mechanism for angular momentum loss to be present in massive stars. Otherwise, the observed rotational velocities of a large population of B supergiants, that are thought to be the evolutionary descendants of O stars, would remain unexplained.

Understanding the Accretion Engine in Pre-main Sequence Stars

NASA Astrophysics Data System (ADS)

Gómez de Castro, Ana I.

2009-05-01

Planetary systems are angular momentum reservoirs generated during star formation as a result of the joint action of gravity and angular momentum conservation. The accretion process drives to the generation of powerful engines able to drive the optical jets and the molecular outflows. A fraction of the engine energy is released into heating the circumstellar plasma to temperatures between 3000 K to 10 MK depending on the plasma location and density. There are very important unsolved problems concerning the nature of the engine, its evolution and its impact in the chemical evolution of the disk. Of special relevance is the understanding of the shear layer between the stellar photosphere and the disk; this layer controls a significant fraction of the magnetic field building up and the subsequent dissipative processes ought to be studied in the UV.

Rejuvenation of the Innocent Bystander: Testing Spin-Up in a Dwarf Carbon Star Sample

NASA Astrophysics Data System (ADS)

Green, Paul

2014-09-01

Carbon stars (C>O) were long assumed to all be giants, because only AGB stars dredge up significant carbon into their atmospheres. We now know that dwarf carbon (dC) stars are actually far more common than C giants. These dC stars are hypothesized to have accreted C-rich envelope material from an AGB companion, in systems that have likely undergone a planetary nebula phase, eventually yielding a white dwarf and a dC star that has gained both significant mass and angular momentum. To test whether the X-ray emission strength and spectral properties are consistent with a rejuvenated dynamo, we propose a Chandra pilot study of dCs selected from the SDSS; some have hot white dwarf companions (indicating more recent mass transfer), and all show Balmer emission lines (a sign of activity).

Is the dark halo of the Milky Way prolate?

NASA Astrophysics Data System (ADS)

Bowden, A.; Evans, N. W.; Williams, A. A.

2016-07-01

We introduce the flattening equation, which relates the shape of the dark halo to the angular velocity dispersions and the density of a tracer population of stars. It assumes spherical alignment of the velocity dispersion tensor, as seen in the data on stellar halo stars in the Milky Way. The angular anisotropy and gradients in the angular velocity dispersions drive the solutions towards prolateness, whilst the gradient in the stellar density is a competing effect favouring oblateness. We provide an efficient numerical algorithm to integrate the flattening equation. Using tests on mock data, we show that there is a strong degeneracy between circular speed and flattening, which can be circumvented with informative priors. Therefore, we advocate the use of the flattening equation to test for oblateness or prolateness, though the precise value of q can only be measured with the addition of the radial Jeans equation. We apply the flattening equation to a sample extracted from the Sloan Digital Sky Survey of ˜15 000 halo stars with full phase space information and errors. We find that between Galactocentric radii of 5 and 10 kpc, the shape of the dark halo is prolate, whilst even mildly oblate models are disfavoured. Strongly oblate models are ruled out. Specifically, for a logarithmic halo model, if the asymptotic circular speed v0 lies between 210 and 250 km s-1, then we find the axis ratio of the equipotentials q satisfies 1.5 ≲ q ≲ 2.

A new photometric study of the triple star system EF Draconis

NASA Astrophysics Data System (ADS)

Yang, Yuan-Gui

2012-04-01

We present new charge-coupled device (CCD) photometry for the triple star EF Draconis, obtained in 2009 and 2011. Using the updated Wilson-Devinney program, the photometric solutions were deduced from two sets of light curves. The results indicate that EF Dra is an A-type W UMa binary with a contact degree of f = 46.7%(±0.6%) and a third light of l3 ≃ 1.5%. Through analyzing the O — C curve, it is found that the orbital period shows a long-time increase with a light-time orbit. The period, semi-amplitude and eccentricity of the third body are Pmod = 17.20(±0.18) yr, A = 0.0039d(±0.0002d) and e = 0.49(±0.02) respectively. This kind of tertiary companion may extract angular momentum from the central system. The orbital period of EF Dra secularly increases at a rate of dP/dt = +3.72(±0.07) × 10-7 d yr-1, which may be interpreted by mass transfer from the less massive to the more massive component. As period increases, the separation between components may increase, which will cause the contact degree to decrease. With mass transferring, the spin angular momentum will increase, while the orbital angular momentum will decrease. Only if the contact configuration would merge at could this kind of deep-contact binary with period increasing, such as EF Dra, evolve into a rapidly-rotating single star.

Radial Angular Momentum Transfer and Magnetic Barrier for Short-type Gamma-Ray-burst Central Engine Activity

NASA Astrophysics Data System (ADS)

Liu, Tong; Liang, En-Wei; Gu, Wei-Min; Hou, Shu-Jin; Lei, Wei-Hua; Lin, Lin; Dai, Zi-Gao; Zhang, Shuang-Nan

2012-11-01

Soft extended emission (EE) following initial hard spikes up to 100 s was observed with Swift/BAT for about half of known short-type gamma-ray bursts (SGRBs). This challenges the conversional central engine models of SGRBs, i.e., compact star merger models. In the framework of black-hole-neutron-star merger models, we study the roles of radial angular momentum transfer in the disk and the magnetic barrier around the black hole in the activity of SGRB central engines. We show that radial angular momentum transfer may significantly prolong the lifetime of the accretion process, which may be divided into multiple episodes by the magnetic barrier. Our numerical calculations based on models of neutrino-dominated accretion flows suggest that disk mass is critical for producing the observed EE. In the case of the mass being ~0.8 M ⊙, our model can reproduce the observed timescale and luminosity of both the main and the EE episodes in a reasonable parameter set. The predicted luminosity of the EE component is lower than the observed EE within about one order of magnitude and the timescale is shorter than 20 s if the disk mass is ~0.2 M ⊙. Swift/BAT-like instruments may be not sensitive enough to detect the EE component in this case. We argue that the EE component could be a probe for the merger process and disk formation for compact star mergers.

Photogrammetry Of A Parabolic Antenna

NASA Technical Reports Server (NTRS)

Merrick, W. D.; Lansing, F. L.; Stoller, F. W.; Lobb, V. B.

1988-01-01

Surface measured with accuracy better than 10 to the negative fifth power times diameter. Report describes use of advanced close-range photogrammetry to determine deviations of 34-m-diameter antenna main reflector and subreflector from nominal paraboloidal shapes. Measurements enable removal of linear offsets and angular misalignments of subreflector, with consequent increase of 4 percent in aperture efficiency.

VLTI/AMBER observations of cold giant stars: atmospheric structures and fundamental parameters

NASA Astrophysics Data System (ADS)

Arroyo-Torres, B.; Martí-Vidal, I.; Marcaide, J. M.; Wittkowski, M.; Guirado, J. C.; Hauschildt, P. H.; Quirrenbach, A.; Fabregat, J.

2014-06-01

Aims: The main goal of this research is to determine the angular size and the atmospheric structures of cool giant stars (ɛ Oct, β Peg, NU Pav, ψ Peg, and γ Hya) and to compare them with hydrostatic stellar model atmospheres, to estimate the fundamental parameters, and to obtain a better understanding of the circumstellar environment. Methods: We conducted spectro-interferometric observations of ɛ Oct, β Peg, NU Pav, and ψ Peg in the near-infrared K band (2.13-2.47 μm), and γ Hya (1.9-2.47 μm) with the VLTI/AMBER instrument at medium spectral resolution (~1500). To obtain the fundamental parameters, we compared our data with hydrostatic atmosphere models (PHOENIX). Results: We estimated the Rosseland angular diameters of ɛ Oct, β Peg, NU Pav, ψ Peg, and γ Hya to be 11.66±1.50 mas, 16.87±1.00 mas, 13.03±1.75 mas, 6.31±0.35 mas, and 3.78±0.65 mas, respectively. Together with distances and bolometric fluxes (obtained from the literature), we estimated radii, effective temperatures, and luminosities of our targets. In the β Peg visibility, we observed a molecular layer of CO with a size similar to that modeled with PHOENIX. However, there is an additional slope in absorption starting around 2.3 μm. This slope is possibly due to a shell of H2O that is not modeled with PHOENIX (the size of the layer increases to about 5% with respect to the near-continuum level). The visibility of ψ Peg shows a low increase in the CO bands, compatible with the modeling of the PHOENIX model. The visibility data of ɛ Oct, NU Pav, and γ Hya show no increase in molecular bands. Conclusions: The spectra and visibilities predicted by the PHOENIX atmospheres agree with the spectra and the visibilities observed in our stars (except for β Peg). This indicates that the opacity of the molecular bands is adequately included in the model, and the atmospheres of our targets have an extension similar to the modeled atmospheres. The atmosphere of β Peg is more extended than that predicted by the model. The role of pulsations, if relevant in other cases and unmodeled by PHOENIX, therefore seems negligible for the atmospheric structures of our sample. The targets are located close to the red limits of the evolutionary tracks of the STAREVOL model, corresponding to masses between 1 M⊙ and 3 M⊙. The STAREVOL model fits the position of our stars in the Hertzsprung-Russell (HR) diagram better than the Ekström model does. STAREVOL includes thermohaline mixing, unlike the Ekström model, and complements the latter for intermediate-mass stars. Based on observations made with the VLT Interferometer (VLTI) at Paranal Observatory under programme ID 089.D-0801.Figures 2-4 are available in electronic form at http://www.aanda.org

VizieR Online Data Catalog: Pulsation model data for delta Cep and eta Aql (Merand+, 2015)

NASA Astrophysics Data System (ADS)

Merand, A.; Kervella, P.; Breitfelder, J.; Gallenne, A.; Coude du Foresto, V.; ten Brummelaar, T. A.; McAlister, H. A.; Ridgway, S.; Sturmann, L.; Sturmann, J.; Turner, N. H.

2015-09-01

FITS files containing the stars' (delta Cep and eta Aql) data and model presented in the paper. Each fits file has 3 HDU: 1- primary HDU: contains no data apart from the header. The header has the parameters of the model (keywords 'HIERARCH PARAM') as well as some other quantities derived from the modeling (keywords 'HIERARCH MODEL'). These quantities are aimed at people who would like to reproduce or compare their results with us. 2- 'DATA' HDU: this contains the data used for the fit. Each line is a scalar measurement described as follow: col1='MJD' (E) modified Julian date of the observations col2='OBS' (A50) description of the data point: the string before ";" defines the type, after ";" is the source. after | are anciliary data: for diam, UDdiam: [wavelengthum, interfbaseline_m] for mag: photometric band for color: photometric band1 - photometric band2 col3='MEAS' (E) the actual measurements. units are km/s for Vpuls or Vrad (which includes the p-factor correction), and mas (milli-arcseconds) for diameters (diam of UDdiam). col4='ERR' (E) the uncertainty on the measurement. col5='MODEL' (E) corresponding value predicted by the model col6='PHASE' (E) pulsation phase computed from the model ranges from 0 to 1. col7='PERIOD' (E) pulsation period computed from the model in days 3- 'MODEL' HDU: a tabulation of the pulsation model, as a function of pulsation phase. col1='PHASE' (E) phase from 0 to 1. col2='Vpuls' (E) pulsation velocity, in km/s. col3='Vrad' (E) radial velocity, in km/s. It is Vpuls/p-factor + Vgamma. col4='diam' (E) Rosseland angular diameter, in milliarcseconds (mas). col5='Teff' (E) effective temperature, in Kelvin. col6='Lum' (E) Luminosity in solar luminosities. col7='logg' (E) surface gravity, in log_10(cm/s2). col8,9,10='diamK xxxm' (E) biased angular diameters measured by an interferometer at baselines xxx (in m), for xxx=[100, 200, 300]. In milliarcseconds col>=11= 'MAG ...' or 'COLOR ...' (E) reddenned magnitudes or colors in various bands, depending on the data entry. '...' is the name of band for magnitudes, and pair of bands for colors. (6 data files).

Magnetorotational instability in decretion disks of critically rotating stars and the outer structure of Be and Be/X-ray disks

NASA Astrophysics Data System (ADS)

Krtička, J.; Kurfürst, P.; Krtičková, I.

2015-01-01

Context. Evolutionary models of fast-rotating stars show that the stellar rotational velocity may approach the critical speed. Critically rotating stars cannot spin up more, therefore they lose their excess angular momentum through an equatorial outflowing disk. The radial extension of such disks is unknown, partly because we lack information about the radial variations of the viscosity. Aims: We study the magnetorotational instability, which is considered to be the origin of anomalous viscosity in outflowing disks. Methods: We used analytic calculations to study the stability of outflowing disks submerged in the magnetic field. Results: The magnetorotational instability develops close to the star if the plasma parameter is large enough. At large radii the instability disappears in the region where the disk orbital velocity is roughly equal to the sound speed. Conclusions: The magnetorotational instability is a plausible source of anomalous viscosity in outflowing disks. This is also true in the region where the disk radial velocity approaches the sound speed. The disk sonic radius can therefore be roughly considered as an effective outer disk radius, although disk material may escape from the star to the insterstellar medium. The radial profile of the angular momentum-loss rate already flattens there, consequently, the disk mass-loss rate can be calculated with the sonic radius as the effective disk outer radius. We discuss a possible observation determination of the outer disk radius by using Be and Be/X-ray binaries.

THE ABSENCE OF EX-COMPANIONS IN TYPE Ia SUPERNOVA REMNANTS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Di Stefano, R.; Kilic, Mukremin, E-mail: rd@cfa.harvard.edu, E-mail: kilic@ou.edu

Type Ia supernovae (SNe Ia) play important roles in our study of the expansion and acceleration of the universe, but because we do not know the exact nature or natures of the progenitors, there is a systematic uncertainty that must be resolved if SNe Ia are to become more precise cosmic probes. No progenitor system has ever been identified either in the pre- or post-explosion images of a Ia event. There have been recent claims for and against the detection of ex-companion stars in several SNe Ia remnants. These studies, however, usually ignore the angular momentum gain of the progenitormore » white dwarf (WD), which leads to a spin-up phase and a subsequent spin-down phase before explosion. For spin-down timescales greater than 10{sup 5} years, the donor star could be too dim to detect by the time of explosion. Here we revisit the current limits on ex-companion stars to SNR 0509-67.5, a 400-year-old remnant in the Large Magellanic Cloud. If the effects of possible angular momentum gain on the WD are included, a wide range of single-degenerate progenitor models are allowed for this remnant. We demonstrate that the current absence of evidence for ex-companion stars in this remnant, as well as other SNe Ia remnants, does not necessarily provide the evidence of absence for ex-companions. We discuss potential ways to identify such ex-companion stars through deep imaging observations.« less

A Disk Origin for S-Stars in the Galactic Center?

NASA Astrophysics Data System (ADS)

Haislip, G.; Youdin, A. N.

2005-12-01

Young massive stars in the central 0.5" of our Galaxy probe dynamics around supermassive black holes, and challenge our understanding of star formation in extreme environments. Recent observations (Ghez et al. 2005, Eisenhauer et al. 2005) show large eccentricities and a seemingly random distribution of inclinations, which seems to contradict formation in a disk. We investigate scenarios in which the massive S-stars are born with circular, coplanar orbits and perturbed to their current relaxed state. John Chambers' MERCURY code is modified to include post-Newtonian corrections to the gravitational central force of a Schwarzchild hole and Lense-Thirring precession about a Kerr black hole. The role of resonant relaxation (Rauch & Tremaine, 1996) of angular momentum between S-stars and a background stellar halo is studied in this context.

VizieR Online Data Catalog: Calibrator stars catalog for interferometers (Swihart+, 2017)

NASA Astrophysics Data System (ADS)

Swihart, S. J.; Garcia, E. V.; Stassun, K. G.; van Belle, G.; Mutterspaugh, M. W.; Elias, N.

2017-04-01

In order to obtain accurate stellar angular sizes, each star was fit with a model SED using the fitting routine sedFit, written by A. Boden (van Belle & von Braun, 2009, Cat. J/ApJ/694/1085; van Belle et al. 2016, Cat. J/AJ/152/16). We compiled a list of positions, spectral types and visual magnitudes for ~3000 bright (Vmag<6) stars in the northern hemisphere with declinations -15°<δ<82° using the SIMBAD database (Wenger et al., 2000A&AS..143....9W). We chose a brightness limit (Vmag<6) given that most visible light interferometers can obtain scientifically useful data on bright stars. We also removed any stars which appear in the JMMC bad calibrator list. (4 data files).

What Makes the Foucault Pendulum Move among the Stars?

NASA Astrophysics Data System (ADS)

Phillips, Norman

2004-11-01

Foucault's pendulum exhibition in 1851 occurred in an era now known by development of the theorems of Coriolis and the formulation of dynamical meteorology by Ferrel. Yet today the behavior of the pendulum is often misunderstood. The existence of a horizontal component of Newtonian gravitation is essential for understanding the behavior with respect to the stars. Two simple mechanical principles describe why the path of oscillation is fixed only at the poles; the principle of centripetal acceleration and the principle of conservation of angular momentum. A sky map is used to describe the elegant path among the stars produced by these principles.

VizieR Online Data Catalog: 1992-1997 binary star speckle measurements (Balega+, 1999)

NASA Astrophysics Data System (ADS)

Balega, I. I.; Balega, Y. Y.; Maksimov, A. F.; Pluzhnik, E. A.; Shkhagosheva, Z. U.; Vasyuk, V. A.

2000-11-01

We present the results of speckle interferometric measurements of binary stars made with the television photon-counting camera at the 6-m Big Azimuthal Telescope (BTA) and 1-m telescope of the Special Astrophysical Observatory (SAO) between August 1992 and May 1997. The data contain 89 observations of 62 star systems on the large telescope and 21 on the smaller one. For the 6-m aperture 18 systems remained unresolved. The measured angular separation ranged from 39 mas, two times above the BTA diffraction limit, to 1593 mas. (3 data files).

Binary star speckle measurements during 1992-1997 from the SAO 6-m and 1-m telescopes in Zelenchuk

NASA Astrophysics Data System (ADS)

Balega, I. I.; Balega, Y. Y.; Maksimov, A. F.; Pluzhnik, E. A.; Shkhagosheva, Z. U.; Vasyuk, V. A.

1999-12-01

We present the results of speckle interferometric measurements of binary stars made with the television photon-counting camera at the 6-m Big Azimuthal Telescope (BTA) and 1-m telescope of the Special Astrophysical Observatory (SAO) between August 1992 and May 1997. The data contain 89 observations of 62 star systems on the large telescope and 21 on the smaller one. For the 6-m aperture 18 systems remained unresolved. The measured angular separation ranged from 39 mas, two times above the BTA diffraction limit, to 1593 mas.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Jee, I.; Komatsu, E.; Suyu, S.H., E-mail: ijee@mpa-garching.mpg.de, E-mail: komatsu@mpa-garching.mpg.de, E-mail: suyu@asiaa.sinica.edu.tw

The distance-redshift relation plays a fundamental role in constraining cosmological models. In this paper, we show that measurements of positions and time delays of strongly lensed images of a background galaxy, as well as those of the velocity dispersion and mass profile of a lens galaxy, can be combined to extract the angular diameter distance of the lens galaxy. Physically, as the velocity dispersion and the time delay give a gravitational potential (GM/r) and a mass (GM) of the lens, respectively, dividing them gives a physical size (r) of the lens. Comparing the physical size with the image positions ofmore » a lensed galaxy gives the angular diameter distance to the lens. A mismatch between the exact locations at which these measurements are made can be corrected by measuring a local slope of the mass profile. We expand on the original idea put forward by Paraficz and Hjorth, who analyzed singular isothermal lenses, by allowing for an arbitrary slope of a power-law spherical mass density profile, an external convergence, and an anisotropic velocity dispersion. We find that the effect of external convergence cancels out when dividing the time delays and velocity dispersion measurements. We derive a formula for the uncertainty in the angular diameter distance in terms of the uncertainties in the observables. As an application, we use two existing strong lens systems, B1608+656 (z{sub L}=0.6304) and RXJ1131−1231 (z{sub L}=0.295), to show that the uncertainty in the inferred angular diameter distances is dominated by that in the velocity dispersion, σ{sup 2}, and its anisotropy. We find that the current data on these systems should yield about 16% uncertainty in D{sub A} per object. This improves to 13% when we measure σ{sup 2} at the so-called sweet-spot radius. Achieving 7% is possible if we can determine σ{sup 2} with 5% precision.« less

Unsteady Plasma Ejections from Hollow Accretion Columns of Galactic Neutron Stars as a Trigger for Gamma-Ray Bursts

NASA Astrophysics Data System (ADS)

Gvaramadze, V. V.

1995-09-01

We propose a model of gamma-ray bursts (GRBs) based on close Galactic neutron stars with accretion disks. We outline a simple mechanism of unsteady plasma ejections during episodic accretion events. The relative kinetic energy of ejected blobs can be converted into gamma-rays by internal shocks. The beaming of gamma-ray emission can be responsible for the observed isotropic angular distribution of GRBs.

OGLE-2008-BLG-355Lb: A massive planet around a late-type star

DOE Office of Scientific and Technical Information (OSTI.GOV)

Koshimoto, N.; Sumi, T.; Fukagawa, M.

2014-06-20

We report the discovery of a massive planet, OGLE-2008-BLG-355Lb. The light curve analysis indicates a planet:host mass ratio of q = 0.0118 ± 0.0006 at a separation of 0.877 ± 0.010 Einstein radii. We do not measure a significant microlensing parallax signal and do not have high angular resolution images that could detect the planetary host star. Therefore, we do not have a direct measurement of the host star mass. A Bayesian analysis, assuming that all host stars have equal probability to host a planet with the measured mass ratio, implies a host star mass of M{sub h}=0.37{sub −0.17}{sup +0.30}more » M{sub ⊙} and a companion of mass M{sub P}=4.6{sub −2.2}{sup +3.7}M{sub J}, at a projected separation of r{sub ⊥}=1.70{sub −0.30}{sup +0.29} AU. The implied distance to the planetary system is D {sub L} = 6.8 ± 1.1 kpc. A planetary system with the properties preferred by the Bayesian analysis may be a challenge to the core accretion model of planet formation, as the core accretion model predicts that massive planets are far more likely to form around more massive host stars. This core accretion model prediction is not consistent with our Bayesian prior of an equal probability of host stars of all masses to host a planet with the measured mass ratio. Thus, if the core accretion model prediction is right, we should expect that follow-up high angular resolution observations will detect a host star with a mass in the upper part of the range allowed by the Bayesian analysis. That is, the host would probably be a K or G dwarf.« less

On the Lack of Circumbinary Planets Orbiting Isolated Binary Stars

NASA Astrophysics Data System (ADS)

Fleming, David; Barnes, Rory; Graham, David E.; Luger, Rodrigo; Quinn, Thomas R.

2018-04-01

To date, no binary star system with an orbital period less than 7.5 days has been observed to host a circumbinary planet (CBP), a puzzling observation given the thousands of binary stars with orbital periods < 10 days discovered by the Kepler mission (Kirk et al., 2016) and the observational biases that favor their detection (Munoz & Lai, 2015). We outline a mechanism that explains the observed lack of CBPs via coupled stellar-tidal evolution of isolated binary stars. Tidal forces between low-mass, short-period binary stars on the pre-main sequence slow the stellar rotations, transferring rotational angular momentum to the orbit as the stars approach the tidally locked state. This transfer increases the binary orbital period, expanding the region of dynamical instability around the binary, and destabilizing CBPs that tend to preferentially orbit just beyond the initial dynamical stability limit. After the stars tidally lock, we find that angular momentum loss due to magnetic braking can significantly shrink the binary orbit, and hence the region of dynamical stability, over time impacting where surviving CBPs are observed relative to the boundary. We perform simulations over a wide range of parameter space and find that the expansion of the instability region occurs for most plausible initial conditions and that in some cases, the stability semi-major axis doubles from its initial value. We examine the dynamical and observable consequences of a CBP falling within the dynamical instability limit by running N-body simulations of circumbinary planetary systems and find that typically, at least one planet is ejected from the system. We apply our theory to the shortest period Kepler binary that possesses a CBP, Kepler-47, and find that its existence is consistent with our model. Under conservative assumptions, we find that coupled stellar-tidal evolution of pre-main sequence binary stars removes at least one close-in CBP in 87% of multi-planet circumbinary systems.

Feeding supermassive black holes by collisional cascades

NASA Astrophysics Data System (ADS)

Faber, Christian; Dehnen, Walter

2018-05-01

The processes driving gas accretion on to supermassive black holes (SMBHs) are still poorly understood. Angular momentum conservation prevents gas within ˜10 pc of the black hole from reaching radii ˜10-3 pc where viscous accretion becomes efficient. Here we present simulations of the collapse of a clumpy shell of swept-up isothermal gas, which is assumed to have formed as a result of feedback from a previous episode of AGN activity. The gas falls towards the SMBH forming clumps and streams, which intersect, collide, and often form a disc. These collisions promote partial cancellations of angular momenta, resulting in further infall and more collisions. This continued collisional cascade generates a tail of gas with sufficiently small angular momenta and provides a viable route for gas inflow to sub-parsec scales. The efficiency of this process hardly depends on details, such as gas temperature, initial virial ratio and power spectrum of the gas distribution, as long as it is not strongly rotating. Adding star formation to this picture might explain the near-simultaneous formation of the S-stars (from tidally disrupted binaries formed in plunging gas streams) and the sub-parsec young stellar disc around Sgr A⋆.

Feeding supermassive black holes by collisional cascades

NASA Astrophysics Data System (ADS)

Faber, Christian; Dehnen, Walter

2018-07-01

The processes driving gas accretion on to supermassive black holes (SMBHs) are still poorly understood. Angular momentum conservation prevents gas within ˜10 pc of the black hole from reaching radii ˜10-3pc where viscous accretion becomes efficient. Here we present simulations of the collapse of a clumpy shell of swept-up isothermal gas, which is assumed to have formed as a result of feedback from a previous episode of AGN activity. The gas falls towards the SMBH forming clumps and streams, which intersect, collide and often form a disc. These collisions promote partial cancellations of angular momenta, resulting in further infall and more collisions. This continued collisional cascade generates a tail of gas with sufficiently small angular momenta and provides a viable route for gas inflow to sub-parsec scales. The efficiency of this process hardly depends on details, such as gas temperature, initial virial ratio and power spectrum of the gas distribution, as long as it is not strongly rotating. Adding star formation to this picture might explain the near-simultaneous formation of the S-stars (from tidally disrupted binaries formed in plunging gas streams) and the sub-parsec young stellar disc around Sgr A⋆.

The distribution of rotational velocities for low-mass stars in the Pleiades

NASA Technical Reports Server (NTRS)

Stauffer, John R.; Hartmann, Lee W.

1987-01-01

The available spectral type and color data for late-type Pleiades members have been reanalyzed, and new reddening estimates are obtained. New photometry for a small number of stars and a compilation of H-alpha equivalent widths for Pleiades dwarfs are presented. These data are used to examine the location of the rapid rotators in color-magnitude diagrams and the correlation between chromospheric activity and rotation. It is shown that the wide range of angular momenta exhibited by Pleiades K and M dwarfs is not necessarily produced by a combination of main-sequence spin-downs and a large age spread; it can also result from a plausible spread in initial angular momenta, coupled with initial main-sequence spin-down rates that are only weakly dependent on rotation. The new reddening estimates confirm Breger's (1985) finding of large extinctions confined to a small region in the southern portion of the Merope nebula.

