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Sample records for red dwarf stars

  1. Habitability of planets around red dwarf stars.

    PubMed

    Heath, M J; Doyle, L R; Joshi, M M; Haberle, R M

    1999-08-01

    Recent models indicate that relatively moderate climates could exist on Earth-sized planets in synchronous rotation around red dwarf stars. Investigation of the global water cycle, availability of photosynthetically active radiation in red dwarf sunlight, and the biological implications of stellar flares, which can be frequent for red dwarfs, suggests that higher plant habitability of red dwarf planets may be possible.

  2. SEARCH FOR RED DWARF STARS IN GLOBULAR CLUSTER NGC 6397

    NASA Technical Reports Server (NTRS)

    2002-01-01

    Left A NASA Hubble Space Telescope image of a small region (1.4 light-years across) in the globular star cluster NGC 6397. Simulated stars (diamonds) have been added to this view of the same region of the cluster to illustrate what astronomers would have expected to see if faint red dwarf stars were abundant in the Milky Way Galaxy. The field would then contain 500 stars, according to theoretical calculations. Right The unmodified HST image shows far fewer stars than would be expected, according to popular theories of star formation. HST resolves about 200 stars. The stellar density is so low that HST can literally see right through the cluster and resolve far more distant background galaxies. From this observation, scientists have identified the surprising cutoff point below which nature apparently doesn't make many stars smaller that 1/5 the mass of our Sun. These HST findings provide new insights into star formation in our Galaxy. Technical detail:The globular cluster NGC 6397, one of the nearest and densest agglomerations of stars, is located 7,200 light-years away in the southern constellation Ara. This visible-light picture was taken on March 3, 1994 with the Wide Field Planetary Camera 2, as part the HST parallel observing program. Credit: F. Paresce, ST ScI and ESA and NASA

  3. Can Red Dwarf stars support Earth-like vegetation?

    NASA Astrophysics Data System (ADS)

    Gale, Joseph; Wandel, Amri

    2016-07-01

    The Kepler mission has shown that Earthlike planets are common. Of particular interest in our search for extra-solar-system, life-clement conditions, are planets orbiting Red Dwarf (RD) stars, the most numerous stellar type in the Milky Way galaxy. Early considerations indicated that conditions on RD planets would be inimical to life, as their Habitable Zones would be so close as to make planets tidally locked to their star. This was expected to engender tempestuous climates and to expose life forms to flares of ionizing electro-magnetic radiation and charged particles. Moreover, the less photon energy of the radiation of the relatively cool RDs would be too low in the 300-700nm waveband required for Oxygenic Photosynthesis (OP). Recent calculations show that these negative factors are less severe than originally estimated. Many authors have suggested that OP may evolve on RP planets to utilize infrared photons in the 700-1000nm waveband. However, projecting from OP and the vegetation in analogous regions on Earth, we argue that the evolutionary pressure to do so would be small. On RD planets there will be regions receiving continuous illumination, of moderate intensity, containing a significant component of photosynthetic 400-700nm radiation. On Earth, OP has been an essential factor in producing the Biosphere environment that enabled the appearance and evolution of complex life. We conclude that the conditions for OP could exist on RD planets and consequently the evolution of vegetation and complex life is possible (albeit not necessary). Furthermore, the huge number of RDs and their long lifetimes, make advanced vegetation, OP and consequently complex life on RD planets probable, and statistically more likely than on planets of solar type stars.

  4. Survival of a brown dwarf after engulfment by a red giant star.

    PubMed

    Maxted, P F L; Napiwotzki, R; Dobbie, P D; Burleigh, M R

    2006-08-01

    Many sub-stellar companions (usually planets but also some brown dwarfs) orbit solar-type stars. These stars can engulf their sub-stellar companions when they become red giants. This interaction may explain several outstanding problems in astrophysics but it is unclear under what conditions a low mass companion will evaporate, survive the interaction unchanged or gain mass. Observational tests of models for this interaction have been hampered by a lack of positively identified remnants-that is, white dwarf stars with close, sub-stellar companions. The companion to the pre-white dwarf AA Doradus may be a brown dwarf, but the uncertain history of this star and the extreme luminosity difference between the components make it difficult to interpret the observations or to put strong constraints on the models. The magnetic white dwarf SDSS J121209.31 + 013627.7 may have a close brown dwarf companion but little is known about this binary at present. Here we report the discovery of a brown dwarf in a short period orbit around a white dwarf. The properties of both stars in this binary can be directly observed and show that the brown dwarf was engulfed by a red giant but that this had little effect on it. PMID:16885979

  5. Survival of a brown dwarf after engulfment by a red giant star.

    PubMed

    Maxted, P F L; Napiwotzki, R; Dobbie, P D; Burleigh, M R

    2006-08-01

    Many sub-stellar companions (usually planets but also some brown dwarfs) orbit solar-type stars. These stars can engulf their sub-stellar companions when they become red giants. This interaction may explain several outstanding problems in astrophysics but it is unclear under what conditions a low mass companion will evaporate, survive the interaction unchanged or gain mass. Observational tests of models for this interaction have been hampered by a lack of positively identified remnants-that is, white dwarf stars with close, sub-stellar companions. The companion to the pre-white dwarf AA Doradus may be a brown dwarf, but the uncertain history of this star and the extreme luminosity difference between the components make it difficult to interpret the observations or to put strong constraints on the models. The magnetic white dwarf SDSS J121209.31 + 013627.7 may have a close brown dwarf companion but little is known about this binary at present. Here we report the discovery of a brown dwarf in a short period orbit around a white dwarf. The properties of both stars in this binary can be directly observed and show that the brown dwarf was engulfed by a red giant but that this had little effect on it.

  6. Living with a Red Dwarf Star: Radiation and Plasma Environments of Hosted Planets

    NASA Astrophysics Data System (ADS)

    Guinan, Edward F.; Engle, S. G.; Ribas, I.; Schulze-Makuch, D.; McCook, G. P.

    2007-05-01

    Red Dwarf (dM) stars are the most numerous stars in our Galaxy. These cool, faint and low mass stars make up > 75% of all stars. Also dM stars have extremely long life times >50-100 Gyr. Determining the number of red dwarfs with planets and assessing planetary habitability (a planet’s potential to develop and sustain life) is critically important because such studies would indicate how common life is in the universe. Our program - Living with a Red Dwarf - addresses these questions by investigating the long-term nuclear evolution and magnetic-dynamo coronal and chromospheric X-ray to Ultraviolet properties of red dwarf stars with widely different ages. The major focus of the program is to study the magnetic-dynamo generated coronal and chromospheric X-ray-Ultraviolet emissions and flare properties. Also studied is how the stellar emissions and winds affect hosted planets and impact on their habitability. For this program we have selected 15 nearby dM0-5 star as proxies for dM-stars of different ages to characterize their radiation and high energy plasma properties. We are constructing irradiance tables (X-UV fluxes) that are used to model the effects of XUV radiation on planetary atmospheres and on possible life on planetary surfaces. Despite the earlier pessimistic view that red dwarfs stars are not suitable environments for habitable planets mainly because their low luminosities require a host planet to orbit quite close (r <0.3 AU) to be warm enough to support life. Our initial study shows that red dwarf stars (at least stars hotter than dM5) can be suitable as hosts for habitable planets. This research is supported by grants from NASA/FUSE (NNX06AD38G) and NSF (AST-0507542 & AST-0507536) which we gratefully acknowledge. The “Living with a Red Dwarf Star” Program is on the web at: http://astronomy.villanova.edu/livingwithareddwarf/Opener.htm

  7. Panel 1: A pulsating red giant star and a compact, hot white dwarf star orbit each other.

    NASA Technical Reports Server (NTRS)

    2002-01-01

    Panel 1: A pulsating red giant star and a compact, hot white dwarf star orbit each other. Panel 2: The red giant sheds much of its outer layers in a stellar wind. The white dwarf helps concentrate the wind along a thin equatorial plane. The white dwarf accretes some of this escaping gas forming a disk around the itself. Panel 3: When enough gas accumulates on the white dwarf's surface it explodes as a nova outburst. Most of the hot gas forms a pair of expanding bubbles above and below the equatorial disk. Panel 4: A few thousand years after the bubbles expand into space, the white dwarf goes through another nova outburst and makes another pair of bubbles, which form a distinctive hourglass shape.

  8. Oscillations of red dwarfs in evolved low-mass binaries with neutron stars

    NASA Technical Reports Server (NTRS)

    Sarna, Marek J.; Lee, Umin; Muslimov, Alexander G.

    1994-01-01

    We investigate a novel aspect of a problem related to the properties of low-mass binaries (LMBs) with millisecond pulsars: the pulsations of the red dwarf (donor) companion of the neutron star (NS). The illumination of the donor star by the pulsar's high-energy nonthermal radiation and relativistic wind may substantially affect its structure. We present a quantitative analysis of the oscillation spectrum of a red dwarf which has evolved in an LMB and has undergone the stage of evaporation. We calculate the p- and g-modes for red dwarfs with masses in the interval (0.2-0.6) stellar mass. For comparison, similar calculations are presented for zero age main-sequence (ZAMS) stars of the same masses. For less massive donor stars (approximately 0.2 stellar mass) the oscillation spectrum becomes quantitatively different from that of their ZAMS counterparts. The differnce is due to the fact that a ZAMS star of 0.2 stellar mass is fully convective, while the donor star in an LMB is expected to be far from thermal equilibrium and not fully convective. As a result, in contrast to a low-mass ZAMS star, a red dwarf of the same mass in an LMB allows the existence of g-modes. We also consider tidally forced g-modes, and perform a linear analysis of these oscillations for different degrees of nonsynchronism between the orbital and spin rotation of the red dwarf component. We demonstrate the existence of a series of reasonances for the low-order g-modes which may occur in LMBs at a late stage of their evolution. We discuss the possibility that these oscillations may trigger Roche lobe overflow and sudden mass loss by the donor star. Further implications of this effect for gamma- and X-ray burst phenomena are outlined.

  9. Continuum and line emission of flares on red dwarf stars

    NASA Astrophysics Data System (ADS)

    Morchenko, E.; Bychkov, K.; Livshits, M.

    2015-06-01

    The emission spectrum has been calculated of a homogeneous pure hydrogen layer, which parameters are typical for a flare on a red dwarf. The ionization and excitation states were determined by the solution of steady-state equations taking into account the continuum and all discrete hydrogen levels. We consider the following elementary processes: electron-impact transitions, spontaneous and induced radiative transitions, and ionization by the bremsstrahlung and recombination radiation of the layer itself. The Biberman-Holstein approximation was used to calculate the scattering of line radiation. Asymptotic formulae for the escape probability are obtained for a symmetric line profile taking into account the Stark and Doppler effects. The approximation for the core of the H- α line by a Gaussian curve has been substantiated.

  10. Future Interstellar Travel Destinations: Assessing the Suitability of Nearby Red Dwarf Stars as Hosts to Habitable Life-bearing Planets

    NASA Astrophysics Data System (ADS)

    Guinan, Edward F.; Engle, S. G.

    2013-01-01

    As part of our NSF/NASA sponsored “Living with a Red Dwarf Star” program, we are carrying out a comprehensive study of red dwarf stars across the electromagnetic spectrum to assess their suitability as hosts for habitable planets. These cool, dim, long-lived, low mass stars comprise >75% of the stars in our Galaxy. Moreover an increasing number of (potentially habitable) large Earth-size planets are being found hosted by red dwarfs. With intrinsically low luminosities (L < 0.02 Lsun), the habitable zones (HZs) of hosted planets are close to their host stars (typically 0.05 AU < HZ <0.4 AU). Our study indicates red dwarf HZ planets without strong (protective) magnetic fields are especially susceptible to atmospheric erosion & loss by the star’s X-UV and wind fluxes. Also, the frequent flaring of young red dwarf stars and tidal-locking of close-in planets could challenge the development of life. But tidal locking of these planets could have some advantages for the developmenet of life. The long lifetimes of the red dwarfs (> 50 BY) could be favorable for the development of complex (possibly even intelligent) life. We discuss our results in the context of nearby red dwarfs as possible destinations for future interstellar missions program. We illustrate this with examples of the red dwarf exoplanet systems: GJ 581 and HD 85512 (both with large HZ Earth-size planets). Also we discuss the nearest star (4.3 LY) - the red dwarf - Proxima Centauri as a potential destination for future interstellar missions such proposed by Icarus Interstellar and the 100-Year Starship and StarVoyager programs. We gratefully acknowledge the support from NSF-Grant AST-10-09903, Chandra Grants GO1-12124X & GO2-13020X and HST Grant GO-10920.

  11. Red Dwarf Stars: Ages, Rotation, Magnetic Dynamo Activity and the Habitability of Hosted Planets

    NASA Astrophysics Data System (ADS)

    Engle, S. G.; Guinan, E. F.

    2011-12-01

    We report on our continued efforts to understand and delineate the magnetic dynamo-induced behavior/variability of red dwarf (K5 V - M6 V) stars over their long lifetimes. These properties include: rotation, light variations (from star spots), coronal-chromospheric XUV activity and flares. This study is being carried out as part of the NSF-sponsored Living with a Red Dwarf program. The Living with a Red Dwarf program's database of dM stars with photometrically determined rotation rates (from starspot modulations) continues to expand, as does the inventory of archival XUV observations. Recently, the photometric properties of several hundred dM stars from the Kepler database are being analyzed to determine the rotation rates, starspot areal coverage/distributions and stellar flare rates. When all data setsare combined with ages from cluster/population memberships and kinematics, the determination of Age-Rotation-Activity relationships is possible. Such relationships have broad impacts not only on the studies of magnetic dynamo theory and angular momentum loss of low-mass stars with deep convective zones, but also on the suitability of planets hosted by red dwarfs to support life. With intrinsically low luminosities (L< 0.02L⊙), the liquid water habitable zones (HZs) for hosted planets are very close to their host stars - typically at ˜0.1 AU < HZ < 0.4 AU. Planets located close to their host stars risk damage and atmospheric loss from coronal & chromospheric XUV radiation, flares and plasma blasts via strong winds and coronal mass ejections. In addition, our relationships permit the stellar ages to be determined through measures of either the stars' rotation periods (best way) or XUV activity levels. This also permits a determination of the ages of their hosted planets. We illustrate this with examples of age determinations of the exoplanet systems: GJ 581 and HD 85512 (both with large Earth-size planets within the host star's HZ), GJ 1214 (hot, close

  12. Reconnaissance of Stars within Twenty-Five Parsecs: Red Dwarfs Rule the Galaxy

    NASA Astrophysics Data System (ADS)

    Henry, Todd J.; Jao, Wei-Chun; Pewett, Tiffany; Riedel, Adric R.; Rodriguez, Justin; Siverstein, Michele L; Slatten, Kenneth J.; Winters, Jennifer G.

    2014-06-01

    The REsearch Consortium On Nearby Stars (RECONS, www.recons.org) team has been mapping the solar neighborhood for 20 years. We continue to collect original astrometric, photometric, and spectroscopic data for the nearest stars and their companions, with significant effort concentrated in the southern hemisphere at the CTIO 0.9m telescope,operated by RECONS for the SMARTS Consortium. These new data are combined with carefully vetted data from classic surveys to paint the most complete portrait to date for the nearby stars.The combined data from RECONS and others have been organized into the RECONS 25 Parsec Database, which as of January 1, 2014 includes 3074 stars, brown dwarfs, and exoplanets in 2168 systems. All of these systems have accurate trigonometric parallaxes in the refereed literature placing them closer than 25.0 parsecs, i.e. parallaxes greater than 40 mas with errors less than 10 mas. Statistical results from this comprehensive Database are outlined, allowing us to make an unprecedented census of the Galaxy's stellar population, of which more than three-quarters are red dwarfs. Fewer than twenty of these red dwarfs are currently known to harbor planets, indicating that a great deal of work remains to be done in the search for the nearest worlds outside our Solar System. It is virtually certain that most planets in the Galaxy are orbiting red dwarfs, and the nearest examples should be among the prime targets in our search for life elsewhere.This effort has been supported by the NSF through grants AST-0908402 and AST-1109445, and via observations made possible by the SMARTS Consortium.

  13. RED DWARF DYNAMO RAISES PUZZLE OVER INTERIORS OF LOWEST-MASS STARS

    NASA Technical Reports Server (NTRS)

    2002-01-01

    NASA's Hubble Space Telescope has uncovered surprising evidence that powerful magnetic fields might exist around the lowest mass stars in the universe, which are near the threshold of stellar burning processes. 'New theories will have to be developed to explain how these strong fields are produced, since conventional models predict that these low mass red dwarfs should have very weak or no magnetic fields,' says Dr. Jeffrey Linsky of the Joint Institute for Laboratory Astrophysics (JILA) in Boulder, Colorado. 'The Hubble observations provide clear evidence that very low mass red dwarf stars must have some form of dynamo to amplify their magnetic fields.' His conclusions are based upon Hubble's detection of a high-temperature outburst, called a flare, on the surface of the extremely small, cool red dwarf star Van Biesbroeck 10 (VB10) also known as Gliese 752B. Stellar flares are caused by intense, twisted magnetic fields that accelerate and contain gasses which are much hotter than a star's surface. Explosive flares are common on the Sun and expected for stars that have internal structures similar to our Sun's. Stars as small as VB10 are predicted to have a simpler internal structure than that of the Sun and so are not expected to generate the electric currents required for magnetic fields that drive flares. Besides leading to a clearer understanding of the interior structure of the smallest red dwarf stars known, these unexpected results might possibly shed light on brown dwarf stars. A brown dwarf is a long-sought class of astronomical object that is too small to shine like a star through nuclear fusion processes, but is too large to be considered a planet. 'Since VB10 is nearly a brown dwarf, it is likely brown dwarfs also have strong magnetic fields,' says Linsky. 'Additional Hubble searches for flares are needed to confirm this prediction.' A QUARTER-MILLION DEGREE TORCH The star VB10 and its companion star Gliese 752A make up a binary system located 19 light

  14. RED DWARF DYNAMO RAISES PUZZLE OVER INTERIORS OF LOWEST-MASS STARS

    NASA Technical Reports Server (NTRS)

    2002-01-01

    NASA's Hubble Space Telescope has uncovered surprising evidence that powerful magnetic fields might exist around the lowest mass stars in the universe, which are near the threshold of stellar burning processes. 'New theories will have to be developed to explain how these strong fields are produced, since conventional models predict that these low mass red dwarfs should have very weak or no magnetic fields,' says Dr. Jeffrey Linsky of the Joint Institute for Laboratory Astrophysics (JILA) in Boulder, Colorado. 'The Hubble observations provide clear evidence that very low mass red dwarf stars must have some form of dynamo to amplify their magnetic fields.' His conclusions are based upon Hubble's detection of a high-temperature outburst, called a flare, on the surface of the extremely small, cool red dwarf star Van Biesbroeck 10 (VB10) also known as Gliese 752B. Stellar flares are caused by intense, twisted magnetic fields that accelerate and contain gasses which are much hotter than a star's surface. Explosive flares are common on the Sun and expected for stars that have internal structures similar to our Sun's. Stars as small as VB10 are predicted to have a simpler internal structure than that of the Sun and so are not expected to generate the electric currents required for magnetic fields that drive flares. Besides leading to a clearer understanding of the interior structure of the smallest red dwarf stars known, these unexpected results might possibly shed light on brown dwarf stars. A brown dwarf is a long-sought class of astronomical object that is too small to shine like a star through nuclear fusion processes, but is too large to be considered a planet. 'Since VB10 is nearly a brown dwarf, it is likely brown dwarfs also have strong magnetic fields,' says Linsky. 'Additional Hubble searches for flares are needed to confirm this prediction.' A QUARTER-MILLION DEGREE TORCH The star VB10 and its companion star Gliese 752A make up a binary system located 19 light

  15. The Living with a Red Dwarf Program: Observing the Decline in dM Star FUV Emissions With Age

    NASA Astrophysics Data System (ADS)

    Engle, Scott G.; Guinan, Edward F.; Mizusawa, Trisha

    2009-05-01

    Red Dwarf (dM) stars are overwhelmingly the most numerous stars in our Galaxy. These cool, faint and low mass stars make up >80% of all stars. Also dM stars have extremely long life times (>50-100 Gyr). Determining the number of red dwarfs with planets and assessing planetary habitability (a planet's potential to develop and sustain life) is critically important because such studies would indicate how common life is in the universe. Our program-``Living with a Red Dwarf''-addresses these questions by investigating the long-term nuclear evolution and the coronal and chromospheric properties of red dwarf stars with widely different ages (~50 Myr-12 Gyr). One major focus of the program is to study the magnetic-dynamo generated coronal and chromospheric X-ray-FUV/UV emissions and flare properties of a sample of dM0-5 stars. Observations carried out by FUSE of a number of young to old dM stars provide important data for understanding transition region heating in these stars with deep convective zones as well as providing measures of FUV irradiances. Also studied are the effects of X-ray-FUV emissions on possible hosted planets and impacts of this radiation on their habitability. Using these data we are constructing irradiance tables (X-UV irradiances) that can be used to model the effects of XUV radiation on planetary atmospheres and possible life on planetary surfaces. The initial results of this program are discussed.

  16. VLT/FLAMES spectroscopy of red giant branch stars in the Fornax dwarf spheroidal galaxy

    NASA Astrophysics Data System (ADS)

    Lemasle, B.; de Boer, T. J. L.; Hill, V.; Tolstoy, E.; Irwin, M. J.; Jablonka, P.; Venn, K.; Battaglia, G.; Starkenburg, E.; Shetrone, M.; Letarte, B.; François, P.; Helmi, A.; Primas, F.; Kaufer, A.; Szeifert, T.

    2014-12-01

    Context. Fornax is one of the most massive dwarf spheroidal galaxies in the Local Group. The Fornax field star population is dominated by intermediate age stars but star formation was going on over almost its entire history. It has been proposed that Fornax experienced a minor merger event. Aims: Despite recent progress, only the high metallicity end of Fornax field stars ([Fe/H] > -1.2 dex) has been sampled in larger number via high resolution spectroscopy. We want to better understand the full chemical evolution of this galaxy by better sampling the whole metallicity range, including more metal poor stars. Methods: We use the VLT-FLAMES multi-fibre spectrograph in high-resolution mode to determine the abundances of several α, iron-peak and neutron-capture elements in a sample of 47 individual red giant branch stars in the Fornax dwarf spheroidal galaxy. We combine these abundances with accurate age estimates derived from the age probability distribution from the colour-magnitude diagram of Fornax. Results: Similar to other dwarf spheroidal galaxies, the old, metal-poor stars of Fornax are typically α-rich while the young metal-rich stars are α-poor. In the classical scenario of the time delay between Type II (SNe II) and Type Ia Supernovae (SNe Ia), we confirm that SNe Ia started to contribute to the chemical enrichment at [Fe/H] between -2.0 and -1.8 dex. We find that the onset of SNe Ia took place between 12-10 Gyr ago. The high values of [Ba/Fe], [La/Fe] reflect the influence of SNe Ia and AGB stars in the abundance pattern of the younger stellar population of Fornax. Conclusions: Our findings of low [α/Fe] and enhanced [Eu/Mg] are compatible with an initial mass function that lacks the most massive stars and with star formation that kept going on throughout the whole history of Fornax. We find that massive stars kept enriching the interstellar medium in α-elements, although they were not the main contributor to the iron enrichment. Based on FLAMES

  17. Testing gravity using dwarf stars

    NASA Astrophysics Data System (ADS)

    Sakstein, Jeremy

    2015-12-01

    Generic scalar-tensor theories of gravity predict deviations from Newtonian physics inside astrophysical bodies. In this paper, we point out that low mass stellar objects, red and brown dwarf stars, are excellent probes of these theories. We calculate two important and potentially observable quantities: the radius of brown dwarfs and the minimum mass for hydrogen burning in red dwarfs. The brown dwarf radius can differ significantly from the general relativity prediction, and upcoming surveys that probe the mass-radius relation for stars with masses red dwarf stars. This places a new and extremely stringent constraint on the parameters that appear in the effective field theory of dark energy and rules out several well-studied dark energy models.

  18. Living with a Red Dwarf: Rotation and X-Ray and Ultraviolet Properties of the Halo Population Kapteyn's Star

    NASA Astrophysics Data System (ADS)

    Guinan, Edward F.; Engle, Scott G.; Durbin, Allyn

    2016-04-01

    As part of Villanova's Living with a Red Dwarf program, we have obtained UV, X-ray, and optical data of the Population II red dwarf -- Kapteyn's Star. Kapteyn's Star is noteworthy for its large proper motions and high radial velocity of ∼+245 km s-1. As the nearest Pop II red dwarf, it serves as an old age anchor for calibrating activity/irradiance-rotation-age relations, and an important test bed for stellar dynamos and the resulting X-ray-UV emissions of slowly rotating, near-fully convective red dwarf stars. Adding to the notoriety, Kapteyn's Star has recently been reported to host two super-Earth candidates, one of which (Kapteyn b) is orbiting within the habitable zone. However, Robertson et al. questioned the planet's existence since its orbital period may be an artifact of activity, related to the star's rotation period. Because of its large Doppler-shift, measures of the important, chromospheric H i Lyα 1215.67 Å emission line can be reliably made, because it is mostly displaced from ISM and geo-coronal sources. Lyα emission dominates the FUV region of cool stars. Our measures can help determine the X-ray-UV effects on planets hosted by Kapteyn's Star, and planets hosted by other old red dwarfs. Stellar X-ray and Lyα emissions have strong influences on the heating and ionization of upper planetary atmospheres and can (with stellar winds and flares) erode or even eliminate planetary atmospheres. Using our program stars, we have reconstructed the past exposures of Kapteyn's Star's planets to coronal - chromospheric XUV emissions over time. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with program #13020. This work is also based on observations obtained with the Chandra X-ray Observatory, a NASA science mission, program #13200633.

  19. The abundance of biotic exoplanets and life on planets of Red Dwarf stars

    NASA Astrophysics Data System (ADS)

    Wandel, Amri; Gale, Joseph

    2016-07-01

    The Kepler mission has shown that Earthlike planets orbiting within the Habitable Zones of their host stars are common. We derive an expression for the abundance of life bearing (biotic) extra-solar-system planets (exoplanets) in terms of the (yet unknown) probability for the evolution of biotic life. This "biotic probability" may be estimated by future missions and observations, e.g. spectral analyses of the atmospheres of exoplanets, looking for biomarkers. We show that a biotic probability in the range 0.001-1 implies that a biotic planet may be expected within ~10-100 light years from Earth. Of particular interest in the search for exolife are planets orbiting Red Dwarf (RD) stars, the most frequent stellar type. Previous researches suggested that conditions on planets near RDs would be inimical to life, e.g. the Habitable Zone of RDs is small, so their habitable planets would be close enough to be tidally locked. Recent calculations show that this and other properties of RDs, presumed hostile for the evolution of life, are less severe than originally estimated. We conclude that RD planets could be hospitable for the evolution of life as we know it, not less so than planets of solar-type stars. This result, together with the large number of RDs and their Kepler planet-statistics, makes finding life on RD planets ~10-1000 times more likely than on planets of solar-type stars. Our nearest biotic RD-planet is likely to be 2-10 times closer than the nearest solar-type one.

  20. High resolution spectroscopy of Red Giant Branch stars and the chemical evolution of the Fornax dwarf spheroidal galaxy

    NASA Astrophysics Data System (ADS)

    Lemasle, B.; de Boer, T. J. L.; Hill, V.; Tolstoy, E.; Irwin, M. J.; Jablonka, P.; Venn, K.; Battaglia, G.; Starkenburg, E.; Shetrone, M.; Letarte, B.; Francois, P.; Helmi, A.; Primas, F.; Kaufer, A.; Szeifert, T.