An All-Sky Search for Wide Binaries in the SUPERBLINK Proper Motion Catalog

NASA Astrophysics Data System (ADS)

Hartman, Zachary; Lepine, Sebastien

2017-01-01

We present initial results from an all-sky search for Common Proper Motion (CPM) binaries in the SUPERBLINK all-sky proper motion catalog of 2.8 million stars with proper motions greater than 40 mas/yr, which has been recently enhanced with data from the GAIA mission. We initially search the SUPERBLINK catalog for pairs of stars with angular separations up to 1 degree and proper motion difference less than 40 mas/yr. In order to determine which of these pairs are real binaries, we develop a Bayesian analysis to calculate probabilities of true companionship based on a combination of proper motion magnitude, angular separation, and proper motion differences. The analysis reveals that the SUPERBLINK catalog most likely contains ~40,000 genuine common proper motion binaries. We provide initial estimates of the distances and projected physical separations of these wide binaries.

Development of a high-precision selenodetic coordinate system for the physical surface of the Moon based on LED beacons on its surface

NASA Astrophysics Data System (ADS)

Shirenin, A. M.; Mazurova, E. M.; Bagrov, A. V.

2016-11-01

The paper presents a mathematical algorithm for processing an array of angular measurements of light beacons on images of the lunar surface onboard a polar artificial lunar satellite (PALS) during the Luna-Glob mission and coordinate-time referencing of the PALS for the development of reference selenocentric coordinate systems. The algorithm makes it possible to obtain angular positions of point light beacons located on the surface of the Moon in selenocentric celestial coordinates. The operation of measurement systems that determine the position and orientation of the PALS during its active existence have been numerically simulated. Recommendations have been made for the optimal use of different types of measurements, including ground radio trajectory measurements, navigational star sensors based on the onboard star catalog, gyroscopic orientation systems, and space videos of the lunar surface.

Interferometry on a Balloon; Paving the Way for Space-based Interferometers

NASA Technical Reports Server (NTRS)

Rinehart, Stephen A.

2008-01-01

Astronomical studies at infrared wavelengths have dramatically improved our understanding of the universe, and observations with Spitzer, the upcoming Herschel mission, and SOFIA will continue to provide exciting new discoveries. The relatively low angular resolution of these missions, however, is insufficient to resolve the physical scale on which mid-to-far-infrared emission arises, resulting in source and structure ambiguities that limit our ability to answer key science questions. Interferometry enables high angular resolution at these wavelengths- a powerful tool for scientific discovery. We will build the Balloon Experimental Twin Telescope for Infrared Interferometry (BETTII), an eight-meter baseline Michelson stellar interferometer to fly on a high-altitude balloon. BETTII's spectral-spatial capability, provided by an instrument using double-Fourier techniques, will address key questions about the nature of disks in young star clusters and active galactic nuclei and the envelopes of evolved stars. BETTII will also lay the technological groundwork for future space interferometers.

The Balloon Experimental Twin Telescope for Infrared Interferometry

NASA Technical Reports Server (NTRS)

Rinehart, Stephen A.

2008-01-01

Astronomical studies at infrared wavelengths have dramatically improved our understanding of the universe, and observations with Spitzer, the upcoming Herschel mission, and SOFIA will continue to provide exciting new discoveries. The relatively low angular resolution of these missions, however, is insufficient to resolve the physical scales on which mid- to far-infrared emission arises, resulting in source and structure ambiguities that limit our ability to answer key science questions. Interferometry enables high angular resolution at these wavelengths, a powerful tool for scientific discovery, We will build the Balloon Experimental Twin Telescope for Infrared Interferometry (BETII), an eight-meter baseline Michelson stellar interferometer to fly on a high-altitude balloon. BETTII's spectral-spatial capability, provided by an instrument using double-Fourier techniques, will address key questions about the nature of disks in young star clusters and active galactic nuclei and the envelopes of evolved stars. BETTII will also lay the technological groundwork for future space interferometers,

Getting the Big Picture: Design Considerations for a ngVLA Short Spacing Array

NASA Astrophysics Data System (ADS)

Mason, Brian Scott; Cotton, William; Condon, James; Kepley, Amanda; Selina, Rob; Murphy, Eric Joseph

2018-01-01

The Next Generation VLA (ngVLA) aims to provide a revolutionary increase in cm-wavelength collecting area and sensitivity while at the same time providing excellent image fidelity for a broad spectrum of science cases. Likely ngVLA configurations currently envisioned provide sensitivity over a very wide range of spatial scales. The antenna diameter (notionally 18 meters) fundamentally limits the largest angular scales that can be reached. One simple and powerful way to image larger angular scales is to build a complementary interferometer comprising a smaller number of smaller-diameter dishes.We have investigated the requirements that such an array would need to meet in order to usefully scientifically complement the ngVLA; this poster presents the results of our investigation.

Seismic constraints on the radial dependence of the internal rotation profiles of six Kepler subgiants and young red giants

NASA Astrophysics Data System (ADS)

Deheuvels, S.; Doğan, G.; Goupil, M. J.; Appourchaux, T.; Benomar, O.; Bruntt, H.; Campante, T. L.; Casagrande, L.; Ceillier, T.; Davies, G. R.; De Cat, P.; Fu, J. N.; García, R. A.; Lobel, A.; Mosser, B.; Reese, D. R.; Regulo, C.; Schou, J.; Stahn, T.; Thygesen, A. O.; Yang, X. H.; Chaplin, W. J.; Christensen-Dalsgaard, J.; Eggenberger, P.; Gizon, L.; Mathis, S.; Molenda-Żakowicz, J.; Pinsonneault, M.

2014-04-01

Context. We still do not understand which physical mechanisms are responsible for the transport of angular momentum inside stars. The recent detection of mixed modes that contain the clear signature of rotation in the spectra of Kepler subgiants and red giants gives us the opportunity to make progress on this question. Aims: Our aim is to probe the radial dependence of the rotation profiles for a sample of Kepler targets. For this purpose, subgiants and early red giants are particularly interesting targets because their rotational splittings are more sensitive to the rotation outside the deeper core than is the case for their more evolved counterparts. Methods: We first extracted the rotational splittings and frequencies of the modes for six young Kepler red giants. We then performed a seismic modeling of these stars using the evolutionary codes Cesam2k and astec. By using the observed splittings and the rotational kernels of the optimal models, we inverted the internal rotation profiles of the six stars. Results: We obtain estimates of the core rotation rates for these stars, and upper limits to the rotation in their convective envelope. We show that the rotation contrast between the core and the envelope increases during the subgiant branch. Our results also suggest that the core of subgiants spins up with time, while their envelope spins down. For two of the stars, we show that a discontinuous rotation profile with a deep discontinuity reproduces the observed splittings significantly better than a smooth rotation profile. Interestingly, the depths that are found to be most probable for the discontinuities roughly coincide with the location of the H-burning shell, which separates the layers that contract from those that expand. Conclusions: We characterized the differential rotation pattern of six young giants with a range of metallicities, and with both radiative and convective cores on the main sequence. This will bring observational constraints to the scenarios of angular momentum transport in stars. Moreover, if the existence of sharp gradients in the rotation profiles of young red giants is confirmed, it is expected to help in distinguishing between the physical processes that could transport angular momentum in the subgiant and red giant branches. Appendices and Tables 3-9 are available in electronic form at http://www.aanda.org

Variable rotational line broadening in the Be star Achernar

NASA Astrophysics Data System (ADS)

Rivinius, Th.; Baade, D.; Townsend, R. H. D.; Carciofi, A. C.; Štefl, S.

2013-11-01

Aims: The main theoretical problem for the formation of a Keplerian disk around Be stars is how angular momentum is supplied from the star to the disk, even more so since Be stars probably rotate somewhat subcritically. For instance, nonradial pulsation may transport angular momentum to the stellar surface until (part of) this excess supports the disk-formation/replenishment. The nearby Be star Achernar is presently building a new disk and offers an excellent opportunity to observe this process from relatively close-up. Methods: Spectra from various sources and epochs are scrutinized to identify the salient stellar parameters characterizing the disk life cycle as defined by Hα emission. The variable strength of the non-radial pulsation is confirmed, but does not affect the other results. Results: For the first time it is demonstrated that the photospheric line width does vary in a Be star, by as much as Δv sini ≲ 35 km s-1. However, unlike assumptions in which a photospheric spin-up accumulates during the diskless phase and then is released into the disk as it is fed, the apparent photospheric spin-up is positively correlated with the appearance of Hα line emission. The photospheric line widths and circumstellar emission increase together, and the apparent stellar rotation declines to the value at quiescence after the Hα line emission becomes undetectable. Based on observations collected at the European Southern Observatory at La Silla and Paranal, Chile, Prog. IDs: 62.H-0319, 64.H-0548, 072.C-0513, 073.C-0784, 074.C-0012, 073.D-0547, 076.C-0431, 077.D-0390, 077.D-0605, and the technical program IDs 60.A-9120 and 60.A-9036.Appendices are available in electronic form at http://www.aanda.org

Amylose-Based Cationic Star Polymers for siRNA Delivery.

PubMed

Nishimura, Tomoki; Umezaki, Kaori; Mukai, Sada-atsu; Sawada, Shin-ichi; Akiyoshi, Kazunari

2015-01-01

A new siRNA delivery system using a cationic glyco-star polymer is described. Spermine-modified 8-arm amylose star polymer (with a degree of polymerization of approximately 60 per arm) was synthesized by chemoenzymatic methods. The cationic star polymer effectively bound to siRNA and formed spherical complexes with an average hydrodynamic diameter of 230 nm. The cationic 8-arm star polymer complexes showed superior cellular uptake characteristics and higher gene silencing effects than a cationic 1-arm polymer. These results suggest that amylose-based star polymers are a promising nanoplatform for glycobiomaterials.

The environment of the fast rotating star Achernar. III. Photospheric parameters revealed by the VLTI

NASA Astrophysics Data System (ADS)

Domiciano de Souza, A.; Kervella, P.; Moser Faes, D.; Dalla Vedova, G.; Mérand, A.; Le Bouquin, J.-B.; Espinosa Lara, F.; Rieutord, M.; Bendjoya, P.; Carciofi, A. C.; Hadjara, M.; Millour, F.; Vakili, F.

2014-09-01

Context. Rotation significantly impacts on the structure and life of stars. In phases of high rotation velocity (close to critical), the photospheric structure can be highly modified, and present in particular geometrical deformation (rotation flattening) and latitudinal-dependent flux (gravity darkening). The fastest known rotators among the nondegenerate stars close to the main sequence, Be stars, are key targets for studying the effects of fast rotation on stellar photospheres. Aims: We seek to determine the purely photospheric parameters of Achernar based on observations recorded during an emission-free phase (normal B phase). Methods: Several recent works proved that optical/IR long-baseline interferometry is the only technique able to sufficiently spatially resolve and measure photospheric parameters of fast rotating stars. We thus analyzed ESO-VLTI (PIONIER and AMBER) interferometric observations of Achernar to measure its photospheric parameters by fitting our physical model CHARRON using a Markov chain Monte Carlo method. This analysis was also complemented by spectroscopic, polarimetric, and photometric observations to investigate the status of the circumstellar environment of Achernar during the VLTI observations and to cross-check our model-fitting results. Results: Based on VLTI observations that partially resolve Achernar, we simultaneously measured five photospheric parameters of a Be star for the first time: equatorial radius (equatorial angular diameter), equatorial rotation velocity, polar inclination, position angle of the rotation axis projected on the sky, and the gravity darkening β coefficient (effective temperature distribution). The close circumstellar environment of Achernar was also investigated based on contemporaneous polarimetry, spectroscopy, and interferometry, including image reconstruction. This analysis did not reveal any important circumstellar contribution, so that Achernar was essentially in a normal B phase at least from mid-2009 to end-2012, and the model parameters derived in this work provide a fair description of its photosphere. Finally, because Achernar is the flattest interferometrically resolved fast rotator to-date, the measured β and flattening, combined with values from previous works, provide a crucial test for a recently proposed gravity darkening model. This model offers a promising explanation to the fact that the measured β parameter decreases with flattening and shows significantly lower values than the classical prediction of von Zeipel. Based on observations performed at ESO, Chile under VLTI PIONIER and AMBER programme IDs 087.D-0150 and 084.D-0456.

Exposure Time Optimization for Highly Dynamic Star Trackers

PubMed Central

Wei, Xinguo; Tan, Wei; Li, Jian; Zhang, Guangjun

2014-01-01

Under highly dynamic conditions, the star-spots on the image sensor of a star tracker move across many pixels during the exposure time, which will reduce star detection sensitivity and increase star location errors. However, this kind of effect can be compensated well by setting an appropriate exposure time. This paper focuses on how exposure time affects the star tracker under highly dynamic conditions and how to determine the most appropriate exposure time for this case. Firstly, the effect of exposure time on star detection sensitivity is analyzed by establishing the dynamic star-spot imaging model. Then the star location error is deduced based on the error analysis of the sub-pixel centroiding algorithm. Combining these analyses, the effect of exposure time on attitude accuracy is finally determined. Some simulations are carried out to validate these effects, and the results show that there are different optimal exposure times for different angular velocities of a star tracker with a given configuration. In addition, the results of night sky experiments using a real star tracker agree with the simulation results. The summarized regularities in this paper should prove helpful in the system design and dynamic performance evaluation of the highly dynamic star trackers. PMID:24618776

Planck 2013 results. XXX. Cosmic infrared background measurements and implications for star formation

NASA Astrophysics Data System (ADS)

Planck Collaboration; Ade, P. A. R.; Aghanim, N.; Armitage-Caplan, C.; Arnaud, M.; Ashdown, M.; Atrio-Barandela, F.; Aumont, J.; Baccigalupi, C.; Banday, A. J.; Barreiro, R. B.; Bartlett, J. G.; Battaner, E.; Benabed, K.; Benoît, A.; Benoit-Lévy, A.; Bernard, J.-P.; Bersanelli, M.; Bethermin, M.; Bielewicz, P.; Blagrave, K.; Bobin, J.; Bock, J. J.; Bonaldi, A.; Bond, J. R.; Borrill, J.; Bouchet, F. R.; Boulanger, F.; Bridges, M.; Bucher, M.; Burigana, C.; Butler, R. C.; Cardoso, J.-F.; Catalano, A.; Challinor, A.; Chamballu, A.; Chen, X.; Chiang, H. C.; Chiang, L.-Y.; Christensen, P. R.; Church, S.; Clements, D. L.; Colombi, S.; Colombo, L. P. L.; Couchot, F.; Coulais, A.; Crill, B. P.; Curto, A.; Cuttaia, F.; Danese, L.; Davies, R. D.; Davis, R. J.; de Bernardis, P.; de Rosa, A.; de Zotti, G.; Delabrouille, J.; Delouis, J.-M.; Désert, F.-X.; Dickinson, C.; Diego, J. M.; Dole, H.; Donzelli, S.; Doré, O.; Douspis, M.; Dupac, X.; Efstathiou, G.; Enßlin, T. A.; Eriksen, H. K.; Finelli, F.; Forni, O.; Frailis, M.; Franceschi, E.; Galeotta, S.; Ganga, K.; Ghosh, T.; Giard, M.; Giraud-Héraud, Y.; González-Nuevo, J.; Górski, K. M.; Gratton, S.; Gregorio, A.; Gruppuso, A.; Hansen, F. K.; Hanson, D.; Harrison, D.; Helou, G.; Henrot-Versillé, S.; Hernández-Monteagudo, C.; Herranz, D.; Hildebrandt, S. R.; Hivon, E.; Hobson, M.; Holmes, W. A.; Hornstrup, A.; Hovest, W.; Huffenberger, K. M.; Jaffe, A. H.; Jaffe, T. R.; Jones, W. C.; Juvela, M.; Kalberla, P.; Keihänen, E.; Kerp, J.; Keskitalo, R.; Kisner, T. S.; Kneissl, R.; Knoche, J.; Knox, L.; Kunz, M.; Kurki-Suonio, H.; Lacasa, F.; Lagache, G.; Lähteenmäki, A.; Lamarre, J.-M.; Langer, M.; Lasenby, A.; Laureijs, R. J.; Lawrence, C. R.; Leonardi, R.; León-Tavares, J.; Lesgourgues, J.; Liguori, M.; Lilje, P. B.; Linden-Vørnle, M.; López-Caniego, M.; Lubin, P. M.; Macías-Pérez, J. F.; Maffei, B.; Maino, D.; Mandolesi, N.; Maris, M.; Marshall, D. J.; Martin, P. G.; Martínez-González, E.; Masi, S.; Massardi, M.; Matarrese, S.; Matthai, F.; Mazzotta, P.; Melchiorri, A.; Mendes, L.; Mennella, A.; Migliaccio, M.; Mitra, S.; Miville-Deschênes, M.-A.; Moneti, A.; Montier, L.; Morgante, G.; Mortlock, D.; Munshi, D.; Murphy, J. A.; Naselsky, P.; Nati, F.; Natoli, P.; Netterfield, C. B.; Nørgaard-Nielsen, H. U.; Noviello, F.; Novikov, D.; Novikov, I.; Osborne, S.; Oxborrow, C. A.; Paci, F.; Pagano, L.; Pajot, F.; Paladini, R.; Paoletti, D.; Partridge, B.; Pasian, F.; Patanchon, G.; Perdereau, O.; Perotto, L.; Perrotta, F.; Piacentini, F.; Piat, M.; Pierpaoli, E.; Pietrobon, D.; Plaszczynski, S.; Pointecouteau, E.; Polenta, G.; Ponthieu, N.; Popa, L.; Poutanen, T.; Pratt, G. W.; Prézeau, G.; Prunet, S.; Puget, J.-L.; Rachen, J. P.; Reach, W. T.; Rebolo, R.; Reinecke, M.; Remazeilles, M.; Renault, C.; Ricciardi, S.; Riller, T.; Ristorcelli, I.; Rocha, G.; Rosset, C.; Roudier, G.; Rowan-Robinson, M.; Rubiño-Martín, J. A.; Rusholme, B.; Sandri, M.; Santos, D.; Savini, G.; Scott, D.; Seiffert, M. D.; Serra, P.; Shellard, E. P. S.; Spencer, L. D.; Starck, J.-L.; Stolyarov, V.; Stompor, R.; Sudiwala, R.; Sunyaev, R.; Sureau, F.; Sutton, D.; Suur-Uski, A.-S.; Sygnet, J.-F.; Tauber, J. A.; Tavagnacco, D.; Terenzi, L.; Toffolatti, L.; Tomasi, M.; Tristram, M.; Tucci, M.; Tuovinen, J.; Türler, M.; Valenziano, L.; Valiviita, J.; Van Tent, B.; Vielva, P.; Villa, F.; Vittorio, N.; Wade, L. A.; Wandelt, B. D.; Welikala, N.; White, M.; White, S. D. M.; Winkel, B.; Yvon, D.; Zacchei, A.; Zonca, A.

2014-11-01

We present new measurements of cosmic infrared background (CIB) anisotropies using Planck. Combining HFI data with IRAS, the angular auto- and cross-frequency power spectrum is measured from 143 to 3000 GHz, and the auto-bispectrum from 217 to 545 GHz. The total areas used to compute the CIB power spectrum and bispectrum are about 2240 and 4400 deg2, respectively. After careful removal of the contaminants (cosmic microwave background anisotropies, Galactic dust, and Sunyaev-Zeldovich emission), and a complete study of systematics, the CIB power spectrum is measured with unprecedented signal to noise ratio from angular multipoles ℓ ~ 150 to 2500. The bispectrum due to the clustering of dusty, star-forming galaxies is measured from ℓ ~ 130 to 1100, with a total signal to noise ratio of around 6, 19, and 29 at 217, 353, and 545 GHz, respectively. Two approaches are developed for modelling CIB power spectrum anisotropies. The first approach takes advantage of the unique measurements by Planck at large angular scales, and models only the linear part of the power spectrum, with a mean bias of dark matter haloes hosting dusty galaxies at a given redshift weighted by their contribution to the emissivities. The second approach is based on a model that associates star-forming galaxies with dark matter haloes and their subhaloes, using a parametrized relation between the dust-processed infrared luminosity and (sub-)halo mass. The two approaches simultaneously fit all auto- and cross-power spectra very well. We find that the star formation history is well constrained up to redshifts around 2, and agrees with recent estimates of the obscured star-formation density using Spitzer and Herschel. However, at higher redshift, the accuracy of the star formation history measurement is strongly degraded by the uncertainty in the spectral energy distribution of CIB galaxies. We also find that the mean halo mass which is most efficient at hosting star formation is log (Meff/M⊙) = 12.6 and that CIB galaxies have warmer temperatures as redshift increases. The CIB bispectrum is steeper than that expected from the power spectrum, although well fitted by a power law; this gives some information about the contribution of massive haloes to the CIB bispectrum. Finally, we show that the same halo occupation distribution can fit all power spectra simultaneously. The precise measurements enabled by Planck pose new challenges for the modelling of CIB anisotropies, indicating the power of using CIB anisotropies to understand the process of galaxy formation.

A Study of Mechanisms Producing Astrophysical Jets.

DTIC Science & Technology

1988-03-01

REPORT NUMBER(S) IS0001f A FAL- T R-88-007 6a. NAME OF PERFORMING ORGANIZATION 6b. OFFICE SYMBOL 7a. NAME OF MONITORING ORGANIZATION (if applicable...entire rest energy by some , ,, . , , , - , . , . ,4 . ,... .- t . mechanism or other. If matter is in-falling in this fashion, conservation of angular...they are still enveloped in the dense gas and dust from which they are condensing. Associated with a class of these stars, the T Tauri stars, are

Evaluation of the table Mountain Ronchi telescope for angular tracking

NASA Technical Reports Server (NTRS)

Lanyi, G.; Purcell, G.; Treuhaft, R.; Buffington, A.

1992-01-01

The performance of the University of California at San Diego (UCSD) Table Mountain telescope was evaluated to determine the potential of such an instrument for optical angular tracking. This telescope uses a Ronchi ruling to measure differential positions of stars at the meridian. The Ronchi technique is summarized and the operational features of the Table Mountain instrument are described. Results from an analytic model, simulations, and actual data are presented that characterize the telescope's current performance. For a star pair of visual magnitude 7, the differential uncertainty of a 5-min observation is about 50 nrad (10 marcsec), and tropospheric fluctuations are the dominant error source. At magnitude 11, the current differential uncertainty is approximately 800 nrad (approximately 170 marcsec). This magnitude is equivalent to that of a 2-W laser with a 0.4-m aperture transmitting to Earth from a spacecraft at Saturn. Photoelectron noise is the dominant error source for stars of visual magnitude 8.5 and fainter. If the photoelectron noise is reduced, ultimately tropospheric fluctuations will be the limiting source of error at an average level of 35 nrad (7 marcsec) for stars approximately 0.25 deg apart. Three near-term strategies are proposed for improving the performance of the telescope to the 10-nrad level: improving the efficiency of the optics, masking background starlight, and averaging tropospheric fluctuations over multiple observations.

Precise attitude rate estimation using star images obtained by mission telescope for satellite missions

NASA Astrophysics Data System (ADS)

Inamori, Takaya; Hosonuma, Takayuki; Ikari, Satoshi; Saisutjarit, Phongsatorn; Sako, Nobutada; Nakasuka, Shinichi

2015-02-01

Recently, small satellites have been employed in various satellite missions such as astronomical observation and remote sensing. During these missions, the attitudes of small satellites should be stabilized to a higher accuracy to obtain accurate science data and images. To achieve precise attitude stabilization, these small satellites should estimate their attitude rate under the strict constraints of mass, space, and cost. This research presents a new method for small satellites to precisely estimate angular rate using star blurred images by employing a mission telescope to achieve precise attitude stabilization. In this method, the angular velocity is estimated by assessing the quality of a star image, based on how blurred it appears to be. Because the proposed method utilizes existing mission devices, a satellite does not require additional precise rate sensors, which makes it easier to achieve precise stabilization given the strict constraints possessed by small satellites. The research studied the relationship between estimation accuracy and parameters used to achieve an attitude rate estimation, which has a precision greater than 1 × 10-6 rad/s. The method can be applied to all attitude sensors, which use optics systems such as sun sensors and star trackers (STTs). Finally, the method is applied to the nano astrometry satellite Nano-JASMINE, and we investigate the problems that are expected to arise with real small satellites by performing numerical simulations.

Report on carbon and nitrogen abundance studies

NASA Technical Reports Server (NTRS)

Boehm-Vitense, Erika

1991-01-01

The aim of the proposal was to determine the nitrogen to carbon abundance ratios from transition layer lines in stars with different T(sub eff) and luminosities. The equations which give the surface emission line fluxes and the measured ratio of the NV to CIV emission line fluxes are presented and explained. The abundance results are compared with those of photospheric abundance studies for stars in common with the photospheric investigations. The results show that the analyses are at least as accurate as the photospheric determinations. These studies can be extended to F and early G stars for which photospheric abundance determinations for giants are hard to do because molecular bands become too weak. The abundance determination in the context of stellar evolution is addressed. The N/C abundance ratio increases steeply at the point of evolution for which the convection zone reaches deepest. Looking at the evolution of the rotation velocities v sin i, a steep decrease in v sin i is related to the increasing depth of the convection zone. It is concluded that the decrease in v sin i for T(sub eff) less than or approximately = 5800 K is most probably due to the rearrangement of the angular momentum in the stars due to deep convective mixing. It appears that the convection zone is rotating with nearly depth independent angular momentum. Other research results and ongoing projects are discussed.

Toward a renewed Galactic Cepheid distance scale from Gaia and optical interferometry

NASA Astrophysics Data System (ADS)

Kervella, Pierre; Mérand, Antoine; Gallenne, Alexandre; Trahin, Boris; Nardetto, Nicolas; Anderson, Richard I.; Breitfelder, Joanne; Szabados, Laszlo; Bond, Howard E.; Borgniet, Simon; Gieren, Wolfgang; Pietrzyński, Grzegorz

2017-09-01

Through an innovative combination of multiple observing techniques and modeling, we are assembling a comprehensive understanding of the pulsation and close environment of Cepheids. We developed the SPIPS modeling tool that combines all observables (radial velocimetry, photometry, angular diameters from interferometry) to derive the relevant physical parameters of the star (effective temperature, infrared excess, reddening, …) and the ratio of the distance and the projection factor d/p. We present the application of SPIPS to the long-period Cepheid RS Pup, for which we derive p = 1.25±0.06. The addition of this massive Cepheid consolidates the existing sample of p-factor measurements towards long-period pulsators. This allows us to conclude that p is constant or mildly variable around p = 1.29±0.04 (±3%) as a function of the pulsation period. The forthcoming Gaia DR2 will provide a considerable improvement in quantity and accuracy of the trigonometric parallaxes of Cepheids. From this sample, the SPIPS modeling tool will enable a robust calibration of the Cepheid distance scale.

SOUTHERN MASSIVE STARS AT HIGH ANGULAR RESOLUTION: OBSERVATIONAL CAMPAIGN AND COMPANION DETECTION

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sana, H.; Le Bouquin, J.-B.; Duvert, G.