    2014-12-01

    From VLT-FLAMES high-resolution spectra, we determine the abundances of several α, iron-peak and neutron-capture elements in 47 Red Giant Branch stars in the Fornax dwarf spheroidal galaxy. We confirm that SNe Ia started to contribute to the chemical enrichment of Fornax at [Fe/H] between --2.0 and --1.8 dex. Combining these abundances with accurate age estimates, we date the onset of SNe Ia to ≈ 12--10 Gyrs ago. Our results are compatible with an initial mass function that lacks the most massive stars and with a star formation going on throughout the whole history of Fornax.

  1. A white dwarf companion to the main-sequence star 4 Omicron(1) Orionis and the binary hypothesis for the origin of peculiar red giants

    NASA Technical Reports Server (NTRS)

    Ake, Thomas B.; Johnson, Hollis R.

    1988-01-01

    Ultraviolet spectra of the peculiar red giants (PRGs) called MS stars are investigated, and the discovery of a white dwarf (WD) companion to the MS star 4 Omicron(1) Orionis is reported. The observations and data analysis are discussed and compared with those for field WDs in order to derive parameters for the WD and the luminosity of the primary. Detection limits for the other MS stars investigated are derived, and the binary hypothesis for PRGs is reviewed.

  2. News and Views: Low-mass stars pull weight in globular clusters; Red dwarf planets are common, too; More planets than stars in the Milky Way? After Bullet comes Musket Ball; Planets survive red giant phase

    NASA Astrophysics Data System (ADS)

    2012-02-01

    Gravitational microlensing techniques have uncovered the first low-mass star found in a globular cluster, suggesting that previously undetectable stars may contribute to cluster masses, meaning that there is less dark matter to find. Data from NASA's Kepler mission suggest that small rocky planets may be common orbiting red dwarf stars - and because red dwarfs are common types of star, this means that rocky planets may be commonplace in the Milky Way. A survey using gravitational microlensing suggest that exoplanets are the exception rather than the rule in the Milky Way - and that small planets like Earth are more common than gas and ice giants. The Bullet Cluster famously allows mapping of the dark matter distribution during the merger of two clusters. Now a merging cluster named the Musket Ball shows a later stage in the process. Planets are not necessarily vaporized when a red giant star expands; the cores of gas giants may survive, but they would not be pleasant places to live. Data from NASA's Kepler mission has revealed two small planets orbiting a star after its red giant phase.

  3. Milky Way red dwarfs in the BoRG survey; galactic scale-height and the distribution of dwarf stars in WFC3 imaging

    SciTech Connect

    Holwerda, B. W.; Bouwens, R.; Trenti, M.; Clarkson, W.; Sahu, K.; Bradley, L.; Stiavelli, M.; Pirzkal, N.; Ryan, R.; De Marchi, G.; Andersen, M.

    2014-06-10

    We present a tally of Milky Way late-type dwarf stars in 68 Wide Field Camera 3 (WFC3) pure-parallel fields (227 arcmin{sup 2}) from the Brightest of Reionizing Galaxies survey for high-redshift galaxies. Using spectroscopically identified M-dwarfs in two public surveys, the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey and the Early Release Science mosaics, we identify a morphological selection criterion using the half-light radius (r {sub 50}), a near-infrared J – H, G – J color region where M-dwarfs are found, and a V – J relation with M-dwarf subtype. We apply this morphological selection of stellar objects, color-color selection of M-dwarfs, and optical-near-infrared color subtyping to compile a catalog of 274 M-dwarfs belonging to the disk of the Milky Way with a limiting magnitude of m {sub F125W} < 24(AB). Based on the M-dwarf statistics, we conclude that (1) the previously identified north-south discrepancy in M-dwarf numbers persists in our sample; there are more M-dwarfs in the northern fields on average than in southern ones, (2) the Milky Way's single disk scale-height for M-dwarfs is 0.3-4 kpc, depending on subtype, (3) the scale-height depends on M-dwarf subtype with early types (M0-4) high scale-height (z {sub 0} = 3-4 kpc) and later types M5 and above in the thin disk (z {sub 0} = 0.3-0.5 kpc), (4) a second component is visible in the vertical distribution, with a different, much higher scale-height in the southern fields compared to the northern ones. We report the M-dwarf component of the Sagittarius stream in one of our fields with 11 confirmed M-dwarfs, seven of which are at the stream's distance. In addition to the M-dwarf catalog, we report the discovery of 1 T-dwarfs and 30 L-dwarfs from their near-infrared colors. The dwarf scale-height and the relative low incidence in our fields of L- and T-dwarfs in these fields makes it unlikely that these stars will be interlopers in great numbers in color-selected samples of high

  4. White Dwarf Stars

    NASA Astrophysics Data System (ADS)

    Kepler, S. O.

    2014-10-01

    White dwarfs are the evolutionary endpoint for nearly 95% of all stars born in our Galaxy, the final stages of evolution of all low- and intermediate mass stars, i.e., main sequence stars with masses below (8.5± 1.5) M_{odot}, depending on metallicity of the progenitor, mass loss and core overshoot. Massive white dwarfs are intrinsically rare objects, tand produce a gap in the determination of the initial vs. final mass relation at the high mass end (e.g. Weidemann 2000 A&A, 363, 647; Kalirai et al. 2008, ApJ, 676, 594; Williams, Bolte & Koester 2009, ApJ, 693, 355). Main sequences stars with higher masses will explode as SNII (Smartt S. 2009 ARA&A, 47, 63), but the limit does depend on the metallicity of the progenitor. Massive white dwarfs are probably SNIa progenitors through accretion or merger. They are rare, being the final product of massive stars (less common) and have smaller radius (less luminous). Kepler et al. 2007 (MNRAS, 375, 1315), Kleinman et al. 2013 (ApJS, 204, 5) estimate only 1-2% white dwarfs have masses above 1 M_{odot}. The final stages of evolution after helium burning are a race between core growth and loss of the H-rich envelope in a stellar wind. When the burning shell is exposed, the star rapidly cools and burning ceases, leaving a white dwarf. As they cool down, the magnetic field freezes in, ranging from a few kilogauss to a gigagauss. Peculiar type Ia SN 2006gz, SN 2007if, SN 2009dc, SN 2003fg suggest progenitors in the range 2.4-2.8 M_{odot}, and Das U. & Mukhopadhyay B. (2012, Phys. Rev. D, 86, 042001) estimate that the Chandrasekhar limit increases to 2.3-2.6 M_{odot} for extremely high magnetic field stars, but differential rotation induced by accretion could also increase it, according to Hachisu I. et al. 2012 (ApJ, 744, 69). García-Berro et al. 2012, ApJ, 749, 25, for example, proposes double degenerate mergers are the progenitors of high-field magnetic white dwarfs. We propose magnetic fields enhance the line broadening in

  5. Asteroseismology of White Dwarf Stars

    NASA Technical Reports Server (NTRS)

    Hansen, Carl J.

    1997-01-01

    The primary purpose of this investigation has been to study various aspects of multimode pulsations in variable white dwarfs. In particular, nonlinear interactions among pulsation modes in white dwarfs (and, to some extent, in other variable stars), analysis of recent observations where such interactions are important, and preliminary work on the effects of crystallization in cool white dwarfs are reported.

  6. White Dwarf Stars

    NASA Technical Reports Server (NTRS)

    1999-01-01

    Peering deep inside a cluster of several hundred thousand stars, NASA's Hubble Space Telescope has uncovered the oldest burned-out stars in our Milky Way Galaxy, giving astronomers a fresh reading on the age of the universe.

    Located in the globular cluster M4, these small, burned-out stars -- called white dwarfs -- are about 12 to 13 billion years old. By adding the one billion years it took the cluster to form after the Big Bang, astronomers found that the age of the white dwarfs agrees with previous estimates that the universe is 13 to 14 billion years old.

    The images, including some taken by Hubble's Wide Field and Planetary Camera 2, are available online at

    http://oposite.stsci.edu/pubinfo/pr/2002/10/ or

    http://www.jpl.nasa.gov/images/wfpc .

    The camera was designed and built by NASA's Jet Propulsion Laboratory, Pasadena, Calif.

    In the top panel, a ground-based observatory snapped a panoramic view of the entire cluster, which contains several hundred thousand stars within a volume of 10 to 30 light-years across. The Kitt Peak National Observatory's .9-meter telescope took this picture in March 1995. The box at left indicates the region observed by the Hubble telescope.

    The Hubble telescope studied a small region of the cluster. A section of that region is seen in the picture at bottom left. A sampling of an even smaller region is shown at bottom right. This region is only about one light-year across. In this smaller region, Hubble pinpointed a number of faint white dwarfs. The blue circles indicate the dwarfs. It took nearly eight days of exposure time over a 67-day period to find these extremely faint stars.

    Globular clusters are among the oldest clusters of stars in the universe. The faintest and coolest white dwarfs within globular clusters can yield a globular cluster's age. Earlier Hubble observations showed that the first stars formed less than 1 billion years after the universe's birth in the big bang. So, finding the

  7. Gravitational Interactions of White Dwarf Double Stars

    NASA Astrophysics Data System (ADS)

    McKeough, James; Robinson, Chloe; Ortiz, Bridget; Hira, Ajit

    2016-03-01

    In the light of the possible role of White Dwarf stars as progenitors of Type Ia supernovas, we present computational simulations of some astrophysical phenomena associated with a study of gravitationally-bound binary stars, composed of at least one white dwarf star. Of particular interest to astrophysicists are the conditions inside a white dwarf star in the time frame leading up to its explosive end as a Type Ia supernova, for an understanding of the massive stellar explosions. In addition, the studies of the evolution of white dwarfs could serve as promising probes of theories of gravitation. We developed FORTRAN computer programs to implement our models for white dwarfs and other stars. These codes allow for different sizes and masses of stars. Simulations were done in the mass interval from 0.1 to 2.5 solar masses. Our goal was to obtain both atmospheric and orbital parameters. The computational results thus obtained are compared with relevant observational data. The data are further analyzed to identify trends in terms of sizes and masses of stars. We will extend our computational studies to blue giant and red giant stars in the future. Funding from National Science Foundation.

  8. Suppression of the water ice and snow albedo feedback on planets orbiting red dwarf stars and the subsequent widening of the habitable zone.

    PubMed

    Joshi, Manoj M; Haberle, Robert M

    2012-01-01

    M stars comprise 80% of main sequence stars, so their planetary systems provide the best chance for finding habitable planets, that is, those with surface liquid water. We have modeled the broadband albedo or reflectivity of water ice and snow for simulated planetary surfaces orbiting two observed red dwarf stars (or M stars), using spectrally resolved data of Earth's cryosphere. The gradual reduction of the albedos of snow and ice at wavelengths greater than 1 μm, combined with M stars emitting a significant fraction of their radiation at these same longer wavelengths, means that the albedos of ice and snow on planets orbiting M stars are much lower than their values on Earth. Our results imply that the ice/snow albedo climate feedback is significantly weaker for planets orbiting M stars than for planets orbiting G-type stars such as the Sun. In addition, planets with significant ice and snow cover will have significantly higher surface temperatures for a given stellar flux if the spectral variation of cryospheric albedo is considered, which in turn implies that the outer edge of the habitable zone around M stars may be 10-30% farther away from the parent star than previously thought.

  9. 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.

  10. 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.

  11. Star formation in dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Dong, Shawfeng

    In this thesis, we examine the star formation history and stellar feedback effects of dwarf galaxies under the influence of extragalactic ultraviolet radiation, as well as the evolution of residual gas within tidally-limited dwarf galaxies and globular clusters. Previous work has indicated that the background UV flux can easily ionize the gas within typical dwarf galaxies, delaying or even preventing cooling and star formation within them. Many dwarf galaxies within the Local Group are, however, observed to contain multiple generations of stars, the oldest of which formed in the early epochs of cosmic evolution, when the background UV flux was intense. In order to address this paradox, we consider the dynamical evolution of gas in dwarf galaxies using a one-dimensional, spherically symmetric, Lagrangian numerical scheme which also computes the effects of radiative transfer and photoionization. We include in the scheme a physically-motivated star formation recipe and consider the effects of feedback. This scheme allows us to follow the history of the gas and of star formation within dwarf galaxies, as influenced by both external and internal UV radiation. Our results indicate that star formation in the severe environment of dwarf galaxies is a difficult and inefficient process. In potentials with total mass less than a few 106 M⊙ , and velocity dispersion less than a few km s-1 , residual gas is efficiently photoionized by cosmic background UV radiation. For intermediate mass systems, such as the dSphs around the Galaxy, star formation can proceed within early cosmic epochs despite the intense background UV flux. Triggering processes such as merger events, collisions, and tidal disturbance can lead to density enhancements, reducing the recombination timescale, allowing gas to cool and star formation to proceed. However, the star formation and gas retention efficiency may vary widely in galaxies with similar dark matter potentials, because they depend on many

  12. Star Formation in Dwarf-Dwarf Mergers: Fueling Hierarchical Assembly

    NASA Astrophysics Data System (ADS)

    Stierwalt, Sabrina; Johnson, K. E.; Kallivayalil, N.; Patton, D. R.; Putman, M. E.; Besla, G.; Geha, M. C.

    2014-01-01

    We present early results from the first systematic study a sample of isolated interacting dwarf pairs and the mechanisms governing their star formation. Low mass dwarf galaxies are ubiquitous in the local universe, yet the efficiency of gas removal and the enhancement of star formation in dwarfs via pre-processing (i.e. dwarf-dwarf interactions occurring before the accretion by a massive host) are currently unconstrained. Studies of Local Group dwarfs credit stochastic internal processes for their complicated star formation histories, but a few intriguing examples suggest interactions among dwarfs may produce enhanced star formation. We combine archival UV imaging from GALEX with deep optical broad- and narrow-band (Halpha) imaging taken with the pre- One Degree Imager (pODI) on the WIYN 3.5-m telescope and with the 2.3-m Bok telescope at Steward Observatory to confirm the presence of stellar bridges and tidal tails and to determine whether dwarf-dwarf interactions alone can trigger significant levels of star formation. We investigate star formation rates and global galaxy colors as a function of dwarf pair separation (i.e. the dwarf merger sequence) and dwarf-dwarf mass ratio. This project is a precursor to an ongoing effort to obtain high spatial resolution HI imaging to assess the importance of sequential triggering caused by dwarf-dwarf interactions and the subsequent affect on the more massive hosts that later accrete the low mass systems.

  13. THE ASYMPTOTIC GIANT BRANCH AND THE TIP OF THE RED GIANT BRANCH AS PROBES OF STAR FORMATION HISTORY: THE NEARBY DWARF IRREGULAR GALAXY KKH 98

    SciTech Connect

    Melbourne, J.; Williams, B.; Dalcanton, J.; Ammons, S. M.; Max, C.; Koo, D. C.; Dolphin, A. E-mail: ben@astro.washington.ed E-mail: ammons@ucolick.or E-mail: koo@ucolick.or E-mail: adolphin@raytheon.co

    2010-03-20

    We investigate the utility of the asymptotic giant branch (AGB) and the red giant branch (RGB) as probes of the star formation history (SFH) of the nearby (D = 2.5 Mpc) dwarf irregular galaxy, KKH 98. Near-infrared (near-IR) Keck Laser Guide Star Adaptive Optics (AO) images resolve 592 IR-bright stars reaching over 1 mag below the tip of the RGB. Significantly deeper optical (F475W and F814W) Hubble Space Telescope images of the same field contain over 2500 stars, reaching to the red clump and the main-sequence turnoff for 0.5 Gyr old populations. Compared to the optical color-magnitude diagram (CMD), the near-IR CMD shows significantly tighter AGB sequences, providing a good probe of the intermediate-age (0.5-5 Gyr) populations. We match observed CMDs with stellar evolution models to recover the SFH of KKH 98. On average, the galaxy has experienced relatively constant low-level star formation (5 x 10{sup -4} M{sub sun} yr{sup -1}) for much of cosmic time. Except for the youngest main-sequence populations (age <0.1 Gyr), which are typically fainter than the AO data flux limit, the SFH estimated from the 592 IR-bright stars is a reasonable match to that derived from the much larger optical data set. Differences between the optical- and IR-derived SFHs for 0.1-1 Gyr populations suggest that current stellar evolution models may be overproducing the AGB by as much as a factor of 3 in this galaxy. At the depth of the AO data, the IR-luminous stars are not crowded. Therefore, these techniques can potentially be used to determine the stellar populations of galaxies at significantly further distances.

  14. Red Clump Stars

    NASA Astrophysics Data System (ADS)

    Girardi, Léo

    2016-09-01

    Low-mass stars in their core-helium-burning stage define the sharpest feature present in the color-magnitude diagrams of nearby galaxy systems: the red clump (RC). This feature has given rise to a series of methods aimed at measuring the distributions of stellar distances and extinctions, especially in the Magellanic Clouds and Milky Way Bulge. Because the RC is easily recognizable within the data of large spectroscopic and asteroseismic surveys, it is a useful probe of stellar densities, kinematics, and chemical abundances across the Milky Way disk; it can be applied up to larger distances than that allowed by dwarfs; and it has better accuracy than is possible with other kinds of giants. Here, we discuss the reasons for the RC narrowness in several sets of observational data, its fine structure, and the presence of systematic changes in the RC properties as regards age, metallicity, and the observed passband. These factors set the limits on the validity and accuracy of several RC methods defined in the literature.

  15. Nearby Red Dwarfs are Sexy for Planets and Life

    NASA Astrophysics Data System (ADS)

    Henry, T. J.; Jao, W.-C.; Subasavage, J. P.; RECONS Team

    2005-12-01

    The RECONS group continues to discover many nearby red dwarfs in the southern sky through a combination of proper motion surveys, literature review, and ultimately, our parallax program CTIOPI. Already, we have measured the first accurate parallaxes for 11 of the nearest 100 stellar systems, including four within 5 parsecs of the Sun. These nearby red dwarfs are prime candidates for NASA's Space Interferometry Mission (SIM) because the astrometric perturbations are largest for planets orbiting stars of low mass that are nearby. In addition, new multiple red dwarf systems can be targeted for mass determinations, thereby providing points on a comprehensive mass-luminosity relation for the most populous members of the Galaxy. Recent atmospheric modeling of planets orbiting red dwarfs indicates that even if the planets are tidally locked, heat distribution is highly effective in keeping the worlds balmy over the entire surface. Red dwarfs are therefore "back on the table" as viable hosts of life-bearing planets. Given their ubiquity, red dwarfs are being seriously considered as prime SETI targets, and will allow us to answer not only the question "Are We Alone?" but "Just How Alone Are We?" This work has been supported by the National Science Foundation, NASA's Space Interferometry Mission, and Georgia State University.

  16. A Search for Fine Wines: Discovering Close Red Dwarf-White Dwarf Binaries

    NASA Astrophysics Data System (ADS)

    Boyd, Mark; Finch, C. T.; Hambly, N. C.; Henry, T. J.; Jao, W.; Riedel, A. R.; Subasavage, J. P.; Winters, J. G.; RECONS

    2012-01-01

    Like fine wines, stars come in both red and white varieties. Here we present initial results of the Fine Wines Project that targets red dwarf-white dwarf pairs. The two scientific goals of Fine Wines are (1) to develop methods to estimate ages for red dwarfs based on the cooling ages of the white dwarfs, and (2) to identify suitable pairs for dynamical mass determinations of white dwarfs to probe their interior structures. Here we focus on the search for Fine Wines, including sample selection, elimination of false positives, and initial reconnaissance. The sample was extracted via color-color plots from a pool of more than 30,000 proper motion systems examined during the SuperCOSMOS-RECONS (SCR) and UCAC3 Proper Motion (UPM) surveys. The initial sample of 75 best candidates is being observed for BVRI photometry and 3500-9500 A spectroscopy to confirm whether or not the systems are red dwarf-white dwarf pairs. Early results indicate that roughly 50% of the candidates selected are indeed Fine Wine systems. This effort is supported by the NSF through grant AST 09-08402 and via observations made possible by the SMARTS Consortium.

  17. White Dwarfs, Neutron Stars and Black Holes

    ERIC Educational Resources Information Center

    Szekeres, P.

    1977-01-01

    The three possible fates of burned-out stars: white dwarfs, neutron stars and black holes, are described in elementary terms. Characteristics of these celestial bodies, as provided by Einstein's work, are described. (CP)

  18. The Kinematics of Dwarf Carbon Stars

    NASA Astrophysics Data System (ADS)

    Plant, Kathryn A.; Margon, Bruce H.; Guhathakurta, Puragra; Rockosi, Constance M.

    2015-01-01

    The presence of molecular carbon absorption bands in the spectra of main sequence dwarfs is remarkable, as these stars have not yet evolved through the helium-burning and dredge-up phases that deposit carbon in a stellar photosphere. Dwarf carbon stars are thus generally considered members of post-mass transfer binaries, with the main sequence star polluted by an evolved, often now invisible, companion. For decades only a handful were known. Now it is recognized that carbon dwarfs likely outnumber the better-understood giant carbon stars. Green (2013) has identified more than 700 carbon dwarfs from the Sloan Digital Sky Survey (SDSS). This large sample- distributed nearly evenly throughout the SDSS footprint- makes a study of stellar kinematics possible for dwarf carbon stars as a class.We examine the proper motions and radial velocities of ~700 carbon dwarfs and compare to a sample of 2×104 non-carbon main sequence stars from the SDSS archive. The spectra of carbon dwarfs and giants can appear indistinguishable, and so the relatively faint carbon dwarfs are recognized only if they have a sufficiently large proper motion to exclude the possibility of their being distant giants. We build our non-carbon control sample by the same proper motion criteria and additionally require that the control stars match the carbon dwarf selection with respect to properties such as photometric colors. In order to examine the kinematics of a sample spread across a large portion of sky, we compare each carbon dwarf with a group of control stars separated from it by less than three degrees. Preliminary results suggest that carbon dwarfs' kinematics are similar to the distributions of their neighboring control stars. We will present the results of detailed tests, including an investigation of several carbon dwarfs with atypical radial velocities.

  19. White Dwarfs in Local Star Streams

    NASA Astrophysics Data System (ADS)

    Fuchs, Burkhard; Dettbarn, Christian

    2011-01-01

    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 these 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.

  20. Variable stars in the dwarf galaxy GR 8 (DDO 155)

    NASA Technical Reports Server (NTRS)

    Tolstoy, Eline; Saha, A.; Hoessel, John G.; Danielson, G. Edward

    1995-01-01

    Observations of the resolved stars in dwarf galaxy GR 8, obtained over the period 1980 February to 1994 March, are presented. Thirty-four separate epochs were searched for variable stars, and a total of six were found, of which one has Cepheid characteristics. After correction for Galactic extinction this single Cepheid yields a distance modulus of m - M = 26.75 +/- 0.35. This corresponds to a distance of 2.24 Mpc, placing GR 8 near the Local Group (LG) zero-velocity surface. The other five variable stars are very red, and possibly have long periods of order 100 days or more.

  1. White dwarf stars with carbon atmospheres.

    PubMed

    Dufour, P; Liebert, J; Fontaine, G; Behara, N

    2007-11-22

    White dwarfs represent the endpoint of stellar evolution for stars with initial masses between approximately 0.07 and 8-10, where is the mass of the Sun (more massive stars end their life as either black holes or neutron stars). The theory of stellar evolution predicts that the majority of white dwarfs have a core made of carbon and oxygen, which itself is surrounded by a helium layer and, for approximately 80 per cent of known white dwarfs, by an additional hydrogen layer. All white dwarfs therefore have been traditionally found to belong to one of two categories: those with a hydrogen-rich atmosphere (the DA spectral type) and those with a helium-rich atmosphere (the non-DAs). Here we report the discovery of several white dwarfs with atmospheres primarily composed of carbon, with little or no trace of hydrogen or helium. Our analysis shows that the atmospheric parameters found for these stars do not fit satisfactorily in any of the currently known theories of post-asymptotic giant branch evolution, although these objects might be the cooler counterpart of the unique and extensively studied PG 1159 star H1504+65 (refs 4-7). These stars, together with H1504+65, might accordingly form a new evolutionary sequence that follows the asymptotic giant branch. PMID:18033290

  2. White dwarf stars with carbon atmospheres.

    PubMed

    Dufour, P; Liebert, J; Fontaine, G; Behara, N

    2007-11-22

    White dwarfs represent the endpoint of stellar evolution for stars with initial masses between approximately 0.07 and 8-10, where is the mass of the Sun (more massive stars end their life as either black holes or neutron stars). The theory of stellar evolution predicts that the majority of white dwarfs have a core made of carbon and oxygen, which itself is surrounded by a helium layer and, for approximately 80 per cent of known white dwarfs, by an additional hydrogen layer. All white dwarfs therefore have been traditionally found to belong to one of two categories: those with a hydrogen-rich atmosphere (the DA spectral type) and those with a helium-rich atmosphere (the non-DAs). Here we report the discovery of several white dwarfs with atmospheres primarily composed of carbon, with little or no trace of hydrogen or helium. Our analysis shows that the atmospheric parameters found for these stars do not fit satisfactorily in any of the currently known theories of post-asymptotic giant branch evolution, although these objects might be the cooler counterpart of the unique and extensively studied PG 1159 star H1504+65 (refs 4-7). These stars, together with H1504+65, might accordingly form a new evolutionary sequence that follows the asymptotic giant branch.

  3. Star Formation and Feedback in Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Dong, Shawfeng; Lin, D. N. C.; Murray, S. D.

    2003-10-01

    We examine the star formation history and stellar feedback effects of dwarf galaxies under the influence of extragalactic ultraviolet radiation. Previous work has indicated that the background UV flux can easily ionize the gas within typical dwarf galaxies, delaying or even preventing cooling and star formation within them. Many dwarf galaxies within the Local Group are, however, observed to contain multiple generations of stars, the oldest of which formed in the early epochs of cosmic evolution, when the background UV flux was intense. In order to address this paradox, we consider the dynamical evolution of gas in dwarf galaxies using a one-dimensional, spherically symmetric, Lagrangian numerical scheme to compute the effects of radiative transfer and photoionization. We include a physically motivated star formation recipe and consider the effects of feedback. This scheme allows us to follow the history of the gas and of star formation within dwarf galaxies, as influenced by both external and internal UV radiation. Our results indicate that star formation in the severe environment of dwarf galaxies is a difficult and inefficient process. In potentials with total mass less than a few times 106 Msolar and velocity dispersion less than a few kilometers per second, residual gas is efficiently photoionized by cosmic background UV radiation. Since the density scale height of the gas within these galaxies is comparable to their size, gas may be tidally removed from them, leaving behind starless residual dark matter clumps. For intermediate-mass systems, such as the dSphs around the Galaxy, star formation can proceed within early cosmic epochs despite the intense background UV flux. Triggering processes such as merger events, collisions, and tidal disturbance can lead to density enhancements, reducing the recombination timescale, allowing gas to cool and star formation to proceed. However, the star formation and gas retention efficiency may vary widely in galaxies with

  4. Theoretical Study of White Dwarf Double Stars

    NASA Astrophysics Data System (ADS)

    Hira, Ajit; Koetter, Ted; Rivera, Ruben; Diaz, Juan

    2015-04-01

    We continue our interest in the computational simulation of the astrophysical phenomena with a study of gravitationally-bound binary stars, composed of at least one white dwarf star. Of particular interest to astrophysicists are the conditions inside a white dwarf star in the time frame leading up to its explosive end as a Type Ia supernova, for an understanding of the massive stellar explosions. In addition, the studies of the evolution of white dwarfs could serve as promising probes of theories of gravitation. We developed FORTRAN computer programs to implement our models for white dwarfs and other stars. These codes allow for different sizes and masses of stars. Simulations were done in the mass interval from 0.1 to 2.0 solar masses. Our goal was to obtain both atmospheric and orbital parameters. The computational results thus obtained are compared with relevant observational data. The data are further analyzed to identify trends in terms of sizes and masses of stars. We hope to extend our computational studies to blue giant stars in the future. Research Supported by National Science Foundation.

  5. Star formation history in forming dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Berczik, P.; Kravchuk, S. G.

    The processes of formation and evolution of isolated dwarf galaxies over the Hubble timescale is followed by means of SPH techniques. As an initial protogalaxy perturbation we consider an isolated, uniform, solid -- body rotated sphere involved into the Hubble flow and made of dark and baryonic matter in a 10:1 ratio. The simulations are carried out for the set of models having spin parameters lambda in the range from 0.01 to 0.08 and the total mass of dark matter 1011 M_odot . Our model includes gasdynamics, radiative processes, star formation, supernova feedback and simplified chemistry. The application of modified star formation criterion which accounts for chaotic motions and the time lag between initial development of suitable conditions for star formation and star formation itself (Berczik P.P, Kravchuk S.G. 1997, Ap.Sp.Sci.) provides the realistic description of the process of galaxy formation and evolution. Two parameters: total mass and initial angular momentum of the dwarf protogalaxy play the crucial role in its star formation activity. After the 15 Gyr of the evolution the rapidly rotated dwarf galaxies manifest themselves as an extremly gasrich, heavy element deficient objects showing the initial burst of star formation activity in several spatially separated regions. Slowly rotating objects manifest themselves finally as typical evolved dwarf galaxies.

  6. Detection of starquakes on magnetically active red dwarfs

    NASA Astrophysics Data System (ADS)

    Contadakis, M. E.; Avgoloupis, S. J.; Seiradakis, J. H.; Papantoniou, Ch.

    2015-07-01

    The scientific team of the Stephanion Observatory, University of Thessaloniki contributed to the research of high frequency optical oscillations on red dwarfs by participating in international programs for Multiwavelength observation of strong Flares of selected flare stars ([12]). These joined research shed plenty of light on the phenomenon of high frequency optical oscillations. Nevertheless a better understanding of the high-frequency oscillations demand a unified analysis of the flare light-curve for a wider time window covering pre-flare, flare and post flare and a broader band of frequencies. Thus in addition to the international campaign research the Stephanion Observatory group observe and analysis one colour (B, or U) observations of the Stephanion Observatory of different red dwarfs: EV Lac([1], [2] and [7]), AD Leo ([4] and [5]),YZ CMin ([3],[9]), V 390 Auri ([6],[10]), UV Cet([8]), at any stage of their activity (quiescence, weak flares, strong flare! s).In this paper we present the analysis of the quiet state observations of the stars EV Lac, BY Drac , AD Leo, YZ Cmin in order to realize if starquakes appear far apart from the observed flares, during the quiet state of the stars, as a result of the general magnetic activity of the star.

  7. Determining the Locations of Brown Dwarfs in Young Star Clusters

    NASA Technical Reports Server (NTRS)

    Porter, Lauren A.

    2005-01-01

    Brown dwarfs are stellar objects with masses less than 0.08 times that of the Sun that are unable to sustain nuclear fusion. Because of the lack of fusion, they are relatively cold, allowing the formation of methane and water molecules in their atmospheres. Brown dwarfs can be detected by examining stars' absorption spectra in the near-infrared to see whether methane and water are present. The objective of this research is to determine the locations of brown dwarfs in Rho Ophiuchus, a star cluster that is only 1 million years old. The cluster was observed in four filters in the near-infrared range using the Wide-Field Infra-Red Camera (WIRC) on the 100" DuPont Telescope and Persson's Auxiliary Nasymith Infrared Camera (PANIC) on the 6.5-m Magellan Telescope. By comparing the magnitude of a star in each of the four filters, an absorption spectrum can be formed. This project uses standard astronomical techniques to reduce raw frames into final images and perform photometry on them to obtain publishable data. Once this is done, it will be possible to determine the locations and magnitudes of brown dwarfs within the cluster.

  8. White dwarfs in Be star binary systems

    NASA Technical Reports Server (NTRS)

    Apparao, K. M. V.

    1991-01-01

    An evaluation is made of possible reasons for the persistent inability to identify white dwarf stars in the Be binary systems. It is noted that many Be stars exhibiting large optical enhancements may be Be + WD and Be + He systems, and that observations of pulsations in the H-alpha emission, as well as observation of time delays between enhancements of optical line and continuum, can identify such systems.

  9. Chemical Abundances of Metal-poor stars in Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Venn, Kim A.; Jablonka, Pascale; Hill, Vanessa; Starkenburg, Else; Lemasle, Bertrand; Shetrone, Matthew; Irwin, Mike; Norris, John; Yong, David; Gilmore, Gerry; Salvadori, Stephania; Skuladottir, Asa; Tolstoy, Eline

    2016-08-01

    Stars in low-mass dwarf galaxies show a larger range in their chemical properties than those in the Milky Way halo. The slower star formation efficiency make dwarf galaxies ideal systems for testing nucleosynthetic yields. Not only are alpha-poor stars found at lower metallicities, and a higher fraction of carbon-enhanced stars, but we are also finding stars in dwarf galaxies that appear to be iron-rich. These are compared with yields from a variety of supernova predictions.

  10. Massive Star Clusters in Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Larsen, Soeren

    2015-08-01

    Dwarf galaxies are often characterized by very high globular cluster specific frequencies, in some cases exceeding that of the Milky Way by a factor of 100 or more. Moreover, the GCs are typically much more metal-poor than the bulk of the field stars, so that a substantial fraction (up to 20-25% or more) of all metal-poor stars in some dwarf galaxies are associated with GCs. The metal-poor components of these galaxies thus represent an extreme case of the "specific frequency problem". In this talk I will review the current status of our understanding of GC systems in dwarf galaxies. Particular emphasis will be placed on the implications of the high GC specific frequencies for the amount of mass loss the clusters could have experienced and the constraints this provides on theories for the origin of multiple populations in globular clusters.

  11. A Search for Close Red Dwarf-White Dwarf Binaries

    NASA Astrophysics Data System (ADS)

    Boyd, Mark R.; Henry, Todd J.; Subasavage, John P.

    2011-08-01

    We propose to observe 59 objects suspected to be red dwarf-white dwarf (RD-WD) binaries with separations < 3 arcsec using the CTIO 1.0m. Our goals are to use images of these objects to both resolve the systems and to obtain accurate BVRI photometry. The systems have been selected based on positions in three different color-color plots using SuperCOSMOS BRI plate photometry and 2MASS JHK photometry in accordance with the positions of known RD-WD binaries. This effort will identify candidates for detailed observations as part of the RECONS astrometric program on the CTIO 0.9m to yield accurate parallaxes and photocentric orbits. The parallaxes will then be used to determine the ages of the systems from WD cooling curves, and the orbits will eventually be used to measure dynamical masses. Ultimately, we aim to increase significantly the number of dynamical masses for white dwarfs because currently only three have been determined to 5% accuracy. The first observational step outlined here will allow us to identify appropriate systems for long-term work. This 1.0m project is likely to become the undergraduate senior thesis work of the PI.

  12. About Exobiology: The Case for Dwarf K Stars

    NASA Astrophysics Data System (ADS)

    Cuntz, M.; Guinan, E. F.

    2016-08-01

    One of the most fundamental topics of exobiology concerns the identification of stars with environments consistent with life. Although it is believed that most types of main-sequence stars might be able to support life, particularly extremophiles, special requirements appear to be necessary for the development and sustainability of advanced life forms. From our study, orange main-sequence stars, ranging from spectral type late-G to mid-K (with a maximum at early K), are most promising. Our analysis considers a variety of aspects, including (1) the frequency of the various types of stars, (2) the speed of stellar evolution in their lifetimes, (3) the size of the stellar climatological habitable zones (CLI-HZs), (4) the strengths and persistence of their magnetic-dynamo-generated X-ray–UV emissions, and (5) the frequency and severity of flares, including superflares; both (4) and (5) greatly reduce the suitability of red dwarfs to host life-bearing planets. The various phenomena show pronounced dependencies on the stellar key parameters such as effective temperature and mass, permitting the assessment of the astrobiological significance of various types of stars. Thus, we developed a “Habitable-Planetary-Real-Estate Parameter” (HabPREP) that provides a measure for stars that are most suitable for planets with life. Early K stars are found to have the highest HabPREP values, indicating that they may be “Goldilocks” stars for life-hosting planets. Red dwarfs are numerous, with long lifetimes, but their narrow CLI-HZs and hazards from magnetic activity make them less suitable for hosting exolife. Moreover, we provide X-ray–far-UV irradiances for G0 V–M5 V stars over a wide range of ages.

  13. About Exobiology: The Case for Dwarf K Stars

    NASA Astrophysics Data System (ADS)

    Cuntz, M.; Guinan, E. F.

    2016-08-01

    One of the most fundamental topics of exobiology concerns the identification of stars with environments consistent with life. Although it is believed that most types of main-sequence stars might be able to support life, particularly extremophiles, special requirements appear to be necessary for the development and sustainability of advanced life forms. From our study, orange main-sequence stars, ranging from spectral type late-G to mid-K (with a maximum at early K), are most promising. Our analysis considers a variety of aspects, including (1) the frequency of the various types of stars, (2) the speed of stellar evolution in their lifetimes, (3) the size of the stellar climatological habitable zones (CLI-HZs), (4) the strengths and persistence of their magnetic-dynamo-generated X-ray-UV emissions, and (5) the frequency and severity of flares, including superflares; both (4) and (5) greatly reduce the suitability of red dwarfs to host life-bearing planets. The various phenomena show pronounced dependencies on the stellar key parameters such as effective temperature and mass, permitting the assessment of the astrobiological significance of various types of stars. Thus, we developed a “Habitable-Planetary-Real-Estate Parameter” (HabPREP) that provides a measure for stars that are most suitable for planets with life. Early K stars are found to have the highest HabPREP values, indicating that they may be “Goldilocks” stars for life-hosting planets. Red dwarfs are numerous, with long lifetimes, but their narrow CLI-HZs and hazards from magnetic activity make them less suitable for hosting exolife. Moreover, we provide X-ray-far-UV irradiances for G0 V-M5 V stars over a wide range of ages.

  14. Pulsating White Dwarf Star GD99

    NASA Astrophysics Data System (ADS)

    Chynoweth, K. M.; Thompson, S.; Mullally, F.; Yeates, C.

    2004-12-01

    We present 15 hours of time-series photometry of the variable white dwarf star GD99. These data were obtained at the McDonald Observatory 2.1m Otto Struve Telescope in January 2003, using the Argos CCD photometer. We achieved a noise level as low as 0.07 %, as measured from the power spectrum of our first night. Our observations confirm that GD99 is a unique pulsating white dwarf whose modes show characteristics of both the hot and cold type of DA variable stars. Additionally, GD99 has a large number of modes, making it a good candidate for asteroseismological study. Our preliminary results indicate that this star merits further study to decipher its abundant set of unusual modes. With such a rich period structure, longer continuous data sets will be required to fully resolve the pulsation spectrum.

  15. Accreting neutron stars, black holes, and degenerate dwarf stars.

    PubMed

    Pines, D

    1980-02-01

    During the past 8 years, extended temporal and broadband spectroscopic studies carried out by x-ray astronomical satellites have led to the identification of specific compact x-ray sources as accreting neutron stars, black holes, and degenerate dwarf stars in close binary systems. Such sources provide a unique opportunity to study matter under extreme conditions not accessible in the terrestrial laboratory. Quantitative theoretical models have been developed which demonstrate that detailed studies of these sources will lead to a greatly increased understanding of dense and superdense hadron matter, hadron superfluidity, high-temperature plasma in superstrong magnetic fields, and physical processes in strong gravitational fields. Through a combination of theory and observation such studies will make possible the determination of the mass, radius, magnetic field, and structure of neutron stars and degenerate dwarf stars and the identification of further candidate black holes, and will contribute appreciably to our understanding of the physics of accretion by compact astronomical objects. PMID:17749313

  16. Be stars with white dwarf companions

    NASA Astrophysics Data System (ADS)

    Orio, Marina; Luna, Gerardo; Zemko, Polina; Kotulla, Ralf; Gallagher, Jay; Harbeck, Daniel

    2016-07-01

    A handful of supersoft X-ray sources in the Magellanic Clouds that could not be identified with transient nova outbursts turned out to be mainly massive close binaries. Recently, we have clearly identified a Be binary in M31, and are currently collecting data for another candidate in that galaxy. Work is in progress to assess whether the compact object companion really is a hydrogen burning white dwarf (the alternative being a massive stellar-mass black hole). If we can prove that Be+white dwarf interacting close binaries are common, and that hydrogen is often ignited on the white dwarf in these systems, we have discovered a new promising channel towards the explosion of supernovae of type Ia in star forming regions, without invoking double degenerate systems

  17. White dwarf stars with some hydrogen

    NASA Astrophysics Data System (ADS)

    Chen, Eugene Yu-Yu

    This dissertation focuses on white dwarf stars with some hydrogen, that possess a mass of surface hydrogen from 1 x 10--11 M⊙ to 1 x 10--7 M⊙ . The chemical structure of such white dwarfs is mainly controlled by convection and has never been studied in a fully proper manner. We present the first comprehensive study of such objects, including: (1) The relation between surface chemical composition and stellar structure. (2) The cooling curve and curve of surface chemical evolution. (3) The "Teff function" of the galactic disc derived from such cooling curves. The study enables us to: (1) Infer physical quantities such as total hydrogen mass (mH) and core temperature ( Tcore) from spectroscopy. (2) Understand the correlation between the non-DA to DA abundance ratio and Teff, such as the "non-DA gap" proposed by Bergeron, Ruiz, & Leggett (1997). (3) Better understand the cosmo-chronological meaning of the white dwarf luminosity function.

  18. HOT WHITE DWARF SHINES IN YOUNG STAR CLUSTER

    NASA Technical Reports Server (NTRS)

    2002-01-01

    A dazzling 'jewel-box' collection of over 20,000 stars can be seen in crystal clarity in this NASA Hubble Space Telescope image, taken with the Wide Field and Planetary Camera 2. The young (40 million year old) cluster, called NGC 1818, is 164,000 light-years away in the Large Magellanic Cloud (LMC), a satellite galaxy of our Milky Way. The LMC, a site of vigorous current star formation, is an ideal nearby laboratory for studying stellar evolution. In the cluster, astronomers have found a young white dwarf star, which has only very recently formed following the burnout of a red giant. Based on this observation astronomers conclude that the red giant progenitor star was 7.6 times the mass of our Sun. Previously, astronomers have estimated that stars anywhere from 6 to 10 solar masses would not just quietly fade away as white dwarfs but abruptly self-destruct in torrential explosions. Hubble can easily resolve the star in the crowded cluster, and detect its intense blue-white glow from a sizzling surface temperature of 50,000 degrees Fahrenheit. IMAGE DATA Date taken: December 1995 Wavelength: natural color reconstruction from three filters (I,B,U) Field of view: 100 light-years, 2.2 arc minutes TARGET DATA Name: NGC 1818 Distance: 164,000 light-years Constellation: Dorado Age: 40 million years Class: Rich star cluster Apparent magnitude: 9.7 Apparent diameter: 7 arc minutes Credit: Rebecca Elson and Richard Sword, Cambridge UK, and NASA (Original WFPC2 image courtesy J. Westphal, Caltech) Image files are available electronically via the World Wide Web at: http://oposite.stsci.edu/pubinfo/1998/16 and via links in http://oposite.stsci.edu/pubinfo/latest.html or http://oposite.stsci.edu/pubinfo/pictures.html. GIF and JPEG images are available via anonymous ftp to oposite.stsci.edu in /pubinfo/GIF/9816.GIF and /pubinfo/JPEG/9816.jpg.

  19. Asteroseismology of DAV White Dwarf Stars

    SciTech Connect

    Bradley, Paul A.

    1997-12-31

    The author reviews the seismological structural determinations of ZZ Ceti stars done to date, and supplement these with additional preliminary determinations of his own. He compares the constraints on the hydrogen layer mass to see what trends emerge and also determines if the observed hydrogen layer masses are consistent with proposed theories. He then looks ahead to the prospects of further DAV white dwarf seismology.

  20. Turbulence and Star Formation in Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Hollyday, Gigja; Hunter, Deidre Ann; Little Things Team

    2015-01-01

    We are interested in understanding the nature and role of turbulence in the interstellar medium of dwarf irregular galaxies. Turbulence, resulting from a variety of processes, is a potential source for cloud formation, and thus star formation. We have undertaken an indirect analysis of turbulence via the third (skewness) and fourth (kurtosis) moments of the distribution of atomic hydrogen gas densities using the LITTLE THINGS data for a 40-count sample of nearby (<10.3 Mpc) dwarf galaxies. We followed the formulism used by Burkhart et al. (2010) in a study of the SMC. We found that there is evidence of turbulence in dwarf galaxies at a level comparable to that found in the SMC, but we have found no correlation between integrated star formation rates and integrated kurtosis values nor a clear correlation between kurtosis as a function of radius with gas surface density and star formation profiles. We are grateful for a summer internship provided by the Research Experiences for Undergraduates program at Northern Arizona University, run by Dr. Kathy Eastwood and Dr. David Trilling and funded by the National Science Foundation through grant AST-1004107.

  1. Reliable Radii for M Dwarf Stars

    NASA Astrophysics Data System (ADS)

    Mann, Andrew; Feiden, Gregory A.; Gaidos, Eric

    2015-01-01

    Precise and accurate parameters for late-type (late K and M) dwarf stars are critical for characterizing their planets. A deluge of planets discovered by Kepler has driven the need for even more precise stellar radii. We present our efforts to better constrain the luminosity-radius and Teff-radius relations for late-type (K5-M6) stars, taking advantage of improved techniques to calculate bolometric fluxes and [Fe/H] for M dwarfs. We determine effective temperatures for these stars by comparing observed spectra to atmospheric models, and confirm the accuracy of these temperatures using stars with temperatures determined from long-baseline optical interferometry. Using the Stefan-Boltzmann law we can empirically determine radii for these stars to better than 5%. We find the Teff-radius relation depends strongly on [Fe/H], which was missed in earlier studies that used smaller samples or less precise methods. We expect our empirical relations to be increasingly useful with the arrival of Gaia parallaxes in the near future.

  2. An unsuccessful search for brown dwarf companions to white dwarf stars

    NASA Technical Reports Server (NTRS)

    Shipman, Harry L.

    1986-01-01

    The results of a survey to detect excess infrared emission from white dwarf stars which would be attributable to a low mass companion are reviewed. Neither a simple comparison of spectroscopically identified white dwarf stars with the IRAS Point Source Catalog nor the coadding of IRAS survey data resulted in a detection of a brown dwarf. The seven nearest stars where the most stringent limits to the presence of a brown dwarf were obtained are listed, and an effort to detect brown dwarfs in the solar neighborhood is discussed.

  3. Gas, Stars, and Star Formation in Alfalfa Dwarf Galaxies

    NASA Technical Reports Server (NTRS)

    Huang, Shan; Haynes, Martha P.; Giovanelli, Riccardo; Brinchmann, Jarle; Stierwalt, Sabrina; Neff, Susan G.

    2012-01-01

    We examine the global properties of the stellar and Hi components of 229 low H i mass dwarf galaxies extracted from the ALFALFA survey, including a complete sample of 176 galaxies with H i masses <10(sup 7.7) solar mass and Hi line widths <80 kilometers per second. Sloan Digital Sky Survey (SDSS) data are combined with photometric properties derived from Galaxy Evolution Explorer to derive stellar masses (M*) and star formation rates (SFRs) by fitting their UV-optical spectral energy distributions (SEDs). In optical images, many of the ALFALFA dwarfs are faint and of low surface brightness; only 56% of those within the SDSS footprint have a counterpart in the SDSS spectroscopic survey. A large fraction of the dwarfs have high specific star formation rates (SSFRs), and estimates of their SFRs and M* obtained by SED fitting are systematically smaller than ones derived via standard formulae assuming a constant SFR. The increased dispersion of the SSFR distribution at M* approximately less than10(exp 8)M(sub 0) is driven by a set of dwarf galaxies that have low gas fractions and SSFRs; some of these are dE/dSphs in the Virgo Cluster. The imposition of an upper Hi mass limit yields the selection of a sample with lower gas fractions for their M* than found for the overall ALFALFA population. Many of the ALFALFA dwarfs, particularly the Virgo members, have H i depletion timescales shorter than a Hubble time. An examination of the dwarf galaxies within the full ALFALFA population in the context of global star formation (SF) laws is consistent with the general assumptions that gas-rich galaxies have lower SF efficiencies than do optically selected populations and that Hi disks are more extended than stellar ones.

  4. NTT Observations Indicate that Brown Dwarfs Form Like Stars

    NASA Astrophysics Data System (ADS)

    2001-06-01

    -floating Brown Dwarfs in the Milky Way galaxy. Both facts would appear to imply a stellar, rather than a planet-like origin for these objects. However, one might also explain these observations if most Brown Dwarfs initially formed as companions to stars (within circumstellar disks), but were later ejected from the systems, e.g., because of gravitational effects during encounters with other stars. So the issue of Brown Dwarf origin is still unsettled. NTT observations of substellar objects in the Orion Nebula ESO PR Photo 22a/01 ESO PR Photo 22a/01 [Preview - JPEG: 400 x 434 pix - 192k] [Normal - JPEG: 800 x 877 pix - 496k] [Full Resolution - JPEG: 1772 x 1943 pix - 1.2Mb Caption : PR Photo 22a/01 shows a colour composite of near-infrared images of the central regions of the Orion Nebula, obtained on March 14, 2000, with the SOFI instrument at the ESO 3.5-m New Technology Telescope (NTT) at La Silla. Three exposures were made through J- (wavelength 1.25 µm here colour-coded as "blue"), H- (1.65 µm; "green") and Ks-filters (2.16 µm; "red"), respectively. The central group of bright stars is the famous "Trapezium" . The total effective exposure time was 86.4 seconds per band. The sky field measures about 4.9 x 4.9 arcmin 2 (1024 x 1024 pix 2 ). North is up and East is left. ESO PR Photo 22b/01 ESO PR Photo 22b/01 [Preview - JPEG: 400 x 439 pix - 35k] [Normal - JPEG: 800 x 877 pix - 90k] Caption : PR Photo 22b/01 contains the corresponding "finding chart" with the positions of the very young Brown Dwarfs in the Orion Nebula that were studied during the present investigation. The starlike symbols represent the brightest stars in PR Photo 22a/01 and are plotted for reference. In this chart, very young Brown Dwarfs are represented by a double open circle (if a dusty disk was detected) or with a single open circle (if no dusty disk was detected). The scale is exactly as in PR Photo 22a/01 . ESO PR Photo 22c/01 ESO PR Photo 22c/01 [Animated GIF: 482 x 465 pix - 248k] Caption : PR

  5. CARMENES: Blue planets orbiting red dwarfs

    NASA Astrophysics Data System (ADS)

    Quirrenbach, A.; Amado, P. J.; Caballero, J. A.; Mandel, H.; Mundt, R.; Reiners, A.; Ribas, I.; Sánchez Carrasco, M. A.; Seifert, W.; Azzaro, M.; Galadí, D.; Alonso-Floriano, F. J.; Dreizler, S.; Montes, D.; Rhode, P.; Stürmer, J.

    2013-04-01

    The CARMENES (Calar Alto high-Resolution search for M dwarfs with Exo-earths with Near-infrared and optical Echelle Spectrographs) consortium, consisting of eleven Spanish and German institutions, has been established to conduct a radial-velocity survey of M dwarfs with the 3.5 m telescope at the Calar Alto Observatory. This survey will target ˜300 M stars, with emphasis on spectral types M4V and later. The CARMENES instrument is currently under construction; it consists of two independent échelle spectrographs covering the wavelength ranges 0.55 …1.05 μm and 0.95 …1.7 μm, respectively, at a spectral resolution of R = 82,000. The spectrographs are fed by fibers from the Cassegrain focus of the telescope; calibration is performed simultaneously with emission-line lamps. The optical benches of the spectrographs are housed in vacuum tanks and climatic chambers, which provide the temperature-stabilized environments necessary to enable a 1 m/s radial velocity precision.

  6. Detection of starquakes on the red dwarf AD Leo

    NASA Astrophysics Data System (ADS)

    Contadakis, M. E.; Avgoloupis, S. J.; Seiradakis, J. H.; Papantoniou, Ch.

    2016-07-01

    The results of the analysis of the one color (B) observations of the Stefanion Observatory for the red dwarf AD Leo at any stage of the stellar activity (quiescence, weak flares, strong flares),indicate that: (1) Transient high frequency oscillations occur during the flare event and during the quiet-star phase as well; (2) The Observed frequencies range between 0.0005Hz (period 33min) and 0.3 Hz (period 3s) not rigorously bounded; However, the quiescence parts of the light-curve which were analyzed belong to the pre- or after- flare state i.e. are connected with a major magnetic even (the observed flare). In this work we find that transient oscillations appear far apart from the observed flares, during the quiet state of the stars, as a result of the general magnetic activity of the star. The power spectrum of these oscillations resembles that of the solar like oscillation spectra i.e the sunquakes spectra. Finally a tentative estimation of the main physical parameters of the star, using asteroseismic analysis, was performed.

  7. On oxygenic photosynthesis in planets of Red Dwarfs

    NASA Astrophysics Data System (ADS)

    Wandel, Amri; Gale, Joseph

    2015-08-01

    The results of the Kepler mission indicate that Earthlike planets are common not only around solar-type stars but also among planets orbiting Red Dwarf (RD) stars, the most numerous stellar type in the Milky Way galaxy. Early considerations indicated that conditions on RD planets would be inimical to life, as their Habitable Zones would be so close as to make planets tidally locked to their star. This was thought to cause an erratic climate and expose life forms to flares of ionizing electro-magnetic radiation and charged particles. It has also been argued that the lesser photon energy of the radiation of the relatively cool RDs would not suffice for oxygenic photosynthesis. However, recent calculations show that these negative factors are less severe than originally estimated, hence conditions for photosynthesis could exist on RD planets. Furthermore, the huge number and the long Main-Sequence lifetime of RDs could make photosynthesis and biotic life on RD planets statistically even more abundant than on planets of solar type stars.

  8. Gas, Stars, and Star Formation in ALFALFA Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Huang, Shan; Haynes, Martha P.; Giovanelli, Riccardo; Brinchmann, Jarle; Stierwalt, Sabrina; Neff, Susan G.

    2012-06-01

    We examine the global properties of the stellar and H I components of 229 low H I mass dwarf galaxies extracted from the ALFALFA survey, including a complete sample of 176 galaxies with H I masses <107.7 M ⊙ and H I line widths <80 km s-1. Sloan Digital Sky Survey (SDSS) data are combined with photometric properties derived from Galaxy Evolution Explorer to derive stellar masses (M *) and star formation rates (SFRs) by fitting their UV-optical spectral energy distributions (SEDs). In optical images, many of the ALFALFA dwarfs are faint and of low surface brightness; only 56% of those within the SDSS footprint have a counterpart in the SDSS spectroscopic survey. A large fraction of the dwarfs have high specific star formation rates (SSFRs), and estimates of their SFRs and M * obtained by SED fitting are systematically smaller than ones derived via standard formulae assuming a constant SFR. The increased dispersion of the SSFR distribution at M * <~ 108 M ⊙ is driven by a set of dwarf galaxies that have low gas fractions and SSFRs; some of these are dE/dSphs in the Virgo Cluster. The imposition of an upper H I mass limit yields the selection of a sample with lower gas fractions for their M * than found for the overall ALFALFA population. Many of the ALFALFA dwarfs, particularly the Virgo members, have H I depletion timescales shorter than a Hubble time. An examination of the dwarf galaxies within the full ALFALFA population in the context of global star formation (SF) laws is consistent with the general assumptions that gas-rich galaxies have lower SF efficiencies than do optically selected populations and that H I disks are more extended than stellar ones. Based on observations made with the Arecibo Observatory and the NASA Galaxy Evolution Explorer (GALEX). The Arecibo Observatory is operated by SRI International under a cooperative agreement with the National Science Foundation (AST-1100968), and in alliance with Ana G. Méndez-Universidad Metropolitana and

  9. White dwarf stars in D dimensions

    NASA Astrophysics Data System (ADS)

    Chavanis, P.-H.