2014-11-01

Multiplicity is one of the most fundamental observable properties of massive O-type stars and offers a promising way to discriminate between massive star formation theories. Nevertheless, companions at separations between 1 and 100 milliarcsec (mas) remain mostly unknown due to intrinsic observational limitations. At a typical distance of 2 kpc, this corresponds to projected physical separations of 2-200 AU. The Southern MAssive Stars at High angular resolution survey (SMaSH+) was designed to fill this gap by providing the first systematic interferometric survey of Galactic massive stars. We observed 117 O-type stars with VLTI/PIONIER and 162 O-type stars with NACO/Sparse Aperturemore » Masking (SAM), probing the separation ranges 1-45 and 30-250 mas and brightness contrasts of ΔH < 4 and ΔH < 5, respectively. Taking advantage of NACO's field of view, we further uniformly searched for visual companions in an 8'' radius down to ΔH = 8. This paper describes observations and data analysis, reports the discovery of almost 200 new companions in the separation range from 1 mas to 8'' and presents a catalog of detections, including the first resolved measurements of over a dozen known long-period spectroscopic binaries. Excluding known runaway stars for which no companions are detected, 96 objects in our main sample (δ < 0°; H < 7.5) were observed both with PIONIER and NACO/SAM. The fraction of these stars with at least one resolved companion within 200 mas is 0.53. Accounting for known but unresolved spectroscopic or eclipsing companions, the multiplicity fraction at separation ρ < 8'' increases to f {sub m} = 0.91 ± 0.03. The fraction of luminosity class V stars that have a bound companion reaches 100% at 30 mas while their average number of physically connected companions within 8'' is f {sub c} = 2.2 ± 0.3. This demonstrates that massive stars form nearly exclusively in multiple systems. The nine non-thermal radio emitters observed by SMaSH+ are all resolved, including the newly discovered pairs HD 168112 and CPD–47°2963. This lends strong support to the universality of the wind-wind collision scenario to explain the non-thermal emission from O-type stars.« less

EVIDENCE FOR CLUSTER TO CLUSTER VARIATIONS IN LOW-MASS STELLAR ROTATIONAL EVOLUTION

DOE Office of Scientific and Technical Information (OSTI.GOV)

Coker, Carl T.; Pinsonneault, Marc; Terndrup, Donald M., E-mail: coker@astronomy.ohio-state.edu, E-mail: pinsono@astronomy.ohio-state.edu, E-mail: terndrup@astronomy.ohio-state.edu

2016-12-10

The concordance model for angular momentum evolution postulates that star-forming regions and clusters are an evolutionary sequence that can be modeled with assumptions about protostar–disk coupling, angular momentum loss from magnetized winds that saturates in a mass-dependent fashion at high rotation rates, and core-envelope decoupling for solar analogs. We test this approach by combining established data with the large h Per data set from the MONITOR project and new low-mass Pleiades data. We confirm prior results that young low-mass stars can be used to test star–disk coupling and angular momentum loss independent of the treatment of internal angular momentum transport.more » For slow rotators, we confirm the need for star–disk interactions to evolve the ONC to older systems, using h Per (age 13 Myr) as our natural post-disk case. There is no evidence for extremely long-lived disks as an alternative to core-envelope decoupling. However, our wind models cannot evolve rapid rotators from h Per to older systems consistently, and we find that this result is robust with respect to the choice of angular momentum loss prescription. We outline two possible solutions: either there is cosmic variance in the distribution of stellar rotation rates in different clusters or there are substantially enhanced torques in low-mass rapid rotators. We favor the former explanation and discuss observational tests that could be used to distinguish them. If the distribution of initial conditions depends on environment, models that test parameters by assuming a universal underlying distribution of initial conditions will need to be re-evaluated.« less

LITHIUM DEPLETION IS A STRONG TEST OF CORE-ENVELOPE RECOUPLING

DOE Office of Scientific and Technical Information (OSTI.GOV)

Somers, Garrett; Pinsonneault, Marc H., E-mail: somers@astronomy.ohio-state.edu

2016-09-20

Rotational mixing is a prime candidate for explaining the gradual depletion of lithium from the photospheres of cool stars during the main sequence. However, previous mixing calculations have relied primarily on treatments of angular momentum transport in stellar interiors incompatible with solar and stellar data in the sense that they overestimate the internal differential rotation. Instead, recent studies suggest that stars are strongly differentially rotating at young ages but approach a solid body rotation during their lifetimes. We modify our rotating stellar evolution code to include an additional source of angular momentum transport, a necessary ingredient for explaining the openmore » cluster rotation pattern, and examine the consequences for mixing. We confirm that core-envelope recoupling with a ∼20 Myr timescale is required to explain the evolution of the mean rotation pattern along the main sequence, and demonstrate that it also provides a more accurate description of the Li depletion pattern seen in open clusters. Recoupling produces a characteristic pattern of efficient mixing at early ages and little mixing at late ages, thus predicting a flattening of Li depletion at a few Gyr, in agreement with the observed late-time evolution. Using Li abundances we argue that the timescale for core-envelope recoupling during the main sequence decreases sharply with increasing mass. We discuss the implications of this finding for stellar physics, including the viability of gravity waves and magnetic fields as agents of angular momentum transport. We also raise the possibility of intrinsic differences in initial conditions in star clusters using M67 as an example.« less

Is the Young UY Auriga System a Triple?

NASA Astrophysics Data System (ADS)

Wittal, Matthew; Prato, Lisa A.; Schaefer, Gail; Ciardi, David R.; Thomas, Allen; Biddle, Lauren; Avilez, Ian; Muzzio, Ryan; Patience, Jennifer; Beichman, Charles

2017-01-01

In an effort to understand the nature of the young binary, UY Aur, we examined the variable behavior of the entire, unresolved 0.9 arcsecond system, as well as the behavior of the angularly resolved, individual A and B components. UY Aur is an approximately 2 Myr old, classical T Tauri in the Taurus-Auriga star forming region and is one of a handful of young systems to host a primordial circumbinary disk, as well as individual circumstellar disks. Using the the facility infrared, high-resolution NIRSPEC spectrograph behind the adaptive optics system at the 10-meter Keck II telescope, we observed a dramatic change in the spectra of UY Aur B between 2003 and 2010. We also identified flux variability in the individual components of 1—2 magnitudes, particularly in the secondary star, on the basis of historical photometry. Thermal dust and line emission observed with millimeter interferometry indicates complex dynamical behavior of the circumbinary and circumstellar dust and led Tang et al. (2014) to speculate that UY Aur B may itself be a binary. Our adaptive optics imaging with the Keck II telescope showed no evidence for a close companion to the B component, although the marked change in our spectra of this star suggest that it could be a spectroscopic binary. We are currently limited by the paucity of angularly resolved observations, both photometric and spectroscopic, hampering the interpretation of the data. High-cadence, angularly resolved spectroscopy and photometry will be required to confirm the potential higher-order multiplicity of this system. This research was supported in part by NSF grants AST-1461200 and AST-1313399.

The origin of kinematically distinct cores and misaligned gas discs in galaxies from cosmological simulations

NASA Astrophysics Data System (ADS)

Taylor, Philip; Federrath, Christoph; Kobayashi, Chiaki

2018-06-01

Integral field spectroscopy surveys provide spatially resolved gas and stellar kinematics of galaxies. They have unveiled a range of atypical kinematic phenomena, which require detailed modelling to understand. We present results from a cosmological simulation that includes stellar and AGN feedback. We find that the distribution of angles between the gas and stellar angular momenta of galaxies is not affected by projection effects. We examine five galaxies (≈6 per cent of well resolved galaxies) that display atypical kinematics; two of the galaxies have kinematically distinct cores (KDC), while the other three have counter-rotating gas and stars. All five form the majority of their stars in the field, subsequently falling into cosmological filaments where the relative orientation of the stellar angular momentum and the bulk gas flow leads to the formation of a counter-rotating gas disc. The accreted gas exchanges angular momentum with pre-existing co-rotating gas causing it to fall to the centre of the galaxy. This triggers low-level AGN feedback, which reduces star formation. Later, two of the galaxies experience a minor merger (stellar mass ratio ˜1/10) with a galaxy on a retrograde orbit compared to the spin of the stellar component of the primary. This produces the KDCs, and is a different mechanism than suggested by other works. The role of minor mergers in the kinematic evolution of galaxies may have been under-appreciated in the past, and large, high-resolution cosmological simulations will be necessary to gain a better understanding in this area.

Interferometric Gravity Darkening Observations of Vega with the CHARA Array

NASA Astrophysics Data System (ADS)

Aufdenberg, J. P.; Merand, A.; Coude Foresto, V.; Absil, O.; Di Folco, E.; Kervella, P.; Ridgway, S. T.; Sturmann, J.; Sturmann, L.; ten Brummelaar, T. A.; Turner, N. H.; Berger, D. H.; McAlister, H. A.

2005-12-01

We have obtained high-precision interferometric measurements of the A0 V standard star Vega with the Center for High Angular Resolution Astronomy (CHARA) Array and the Fiber Linked Unit for Optical Recombination (FLUOR) beam combiner in the K' band at projected baselines between 103 m and 273 m. The measured squared visibility amplitudes beyond the first lobe are significantly weaker than expected for a slowly rotating star and provide strong evidence for the model of Vega as a rapidly rotating star viewed very nearly pole on. We have constructed a Roche-von Zeipel gravity-darkened model atmosphere which is in generally good agreement with both our interferometric data and archival spectrophotometry. Our model indicates Vega is rotating at ˜92% of its angular break-up rate with an equatorial velocity of ˜275 km s-1. We find a polar effective temperature of ˜10150 K and a pole-to-equator effective temperature difference of ˜2500 K, much larger than the ˜300 K derived by Gulliver, Hill, and Adelman. Our model suggests that Vega's cool equatorial atmosphere may have significant convective flux and predicts a significantly cooler spectral energy distribution for Vega as seen by its surrounding debris disk. This work was performed in part under contract with the Jet Propulsion Laboratory (JPL) funded by NASA through the Michelson Fellowship Program. JPL is managed for NASA by the California Institute of Technology. The CHARA Array is operated by the Center for High Angular Resolution Astronomy with support from Georgia State University and the National Science Foundation, the Keck Foundation and the Packard Foundation.

A Universal Spin–Mass Relation for Brown Dwarfs and Planets

NASA Astrophysics Data System (ADS)

Scholz, Aleks; Moore, Keavin; Jayawardhana, Ray; Aigrain, Suzanne; Peterson, Dawn; Stelzer, Beate

2018-06-01

While brown dwarfs show similarities to stars early in their lives, their spin evolutions are much more akin to those of planets. We have used light curves from the K2 mission to measure new rotation periods for 18 young brown dwarfs in the Taurus star-forming region. Our sample spans masses from 0.02 to 0.08 M ⊙ and has been characterized extensively in the past. To search for periods, we utilize three different methods (autocorrelation, periodogram, Gaussian processes). The median period for brown dwarfs with disks is twice as long as for those without (3.1 versus 1.6 days), a signature of rotational braking by the disk, albeit with small numbers. With an overall median period of 1.9 days, brown dwarfs in Taurus rotate slower than their counterparts in somewhat older (3–10 Myr) star-forming regions, consistent with spin-up of the latter due to contraction and angular momentum conservation, a clear sign that disk braking overall is inefficient and/or temporary in this mass domain. We confirm the presence of a linear increase of the typical rotation period as a function of mass in the substellar regime. The rotational velocities, when calculated forward to the age of the solar system, assuming angular momentum conservation, fit the known spin–mass relation for solar system planets and extra-solar planetary-mass objects. This spin–mass trend holds over six orders of magnitude in mass, including objects from several different formation paths. Our result implies that brown dwarfs by and large retain their primordial angular momentum through the first few Myr of their evolution.

SIM Lite Detection of Habitable Planets in P-Type Binary-Planetary Systems

NASA Technical Reports Server (NTRS)

Pan, Xiaopei; Shao, Michael; Shaklan, Stuart; Goullioud, Renaud

2010-01-01

Close binary stars like spectroscopic binaries create a completely different environment than single stars for the evolution of a protoplanetary disk. Dynamical interactions between one star and protoplanets in such systems provide more challenges for theorists to model giant planet migration and formation of multiple planets. For habitable planets the majority of host stars are in binary star systems. So far only a small amount of Jupiter-size planets have been discovered in binary stars, whose minimum separations are 20 AU and the median value is about 1000 AU (because of difficulties in radial velocity measurements). The SIM Lite mission, a space-based astrometric observatory, has a unique capability to detect habitable planets in binary star systems. This work analyzed responses of the optical system to the field stop for companion stars and demonstrated that SIM Lite can observe exoplanets in visual binaries with small angular separations. In particular we investigated the issues for the search for terrestrial planets in P-type binary-planetary systems, where the planets move around both stars in a relatively distant orbit.

Gravitational radiation from rapidly rotating nascent neutron stars

NASA Technical Reports Server (NTRS)

Lai, Dong; Shapiro, Stuart L.

1995-01-01

We study the secular evolution and gravitational wave signature of a newly formed, rapidly rotating neutron star. The neutron star may arise from core collapse in a massive star or from the accretion-induced collapse of a white dwarf. After a brief dynamical phase, the nascent neutron star settles into an axisymmetric, secularly unstable state. Gravitational radiation drives the star to a nonaxisymmetric, stationary equilibrium configuration via the bar-mode instability. The emitted quasi-periodic gravitational waves have a unique signature: the wave frequency sweeps downward from a few hundred Hertz to zero, while the wave amplitude increase from zero to a maximum and then decays back to zero. Such a wave signal could detected by broadband gravitational wave interferometers currently being constructed. We also characterize two other types of gravitational wave signals that could arise in principle from a rapidly rotating, secularly unstable neutron star: a high-frequency (f greater than or approximately = 1000 Hz) wave which increases the pattern-speed of the star, and a wave that actually increases the angular momentum of the star.

THE TAOS PROJECT: RESULTS FROM SEVEN YEARS OF SURVEY DATA

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhang, Z.-W.; Lehner, M. J.; Wang, J.-H.

2013-07-01

The Taiwanese-American Occultation Survey (TAOS) aims to detect serendipitous occultations of stars by small ({approx}1 km diameter) objects in the Kuiper Belt and beyond. Such events are very rare (<10{sup -3} events per star per year) and short in duration ({approx}200 ms), so many stars must be monitored at a high readout cadence. TAOS monitors typically {approx}500 stars simultaneously at a 5 Hz readout cadence with four telescopes located at Lulin Observatory in central Taiwan. In this paper, we report the results of the search for small Kuiper Belt objects (KBOs) in seven years of data. No occultation events weremore » found, resulting in a 95% c.l. upper limit on the slope of the faint end of the KBO size distribution of q = 3.34-3.82, depending on the surface density at the break in the size distribution at a diameter of about 90 km.« less

Cool stars, stellar systems, and the sun; Proceedings of the 7th Cambridge Workshop, Tucson, AZ, Oct. 9-12, 1991

NASA Technical Reports Server (NTRS)

Giampapa, Mark S. (Editor); Bookbinder, Jay A. (Editor)

1992-01-01

Consideration is given to HST observations of late-type stars, molecular absorption in the UV spectrum of Alpha Ori, EUV emission from late-type stars, Rosat observations of the Pleiades cluster, a deep ROSAT observation of the Hyades cluster, optical spectroscopy detected by EXOSAT, stellar photospheric convection, a structure of the solar X-ray corona, magnetic surface images of the BY Dra Star HD 82558, a Zebra interpretatin of BY Dra stars, optical flares on II Peg, a low-resolution spectroscopic survey of post-T tauri candidates, millimeter and sub-millimeter emission from flare stars, and activity in tidally interacting binaries. Attention is also given to modeling stellar angular momentum evolution, extended 60-micron emission from nearby Mira variables, the PANDORA atmosphere program, the global properties of active regions, oscillations in a stratified atmosphere, lithium abundances in northern RS CVn binaries, a new catalog of cool dwarf stars, the Far UV Spectrograph Explorer, and development of reflecting coronagraphs.

RUNAWAY DWARF CARBON STARS AS CANDIDATE SUPERNOVA EJECTA

DOE Office of Scientific and Technical Information (OSTI.GOV)

Plant, Kathryn A.; Margon, Bruce; Guhathakurta, Puragra

2016-12-20

The dwarf carbon (dC) star SDSS J112801.67+004034.6 has an unusually high radial velocity, 531 ± 4 km s{sup −1}. We present proper motion and new spectroscopic observations which imply a large Galactic rest frame velocity, 425 ± 9 km s{sup −1}. Several other SDSS dC stars are also inferred to have very high galactocentric velocities, again each based on both high heliocentric radial velocity and also confidently detected proper motions. Extreme velocities and the presence of C {sub 2} bands in the spectra of dwarf stars are both rare. Passage near the Galactic center can accelerate stars to such extreme velocities, but the largemore » orbital angular momentum of SDSS J1128 precludes this explanation. Ejection from a supernova in a binary system or disruption of a binary by other stars are possibilities, particularly as dC stars are thought to obtain their photospheric C {sub 2} via mass transfer from an evolved companion.« less

OB Stars and Cepheids From the Gaia TGAS Catalogue: Test of their Distances and Proper Motions

NASA Astrophysics Data System (ADS)

Bobylev, Vadim V.; Bajkova, Anisa T.

2017-12-01

We consider young distant stars from the Gaia TGAS catalog. These are 250 classical Cepheids and 244 OB stars located at distances up to 4 kpc from the Sun. These stars are used to determine the Galactic rotation parameters using both trigonometric parallaxes and proper motions of the TGAS stars. In this case the considered stars have relative parallax errors less than 200%. Following the well-known statistical approach, we assume that the kinematic parameters found from the line-of-sight velocities Vr are less dependent on errors of distances than the found from the velocity components Vl. From values of the first derivative of the Galactic rotation angular velocity '0, found from the analysis of velocities Vr and Vl separately, the scale factor of distances is determined.We found that from the sample of Cepheids the scale of distances of the TGAS should be reduced by 3%, and from the sample of OB stars, on the contrary, the scale should be increased by 9%.

Three-dimensional Hydrodynamical Simulations of Mass Transfer in Binary Systems by a Free Wind

DOE Office of Scientific and Technical Information (OSTI.GOV)

Liu, Zheng-Wei; Stancliffe, Richard J.; Abate, Carlo

A large fraction of stars in binary systems are expected to undergo mass and angular momentum exchange at some point in their evolution, which can drastically alter the chemical and dynamical properties and fates of the systems. Interaction by stellar wind is an important process in wide binaries. However, the details of wind mass transfer are still not well understood. We perform three-dimensional hydrodynamical simulations of wind mass transfer in binary systems to explore mass-accretion efficiencies and geometries of mass outflows, for a range of mass ratios from 0.05 to 1.0. In particular, we focus on the case of amore » free wind, in which some physical mechanism accelerates the expelled wind material balancing the gravity of the mass-losing star with the wind velocity comparable to the orbital velocity of the system. We find that the mass-accretion efficiency and accreted specific angular momentum increase with the mass ratio of the system. For an adiabatic wind, we obtain that the accretion efficiency onto the secondary star varies from about 0.1% to 8% for mass ratios between 0.05 and 1.0.« less

The Milli-Arc-Second Structure Imager, MASSIM: A New Concept for a High Angular Resolution X-ray Telescope

NASA Technical Reports Server (NTRS)

Skinner, Gerry; Arzoumanian, Z.; Cash, W.; Gehrels, N.; Gendreau, K.; Gorenstein, P.; Krizmanic, J.; Leitner, J.; Miller, M.; Reasenberg, R.;

Skyrmions, Skyrme stars and black holes with Skyrme hair in five spacetime dimension

NASA Astrophysics Data System (ADS)

Brihaye, Yves; Herdeiro, Carlos; Radu, Eugen; Tchrakian, D. H.

2017-11-01

We consider a class of generalizations of the Skyrme model to five spacetime dimensions ( d = 5), which is defined in terms of an O(5) sigma model. A special ansatz for the Skyrme field allows angular momentum to be present and equations of motion with a radial dependence only. Using it, we obtain: 1) everywhere regular solutions describing localised energy lumps ( Skyrmions); 2) Self-gravitating, asymptotically flat, everywhere non-singular solitonic solutions ( Skyrme stars), upon minimally coupling the model to Einstein's gravity; 3) both static and spinning black holes with Skyrme hair, the latter with rotation in two orthogonal planes, with both angular momenta of equal magnitude. In the absence of gravity we present an analytic solution that satisfies a BPS-type bound and explore numerically some of the non-BPS solutions. In the presence of gravity, we contrast the solutions to this model with solutions to a complex scalar field model, namely boson stars and black holes with synchronised hair. Remarkably, even though the two models present key differences, and in particular the Skyrme model allows static hairy black holes, when introducing rotation, the synchronisation condition becomes mandatory, providing further evidence for its generality in obtaining rotating hairy black holes.

High angular resolution and position determinations by infrared interferometry

NASA Technical Reports Server (NTRS)

1974-01-01

Interferometer systems are described in the form of publications and reports. 'Distance Meter Helps Track the Stars', 'Berkeley Heterodyne Interferometer', 'Infrared Heterodyne Spectroscopy of CO2 on Mars', and 'A 10 micron Heterodyne Stellar Interferometer' are papers reported.

Optical spring stabilization

NASA Astrophysics Data System (ADS)

Lough, James D.

The Advanced LIGO detectors will soon be online with enough sensitivity to begin detecting gravitational waves, based on conservative estimates of the rate of neutron star inspirals. These first detections are sure to be significant, however, we will always strive to do better. More questions will be asked about the nature of neutron star material, rates of black hole inspirals, electromagnetic counterparts, etc. To begin to answer all of the questions aLIGO will bring us we will need even better sensitivity in future gravitational wave detectors. This thesis addresses one aspect that will limit us in the future: angular stability of the test masses. Angular stability in advanced LIGO uses an active feedback system. We are proposing to replace the active feedback system with a passive one, eliminating sensing noise contributions. This technique uses the radiation pressure of light inside a cavity as a stable optical spring, fundamentally the same as technique developed by Corbitt, et al. with an additional degree of freedom. I will review the theory of the one dimensional technique and discuss the multidimensional control theory and angular trap setup. I will then present results from the one-dimensional trap which we have built and tested. And propose improvements for the angular trap experiment. Along the way we have discovered an interesting coupling with thermal expansion due to round trip absorption in the high reflective coatings. The front surface HR coating limits our spring stability in this experiment due to the high circulating power and small beam spot size.

Rotational evolution of slow-rotator sequence stars

NASA Astrophysics Data System (ADS)

Lanzafame, A. C.; Spada, F.

2015-12-01

Context. The observed relationship between mass, age and rotation in open clusters shows the progressive development of a slow-rotator sequence among stars possessing a radiative interior and a convective envelope during their pre-main sequence and main-sequence evolution. After 0.6 Gyr, most cluster members of this type have settled on this sequence. Aims: The observed clustering on this sequence suggests that it corresponds to some equilibrium or asymptotic condition that still lacks a complete theoretical interpretation, and which is crucial to our understanding of the stellar angular momentum evolution. Methods: We couple a rotational evolution model, which takes internal differential rotation into account, with classical and new proposals for the wind braking law, and fit models to the data using a Monte Carlo Markov chain (MCMC) method tailored to the problem at hand. We explore to what extent these models are able to reproduce the mass and time dependence of the stellar rotational evolution on the slow-rotator sequence. Results: The description of the evolution of the slow-rotator sequence requires taking the transfer of angular momentum from the radiative core to the convective envelope into account. We find that, in the mass range 0.85-1.10 M⊙, the core-envelope coupling timescale for stars in the slow-rotator sequence scales as M-7.28. Quasi-solid body rotation is achieved only after 1-2 Gyr, depending on stellar mass, which implies that observing small deviations from the Skumanich law (P ∝ √{t}) would require period data of older open clusters than is available to date. The observed evolution in the 0.1-2.5 Gyr age range and in the 0.85-1.10 M⊙ mass range is best reproduced by assuming an empirical mass dependence of the wind angular momentum loss proportional to the convective turnover timescale and to the stellar moment of inertia. Period isochrones based on our MCMC fit provide a tool for inferring stellar ages of solar-like main-sequence stars from their mass and rotation period that is largely independent of the wind braking model adopted. These effectively represent gyro-chronology relationships that take the physics of the two-zone model for the stellar angular momentum evolution into account.

GALACTIC ANGULAR MOMENTUM IN THE ILLUSTRIS SIMULATION: FEEDBACK AND THE HUBBLE SEQUENCE

DOE Office of Scientific and Technical Information (OSTI.GOV)

Genel, Shy; Fall, S. Michael; Snyder, Gregory F.

We study the stellar angular momentum of thousands of galaxies in the Illustris cosmological simulation, which captures gravitational and gas dynamics within galaxies, as well as feedback from stars and black holes. We find that the angular momentum of the simulated galaxies matches observations well, and in particular two distinct relations are found for late-type versus early-type galaxies. The relation for late-type galaxies corresponds to the value expected from full conservation of the specific angular momentum generated by cosmological tidal torques. The relation for early-type galaxies corresponds to retention of only ∼30% of that, but we find that those early-typemore » galaxies with low angular momentum at z = 0 nevertheless reside at high redshift on the late-type relation. Some of them abruptly lose angular momentum during major mergers. To gain further insight, we explore the scaling relations in simulations where the galaxy formation physics is modified with respect to the fiducial model. We find that galactic winds with high mass-loading factors are essential for obtaining the high angular momentum relation typical for late-type galaxies, while active galactic nucleus feedback largely operates in the opposite direction. Hence, feedback controls the stellar angular momentum of galaxies, and appears to be instrumental for establishing the Hubble sequence.« less

The Rise and Fall of μ Velorum: A Remarkable Flare on a Yellow Giant Star Observed with the Extreme Ultraviolet Explorer

NASA Astrophysics Data System (ADS)

Ayres, Thomas R.; Osten, Rachel A.; Brown, Alexander

1999-11-01

The close visual double μ Velorum (HD 93497; G6 III+dF) consists of a yellow giant and a fainter companion currently 2" apart. Recently μ Vel was the source of a large flare recorded by the Extreme Ultraviolet Explorer. The long 1.5 day decay phase was like the extremes seen on hyperactive RS CVn-type binaries. The primary, μ Vel A is a 3 Msolar star, in the ``rapid braking zone'' redward of G0 III. Yellow giants are not commonly reported as flare stars, perhaps because the first-crossers are relatively rare and not well represented in the observational samples. The secondary star is classified G2 V, but the 1700 Å energy distribution places it earlier on the main sequence, probably F4 or F5 V, in a class also not usually known for coronal variability. The long duration of the μ Vel event suggests that it occurred in a significantly elongated structure of moderate density, ne<~109 cm-3. If it was a magnetic plasmoid, like a coronal mass ejection on the Sun, then such events might play a role in shedding angular momentum from active evolved stars. The associated spin-down could control the activity survival time of red giants (in later stages of evolution than the first-crosser μ Vel) whose dynamos were rejunvenated by dredge-up of angular momentum from the interior, or more exotic sources, such as cannibalism of close-in substellar companions during the first or second ascent.

Improved compensation of atmospheric turbulence effects by multiple adaptive mirror systems.