    2007-07-01

    We derive the mass-radius relation of relativistic white dwarf stars (modeled as a self-gravitating degenerate Fermi gas at T=0) in a D-dimensional universe and study the influence of the dimension of space on the laws of physics when we combine quantum mechanics, special relativity, and gravity. We exhibit characteristic dimensions D=1, D=2, D=3, D=(3+17)/2, D=4, D=2(1+2) and show that quantum mechanics cannot balance gravitational collapse for D≥4. This is similar to a result found by Ehrenfest (1917) at the atomic level for Coulomb forces (in Bohr’s model) and for the Kepler problem. This makes the dimension of our universe D=3 very particular with possible implications regarding the anthropic principle. We discuss some historic aspects concerning the discovery of the Chandrasekhar (1931) limiting mass in relation to previous investigations by Anderson (1929) and Stoner (1930). We also propose different derivations of the stability limits of polytropic distributions and consider their application to classical and relativistic white dwarf stars.

  10. Star Formation in Dwarf Galaxies: Life in a Rough Neighborhood

    SciTech Connect

    Murray, S

    2003-10-16

    Star formation within dwarf galaxies is governed by several factors. Many of these factors are external, including ram-pressure stripping, tidal stripping, and heating by external UV radiation. The latter, in particular, may prevent star formation in the smallest systems. Internal factors include negative feedback in the form of UV radiation, winds and supernovae from massive stars. These act to reduce the star formation efficiency within dwarf systems, which may, in turn, solve several theoretical and observational problems associated with galaxy formation. In this contribution, we discuss our recent work being done to examine the importance of the many factors in the evolution of dwarf galaxies.

  11. A low-temperature companion to a white dwarf star

    NASA Technical Reports Server (NTRS)

    Becklin, E. E.; Zuckerman, B.

    1988-01-01

    An infrared object located about 120 AU from the white dwarf GD165 has been discovered. With the exception of the possible brown dwarf companion to Giclas 29-38 reported last year, the companion to GD165 is the coolest (2100 K) dwarf star ever reported and, according to some theoretical models, it should be a substellar brown dwarf with a mass between 0.06 and 0.08 solar mass. These results, together with newly discovered low-mass stellar companions to white dwarfs, change the investigation of very low-mass stars from the study of a few chance objects to that of a statistical distribution. In particular, it appears that very low-mass stars and perhaps even brown dwarfs could be quite common in the Galaxy.

  12. DISCOVERY OF SUPER-Li-RICH RED GIANTS IN DWARF SPHEROIDAL GALAXIES

    SciTech Connect

    Kirby, Evan N.; Fu, Xiaoting; Deng, Licai; Guhathakurta, Puragra

    2012-06-10

    Stars destroy lithium (Li) in their normal evolution. The convective envelopes of evolved red giants reach temperatures of millions of kelvin, hot enough for the {sup 7}Li(p, {alpha}){sup 4}He reaction to burn Li efficiently. Only about 1% of first-ascent red giants more luminous than the luminosity function bump in the red giant branch exhibit A(Li) > 1.5. Nonetheless, Li-rich red giants do exist. We present 15 Li-rich red giants-14 of which are new discoveries-among a sample of 2054 red giants in Milky Way dwarf satellite galaxies. Our sample more than doubles the number of low-mass, metal-poor ([Fe/H] {approx}< -0.7) Li-rich red giants, and it includes the most-metal-poor Li-enhanced star known ([Fe/H] = -2.82, A(Li){sub NLTE} = 3.15). Because most of the stars have Li abundances larger than the universe's primordial value, the Li in these stars must have been created rather than saved from destruction. These Li-rich stars appear like other stars in the same galaxies in every measurable regard other than Li abundance. We consider the possibility that Li enrichment is a universal phase of evolution that affects all stars, and it seems rare only because it is brief.

  13. Diffusion of neon in white dwarf stars.

    PubMed

    Hughto, J; Schneider, A S; Horowitz, C J; Berry, D K

    2010-12-01

    Sedimentation of the neutron rich isotope 22Ne may be an important source of gravitational energy during the cooling of white dwarf stars. This depends on the diffusion constant for 22Ne in strongly coupled plasma mixtures. We calculate self-diffusion constants D(i) from molecular dynamics simulations of carbon, oxygen, and neon mixtures. We find that D(i) in a mixture does not differ greatly from earlier one component plasma results. For strong coupling (coulomb parameter Γ> few), D(i) has a modest dependence on the charge Z(i) of the ion species, D(i)∝Z(i)(-2/3). However, D(i) depends more strongly on Z(i) for weak coupling (smaller Γ). We conclude that the self-diffusion constant D(Ne) for 22Ne in carbon, oxygen, and neon plasma mixtures is accurately known so that uncertainties in D(Ne) should be unimportant for simulations of white dwarf cooling. PMID:21230741

  14. Diffusion of neon in white dwarf stars.

    PubMed

    Hughto, J; Schneider, A S; Horowitz, C J; Berry, D K

    2010-12-01

    Sedimentation of the neutron rich isotope 22Ne may be an important source of gravitational energy during the cooling of white dwarf stars. This depends on the diffusion constant for 22Ne in strongly coupled plasma mixtures. We calculate self-diffusion constants D(i) from molecular dynamics simulations of carbon, oxygen, and neon mixtures. We find that D(i) in a mixture does not differ greatly from earlier one component plasma results. For strong coupling (coulomb parameter Γ> few), D(i) has a modest dependence on the charge Z(i) of the ion species, D(i)∝Z(i)(-2/3). However, D(i) depends more strongly on Z(i) for weak coupling (smaller Γ). We conclude that the self-diffusion constant D(Ne) for 22Ne in carbon, oxygen, and neon plasma mixtures is accurately known so that uncertainties in D(Ne) should be unimportant for simulations of white dwarf cooling.

  15. Discovery of a brown dwarf in the Pleiades star cluster

    NASA Astrophysics Data System (ADS)

    Rebolo, R.; Zapatero Osorio, M. R.; Martín, E. L.

    1995-09-01

    BROWN dwarfs are cool star-like objects that have insufficient mass to maintain stable nuclear fusion in their interiors. Although brown dwarfs are not stars, they are expected to form in the same way, and their frequency of occurrence should reflect the trends seen in the birthrates of low-mass stars. But finding brown dwarfs has proved to be difficult, because of their low intrinsic luminosity. The nearby Pleiades star cluster is widely recognized as a likely host for detectable brown dwarfs because of its young age - the still-contracting brown dwarfs should radiate a large fraction of their gravitational energy at near-infrared wavelengths. Here we report the discovery of a brown dwarf near the centre of the Pleiades. The luminosity and temperature of this object are so low that its mass must be less than 0.08 solar masses, the accepted lower limit on the mass of a true star1-3. The detection of only one brown dwarf within our survey area is consistent with a smooth extrapolation of the stellar mass function of the Pleiades4, suggesting that brown dwarfs, although probably quite numerous in the Galactic disk, are unlikely to comprise more than ~1% of its mass.

  16. Fundamental Parameters of Nearby Red Dwarfs: Stellar Radius as an Indicator of Age

    NASA Astrophysics Data System (ADS)

    Silverstein, Michele L.; Henry, Todd J.; Winters, Jennifer G.; Jao, Wei-Chun; Riedel, Adric R.; Dieterich, Sergio; RECONS Team

    2016-01-01

    Red dwarfs dominate the Galactic population, yet determining one of their most fundamental characteristics --- age --- has proven difficult. The characterization of red dwarfs in terms of their age is fundamental to mapping the history of star and, ultimately, planet formation in the Milky Way. Here we report on a compelling technique to evaluate the radii of red dwarfs, which can be used to provide leverage in estimating their ages. These radii are also particularly valuable in the cases of transiting exoplanet hosts because accurate stellar radii are required to determine accurate planetary radii.In this work, we use the BT-Settl models in combination with Johnson-Kron-Cousins VRI, 2MASS JHK, and WISE All-Sky Release photometry to produce spectral energy distributions (SEDs) to determine the temperatures and bolometric fluxes for 500 red dwarfs, most of which are in the southern sky. The full suites of our photometric and astrometric data (including hundreds of accurate new parallaxes from the RECONS team at the CTIO/SMARTS 0.9m) allow us to also determine the bolometric luminosities and radii. This method of radius determination is validated by a comparison of our measurements to those found using the CHARA Array (Boyajian et al. 2012), which match within a few percent.In addition to a compilation of red dwarf fundamental parameters, our findings provide a snapshot of relative stellar ages in the solar neighborhood. Of particular interest are the cohorts of very young and very old stars identified within 50 pc. These outliers exemplify the demographic extremes of the nearest stars.This effort has been supported by the NSF through grants AST-0908402, AST-1109445, and AST-1412026, and via observations made possible by the SMARTS Consortium.

  17. Do all barium stars have a white dwarf companion?

    NASA Technical Reports Server (NTRS)

    Dominy, J. F.; Lambert, D. L.

    1983-01-01

    International Ultraviolet Explorer short-wavelength, low-dispersion spectra were analyzed for four barium, two mild barium, and one R-type carbon star in order to test the hypothesis that the barium and related giants are produced by mass transfer from a companion now present as a white dwarf. An earlier tentative identification of a white dwarf companion to the mild barium star Zeta Cyg is confirmed. For the other stars, no ultraviolet excess attributable to a white dwarf is seen. Limits are set on the bolometric magnitude and age of a possible white dwarf companion. Since the barium stars do not have obvious progenitors among main-sequence and subgiant stars, mass transfer must be presumed to occur when the mass-gaining star is already on the giant branch. This restriction, and the white dwarf's minimum age, which is greater than 8 x 10 to the 8th yr, determined for several stars, effectively eliminates the hypothesis that mass transfer from an asymptotic giant branch star creates a barium star. Speculations are presented on alternative methods of producing a barium star in a binary system.

  18. Magnetic activity of red secondaries: clues from the outburst cycle variations of dwarf novae

    NASA Astrophysics Data System (ADS)

    Chinarova, L. L.

    Photometric variations of 6 dwarf novae stars are studied based on the photographic observations from the Odessa, Moscow and Sonneberg plate collections and published visual monitoring data from the AFOEV database (Schweitzer E.: 1993, Bull. AFOEV, 64, 14). The moments of maxima are determined by using the "running parabola" fit (Andronov I.L., 1990, Kinematika Fizika Nebesn. Tel., v.6,,N 6, 87) with automatically determined filter half-width (Andronov I.L., 1997, As.Ap. Suppl., in press). All investigated stars exhibit significant changes not only from cycle-to-cycle, but from season-to-season as well. Secondary decade-scale cycles of smooth variations (Bianchini A., 1990, AJ 99, 1941) and abrupt switchings (Andronov I.L., Shakun L.I., 1990, ASS 169, 237) were interpreted by a solar-type activity of the red dwarf secondary in a binary system and may argue for existence of two different subgroups of the dwarf novae.

  19. Supernova SN 2011fe from an exploding carbon-oxygen white dwarf star.

    PubMed

    Nugent, Peter E; Sullivan, Mark; Cenko, S Bradley; Thomas, Rollin C; Kasen, Daniel; Howell, D Andrew; Bersier, David; Bloom, Joshua S; Kulkarni, S R; Kandrashoff, Michael T; Filippenko, Alexei V; Silverman, Jeffrey M; Marcy, Geoffrey W; Howard, Andrew W; Isaacson, Howard T; Maguire, Kate; Suzuki, Nao; Tarlton, James E; Pan, Yen-Chen; Bildsten, Lars; Fulton, Benjamin J; Parrent, Jerod T; Sand, David; Podsiadlowski, Philipp; Bianco, Federica B; Dilday, Benjamin; Graham, Melissa L; Lyman, Joe; James, Phil; Kasliwal, Mansi M; Law, Nicholas M; Quimby, Robert M; Hook, Isobel M; Walker, Emma S; Mazzali, Paolo; Pian, Elena; Ofek, Eran O; Gal-Yam, Avishay; Poznanski, Dovi

    2011-12-15

    Type Ia supernovae have been used empirically as 'standard candles' to demonstrate the acceleration of the expansion of the Universe even though fundamental details, such as the nature of their progenitor systems and how the stars explode, remain a mystery. There is consensus that a white dwarf star explodes after accreting matter in a binary system, but the secondary body could be anything from a main-sequence star to a red giant, or even another white dwarf. This uncertainty stems from the fact that no recent type Ia supernova has been discovered close enough to Earth to detect the stars before explosion. Here we report early observations of supernova SN 2011fe in the galaxy M101 at a distance from Earth of 6.4 megaparsecs. We find that the exploding star was probably a carbon-oxygen white dwarf, and from the lack of an early shock we conclude that the companion was probably a main-sequence star. Early spectroscopy shows high-velocity oxygen that slows rapidly, on a timescale of hours, and extensive mixing of newly synthesized intermediate-mass elements in the outermost layers of the supernova. A companion paper uses pre-explosion images to rule out luminous red giants and most helium stars as companions to the progenitor. PMID:22170680

  20. Supernova SN 2011fe from an exploding carbon-oxygen white dwarf star.

    PubMed

    Nugent, Peter E; Sullivan, Mark; Cenko, S Bradley; Thomas, Rollin C; Kasen, Daniel; Howell, D Andrew; Bersier, David; Bloom, Joshua S; Kulkarni, S R; Kandrashoff, Michael T; Filippenko, Alexei V; Silverman, Jeffrey M; Marcy, Geoffrey W; Howard, Andrew W; Isaacson, Howard T; Maguire, Kate; Suzuki, Nao; Tarlton, James E; Pan, Yen-Chen; Bildsten, Lars; Fulton, Benjamin J; Parrent, Jerod T; Sand, David; Podsiadlowski, Philipp; Bianco, Federica B; Dilday, Benjamin; Graham, Melissa L; Lyman, Joe; James, Phil; Kasliwal, Mansi M; Law, Nicholas M; Quimby, Robert M; Hook, Isobel M; Walker, Emma S; Mazzali, Paolo; Pian, Elena; Ofek, Eran O; Gal-Yam, Avishay; Poznanski, Dovi

    2011-12-14

    Type Ia supernovae have been used empirically as 'standard candles' to demonstrate the acceleration of the expansion of the Universe even though fundamental details, such as the nature of their progenitor systems and how the stars explode, remain a mystery. There is consensus that a white dwarf star explodes after accreting matter in a binary system, but the secondary body could be anything from a main-sequence star to a red giant, or even another white dwarf. This uncertainty stems from the fact that no recent type Ia supernova has been discovered close enough to Earth to detect the stars before explosion. Here we report early observations of supernova SN 2011fe in the galaxy M101 at a distance from Earth of 6.4 megaparsecs. We find that the exploding star was probably a carbon-oxygen white dwarf, and from the lack of an early shock we conclude that the companion was probably a main-sequence star. Early spectroscopy shows high-velocity oxygen that slows rapidly, on a timescale of hours, and extensive mixing of newly synthesized intermediate-mass elements in the outermost layers of the supernova. A companion paper uses pre-explosion images to rule out luminous red giants and most helium stars as companions to the progenitor.

  1. Variable Stars in the Sextans Dwarf Spheroidal Galaxy

    NASA Astrophysics Data System (ADS)

    Mateo, Mario; Fischer, Philippe; Krzeminski, Wojtek

    1995-11-01

    We describe a survey for variable stars in the Sextans dwarf spheroidal (dSph) galaxy based on the analysis of 113 B and 48 V CCD images of four fields covering a total area of 18' x 18'. We have identified 44 variables: 36 RR Lyr star, 6 anomalous Cepheids, one long-period red variable, all probable members of Sextans, and one foreground contact binary. We have used the pulsating stars to derive a true distance modulus of 19.67 +/- 0.15 for Sextans (or D = 86 +/-6 kpc), where the error is primarily due to uncertainties in the luminosity-metallicity relation for RR Lyr stars. Based on our new data we conclude that [Fe/H]_Sex_ = -1.6 +/- 0.2, somewhat higher than the value from Suntzeff et al. (ApJ, 418,208(1993)] obtained from the analysis of fiber spectroscopy of the near-IR Calcium triplet. We present a new deep color- magnitude diagram for Sextans which reveals the presence of a metal-poor population containing stars as young as 2-4 Gyr, consistent with the presence of anomalous Cepheids in the galaxy. This young population may represent as much as 25% of the total stellar content of Sextans. We find a surprisingly strong correlation between the frequency of anomalous Cepheids in dSph galaxies and galaxian luminosity and speculate on the possible origin of this strange effect. The RR Lyr stars in Sextans do not exhibit the Oosterhoff dichotomy observed in globular clusters and in the Galactic halo field.

  2. Surprising Rapid Collapse of Sirius B from Red Giant to White Dwarf Through Mass Transfer to Sirius a

    NASA Astrophysics Data System (ADS)

    Yousef, Shahinaz; Ali, Ola

    2013-03-01

    Sirius was observed in antiquity as a red star. In his famous astronomy textbook the Almagest written 140 AD, Ptolemy described the star Sirius as fiery red. He curiously depicted it as one of six red-colored stars. The other five are class M and K stars, such as Arcturus and Betelgeuse. Apparent confirmation in ancient Greek and Roman sources are found and Sirius was also reported red in Europe about 1400 years ago. Sirius must have changed to a white dwarf in the night of Ascension. The star chapter in the Quran started with "by the star as it collapsed (1) your companion have not gone astray nor being misled (2), and in verse 49 which is the rotation period of the companion Sirius B around Sirius A, it is said" He is the Lord of Sirius (49). If Sirius actually was red what could have caused it to change into the brilliant bluish-white star we see today? What the naked eye perceives as a single star is actually a binary star system, consisting of a white main sequence star of spectral type A1V, termed Sirius A, and a faint white dwarf companion of spectral type DA2, termed Sirius B. The red color indicates that the star seen then was a red giant. It looks that what they have seen in antiquity was Sirius B which was then a red giant and it collapsed to form a white dwarf. Since there is no evidence of a planetary nebula, then the red Sirius paradox can be solved in terms of stellar evolution with mass transfer. Sirius B was the most massive star which evolved to a red giant and filled the Roche lobe. Mass transfer to Sirius A occurred through the Lagrangian point. Sirius A then became more massive while Sirius B lost mass and shrank. Sirius B then collapsed abruptly into a white dwarf. In the case of Algol, Ptolmy observed it as white star but it was red at the time of El sufi. At present it is white. The rate of mass transfer from Sirius B to Sirius A, and from Algol B to A is estimated from observational data of colour change from red to bullish white to be 0

  3. Infrared spectrum of an extremely cool white-dwarf star

    PubMed

    Hodgkin; Oppenheimer; Hambly; Jameson; Smartt; Steele

    2000-01-01

    White dwarfs are the remnant cores of stars that initially had masses of less than 8 solar masses. They cool gradually over billions of years, and have been suggested to make up much of the 'dark matter' in the halo of the Milky Way. But extremely cool white dwarfs have proved difficult to detect, owing to both their faintness and their anticipated similarity in colour to other classes of dwarf stars. Recent improved models indicate that white dwarfs are much more blue than previously supposed, suggesting that the earlier searches may have been looking for the wrong kinds of objects. Here we report an infrared spectrum of an extremely cool white dwarf that is consistent with the new models. We determine the star's temperature to be 3,500 +/- 200 K, making it the coolest known white dwarf. The kinematics of this star indicate that it is in the halo of the Milky Way, and the density of such objects implied by the serendipitous discovery of this star is consistent with white dwarfs dominating the dark matter in the halo. PMID:10638748

  4. Infrared spectrum of an extremely cool white-dwarf star

    PubMed

    Hodgkin; Oppenheimer; Hambly; Jameson; Smartt; Steele

    2000-01-01

    White dwarfs are the remnant cores of stars that initially had masses of less than 8 solar masses. They cool gradually over billions of years, and have been suggested to make up much of the 'dark matter' in the halo of the Milky Way. But extremely cool white dwarfs have proved difficult to detect, owing to both their faintness and their anticipated similarity in colour to other classes of dwarf stars. Recent improved models indicate that white dwarfs are much more blue than previously supposed, suggesting that the earlier searches may have been looking for the wrong kinds of objects. Here we report an infrared spectrum of an extremely cool white dwarf that is consistent with the new models. We determine the star's temperature to be 3,500 +/- 200 K, making it the coolest known white dwarf. The kinematics of this star indicate that it is in the halo of the Milky Way, and the density of such objects implied by the serendipitous discovery of this star is consistent with white dwarfs dominating the dark matter in the halo.

  5. A radio-pulsing white dwarf binary star

    NASA Astrophysics Data System (ADS)

    Marsh, T. R.; Gänsicke, B. T.; Hümmerich, S.; Hambsch, F.-J.; Bernhard, K.; Lloyd, C.; Breedt, E.; Stanway, E. R.; Steeghs, D. T.; Parsons, S. G.; Toloza, O.; Schreiber, M. R.; Jonker, P. G.; van Roestel, J.; Kupfer, T.; Pala, A. F.; Dhillon, V. S.; Hardy, L. K.; Littlefair, S. P.; Aungwerojwit, A.; Arjyotha, S.; Koester, D.; Bochinski, J. J.; Haswell, C. A.; Frank, P.; Wheatley, P. J.

    2016-09-01

    White dwarfs are compact stars, similar in size to Earth but approximately 200,000 times more massive. Isolated white dwarfs emit most of their power from ultraviolet to near-infrared wavelengths, but when in close orbits with less dense stars, white dwarfs can strip material from their companions and the resulting mass transfer can generate atomic line and X-ray emission, as well as near- and mid-infrared radiation if the white dwarf is magnetic. However, even in binaries, white dwarfs are rarely detected at far-infrared or radio frequencies. Here we report the discovery of a white dwarf/cool star binary that emits from X-ray to radio wavelengths. The star, AR Scorpii (henceforth AR Sco), was classified in the early 1970s as a δ-Scuti star, a common variety of periodic variable star. Our observations reveal instead a 3.56-hour period close binary, pulsing in brightness on a period of 1.97 minutes. The pulses are so intense that AR Sco’s optical flux can increase by a factor of four within 30 seconds, and they are also detectable at radio frequencies. They reflect the spin of a magnetic white dwarf, which we find to be slowing down on a 107-year timescale. The spin-down power is an order of magnitude larger than that seen in electromagnetic radiation, which, together with an absence of obvious signs of accretion, suggests that AR Sco is primarily spin-powered. Although the pulsations are driven by the white dwarf’s spin, they mainly originate from the cool star. AR Sco’s broadband spectrum is characteristic of synchrotron radiation, requiring relativistic electrons. These must either originate from near the white dwarf or be generated in situ at the M star through direct interaction with the white dwarf’s magnetosphere.

  6. Multi-periodic pulsations of a stripped red-giant star in an eclipsing binary system.

    PubMed

    Maxted, Pierre F L; Serenelli, Aldo M; Miglio, Andrea; Marsh, Thomas R; Heber, Ulrich; Dhillon, Vikram S; Littlefair, Stuart; Copperwheat, Chris; Smalley, Barry; Breedt, Elmé; Schaffenroth, Veronika

    2013-06-27

    Low-mass white-dwarf stars are the remnants of disrupted red-giant stars in binary millisecond pulsars and other exotic binary star systems. Some low-mass white dwarfs cool rapidly, whereas others stay bright for millions of years because of stable fusion in thick surface hydrogen layers. This dichotomy is not well understood, so the potential use of low-mass white dwarfs as independent clocks with which to test the spin-down ages of pulsars or as probes of the extreme environments in which low-mass white dwarfs form cannot fully be exploited. Here we report precise mass and radius measurements for the precursor to a low-mass white dwarf. We find that only models in which this disrupted red-giant star has a thick hydrogen envelope can match the strong constraints provided by our data. Very cool low-mass white dwarfs must therefore have lost their thick hydrogen envelopes by irradiation from pulsar companions or by episodes of unstable hydrogen fusion (shell flashes). We also find that this low-mass white-dwarf precursor is a type of pulsating star not hitherto seen. The observed pulsation frequencies are sensitive to internal processes that determine whether this star will undergo shell flashes. PMID:23803845

  7. Multi-periodic pulsations of a stripped red-giant star in an eclipsing binary system.

    PubMed

    Maxted, Pierre F L; Serenelli, Aldo M; Miglio, Andrea; Marsh, Thomas R; Heber, Ulrich; Dhillon, Vikram S; Littlefair, Stuart; Copperwheat, Chris; Smalley, Barry; Breedt, Elmé; Schaffenroth, Veronika

    2013-06-27

    Low-mass white-dwarf stars are the remnants of disrupted red-giant stars in binary millisecond pulsars and other exotic binary star systems. Some low-mass white dwarfs cool rapidly, whereas others stay bright for millions of years because of stable fusion in thick surface hydrogen layers. This dichotomy is not well understood, so the potential use of low-mass white dwarfs as independent clocks with which to test the spin-down ages of pulsars or as probes of the extreme environments in which low-mass white dwarfs form cannot fully be exploited. Here we report precise mass and radius measurements for the precursor to a low-mass white dwarf. We find that only models in which this disrupted red-giant star has a thick hydrogen envelope can match the strong constraints provided by our data. Very cool low-mass white dwarfs must therefore have lost their thick hydrogen envelopes by irradiation from pulsar companions or by episodes of unstable hydrogen fusion (shell flashes). We also find that this low-mass white-dwarf precursor is a type of pulsating star not hitherto seen. The observed pulsation frequencies are sensitive to internal processes that determine whether this star will undergo shell flashes.

  8. Erratum: “Milky Way Red Dwarfs in the Borg Survey; Galactic Scale-Height and the Distribution of Dwarfs Stars in WFC3 Imaging" (2014, ApJ, 788, 77)

    NASA Astrophysics Data System (ADS)

    Holwerda, B. W.; Trenti, M.; Clarkson, W.; Sahu, K.; Bradley, L.; Stiavelli, M.; Pirzkal, N.; De Marchi, G.; Andersen, M.; Bouwens, R.; Ryan, R.; van Vledder, I.; van der Vlugt, D.

    2016-07-01

    In the catalog of M-dwarfs presented in Holwerda et al. (2014, H14 hereafter), there is an issue with the conversion from celestial coordinates to Galactic ones, done with pyephem a wrapper around a trusted and vetted library ephermis. Here we present the corrected coordinates (using AstroPy) and distances based on AB magnitudes. We have amended the tables and figures accordingly. The relation between vertical scale-height (z0) and M- dwarf subtype found in H14 is no longer present. We find a scale-height of 600 pc for all types, in part due to the presence of a second Galactic structural component.

  9. Do Some X-ray Stars Have White Dwarf Companions?

    NASA Technical Reports Server (NTRS)

    McCollum, Bruce

    1995-01-01

    Some Be stars which are intermittent C-ray sources may have white dwarf companions rather than neutron stars. It is not possible to prove or rule out the existence of Be+WD systems using X-ray or optical data. However, the presence of a white dwarf could be established by the detection of its EUV continuum shortward of the Be star's continuum turnover at 1OOOA. Either the detection or the nondetection of Be+WD systems would have implications for models of Be star variability, models of Be binary system formation and evolution, and models of wind-fed accretion.

  10. Do some x-ray stars have white dwarf companions

    NASA Technical Reports Server (NTRS)

    Mccollum, Bruce

    1995-01-01

    Some Be stars which are intermittent X-ray sources may have white dwarf companions rather than neutron stars. It is not possible to prove or rule out the existence of Be + WD systems using X-ray or optical data. However, the presence of a white dwarf could be established by the detection of its EUV continuum shortward of the Be star's continuum turnover at 100 A. Either the detection or the nondetection of Be + WD systems would have implications for models of Be star variability, models of Be binary system formation and evolution, and models of wind-fed accretion.