PubMed

Shamir, J; Crowe, D G; Beletic, J W

1993-08-20

Optical wave-front propagation in a layered model for the atmosphere is analyzed by the use of diffraction theory, leading to a novel approach for utilizing artificial guide stars. Considering recent observations of layering in the atmospheric turbulence, the results of this paper indicate that, even for very large telescopes, a substantial enlargement of the compensated angular field of view is possible when two adaptive mirrors and four or five artificial guide stars are employed. The required number of guide stars increases as the thickness of the turbulent layers increases, converging to the conventional results at the limit of continuously turbulent atmosphere.

Non-Equilibrium Chemistry of O-Rich AGB Stars as Revealed by ALMA

NASA Astrophysics Data System (ADS)

Wong, Ka Tat

2018-04-01

Chemical models suggest that pulsation driven shocks propagating from the stellar surfaces of oxygen-rich evolved stars to the dust formation zone trigger non-equilibrium chemistry in the shocked gas near the star, including the formation of carbon-bearing molecules in the stellar winds dominated by oxygen-rich chemistry. Recent long-baseline ALMA observations are able to give us a detailed view of the molecular line emission and absorption at an angular resolution of a few stellar radii. I am going to present the latest results from the ALMA observations of IK Tau and o Cet in late 2017, with a particular focus on HCN.

Triple system HD 201433 with a SPB star component seen by BRITE - Constellation: Pulsation, differential rotation, and angular momentum transfer

NASA Astrophysics Data System (ADS)

Kallinger, T.; Weiss, W. W.; Beck, P. G.; Pigulski, A.; Kuschnig, R.; Tkachenko, A.; Pakhomov, Y.; Ryabchikova, T.; Lüftinger, T.; Palle, , P. L.; Semenko, E.; Handler, G.; Koudelka, O.; Matthews, J. M.; Moffat, A. F. J.; Pablo, H.; Popowicz, A.; Rucinski, S.; Wade, G. A.; Zwintz, K.

2017-07-01

Context. Stellar rotation affects the transport of chemical elements and angular momentum and is therefore a key process during stellar evolution, which is still not fully understood. This is especially true for massive OB-type stars, which are important for the chemical enrichment of the Universe. It is therefore important to constrain the physical parameters and internal angular momentum distribution of massive OB-type stars to calibrate stellar structure and evolution models. Stellar internal rotation can be probed through asteroseismic studies of rotationally split non radial oscillations but such results are still quite rare, especially for stars more massive than the Sun. The slowly pulsating B9V star HD 201433 is known to be part of a single-lined spectroscopic triple system, with two low-mass companions orbiting with periods of about 3.3 and 154 days. Aims: Our goal is to measure the internal rotation profile of HD 201433 and investigate the tidal interaction with the close companion. Methods: We used probabilistic methods to analyse the BRITE - Constellation photometry and radial velocity measurements, to identify a representative stellar model, and to determine the internal rotation profile of the star. Results: Our results are based on photometric observations made by BRITE - Constellation and the Solar Mass Ejection Imager on board the Coriolis satellite, high-resolution spectroscopy, and more than 96 yr of radial velocity measurements. We identify a sequence of nine frequency doublets in the photometric time series, consistent with rotationally split dipole modes with a period spacing of about 5030 s. We establish that HD 201433 is in principle a solid-body rotator with a very slow rotation period of 297 ± 76 days. Tidal interaction with the inner companion has, however, significantly accelerated the spin of the surface layers by a factor of approximately one hundred. The angular momentum transfer onto the surface of HD 201433 is also reflected by the statistically significant decrease of the orbital period of about 0.9 s during the last 96 yr. Conclusions: Combining the asteroseismic inferences with the spectroscopic measurements and the orbital analysis of the inner binary system, we conclude that tidal interactions between the central SPB star and its inner companion have almost circularised the orbit. They have, however, not yet aligned all spins of the system and have just begun to synchronise rotation. Based on data collected by the BRITE - Constellation satellite mission, built, launched and operated thanks to support from the Austrian Aeronautics and Space Agency and the University of Vienna, the Canadian Space Agency (CSA), and the Foundation for Polish Science & Technology (FNiTP MNiSW) and National Science Centre (NCN), the Hermes spectrograph mounted on the 1.2 m Mercator Telescope at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, and the Solar Mass Ejection Imager, which is a joint project of the University of California San Diego, Boston College, the University of Birmingham (UK), and the Air Force Research Laboratory.

Angular and radial dependence of the energy response factor for LIF-TLD micro-rods in 125L permanent implant source.

PubMed

Mobit, Paul; Badragan, Iulian

2006-01-01

EGSnrc Monte Carlo simulations were used to calculate the angular and radial dependence of the energy response factor for LiF-thermoluminescence dosemeters (TLDs) irradiated with a commercially available (125)I permanent brachytherapy source. The LiF-TLDs were modelled as cylindrical micro-rods of length 6 mm and with diameters of 1 mm and 5 mm. The results show that for a LiF-TLD micro-rod of 1 mm diameter, the energy response relative to (60)Co gamma rays is 1.406 +/- 0.3% for a polar angle of 90 degrees and radial distance of 1.0 cm. When the diameter of the micro-rod is increased from 1 to 5 mm, the energy response decreases to 1.32 +/- 0.3% at the same point. The variation with position of the energy response factor is not >5% in a 6 cm x 6 cm x 6 cm calculation grid for the 5 mm diameter micro-rod. The results show that there is a change in the photon spectrum with angle and radial distance, which causes the variation of the energy response.

Angular dependence of the magnetic properties of permalloy and nickel nanowires as a function of their diameters

NASA Astrophysics Data System (ADS)

Raviolo, Sofía; Tejo, Felipe; Bajales, Noelia; Escrig, Juan

2018-01-01

In this paper we have compared the angular dependence of the magnetic properties of permalloy (Ni80Fe20) and nickel nanowires by means of micromagnetic simulations. For each material we have chosen two diameters, 40 and 100 nm. Permalloy nanowires with smaller diameters (d = 40 nm) exhibit greater coercivity than nickel nanowires, regardless of the angle at which the external magnetic field is applied. In addition, both Py and Ni nanowires exhibit the same remanence values. However, the nanowires of larger diameters (d = 100 nm) exhibit a more complex behavior, noting that for small angles, nickel nanowires are those that now exhibit a greater coercivity in comparison to those of permalloy. The magnetization reversal modes vary as a function of the angle at which the external field is applied. When the field is applied parallel to the wire axis, it reverts through nucleation and propagation of domain walls, whereas when the field is applied perpendicular to the axis, it reverts by a pseudo-coherent rotation. These results may provide a guide to control the magnetic properties of nanowires for use in potential applications.

Pseudo-Linear Attitude Determination of Spinning Spacecraft

NASA Technical Reports Server (NTRS)

Bar-Itzhack, Itzhack Y.; Harman, Richard R.

2004-01-01

This paper presents the overall mathematical model and results from pseudo linear recursive estimators of attitude and rate for a spinning spacecraft. The measurements considered are vector measurements obtained by sun-sensors, fixed head star trackers, horizon sensors, and three axis magnetometers. Two filters are proposed for estimating the attitude as well as the angular rate vector. One filter, called the q-Filter, yields the attitude estimate as a quaternion estimate, and the other filter, called the D-Filter, yields the estimated direction cosine matrix. Because the spacecraft is gyro-less, Euler s equation of angular motion of rigid bodies is used to enable the estimation of the angular velocity. A simpler Markov model is suggested as a replacement for Euler's equation in the case where the vector measurements are obtained at high rates relative to the spacecraft angular rate. The performance of the two filters is examined using simulated data.

Magnetocentrifugally Driven Flows from Young Stars and Disks. IV. The Accretion Funnel and Dead Zone

NASA Astrophysics Data System (ADS)

Ostriker, Eve C.; Shu, Frank H.

1995-07-01

We formulate the time-steady, axisymmetric problem of stellar magnetospheric inflow of gas from a surrounding accretion disk. The computational domain is bounded on the outside by a surface of given shape containing the open field lines associated with an induced disk wind. The mechanism for this wind has been investigated in previous publications in this journal. Our zeroth-order solution incorporates an acceptable accounting of the pressure balance between the magnetic field lines loaded with accreting gas (funnel flow) and those empty of matter (dead zone). In comparison with previous models, our funnel-flow/dead-zone solution has the following novel features: (1) Because of a natural tendency for the trapped stellar magnetic flux to pinch toward the corotation radius Rx (X-point of the effective potential), most of the interesting magnetohydrodynamics is initiated within a small neighborhood of Rx (X-region), where the Keplerian angular speed of rotation in the disk equals the spin rate of the star. (2) Unimpeded funnel flow from the inner portion of the X-region to the star can occur when the amount of trapped magnetic flux equals or exceeds 1.5 times the unperturbed dipole flux that would lie outside Rx in the absence of an accretion disk. (3). Near the equatorial plane, radial infall from the X-point is terminated at a "kink" point Rk = 0.74Rx that deflects the flow away from the midplane, mediating thereby between the field topology imposed by a magnetic fan of trapped flux at Rx and the geometry of a strong stellar dipole. (4) The excess angular momentum of accretion that would otherwise spin up the star rapidly is deposited by the magnetic torques of the funnel flow into the inner portion of the X-region of the disk. (5) An induced disk wind arises in the outer portion of the .X-region, where the stellar field lines have been blown open, and removes whatever excess angular momentum that viscous torques do not transport to the outer disk. (6) The interface between open field lines loaded with outflowing matter (connected to the disk) and those not loaded (connected to the star) forms a "helmet streamer," along which major mass-ejection and reconnection events may arise in response to changing boundary conditions (e.g., stellar magnetic cycles), much the way that such events occur in the active Sun. (7) Pressure balance across the dead-zone/wind interface will probably yield an asymptotically vertical (i.e., "jetlike") trajectory for the matter ejected along the helmet streamer, but mathematical demonstration of this fact is left for future studies. (8) In steady state the overall balance of angular momentum in the star/disk/ magnetosphere system fixes the fractions, f and 1 - f, of the disk mass accretion rate into the X-region carried away, respectively, by the wind and funnel flows.

Imaging Stellar Surface with The CHARA Array

NASA Astrophysics Data System (ADS)

Schaefer, Gail

2018-04-01

I will provide an overview of results on imaging stellar surfaces with the CHARA Array. These include imaging gravity darkening on rapid rotators, starspots on magnetically active stars, convective cells on red supergiants, and stellar winds from massive stars. In binary systems, the CHARA Array has been used to observe tidal distortions from Roche lobe filling in interactive binaries, transiting companions as they move through eclipse, and the angular expansion of novae explosions. I will discuss the impact of these results in an astrophysical context.

Science with MATISSE

NASA Astrophysics Data System (ADS)

Wolf, Sebastian; Lopez, Bruno; Augereau, Jean-Charles; Delbo, Marco; Dominik, Carsten; Henning, Thomas; Hofmann, Karl-Heinz; Hogerheijde, Michiel; Hron, Josef; Jaffe, Walter; Lanz, Thierry; Meisenheimer, Klaus; Millour, Florentin; Pantin, Eric; Petrov, Roman; Schertl, Dieter; van Boekel, Roy; Weigelt, Gerd; Chiavassa, Andrea; Juhasz, Attila; Matter, Alexis; Meilland, Anthony; Nardetto, Nicolas; Paladini, Claudia

2016-07-01

We present an overview of the scientific potential of MATISSE, the Multi Aperture mid-Infrared SpectroScopic Experiment for the Very Large Telescope Interferometer. For this purpose we outline selected case studies from various areas, such as star and planet formation, active galactic nuclei, evolved stars, extrasolar planets, and solar system minor bodies and discuss strategies for the planning and analysis of future MATISSE observations. Moreover, the importance of MATISSE observations in the context of complementary high-angular resolution observations at near-infrared and submillimeter/millimeter wavelengths is highlighted.

REVIEWS OF TOPICAL PROBLEMS: The nature of accretion disks of close binary stars: overreflection instability and developed turbulence

NASA Astrophysics Data System (ADS)

Fridman, A. M.; Bisikalo, D. V.

2008-06-01

The current status of the physics of accretion disks in close binary stars is reviewed, with an emphasis on the hydrodynamic overreflection instability, which is a factor leading to the accretion disk turbulence. The estimated turbulent viscosity coefficients are in good agreement with observations and explain the high angular momentum transfer rate and the measured accretion rate. Based on the observations, a power-law spectrum for the developed turbulence is obtained.

High angular resolution observations of the cool giant V Hya

NASA Astrophysics Data System (ADS)

Pedretti, E.; Monnier, J. D.; Millan Gabet, R.; Traub, W. A.; Tuthill, P.; Danchi, W.; Berger, J.; Schloerb, F. P.; Thureau, N. D.; Carleton, N. P.; Lacasse, M. G.; Schuller, P. A.; Ragland, S.; Brewer, M.

2005-12-01

We present the preliminary interferometric observations of the cool giant star V Hya. V Hya, which is known to have mass-loss and to be surrounded by a dust shell,was observed in three narrow-band filters in the H bandpass at the infrared optical telescope array (IOTA), using the IONIC three-telescope beam combiner. The star was also observed at the Keck telescope using an aperture mask. We discuss the results and try to fit simple models to the observed data.

Amylose-Based Cationic Star Polymers for siRNA Delivery

PubMed Central

Nishimura, Tomoki; Umezaki, Kaori; Mukai, Sada-atsu; Sawada, Shin-ichi; Akiyoshi, Kazunari

2015-01-01

A new siRNA delivery system using a cationic glyco-star polymer is described. Spermine-modified 8-arm amylose star polymer (with a degree of polymerization of approximately 60 per arm) was synthesized by chemoenzymatic methods. The cationic star polymer effectively bound to siRNA and formed spherical complexes with an average hydrodynamic diameter of 230 nm. The cationic 8-arm star polymer complexes showed superior cellular uptake characteristics and higher gene silencing effects than a cationic 1-arm polymer. These results suggest that amylose-based star polymers are a promising nanoplatform for glycobiomaterials. PMID:26539548

Kinematics of the inner thousand AU region around the young massive star AFGL 2591-VLA3: a massive disk candidate?

NASA Astrophysics Data System (ADS)

Wang, K.-S.; van der Tak, F. F. S.; Hogerheijde, M. R.

2012-07-01

Context. Recent detections of disks around young high-mass stars support the idea of massive star formation through accretion rather than coalescence, but the detailed kinematics in the equatorial region of the disk candidates is not well known, which limits our understanding of the accretion process. Aims: This paper explores the kinematics of the gas around a young massive star with millimeter-wave interferometry to improve our understanding of the formation of massive stars though accretion. Methods: We use Plateau de Bure interferometric images to probe the environment of the nearby (~1 kpc) and luminous (~20 000 L⊙) high-mass (10-16 M⊙) young star AFGL 2591-VLA3 in continuum and in lines of HDO, H_218O and SO2 in the 115 and 230 GHz bands. Radiative transfer calculations are employed to investigate the kinematics of the source. Results: At ~0.5″ (500 AU) resolution, the line images clearly resolve the velocity field of the central compact source (diameter of ~800 AU) and show linear velocity gradients in the northeast-southwest direction. Judging from the disk-outflow geometry, the observed velocity gradient results from rotation and radial expansion in the equatorial region of VLA3. Radiative transfer calculations suggest that the velocity field is consistent with sub-Keplerian rotation plus Hubble-law like expansion. The line profiles of the observed molecules suggest a layered structure, with HDO emission arising from the disk mid-plane, H_218O from the warm mid-layer, and SO2 from the upper disk. Conclusions: We propose AFGL 2591-VLA3 as a new massive disk candidate, with peculiar kinematics. The rotation of this disk is sub-Keplerian, probably due to magnetic braking, while the stellar wind may be responsible for the expansion of the disk. The expansion motion may also be an indirect evidence of disk accretion in the very inner region because of the conservation of angular momentum. The sub-Keplerian rotation discovered in our work suggests that AFGL 2591-VLA3 may be a special case linking transition of velocity field of massive disks from pure Keplerian rotation to solid-body rotation though definitely more new detections of circumstellar disks around high-mass YSOs are required to examine this hypothesis. Our results support the idea that early B-type stars could be formed with a circumstellar disk from the point of view of the disk-outflow geometry, though the accretion processes in the disk need to be further investigated.

Report on the ESO Workshop ''Astronomy at High Angular Resolution''

NASA Astrophysics Data System (ADS)

Boffin, H.; Schmidtobreick, L.; Hussain, G.; Berger, J.-Ph.

2015-03-01

A workshop took place in Brussels in 2000 on astrotomography, a generic term for indirect mapping techniques that can be applied to a huge variety of astrophysical systems, ranging from planets, single stars and binaries to active galactic nuclei. It appeared to be timely to revisit the topic given the many past, recent and forthcoming improvements in telescopes and instrumentation. We therefore decided to repeat the astrotomography workshop, but to put it into the much broader context of high angular resolution astronomy. Many techniques, from lucky and speckle imaging, adaptive optics to interferometry, are now widely employed to achieve high angular resolution and they have led to an amazing number of new discoveries. A summary of the workshop themes is presented.

Terrestrial-passage theory: failing a test.

PubMed

Reed, Charles F; Krupinski, Elizabeth A

2009-01-01

Terrestrial-passage theory proposes that the 'moon' and 'sky' illusions occur because observers learn to expect an elevation-dependent transformation of visual angle. The transformation accompanies daily movement through ordinary environments of fixed-altitude objects. Celestial objects display the same visual angle at all elevations, and hence are necessarily non-conforming with the ordinary transformation. On hypothesis, observers should target angular sizes to appear greater at elevation than at horizon. However, in a sample of forty-eight observers there was no significant difference between the perceived angular size of a constellation of stars at horizon and that predicted for a specific elevation. Occurrence of the illusion was not restricted to those observers who expected angular expansion. These findings fail to support the terrestrial-passage theory of the illusion.

Cartwheel Galaxy

NASA Astrophysics Data System (ADS)

Murdin, P.

2000-11-01

A peculiar galaxy in the constellation of Sculptor that derives its name from its wheel-like appearance. The Cartwheel has a ring-shaped `rim', 150 000 light-years in diameter, that contains billions of recently formed stars and which is dominated by massive clusters of bright blue stars and HII regions. The nucleus, or `hub' of the galaxy contains a predominantly older population of stars and is...

Prominence formation and ejection in cool stars

NASA Astrophysics Data System (ADS)

Villarreal D'Angelo, Carolina; Jardine, Moira; See, Victor

2018-03-01

The observational signatures of prominences have been detected in single and binary G and K type stars for many years now, but recently this has been extended to the M dwarf regime. Prominences carry away both mass and angular momentum when they are ejected and the impact of this mass on any orbiting planets may be important for the evolution of exoplanetary atmospheres. By means of the classification used in the massive star community, that involves knowledge of two parameters (the co-rotation and Alfvén radii, rK and rA), we have determined which cool stars could support prominences. From a model of mechanical support, we have determined that the prominence mass mp/M⋆ = (EM/EG)(r⋆/rK)2F where E_MB_\\star ^2r_\\star ^3 and E_G = GM_\\star ^2/r_\\star are magnetic and gravitational energies and F is a geometric factor. Our calculated masses and ejection frequencies (typically 1016 - 1017 g and 0.4 d, respectively) are consistent with observations and are sufficient to ensure that an exoplanet orbiting in the habitable zone of an M dwarf could suffer frequent impacts.

Investigation of star polymer nanoshells for use in diagnostic imaging and photothermal cancer therapy applications

NASA Astrophysics Data System (ADS)

Gomez, Lizabeth

Gold nanoshells can be designed to possess high light scattering and strong absorption of near-infrared light. Thus, they have the potential to be used in biological applications as contrast agents for diagnostic imaging as well as for thermal ablation of tumor cells in future cancer treatments. In this study, gold nanoshells with dye-loaded star polymer cores were investigated. Uniform near-infrared gold nanoshells with 100 nm diameters were successfully generated using different batches of star polymer templates and were characterized by UV-visible spectroscopy and scanning electron microscopy. The star polymers used were block copolymer structures with a hydrophobic polystyrene (PS) core and a hydrophilic poly(N,N-dimethylaminoethylmethracrylate) (DMAEMA) outer shell. Within this work, a general procedure was established in order to achieve a desired gold nanoshell size regardless of the star polymer batch used, since the synthesis process conditions can cause star polymers to vary in size as well in the number and length of amino-functionalized arms. Control of the gold nanoshell diameter was optimized after an in-depth analysis of the synthesis parameters that affected the formation and final size of the dye-loaded star polymer gold nanoshells. The main parameters examined were pH of the gold seeds used to nucleate the templates and the ratio of star polymer to gold hydroxide used during the growth of the outer gold shell.

Searching for Low-mass Companions of Cepheids, Part II

NASA Astrophysics Data System (ADS)

Remage Evans, Nancy; Tingle, E.; Bond, H. E.; Schaefer, G. H.; Mason, B.; Karovska, M.; Wolk, S.; Pillitteri, I.; DePasquale, J.; Guinan, E.; Engle, S.

2012-01-01

The formation of a binary/multiple system is an effective way to manipulate angular momentum during the star-formation process. The properties of binary systems (separations and mass ratios) are thus the ``fingerprints" of the process. Low mass companions are the most difficult to identify particularly for massive stars. We are conducting a snapshot survey of the nearest Cepheids (5 Msun stars) using the Hubble Space Telescope Wide Field Camera 3 (WFC3) to discover possible resolved low mass companions. The color-magnitude combination is the first approach to identifying probable physical companions. The distributions of mass and separation for these stars will be discussed. Financial suppoet was provided by Hubble grant GO-12215.01-A and the Chandra X-ray Center NASA contract NAS8-03060.

Rotational velocities of newly discovered, low-mass members of the Alpha Persei cluster

NASA Technical Reports Server (NTRS)

Stauffer, John R.; Hartmann, Lee W.; Jones, Burton F.

1989-01-01

About 30 new, low-mass members of the young open cluster Alpha Persei are identified via a proper-motion study and subsequent photometric and spectroscopic observations. Membership in the cluster is confirmed for a number of the fainter proper-motion candidates from Heckman, Dieckvoss, and Kox (1956). Coordinates, finding charts, BVRI photometry, and rotational velocities are provided for most of the stars. At least two of the stars show peculiar H-alpha emission profiles, with weak but very broad emission wings, and relatively narrow absorption reversals. The rotational velocity distribution for low-mass stars in the Alpha Per cluster are compared with recently derived rotational velocity distributions for T Tauri stars, placing strong constraints on the mechanisms for angular momentum loss during pre-main-sequence evolution.

The shortest-known-period star orbiting our Galaxy's supermassive black hole.

PubMed

Meyer, L; Ghez, A M; Schödel, R; Yelda, S; Boehle, A; Lu, J R; Do, T; Morris, M R; Becklin, E E; Matthews, K

2012-10-05

Stars with short orbital periods at the center of our Galaxy offer a powerful probe of a supermassive black hole. Over the past 17 years, the W. M. Keck Observatory has been used to image the galactic center at the highest angular resolution possible today. By adding to this data set and advancing methodologies, we have detected S0-102, a star orbiting our Galaxy's supermassive black hole with a period of just 11.5 years. S0-102 doubles the number of known stars with full phase coverage and periods of less than 20 years. It thereby provides the opportunity, with future measurements, to resolve degeneracies in the parameters describing the central gravitational potential and to test Einstein's theory of general relativity in an unexplored regime.

New measurements of photospheric magnetic fields in late-type stars and emerging trends

NASA Technical Reports Server (NTRS)

Saar, S. H.; Linsky, J. L.

1986-01-01

The magnetic fields of late-type stars are measured using the method of Saar et al. (1986). The method includes radiative transfer effects and compensation for line blending; the photospheric magnetic field parameters are derived by comparing observed and theoretical line profiles using an LTE code that includes line saturation and full Zeeman pattern. The preliminary mean active region magnetic field strengths (B) and surface area coverages for 20 stars are discussed. It is observed that there is a trend of increasing B towards the cooler dwarfs stars, and the linear correlation between B and the equipartition value of the magnetic field strength suggests that the photospheric gas pressure determines the photospheric magnetic field strengths. A tendency toward larger filling factors at larger stellar angular velocities is also detected.

Galactic Astronomy in the Ultraviolet

NASA Astrophysics Data System (ADS)

Rastorguev, A. S.; Sachkov, M. E.; Zabolotskikh, M. V.

2017-12-01

We propose a number of prospective observational programs for the ultraviolet space observatory WSO-UV, which seem to be of great importance to modern galactic astronomy. The programs include the search for binary Cepheids; the search and detailed photometric study and the analysis of radial distribution of UV-bright stars in globular clusters ("blue stragglers", blue horizontal-branch stars, RR Lyrae variables, white dwarfs, and stars with UV excesses); the investigation of stellar content and kinematics of young open clusters and associations; the study of spectral energy distribution in hot stars, including calculation of the extinction curves in the UV, optical and NIR; and accurate definition of the relations between the UV-colors and effective temperature. The high angular resolution of the observatory allows accurate astrometric measurements of stellar proper motions and their kinematic analysis.

Multiplicity At Early Stages Of Star Formation, Small Clusters. Observations Overview

NASA Astrophysics Data System (ADS)

Saito, Masao

2017-07-01

The SOLA (Soul of Lupus with ALMA) project is conducting comprehensive studies of the Lupus Molecular Clouds and their star formation processes covering 10-10^4 AU scale. Our goal is to exploit ALMA and other facilities over a wide wavelength range to establish a prototypical low-mass star forming scenario based on the Lupus region. In the presentation, we will focus on angular momentum in dense cores in a filament, molecular outflows from young stars, and Class 0/I binary survey in Lupus as well as overview of our projects. Our binary survey was conducted in ALMA cycle 2 and achieved at 0.2-0.3 arcsec resolution discovering new binary systems in Lupus. At the same time, we obtained EX Lup, EXor type burst source, data in ALMA Cycle 3.

Multiplicity at Early Stages of Star Formation, Small Clusters. Observations Overview

NASA Astrophysics Data System (ADS)

Saito, Masao

2017-06-01

The SOLA (Soul of Lupus with ALMA) project is conducting comprehensive studies of the Lupus Molecular Clouds and their star formation processes covering 10-10^4 AU scale. Our goal is to exploit ALMA and other facilities over a wide wavelength range to establish a prototypical low-mass star forming scenario based on the Lupus region. In the presentation, we will focus on angular momentum in dense cores in a filament, molecular outflows from young stars, and Class 0/I binary survey in Lupus as well as overview of our projects. Our binary survey was conducted in ALMA cycle 2 and achieved at 0.2-0.3 arcsec resolution discovering new binary systems in Lupus. At the same time, we obtained EX Lup, EXor type burst source, data in ALMA Cycle 3.

A robust star identification algorithm with star shortlisting

NASA Astrophysics Data System (ADS)

Mehta, Deval Samirbhai; Chen, Shoushun; Low, Kay Soon

2018-05-01

A star tracker provides the most accurate attitude solution in terms of arc seconds compared to the other existing attitude sensors. When no prior attitude information is available, it operates in "Lost-In-Space (LIS)" mode. Star pattern recognition, also known as star identification algorithm, forms the most crucial part of a star tracker in the LIS mode. Recognition reliability and speed are the two most important parameters of a star pattern recognition technique. In this paper, a novel star identification algorithm with star ID shortlisting is proposed. Firstly, the star IDs are shortlisted based on worst-case patch mismatch, and later stars are identified in the image by an initial match confirmed with a running sequential angular match technique. The proposed idea is tested on 16,200 simulated star images having magnitude uncertainty, noise stars, positional deviation, and varying size of the field of view. The proposed idea is also benchmarked with the state-of-the-art star pattern recognition techniques. Finally, the real-time performance of the proposed technique is tested on the 3104 real star images captured by a star tracker SST-20S currently mounted on a satellite. The proposed technique can achieve an identification accuracy of 98% and takes only 8.2 ms for identification on real images. Simulation and real-time results depict that the proposed technique is highly robust and achieves a high speed of identification suitable for actual space applications.

Nearby Dwarf Stars: Duplicity, Binarity, and Masses

NASA Astrophysics Data System (ADS)

Mason, Brian D.; Hartkopf, William I.; Henry, Todd J.; Jao, Wei-Chun; Subasavage, John; Riedel, Adric; Winters, Jennifer

2010-02-01

Double stars have proven to be both a blessing and a curse for astronomers since their discovery over two centuries ago. They remain the only reliable source of masses, the most fundamental parameter defining stars. On the other hand, their sobriquet ``vermin of the sky'' is well-earned, due to the complications they present to both observers and theoreticians. These range from non-linear proper motions to stray light in detectors, to confusion in pointing of instruments due to non-symmetric point spread functions, to angular momentum conservation in multiple stars which results in binaries closer than allowed by evolution of two single stars. This proposal is primarily focused on targets where precise astrophysical information is sorely lacking: white dwarfs, red dwarfs, and subdwarfs. The proposed work will refine current statistics regarding duplicity (chance alignments of nearby point sources) and binarity (actual physical relationships), and improve the precisions and accuracies of stellar masses. Several targets support Riedel's and Winters' theses.

Nearby Dwarf Stars: Duplicity, Binarity, and Masses

NASA Astrophysics Data System (ADS)

Mason, Brian D.; Hartkopf, William I.; Henry, Todd J.; Jao, Wei-Chun; Subasavage, John; Riedel, Adric; Winters, Jennifer

2009-08-01

Double stars have proven to be both a blessing and a curse for astronomers since their discovery over two centuries ago. They remain the only reliable source of masses, the most fundamental parameter defining stars. On the other hand, their sobriquet ``vermin of the sky'' is well-earned, due to the complications they present to both observers and theoreticians. These range from non-linear proper motions to stray light in detectors, to confusion in pointing of instruments due to non-symmetric point spread functions, to angular momentum conservation in multiple stars which results in binaries closer than allowed by evolution of two single stars. This proposal is primarily focused on targets where precise astrophysical information is sorely lacking: white dwarfs, red dwarfs, and subdwarfs. The proposed work will refine current statistics regarding duplicity (chance alignments of nearby point sources) and binarity (actual physical relationships), and improve the precisions and accuracies of stellar masses. Several targets support Riedel's and Winters' theses.

Simulating Shock Triggered Star Formation with AstroBEAR2.0

NASA Astrophysics Data System (ADS)

Li, Shule; Frank, Adam; Blackman, Eric

2013-07-01

Star formation can be triggered by the compression from shocks running over stable clouds. Triggered star formation is a favored explanation for the traces of SLRI's in our solar system. Previous research has shown that when parameters such as shock speed are within a certain range, the gravitational collapse of otherwise stable, dense cloud cores is possible. However, these studies usually focus on the precursors of star formation, and the conditions for the triggering. We use AstroBEAR2.0 code to simulate the collapse and subsequent evolution of a stable Bonnor-Ebert cloud by an incoming shock. Through our simulations, we show that interesting physics happens when the newly formed star interacts with the cloud residue and the post-shock flow. We identify these interactions as controlled by the initial conditions of the triggering and study the flow pattern as well as the evolution of important physics quantities such as accretion rate and angular momentum.

The evolution of magnetic hot massive stars: Implementation of the quantitative influence of surface magnetic fields in modern models of stellar evolution

NASA Astrophysics Data System (ADS)

Keszthelyi, Zsolt; Wade, Gregg A.; Petit, Veronique

2017-11-01

Large-scale dipolar surface magnetic fields have been detected in a fraction of OB stars, however only few stellar evolution models of massive stars have considered the impact of these fossil fields. We are performing 1D hydrodynamical model calculations taking into account evolutionary consequences of the magnetospheric-wind interactions in a simplified parametric way. Two effects are considered: i) the global mass-loss rates are reduced due to mass-loss quenching, and ii) the surface angular momentum loss is enhanced due to magnetic braking. As a result of the magnetic mass-loss quenching, the mass of magnetic massive stars remains close to their initial masses. Thus magnetic massive stars - even at Galactic metallicity - have the potential to be progenitors of "heavy" stellar mass black holes. Similarly, at Galactic metallicity, the formation of pair instability supernovae is plausible with a magnetic progenitor.

Miniaturized star tracker for micro spacecraft with high angular rate

NASA Astrophysics Data System (ADS)

Li, Jianhua; Li, Zhifeng; Niu, Zhenhong; Liu, Jiaqi

2017-10-01

There is a clear need for miniaturized, lightweight, accurate and inexpensive star tracker for spacecraft with large anglar rate. To face these new constraints, the Beijing Institute of Space Long March Vehicle has designed, built and flown a low cost miniaturized star tracker that provides autonomous ("Lost in Space") inertial attitude determination, 2 Hz 3-axis star tracking, and digital imaging with embedded compression. Detector with high sensitivity is adopted to meet the dynamic and miniature requirement. A Sun and Moon avoiding method based on the calculation of Sun and Moon's vector by astronomical theory is proposed. The produced prototype weight 0.84kg, and can be used for a spacecraft with 6°/s anglar rate. The average angle measure error is less than 43 arc second. The ground verification and application of the star tracker during the pick-up flight test showed that the capability of the product meet the requirement.

Braking Index of Isolated Pulsars

NASA Astrophysics Data System (ADS)

Hamil, Oliver; Stone, Jirina; Urbanec, Martin; Urbancova, Gabriela

2015-04-01

Isolated pulsars are rotating neutron stars with accurately measured angular velocities Ω, and their time derivatives which show unambiguously that the pulsars are slowing down. The exact mechanism of the spin-down is a question of debate in detail, but the commonly accepted view is that it arises through emission of magnetic dipole radiation (MDR). The energy loss by a rotating pulsar is proportional to a model dependent power of Ω. This relation leads to the power law Ω˙ = -K Ωn where n is called the braking index, equal to the ratio (ΩΩ̈)/ Ω˙2 . The simple MDR model predicts the value of n = 3, but observations of isolated pulsars provide rather precise values of n, individually accurate to a few percent or better, in the range 1 < n < 2.8, which is consistently less than the predictions of the MDR model. In this work, we study the dynamical limits of the MDR model as a function of angular velocity. The effects of variation in the rest mass, the moment of inertia, and the dependence on a realistic Equation of State of the rotating star are considered. Furthermore, we introduce a simulated superfluid effect by which the angular momentum of the core is eliminated from the calculation.

POET: A Model for Planetary Orbital Evolution Due to Tides on Evolving Stars

NASA Astrophysics Data System (ADS)

Penev, Kaloyan; Zhang, Michael; Jackson, Brian

2014-06-01

We make publicly available an efficient, versatile, easy to use and extend tool for calculating the evolution of circular aligned planetary orbits due to the tidal dissipation in the host star. This is the first model to fully account for the evolution of the angular momentum of the stellar convective envelope by the tidal coupling, the transfer of angular momentum between the stellar convective and radiative zones, the effects of the stellar evolution on the tidal dissipation efficiency and stellar core and envelope spins, the loss of stellar convective zone angular momentum to a magnetically launched wind and frequency dependent tidal dissipation. This is only a first release and further development is under way to allow calculating the evolution of inclined and eccentric orbits, with the latter including the tidal dissipation in the planet and its feedback on planetary structure. Considerable effort has been devoted to providing extensive documentation detailing both the usage and the complete implementation details, in order to make it as easy as possible for independent groups to use and/or extend the code for their purposes. POET represents a significant improvement over some previous models for planetary tidal evolution and so has many astrophysical applications. In this article, we describe and illustrate several key examples.

The growth of discs and bulges during hierarchical galaxy formation - II. Metallicity, stellar populations and dynamical evolution

NASA Astrophysics Data System (ADS)

Tonini, C.; Mutch, S. J.; Wyithe, J. S. B.; Croton, D. J.

2017-03-01

We investigate the properties of the stellar populations of model galaxies as a function of galaxy evolutionary history and angular momentum content. We use the new semi-analytic model presented in Tonini et al. This new model follows the angular momentum evolution of gas and stars, providing the base for a new star formation recipe, and treatment of the effects of mergers that depends on the central galaxy dynamical structure. We find that the new recipes have the effect of boosting the efficiency of the baryonic cycle in producing and recycling metals, as well as preventing minor mergers from diluting the metallicity of bulges and ellipticals. The model reproduces the stellar mass-stellar metallicity relation for galaxies above 1010 solar masses, including Brightest Cluster Galaxies. Model discs, galaxies dominated by instability-driven components, and merger-driven objects each stem from different evolutionary channels. These model galaxies therefore occupy different loci in the galaxy mass-size relation, which we find to be in accord with the ATLAS 3D classification of disc galaxies, fast rotators and slow rotators. We find that the stellar populations' properties depend on the galaxy evolutionary type, with more evolved stellar populations being part of systems that have lost or dissipated more angular momentum during their assembly history.

Effects of general relativity on glitch amplitudes and pulsar mass upper bounds

NASA Astrophysics Data System (ADS)

Antonelli, M.; Montoli, A.; Pizzochero, P. M.

2018-04-01

Pinning of vortex lines in the inner crust of a spinning neutron star may be the mechanism that enhances the differential rotation of the internal neutron superfluid, making it possible to freeze some amount of angular momentum which eventually can be released, thus causing a pulsar glitch. We investigate the general relativistic corrections to pulsar glitch amplitudes in the slow-rotation approximation, consistently with the stratified structure of the star. We thus provide a relativistic generalization of a previous Newtonian model that was recently used to estimate upper bounds on the masses of glitching pulsars. We find that the effect of general relativity on the glitch amplitudes obtained by emptying the whole angular momentum reservoir is less than 30 per cent. Moreover, we show that the Newtonian upper bounds on the masses of large glitchers obtained from observations of their maximum recorded event differ by less than a few percent from those calculated within the relativistic framework. This work can also serve as a basis to construct more sophisticated models of angular momentum reservoir in a relativistic context: in particular, we present two alternative scenarios for macroscopically rigid and slack pinned vortex lines, and we generalize the Feynman-Onsager relation to the case when both entrainment coupling between the fluids and a strong axisymmetric gravitational field are present.

Polarized Continuum Radiation from Stellar Atmospheres

NASA Astrophysics Data System (ADS)

Harrington, J. Patrick

2015-10-01

Continuum scattering by free electrons can be significant in early type stars, while in late type stars Rayleigh scattering by hydrogen atoms or molecules may be important. Computer programs used to construct models of stellar atmospheres generally treat the scattering of the continuum radiation as isotropic and unpolarized, but this scattering has a dipole angular dependence and will produce polarization. We review an accurate method for evaluating the polarization and limb darkening of the radiation from model stellar atmospheres. We use this method to obtain results for: (i) Late type stars, based on the MARCS code models (Gustafsson et al. 2008), and (ii) Early type stars, based on the NLTE code TLUSTY (Lanz and Hubeny 2003). These results are tabulated at http://www.astro.umd.edu/~jph/Stellar_Polarization.html. While the net polarization vanishes for an unresolved spherical star, this symmetry is broken by rapid rotation or by the masking of part of the star by a binary companion or during the transit of an exoplanet. We give some numerical results for these last cases.

Formation and Evolution of X-ray Binaries

NASA Astrophysics Data System (ADS)

Shao, Y.

2017-07-01

X-ray binaries are a class of binary systems, in which the accretor is a compact star (i.e., black hole, neutron star, or white dwarf). They are one of the most important objects in the universe, which can be used to study not only binary evolution but also accretion disks and compact stars. Statistical investigations of these binaries help to understand the formation and evolution of galaxies, and sometimes provide useful constraints on the cosmological models. The goal of this thesis is to investigate the formation and evolution processes of X-ray binaries including Be/X-ray binaries, low-mass X-ray binaries (LMXBs), ultraluminous X-ray sources (ULXs), and cataclysmic variables. In Chapter 1 we give a brief review on the basic knowledge of the binary evolution. In Chapter 2 we discuss the formation of Be stars through binary interaction. In this chapter we investigate the formation of Be stars resulting from mass transfer in binaries in the Galaxy. Using binary evolution and population synthesis calculations, we find that in Be/neutron star binaries the Be stars have a lower limit of mass ˜ 8 M⊙ if they are formed by a stable (i.e., without the occurrence of common envelope evolution) and nonconservative mass transfer. We demonstrate that the isolated Be stars may originate from both mergers of two main-sequence stars and disrupted Be binaries during the supernova explosions of the primary stars, but mergers seem to play a much more important role. Finally the fraction of Be stars produced by binary interactions in all B type stars can be as high as ˜ 13%-30% , implying that most of Be stars may result from binary interaction. In Chapter 3 we show the evolution of intermediate- and low-mass X-ray binaries (I/LMXBs) and the formation of millisecond pulsars. Comparing the calculated results with the observations of binary radio pulsars, we report the following results: (1) The allowed parameter space for forming binary pulsars in the initial orbital period-donor mass plane increases with the increasing neutron star mass. This may help to explain why some millisecond pulsars with orbital periods longer than ˜ 60 d seem to have less massive white dwarfs than expected. Alternatively, some of these wide binary pulsars may be formed through mass transfer driven by planet/brown dwarf-involved common envelope evolution; (2) Some of the pulsars in compact binaries might have evolved from intermediate-mass X-ray binaries with an anomalous magnetic braking; (3) The equilibrium spin periods of neutron stars in low-mass X-ray binaries are in general shorter than the observed spin periods of binary pulsars by more than one order of magnitude, suggesting that either the simple equilibrium spin model does not apply, or there are other mechanisms/processes spinning down the neutron stars. In Chapter 4, angular momentum loss mechanisms in the cataclysmic variables below the period gap are presented. By considering several kinds of consequential angular momentum loss mechanisms, we find that neither isotropic wind from the white dwarf nor outflow from the L1 point can explain the extra angular momentum loss rate, while an ouflow from the L2 point or a circumbinary disk can effectively extract the angular momentum provided that ˜ 15%-45% of the transferred mass is lost from the binary. A more promising mechanism is a circumbinary disk exerting a gravitational torque on the binary. In this case the mass loss fraction can be as low as ≲ 10-3. In Chapter 5 we present a study on the population of ultraluminous X-ray sources with an accreting neutron star. Most ULXs are believed to be X-ray binary systems, but previous observational and theoretical studies tend to prefer a black hole rather than a neutron star accretor. The recent discovery of 1.37 s pulsations from the ULX M82 X-2 has established its nature as a magnetized neutron star. In this chapter we model the formation history of neutron star ULXs in an M82- or Milky Way-like galaxy, by use of both binary population synthesis and detailed binary evolution calculations. We find that the birthrate is around 10-4 yr-1 for the incipient X-ray binaries in both cases. We demonstrate the distribution of the ULX population in the donor mass - orbital period plane. Our results suggest that, compared with black hole X-ray binaries, neutron star X-ray binaries may significantly contribute to the ULX population, and high/intermediate-mass X-ray binaries dominate the neutron star ULX population in M82/Milky Way-like galaxies, respectively. In Chapter 6, the population of intermediate- and low-mass X-ray binaries in the Galaxy is explored. We investigate the formation and evolutionary sequences of Galactic intermediate- and low-mass X-ray binaries by combining binary population synthesis (BPS) and detailed stellar evolutionary calculations. Using an updated BPS code we compute the evolution of massive binaries that leads to the formation of incipient I/LMXBs, and present their distribution in the initial donor mass vs. initial orbital period diagram. We then follow the evolution of I/LMXBs until the formation of binary millisecond pulsars (BMSPs). We show that during the evolution of I/LMXBs they are likely to be observed as relatively compact binaries. The resultant BMSPs have orbital periods ranging from about 1 day to a few hundred days. These features are consistent with observations of LMXBs and BMSPs. We also confirm the discrepancies between theoretical predictions and observations mentioned in the literature, that is, the theoretical average mass transfer rates of LMXBs are considerably lower than observed, and the number of BMSPs with orbital periods ˜ 0.1-1 \\unit{d} is severely underestimated. Both imply that something is missing in the modeling of LMXBs, which is likely to be related to the mechanisms of the orbital angular momentum loss. Finally in Chapter 7 we summarize our results and give the prospects for the future work.

A grid of MHD models for stellar mass loss and spin-down rates of solar analogs

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cohen, O.; Drake, J. J.

2014-03-01

Stellar winds are believed to be the dominant factor in the spin-down of stars over time. However, stellar winds of solar analogs are poorly constrained due to observational challenges. In this paper, we present a grid of magnetohydrodynamic models to study and quantify the values of stellar mass loss and angular momentum loss rates as a function of the stellar rotation period, magnetic dipole component, and coronal base density. We derive simple scaling laws for the loss rates as a function of these parameters, and constrain the possible mass loss rate of stars with thermally driven winds. Despite the successmore » of our scaling law in matching the results of the model, we find a deviation between the 'solar dipole' case and a real case based on solar observations that overestimates the actual solar mass loss rate by a factor of three. This implies that the model for stellar fields might require a further investigation with additional complexity. Mass loss rates in general are largely controlled by the magnetic field strength, with the wind density varying in proportion to the confining magnetic pressure B {sup 2}. We also find that the mass loss rates obtained using our grid models drop much faster with the increase in rotation period than scaling laws derived using observed stellar activity. For main-sequence solar-like stars, our scaling law for angular momentum loss versus poloidal magnetic field strength retrieves the well-known Skumanich decline of angular velocity with time, Ω{sub *}∝t {sup –1/2}, if the large-scale poloidal magnetic field scales with rotation rate as B{sub p}∝Ω{sub ⋆}{sup 2}.« less

On the Hipparcos Link to the ICRF derived from VLA and MERLIN radio astrometry

NASA Astrophysics Data System (ADS)

Hering, R.; Walter, H. G.

2007-06-01

Positions and proper motions obtained from observations by the very large array (VLA) and the multi-element radio-linked interferometer network (MERLIN) are used to establish the link of the Hipparcos Celestial Reference Frame (HCRF) to the International Celestial Reference Frame (ICRF). The VLA and MERLIN data are apparently the latest ones published in the literature. Their mean epoch at around 2001 is about 10 years after the epoch of the Hipparcos catalogue and, therefore, the data are considered suitable to check the Hipparcos link established at epoch 1991.25. The parameters of the link, i.e., the angles of frame orientation and the angular rates of frame rotation, are estimated by fitting these parameters to the differences of the optical and radio positions and proper motions of stars common to the Hipparcos catalogue and the VLA and MERLIN data. Both the estimates of the angles of orientation and the angular rates of rotation show nearly consistent but insignificant results for all samples of stars treated. We conclude that not only the size of the samples of 9 15 stars is too small, but also that the accuracy of the radio positions and, above all, of the radio proper motions is insufficient, the latter being based on early-epoch star positions of low accuracy. The present observational data at epoch 2001 suggest that maintenance of the Hipparcos frame is not feasible at this stage.

Accretion torques in X-ray pulsars

NASA Technical Reports Server (NTRS)

Rappaport, S.; Joss, P. C.

1977-01-01

An analysis of the accretion process in an X-ray pulsar, whereby angular momentum is transferred to the star and its rotation period is changed, is presented, and an expression for the fractional rate of change of the pulse period in terms of X-ray luminosity and other star parameters is derived. It is shown that observed characteristic spin-up time scales for seven X-ray pulsars strongly support the view that in every source (1) the pulse period reflects the rotation period of a compact object, (2) the accretion is mediated by a disk surrounding the compact object and rotating in the same sense, and (3) the compact object is a neutron star rather than a white dwarf.

The Frame of Fixed Stars in Relational Mechanics

NASA Astrophysics Data System (ADS)

Ferraro, Rafael

2017-01-01

Relational mechanics is a gauge theory of classical mechanics whose laws do not govern the motion of individual particles but the evolution of the distances between particles. Its formulation gives a satisfactory answer to Leibniz's and Mach's criticisms of Newton's mechanics: relational mechanics does not rely on the idea of an absolute space. When describing the behavior of small subsystems with respect to the so called "fixed stars", relational mechanics basically agrees with Newtonian mechanics. However, those subsystems having huge angular momentum will deviate from the Newtonian behavior if they are described in the frame of fixed stars. Such subsystems naturally belong to the field of astronomy; they can be used to test the relational theory.

A spin-down clock for cool stars from observations of a 2.5-billion-year-old cluster.

PubMed

Meibom, Søren; Barnes, Sydney A; Platais, Imants; Gilliland, Ronald L; Latham, David W; Mathieu, Robert D

2015-01-29

The ages of the most common stars--low-mass (cool) stars like the Sun, and smaller--are difficult to derive because traditional dating methods use stellar properties that either change little as the stars age or are hard to measure. The rotation rates of all cool stars decrease substantially with time as the stars steadily lose their angular momenta. If properly calibrated, rotation therefore can act as a reliable determinant of their ages based on the method of gyrochronology. To calibrate gyrochronology, the relationship between rotation period and age must be determined for cool stars of different masses, which is best accomplished with rotation period measurements for stars in clusters with well-known ages. Hitherto, such measurements have been possible only in clusters with ages of less than about one billion years, and gyrochronology ages for older stars have been inferred from model predictions. Here we report rotation period measurements for 30 cool stars in the 2.5-billion-year-old cluster NGC 6819. The periods reveal a well-defined relationship between rotation period and stellar mass at the cluster age, suggesting that ages with a precision of order 10 per cent can be derived for large numbers of cool Galactic field stars.

Angular Momentum Evolution of Young Stars in the nearby Scorpius-Centaurus OB Association

NASA Astrophysics Data System (ADS)

Mellon, Samuel N.; Mamajek, Eric E.; Oberst, Thomas E.; Pecaut, Mark J.

2017-07-01

We report the results of a study of archival SuperWASP light curves for stars in Scorpius-Centaurus (Sco-Cen), the nearest OB association. We use SuperWASP time-series photometry to extract rotation periods for 189 candidate members of the Sco-Cen complex and verify that 162 of those are members of the classic Sco-Cen subgroups of Upper Scorpius (US), Upper Centaurus-Lupus (UCL), and Lower Centaurus-Crux (LCC). This study provides the first measurements of rotation periods explicitly for large samples of pre-main-sequence (pre-MS) stars spanning the UCL and LCC subgroups. Our final sample of 157 well-characterized pre-MS stars spans ages of ˜10-20 Myr, spectral types of ˜F3-M0, and masses of M ≃ 0.3-1.5 {{ M }}⊙ {{N}}. For this sample, we find a distribution of stellar rotation periods with a median of P rot ≃ 2.4 days, an overall range of 0.2 < P rot < 8 days, and a fairly well-defined mass-dependent upper envelope of rotation periods. This distribution of periods is consistent with recently developed stellar angular momentum evolution models. These data are significant because they represent an undersampled age range and the number of measurable rotation periods is large compared to recent studies of other regions. We also search for new examples of eclipsing disk or ring systems analogous to 1SWASP J140747.93-394542.6 (J1407), but find none. Our survey yielded five eclipsing binaries, but only one appears to be physically associated with the Sco-Cen complex. V2394 Oph is a heavily reddened (A V ≃ 5 mag) massive contact binary in the LDN 1689 cloud whose Gaia astrometry is clearly consistent with kinematic membership with the Ophiuchus star-forming region.

Milliarcsecond resolution infrared observations of young stars in Taurus and Ophiuchus

NASA Astrophysics Data System (ADS)

Simon, M.; Howell, R. R.; Longmore, A. J.; Wilking, B. A.; Peterson, D. M.; Chen, W.-P.

1987-09-01

The paper reports K-band lunar occultation observations of 18 stars in the Taurus and Ophiuchus star-forming regions. Four of the systems, HQ Tau, FF Tau, and SR 12 and ROX 31 in Ophiuchus, are binaries. Their separations, as observed in the projection along the directions of their occultations, range from about 5 to 186 milliarcseconds (mas). SR 12 was also observed by the technique of speckle interferometry in the J, H, and K bands. These observations, taken together with the lunar occultation results, show that SR 12 is an about 0.30 arcsec binary system whose components are late-type stars still approaching the main sequence. The lunar occultation observations reveal extended structure associated with two objects. Elias 29 in Ophiuchus contains a central component about 7 mas in diameter, that radiates most of the flux, and a much larger diffuse component. YLW 16A, also in Ophiuchus, is an extended object about 0.5 arcsec in diameter.

Universal relations for differentially rotating relativistic stars at the threshold to collapse

NASA Astrophysics Data System (ADS)

Bozzola, Gabriele; Stergioulas, Nikolaos; Bauswein, Andreas

2018-03-01

A binary neutron star merger produces a rapidly and differentially rotating compact remnant whose lifespan heavily affects the electromagnetic and gravitational emissions. Its stability depends on both the equation of state (EOS) and the rotation law and it is usually investigated through numerical simulations. Nevertheless, by means of a sufficient criterion for secular instability, equilibrium sequences can be used as a computational inexpensive way to estimate the onset of dynamical instability, which, in general, is close to the secular one. This method works well for uniform rotation and relies on the location of turning points: stellar models that are stationary points in a sequence of equilibrium solutions with constant rest mass or angular momentum. Here, we investigate differentially rotating models (using a large number of EOSs and different rotation laws) and find that several universal relations between properly scaled gravitational mass, rest mass and angular momentum of the turning-point models that are valid for uniform rotation are insensitive to the degree of differential rotation, to high accuracy.

Dynamical Scaling Relations and the Angular Momentum Problem in the FIRE Simulations

NASA Astrophysics Data System (ADS)

Schmitz, Denise; Hopkins, Philip F.; Quataert, Eliot; Keres, Dusan; Faucher-Giguere, Claude-Andre

2015-01-01

Simulations are an extremely important tool with which to study galaxy formation and evolution. However, even state-of-the-art simulations still fail to accurately predict important galaxy properties such as star formation rates and dynamical scaling relations. One possible explanation is the inadequacy of sub-grid models to capture the range of stellar feedback mechanisms which operate below the resolution limit of simulations. FIRE (Feedback in Realistic Environments) is a set of high-resolution cosmological galaxy simulations run using the code GIZMO. It includes more realistic models for various types of feedback including radiation pressure, supernovae, stellar winds, and photoionization and photoelectric heating. Recent FIRE results have demonstrated good agreement with the observed stellar mass-halo mass relation as well as more realistic star formation histories than previous simulations. We investigate the effects of FIRE's improved feedback prescriptions on the simulation "angular momentum problem," i.e., whether FIRE can reproduce observed scaling relations between galaxy stellar mass and rotational/dispersion velocities.