  11. IUE spectrophotometry of the DA4 primary in the short-period white dwarf-red dwarf spectroscopic binary Case 1

    NASA Technical Reports Server (NTRS)

    Sion, E. M.; Guinan, E. F.; Wesemael, F.

    1984-01-01

    Low-resolution ultraviolet International Ultraviolet Explorer spectra of the DA white dwarf Case 1 are presented. The spectra show the presence of the 1400 A feature, already discovered in several other DA stars, and of a shallower trough in the 1550-1700 A range. A model atmosphere analysis of the ultraviolet energy distribution of the Ly-alpha red wing yields T(e) = 13,000 + or - 500 K. Possible interpretations of the 1400 A feature are reviewed. Case 1 is the coolest white dwarf found in a short-period, detached white dwarf-red dwarf binary, and its cooling time is consistent with estimates of the efficiency of angular momentum removal mechanisms in the phases subsequent to common envelope binary evolution.

  12. V and K-band Mass-Luminosity Relations for M Dwarf Stars

    NASA Astrophysics Data System (ADS)

    Benedict, George Frederick; Henry, Todd J.; McArthur, Barbara E.; Franz, Otto; Wasserman, Larry H.; Dieterich, Sergio

    2015-08-01

    Applying Hubble Space Telescope Fine Guidance Sensor astrometric techniques developed to establish relative orbits for binary stars (Franz et al. 1998, AJ, 116, 1432), determine masses of binary components (Benedict et al. 2001, AJ, 121, 1607), and measure companion masses of exoplanet host stars (McArthur et al. 2010, ApJ, 715, 1203), we derive masses with an average 2% error for 28 components of 14 M dwarf binary star systems. With these and other published masses we update the lower Main Sequence V-band Mass-Luminosity Relation first shown in Henry et al. 1999, ApJ, 512, 864. We demonstrate that a Mass-Luminosity Relation in the K-band has far less scatter. These relations can be used to estimate the masses of the ubiquitous red dwarfs (75% of all stars) to an accuracy of better than 5%.

  13. A coronagraphic search for brown dwarfs around nearby stars

    NASA Technical Reports Server (NTRS)

    Nakajima, T.; Durrance, S. T.; Golimowski, D. A.; Kulkarni, S. R.

    1994-01-01

    Brown dwarf companions have been searched for around stars within 10 pc of the Sun using the Johns-Hopkins University Adaptive Optics Coronagraph (AOC), a stellar coronagraph with an image stabilizer. The AOC covers the field around the target star with a minimum search radius of 1 sec .5 and a field of view of 1 arcmin sq. We have reached an unprecedented dynamic range of Delta m = 13 in our search for faint companions at I band. Comparison of our survey with other brown dwarf searches shows that the AOC technique is unique in its dynamic range while at the same time just as sensitive to brown dwarfs as the recent brown dwarf surveys. The present survey covered 24 target stars selected from the Gliese catalog. A total of 94 stars were detected in 16 fields. The low-latitude fields are completely dominated by background star contamination. Kolmogorov-Smirnov tests were carried out for a sample restricted to high latitudes and a sample with small angular separations. The high-latitude sample (b greater than or equal to 44 deg) appears to show spatial concentration toward target stars. The small separation sample (Delta Theta less than 20 sec) shows weaker dependence on Galactic coordinates than field stars. These statistical tests suggest that both the high-latitude sample and the small separation sample can include a substantial fraction of true companions. However, the nature of these putative companions is mysterious. They are too faint to be white dwarfs and too blue for brown dwarfs. Ignoring the signif icance of the statistical tests, we can reconcile most of the detections with distant main-sequence stars or white dwarfs except for a candidate next to GL 475. Given the small size of our sample, we conclude that considerably more targets need to be surveyed before a firm conclusion on the possibility of a new class of companions can be made.

  14. The hinterland: compilation of nearby brown dwarfs and ultracool stars

    NASA Astrophysics Data System (ADS)

    Ramos, Christopher David

    This work is a compilation and analysis of ultracool dwarfs (UCDs) and brown dwarfs within 25 parsecs. It supplements the work of Stauffer et al. [2010] who updated the reputable and widely relied upon Third Catalog of Nearby Stars [Gliese & Jahreiß 1991] with revised coordinates and cross-matched each object with the 2MASS point source catalog [Cutri et al. 2003]. I began by incorporating newly discovered (post 1991) cool companions to Gliese-Jahreiß stars that had been previously undetectable. I then expanded the compilation to include isolated UCDs and other nearby systems with at least one UCD component. Multiple systems are a panacea for astrophysical problems: by applying Kepler's laws, the model-independent mass of brown dwarfs and low mass stars can be determined and hence serve to constrain theory. This work puts this data into context by exploring the history of brown dwarf theory and reviewing open questions concerning their nature.

  15. Inhomogeneous structure in the chromospheres of dwarf M stars

    NASA Technical Reports Server (NTRS)

    Turner, N. J.; Cram, L. E.; Robinson, R. D.

    1991-01-01

    Linear combinations of observed spectra of the H-alpha and Ca-II resonance and IR lines from the chromospheres of a quiet (Gl 1) and an active (Gl 735) dwarf-M star are compared with the corresponding spectra from a star of intermediate activity (Gl 887). It is shown that the intermediate spectra cannot be explained as a simple juxtaposition of the extreme chromospheric states. It is concluded that the range of observed strengths of chromospheric activity indicators in dwarf-M stars is due, at least in part, to changes in the radial structure of the chromospheric heating function and not to changes in the area filling factor.

  16. The White Dwarf Binary Pathways Survey I: A sample of FGK stars with white dwarf companions

    NASA Astrophysics Data System (ADS)

    Parsons, S. G.; Rebassa-Mansergas, A.; Schreiber, M. R.; Gänsicke, B. T.; Zorotovic, M.; Ren, J. J.

    2016-08-01

    The number of spatially unresolved white dwarf plus main-sequence star binaries has increased rapidly in the last decade, jumping from only ˜30 in 2003 to over 3000. However, in the majority of known systems the companion to the white dwarf is a low mass M dwarf, since these are relatively easy to identify from optical colours and spectra. White dwarfs with more massive FGK type companions have remained elusive due to the large difference in optical brightness between the two stars. In this paper we identify 934 main-sequence FGK stars from the Radial Velocity Experiment (RAVE) survey in the southern hemisphere and the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) survey in the northern hemisphere, that show excess flux at ultraviolet wavelengths which we interpret as the likely presence of a white dwarf companion. We obtained Hubble Space Telescope ultraviolet spectra for nine systems which confirmed that the excess is indeed caused, in all cases, by a hot compact companion, eight being white dwarfs and one a hot subdwarf or pre-helium white dwarf, demonstrating that this sample is very clean. We also address the potential of this sample to test binary evolution models and type Ia supernovae formation channels.

  17. DISCOVERY OF AN UNUSUALLY RED L-TYPE BROWN DWARF

    SciTech Connect

    Gizis, John E.; Castro, Philip J.; Faherty, Jacqueline K.; Liu, Michael C.; Aller, Kimberly M.; Shaw, John D.; Vrba, Frederick J.; Harris, Hugh C.; Deacon, Niall R.

    2012-10-01

    We report the discovery of an unusually red brown dwarf found in a search for high proper motion objects using WISE and 2MASS data. WISEP J004701.06+680352.1 is moving at 0.''44 yr{sup -1} and lies relatively close to the Galactic plane (b = 5.{sup 0}2). Near-infrared photometry and spectroscopy reveals that this is one of the reddest (2MASS J - K{sub s} 2.55 {+-} 0.08 mag) field L dwarfs yet detected, making this object an important member of the class of unusually red L dwarfs. We discuss evidence for thick condensate clouds and speculate on the age of the object. Although models by different research groups agree that thick clouds can explain the red spectrum, they predict dramatically different effective temperatures, ranging from 1100 K to 1600 K. This brown dwarf is well suited for additional studies of extremely dusty substellar atmospheres because it is relatively bright (K{sub s} = 13.05 {+-} 0.03 mag), which should also contribute to an improved understanding of young gas-giant planets and the transition between L and T brown dwarfs.

  18. Potential Nearby M Dwarf Stars Selected from the 2MASS Catalogs

    NASA Astrophysics Data System (ADS)

    Robertson, Thomas H.; Thompson, Dayna L.

    2016-01-01

    Potential nearby red dwarf stars have been selected from the 2MASS catalogs using assumptions about apparent magnitudes and colors. Candidate stars in this study are north of the celestial equator and have been restricted to galactic latitudes greater than 20 degrees from the galactic plane to permit subsequent aperture photometry with small telescopes. Stars with close companions have also been eliminated. Most probable M giant stars were eliminated using the (J-H) - (H-K) two-color diagram. Proper motions were obtained from the USNO-B catalog. Additional potential M giant stars were eliminated by removing stars with very low proper motions. Known nearby stars were removed from the list and stars with proper motions greater than 0.175 arcsec yr-1 were also removed, since such stars will likely be studied in other programs devoted to stars of known proper motion. Photometric parallaxes for the candidate stars were computed using 2MASS photometry and stars having average photometric distances of 25 pc or less were retained. A sample of 121 stars was produced. These stars are being observed using Kron-Cousins R, I and CaH photometry. To date about 75% of the program stars have been observed. All are confirmed dwarf stars and about 50% have distances of 25 pc or less based on photometric parallaxes using Kron-Cousins photometry.This publication makes use of data products from the Two Micron All Sky Survey and the U.S. Naval Observatory B1.0 Catalog. Services and products provided by the Strasbourg Astronomical Data Center (CDS) and US Virtual Astronomical Observatory (VAO) were used in processing the data. Observations have been obtained using the telescopes of the Southeastern Association for Research in Astronomy (SARA).

  19. Discovery of Super-Li-rich Red Giants in Dwarf Spheroidal Galaxies

    NASA Astrophysics Data System (ADS)

    Kirby, Evan N.; Fu, Xiaoting; Guhathakurta, Puragra; Deng, Licai

    2012-06-01

    Stars destroy lithium (Li) in their normal evolution. The convective envelopes of evolved red giants reach temperatures of millions of kelvin, hot enough for the 7Li(p, α)4He reaction to burn Li efficiently. Only about 1% of first-ascent red giants more luminous than the luminosity function bump in the red giant branch exhibit A(Li) > 1.5. Nonetheless, Li-rich red giants do exist. We present 15 Li-rich red giants—14 of which are new discoveries—among a sample of 2054 red giants in Milky Way dwarf satellite galaxies. Our sample more than doubles the number of low-mass, metal-poor ([Fe/H] <~ -0.7) Li-rich red giants, and it includes the most-metal-poor Li-enhanced star known ([Fe/H] = -2.82, A(Li)NLTE = 3.15). Because most of the stars have Li abundances larger than the universe's primordial value, the Li in these stars must have been created rather than saved from destruction. These Li-rich stars appear like other stars in the same galaxies in every measurable regard other than Li abundance. We consider the possibility that Li enrichment is a universal phase of evolution that affects all stars, and it seems rare only because it is brief. The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.

  20. Can brown dwarfs survive on close orbits around convective stars?

    NASA Astrophysics Data System (ADS)

    Damiani, C.; Díaz, R. F.

    2016-05-01

    Context. The mass range of brown dwarfs extends across the planetary domain to stellar objects. There is a relative paucity of brown dwarfs companions around FGKM-type stars compared to exoplanets for orbital periods of less than a few years, but most of the short-period brown dwarf companions that are fully characterised by transits and radial velocities are found around F-type stars. Aims: We examine the hypothesis that brown dwarf companions could not survive on close orbit around stars with important convective envelopes because the tides and angular momentum loss, the result of magnetic braking, would lead to a rapid orbital decay with the companion being quickly engulfed. Methods: We use a classical Skumanich-type braking law and constant time-lag tidal theory to assess the characteristic timescale for orbital decay for the brown dwarf mass range as a function of the host properties. Results: We find that F-type stars may host massive companions for a significantly longer time than G-type stars for a given orbital period, which may explain the paucity of G-type hosts for brown dwarfs with an orbital period less than five days. On the other hand, we show that the small radius of early M-type stars contributes to orbital decay timescales that are only half those of F-type stars, despite their more efficient tidal dissipation and magnetic braking. For fully convective later type M-dwarfs, orbital decay timescales could be orders of magnitude greater than for F-type stars. Moreover, we find that, for a wide range of values of tidal dissipation efficiency and magnetic braking, it is safe to assume that orbital decay for massive companions can be neglected for orbital periods greater than ten days. Conclusions: For orbital periods greater than ten days, brown dwarf occurrence should largely be unaffected by tidal decay, whatever the mass of the host. On closer orbital periods, the rapid engulfment of massive companions could explain the lack of G and K-type hosts

  1. An extrasolar extreme-ultraviolet object. II - The nature of HZ 43. [hot white dwarf star

    NASA Technical Reports Server (NTRS)

    Margon, B.; Liebert, J.; Lampton, M.; Spinrad, H.; Bowyer, S.; Gatewood, G.

    1976-01-01

    A variety of data are presented concerning the spectrum, distance, temperature, and evolutionary state of the hot white dwarf HZ 43, the first extrasolar object to be detected in the EUV band. The data include spectrophotometry of the star and its red dwarf companion (HZ 43B), a trigonometric parallax for the star, its tangential velocity, and results of soft X-ray and EUV observations. The main conclusions are that: (1) the spectrum of HZ 43A is that of a hot DAwk star, (2) HZ 43B is a dM3.5e star, (3) the distance of the system is about 65 pc, (4) the tangential velocity is not atypical of white dwarfs, and (5) the stellar energy distribution of HZ 43A is well fitted by a black body with an effective temperature of approximately 110,000 K. Evolutionary implications of the existence of an object as hot as HZ 43A are briefly considered, and it is suggested that the progenitors of hot DA stars must include objects hotter than spectral type sdB, with logical possibilities being nuclei of planetary nebulae and sdO stars.

  2. Asteroseismology of Red Giant stars

    NASA Astrophysics Data System (ADS)

    Tarrant, N. J.; Chaplin, W. J.; Elsworth, Y. P.; Spreckley, S. A.; Stevens, I. R.

    2008-12-01

    Sun-like oscillations, that is p-modes excited stochastically by convective noise, have now been observed in a number of Red Giant stars. Compared to those seen in the Sun, these modes are of large amplitude and long period, making the oscillations attractive prospects for observation. However, the low Q-factor of these modes, and issues relating to the rising background at low frequencies, present some interesting challenges for identifying modes and determining the related asteroseismic parameters. We report on the analysis procedure adopted for peak-bagging by our group at Birming- ham, and the techniques used to robustly ensure these are not a product of noise. I also show results from a number of giants extracted from multi-year observations with the SMEI instrument

  3. Infrared Spectral Energy Distributions of Nearby Dwarf Carbon Stars

    NASA Astrophysics Data System (ADS)

    Lowrance, Patrick

    2014-06-01

    The discovery of G77-61 (Dahn et al. 1977) -- a star with a carbon-rich spectrum a mere 58 pc away and therefore of relatively low luminosity -- led to the recognition that _dwarf_ carbon (dC) stars exist. As more dCs are now known, the accepted paradigm of the presence of atmospheric carbon is that dCs must contain a white dwarf secondary. While the white dwarf companion was going through an AGB stage, it deposited carbon-rich material in the atmosphere of the lower-mass (and now brighter) dwarf star. Indeed, a handful of the dC's have exhibited radial velocity signatures consistent with this picture. To allow for the carbon to still be present in the atmosphere past the AGB stage, a replenishing outer shell or disk has been proposed. Current understanding of the formation and evolution of a dC is, however, limited by the small number of objects and observations. We present a full range of fluxes and flux limits from 1 - 160 um including 2MASS, WISE, Spitzer, and Herschel observations for a list of the nearest carbon dwarfs. We reconstruct the spectral energy distribution exploring the mid-infrared region where any residual debris disks would be detectable. The carbon dwarfs have been historically studied in the visible, and these new infrared observations provide a picture of the circumstellar dust.

  4. Einstein solid state spectrometer observation of the peculiar red dwarf Wolf 630 AB

    NASA Technical Reports Server (NTRS)

    Swank, J. H.; Johnson, H. M.

    1982-01-01

    Wolf 630 AB is a double and perhaps triple star with a predominant dM 3.5e spectrum. It is one of the relatively strong red dwarf X-ray sources. The 0.5 to 4 keV spectral data for a steady, non-flaring flux are interpreted in terms of emission from thin thermal plasma with a dominant temperature of approximately 6,500,000 K. Both in temperature and average surface flux the quiescent corona is similar to that of the low temperature component found for RS Canum Venaticorum binaries. There is an indication of additional emission above 10 to the 7th power K, but the ratio of high to low temperature emission is smaller than for typical RS CVn systems. The solid state spectrometer observed the spectrum of only one other red dwarf, Ad Leo, which is very similar to that observed for Wolf 630 AB.

  5. Evidence for dwarf stars at D of about 100 kiloparsecs near the Sextans dwarf spheroidal galaxy

    NASA Astrophysics Data System (ADS)

    Gould, Andrew; Guhathakurta, Puragra; Richstone, Douglas; Flynn, Chris

    1992-04-01

    A method is presented for detecting individual, metal-poor, dwarf stars at distances less than about 150 kpc - a method specifically designed to filter out stars from among the much more numerous faint background field galaxies on the basis of broad-band colors. This technique is applied to two fields at high Galactic latitude, for which there are deep CCD data in four bands ranging from 3600 to 9000 A. The field in Sextans probably contains more than about five dwarf stars with BJ not greater than 25.5. These are consistent with being at a common distance about 100 kpc and lie about 1.7 deg from the newly discovered dwarf galaxy in Sextans whose distance is about 85 +/- 10 kpc. The stars lie near the major axis of the galaxy and are near or beyond the tidal radius. The second field, toward the south Galactic pole, may contain up to about five extra-Galactic stars, but these show no evidence for being at a common distance. Possible applications of this type technique are discussed, and it is shown that even very low surface brightness star clusters or dwarf galaxies may be detected at distances less than about 1 Mpc.

  6. Evidence for dwarf stars at D of about 100 kiloparsecs near the Sextans dwarf spheroidal galaxy

    NASA Technical Reports Server (NTRS)

    Gould, Andrew; Guhathakurta, Puragra; Richstone, Douglas; Flynn, Chris

    1992-01-01

    A method is presented for detecting individual, metal-poor, dwarf stars at distances less than about 150 kpc - a method specifically designed to filter out stars from among the much more numerous faint background field galaxies on the basis of broad-band colors. This technique is applied to two fields at high Galactic latitude, for which there are deep CCD data in four bands ranging from 3600 to 9000 A. The field in Sextans probably contains more than about five dwarf stars with BJ not greater than 25.5. These are consistent with being at a common distance about 100 kpc and lie about 1.7 deg from the newly discovered dwarf galaxy in Sextans whose distance is about 85 +/- 10 kpc. The stars lie near the major axis of the galaxy and are near or beyond the tidal radius. The second field, toward the south Galactic pole, may contain up to about five extra-Galactic stars, but these show no evidence for being at a common distance. Possible applications of this type technique are discussed, and it is shown that even very low surface brightness star clusters or dwarf galaxies may be detected at distances less than about 1 Mpc.

  7. M-dwarf binaries as tracers of star and brown dwarf formation

    NASA Astrophysics Data System (ADS)

    Marks, Michael; Janson, Markus; Kroupa, Pavel; Leigh, Nathan; Thies, Ingo

    2015-09-01

    The separation distribution for M-dwarf binaries in the AstraLux survey is narrower and peaking at smaller separations than the distribution for solar-type binaries. This is often interpreted to mean that M-dwarfs constitute a continuous transition from brown dwarfs (BDs) to stars. Here, a prediction for the M-dwarf separation distribution is presented, using a dynamical population synthesis (DPS) model in which `star-like' binaries with late-type primaries (≲1.5 M⊙) follow universal initial distribution functions and are dynamically processed in their birth embedded clusters. A separate `BD-like' population has both its own distribution functions for binaries and initial mass function (IMF), which overlaps in mass with the IMF for stars. Combining these two formation modes results in a peak on top of a wider separation distribution for late M-dwarfs consistent with the late AstraLux sample. The DPS separation distribution for early M-dwarfs shows no such peak and is in agreement with the M-dwarfs in Multiples (MinMS) data. We note that the latter survey is potentially in tension with the early AstraLux data. Concluding, the AstraLux and MinMS data are unable to unambiguously distinguish whether or not BDs are a continuous extension of the stellar IMF. Future observational efforts are needed to fully answer this interesting question. The DPS model predicts that binaries outside the sensitivity range of the AstraLux survey remain to be detected. For application to future data, we present a means to observationally measure the overlap of the putative BD-like branch and the stellar branch. We discuss the meaning of universal star formation and distribution functions.

  8. Inclusion of horizontal branch stars in the derivation of star formation histories of dwarf galaxies: The Carina dSph

    NASA Astrophysics Data System (ADS)

    Savino, Alessandro; Salaris, Maurizio; Tolstoy, Eline

    2015-11-01

    We present a detailed analysis of the horizontal branch of the Carina dwarf spheroidal galaxy by means of synthetic modelling techniques, taking consistently into account the star formation history and metallicity evolution as determined from main sequence and red giant branch spectroscopic observations. We found that a range of integrated red giant branch mass loss values of 0.1-0.14 M⊙ increasing with metallicity is able to reproduce the colour extension of the old horizontal branch. Nonetheless, leaving the mass loss as the only free parameter is not enough to match the detailed morphology of Carina horizontal branch. We then investigated the role played by the star formation history on the discrepancies between synthetic and observed horizontal branches. We derived a "toy" bursty star formation history that reproduces well the observed horizontal branch star counts, and also matches qualitatively the red giant and the turn-off regions. This bursty star formation history is made of a subset of age and [M/H] components of the star formation history based on turn off and red giants only, and entails four separate bursts of star formation of different strengths, centred at 2, 5, 8.6, and 11.5 Gyr, respectively, with mean [M/H] decreasing from ~-1.7 to ~-2.2 when the age of the burst increases, and with a Gaussian spread of σ 0.1 dex around these mean values. The comparison between the metallicity distribution function of our bursty star formation history and the one measured from the infrared CaT feature using a CaT-[Fe/H] calibration shows a qualitative agreement, once the range of [Ca/Fe] abundances measured in a sample of Carina stars have been taken into account, that causes a bias of the derived [Fe/H] distribution toward values that are too low. In conclusion, we show how the information contained within the horizontal branch of Carina (and dwarf galaxies in general) can be extracted and interpreted to refine the star formation history derived exclusively

  9. Circumstellar debris and pollution at white dwarf stars

    NASA Astrophysics Data System (ADS)

    Farihi, J.

    2016-04-01

    Circumstellar disks of planetary debris are now known or suspected to closely orbit hundreds of white dwarf stars. To date, both data and theory support disks that are entirely contained within the preceding giant stellar radii, and hence must have been produced during the white dwarf phase. This picture is strengthened by the signature of material falling onto the pristine stellar surfaces; disks are always detected together with atmospheric heavy elements. The physical link between this debris and the white dwarf host abundances enables unique insight into the bulk chemistry of extrasolar planetary systems via their remnants. This review summarizes the body of evidence supporting dynamically active planetary systems at a large fraction of all white dwarfs, the remnants of first generation, main-sequence planetary systems, and hence provide insight into initial conditions as well as long-term dynamics and evolution.

  10. Star Formation History of the Fornax Dwarf Spheroidal Galaxy

    NASA Astrophysics Data System (ADS)

    Del Pino, A.; Aparicio, A.; Gallart, C.; Hidalgo, S.

    2011-07-01

    We present color-magnitude diagrams (CMD) reaching the oldest main sequence turn offs for three fields in the Fornax dwarf spheroidal galaxy, obtained with FORS1 at the VLT. One of them is situated at the center of the galaxy while the other two are located at a distance of 10' form the center. We determine the full star formation history (SFH) of the central field, extending to the first star formation events.

  11. White dwarf stars with chemically stratified atmospheres

    NASA Technical Reports Server (NTRS)

    Muchmore, D.

    1982-01-01

    Recent observations and theory suggest that some white dwarfs may have chemically stratified atmospheres - thin layers of hydrogen lying above helium-rich envelopes. Models of such atmospheres show that a discontinuous temperature inversion can occur at the boundary between the layers. Model spectra for layered atmospheres at 30,000 K and 50,000 K tend to have smaller decrements at 912 A, 504 A, and 228 A than uniform atmospheres would have. On the basis of their continuous extreme ultraviolet spectra, it is possible to distinguish observationally between uniform and layered atmospheres for hot white dwarfs.

  12. White Dwarf Stars in the HET Dark Energy Experiment

    NASA Astrophysics Data System (ADS)

    Castanheira, Barbara; Winget, D.; Gebhardt, K.; Allende Prieto, C.; Shetrone, M.; Odewahn, S.; Montgomery, M. H.

    2012-01-01

    In this poster, we present the project that will survey all white dwarf stars observed in the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) and the Visible Integral-field Replicable Unit Spectrograph (VIRUS) observations in parallel mode. The final product will be a unique magnitude-limited catalog of as many as 10,000 stars. Since we will use data from an Integral-field Units, our survey will be free of the selection biases that plagued preceding surveys, e.g. the Sloan Digital Sky Survey (SDSS). The critical advantages of our program are our ability to produce a white dwarf luminosity function five magnitudes fainter than the one derived from the Palomar-Green survey and with a similar number of faint stars as the one from SDSS. Our project will help to derive a more precise age of the Galactic disk, and will provide fundamental information about the white dwarf population and the star formation history of the Milky Way, impacting the white dwarf field and many other fields of astronomy.

  13. Models of very-low-mass stars, brown dwarfs and exoplanets.

    PubMed

    Allard, F; Homeier, D; Freytag, B

    2012-06-13

    Within the next few years, GAIA and several instruments aiming to image extrasolar planets will be ready. In parallel, low-mass planets are being sought around red dwarfs, which offer more favourable conditions, for both radial velocity detection and transit studies, than solar-type stars. In this paper, the authors of a model atmosphere code that has allowed the detection of water vapour in the atmosphere of hot Jupiters review recent advances in modelling the stellar to substellar transition. The revised solar oxygen abundances and cloud model allow the photometric and spectroscopic properties of this transition to be reproduced for the first time. Also presented are highlight results of a model atmosphere grid for stars, brown dwarfs and extrasolar planets. PMID:22547243

  14. Models of very-low-mass stars, brown dwarfs and exoplanets

    PubMed Central

    Allard, F.; Homeier, D.; Freytag, B.

    2012-01-01

    Within the next few years, GAIA and several instruments aiming to image extrasolar planets will be ready. In parallel, low-mass planets are being sought around red dwarfs, which offer more favourable conditions, for both radial velocity detection and transit studies, than solar-type stars. In this paper, the authors of a model atmosphere code that has allowed the detection of water vapour in the atmosphere of hot Jupiters review recent advances in modelling the stellar to substellar transition. The revised solar oxygen abundances and cloud model allow the photometric and spectroscopic properties of this transition to be reproduced for the first time. Also presented are highlight results of a model atmosphere grid for stars, brown dwarfs and extrasolar planets. PMID:22547243

  15. A reappraisal of the habitability of planets around M dwarf stars.

    PubMed

    Tarter, Jill C; Backus, Peter R; Mancinelli, Rocco L; Aurnou, Jonathan M; Backman, Dana E; Basri, Gibor S; Boss, Alan P; Clarke, Andrew; Deming, Drake; Doyle, Laurance R; Feigelson, Eric D; Freund, Friedmann; Grinspoon, David H; Haberle, Robert M; Hauck, Steven A; Heath, Martin J; Henry, Todd J; Hollingsworth, Jeffery L; Joshi, Manoj M; Kilston, Steven; Liu, Michael C; Meikle, Eric; Reid, I Neill; Rothschild, Lynn J; Scalo, John; Segura, Antigona; Tang, Carol M; Tiedje, James M; Turnbull, Margaret C; Walkowicz, Lucianne M; Weber, Arthur L; Young, Richard E

    2007-02-01

    Stable, hydrogen-burning, M dwarf stars make up about 75% of all stars in the Galaxy. They are extremely long-lived, and because they are much smaller in mass than the Sun (between 0.5 and 0.08 M(Sun)), their temperature and stellar luminosity are low and peaked in the red. We have re-examined what is known at present about the potential for a terrestrial planet forming within, or migrating into, the classic liquid-surface-water habitable zone close to an M dwarf star. Observations of protoplanetary disks suggest that planet-building materials are common around M dwarfs, but N-body simulations differ in their estimations of the likelihood of potentially habitable, wet planets that reside within their habitable zones, which are only about one-fifth to 1/50th of the width of that for a G star. Particularly in light of the claimed detection of the planets with masses as small as 5.5 and 7.5 M(Earth) orbiting M stars, there seems no reason to exclude the possibility of terrestrial planets. Tidally locked synchronous rotation within the narrow habitable zone does not necessarily lead to atmospheric collapse, and active stellar flaring may not be as much of an evolutionarily disadvantageous factor as has previously been supposed. We conclude that M dwarf stars may indeed be viable hosts for planets on which the origin and evolution of life can occur. A number of planetary processes such as cessation of geothermal activity or thermal and nonthermal atmospheric loss processes may limit the duration of planetary habitability to periods far shorter than the extreme lifetime of the M dwarf star. Nevertheless, it makes sense to include M dwarf stars in programs that seek to find habitable worlds and evidence of life. This paper presents the summary conclusions of an interdisciplinary workshop (http://mstars.seti.org) sponsored by the NASA Astrobiology Institute and convened at the SETI Institute.

  16. Star formation history of And XVIII: a dwarf spheroidal galaxy in isolation

    NASA Astrophysics Data System (ADS)

    Makarova, L. N.; Makarov, D. I.; Karachentsev, I. D.; Tully, R. B.; Rizzi, L.

    2016-10-01

    We present a photometric study of the Andromeda XVIII dwarf spheroidal galaxy associated with M31, and situated well outside of the virial radius of the M31 halo. The galaxy was resolved into stars with Hubble Space Telescope/Advanced Camera for Surveys revealing the old red giant branch and red clump. With the new observational data we determined the Andromeda XVIII distance to be D = 1.33_{-0.09}^{+0.06} Mpc using the tip of red giant branch method. Thus, the dwarf is situated at the distance of 579 kpc from M31. We model the star formation history of Andromeda XVIII from the stellar photometry and Padova theoretical stellar isochrones. An ancient burst of star formation occurred 12-14 Gyr ago. There is no sign of recent/ongoing star formation in the last 1.5 Gyr. The mass fractions of the ancient and intermediate age stars are 34 and 66 per cent, respectively, and the total stellar mass is 4.2 × 106 M⊙. It is probable that the galaxy has not experienced an interaction with M31 in the past. We also discuss star formation processes of dSphs KKR 25, KKs 03, as well as dTr KK 258. Their star formation histories were uniformly measured by us from HST/ACS observations. All the galaxies are situated well beyond the Local Group and the two dSphs KKR 25 and KKs 03 are extremely isolated. Evidently, the evolution of these objects has proceeded without influence of neighbours.

  17. The Star Formation Properties of Void Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Moorman, Crystal; Vogeley, Michael S.

    2016-01-01

    We measure the star formation properties of two large samples of galaxies from the SDSS in large-scale cosmic voids on time scales of 10 Myr and 100 Myr, using Ha emission line strengths and GALEX FUV fluxes, respectively. The first sample consists of 109,818 optically selected galaxies. We find that void galaxies in this sample have higher specific star formation rates (SSFRs; star formation rates per unit stellar mass) than similar stellar mass galaxies in denser regions. The second sample is a subset of the optically selected sample containing 8070 galaxies with reliable S/N HI detections from ALFALFA. For the HI detected sample, SSFRs are similar regardless of large-scale environment. Investigating only the HI detected dwarf galaxies reveals a trend towards higher SSFRs in voids. Furthermore, we estimate the star formation rate per unit HI mass, known as the star formation efficiency (SFE) of a galaxy, as a function of environment. For the overall HI detected population, we notice no environmental dependence. Limiting the sample to dwarf galaxies again reveals a trend towards higher SFEs in voids. These results suggest that void environments provide a nurturing environment for dwarf galaxy evolution.

  18. Variable stars in the Pegasus dwarf galaxy (DDO 216)

    NASA Technical Reports Server (NTRS)

    Hoessel, J. G.; Abbott, Mark J.; Saha, A.; Mossman, Amy E.; Danielson, G. Edward

    1990-01-01

    Observations obtained over a period of five years of the resolved stars in the Pegasus dwarf irregular galaxy (DDO 216) have been searched for variable stars. Thirty-one variables were found, and periods established for 12. Two of these variable stars are clearly eclipsing variables, seven are very likely Cepheid variables, and the remaining three are probable Cepheids. The period-luminosity relation for the Cepheids indicates a distance modulus for Pegasus of m - M = 26.22 + or - 0.20. This places Pegasus very near the zero-velocity surface of the Local Group.

  19. Scl-1013644: a CEMP-s star in the Sculptor Dwarf Spheroidal Galaxy

    NASA Astrophysics Data System (ADS)

    Salgado, C.; Da Costa, G. S.; Yong, D.; Norris, J. E.

    2016-08-01

    Recent studies of the Milky Way and its satellites have paid special attention to the importance of carbon-enhanced metal-poor (CEMP) stars due to their involvement in Galactic formation history and their possible connection with the chemical elements originating in the first stellar generation. In an ongoing study of red giants in the Sculptor dwarf galaxy we have discovered a star with extremely strong CN and CH molecular bands. This star, Scl-1013644, has previously been identified by Geisler et al. (2005) as a star with an enrichment in the heavy elements. Spectrum synthesis has been used to derive the carbon, nitrogen and barium abundances for Scl-1013644. Our findings are [C/Fe] = +0.8, [N/Fe] = -0.3 and [Ba/Fe] = +2.1 with the latter result consistent with the value found by Geisler et al. (2005). These results reveal Scl-1013644 as a CEMP-s star, the third such star discovered in this dwarf galaxy.

  20. Scl-1013644: a CEMP-s star in the Sculptor dwarf spheroidal galaxy

    NASA Astrophysics Data System (ADS)

    Salgado, C.; Da Costa, G. S.; Yong, D.; Norris, J. E.

    2016-11-01

    Recent studies of the Milky Way and its satellites have paid special attention to the importance of carbon-enhanced metal-poor (CEMP) stars due to their involvement in Galactic formation history and their possible connection with the chemical elements originating in the first stellar generation. In an ongoing study of red giants in the Sculptor dwarf galaxy, we have discovered a star with extremely strong CN and CH molecular bands. This star, Scl-1013644, has previously been identified by Geisler et al. as a star with an enrichment in the heavy elements. Spectrum synthesis has been used to derive the carbon, nitrogen and barium abundances for Scl-1013644. Our findings are [C/Fe] = +0.8, [N/Fe] = -0.3 and [Ba/Fe] = +2.1 with the latter result consistent with the value found by Geisler et al. These results reveal Scl-1013644 as a CEMP-s star, the third such star discovered in this dwarf galaxy.

  1. SWP Echelle Spectra of Chromospherically Active Dwarf Stars

    NASA Astrophysics Data System (ADS)

    Ayres, Thomas R.

    High resolution spectra of the 1150-2000 A region are enormously valuable for probing outer- atmosphere structure in cool stars. For example, such data can be used to separate blends, identify individual emission components in short-period binary systems, determine intensity ratios in close multiplets, estimate reliable emission strengths of lines superimposed on bright stellar continua, and test for the presence or absence of stellar winds at 105 K temperatures. These possibilities are not practical with IUE low-dispersion spectra. However, one must pay a steep-price to obtain useable high-dispersion IUE spectra and the additional dimension of diagnostic information, namely only a handful of the brightest UV sources are accessible even with shift-long exposures. We propose below an observing program to obtain echelle spectra of chromospherically active dwarf stars in the 1150-2000 A shortwavelength region. This program is intended to explore a particular class of objects that heretofore have not been observed at high dispersion with the SWP camera. Futhermore, this program complements previous SWP echelle studies by our group at the University of Colorado of quiet-chromosphere dwarf stars (alpha Cen A, alpha Cen B), active giants (alpha Aur A, lambda And, beta Dra), and the extreme case of the very active RS CVn-type system HR 1099. As described below, highdispersion spectra of these targets have provided a critical interpretive dimension that was lacking in previous low-dispersion studies. However, several fundamental questions have been raised in the course of our exploratory SWP work on what, in practice, are two distinct classes of chromospheric stars: the quiet dwarfs and the active giants. We feel that many of these questions can be answered by bridging the interpretive gap with a careful study of the active dwarfs. Our recent experience with shift-long SWP echelle exposures of chromospheric emission stars has suggested that our previous estimates of

  2. The Chromospheric Activity-Age Relation for M Dwarf Stars

    NASA Astrophysics Data System (ADS)

    Silvestri, N. M.; Oswalt, T. D.; Hawley, S. L.

    2000-12-01

    We present preliminary results from our study in which we use moderate resolution spectroscopy to determine the correlation between the chromospheric activity and age of M dwarf stars in wide binary systems. We have observed ~50 M dwarf stars from our sample with the Apache Point Observatory 3.5-m telescope. We measure the ratio of Hα luminosity to the bolometric luminosity (LHα /Lbol) of the M dwarf---a measure of activity that is proven to correlate well with age. This project is unique in that it will extend the chromospheric activity-age relation of low-mass main sequence stars beyond the ages provided by cluster methods. The ages so determined are also independent of the uncertainties in cluster age determinations. The technique has the potential to improve by at least a factor of two the precision and the range over which ages can currently be determined for main sequence stars. Work on this project is supported by the NASA Graduate Student Researchers Program grant NGT-50290 (N.M.S.).

  3. Formation of Low-Mass Stars and Brown Dwarfs

    NASA Astrophysics Data System (ADS)

    Hennebelle, P.

    2012-11-01

    These lectures attempt to expose the most important ideas, which have been proposed to explain the formation of stars with particular emphasis on the formation of brown dwarfs and low-mass stars. We first describe the important physical processes which trigger the collapse of a self-gravitating piece of fluid and regulate the star formation rate in molecular clouds. Then we review the various theories which have been proposed along the years to explain the origin of the stellar initial mass function paying particular attention to four models, namely the competitive accretion and the theories based respectively on stopped accretion, MHD shocks and turbulent dispersion. As it is yet unsettled whether the brown dwarfs form as low-mass stars, we present the theory of brown dwarfs based on disk fragmentation stressing all the uncertainties due to the radiative feedback and magnetic field. Finally, we describe the results of large scale simulations performed to explain the collapse and fragmentation of molecular clouds.

  4. Stellar, brown dwarf and multiple star properties from a radiation hydrodynamical simulation of star cluster formation

    NASA Astrophysics Data System (ADS)

    Bate, Matthew R.

    2012-02-01

    We report the statistical properties of stars, brown dwarfs and multiple systems obtained from the largest radiation hydrodynamical simulation of star cluster formation to date that resolves masses down to the opacity limit for fragmentation (a few Jupiter masses). The initial conditions are identical to those of previous barotropic calculations published by Bate, but this time the calculation is performed using a realistic equation of state and radiation hydrodynamics. The calculation uses sink particles to model 183 stars and brown dwarfs, including 28 binaries and 12 higher-order multiple systems, the properties of which are compared to the results from observational surveys. We find that the radiation hydrodynamical/sink particle simulation reproduces many observed stellar properties very well. In particular, whereas using a barotropic equation of state produces more brown dwarfs than stars, the inclusion of radiative feedback results in a stellar mass function and a ratio of brown dwarfs to stars in good agreement with observations of Galactic star-forming regions. In addition, many of the other statistical properties of the stars and brown dwarfs are in reasonable agreement with observations, including multiplicity as a function of primary mass, the frequency of very low mass binaries, and general trends for the mass ratio and separation distributions of binaries. We also examine the velocity dispersion of the stars, the distributions of disc truncation radii due to dynamical interactions, and coplanarity of orbits and sink particle spins in multiple systems. Overall, the calculation produces a cluster of stars whose statistical properties are difficult to distinguish from observed systems, implying that gravity, hydrodynamics and radiative feedback are the primary ingredients for determining the origin of the statistical properties of low-mass stars.

  5. Delayed star formation in isolated dwarf galaxies: Hubble space telescope star formation history of the Aquarius dwarf irregular

    SciTech Connect

    Cole, Andrew A.; Weisz, Daniel R.; Dolphin, Andrew E.; Skillman, Evan D.; McConnachie, Alan W.; Brooks, Alyson M.; Leaman, Ryan E-mail: drw@ucsc.edu E-mail: skillman@astro.umn.edu E-mail: abrooks@physics.rutgers.edu

    2014-11-01

    We have obtained deep images of the highly isolated (d = 1 Mpc) Aquarius dwarf irregular galaxy (DDO 210) with the Hubble Space Telescope Advanced Camera for Surveys. The resulting color-magnitude diagram (CMD) reaches more than a magnitude below the oldest main-sequence turnoff, allowing us to derive the star formation history (SFH) over the entire lifetime of the galaxy with a timing precision of ≈10% of the lookback time. Using a maximum likelihood fit to the CMD we find that only ≈10% of all star formation in Aquarius took place more than 10 Gyr ago (lookback time equivalent to redshift z ≈ 2). The star formation rate increased dramatically ≈6-8 Gyr ago (z ≈ 0.7-1.1) and then declined until the present time. The only known galaxy with a more extreme confirmed delay in star formation is Leo A, a galaxy of similar M {sub H} {sub I}/M {sub *}, dynamical mass, mean metallicity, and degree of isolation. The delayed stellar mass growth in these galaxies does not track the mean dark matter accretion rate from CDM simulations. The similarities between Leo A and Aquarius suggest that if gas is not removed from dwarf galaxies by interactions or feedback, it can linger for several gigayears without cooling in sufficient quantity to form stars efficiently. We discuss possible causes for the delay in star formation including suppression by reionization and late-time mergers. We find reasonable agreement between our measured SFHs and select cosmological simulations of isolated dwarfs. Because star formation and merger processes are both stochastic in nature, delayed star formation in various degrees is predicted to be a characteristic (but not a universal) feature of isolated small galaxies.

  6. Dwarf galaxies in the coma cluster: Star formation properties and evolution

    NASA Astrophysics Data System (ADS)

    Hammer, Derek M.

    in the process of being quenched or were only recently quenched. We modeled the quenching timescales for transition galaxies, or “green valley” objects, and found that the majority are quenched in less than 1 Gyr. This timescale is consistent with rapid dynamical processes that are active in the cluster environment as opposed to the more gradual quenching mechanisms that exist in the group environment. For the passive galaxy population, we have measured an average stellar age of 6-8 Gyr for the red sequence which is consistent with previous studies based on spectroscopic observations. We note that the star formation properties of Coma member galaxies were established from photometry alone, as opposed to using spectroscopic data which are more challenging to obtain for dwarf galaxies. We have measured the faintest UV luminosity functions (LFs) presented for a rich galaxy cluster thus far. The Coma UV LFs are 3.5 mag fainter than previous studies in Coma, and are sufficiently deep that we reach the dwarf passive galaxy population for the first time. We have introduced a new technique for measuring the LF which avoids color selection effects associated with previous methods. The UV LFs constructed separately for star-forming and passive galaxies follow a similar distribution at faint magnitudes, which suggests that the recent quenching of infalling dwarf star-forming galaxies is sufficient to build the dwarf passive population in Coma. The Coma UV LFs show a turnover at faint magnitudes as compared to the field, owing to a deficit of dwarf galaxies with stellar masses below M∗ = 108 M⊙ . We show that the UV LFs for the field behind the Coma cluster are nearly identical to the average field environment, and do not show evidence for a turnover at faint magnitudes. We suspect that the missing dwarf galaxies in Coma are severely disrupted by tidal processes as they are accreted onto the cluster, just prior to reaching the infall region studied here.

  7. 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.

  8. Spectroscopy of Six Red Giants in the Draco Dwarf Spheroidal Galaxy

    NASA Astrophysics Data System (ADS)

    Smith, Graeme H.; Siegel, Michael H.; Shetrone, Matthew D.; Winnick, Rebeccah

    2006-10-01

    Keck Observatory LRIS-B (Low Resolution Imaging Spectrometer) spectra are reported for six red giant stars in the Draco dwarf spheroidal galaxy and several comparison giants in the globular cluster M13. Indexes that quantify the strengths of the Ca II H and K lines, the λ3883 and λ4215 CN bands, and the λ4300 G band have been measured. These data confirm evidence of metallicity inhomogeneity within Draco obtained by previous authors. The four brightest giants in the sample have absolute magnitudes in the range -2.6stars may have higher [C/Fe] ratios than globular cluster red giants: deep mixing might be inhibited in these Draco stars, they may formerly have been mass-transfer binaries that acquired carbon from a more massive companion, or the Draco dwarf galaxy may have experienced relatively slow chemical evolution over a period of several billion years, allowing carbon-enhanced ejecta from intermediate-mass asymptotic giant branch stars to enrich the interstellar medium while star formation was still occurring. The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.

  9. Sulfur and zinc abundances of red giant stars

    NASA Astrophysics Data System (ADS)

    Takeda, Yoichi; Omiya, Masashi; Harakawa, Hiroki; Sato, Bun'ei

    2016-08-01

    Sulfur and zinc are chemically volatile elements, which play significant roles as depletion-free tracers in studying galactic chemical evolution. However, regarding red giants having evolved off the main sequence, reliable abundance determinations of S and Zn seem to be difficult, despite the several studies that have been reported so far. Given this situation, we tried to establish the abundances of these elements for an extensive sample of 239 field GK giants ( - 0.8 ≲ [Fe/H] ≲ +0.2), by applying the spectrum-fitting technique to S I 8694-5, S I 6757, and Zn I 6362 lines and by taking into account the non-LTE effect. Besides, similar abundance analysis was done for 160 FGK dwarfs to be used for comparison. The non-LTE corrections for the S and Zn abundances derived from these lines turned out to be ≲ 0.1(-0.2) dex for most cases and not very significant. It revealed that the S I 6757 feature is more reliable as an abundance indicator than S I 8694-5 for the case of red giants, because the latter suffers blending of unidentified lines. The finally resulting [S/Fe]-[Fe/H] and [Zn/Fe]-[Fe/H] relations for GK giants were confirmed to be in good agreement with those for FGK dwarfs, indicating that S and Zn abundances of red giants are reliably determinable from the S I 6757 and Zn I 6362 lines. Accordingly, not only main-sequence stars but also evolved red giant stars are usable for tracing the chemical evolution history of S and Zn in the regime of disk metallicity by using these lines.

  10. 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.

  11. 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.

  12. Neutron star formation in theoretical supernovae. Low mass stars and white dwarfs

    SciTech Connect

    Nomoto, K.

    1986-01-01

    The presupernova evolution of stars that form semi-degenerate or strongly degenerate O + Ne + Mg cores is discussed. For the 10 to 13 Msub solar stars, behavior of off-center neon flashes is crucial. The 8 to 10 m/sub solar stars do not ignite neon and eventually collapse due to electron captures. Properties of supernova explosions and neutron stars expected from these low mass progenitors are compared with the Crab nebula. The conditions for which neutron stars form from accretion-induced collapse of white dwarfs in clsoe binary systems is also examined.

  13. New white dwarf and subdwarf stars in the Sloan Digital Sky Survey Data Release 12

    NASA Astrophysics Data System (ADS)

    Kepler, S. O.; Pelisoli, I.; Koester, D.; Ourique, G.; Romero, A. D.; Reindl, N.; Kleinman, S. J.; Eisenstein, D. J.; Valois, A. D. M.; Amaral, L. A.

    2016-02-01

    We report the discovery of 6576 new spectroscopically confirmed white dwarf and subdwarf stars in the Sloan Digital Sky Survey Data Release 12. We obtain Teff, log g and mass for hydrogen atmosphere white dwarf stars (DAs) and helium atmosphere white dwarf stars (DBs), estimate the calcium/helium abundances for the white dwarf stars with metallic lines (DZs) and carbon/helium for carbon-dominated spectra (DQs). We found one central star of a planetary nebula, one ultracompact helium binary (AM CVn), one oxygen line-dominated white dwarf, 15 hot DO/PG1159s, 12 new cataclysmic variables, 36 magnetic white dwarf stars, 54 DQs, 115 helium-dominated white dwarfs, 148 white dwarf + main-sequence star binaries, 236 metal-polluted white dwarfs, 300 continuum spectra DCs, 230 hot subdwarfs, 2936 new hydrogen-dominated white dwarf stars, and 2675 cool hydrogen-dominated subdwarf stars. We calculate the mass distribution of all 5883 DAs with S/N ≥ 15 in DR12, including the ones in DR7 and DR10, with an average S/N = 26, corrected to the 3D convection scale, and also the distribution after correcting for the observed volume, using 1/Vmax.

  14. Variable stars in the Leo A dwarf galaxy (DDO 69)

    NASA Technical Reports Server (NTRS)

    Hoessel, John G.; Saha, A.; Krist, John; Danielson, G. Edward

    1994-01-01

    Observations of the Leo A dwarf galaxy, obtained over the period from 1980 to 1991 are reported. Forty two separate Charge Coupled Devices (CCD) frames were searched for variable stars. A total of 14 suspected variables were found, 9 had sufficient coverage for period determination, and 5 had Cepheid light curves. Four of these stars fit well on a P-L relation and yield a distance modulus, after correction for Galactic foreground extinction, of m-M = 26.74. This corresponds to a distance of 2.2 Mpc, placing Leo A near the Local Group zero-velocity surface.

  15. Luminosity functions for very low mass stars and brown dwarfs

    NASA Technical Reports Server (NTRS)

    Laughlin, Gregory; Bodenheimer, Peter

    1993-01-01

    A theoretical investigation of the luminosity function for low-mass objects to constrain the stellar initial mass function at the low-mass end is reported. The ways in which luminosity functions for low-mass stars are affected by star formation histories, brown dwarf and premain-sequence cooling rates and main-sequence mass luminosity relations, and the IMF are examined. Cooling rates and the mass-luminosity relation are determined through a new series of evolutionary calculations for very low mass stars and brown dwarfs in the range 0.05-0.50 solar mass. Model luminosity functions are constructed for specific comparison with the results of four recent observational surveys. The likelihood that the stellar mass function in the solar neighborhood is increasing at masses near the bottom of the main sequence and perhaps at lower masses is confirmed. In the most optimistic case, brown dwarfs contribute half of the local missing disk mass. The actual contribution is likely to be considerably less.

  16. The history of star formation in nearby dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Weisz, Daniel Ray

    2010-11-01

    We present detailed analysis of color-magnitude diagrams (CMDs) of resolved stellar populations in nearby dwarf galaxies based on observations taken with the Hubble Space Telescope (HST). From the positions of individual stars on a CMD, we are able to derive the star formation histories (SFHs), i.e., the star formation rate (SFR) as a function of time and metallicity, of the observed stellar populations. Specifically, we apply this technique to a number of nearby dwarf galaxies to better understand the mechanisms driving their evolution. The ACS Nearby Galaxy Survey Treasury program (ANGST) provides multi-color photometry of resolved stars in ˜ 60 nearby dwarf galaxies from images taken with HST. This sample contains 12 dSph, 5 dwarf spiral, 28 dIrr, 12 dSph/dIrr (transition), and 3 tidal dwarf galaxies. The sample spans a range of ˜ 10 in MB and covers a wide range of environments, from highly interacting to truly isolated. From the best fit lifetime SFHs we find three significant results: (1) the average dwarf galaxy formed ˜ 60% of its stars by z ˜ 2 and 70% of its stars by z ˜ 1, regardless of morphological type, (2) the only statistically significant difference between the SFHs of different morphological types is within the most recent 1 Gyr (excluding tidal dwarf galaxies), and (3) the SFHs are complex and the mean values are inconsistent with simple SFH models, e.g., single epoch SF or constant SFH. We then present the recent ( ≲ 1 Gyr) SFHs of nine M81 Group Dwarf Galaxies. Comparing the SFHs, birthrate parameters, fraction of stars formed per time interval, and spatial distribution of stellar components as a function of luminosity, we find only minor differences in SF characteristics among the M81 Group dIs despite a wide range of physical properties. We extend our comparison to select dIs in the Local Group (LG), with similar quality photometry, and again find only minor differences in SF parameters. The lack of a clear trend in SF parameters over

  17. The Ultraviolet Radiation Environment around M Dwarf Exoplanet Host Stars

    NASA Technical Reports Server (NTRS)

    France, Kevin; Froning, Cynthia S.; Linsky, Jeffrey L.; Roberge, Aki; Stocke, John T.; Tian, Feng; Bushinsky, Rachel; Desert, Jean-Michel; Mauas, Pablo; Mauas, Pablo; Walkowicz, Lucianne M.

    2013-01-01

    The spectral and temporal behavior of exoplanet host stars is a critical input to models of the chemistry and evolution of planetary atmospheres. Ultraviolet photons influence the atmospheric temperature profiles and production of potential biomarkers on Earth-like planets around these stars. At present, little observational or theoretical basis exists for understanding the ultraviolet spectra of M dwarfs, despite their critical importance to predicting and interpreting the spectra of potentially habitable planets as they are obtained in the coming decades. Using observations from the Hubble Space Telescope, we present a study of the UV radiation fields around nearby M dwarf planet hosts that covers both far-UV (FUV) and near-UV (NUV) wavelengths. The combined FUV+NUV spectra are publicly available in machine-readable format. We find that all six exoplanet host stars in our sample (GJ 581, GJ 876, GJ 436, GJ 832, GJ 667C, and GJ 1214) exhibit some level of chromospheric and transition region UV emission. No "UV-quiet" M dwarfs are observed. The bright stellar Lyman-alpha emission lines are reconstructed, and we find that the Lyman-alpha line fluxes comprise approximately 37%-75% of the total 1150-3100 A flux from most M dwarfs; approximately greater than 10(exp3) times the solar value. We develop an empirical scaling relation between Lyman-alpha and Mg II emission, to be used when interstellar H I attenuation precludes the direct observation of Lyman-alpha. The intrinsic unreddened flux ratio is F(Lyman-alpha)/F(Mg II) = 10(exp3). The F(FUV)/F(NUV) flux ratio, a driver for abiotic production of the suggested biomarkers O2 and O3, is shown to be approximately 0.5-3 for all M dwarfs in our sample, greather than 10(exp3) times the solar ratio. For the four stars with moderate signal-to-noise Cosmic Origins Spectrograph time-resolved spectra, we find UV emission line variability with amplitudes of 50%.500% on 10(exp2)-10(exp3) s timescales. This effect should be taken

  18. THE ULTRAVIOLET RADIATION ENVIRONMENT AROUND M DWARF EXOPLANET HOST STARS

    SciTech Connect

    France, Kevin; Froning, Cynthia S.; Stocke, John T.; Bushinsky, Rachel; Linsky, Jeffrey L.; Roberge, Aki; Tian, Feng; Desert, Jean-Michel; Mauas, Pablo; Vieytes, Mariela; Walkowicz, Lucianne M.