The Balloon Experimental Twin Telescope for Infrared Interferometry (BETTII): Spatially Resolved Spectroscopy in the Far-Infrared

NASA Technical Reports Server (NTRS)

Rinehart, Stephen

2009-01-01

Astronomical studies at infrared wavelengths have dramatically improved our understanding of the universe, and observations with Spitzer, the upcoming Herschel mission, and SOFIA will continue to provide exciting new discoveries. The relatively low angular resolution of these missions, however, is insufficient to resolve the physical scale on which mid-to far-infrared emission arises, resulting in source and structure ambiguities that limit our ability to answer key science questions. Interferometry enables high angular resolution at these wavelengths - a powerful tool for scientific discovery. We will build the Balloon Experimental Twin Telescope for Infrared Interferometry (BETTII), an eight-meter baseline Michelson stellar interferometer to fly on a high-altitude balloon. BETTII's spectral-spatial capability, provided by an instrument using double-Fourier techniques, will address key questions about the nature of disks in young star clusters and active galactic nuclei and the envelopes of evolved stars. BETTII will also lay the technological groundwork for future space interferometers and for suborbital programs optimized for studying extrasolar planets.

On the Peculiarities of Evolutionary History of EHB Objects in Binary Systems with Hot Subdwarf Companions

NASA Astrophysics Data System (ADS)

Pustynski, V.-V.; Pustylnik, I.

2006-03-01

It has been shown quite recently (Maxted etal 2001, Morales-Rueda etal 2003) that dB stars, extreme horizontal branch (EHB) objects, likely all belong to binary systems. We study in detail the mass and angular momentum loss in the giant progenitors of sdB stars in an attempt to clarify why binarity must be a crucial factor in producing EHB objects. Assuming that the progenitors of EHB objects belong to the binaries with initial separations of 100-150 R_odot and fill in their critical Roche lobes while being close to the RGB tip we have found that considerable shrinkage of the orbit can be achieved due to the combined effect of angular momentum loss from the red giant and appreciable accretion on its low mass companion on the hydrodynamical time scale of the donor resulting in formation of helium white dwarfs with masses about 0.5 M_odot and thus evading the common envelope stage.

Lunar Occultations, Setting the Stage for VLTI: The Case Study of CW-Leo (aka IRC+10216)

NASA Astrophysics Data System (ADS)

Käufl, Hans Ulrich; Stecklum, Bringfried; Richter, Steffen; Richichi, Andrea

Lunar occultation allows for a sneak preview of what the VLTI will observe, both with comparable angular resolution and sensitivity. In the thermal infrared ( λ ≈ 10μ m, angular resolution ≤ 0.03^' ') the technique has been pioneered with TIMMI on La Silla. Using this technique several dust shells around Asymptotic Giant Branch stars have been resolved. For the Carbon star CW-Leo (IRC+10 216) high S/N scans will allow for `11/2-dimensional' imaging of the source. At the present state of data reduction the light curves already provide for a very convincing proof of theories on the milli-arcsec scale. In combination with VLTI the technique allows for checks of the visibility calibration and related issues. Moreover, in the (u,v)-plane both techniques are extremely complementary, so that a merging of the data sets appear highly desirable. At La Silla and Paranal ESO a suite of instruments which can be (ab)used for this project is under construction.

ROTATING STARS FROM KEPLER OBSERVED WITH GAIA DR1

DOE Office of Scientific and Technical Information (OSTI.GOV)

Davenport, James R. A.

2017-01-20

Astrometric data from the recent Gaia Data Release 1 have been matched against the sample of stars from Kepler with known rotation periods. A total of 1299 bright rotating stars were recovered from the subset of Gaia sources with good astrometric solutions, most with temperatures above 5000 K. From these sources, 894 were selected as lying near the main sequence using their absolute G -band magnitudes. These main-sequence stars show a bimodality in their rotation period distribution, centered roughly around a 600 Myr rotation isochrone. This feature matches the bimodal period distribution found in cooler stars with Kepler , butmore » was previously undetected for solar-type stars due to sample contamination by subgiants. A tenuous connection between the rotation period and total proper motion is found, suggesting that the period bimodality is due to the age distribution of stars within ∼300 pc of the Sun, rather than a phase of rapid angular momentum loss. This work emphasizes the unique power for understanding stellar populations that is created by combining temporal monitoring from Kepler with astrometric data from Gaia .« less

Radiative feedback and cosmic molecular gas: the role of different radiative sources

NASA Astrophysics Data System (ADS)

Maio, Umberto; Petkova, Margarita; De Lucia, Gabriella; Borgani, Stefano

2016-08-01

We present results from multifrequency radiative hydrodynamical chemistry simulations addressing primordial star formation and related stellar feedback from various populations of stars, stellar spectral energy distributions (SEDs) and initial mass functions. Spectra for massive stars, intermediate-mass stars and regular solar-like stars are adopted over a grid of 150 frequency bins and consistently coupled with hydrodynamics, heavy-element pollution and non-equilibrium species calculations. Powerful massive Population III stars are found to be able to largely ionize H and, subsequently, He and He+, causing an inversion of the equation of state and a boost of the Jeans masses in the early intergalactic medium. Radiative effects on star formation rates are between a factor of a few and 1 dex, depending on the SED. Radiative processes are responsible for gas heating and photoevaporation, although emission from soft SEDs has minor impacts. These findings have implications for cosmic gas preheating, primordial direct-collapse black holes, the build-up of `cosmic fossils' such as low-mass dwarf galaxies, the role of active galactic nuclei during reionization, the early formation of extended discs and angular-momentum catastrophe.

Random forest classification of stars in the Galactic Centre

NASA Astrophysics Data System (ADS)

Plewa, P. M.

2018-05-01

Near-infrared high-angular resolution imaging observations of the Milky Way's nuclear star cluster have revealed all luminous members of the existing stellar population within the central parsec. Generally, these stars are either evolved late-type giants or massive young, early-type stars. We revisit the problem of stellar classification based on intermediate-band photometry in the K band, with the primary aim of identifying faint early-type candidate stars in the extended vicinity of the central massive black hole. A random forest classifier, trained on a subsample of spectroscopically identified stars, performs similarly well as competitive methods (F1 = 0.85), without involving any model of stellar spectral energy distributions. Advantages of using such a machine-trained classifier are a minimum of required calibration effort, a predictive accuracy expected to improve as more training data become available, and the ease of application to future, larger data sets. By applying this classifier to archive data, we are also able to reproduce the results of previous studies of the spatial distribution and the K-band luminosity function of both the early- and late-type stars.

SMA Spectral Line Survey of the Proto-Planetary Nebula CRL 618

NASA Astrophysics Data System (ADS)

Patel, Nimesh A.; Gottlieb, Carl; Young, Ken; Kaminski, Tomasz Tomek; McCarthy, Michael; Menten, Karl; Primiani, Rurik; Lee, Chin-Fei; Gupta, Harshal

2018-01-01

Carbon-rich Asymptotic Giant Branch (AGB) stars are major sources of gas and dust in the interstellar medium. AGB stars remain in their evolutionary stage for 1 to 10 Myrs, during which they have very high mass loss rates that increase at the end. During the brief (~1000 yr) period in the evolution from the AGB to the Planetary Nebula (PN) stage there are dramatic changes in the morphology from nearly spherical symmetry, to bipolar, quadrupolar and more complex structures, with the development of both slow and fast (100 km/s) outflows. The molecular composition of these objects' cirumstellar envelopes also evolves from being similar to that of parent AGB star (mainly diatomic and small polyatomic species), to more complex molecules (including ions).We have started an observational study of a sample of Proto-Planetary nebulae (PPN) with the Submillimeter Array to carry out spectral-line surveys of ~60 GHz frequency coverage in the 345 GHz band (similar to our published IRC+10216 line survey of 2011). Here we present preliminary results from the line survey of the carbon-rich PPN CRL 618, covering a frequency range of 281.9 to 359.4 GHz. Observations were carried out in January 2016 and September 2017, with the SMA in compact (3" angular resolution) and very extended (0.5") configurations, respectively.More than 1100 lines were detected in CRL 618. The majority of them can be attributed to HC3N and c-C3H2, and their isotopologues. About 350 lines are as yet unassigned. The continuum emission is unresolved even at 0.5" resolution. Several hydrogen recombination lines are detected from the central HII region. Lines of CO, HCO+, CS show the fast outflow wings, while the majority of molecular emission arises from a compact region of about 1" diameter. We present LTEmodeling and rotation temperature diagram analysis of HC3N, c-C3H2, CH3CN, and their isotopologues. We plan to observe another PPN, CRL 2688 with the SMA in 2018. Together, these imaging line surveys will provide observational constraints on models of the chemical evolution from AGB stars to Planetary Nebulae.

The close circumstellar environment of the semi-regular S-type star π 1 Gruis

NASA Astrophysics Data System (ADS)

Sacuto, S.; Jorissen, A.; Cruzalèbes, P.; Chesneau, O.; Ohnaka, K.; Quirrenbach, A.; Lopez, B.

2008-05-01

Aims: We study the close circumstellar environment of the nearby S-type star π1 Gruis using high spatial-resolution, mid-infrared observations from the ESO/VLTI. Methods: Spectra and visibilities were obtained with the MIDI interferometer on the VLT Auxiliary Telescopes. The cool M5III giant β Gruis was used as bright primary calibrator, and a dedicated spectro-interferometric study was undertaken to determine its angular diameter accurately. The MIDI measurements were fitted with the 1D numerical radiative transfer code DUSTY to determine the dust shell parameters of π1 Gruis. Taking into account the low spatial extension of the model in the 8-9 μm spectral band for the smallest projected baselines, we consider the possibility of a supplementary molecular shell. Results: The MIDI visibility and phase data are mostly dominated by the spherical 21 mas (694 R_⊙) central star, while the extended dusty environment is over-resolved even with the shortest baselines. No obvious departure from spherical symmetry is found on the milliarcsecond scale. The spectro-interferometric observations are well-fitted by an optically thin (τ_dust<0.01 in the N band) dust shell that is located at about 14 stellar radii with a typical temperature of 700 K and composed of 70% silicate and 30% of amorphous alumina grains. An optically thin (τ_mol<0.1 in the N band) H{2}O + SiO molecular shell extending from the photosphere of the star up to 4.4 stellar radii with a typical temperature of 1000 K is added to the model to improve the fit in the 8-9 μm spectral band. We discuss the probable binary origin of asymmetries as revealed by millimetric observations. Based on observations made with the Very Large Telescope Interferometer at Paranal Observatory under programs 077.D-0294(D/E/F). Reduced visibilities and differential phases are available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/482/561

Star clusters and K2

NASA Astrophysics Data System (ADS)

Dotson, Jessie; Barentsen, Geert; Cody, Ann Marie

2018-01-01

The K2 survey has expanded the Kepler legacy by using the repurposed spacecraft to observe over 20 star clusters. The sample includes open and globular clusters at all ages, including very young (1-10 Myr, e.g. Taurus, Upper Sco, NGC 6530), moderately young (0.1-1 Gyr, e.g. M35, M44, Pleiades, Hyades), middle-aged (e.g. M67, Ruprecht 147, NGC 2158), and old globular clusters (e.g. M9, M19, Terzan 5). K2 observations of stellar clusters are exploring the rotation period-mass relationship to significantly lower masses than was previously possible, shedding light on the angular momentum budget and its dependence on mass and circumstellar disk properties, and illuminating the role of multiplicity in stellar angular momentum. Exoplanets discovered by K2 in stellar clusters provides planetary systems ripe for modeling given the extensive information available about their ages and environment. I will review the star clusters sampled by K2 across 16 fields so far, highlighting several characteristics, caveats, and unexplored uses of the public data set along the way. With fuel expected to run out in 2018, I will discuss the closing Campaigns, highlight the final target selection opportunities, and explain the data archive and TESS-compatible software tools the K2 mission intends to leave behind for posterity.

In-N-Out: The Gas Cycle from Dwarfs to Spiral Galaxies

NASA Astrophysics Data System (ADS)

Christensen, Charlotte R.; Davé, Romeel; Governato, Fabio; Pontzen, Andrew; Brooks, Alyson; Munshi, Ferah; Quinn, Thomas; Wadsley, James

2016-06-01

We examine the scalings of galactic outflows with halo mass across a suite of 20 high-resolution cosmological zoom galaxy simulations covering halo masses in the range {10}9.5{--}{10}12 {M}⊙ . These simulations self-consistently generate outflows from the available supernova energy in a manner that successfully reproduces key galaxy observables, including the stellar mass-halo mass, Tully-Fisher, and mass-metallicity relations. We quantify the importance of ejective feedback to setting the stellar mass relative to the efficiency of gas accretion and star formation. Ejective feedback is increasingly important as galaxy mass decreases; we find an effective mass loading factor that scales as {v}{{circ}}-2.2, with an amplitude and shape that are invariant with redshift. These scalings are consistent with analytic models for energy-driven wind, based solely on the halo potential. Recycling is common: about half of the outflow mass across all galaxy masses is later reaccreted. The recycling timescale is typically ˜1 Gyr, virtually independent of halo mass. Recycled material is reaccreted farther out in the disk and with typically ˜2-3 times more angular momentum. These results elucidate and quantify how the baryon cycle plausibly regulates star formation and alters the angular momentum distribution of disk material across the halo mass range where most cosmic star formation occurs.

ALMA detection of a disk wind from HD 163296

NASA Astrophysics Data System (ADS)

Klaassen, Pamela; Juhasz, Attila; Mathews, Geoffrey; Mottram, Joseph; De Gregorio-Monsalvo, Itziar; van Dishoeck, Ewine; Takahashi, Satoko; Akiyama, Eiji; Chapillon, Edwige; Espada, Daniel; Hales, Antonio; Hogerheijde, Michiel; Rawlings, Mark; Schmalzl, Markus; Testi, Leonardo

2013-07-01

Disk winds have been postulated as a mechanism for angular momentum release in protostellar systems for decades. HD 163296 is a Herbig Ae star surrounded by a disk and has been shown to host a series of HH knots (HH 409) with bow shocks associated with the farthest knots. Here we present ALMA Science Verification data of CO J=2-1 and J=3-2 emission which are spatially coincident with the blue shifted jet of HH knots, and offset from the disk by -18.6 km/s. The emission has a double corkscrew morphology and extends more than 10'' from the disk with embedded emission clumps coincident with jet knots. We interpret this double corkscrew as emission from material in a molecular disk wind, and that the compact emission near the jet knots is being heated by the jet which is moving at much higher velocities. We show that the J=3-2 emission is likely heavily filtered by the interferometer, but the J=2-1 emission suffers less due to the larger beam and measurable angular scales. Excitation analysis suggests temperatures exceeding 900 K in these compact features. The high mass loss rate suggests that this star is dispersing the disk faster than it is funneling mass onto the star, signaling the end of the main accretion phase.

Precise Selenodetic Coordinate System on Artificial Light Refers

NASA Astrophysics Data System (ADS)

Bagrov, Alexander; Pichkhadze, Konstantin M.; Sysoev, Valentin

Historically a coordinate system for the Moon was established on the base of telescopic observations from the Earth. As the angular resolution of Earth-to-Space telescopic observations is limited by Earth atmosphere, and is ordinary worse then 1 ang. second, the mean accuracy of selenodetic coordinates is some angular minutes, which corresponds to errors about 900 meters for positions of lunar objects near center of visible lunar disk, and at least twice more when objects are near lunar poles. As there are no Global Positioning System nor any astronomical observation instruments on the Moon, we proposed to use an autonomous light beacon on the Luna-Globe landing module to fix its position on the surface of the moon ant to use it as refer point for fixation of spherical coordinates system for the Moon. The light beacon is designed to be surely visible by orbiting probe TV-camera. As any space probe has its own stars-orientation system, there is not a problem to calculate a set of directions to the beacon and to the referent stars in probe-centered coordinate system during flight over the beacon. Large number of measured angular positions and time of each observation will be enough to calculate both orbital parameters of the probe and selenodetic coordinates of the beacon by methods of geodesy. All this will allow fixing angular coordinates of any feature of lunar surface in one global coordinate system, referred to the beacon. The satellite’s orbit plane contains ever the center mass of main body, so if the beacon will be placed closely to a lunar pole, we shall determine pole point position of the Moon with accuracy tens times better then it is known now. When angular accuracy of self-orientation by stars of the orbital module of Luna-Glob mission will be 6 angular seconds, then being in circular orbit with height of 200 km the on-board TV-camera will allow calculation of the beacon position as well as 6" corresponding to spatial resolution of the camera. It mean that coordinates of the beacon will be determined with accuracy not worse then 6 meters on the lunar surface. Much more accuracy can be achieved if orbital probe will use as precise angular measurer as optical interferometer. The limiting accuracy of proposed method is far above any reasonable level, because it may be sub-millimeter one. Theoretical analysis shows that for achievement of 1-meter accuracy of coordinate measuring over lunar globe it will be enough to disperse over it surface some 60 light beacons. Designed by Lavochkin Association light beacon is autonomous one, and it will work at least 10 years, so coordinate frame of any other lunar mission could use established selenodetic coordinates during this period. The same approach may be used for establishing Martial coordinates system.

The Physics and Chemistry of Oxygen-Rich Circumstellar Envelopes as Traced by Simple Molecules

NASA Astrophysics Data System (ADS)

Wong, Ka Tat

2018-04-01

The physics and chemistry of the circumstellar envelopes (CSEs) of evolved stars are not fully understood despite decades of research. This thesis addresses two issues in the study of the CSEs of oxygen-rich (O-rich) evolved stars. In the first project, the ammonia (NH3) chemistry of O-rich stars is investigated with multi-wavelength observations; in the second project, the extended atmosphere and inner wind of the archetypal asymptotic giant branch (AGB) star o Ceti (Mira) is studied with high-angular resolution observations. One of the long-standing mysteries in circumstellar chemistry is the perplexing overabundance of the NH3 molecule. NH3 in O-rich evolved stars has been found in much higher abundance, by several orders of magnitude, than that expected in equilibrium chemistry. Several mechanisms have been suggested in the literature to explain this high NH3 abundance, including shocks in the inner wind, photodissociation of nitrogen by interstellar ultraviolet radiation, and nitrogen enrichment in stellar nucleosynthesis; however, none of these suggestions can fully explain the abundances of NH3 and various other molecular species in the CSEs of O-rich stars. In order to investigate the distribution of NH3 in O-rich CSEs, observations of the spectral lines of NH3 from a diverse sample of evolved stars and in different wavelength regimes are necessary. In this thesis, the NH3 line emission and absorption from four O-rich stars are studied. These targets include the AGB star IK Tauri, the pre-planetary nebula OH 231.8+4.2, the red supergiant VY Canis Majoris, and the yellow hypergiant IRC +10420. The amount of NH3 observational data has increased drastically thanks to the recent advancement of instrumentation. Observations of NH3 rotational line emission at submillimetre/far-infrared wavelengths were possible with the Herschel Space Observatory (2009–2013). The new wideband correlator in the upgraded Karl G. Janksy Very Large Array (VLA) provided data of multiple radio inversion lines of NH3. Furthermore, mid-infrared absorption of NH3 has been observed by the NASA Infrared Telescope Facility (IRTF) for IK Tau and VY CMa. Full radiative transfer modelling including mid-infrared pumping to the first vibrationally excited state (v2=1) has been carried out to reproduce the observed emission and absorption spectra and to retrieve the NH3 abundances in the targets. It is found that the NH3 emission in the CSEs of the targets arises from localised spatial-kinematic structures in which the gas density may be higher than in the surrounding gas. Circumstellar shocks may contribute to, but cannot fully account for, the formation of the molecule. Besides circumstellar chemistry, our understanding of the dust formation and wind-driving mechanisms of oxygen-rich evolved stars is still incomplete. One of the obstacles in the past was the difficulty in imaging the dust condensation and wind acceleration zones due to the lack of high-angular resolution instruments. Thanks to the Atacama Large Millimeter/submillimeter Array (ALMA), which has the longest baseline of about 15 km, we are now able to produce high-fidelity images at unprecedented angular resolutions of tens of milliarcseconds (mas) in the (sub)millimetre regime. Such angular resolutions, which are comparable to the stellar radii of nearby objects, are necessary to understand the gas dynamics and chemical evolution in the pulsating atmosphere and dust formation zone of nearby AGB stars. The eponymous Mira-type long-period variable, o Cet, was observed as a Science Verification target during the first ALMA Long Baseline Campaign that took place in 2014. The observations produced images of the stellar radio photosphere and the molecular transitions of SiO and H2O at an angular resolution of about 30 mas near 220 GHz (1.3 mm). The millimetre stellar disc of o Cet was resolved and modelled. More importantly, this is the first time that molecular line absorption against the background stellar continuum has been clearly imaged in the (sub)millimetre wavelength regime. Through radiative transfer modelling of the SiO and H2O line absorption and emission, it is found that during the ALMA observations, the extended atmosphere of the star exhibited infall motions in general with a shock front of velocity

WHITE DWARFS IN LOCAL STAR STREAMS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Fuchs, Burkhard; Dettbarn, Christian

2011-01-15

We have studied the fine structure of the phase space distribution of white dwarfs in the solar neighborhood. White dwarfs have kinematics that are typical for the stellar population of the old thin disk of the Milky Way. Using a projection of the space velocities of stars onto vertical angular momentum components and eccentricities of the stellar orbits we demonstrate that stellar streams can be identified in the phase space distribution of the white dwarfs. These correspond to the well-known Sirius, Pleiades, and Hercules star streams. Membership of white dwarfs, which represent the oldest population in the Galaxy, in thesemore » streams lends support to the interpretation that the streams owe their existence to dynamical resonance effects of the stars with Galactic spiral arms or the Galactic bar, because these indiscriminately affect all stellar populations.« less

Fragmentation of interstellar clouds and star formation

NASA Technical Reports Server (NTRS)

Silk, J.

1982-01-01

The principal issues are addressed: the fragmentation of molecular clouds into units of stellar mass and the impact of star formation on molecular clouds. The observational evidence for fragmentation is summarized, and the gravitational instability described of a uniform spherical cloud collapsing from rest. The implications are considered of a finite pressure for the minimum fragment mass that is attainable in opacity-limited fragmentation. The role of magnetic fields is discussed in resolving the angular momentum problem and in making the collapse anisotropic, with notable consequences for fragmentation theory. Interactions between fragments are described, with emphasis on the effect of protostellar winds on the ambient cloud matter and on inhibiting further star formation. Such interactions are likely to have profound consequences for regulating the rate of star formation and on the energetics and dynamics of molecular clouds.

Modelling of celestial backgrounds

NASA Astrophysics Data System (ADS)

Hickman, Duncan L.; Smith, Moira I.; Lim, Jae-Wan; Jeon, Yun-Ho

2018-05-01

For applications where a sensor's image includes the celestial background, stars and Solar System Bodies compromise the ability of the sensor system to correctly classify a target. Such false targets are particularly significant for the detection of weak target signatures which only have a small relative angular motion. The detection of celestial features is well established in the visible spectral band. However, given the increasing sensitivity and low noise afforded by emergent infrared focal plane array technology together with larger and more efficient optics, the signatures of celestial features can also impact performance at infrared wavelengths. A methodology has been developed which allows the rapid generation of celestial signatures in any required spectral band using star data from star catalogues and other open-source information. Within this paper, the radiometric calculations are presented to determine the irradiance values of stars and planets in any spectral band.

Rejuvenation of the Innocent Bystander: Testing Spin-Up in Dwarf Carbon Stars

NASA Astrophysics Data System (ADS)

Green, Paul

2013-09-01

Carbon stars (C>O) were long assumed to all be giants, because only AGB stars dredge up significant carbon into their atmospheres. We now know that dwarf carbon (dC) stars are actually far more common than C giants. These dCs are hypothesized to have accreted C-rich envelope material from an AGB companion, in systems that have likely undergone a planetary nebula phase, eventually yielding a white dwarf and a dC that has gained both significant mass and angular momentum. To test whether the X-ray emission strength and spectral properties are consistent with a rejuvenated dynamo, we propose a Chandra pilot study of dCs selected from the SDSS; some have hot white dwarf companions (indicating more recent mass transfer), and all show Balmer emission lines (a sign of activity).

Parametric Modeling in Action: High Accuracy Seismology of Kepler DAV Stars

NASA Astrophysics Data System (ADS)

Giammichele, N.; Fontaine, G.; Charpinet, S.; Brassard, P.; Greiss, S.

2015-06-01

We summarize here the efforts made on the quantitative seismic analyses performed on two ZZ Ceti stars observed with the Kepler satellite. One of them, KIC 11911480, is located close to the blue edge of the instability strip, while the other, GD 1212, is found at the red edge. We emphasize the need for parameterized modeling and the forward approach to uniquely establish the fundamental parameters of the stars. We show how the internal structures as well as rotation profiles are unravelled to surprisingly large depths for degenerates such as ZZ Ceti stars, which further confirms the loss of stellar angular momentum before the white dwarf stage detected previously in GW Vir pulsating white dwarfs. This opens up interesting prospects for the new mission to come, Kepler-2, in the field of white dwarf asteroseismology.

A mysterious dust clump in a disk around an evolved binary star system.

PubMed

Jura, M; Turner, J

1998-09-10

The discovery of planets in orbit around the pulsar PSR1257+12 shows that planets may form around post-main-sequence stars. Other evolved stars, such as HD44179 (an evolved star which is part of the binary system that has expelled the gas and dust that make the Red Rectangle nebula), possess gravitationally bound orbiting dust disks. It is possible that planets might form from gravitational collapse in such disks. Here we report high-angular-resolution observations at millimetre and submillimetre wavelengths of the dusk disk associated with the Red Rectangle. We find a dust clump with an estimated mass near that of Jupiter in the outer region of the disk. The clump is larger than our Solar System, and far beyond where planet formation would normally be expected, so its nature is at present unclear.

Gravitational wave content and stability of uniformly, rotating, triaxial neutron stars in general relativity.

PubMed

Tsokaros, Antonios; Ruiz, Milton; Paschalidis, Vasileios; Shapiro, Stuart L; Baiotti, Luca; Uryū, Kōji

2017-06-15

Targets for ground-based gravitational wave interferometers include continuous, quasiperiodic sources of gravitational radiation, such as isolated, spinning neutron stars. In this work, we perform evolution simulations of uniformly rotating, triaxially deformed stars, the compressible analogs in general relativity of incompressible, Newtonian Jacobi ellipsoids. We investigate their stability and gravitational wave emission. We employ five models, both normal and supramassive, and track their evolution with different grid setups and resolutions, as well as with two different evolution codes. We find that all models are dynamically stable and produce a strain that is approximately one-tenth the average value of a merging binary system. We track their secular evolution and find that all our stars evolve toward axisymmetry, maintaining their uniform rotation, rotational kinetic energy, and angular momentum profiles while losing their triaxiality.

Spatial and kinematic structure of Monoceros star-forming region

NASA Astrophysics Data System (ADS)

Costado, M. T.; Alfaro, E. J.

2018-05-01

The principal aim of this work is to study the velocity field in the Monoceros star-forming region using the radial velocity data available in the literature, as well as astrometric data from the Gaia first release. This region is a large star-forming complex formed by two associations named Monoceros OB1 and OB2. We have collected radial velocity data for more than 400 stars in the area of 8 × 12 deg2 and distance for more than 200 objects. We apply a clustering analysis in the subspace of the phase space formed by angular coordinates and radial velocity or distance data using the Spectrum of Kinematic Grouping methodology. We found four and three spatial groupings in radial velocity and distance variables, respectively, corresponding to the Local arm, the central clusters forming the associations and the Perseus arm, respectively.