    2013-02-15

    The spectral and temporal behavior of exoplanet host stars is a critical input to models of the chemistry and evolution of planetary atmospheres. Ultraviolet photons influence the atmospheric temperature profiles and production of potential biomarkers on Earth-like planets around these stars. At present, little observational or theoretical basis exists for understanding the ultraviolet spectra of M dwarfs, despite their critical importance to predicting and interpreting the spectra of potentially habitable planets as they are obtained in the coming decades. Using observations from the Hubble Space Telescope, we present a study of the UV radiation fields around nearby M dwarf planet hosts that covers both far-UV (FUV) and near-UV (NUV) wavelengths. The combined FUV+NUV spectra are publicly available in machine-readable format. We find that all six exoplanet host stars in our sample (GJ 581, GJ 876, GJ 436, GJ 832, GJ 667C, and GJ 1214) exhibit some level of chromospheric and transition region UV emission. No 'UV-quiet' M dwarfs are observed. The bright stellar Ly{alpha} emission lines are reconstructed, and we find that the Ly{alpha} line fluxes comprise {approx}37%-75% of the total 1150-3100 A flux from most M dwarfs; {approx}>10{sup 3} times the solar value. We develop an empirical scaling relation between Ly{alpha} and Mg II emission, to be used when interstellar H I attenuation precludes the direct observation of Ly{alpha}. The intrinsic unreddened flux ratio is F(Ly{alpha})/F(Mg II) = 10 {+-} 3. The F(FUV)/F(NUV) flux ratio, a driver for abiotic production of the suggested biomarkers O{sub 2} and O{sub 3}, is shown to be {approx}0.5-3 for all M dwarfs in our sample, >10{sup 3} times the solar ratio. For the four stars with moderate signal-to-noise Cosmic Origins Spectrograph time-resolved spectra, we find UV emission line variability with amplitudes of 50%-500% on 10{sup 2}-10{sup 3} s timescales. This effect should be taken into account in future UV transiting

  19. Star Formation at Low Metallicity in Local Dwarf Irregular Galaxies

    NASA Astrophysics Data System (ADS)

    Elmegreen, Bruce; Hunter, Deidre Ann; Rubio, Monica; Brinks, Elias; Cortés, Juan R.; Cigan, Phil

    2016-01-01

    The radial profiles of star formation rates and surface mass densities for gas and stars have been compiled for 20 local dwarf irregular galaxies and converted into disk scale heights and Toomre Q values. The scale heights are relatively large compared to the galaxy sizes (~0.6 times the local radii) and generally increase with radius in a flare. The gaseous Q values are high, ~4, at most radii and even higher for the stars. Star formation proceeds even with these high Q values in a normal exponential disk as viewed in the far ultraviolet. Such normal star formation suggests that Q is not relevant to star formation in dIrrs. The star formation rate per unit area always equals approximately the gas surface density divided by the midplane free fall time with an efficiency factor of about 1% that decreases systematically with radius in approximate proportion to the gas surface density. We view this efficiency variation as a result of a changing molecular fraction in a disk where atomic gas dominates both stars and molecules. In a related study, CO observations with ALMA of star-forming regions at the low metallicities of these dwarfs, which averages 13% solar, shows, in the case of the WLM galaxy, tiny CO clouds inside much larger molecular and atomic hydrogen envelopes. The CO cloud mass fraction within the molecular region is only one percent or so. Nevertheless, the CO clouds have properties that are similar to solar neighborhood clouds: they satisfy the size-linewidth relation observed in the LMC, SMC, and other local dwarfs where CO has been observed, and the same virial mass versus luminosity relation. This uniforming of CO cloud properties seems to be the result of a confining pressure from the weight of the overlying molecular and atomic shielding layers. Star formation at low metallicity therefore appears to be a three dimensional process independent of 2D instabilities involving Q, in highly atomic gas with relatively small CO cores, activated at a rate

  20. MOST Photometry and DDO Spectroscopy of the Eclipsing (White Dwarf + Red Dwarf) Binary V471 Tau

    NASA Astrophysics Data System (ADS)

    Kamiński, Krzysztof Z.; Ruciński, Slavek M.; Matthews, Jaymie M.; Kuschnig, Rainer; Rowe, Jason F.; Guenther, David B.; Moffat, Anthony F. J.; Sasselov, Dimitar; Walker, Gordon A. H.; Weiss, Werner W.

    2007-09-01

    The Hyades K2 V + WD system 471 Tau is a prototype post-common envelope system and a likely cataclysmic binary progenitor. We present 10 days of nearly continuous optical photometry by the MOST (Microvariability and Oscillations of Stars) satellite and partly simultaneous optical spectroscopy from DDO (David Dunlap Observatory) of the binary. The photometric data indicate that the spot coverage of the K dwarf component was less than observed in the past, suggesting that we monitored the star close to a minimum in its activity cycle. Despite the low spot activity, we still detected seven flarelike events whose estimated energies are among the highest ever observed in V471 Tau and whose times of occurrence do not correlate with the binary orbital phase. A detailed O - C analysis of the times of eclipse over the last ~35 years reveals timing variations which could be explained in several ways, including perturbations by an as-yet-undetected third body in the system or by a small orbital eccentricity inducing slow apsidal motion. The DDO spectra result in improved determinations of the K dwarf projected rotation velocity, VK sin i = 92 km s-1, and the orbital amplitude, KK = 150.5 km s-1. The spectra also allow us to measure changes in Hα emission strength and radial velocity variations. We measure a larger Hα velocity amplitude than found previously, suggesting that the source of the emission in V471 Tau was less concentrated around the sub-white dwarf point on the K star than had been observed in previous studies. Based on data from the MOST satellite, a Canadian Space Agency mission jointly operated by Dynacon, Inc., the University of Toronto Institute for Aerospace Studies, and the University of British Columbia, with the assistance of the University of Vienna, and on data obtained at the David Dunlap Observatory, University of Toronto.

  1. The red and blue galaxy populations in the GOODS field: evidence for an excess of red dwarfs

    NASA Astrophysics Data System (ADS)

    Salimbeni, S.; Giallongo, E.; Menci, N.; Castellano, M.; Fontana, A.; Grazian, A.; Pentericci, L.; Trevese, D.; Cristiani, S.; Nonino, M.; Vanzella, E.

    2008-01-01

    Aims: We study the evolution of the galaxy population up to z˜ 3 as a function of its colour properties. In particular, luminosity functions and luminosity densities were derived as a function of redshift for the blue/late and red/early populations. Methods: We use data from the GOODS-MUSIC catalogue, which have typical magnitude limits z850≤ 26 and K_s≤ 23.5 for most of the sample. About 8% of the galaxies have spectroscopic redshifts; the remaining have well calibrated photometric redshifts derived from the extremely wide multi-wavelength coverage in 14 bands (from the U band to the Spitzer 8~ μm band). We have derived a catalogue of galaxies complete in the rest-frame B-band, which has been divided into two subsamples according to their rest-frame U-V colour (or derived specific star formation rate) properties. Results: We confirm a bimodality in the U-V colour and specific star formation rate of the galaxy sample up to z˜ 3. This bimodality is used to compute the luminosity functions of the blue/late and red/early subsamples. The luminosity functions of the blue/late and total samples are well represented by steep Schechter functions evolving in luminosity with increasing redshifts. The volume density of the luminosity functions of the red/early populations decreases with increasing redshift. The shape of the red/early luminosity functions shows an excess of faint red dwarfs with respect to the extrapolation of a flat Schechter function and can be represented by the sum of two Schechter functions. Our model for galaxy formation in the hierarchical clustering scenario, which also includes external feedback due to a diffuse UV background, shows a general broad agreement with the luminosity functions of both populations, the larger discrepancies being present at the faint end for the red population. Hints on the nature of the red dwarf population are given on the basis of their stellar mass and spatial distributions.

  2. The history of star formation in nearby dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Weisz, Daniel Ray

    2010-11-01

    We present detailed analysis of color-magnitude diagrams (CMDs) of resolved stellar populations in nearby dwarf galaxies based on observations taken with the Hubble Space Telescope (HST). From the positions of individual stars on a CMD, we are able to derive the star formation histories (SFHs), i.e., the star formation rate (SFR) as a function of time and metallicity, of the observed stellar populations. Specifically, we apply this technique to a number of nearby dwarf galaxies to better understand the mechanisms driving their evolution. The ACS Nearby Galaxy Survey Treasury program (ANGST) provides multi-color photometry of resolved stars in ˜ 60 nearby dwarf galaxies from images taken with HST. This sample contains 12 dSph, 5 dwarf spiral, 28 dIrr, 12 dSph/dIrr (transition), and 3 tidal dwarf galaxies. The sample spans a range of ˜ 10 in MB and covers a wide range of environments, from highly interacting to truly isolated. From the best fit lifetime SFHs we find three significant results: (1) the average dwarf galaxy formed ˜ 60% of its stars by z ˜ 2 and 70% of its stars by z ˜ 1, regardless of morphological type, (2) the only statistically significant difference between the SFHs of different morphological types is within the most recent 1 Gyr (excluding tidal dwarf galaxies), and (3) the SFHs are complex and the mean values are inconsistent with simple SFH models, e.g., single epoch SF or constant SFH. We then present the recent ( ≲ 1 Gyr) SFHs of nine M81 Group Dwarf Galaxies. Comparing the SFHs, birthrate parameters, fraction of stars formed per time interval, and spatial distribution of stellar components as a function of luminosity, we find only minor differences in SF characteristics among the M81 Group dIs despite a wide range of physical properties. We extend our comparison to select dIs in the Local Group (LG), with similar quality photometry, and again find only minor differences in SF parameters. The lack of a clear trend in SF parameters over

  3. V and K-band Mass-Luminosity Relations for M dwarf Stars

    NASA Astrophysics Data System (ADS)

    Benedict, G. Fritz; Henry, Todd J.; McArthur, Barbara; Franz, Otto G.; Wasserman, Lawrence H.; Dieterich, Sergio

    2015-01-01

    Applying Hubble Space Telescope Fine Guidance Sensor astrometric techniques developed to establish relative orbits for binary stars (Franz et al. 1998, AJ, 116, 1432), determine masses of binary components (Benedict et al. 2001, AJ, 121, 1607), and measure companion masses of exoplanet host stars (McArthur et al. 2010, ApJ, 715, 1203), we derive masses with an average 2.1% error for 24 components of 12 M dwarf binary star systems. Masses range 0.08 to 0.40 solar masses. With these we update the lower Main Sequence V-band Mass-Luminosity Relation first shown in Henry et al. (1999, ApJ, 512, 864). We demonstrate that a Mass-Luminosity Relation in the K-band has far less scatter than in the V-band. For the eight binary components for which we have component magnitude differences in the K-band the RMS residual drops from 0.5 magnitude in the V-band to 0.05 magnitude in the K-band. These relations can be used to estimate the masses of the ubiquitous red dwarfs that account for 75% of all stars, to an accuracy of 5%, which is much better than ever before.

  4. Search for white dwarf companions of cool stars with peculiar element abundances

    NASA Technical Reports Server (NTRS)

    Boehm-Vitense, E.

    1984-01-01

    A search for a white dwarf companions of cool stars with peculiar element abundances was undertaken. One additional star the xi Cet, was found with a white dwarf companion. It was found that HR 1016, 56Uma, 16 Ser, have high excitation emission lines which indicate a high temperature object in the system. It is suggested that since these indications for high temperature companions were seen for all nearby Ba stars, it is highly probable that all Ba stars have white dwarf companions, and that the peculiar element abundances seen in the Ba stars are due to mass transfer. Observations, arguments and conclusions are presented. White dwarf companions were not found. Together with the Li and Be abundances and the chromospheric emission line spectra in these stars were studied. No white dwarf companions were seen for subgiant CH stars.

  5. Neutron stars and white dwarfs in galactic halos

    NASA Technical Reports Server (NTRS)

    Ryu, Dongsu; Olive, Keith A.; Silk, Joseph

    1989-01-01

    The possibility that galactic halos are composed of stellar remnants such as neutron stars and white dwarfs is discussed. On the basis of a simple model for the evolution of galactic halos, researchers follow the history of halo matter, luminosity, and metal and helium abundances. They assume conventional yields for helium and the heavier elements. By comparing with the observational constraints, which may be considered as fairly conservative, it is found that, for an exponentially decreasing star formation rate (SFR) with e-folding time tau, only values between 6 x 10(8) less than similar to tau less than similar to 2 x 10(9) years are allowed together with a very limited range of masses for the initial mass function (IMF). Star formation is allowed for 2 solar mass less than similar to m less than similar to 8 solar mass if tau = 2 x 10(9) years, and for 4 solar mass less than similar to m less than similar to 6 solar mass if tau = 10(9) years. For tau = 6 x 10(8) years, the lower and upper mass limits merge to similar to 5 solar mass. Researchers conclude that, even though the possibility of neutron stars as halo matter may be ruled out, that of white dwarfs may still be a viable hypothesis, though with very stringent constraints on allowed parameters, that merits further consideration.

  6. Neutron stars and white dwarfs in galactic halos?

    NASA Technical Reports Server (NTRS)

    Ryu, Dongsu; Olive, Keith A.; Silk, Joseph

    1990-01-01

    The possibility that galactic halos are composed of stellar remnants such as neutron stars and white dwarfs is discussed. On the basis of a simple model for the evolution of galactic halos, researchers follow the history of halo matter, luminosity, and metal and helium abundances. They assume conventional yields for helium and the heavier elements. By comparing with the observational constraints, which may be considered as fairly conservative, it is found that, for an exponentially decreasing star formation rate (SFR) with e-folding time tau, only values between 6 x 10(8) less than similar to tau less than similar to 2 x 10(9) years are allowed together with a very limited range of masses for the initial mass function (IMF). Star formation is allowed for 2 solar mass less than similar to m less than similar to 8 solar mass if tau = 2 x 10(9) years, and for 4 solar mass less than similar to m less than similar to 6 solar mass if tau = 10(9) years. For tau = 6 x 10(8) years, the lower and upper mass limits merge to similar to 5 solar mass. Researchers conclude that, even though the possibility of neutron stars as halo matter may be ruled out, that of white dwarfs may still be a viable hypothesis, though with very stringent constraints on allowed parameters, that merits further consideration.

  7. White dwarf kicks and implications for barium stars

    NASA Astrophysics Data System (ADS)

    Izzard, R. G.; Church, R. P.; Dermine, T.

    The barium stars have caused much grief in the field of binary stellar evolution. They are often eccentric when they should be circular and are not found to have periods longer than 104 days even though wind accretion should still be efficient at such separations. We address both these problems by introducing a kick to white dwarfs when they are born, thus solving the eccentricity problem, and imposing strong orbital angular momentum loss to shrink barium-star binaries down to the observed periods. Whilst our angular momentum prescription is hard to justify for the barium stars it shows that strong angular momentum loss is necessary to reproduce the observed period-eccentricity distribution. We are investigating whether this can be obtained from a circumbinary disc.

  8. Carbon and nitrogen abundances of individual stars in the Sculptor dwarf spheroidal galaxy

    NASA Astrophysics Data System (ADS)

    Lardo, C.; Battaglia, G.; Pancino, E.; Romano, D.; de Boer, T. J. L.; Starkenburg, E.; Tolstoy, E.; Irwin, M. J.; Jablonka, P.; Tosi, M.

    2016-01-01

    We present [C/Fe] and [N/Fe] abundance ratios and CH(λ4300) and S(λ3883) index measurements for 94 red giant branch (RGB) stars in the Sculptor dwarf spheroidal galaxy from VLT/VIMOS MOS observations at a resolving power R = 1150 at 4020 Å. This is the first time that [N/Fe] abundances are derived for a large number of stars in a dwarf spheroidal. We found a trend for the [C/Fe] abundance to decrease with increasing luminosity on the RGB across the whole metallicity range, a phenomenon observed in both field and globular cluster giants, which can be interpreted in the framework of evolutionary mixing of partially processed CNO material. Both our measurements of [C/Fe] and [N/Fe] are in good agreement with the theoretical predictions for stars at similar luminosity and metallicity. We detected a dispersion in the carbon abundance at a given [Fe/H], which cannot be ascribed to measurement uncertainties alone. We interpret this observational evidence as the result of the contribution of different nucleosynthesis sources over time to a not well-mixed interstellar medium. We report the discovery of two new carbon-enhanced, metal-poor stars. These are likely the result of pollution from material enriched by asymptotic giant branch stars, as indicated by our estimates of [Ba/Fe] >+1. We also attempted a search for dissolved globular clusters in the field of the galaxy by looking for the distinctive C-N pattern of second population globular clusters stars in a previously detected, very metal-poor, chemodynamical substructure. We do not detect chemical anomalies among this group of stars. However, small number statistics and limited spatial coverage do not allow us to exclude the hypotheses that this substructure forms part of a tidally shredded globular cluster. Based on observations made with ESO Telescopes at the La Silla Paranal Observatory under programme ID 091.D-0089

  9. MULTI-ELEMENT ABUNDANCE MEASUREMENTS FROM MEDIUM-RESOLUTION SPECTRA. II. CATALOG OF STARS IN MILKY WAY DWARF SATELLITE GALAXIES

    SciTech Connect

    Kirby, Evan N.; Cohen, Judith G.; Guhathakurta, Puragra; Rockosi, Constance M.; Geha, Marla C.; Sneden, Christopher; Sohn, Sangmo Tony; Majewski, Steven R.; Siegel, Michael

    2010-12-15

    We present a catalog of Fe, Mg, Si, Ca, and Ti abundances for 2961 stars in eight dwarf satellite galaxies of the Milky Way (MW): Sculptor, Fornax, Leo I, Sextans, Leo II, Canes Venatici I, Ursa Minor, and Draco. For the purposes of validating our measurements, we also observed 445 red giants in MW globular clusters and 21 field red giants in the MW halo. The measurements are based on Keck/DEIMOS medium-resolution spectroscopy (MRS) combined with spectral synthesis. We estimate uncertainties in [Fe/H] by quantifying the dispersion of [Fe/H] measurements in a sample of stars in monometallic globular clusters (GCs). We estimate uncertainties in Mg, Si, Ca, and Ti abundances by comparing to high-resolution spectroscopic abundances of the same stars. For this purpose, a sample of 132 stars with published high-resolution spectroscopy in GCs, the MW halo field, and dwarf galaxies has been observed with MRS. The standard deviations of the differences in [Fe/H] and ([{alpha}/Fe]) (the average of [Mg/Fe], [Si/Fe], [Ca/Fe], and [Ti/Fe]) between the two samples is 0.15 and 0.16, respectively. This catalog represents the largest sample of multi-element abundances in dwarf galaxies to date. The next papers in this series draw conclusions on the chemical evolution, gas dynamics, and star formation histories from the catalog presented here. The wide range of dwarf galaxy luminosity reveals the dependence of dwarf galaxy chemical evolution on galaxy stellar mass.

  10. Cleaning spectroscopic samples of stars in nearby dwarf galaxies. The use of the nIR Mg I line to weed out Milky Way contaminants

    NASA Astrophysics Data System (ADS)

    Battaglia, G.; Starkenburg, E.

    2012-03-01

    Dwarf galaxies provide insight into the processes of star formation and chemical enrichment at the low end of the galaxy mass function, as well as into the clustering of dark matter on small scales. In studies of Local Group dwarf galaxies, spectroscopic samples of individual stars are used to derive the internal kinematics and abundance properties of these galaxies. It is therefore important to clean these samples from Milky Way stars, which are not related to the dwarf galaxy, since they can contaminate analysis of the properties of these objects. Here we introduce a new diagnostic for separating Milky Way contaminant stars, which mainly consist of dwarf stars, and red giant branch stars targeted in dwarf galaxies. As discriminator we use the trends in the equivalent width of the nIR Mg I line at 8806.8 Å as a function of the equivalent width of Ca II triplet lines. This method is particularly useful for works dealing with multi-object, intermediate-resolution spectroscopy focusing in the region of the nIR Ca II triplet. We use synthetic spectra to explore how the equivalent width of these lines changes for stars with different properties (gravity, effective temperature, metallicity) and find that a distinction among giants above the horizontal branch and dwarfs can be made with this method at [Fe/H] > -2 dex. For -2 ≤ [Fe/H] ≤ -1, this method is also valid for distinguishing dwarfs and giants down to approximately one magnitude below the horizontal branch. Using a foreground model we make predictions on the use of this new discrimination method for nearby dwarf spheroidal galaxies, including the ultra-faints. We subsequently use VLT/FLAMES data for the Sextans, Sculptor, and Fornax dwarf spheroidal galaxies to verify the predicted theoretical trends. Based on FLAMES observations collected at the ESO, proposals 171.B-0588, 076.B-0391, 079.B-0435.

  11. Stellar, brown dwarf and multiple star properties from hydrodynamical simulations of star cluster formation

    NASA Astrophysics Data System (ADS)

    Bate, Matthew R.

    2009-01-01

    We report the statistical properties of stars, brown dwarfs and multiple systems obtained from the largest hydrodynamical simulation of star cluster formation to date that resolves masses down to the opacity limit for fragmentation (a few Jupiter masses). The simulation is essentially identical to that of Bate, Bonnell & Bromm except that the initial molecular cloud is larger and more massive. It produces more than 1250 stars and brown dwarfs, providing unprecedented statistical information that can be compared with observational surveys. The calculation uses sink particles to model the stars and brown dwarfs. Part of the calculation is rerun with smaller sink particle accretion radii and gravitational softening to investigate the effect of these approximations on the results. We find that hydrodynamical/sink particle simulations can reproduce many of the observed stellar properties very well. Multiplicity as a function of the primary mass, the frequency of very low mass (VLM) binaries, general trends for the separation and mass ratio distributions of binaries and the relative orbital orientations of triples systems are all in reasonable agreement with observations. We also examine the radial variations of binarity, velocity dispersion and mass function in the resulting stellar cluster and the distributions of disc truncation radii due to dynamical interactions. For VLM binaries, because their separations are typically close, we find that their frequency is sensitive to the sink particle accretion radii and gravitational softening used in the calculations. Using small accretion radii and gravitational softening results in a frequency of VLM binaries similar to that expected from observational surveys (~20 per cent). We also find that VLM binaries evolve from wide, unequal-mass systems towards close equal-mass systems as they form. The two main deficiencies of the calculations are that they overproduce brown dwarfs relative to stars and that there are too few

  12. Spot temperatures and area coverages on active dwarf stars

    NASA Technical Reports Server (NTRS)

    Sarr, Steven H.; Neff, James E.

    1990-01-01

    Two active K dwarfs are examined to determine the temperatures of the stars and to estimate the locations and sizes of cool spots on the stellar surfaces. Two wavelength regions with TiO absorption bands at different temperature sensitivities are modeled simultaneously using the method developed by Huenemoerder and Ramsey (1987). The spectrum of BD +26deg730 shows excess absorption in the TiO band, and the absence of the 8860 A band in HD 82558 indicates that its spots are warmer than those of BD +26deg730.

  13. Star Formation Rate in Holmberg IX Dwarf Galaxy

    NASA Astrophysics Data System (ADS)

    Andjelic, M. M.

    2011-12-01

    In this paper we use previously determined Hα fluxes for dwarf galaxy Holmberg IX (Arbutina et al. 2009) to calculate star formation rate (SFR) in this galaxy. We discuss possible contaminations of Hα flux and, for the first time, we take into account optical emission from supernova remnants (SNRs) as a possible source of contamination of Hα flux. Derived SFR for Holmberg IX is 3.4×10-4M_{⊙} yr-1. Our value is lower then in previous studies, due to luminous shock-heated source M&H 9-10, possible hypernova remnant, which we excluded from the total Hα flux in our calculation of SFR.

  14. Thomson scattering in magnetic fields. [of white dwarf stars

    NASA Technical Reports Server (NTRS)

    Whitney, Barbara

    1989-01-01

    The equation of transfer in Thomson scattering atmospheres with magnetic fields is solved using Monte Carlo methods. Two cases, a plane parallel atmosphere with a magnetic field perpendicular to the atmosphere, and a dipole star, are investigated. The wavelength dependence of polarization from plane-parallel atmosphere is qualitatively similar to that observed in the magnetic white dwarf Grw+70 deg 8247, and the field strength determined by the calculation, 320 MG, is quantitatively similar to that determined from the line spectrum. The dipole model does not resemble the data as well as the single plane-parallel atmosphere.

  15. A compact system of small planets around a former red-giant star.

    PubMed

    Charpinet, S; Fontaine, G; Brassard, P; Green, E M; Van Grootel, V; Randall, S K; Silvotti, R; Baran, A S; Ostensen, R H; Kawaler, S D; Telting, J H

    2011-12-22

    Planets that orbit their parent star at less than about one astronomical unit (1 AU is the Earth-Sun distance) are expected to be engulfed when the star becomes a red giant. Previous observations have revealed the existence of post-red-giant host stars with giant planets orbiting as close as 0.116 AU or with brown dwarf companions in tight orbits, showing that these bodies can survive engulfment. What has remained unclear is whether planets can be dragged deeper into the red-giant envelope without being disrupted and whether the evolution of the parent star itself could be affected. Here we report the presence of two nearly Earth-sized bodies orbiting the post-red-giant, hot B subdwarf star KIC 05807616 at distances of 0.0060 and 0.0076 AU, with orbital periods of 5.7625 and 8.2293 hours, respectively. These bodies probably survived deep immersion in the former red-giant envelope. They may be the dense cores of evaporated giant planets that were transported closer to the star during the engulfment and triggered the mass loss necessary for the formation of the hot B subdwarf, which might also explain how some stars of this type did not form in binary systems. PMID:22193103

  16. A compact system of small planets around a former red-giant star.

    PubMed

    Charpinet, S; Fontaine, G; Brassard, P; Green, E M; Van Grootel, V; Randall, S K; Silvotti, R; Baran, A S; Ostensen, R H; Kawaler, S D; Telting, J H

    2011-12-21

    Planets that orbit their parent star at less than about one astronomical unit (1 AU is the Earth-Sun distance) are expected to be engulfed when the star becomes a red giant. Previous observations have revealed the existence of post-red-giant host stars with giant planets orbiting as close as 0.116 AU or with brown dwarf companions in tight orbits, showing that these bodies can survive engulfment. What has remained unclear is whether planets can be dragged deeper into the red-giant envelope without being disrupted and whether the evolution of the parent star itself could be affected. Here we report the presence of two nearly Earth-sized bodies orbiting the post-red-giant, hot B subdwarf star KIC 05807616 at distances of 0.0060 and 0.0076 AU, with orbital periods of 5.7625 and 8.2293 hours, respectively. These bodies probably survived deep immersion in the former red-giant envelope. They may be the dense cores of evaporated giant planets that were transported closer to the star during the engulfment and triggered the mass loss necessary for the formation of the hot B subdwarf, which might also explain how some stars of this type did not form in binary systems.

  17. White dwarf stars and the age of the Galactic disk

    NASA Technical Reports Server (NTRS)

    Wood, M. A.

    1990-01-01

    The history of the Galaxy is written in its oldest stars, the white dwarf (WD) stars. Significant limits can be placed on both the Galactic age and star formation history. A wide range of input WD model sequences is used to derive the current limits to the age estimates suggested by fitting to the observed falloff in the WD luminosity function. The results suggest that the star formation rate over the history of the Galaxy has been relatively constant, and that the disk age lies in the range 6-12 billion years, depending upon the assumed structure of WD stars, and in particular on the core composition and surface helium layer mass. Using plausible mixed C/O core input models, the estimates for the disk age range from 8-10.5 Gyr, i.e.,sustantially younger than most age estimates for the halo globular clusters. After speculating on the significance of the results, expected observational and theoretical refinements which will further enhance the reliability of the method are discussed.

  18. Crystallization of Carbon-Oxygen Mixtures in White Dwarf Stars

    NASA Astrophysics Data System (ADS)

    Horowitz, C. J.; Schneider, A. S.; Berry, D. K.

    2010-06-01

    We determine the phase diagram for dense carbon-oxygen mixtures in white dwarf (WD) star interiors using molecular dynamics simulations involving liquid and solid phases. Our phase diagram agrees well with predictions from Ogata et al. and from Medin and Cumming and gives lower melting temperatures than Segretain et al. Observations of WD crystallization in the globular cluster NGC 6397 by Winget et al. suggest that the melting temperature of WD cores is close to that for pure carbon. If this is true, our phase diagram implies that the central oxygen abundance in these stars is less than about 60%. This constraint, along with assumptions about convection in stellar evolution models, limits the effective S factor for the C12(α,γ)O16 reaction to S300≤170keVb.