The natural moon illusion: a multifactor angular account.

PubMed

Plug, C; Ross, H E

1994-01-01

It is argued that the failure to explain the celestial illusion results from conceptual confusion about perceived size and from disregard of the observational evidence relating to the natural moon illusion. The evidence shows that the illusion consists of a perceived angular size enlargement of horizon objects, by a factor of about 1.5-2.0 in diameter in comparison with elevated objects. Most measurements of the illusion have been made in terms of angular size, although in some proposed explanations an illusion of linear size is assumed. The magnitude of the illusion varies, particularly with the detail of the horizon scene. The illusion can be explained as the sum of several factors that affect perceived angular size: size contrast, vergence commands and eye or head position, aerial perspective, and colour. The relative contributions of these factors are assessed.

Small cell foams containing a modified dense star polymer or dendrimer as a nucleating agent

DOEpatents

Hedstrand, David M.; Tomalia, Donald A.

1995-01-01

A small cell foam having a modified dense star polymer or dendrimer is described. This modified dense star polymer or dendrimer has a highly branched interior of one monomeric composition and an exterior structure of a different monomeric composition capable of providing a hydrophobic outer shell and a particle diameter of from about 5 to about 1,000 nm with a matrix polymer.

Small cell foams containing a modified dense star polymer or dendrimer as a nucleating agent

DOEpatents

Hedstrand, D.M.; Tomalia, D.A.

1995-02-28

A small cell foam having a modified dense star polymer or dendrimer is described. This modified dense star polymer or dendrimer has a highly branched interior of one monomeric composition and an exterior structure of a different monomeric composition capable of providing a hydrophobic outer shell and a particle diameter of from about 5 to about 1,000 nm with a matrix polymer.

Physical properties and catalog of EW-type eclipsing binaries observed by LAMOST

NASA Astrophysics Data System (ADS)

Qian, Sheng-Bang; He, Jia-Jia; Zhang, Jia; Zhu, Li-Ying; Shi, Xiang-Dong; Zhao, Er-Gang; Zhou, Xiao

2017-08-01

EW-type eclipsing binaries (hereafter called EWs) are strong interacting systems in which both component stars usually fill their critical Roche lobes and share a common envelope. Numerous EWs were discovered by several deep photometric surveys and there were about 40 785 EW-type binary systems listed in the international variable star index (VSX) by 2017 March 13. 7938 of them were observed with LAMOST by 2016 November 30 and their spectral types were identified. Stellar atmospheric parameters of 5363 EW-type binary stars were determined based on good spectroscopic observations. In the paper, those EWs are cataloged and their properties are analyzed. The distributions of orbital period (P), effective temperature (T), gravitational acceleration (log(g)), metallicity ([Fe/H]) and radial velocity (RV) are presented for these observed EW-type systems. It is shown that about 80.6% of sample stars have metallicity below zero, indicating that EW-type systems are old stellar populations. This is in agreement with the conclusion that EW binaries are formed from moderately close binaries through angular momentum loss via magnetic braking that takes a few hundred million to a few billion years. The unusually high metallicities of a few percent of EWs may be caused by contamination of material from the evolution of unseen neutron stars or black holes in the systems. The correlations between orbital period and effective temperature, gravitational acceleration and metallicity are presented and their scatters are mainly caused by (i) the presence of third bodies and (ii) sometimes wrongly determined periods. It is shown that some EWs contain evolved component stars and the physical properties of EWs mainly depend on their orbital periods. It is found that extremely short-period EWs may be older than their long-period cousins because they have lower metallicities. This reveals that they have a longer timescale of pre-contact evolution and their formation and evolution aremainly driven by angular momentum loss via magnetic braking.

Accretion disks around neutron and strange stars in R + aR {sup 2} gravity

DOE Office of Scientific and Technical Information (OSTI.GOV)

Staykov, Kalin V.; Yazadjiev, Stoytcho S.; Doneva, Daniela D., E-mail: kstaykov@phys.uni-sofia.bg, E-mail: daniela.doneva@uni-tuebingen.de, E-mail: yazad@phys.uni-sofia.bg

2016-08-01

We study the electromagnetic spectrum of accretion disks around neutron and strange stars in R + aR {sup 2} gravity. Both static and rapidly rotating models are investigated. The results are compared with the General Relativistic results. We found difference between the results in both theories of about 50% for the electromagnetic flux and about 20% in the luminosity for models with equal mass and angular velocity in both theories. The observed differences are much lower for models rotating with Keplerian velocity and with equal masses.

Accretion disks around neutron and strange stars in R+aR2 gravity

NASA Astrophysics Data System (ADS)

Staykov, Kalin V.; Doneva, Daniela D.; Yazadjiev, Stoytcho S.

2016-08-01

We study the electromagnetic spectrum of accretion disks around neutron and strange stars in R+aR2 gravity. Both static and rapidly rotating models are investigated. The results are compared with the General Relativistic results. We found difference between the results in both theories of about 50% for the electromagnetic flux and about 20% in the luminosity for models with equal mass and angular velocity in both theories. The observed differences are much lower for models rotating with Keplerian velocity and with equal masses.

DSLR Double Star Astrometry Using an Alt-Az Telescope

NASA Astrophysics Data System (ADS)

Frey, Thomas; Haworth, David

2014-07-01

The goal of this project was to determine if the double star's angular separation and position angle measurements could be successfully measured with a motor driven, alt-azimuth Dobsonian-mounted Newtonian telescope (without a field rotator), and a digital single-lens reflex (DSLR) camera. Additionally, the project was constrained by using as much existing equipment as much as possible, including an Apple MacBook Pro laptop and a Canon T2i camera. This project was additionally challenging because the first author had no experience with astrophotography.

Cyclotron line resonant transfer through neutron star atmospheres

NASA Technical Reports Server (NTRS)

Wang, John C. L.; Wasserman, Ira M.; Salpeter, Edwin E.

1988-01-01

Monte Carlo methods are used to study in detail the resonant radiative transfer of cyclotron line photons with recoil through a purely scattering neutron star atmosphere for both the polarized and unpolarized cases. For each case, the number of scatters, the path length traveled, the escape frequency shift, the escape direction cosine, the emergent frequency spectra, and the angular distribution of escaping photons are investigated. In the polarized case, transfer is calculated using both the cold plasma e- and o-modes and the magnetic vacuum perpendicular and parallel modes.

Three-dimensional finite volume modelling of blood flow in simulated angular neck abdominal aortic aneurysm

NASA Astrophysics Data System (ADS)

Algabri, Y. A.; Rookkapan, S.; Chatpun, S.

2017-09-01

An abdominal aortic aneurysm (AAA) is considered a deadly cardiovascular disease that defined as a focal dilation of blood artery. The healthy aorta size is between 15 and 24 mm based on gender, bodyweight, and age. When the diameter increased to 30 mm or more, the rupture can occur if it is kept growing or untreated. Moreover, the proximal angular neck of aneurysm is categorized as a significant morphological feature with prime harmful effects on endovascular aneurysm repair (EVAR). Flow pattern in pathological vessel can influence the vascular intervention. The aim of this study is to investigate the blood flow behaviours in angular neck abdominal aortic aneurysm with simulated geometry based on patient’s information using computational fluid dynamics (CFD). The 3D angular neck AAA models have been designed by using SolidWorks Software. Consequently, CFD tools are used for simulating these 3D models of angular neck AAA in ANSYS FLUENT Software. Eventually, based on the results, we summarized that the CFD techniques have shown high performance in explaining and investigating the flow patterns for angular neck abdominal aortic aneurysm.

Optical diffraction by ordered 2D arrays of silica microspheres

NASA Astrophysics Data System (ADS)

Shcherbakov, A. A.; Shavdina, O.; Tishchenko, A. V.; Veillas, C.; Verrier, I.; Dellea, O.; Jourlin, Y.

2017-03-01

The article presents experimental and theoretical studies of angular dependent diffraction properties of 2D monolayer arrays of silica microspheres. High-quality large area defect-free monolayers of 1 μm diameter silica microspheres were deposited by the Langmuir-Blodgett technique under an accurate optical control. Measured angular dependencies of zeroth and one of the first order diffraction efficiencies produced by deposited samples were simulated by the rigorous Generalized Source Method taking into account particle size dispersion and lattice nonideality.

The bright-star masks for the HSC-SSP survey

NASA Astrophysics Data System (ADS)

Coupon, Jean; Czakon, Nicole; Bosch, James; Komiyama, Yutaka; Medezinski, Elinor; Miyazaki, Satoshi; Oguri, Masamune

2018-01-01

We present the procedure to build and validate the bright-star masks for the Hyper-Suprime-Cam Strategic Subaru Proposal (HSC-SSP) survey. To identify and mask the saturated stars in the full HSC-SSP footprint, we rely on the Gaia and Tycho-2 star catalogues. We first assemble a pure star catalogue down to GGaia < 18 after removing ˜1.5% of sources that appear extended in the Sloan Digital Sky Survey (SDSS). We perform visual inspection on the early data from the S16A internal release of HSC-SSP, finding that our star catalogue is 99.2% pure down to GGaia < 18. Second, we build the mask regions in an automated way using stacked detected source measurements around bright stars binned per GGaia magnitude. Finally, we validate those masks by visual inspection and comparison with the literature of galaxy number counts and angular two-point correlation functions. This version (Arcturus) supersedes the previous version (Sirius) used in the S16A internal and DR1 public releases. We publicly release the full masks and tools to flag objects in the entire footprint of the planned HSC-SSP observations at "ftp://obsftp.unige.ch/pub/coupon/brightStarMasks/HSC-SSP/".

Probing Massive Star Cluster Formation with ALMA

NASA Astrophysics Data System (ADS)

Johnson, Kelsey

2015-08-01

Observationally constraining the physical conditions that give rise to massive star clusters has been a long-standing challenge. Now with the ALMA Observatory coming on-line, we can finally begin to probe the birth environments of massive clusters in a variety of galaxies with sufficient angular resolution. In this talk I will give an overview of ALMA observations of galaxies in which candidate proto-super star cluster molecular clouds have been identified. These new data probe the physical conditions that give rise to super star clusters, providing information on their densities, pressures, and temperatures. In particular, the observations indicate that these clouds may be subject to external pressures of P/k > 108 K cm-3, which is consistent with the prevalence of optically observed adolescent super star clusters in interacting galaxy systems and other high pressure environments. ALMA observations also enable an assessement of the molecular cloud chemical abundances in the regions surrounding super star clusters. Molecular clouds associated with existing super star clusters are strongly correlated with HCO+ emission, but appear to have relatively low ratio of CO/HCO+ emission compared to other clouds, indicating that the super star clusters are impacting the molecular abundances in their vicinity.

Using Photometric Variability to Detect Binarity in the Central Stars of Four Planetary Nebulae, A 43, A 74, NGC 6720, and NGC 6853

NASA Astrophysics Data System (ADS)

Smith, Alexander; De Marco, O.

2007-12-01

Recent observational evidence and theoretical models are challenging the classical paradigm of single star planetary nebula (PN) evolution, suggesting instead that binary stars play a significant role in the process of PN formation. In order to shape the 90% of PN that are non-spherical, the central star must be rotating and have a magnetic field; the most-likely source of the angular momentum needed to sustain magnetic fields is a binary companion. More observational evidence is needed to confirm that the fraction of PN with close binary central stars is indeed higher than the currently known value of 10-15%. As part of an international effort to detect binary central stars (PLAN-B - Panetary Nebula Binaries), we are carrying out a new photometric survey to look for close binary central stars of PN. Here we present the findings for 4 objects: A 43, A 74, NGC 6720, and NGC 6853. NGC 6720 and NGC 6853 show evidence of periodic variability, the former of which might even show one eclipse. Once completed, the survey will assess the binarity of about 100 central stars of PN.

First light for GRAVITY: Phase referencing optical interferometry for the Very Large Telescope Interferometer

NASA Astrophysics Data System (ADS)

Gravity Collaboration; Abuter, R.; Accardo, M.; Amorim, A.; Anugu, N.; Ávila, G.; Azouaoui, N.; Benisty, M.; Berger, J. P.; Blind, N.; Bonnet, H.; Bourget, P.; Brandner, W.; Brast, R.; Buron, A.; Burtscher, L.; Cassaing, F.; Chapron, F.; Choquet, É.; Clénet, Y.; Collin, C.; Coudé Du Foresto, V.; de Wit, W.; de Zeeuw, P. T.; Deen, C.; Delplancke-Ströbele, F.; Dembet, R.; Derie, F.; Dexter, J.; Duvert, G.; Ebert, M.; Eckart, A.; Eisenhauer, F.; Esselborn, M.; Fédou, P.; Finger, G.; Garcia, P.; Garcia Dabo, C. E.; Garcia Lopez, R.; Gendron, E.; Genzel, R.; Gillessen, S.; Gonte, F.; Gordo, P.; Grould, M.; Grözinger, U.; Guieu, S.; Haguenauer, P.; Hans, O.; Haubois, X.; Haug, M.; Haussmann, F.; Henning, Th.; Hippler, S.; Horrobin, M.; Huber, A.; Hubert, Z.; Hubin, N.; Hummel, C. A.; Jakob, G.; Janssen, A.; Jochum, L.; Jocou, L.; Kaufer, A.; Kellner, S.; Kendrew, S.; Kern, L.; Kervella, P.; Kiekebusch, M.; Klein, R.; Kok, Y.; Kolb, J.; Kulas, M.; Lacour, S.; Lapeyrère, V.; Lazareff, B.; Le Bouquin, J.-B.; Lèna, P.; Lenzen, R.; Lévêque, S.; Lippa, M.; Magnard, Y.; Mehrgan, L.; Mellein, M.; Mérand, A.; Moreno-Ventas, J.; Moulin, T.; Müller, E.; Müller, F.; Neumann, U.; Oberti, S.; Ott, T.; Pallanca, L.; Panduro, J.; Pasquini, L.; Paumard, T.; Percheron, I.; Perraut, K.; Perrin, G.; Pflüger, A.; Pfuhl, O.; Phan Duc, T.; Plewa, P. M.; Popovic, D.; Rabien, S.; Ramírez, A.; Ramos, J.; Rau, C.; Riquelme, M.; Rohloff, R.-R.; Rousset, G.; Sanchez-Bermudez, J.; Scheithauer, S.; Schöller, M.; Schuhler, N.; Spyromilio, J.; Straubmeier, C.; Sturm, E.; Suarez, M.; Tristram, K. R. W.; Ventura, N.; Vincent, F.; Waisberg, I.; Wank, I.; Weber, J.; Wieprecht, E.; Wiest, M.; Wiezorrek, E.; Wittkowski, M.; Woillez, J.; Wolff, B.; Yazici, S.; Ziegler, D.; Zins, G.

2017-06-01

GRAVITY is a new instrument to coherently combine the light of the European Southern Observatory Very Large Telescope Interferometer to form a telescope with an equivalent 130 m diameter angular resolution and a collecting area of 200 m2. The instrument comprises fiber fed integrated optics beam combination, high resolution spectroscopy, built-in beam analysis and control, near-infrared wavefront sensing, phase-tracking, dual-beam operation, and laser metrology. GRAVITY opens up to optical/infrared interferometry the techniques of phase referenced imaging and narrow angle astrometry, in many aspects following the concepts of radio interferometry. This article gives an overview of GRAVITY and reports on the performance and the first astronomical observations during commissioning in 2015/16. We demonstrate phase-tracking on stars as faint as mK ≈ 10 mag, phase-referenced interferometry of objects fainter than mK ≈ 15 mag with a limiting magnitude of mK ≈ 17 mag, minute long coherent integrations, a visibility accuracy of better than 0.25%, and spectro-differential phase and closure phase accuracy better than 0.5°, corresponding to a differential astrometric precision of better than ten microarcseconds (μas). The dual-beam astrometry, measuring the phase difference of two objects with laser metrology, is still under commissioning. First observations show residuals as low as 50 μas when following objects over several months. We illustrate the instrument performance with the observations of archetypical objects for the different instrument modes. Examples include the Galactic center supermassive black hole and its fast orbiting star S2 for phase referenced dual-beam observations and infrared wavefront sensing, the high mass X-ray binary BP Cru and the active galactic nucleus of PDS 456 for a few μas spectro-differential astrometry, the T Tauri star S CrA for a spectro-differential visibility analysis, ξ Tel and 24 Cap for high accuracy visibility observations, and η Car for interferometric imaging with GRAVITY.

Constraining smoothness parameter and the DD relation of Dyer-Roeder equation with supernovae

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yang, Xi; Yu, Hao-Ran; Zhang, Tong-Jie, E-mail: yangwds@mail.bnu.edu.cn, E-mail: yu@bnu.edu.cn, E-mail: tjzhang@bnu.edu.cn

2013-06-01

Our real universe is locally inhomogeneous. Dyer and Roeder introduced the smoothness parameter α to describe the influence of local inhomogeneity on angular diameter distance, and they obtained the angular diameter distance-redshift approximate relation (Dyer-Roeder equation) for locally inhomogeneous universe. Furthermore, the Distance-Duality (DD) relation, D{sub L}(z)(1+z){sup −2}/D{sub A}(z) = 1, should be valid for all cosmological models that are described by Riemannian geometry, where D{sub L} and D{sub A} are, respectively, the luminosity and angular distance distances. Therefore, it is necessary to test whether if the Dyer-Roeder approximate equation can satisfy the Distance-Duality relation. In this paper, we usemore » Union2.1 SNe Ia data to constrain the smoothness parameter α and test whether the Dyer-Roeder equation meet the DD relation. By using χ{sup 2} minimization, we get α = 0.92{sub −0.32}{sup +0.08} at 1σ and 0.92{sub −0.65}{sup +0.08} at 2σ, and our results show that the Dyer-Roeder equation is in good consistency with the DD relation at 1σ.« less

n l -> n' l' transition rates in electron and proton - Rydberg atom collision

NASA Astrophysics Data System (ADS)

Vrinceanu, Daniel

2017-04-01

Electrons and protons drive the recombination dynamics of highly excited Rydberg atoms in cold rarefied plasmas found in astrophysical conditions such as primordial recombination or star formation in H-II clouds. It has been recognized that collisions induce both energy and angular momentum transitions in Rydberg atoms, although in different proportions, depending on the initial state, temperature and the given species considered in the collision (electron or proton). Most studies focused on one collision type at a time, under the assumption that collision types are independent or their effects are not competing. The classical Monte-Carlo trajectory simulations presented in this work calculate the rates for both energy and angular momentum transfers and show their interdependence. For example, energy transfer with small angular momentum change are more efficient for target states with initial large angular momentum. The author acknowledges support received from the National Science Foundation through a Grant for the Center for Research on Complex Networks (HRD-1137732).

Boring apparatus capable of boring straight holes

DOE Office of Scientific and Technical Information (OSTI.GOV)

Peterson, C.R.

The invention relates to a rock boring assembly for producing a straight hole for use in a drill string above a pilot boring bit of predetermined diameter smaller than the desired final hole size. The boring assembly comprises a small conical boring bit and a larger conical boring, the conical boring bits mounted on lower and upper ends of an enlongated spacer, respectively, and the major effective cutting diameters of each of the conical boring bits being at least 10% greater than the minor effective cutting diameter of the respective bit. The spacer has a cross-section resistant bending and spacesmore » the conical boring bits apart a distance at least 5 times the major cutting diameter of the small conical boring bit, thereby spacing the pivot points provided by the two conical boring bits to limit bodily angular deflection of the assembly and providing a substantial moment arm to resist lateral forces applied to the assembly by the pilot bit and drill string. The spacing between the conical bits is less than about 20 times the major cutting diameter of the lower conical boring bit to enable the spacer to act as a bend-resistant beam to resist angular deflection of the axis of either of the conical boring bits relative to the other when it receives uneven lateral force due to non-uniformity of cutting conditions about the circumference of the bit. Advantageously the boring bits also are self-advancing and feature skewed rollers. 7 claims.« less

Physical conditions near red giant and supergiant stars - An interpretation of SiO VLBI maps

NASA Technical Reports Server (NTRS)

Alcock, Charles; Ross, Randy R.

1986-01-01

Understanding the dynamical structure of circumstellar envelopes around cool giant and supergiant stars depends critically on the knowledge of what happens in the 'near zone' of the envelope, within a few stellar radii of the star. One probe with adequate angular resolution to study the near zone is VLBI observation of the SiO masers. It is shown that VLBI maps of VX Sgr establish that the particle density in the SiO masers is very high (about 10 to the 12th/cu cm), indicating that the masers form in dense cloudlets and not in a spherically expanding wind. The implications of these results for the mechanism of mass loss are discussed.

Magnetic fields in the formation of massive stars.

PubMed

Girart, Josep M; Beltrán, Maria T; Zhang, Qizhou; Rao, Ramprasad; Estalella, Robert

2009-06-12

Massive stars play a crucial role in the production of heavy elements and in the evolution of the interstellar medium, yet how they form is still a matter of debate. We report high-angular-resolution submillimeter observations toward the massive hot molecular core (HMC) in the high-mass star-forming region G31.41+0.31. We find that the evolution of the gravitational collapse of the HMC is controlled by the magnetic field. The HMC is simultaneously contracting and rotating, and the magnetic field lines threading the HMC are deformed along its major axis, acquiring an hourglass shape. The magnetic energy dominates over the centrifugal and turbulence energies, and there is evidence of magnetic braking in the contracting core.

Collapse of magnetized hypermassive neutron stars in general relativity.

PubMed

Duez, Matthew D; Liu, Yuk Tung; Shapiro, Stuart L; Shibata, Masaru; Stephens, Branson C

2006-01-27

Hypermassive neutron stars (HMNSs)--equilibrium configurations supported against collapse by rapid differential rotation--are possible transient remnants of binary neutron-star mergers. Using newly developed codes for magnetohydrodynamic simulations in dynamical spacetimes, we are able to track the evolution of a magnetized HMNS in full general relativity for the first time. We find that secular angular momentum transport due to magnetic braking and the magnetorotational instability results in the collapse of an HMNS to a rotating black hole, accompanied by a gravitational wave burst. The nascent black hole is surrounded by a hot, massive torus undergoing quasistationary accretion and a collimated magnetic field. This scenario suggests that HMNS collapse is a possible candidate for the central engine of short gamma-ray bursts.

A Deep X-ray Survey of Low-Mass PMS Stars in NGC 2264

NASA Technical Reports Server (NTRS)

Simon, Theodore

2005-01-01

Two X-ray images were obtained with the XMM-Newton spacecraft of more than 300 members of the NGC 2264 Open Cluster and its associated molecular cloud in order to investigate their magnetic activity. The X-ray fluxes extracted from those observations were used to study the dependence of stellar dynamo activity upon age and rotation for the optically revealed T Tauri stars and to place empirical constraints on theoretical models of angular momentum/dynamo evolution. The observations were also used to study the role of magnetic fields in the formation of low mass stars through the observation of very young protostars that are deeply embedded in the molecular cloud located behind the visible open cluster.

Gravitational Waves from Gravitational Collapse.

PubMed

Fryer, Chris L; New, Kimberly C B

2011-01-01

Gravitational-wave emission from stellar collapse has been studied for nearly four decades. Current state-of-the-art numerical investigations of collapse include those that use progenitors with more realistic angular momentum profiles, properly treat microphysics issues, account for general relativity, and examine non-axisymmetric effects in three dimensions. Such simulations predict that gravitational waves from various phenomena associated with gravitational collapse could be detectable with ground-based and space-based interferometric observatories. This review covers the entire range of stellar collapse sources of gravitational waves: from the accretion-induced collapse of a white dwarf through the collapse down to neutron stars or black holes of massive stars to the collapse of supermassive stars. Supplementary material is available for this article at 10.12942/lrr-2011-1.

Mass transfer in white dwarf-neutron star binaries

NASA Astrophysics Data System (ADS)

Bobrick, Alexey; Davies, Melvyn B.; Church, Ross P.

2017-05-01

We perform hydrodynamic simulations of mass transfer in binaries that contain a white dwarf and a neutron star (WD-NS binaries), and measure the specific angular momentum of material lost from the binary in disc winds. By incorporating our results within a long-term evolution model, we measure the long-term stability of mass transfer in these binaries. We find that only binaries containing helium white dwarfs (WDs) with masses less than a critical mass of MWD, crit = 0.2 M⊙ undergo stable mass transfer and evolve into ultracompact X-ray binaries. Systems with higher mass WDs experience unstable mass transfer, which leads to tidal disruption of the WD. Our low critical mass compared to the standard jet-only model of mass-loss arises from the efficient removal of angular momentum in the mechanical disc winds, which develop at highly super-Eddington mass-transfer rates. We find that the eccentricities expected for WD-NS binaries when they come into contact do not affect the loss of angular momentum, and can only affect the long-term evolution if they change on shorter time-scales than the mass-transfer rate. Our results are broadly consistent with the observed numbers of both ultracompact X-ray binaries and radio pulsars with WD companions. The observed calcium-rich gap transients are consistent with the merger rate of unstable systems with higher mass WDs.

The Auriga Project: the properties and formation mechanisms of disc galaxies across cosmic time

NASA Astrophysics Data System (ADS)

Grand, Robert J. J.; Gómez, Facundo A.; Marinacci, Federico; Pakmor, Rüdiger; Springel, Volker; Campbell, David J. R.; Frenk, Carlos S.; Jenkins, Adrian; White, Simon D. M.

2017-05-01

We introduce a suite of 30 cosmological magneto-hydrodynamical zoom simulations of the formation of galaxies in isolated Milky Way mass dark haloes. These were carried out with the moving mesh code arepo, together with a comprehensive model for galaxy formation physics, including active galactic nuclei (AGN) feedback and magnetic fields, which produces realistic galaxy populations in large cosmological simulations. We demonstrate that our simulations reproduce a wide range of present-day observables, in particular, two-component disc-dominated galaxies with appropriate stellar masses, sizes, rotation curves, star formation rates and metallicities. We investigate the driving mechanisms that set present-day disc sizes/scalelengths, and find that they are related to the angular momentum of halo material. We show that the largest discs are produced by quiescent mergers that inspiral into the galaxy and deposit high-angular momentum material into the pre-existing disc, simultaneously increasing the spin of dark matter and gas in the halo. More violent mergers and strong AGN feedback play roles in limiting disc size by destroying pre-existing discs and by suppressing gas accretion on to the outer disc, respectively. The most important factor that leads to compact discs, however, is simply a low angular momentum for the halo. In these cases, AGN feedback plays an important role in limiting central star formation and the formation of a massive bulge.

Looking for high-mass young stellar objects: H2O and OH masers in ammonia cores

NASA Astrophysics Data System (ADS)

Codella, C.; Cesaroni, R.; López-Sepulcre, A.; Beltrán, M. T.; Furuya, R.; Testi, L.

2010-02-01

Context. The earliest stages of high-mass star formation have yet to be characterised well, because high-angular resolution observations are required to infer the properties of the molecular gas hosting the newly formed stars. Aims: We search for high-mass molecular cores in a large sample of 15 high-mass star-forming regions that are observed at high-angular resolution, extending a pilot survey based on a smaller number of objects. Methods: The sample was chosen from surveys of H2O and OH masers to favour the earliest phases of high-mass star formation. Each source was first observed with the 32-m single-dish Medicina antenna in the (1, 1) and (2, 2) inversion transitions at 1.3 cm of ammonia, which is an excellent tracer of dense gas. High-resolution maps in the NH3(2, 2) and (3, 3) lines and the 1.3 cm continuum were obtained successively with the VLA interferometer. Results: We detect continuum emission in almost all the observed star-forming regions, which corresponds to extended and UCHii regions created by young stellar objects with typical luminosities of ˜10^4~L⊙. However, only in three cases do we find a projected overlap between Hii regions and H2O and OH maser spots. On the other hand, the VLA images detect eight ammonia cores closely associated with the maser sources. The ammonia cores have sizes of ˜10^4 AU, and high masses (up to 104M⊙), and are very dense (from ˜10^6 to a few ×10^9 cm-3). The typical relative NH3 abundance is ≤10-7, in agreement with previous measurements in high-mass star-forming regions. Conclusions: The statistical analysis of the distribution between H2O and OH masers, NH3 cores, and Hii regions confirms that the earliest stages of high-mass star formation are characterised by high-density molecular cores with temperatures of on average ≥30 K, either without a detectable ionised region or associated with a hypercompact Hii region.

Deep space target location with Hubble Space Telescope (HST) and Hipparcos data

NASA Technical Reports Server (NTRS)

Null, George W.

1988-01-01

Interplanetary spacecraft navigation requires accurate a priori knowledge of target positions. A concept is presented for attaining improved target ephemeris accuracy using two future Earth-orbiting optical observatories, the European Space Agency (ESA) Hipparcos observatory and the Nasa Hubble Space Telescope (HST). Assuming nominal observatory performance, the Hipparcos data reduction will provide an accurate global star catalog, and HST will provide a capability for accurate angular measurements of stars and solar system bodies. The target location concept employs HST to observe solar system bodies relative to Hipparcos catalog stars and to determine the orientation (frame tie) of these stars to compact extragalactic radio sources. The target location process is described, the major error sources discussed, the potential target ephemeris error predicted, and mission applications identified. Preliminary results indicate that ephemeris accuracy comparable to the errors in individual Hipparcos catalog stars may be possible with a more extensive HST observing program. Possible future ground and spacebased replacements for Hipparcos and HST astrometric capabilities are also discussed.

Design of Shrouded Airborne Wind Turbine & CFD Analysis

NASA Astrophysics Data System (ADS)

Anbreen, Faiqa; Faiqa Anbreen Collaboration

2015-11-01

The focus is to design a shrouded airborne wind turbine, capable to generate 70 kW to propel a leisure boat. The idea of designing an airborne turbine is to take the advantage of different velocity layers in the atmosphere. The blades have been designed using NREL S826 airfoil, which has coefficient of lift CL of 1.4 at angle of attack, 6°. The value selected for CP is 0.8. The rotor diameter is 7.4 m. The balloon (shroud) has converging-diverging nozzle design, to increase the mass flow rate through the rotor. The ratio of inlet area to throat area, Ai/At is 1.31 and exit area to throat area, Ae/At is1.15. The Solidworks model has been analyzed numerically using CFD. The software used is StarCCM +. The Unsteady Reynolds Averaged Navier Stokes Simulation (URANS) K- ɛ model has been selected, to study the physical properties of the flow, with emphasis on the performance of the turbine. Stress analysis has been done using Nastran. From the simulations, the torque generated by the turbine is approximately 800N-m and angular velocity is 21 rad/s.

Solar diameter measurements from eclipses as a solar variability proxy

NASA Astrophysics Data System (ADS)

Dunham, David W.; Sofia, Sabatino; Guhl, Konrad; Herald, David

The widths of total solar eclipse paths depends on the diameter of the Sun, so if observations are obtained near both the northern and southern limits of the eclipse path, in principle, the angular diameter of the Sun can be measured. Concerted efforts have been made to obtain contact timings from locations near total solar eclipse path edges since the mid 19th century, and Edmund Halley organized a rather successful first effort in 1715. Members of IOTA have been making increasingly sophisticated observations of the Baily's bead phenomena near central solar eclipse path edges since 1970.

The Role of Binarity in the Angular Momentum Evolution of M Dwarfs

NASA Astrophysics Data System (ADS)

Stauffer, John; Rebull, Luisa; K2 clusters team

2018-01-01

We have analysed K2 light curves for of order a thousand low mass stars in each of the 8 Myr old Upper Sco association, the 125 Myr age Pleiades open cluster and the ~700 Myr old Praesepe cluster. A very large fraction of these stars show well-determined rotation periods with K2, and where the star is a binary, we usually are able to determine periods for both stars. In Upper Sco, where there are ~150 M dwarf binaries with K2 light curves, the binary stars have periods that are much shorter on average and much closer to each other than would be true if drawn at random from the Upper Sco M dwarf single stars. The same is true in the Pleiades,though the size of the differences from the single M dwarf population is smaller. By Praesepe age, the M dwarf binaries are still somewhat rapidly rotating but their period differences are not significantly different from what would be true if drawn by chance from the singles.

Nearby Dwarf Stars: Duplicity, Binarity, and Masses

NASA Astrophysics Data System (ADS)

Mason, Brian D.; Hatkopf, William I.; Raghavan, Deepak

2008-02-01

Double stars have proven to be both a blessing and a curse for astronomers since their discovery over two centuries ago. They remain the only reliable source of masses, the most fundamental parameter defining stars. On the other hand, their sobriquet ``vermin of the sky'' is well-earned, due to the complications they present to both observers and theoreticians. These range from non-linear proper motions to stray light in detectors, to confusion in pointing of instruments due to non-symmetric point spread functions, to angular momentum conservation in multiple stars which results in binaries closer than allowed by evolution of two single stars. This proposal is an effort to address both their positive and negative aspects, through speckle interferometric observations, targeting ~1200 systems where useful information can be obtained with only a single additional observation. The proposed work will refine current statistics regarding duplicity (chance alignments of nearby point sources) and binarity (actual physical relationships), and improve the precisions and accuracies of stellar masses. Several targets support Raghavan's Ph.D. thesis, which is a comprehensive survey aimed at determining the multiplicity fraction among solar-type stars.

Nearby Dwarf Stars: Duplicity, Binarity, and Masses

NASA Astrophysics Data System (ADS)

Mason, Brian D.; Hartkopf, William I.; Raghavan, Deepak

2007-08-01

Double stars have proven to be both a blessing and a curse for astronomers since their discovery over two centuries ago. They remain the only reliable source of masses, the most fundamental parameter defining stars. On the other hand, their sobriquet ``vermin of the sky'' is well-earned, due to the complications they present to both observers and theoreticians. These range from non-linear proper motions to stray light in detectors, to confusion in pointing of instruments due to non-symmetric point spread functions, to angular momentum conservation in multiple stars which results in binaries closer than allowed by evolution of two single stars. This proposal is an effort to address both their positive and negative aspects, through speckle interferometric observations, targeting ~1200 systems where useful information can be obtained with only a single additional observation. The proposed work will refine current statistics regarding duplicity (chance alignments of nearby point sources) and binarity (actual physical relationships), and improve the precisions and accuracies of stellar masses. Several targets support Raghavan's Ph.D. thesis, which is a comprehensive survey aimed at determining the multiplicity fraction among solar-type stars.

Dynamical Asteroseismology: towards improving the theories of stellar structure and (tidal) evolution

NASA Astrophysics Data System (ADS)

Tkachenko, Andrew

2017-10-01

The potential of the dynamical asteroseismology, the research area that builds upon the synergies between the asteroseismology and binary stars research fields, is discussed in this manuscript. We touch upon the following topics: i) the mass discrepancy observed in intermediate-to high-mass main-sequence and evolved binaries as well as in the low mass systems that are still in the pre-main sequence phase of their evolution; ii) the rotationally induced mixing in high-mass stars, in particular how the most recent theoretical predictions and spectroscopic findings compare to the results of asteroseismic investigations; iii) internal gravity waves and their potential role in the evolution of binary star systems and surface nitrogen enrichment in high-mass stars; iv) the tidal evolution theory, in particular how its predictions of spin-orbit synchronisation and orbital circularisation compare to the present-day high-quality observations; v) the tidally-induced pulsations and their role in the angular momentum transport within binary star systems; vi) the scaling relations between fundamental and seismic properties of stars across the entire HR-diagram.

Spectral and spatial imaging of the Be+sdO binary ϕ Persei

NASA Astrophysics Data System (ADS)

Mourard, D.; Monnier, J. D.; Meilland, A.; Gies, D.; Millour, F.; Benisty, M.; Che, X.; Grundstrom, E. D.; Ligi, R.; Schaefer, G.; Baron, F.; Kraus, S.; Zhao, M.; Pedretti, E.; Berio, P.; Clausse, J. M.; Nardetto, N.; Perraut, K.; Spang, A.; Stee, P.; Tallon-Bosc, I.; McAlister, H.; ten Brummelaar, T.; Ridgway, S. T.; Sturmann, J.; Sturmann, L.; Turner, N.; Farrington, C.

2015-05-01

Aims: The rapidly rotating Be star ϕ Persei was spun up by mass and angular momentum transfer from a now stripped-down, hot subdwarf companion. Here we present the first high angular resolution images of ϕ Persei made possible by new capabilities in long-baseline interferometry at near-IR and visible wavelengths. We analyzed these images to search for the companion, to determine the binary orbit, stellar masses, and fluxes, and to examine the geometrical and kinematical properties of the outflowing disk surrounding the Be star. Methods: We observed ϕ Persei with the MIRC and VEGA instruments of the CHARA Array. MIRC was operated in six-telescope mode, whereas VEGA was configured in four-telescope mode with a change of quadruplets of telescopes during two nights to improve the (u,v) plane coverage. Additional MIRC-only observations were performed to track the orbital motion of the companion, and these were fit together with new and existing radial velocity measurements of both stars to derive the complete orbital elements and distance. We also used the MIRC data to reconstruct an image of the Be disk in the near-IR H-band. VEGA visible broadband and spectro-interferometric Hα observations were fit with analytical models for the Be star and disk, and image reconstruction was performed on the spectrally resolved Hα emission line data. Results: The hot subdwarf companion is clearly detected in the near-IR data at each epoch of observation with a flux contribution of 1.5% in the H band, and restricted fits indicate that its flux contribution rises to 3.3% in the visible. A new binary orbital solution is determined by combining the astrometric and radial velocity measurements. The derived stellar masses are 9.6 ± 0.3 M⊙ and 1.2 ± 0.2 M⊙ for the Be primary and subdwarf secondary, respectively. The inferred distance (186 ± 3 pc), kinematical properties, and evolutionary state are consistent with membership of ϕ Persei in the α Per cluster. From the cluster age we deduce significant constraints on the initial masses and evolutionary mass transfer processes that transformed the ϕ Persei binary system. The interferometric data place strong constraints on the Be disk elongation, orientation, and kinematics, and the disk angular momentum vector is coaligned with and has the same sense of rotation as the orbital angular momentum vector. The VEGA visible continuum data indicate an elongated shape for the Be star itself, due to the combined effects of rapid rotation, partial obscuration of the photosphere by the circumstellar disk, and flux from the bright inner disk. Based on observations with MIRC-6T and VEGA-4T instruments on the CHARA Array.Table 2 and Appendix A are available in electronic form at http://www.aanda.org

Angular distribution of ions and extreme ultraviolet emission in laser-produced tin droplet plasma

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chen, Hong; Duan, Lian; Lan, Hui

Angular-resolved ion time-of-flight spectra as well as extreme ultraviolet radiation in laser-produced tin droplet plasma are investigated experimentally and theoretically. Tin droplets with a diameter of 150 μm are irradiated by a pulsed Nd:YAG laser. The ion time-of-flight spectra measured from the plasma formed by laser irradiation of the tin droplets are interpreted in terms of a theoretical elliptical Druyvesteyn distribution to deduce ion density distributions including kinetic temperatures of the plasma. The opacity of the plasma for extreme ultraviolet radiation is calculated based on the deduced ion densities and temperatures, and the angular distribution of extreme ultraviolet radiation is expressedmore » as a function of the opacity using the Beer–Lambert law. Our results show that the calculated angular distribution of extreme ultraviolet radiation is in satisfactory agreement with the experimental data.« less

Angular distribution of ions and extreme ultraviolet emission in laser-produced tin droplet plasma

NASA Astrophysics Data System (ADS)

Chen, Hong; Wang, Xinbing; Duan, Lian; Lan, Hui; Chen, Ziqi; Zuo, Duluo; Lu, Peixiang

2015-05-01

Angular-resolved ion time-of-flight spectra as well as extreme ultraviolet radiation in laser-produced tin droplet plasma are investigated experimentally and theoretically. Tin droplets with a diameter of 150 μm are irradiated by a pulsed Nd:YAG laser. The ion time-of-flight spectra measured from the plasma formed by laser irradiation of the tin droplets are interpreted in terms of a theoretical elliptical Druyvesteyn distribution to deduce ion density distributions including kinetic temperatures of the plasma. The opacity of the plasma for extreme ultraviolet radiation is calculated based on the deduced ion densities and temperatures, and the angular distribution of extreme ultraviolet radiation is expressed as a function of the opacity using the Beer-Lambert law. Our results show that the calculated angular distribution of extreme ultraviolet radiation is in satisfactory agreement with the experimental data.

Determination of the Limiting Magnitude

NASA Technical Reports Server (NTRS)

Kingery, Aaron; Blaauw, Rhiannon

2017-01-01

The limiting magnitude of an optical camera system is an important property to understand since it is used to find the completeness limit of observations. Limiting magnitude depends on the hardware and software of the system, current weather conditions, and the angular speed of the objects observed. If an object exhibits a substantial angular rate during the exposure, its light spreads out over more pixels than the stationary stars. This spreading causes the limiting magnitude to be brighter when compared to the stellar limiting magnitude. The effect, which begins to become important when the object moves a full width at half max during a single exposure or video frame. For targets with high angular speeds or camera systems with narrow field of view or long exposures, this correction can be significant, up to several magnitudes. The stars in an image are often used to measure the limiting magnitude since they are stationary, have known brightness, and are present in large numbers, making the determination of the limiting magnitude fairly simple. In order to transform stellar limiting magnitude to object limiting magnitude, a correction must be applied accounting for the angular velocity. This technique is adopted in meteor and other fast-moving object observations, as the lack of a statistically significant sample of targets makes it virtually impossible to determine the limiting magnitude before the weather conditions change. While the weather is the dominant factor in observing satellites, the limiting magnitude for meteors also changes throughout the night due to the motion of a meteor shower or sporadic source radiant across the sky. This paper presents methods for determining the limiting stellar magnitude and the conversion to the target limiting magnitude.

IRAS observations of R Coronae Borealis - Detection and study of a fossil shell

NASA Technical Reports Server (NTRS)

Gillett, F. C.; Backman, D. E.; Beichman, C.; Neugebauer, G.

1986-01-01

IRAS observations of the extreme hydrogen-deficient supergiant R CrB are presented and discussed. The star is surrounded by an enormous cool dust cloud which is tentatively identified as a fossil remnant of the hydrogen-rich envelope of the star. The angular extent of the emission corresponds to a linear extent of 8 pc, 20 times larger than the largest previously known shell around a late-type star. The radiating material is distributed very symmetrically over a wide range of radial distances from the star. The dust temperature is nearly constant throughout the extended shell. The total mass in the shell is about 0.3 solar mass. The ejection process appears to have occurred in a spherically symmetric fashion with a nearly constant mass loss rate and expansion velocity over a period of about 150,000 yr, terminating about 26,000 yr ago.

Chemistry and Star Formation: A Love-Hate Relationship

NASA Astrophysics Data System (ADS)

Jiménez-Serra, Izaskun; Zhang, Qizhou; Patel, Nimesh; Lu, Xing; Wang, Ke; Testi, Leonardo; Caselli, Paola; Martin-Pintado, Jesus

2014-06-01

The development of the broad bandwidth receivers at the Submillimeter Array (SMA) a decade ago opened up the possibility to observe tens of molecular lines at high angular resolution simultaneously. The unprecedented wealth of molecular line data provided by the SMA allowed for the first time detailed studies of the chemistry in star-forming regions. These studies have revealed that chemistry is a useful tool to pin down the internal physical structure and the physical processes involved in the process of low-mass and high-mass star formation. In this talk, I will review the most important advances in our understanding of the star-formation process through chemistry thanks to the SMA, and I will present the challenges that will be faced in the next decade in this field of research thanks to the advent of new instrumentation such as the Atacama Large Millimeter/Submillimeter Array and the Square Kilometer Array.

Degenerate stars and gravitational collapse in AdS/CFT

NASA Astrophysics Data System (ADS)

Arsiwalla, Xerxes; de Boer, Jan; Papadodimas, Kyriakos; Verlinde, Erik

2011-01-01

We construct composite CFT operators from a large number of fermionic primary fields corresponding to states that are holographically dual to a zero temperature Fermi gas in AdS space. We identify a large N regime in which the fermions behave as free particles. In the hydrodynamic limit the Fermi gas forms a degenerate star with a radius determined by the Fermi level, and a mass and angular momentum that exactly matches the boundary calculations. Next we consider an interacting regime, and calculate the effect of the gravitational back-reaction on the radius and the mass of the star using the Tolman-Oppenheimer-Volkoff equations. Ignoring other interactions, we determine the "Chandrasekhar limit" beyond which the degenerate star (presumably) undergoes gravitational collapse towards a black hole. This is interpreted on the boundary as a high density phase transition from a cold baryonic phase to a hot deconfined phase.

The Dominance of Dynamic Barlike Instabilities in the Evolution of a Massive Stellar Core Collapse That ``Fizzles''

NASA Astrophysics Data System (ADS)

Imamura, James N.; Durisen, Richard H.

2001-03-01

Core collapse in a massive rotating star may halt at subnuclear density if the core contains angular momentum J>~1049 g cm2 s-1. An aborted collapse can lead to the formation of a rapidly rotating equilibrium object, which, because of its high electron fraction, Ye>0.4, and high entropy per baryon, Sb/k~1-2, is secularly and dynamically stable. The further evolution of such a ``fizzler'' is driven by deleptonization and cooling of the hot, dense material. These processes cause the fizzler both to contract toward neutron star densities and to spin up, driving it toward instability points of the barlike modes. Using linear stability analyses to study the latter case, we find that the stability properties of fizzlers are similar to those of Maclaurin spheroids and polytropes despite the nonpolytropic nature and extreme compressibility of the fizzler equation of state. For fizzlers with the specific angular momentum distribution of the Maclaurin spheroids, secular and dynamic barlike instabilities set in at T/|W|~0.14 and 0.27, respectively, where T is the rotational kinetic energy and W is the gravitational energy of the fizzler, the same limits as found for Maclaurin spheroids. For fizzlers in which angular momentum is more concentrated toward the equator, the secular stability limits drop dramatically. For the most extreme angular momentum distribution we consider, the secular stability limit for the barlike modes falls to T/|W|~0.038, compared with T/|W|~0.09-0.10 for the most extreme polytropic cases known previously (Imamura et al.). For fixed equation-of-state parameters, the secular and dynamic stability limits occur at roughly constant mass over the range of typical fizzler central densities. Deleptonization and cooling decrease the limiting masses on timescales shorter than the growth time for secular instability. Consequently, unless an evolving fizzler reaches neutron star densities first, it will always encounter dynamic barlike instabilities before secular instabilities have time to grow. Quasi-linear analysis shows that the angular momentum loss during the early nonlinear evolution of the dynamic barlike instability is dominated by Newtonian self-interaction gravitational torques rather than by the emission of gravitational wave (GW) radiation. GW emission may dominate after the initial dynamic evolutionary phase ends. Nonlinear hydrodynamics simulations with a proper equation of state will be required to determine the ultimate outcome of such evolutions and to refine predictions of GW production by barlike instabilities.

Iridescent Glory of Nearby Helix Nebula

NASA Image and Video Library

2014-04-04

This composite picture is a seamless blend of ultra-sharp NASA Hubble Space Telescope (HST) images combined with the wide view of the Mosaic Camera on the National Science Foundation's 0.9-meter telescope at Kitt Peak National Observatory, part of the National Optical Astronomy Observatory, near Tucson, Ariz. Astronomers at the Space Telescope Science Institute assembled these images into a mosaic. The mosaic was then blended with a wider photograph taken by the Mosaic Camera. The image shows a fine web of filamentary "bicycle-spoke" features embedded in the colorful red and blue gas ring, which is one of the nearest planetary nebulae to Earth. Because the nebula is nearby, it appears as nearly one-half the diameter of the full Moon. This required HST astronomers to take several exposures with the Advanced Camera for Surveys to capture most of the Helix. HST views were then blended with a wider photo taken by the Mosaic Camera. The portrait offers a dizzying look down what is actually a trillion-mile-long tunnel of glowing gases. The fluorescing tube is pointed nearly directly at Earth, so it looks more like a bubble than a cylinder. A forest of thousands of comet-like filaments, embedded along the inner rim of the nebula, points back toward the central star, which is a small, super-hot white dwarf. The tentacles formed when a hot "stellar wind" of gas plowed into colder shells of dust and gas ejected previously by the doomed star. Ground-based telescopes have seen these comet-like filaments for decades, but never before in such detail. The filaments may actually lie in a disk encircling the hot star, like a collar. The radiant tie-die colors correspond to glowing oxygen (blue) and hydrogen and nitrogen (red). Valuable Hubble observing time became available during the November 2002 Leonid meteor storm. To protect the spacecraft, including HST's precise mirror, controllers turned the aft end into the direction of the meteor stream for about half a day. Fortunately, the Helix Nebula was almost exactly in the opposite direction of the meteor stream, so Hubble used nine orbits to photograph the nebula while it waited out the storm. To capture the sprawling nebula, Hubble had to take nine separate snapshots. Planetary nebulae like the Helix are sculpted late in a Sun-like star's life by a torrential gush of gases escaping from the dying star. They have nothing to do with planet formation, but got their name because they look like planetary disks when viewed through a small telescope. With higher magnification, the classic "donut-hole" in the middle of a planetary nebula can be resolved. Based on the nebula's distance of 650 light-years, its angular size corresponds to a huge ring with a diameter of nearly 3 light-years. That's approximately three-quarters of the distance between our Sun and the nearest star. The Helix Nebula is a popular target of amateur astronomers and can be seen with binoculars as a ghostly, greenish cloud in the constellation Aquarius. Larger amateur telescopes can resolve the ring-shaped nebula, but only the largest ground-based telescopes can resolve the radial streaks. After careful analysis, astronomers concluded the nebula really isn't a bubble, but is a cylinder that happens to be pointed toward Earth. http://photojournal.jpl.nasa.gov/catalog/PIA18164

New determination of the solar apex

NASA Astrophysics Data System (ADS)

Fehrenbach, Ch.; Duflot, M.; Burnage, R.

2001-04-01

Many studies recently have been performed to determine the velocity vector of the Sun, mainly using the latest data on proper motions and parallaxes given by the Hipparcos satellite. We wished to carry out a similar study using totally independent data: the numerous radial velocities (RV) obtained with the Fehrenbach Objective Prisms (PO). This method allows the determination of the RVs of all the stars contained in the same field. These RVs are relative to each other but are linked to the IAU standard system by means of at least two calibration stars of known RV belonging to that field. These data are very homogeneous. We discuss the precision of the results, and deduce that this material is relevant for the computation of the movement of the Sun towards its Apex. We have performed several studies: 1) With 6965 stars of magnitudes ranging from 7 to 10, measured with the small PO of 15 cm diameter (PPO), with the whole sample and with the same sample split into blue and red stars. 2) With 11 978 stars of magnitudes ranging from 7 to 11, by adding to the previous sample the stars measured with the 60 cm diameter PO associated with the Schmidt telescope of Observatoire de Haute Provence (SPO). The results of both studies are consistent. 3) We have estimated the distance D of all stars studied and determined U, V, W and S for four groups of stars selected according to their distances: D<100, ~ 100500 parsecs. We have determined the variation of U, V, W and S with respect to the distance of the stars. The variation of U, V and S is linear up to 500 parsecs. We can consider that W is constant.

The ATLASGAL survey: distribution of cold dust in the Galactic plane. Combination with Planck data

NASA Astrophysics Data System (ADS)

Csengeri, T.; Weiss, A.; Wyrowski, F.; Menten, K. M.; Urquhart, J. S.; Leurini, S.; Schuller, F.; Beuther, H.; Bontemps, S.; Bronfman, L.; Henning, Th.; Schneider, N.

2016-01-01

Context. Sensitive ground-based submillimeter surveys, such as ATLASGAL, provide a global view on the distribution of cold dense gas in the Galactic plane at up to two-times better angular-resolution compared to recent space-based surveys with Herschel. However, a drawback of ground-based continuum observations is that they intrinsically filter emission, at angular scales larger than a fraction of the field-of-view of the array, when subtracting the sky noise in the data processing. The lost information on the distribution of diffuse emission can be, however, recovered from space-based, all-sky surveys with Planck. Aims: Here we aim to demonstrate how this information can be used to complement ground-based bolometer data and present reprocessed maps of the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL) survey. Methods: We use the maps at 353 GHz from the Planck/HFI instrument, which performed a high sensitivity all-sky survey at a frequency close to that of the APEX/LABOCA array, which is centred on 345 GHz. Complementing the ground-based observations with information on larger angular scales, the resulting maps reveal the distribution of cold dust in the inner Galaxy with a larger spatial dynamic range. We visually describe the observed features and assess the global properties of dust distribution. Results: Adding information from large angular scales helps to better identify the global properties of the cold Galactic interstellar medium. To illustrate this, we provide mass estimates from the dust towards the W43 star-forming region and estimate a column density contrast of at least a factor of five between a low intensity halo and the star-forming ridge. We also show examples of elongated structures extending over angular scales of 0.5°, which we refer to as thin giant filaments. Corresponding to > 30 pc structures in projection at a distance of 3 kpc, these dust lanes are very extended and show large aspect ratios. We assess the fraction of dense gas by determining the contribution of the APEX/LABOCA maps to the combined maps, and estimate 2-5% for the dense gas fraction (corresponding to Av> 7 mag) on average in the Galactic plane. We also show probability distribution functions of the column density (N-PDF), which reveal the typically observed log-normal distribution for low column density and exhibit an excess at high column densities. As a reference for extragalactic studies, we show the line-of-sight integrated N-PDF of the inner Galaxy, and derive a contribution of this excess to the total column density of ~ 2.2%, corresponding to NH2 = 2.92 × 1022 cm-2. Taking the total flux density observed in the maps, we provide an independent estimate of the mass of molecular gas in the inner Galaxy of ~ 1 × 109 M⊙, which is consistent with previous estimates using CO emission. From the mass and dense gas fraction (fDG), we estimate a Galactic SFR of Ṁ = 1.3 M⊙ yr-1. Conclusions: Retrieving the extended emission helps to better identify massive giant filaments which are elongated and confined structures. We show that the log-normal distribution of low column density gas is ubiquitous in the inner Galaxy. While the distribution of diffuse gas is relatively homogenous in the inner Galaxy, the central molecular zone (CMZ) stands out with a higher dense gas fraction despite its low star formation efficiency.Altogether our findings explain well the observed low star formation efficiency of the Milky Way by the low fDG in the Galactic ISM. In contrast, the high fDG observed towards the CMZ, despite its low star formation activity, suggests that, in that particular region of our Galaxy, high density gas is not the bottleneck for star formation.