  19. Crystallization of carbon-oxygen mixtures in white dwarf stars.

    PubMed

    Horowitz, C J; Schneider, A S; Berry, D K

    2010-06-11

    We determine the phase diagram for dense carbon-oxygen mixtures in white dwarf (WD) star interiors using molecular dynamics simulations involving liquid and solid phases. Our phase diagram agrees well with predictions from Ogata et al. and from Medin and Cumming and gives lower melting temperatures than Segretain et al. Observations of WD crystallization in the globular cluster NGC 6397 by Winget et al. suggest that the melting temperature of WD cores is close to that for pure carbon. If this is true, our phase diagram implies that the central oxygen abundance in these stars is less than about 60%. This constraint, along with assumptions about convection in stellar evolution models, limits the effective S factor for the 12C(α,γ)16O reaction to S(300)≤170  keV b. PMID:20867223

  20. Crystallization of carbon-oxygen mixtures in white dwarf stars.

    PubMed

    Horowitz, C J; Schneider, A S; Berry, D K

    2010-06-11

    We determine the phase diagram for dense carbon-oxygen mixtures in white dwarf (WD) star interiors using molecular dynamics simulations involving liquid and solid phases. Our phase diagram agrees well with predictions from Ogata et al. and from Medin and Cumming and gives lower melting temperatures than Segretain et al. Observations of WD crystallization in the globular cluster NGC 6397 by Winget et al. suggest that the melting temperature of WD cores is close to that for pure carbon. If this is true, our phase diagram implies that the central oxygen abundance in these stars is less than about 60%. This constraint, along with assumptions about convection in stellar evolution models, limits the effective S factor for the 12C(α,γ)16O reaction to S(300)≤170  keV b.

  1. Search for carbon stars and DZ white dwarfs in SDSS spectra survey through machine learning

    NASA Astrophysics Data System (ADS)

    Si, JianMin; Luo, ALi; Li, YinBi; Zhang, JianNan; Wei, Peng; Wu, YiHong; Wu, FuChao; Zhao, YongHeng

    2014-01-01

    Carbon stars and DZ white dwarfs are two types of rare objects in the Galaxy. In this paper, we have applied the label propagation algorithm to search for these two types of stars from Data Release Eight (DR8) of the Sloan Digital Sky Survey (SDSS), which is verified to be efficient by calculating precision and recall. From nearly two million spectra including stars, galaxies and QSOs, we have found 260 new carbon stars in which 96 stars have been identified as dwarfs and 7 identified as giants, and 11 composition spectrum systems (each of them consists of a white dwarf and a carbon star). Similarly, using the label propagation method, we have obtained 29 new DZ white dwarfs from SDSS DR8. Compared with PCA reconstructed spectra, the 29 findings are typical DZ white dwarfs. We have also investigated their proper motions by comparing them with proper motion distribution of 9,374 white dwarfs, and found that they satisfy the current observed white dwarfs by SDSS generally have large proper motions. In addition, we have estimated their effective temperatures by fitting the polynomial relationship between effective temperature and g-r color of known DZ white dwarfs, and found 12 of the 29 new DZ white dwarfs are cool, in which nine are between 6,000 K and 6,600 K, and three are below 6,000 K.

  2. R Coronae Borealis Stars formed from Double White Dwarf Mergers

    NASA Astrophysics Data System (ADS)

    Staff, Jan E.; Herwig, F.; Menon, A.; Even, W.; Tohline, J.; Clayton, G.; Motl, P.; Fryer, C.; Geballe, T.

    2011-01-01

    R Coronae Borealis (RCB) stars are hydrogen-deficient variable stars that suddenly fade by several magnitudes at irregular intervals whereafter they gradually return to their original brightness over a period of some months. The origin of RCBs remain a mystery. It is often thought that they are the result of the merger of a He and a CO white dwarf, while the fading is thought to be due to the formation of dust blocking light from the star. We are working on revealing the secrets behind the origin of RCBs. Here we present the results of 3 dimensional hydrodynamic simulations of the merger of a double white dwarf system where total mass is 0.9 M⊙ and initial mass ratio is q=0.7. We use a zero-temperature plus ideal gas equation of state that allows for heating through shocks. These simulations allow us to follow the evolution of the system for 10-20 initial orbital periods (1000-2000 seconds), from the onset of mass-transfer to a point after merger when the combined object has settled into a nearly axisymmetric, rotationally flattened configuration. The final merged object from the hydrodynamics simulation is then used as input for a stellar evolution code where the object's evolution can be followed over a much longer (thermal and/or nuclear) timescale. A preliminary post-merger stellar evolution simulation shows how an initial configuration of a 0.7 CO WD surrounded by 0.3 M⊙ of dynamically accreted He evolves on a time scale of 105 years to the location of the RCB stars in the H-R diagram at an effective temperature Teff<7000 K and log L 4. We acknowledge support from NASA Astrophysics Theory Program grant number NNX10AC72G.

  3. SDSS J001641-000925: THE FIRST STABLE RED DWARF CONTACT BINARY WITH A CLOSE-IN STELLAR COMPANION

    SciTech Connect

    Qian, S.-B.; Jiang, L.-Q.; Zhu, L.-Y.; Zhao, E. G.; He, J.-J.; Liao, W.-P.; Wang, J.-J.; Liu, L.; Zhou, X.; Liu, N. P.; Fernández Lajús, E.; Soonthornthum, B.; Rattanasoon, S.; Aukkaravittayapun, S.

    2015-01-10

    SDSS J001641-000925 is the first red dwarf contact binary star with an orbital period of 0.19856 days that is one of the shortest known periods among M-dwarf binary systems. The orbital period was detected to be decreasing rapidly at a rate of P-dot ∼8 s yr{sup −1}. This indicated that SDSS J001641-000925 was undergoing coalescence via a dynamical mass transfer or loss and thus this red dwarf contact binary is dynamically unstable. To understand the properties of the period change, we monitored the binary system photometrically from 2011 September 2 to 2014 October 1 by using several telescopes in the world and 25 eclipse times were determined. It is discovered that the rapid decrease of the orbital period is not true. This is contrary to the prediction that the system is merging driven by rapid mass transfer or loss. Our preliminary analysis suggests that the observed minus calculated (O–C) diagram shows a cyclic oscillation with an amplitude of 0.00255 days and a period of 5.7 yr. The cyclic variation can be explained by the light travel time effect via the presence of a cool stellar companion with a mass of M {sub 3}sin i' ∼ 0.14 M {sub ☉}. The orbital separation between the third body and the central binary is about 2.8 AU. These results reveal that the rarity of red dwarf contact binaries could not be explained by rapidly dynamical destruction and the presence of the third body helps to form the red dwarf contact binary.

  4. SPLAT: Using Spectral Indices to Identify and Characterize Ultracool Stars, Brown Dwarfs and Exoplanets in Deep Surveys and as Companions to Nearby Stars

    NASA Astrophysics Data System (ADS)

    Aganze, Christian; Burgasser, Adam J.; Martin, Eduardo; Konopacky, Quinn; Masters, Daniel C.

    2016-06-01

    The majority of ultracool dwarf stars and brown dwarfs currently known were identified in wide-field red optical and infrared surveys, enabling measures of the local, typically isolated, population in a relatively shallow (<100 pc radius) volume. Constraining the properties of the wider Galactic population (scale height, radial distribution, Population II sources), and close brown dwarf and exoplanet companions to nearby stars, requires specialized instrumentation, such as high-contrast, coronagraphic spectrometers (e.g., Gemini/GPI, VLT/Sphere, Project 1640); and deep spectral surveys (e.g., HST/WFC3 parallel fields, Euclid). We present a set of quantitative methodologies to identify and robustly characterize sources for these specific populations, based on templates and tools developed as part of the SpeX Prism Library Analysis Toolkit. In particular, we define and characterize specifically-tuned sets spectral indices that optimize selection of cool dwarfs and distinguish rare populations (subdwarfs, young planetary-mass objects) based on low-resolution, limited-wavelength-coverage spectral data; and present a template-matching classification method for these instruments. We apply these techniques to HST/WFC3 parallel fields data in the WISPS and HST-3D programs, where our spectral index set allows high completeness and low contamination for searches of late M, L and T dwarfs to distances out to ~3 kpc.The material presented here is based on work supported by the National Aeronautics and Space Administration under Grant No. NNX15AI75G.

  5. CALIBRATING UV STAR FORMATION RATES FOR DWARF GALAXIES FROM STARBIRDS

    SciTech Connect

    McQuinn, Kristen B. W.; Skillman, Evan D.; Mitchell, Noah P.; Dolphin, Andrew E.

    2015-08-01

    Integrating our knowledge of star formation (SF) traced by observations at different wavelengths is essential for correctly interpreting and comparing SF activity in a variety of systems and environments. This study compares extinction corrected integrated ultraviolet (UV) emission from resolved galaxies with color–magnitude diagram (CMD) based star formation rates (SFRs) derived from resolved stellar populations and CMD fitting techniques in 19 nearby starburst and post-starburst dwarf galaxies. The data sets are from the panchromatic Starburst Irregular Dwarf Survey and include deep legacy GALEX UV imaging, Hubble Space Telescope optical imaging, and Spitzer MIPS imaging. For the majority of the sample, the integrated near-UV fluxes predicted from the CMD-based SFRs—using four different models—agree with the measured, extinction corrected, integrated near-UV fluxes from GALEX images, but the far-UV (FUV) predicted fluxes do not. Furthermore, we find a systematic deviation between the SFRs based on integrated FUV luminosities and existing scaling relations, and the SFRs based on the resolved stellar populations. This offset is not driven by different SF timescales, variations in SFRs, UV attenuation, nor stochastic effects. This first comparison between CMD-based SFRs and an integrated FUV emission SFR indicator suggests that the most likely cause of the discrepancy is the theoretical FUV–SFR calibration from stellar evolutionary libraries and/or stellar atmospheric models. We present an empirical calibration of the FUV-based SFR relation for dwarf galaxies, with uncertainties, which is ∼53% larger than previous relations.

  6. Stellar Content and Recent Star Formation History of the Local Group Dwarf Irregular Galaxy IC 1613

    NASA Astrophysics Data System (ADS)

    Bernard, Edouard J.; Aparicio, Antonio; Gallart, Carme; Padilla-Torres, Carmen P.; Panniello, Maurizio

    2007-09-01

    We present resolved-star VI photometry of the Local Group dwarf irregular galaxy IC 1613 reaching I ~ 23.5, obtained with the wide-field camera at the 2.5 m Isaac Newton Telescope. A fit to the stellar density distribution shows an exponential profile of scale length 2.9' ± 0.1' and gives a central surface brightness μV,0 = 22.7 ± 0.6. The significant number of red giant branch (RGB) stars present in the outer part of our images (r > 16.5') indicates that the galaxy is actually more extended than previously estimated. A comparison of the color-magnitude diagrams (CMDs) as a function of galactocentric distance shows a clear gradient in the age of its population, the scale length increasing with age, while we find no evidence of a metallicity gradient from the width of the RGB. We present quantitative results of the recent star formation history from a synthetic CMD analysis using IAC-STAR. We find a mean star formation rate of (1.6 ± 0.8) × 10-3 Modot yr-1 kpc-2 in the central r lesssim 2.5' for the last 300 Myr. Based on observations made with the Isaac Newton Telescope, operated on the island of La Palma by the Isaac Newton Group, in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias.

  7. CEPHEID VARIABLE STARS IN THE PEGASUS DWARF IRREGULAR GALAXY: CONSTRAINTS ON THE STAR FORMATION HISTORY

    SciTech Connect

    Meschin, I.; Gallart, C.; Aparicio, A.; Rosenberg, A.; Cassisi, S. E-mail: carme@iac.es E-mail: alf@iac.es

    2009-03-15

    Observations of the resolved stars obtained over a period of 11 years in the Local Group dwarf irregular galaxy Pegasus have been used to search for Cepheid variable stars. Images were obtained in 55 epochs in the V band and in 24 epochs in the I band. We have identified 26 Cepheids and have obtained their light curves and periods. On the basis of their position in the period-luminosity (PL) diagram, we have classified them as 18 fundamental modes and eight first overtone Cepheids. Two PL relations for Cepheids have been used to derive the distance, resulting in 1.07 {+-} 0.05 Mpc. We present the VARFINDER code which finds the variable stars and their predicted periods in a given synthetic color-magnitude diagram computed with IAC-star and we propose the use of the Cepheid population as a constraint of the star formation history of Pegasus.

  8. The Star Formation History of the Local Group Dwarf Elliptical Galaxy NGC 185. I. Stellar Content

    NASA Astrophysics Data System (ADS)

    Martínez-Delgado, D.; Aparicio, A.

    1998-04-01

    We present VI CCD photometry of ~16,000 stars in a 7.2‧ x 7.2‧ field of the Local Group dwarf elliptical galaxy NGC 185. The resulting VI color-magnitude diagram reveals a dominant red giant branch population, an important number of luminous red stars located above the tip of the red giant branch, and a number of blue and yellow stars. Besides the nucleus, our field also covers a large, less crowded area of the galaxy. We show color-magnitude diagrams at six different distances from the nucleus. The red giant branch becomes substantially narrower at larger distances from the nucleus, while the photometry gets deeper. In this paper, we concentrate on investigating the contribution of the observational effects (mainly crowding) to this observed gradient. Although we cannot rule out here the possibility that this trend partially originates in a gradient of the characteristics of the stellar populations of the galaxy with radius, we show that a strong radial gradient exists in the observational effects that can mimic a gradient in the real properties (e.g., age, metallicity) of the stellar population. A distance modulus of m - M = 23.95 +/- 0.10 has been obtained from the tip of the red giant branch, in good agreement with previous estimates. The average stellar metallicity is estimated to be [Fe/H] = -1.43 +/- 0.15, and decreases for increasing galactocentric distance. Based on observations made with the William Herschel Telescope operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias.

  9. A STAR FORMATION LAW FOR DWARF IRREGULAR GALAXIES

    SciTech Connect

    Elmegreen, Bruce G.; Hunter, Deidre A. E-mail: dah@lowell.edu

    2015-06-01

    The radial profiles of gas, stars, and far-ultraviolet radiation in 20 dwarf Irregular galaxies are converted to stability parameters and scale heights for a test of the importance of two-dimensional (2D) instabilities in promoting star formation. A detailed model of this instability involving gaseous and stellar fluids with self-consistent thicknesses and energy dissipation on a perturbation crossing time gives the unstable growth rates. We find that all locations are effectively stable to 2D perturbations, mostly because the disks are thick. We then consider the average volume densities in the midplanes, evaluated from the observed H i surface densities and calculated scale heights. The radial profiles of the star-formation rates are equal to about 1% of the H i surface densities divided by the free fall times at the average midplane densities. This 1% resembles the efficiency per unit free fall time commonly found in other cases. There is a further variation of this efficiency with radius in all of our galaxies, following the exponential disk with a scale length equal to about twice the stellar mass scale length. This additional variation is modeled by the molecular fraction in a diffuse medium using radiative transfer solutions for galaxies with the observed dimensions and properties of our sample. We conclude that star formation is activated by a combination of three-dimensional gaseous gravitational processes and molecule formation. Implications for outer disk structure and formation are discussed.

  10. A Star Formation Law for Dwarf Irregular Galaxies

    NASA Astrophysics Data System (ADS)

    Elmegreen, Bruce G.; Hunter, Deidre A.

    2015-06-01

    The radial profiles of gas, stars, and far-ultraviolet radiation in 20 dwarf Irregular galaxies are converted to stability parameters and scale heights for a test of the importance of two-dimensional (2D) instabilities in promoting star formation. A detailed model of this instability involving gaseous and stellar fluids with self-consistent thicknesses and energy dissipation on a perturbation crossing time gives the unstable growth rates. We find that all locations are effectively stable to 2D perturbations, mostly because the disks are thick. We then consider the average volume densities in the midplanes, evaluated from the observed H i surface densities and calculated scale heights. The radial profiles of the star-formation rates are equal to about 1% of the H i surface densities divided by the free fall times at the average midplane densities. This 1% resembles the efficiency per unit free fall time commonly found in other cases. There is a further variation of this efficiency with radius in all of our galaxies, following the exponential disk with a scale length equal to about twice the stellar mass scale length. This additional variation is modeled by the molecular fraction in a diffuse medium using radiative transfer solutions for galaxies with the observed dimensions and properties of our sample. We conclude that star formation is activated by a combination of three-dimensional gaseous gravitational processes and molecule formation. Implications for outer disk structure and formation are discussed.

  11. Stars of type MS with evidence of white dwarf companions. [IUE, Main Sequence (MS)

    NASA Technical Reports Server (NTRS)

    Peery, Benjamin F., Jr.

    1986-01-01

    A search for white dwarf companions of MS-type stars was conducted, using IUE. The overendowments of these stars in typical S-process nuclides suggest that they, like the Ba II stars, may owe their peculiar compositions to earlier mass transfer. Short-wavelength IUE spectra show striking emission line variability in HD35155, HD61913, and 4 Ori; HD35155 and 4 Ori show evidence of white dwarf companions.

  12. The collapse of white dwarfs to neutron stars

    NASA Technical Reports Server (NTRS)

    Woosley, S. E.; Baron, E.

    1992-01-01

    The observable consequences of an accreting white dwarf collapsing directly to a neutron star are considered. The outcome depends critically upon the nature of the wind that is driven by neutrino absorption in the surface layers as the dwarf collapses. Unlike previous calculations which either ignored mass loss or employed inadequate zoning to resolve it, a characteristic mass-loss rate of about 0.005 solar mass/s and an energy input of 5 x 10 exp 50 ergs/s is found. Such a large mass-loss rate almost completely obscures any prompt electromagnetic display and certainly rules out the production by this model of gamma-ray bursts situated at cosmological distances. The occurrence of such collapses with the Milky Way Galaxy might, however, be detected and limited by their nucleosynthesis and gamma-ray line emission. To avoid the overproduction of rare neutron-rich isotopes heavier than iron, such events must be very infrequent, probably happening no more than once every thousand years.

  13. Origin of the DA and non-DA white dwarf stars

    NASA Technical Reports Server (NTRS)

    Shipman, Harry L.

    1989-01-01

    Various proposals for the bifurcation of the white dwarf cooling sequence are reviewed. 'Primordial' theories, in which the basic bifurcation of the white dwarf sequence is rooted in events predating the white dwarf stage of stellar evolution, are discussed, along with the competing 'mixing' theories in which processes occurring during the white dwarf stage are responsible for the existence of DA or non-DA stars. A new proposal is suggested, representing a two-channel scenario. In the DA channel, some process reduces the hydrogen layer mass to the value of less than 10 to the -7th. The non-DA channel is similar to that in the primordial scenario. These considerations suggest that some mechanism operates in both channels to reduce the thickness of the outermost layer of the white dwarf. It is also noted that accretion from the interstellar medium has little to do with whether a particular white dwarf becomes a DA or a non-DA star.

  14. SPITZER OBSERVATIONS OF WHITE DWARFS: THE MISSING PLANETARY DEBRIS AROUND DZ STARS

    SciTech Connect

    Xu, S.; Jura, M. E-mail: jura@astro.ucla.edu

    2012-01-20

    We report a Spitzer/Infrared Array Camera search for infrared excesses around white dwarfs, including 14 newly observed targets and 16 unpublished archived stars. We find a substantial infrared excess around two warm white dwarfs-J220934.84+122336.5 and WD 0843+516, the latter apparently being the hottest white dwarf known to display a close-in dust disk. Extending previous studies, we find that the fraction of white dwarfs with dust disks increases as the star's temperature increases; for stars cooler than 10,000 K, even the most heavily polluted ones do not have {approx}1000 K dust. There is tentative evidence that the dust disk occurrence is correlated with the volatility of the accreted material. In the Appendix, we modify a previous analysis to clarify how Poynting-Robertson drag might play an important role in transferring materials from a dust disk into a white dwarf's atmosphere.

  15. Very low mass stars and white dwarfs in NGC 6397

    NASA Technical Reports Server (NTRS)

    Paresce, Francesco; De Marchi, Guido; Romaniello, Martino

    1995-01-01

    Deep Wide Field/Planetary Camera 2 (WFPC2) images in wide bands centered at 606 and 802 nm were taken with the Hubble Space Telescope (HST) 4.6 min from the center of the galactic globular cluster NGC 6397. The images were used to accurately position approximately 2120 stars detected in the field on a color magnitude diagram down to a limiting magnitude m(sub 814) approximately = m(sub I) approximately = 26 determined reliably and solely by counting statistics. A white dwarf sequence and a rich, narrow cluster main sequence are detected for the first time, the latter stretching from m(sub 814) = 18.5 to m(sub 814) = 24.0 where it becomes indistinguishable from the field population. Two changes of slope of the main sequence at m(sub 814) approximately = 20 and m(sub 814) approximately = 22.5 are evident. The corresponding luminosity function increases slowly from M(sub 814) approximately = 6.5 to 8.5 are expected from ground-based observations but then drops sharply from there dwon to the measurement limit. The corresponding mass function obtained bu using the only presently available mass-luminosity function for the cluster's metallicity rises to a plateau between approximately 0.25 and approximately 0.15 solar mass, but drops toward the expected mass limit of the normal hydrogen burning main sequence at approximately 0.1 solar mass. This result is in clear contrast to that obtained from the ground and implies either a substantial modification of the cluster's initial mass function due to dynamical evolution in its lifetime, or that very low mass stars are not produced in any dynamically significant amount by clusters of this type. The white dwarf sequence is in reasonable agreement with a cooling sequence of models of mass 0.5 solar mass at the canonical distance of NGC 6397 with a scatter that is most likely due to photometric errors, but may also reflect real differences in mass or chemical composition. Contamination from unresolved galaxies, which cannot be

  16. An historical perspective - Brown is not a color. [astrophysics of infrared dwarf stars

    NASA Technical Reports Server (NTRS)

    Tarter, J. C.

    1986-01-01

    Major shifts in theoretical understanding of the star formation process and the possible components of the local mass density are reviewed. Those aspects of brown dwarf structure and evolution that are still not well enough understood are outlined, and the types of observations that might force the modification of current theories to accommodate the existence of brown dwarfs are suggested. The appropriateness of the name 'brown dwarf' is defended.

  17. Limits from the Ongoing Search for Planets Around White Dwarf Stars Using Pulsation Timings

    NASA Astrophysics Data System (ADS)

    Winget, D. E.; Hermes, J. J.; Mullally, Fergal; Bell, K. J.; Montgomery, M. H.; Williams, S. G.; Harrold, S. T.; Kepler, S. O.; Castanheira, B.; Chandler, D. W.; Winget, K. I.; Mukadam, A. S.; Nather, R. E.

    2015-06-01

    Evidence from searches of stars in our galaxy for exoplanet companions suggests that most lower main sequence stars likely have one or more planets; the vast majority of these planet-hosting stars will evolve into white dwarf stars. Some planets may survive this process and new ones may form in a sort of second generation from the cast-off material. If we combine this argument with evidence of a substantial population of metal polluted white dwarf stars, we may plausibly expect that planets may be common around white dwarf stars. Empirically, however, little is known about the presence of planets, new or old around white dwarf stars. Our search is small (˜15 white dwarf stars), but sensitive. Using pulsation arrival times we reach a large search volume around each star: we are sensitive to 1 MJupiter planets at distances ranging from 1- 100AU. In this context, our tightening constraints from pulsation timings become increasingly important to the broader study of planet formation, dynamical evolution, and ultimate survival.

  18. Brown dwarfs: at last filling the gap between stars and planets.

    PubMed

    Zuckerman, B

    2000-02-01

    Until the mid-1990s a person could not point to any celestial object and say with assurance that "here is a brown dwarf." Now dozens are known, and the study of brown dwarfs has come of age, touching upon major issues in astrophysics, including the nature of dark matter, the properties of substellar objects, and the origin of binary stars and planetary systems.

  19. Fundamental Properties of Low-Mass Stars and Brown Dwarfs

    SciTech Connect

    Liu, Michael C.; Dupuy, Trent J.; Stassun, Keivan G.; Allard, France; Blake, Cullen H.; Bonnefoy, M.; Cody, Ann Marie; Kraus, Adam; Day-Jones, A. C.; Lopez-Morales, Mercedes

    2009-02-16

    Precise measurements of the fundamental properties of low-mass stars and brown dwarfs are key to understanding the physics underlying their formation and evolution. While there has been great progress over the last decade in studying the bulk spectrophotometric properties of low-mass objects, direct determination of their masses, radii, and temperatures have been very sparse. Thus, theoretical predictions of low-mass evolution and ultracool atmospheres remain to be rigorously tested. The situation is alarming given that such models are widely used, from the determination of the low-mass end of the initial mass function to the characterization of exoplanets.An increasing number of mass, radius, and age determinations are placing critical constraints on the physics of low-mass objects. A wide variety of approaches are being pursued, including eclipsing binary studies, astrometric-spectroscopic orbital solutions, interferometry, and characterization of benchmark systems. In parallel, many more systems suitable for concerted study are now being found, thanks to new capabilities spanning both the very widest (all-sky surveys) and very narrowest (diffraction-limited adaptive optics) areas of the sky. This Cool Stars 15 splinter session highlighted the current successes and limitations of this rapidly growing area of precision astrophysics.

  20. Hunting for exploding red supergiant stars

    NASA Astrophysics Data System (ADS)

    Messineo, Maria; Menten, Karl M.; Figer, Donald F.; Ivanov, Valentin D.; Zhu, Qingfeng; Kudritzki, Rolf-Peter; Davies, Ben; Clark, J. Simon; Rich, Michael; Chen, Rosie; Trombley, Christine; MacKenty, John W.; Habing, Harm; Churchwell, Edward

    2015-08-01

    Red supergiants (RSGs) are among the brightest Galactic stars at infrared wavelengths. They lose mass at high-rates and, eventually, explode as supernovae, enriching the interstellar medium. I would like to present results on our ongoing searches for candidate obscured-far-luminous late-type stars, which are based on 2MASS, UKIDSS, and GLIMPSE data, on extinction-free colors(Messineo et al. 2012, A&A, 537) and on the analysis of the extinction curve along a given line-of-sight with clump stars. Messineo et al. (2014, A&A, 571, 43) spectroscopically confirmed two clusters of red supergiants, one on the Sagittarius-Carina spiral arm at a distance of ~7 kpc, and another on the Scutum-Crux arm at a distance of ~4 kpc; while Messineo et al. (2014, A&A, 569, 20) have, found several RSGs in the core of SNRs W41 and within the area covered by the SNR G22.7-0.2 in the GMC G23.3-0.3. SNR G22.7-0.2 appears to be most likely a type II SNR.Messineo , M.; Menten, K. M.; Churchwell, E.; Habing, H. 2012A&A...537A..10MMessineo, Maria; Zhu, Qingfeng; Ivanov, Valentin D.; Figer, Donald F.; Davies, Ben; Menten, Karl M.; Kudritzki, Rolf P.; Chen, C.-H. Rosie 2014A&A...571A..43MMessineo, Maria; Menten, Karl M.; Figer, Donald F.; Davies, Ben; Clark, J. Simon; Ivanov, Valentin D.; Kudritzki, Rolf-Peter; Rich, R. Michael; MacKenty, John W.; Trombley, Christine; 2014A&A...569A..20M

  1. Low-Metallicity Star Formation: From the First Stars to Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Hunt, Leslie K.; Madden, Suzanne C.; Schneider, Raffaella

    2008-12-01

    'Shea and Michael L. Norman; 16. Damped Lyα systems as probes of chemical evolution over cosmological timescales Miroslava Dessauges-Zavadsky; 17. Connecting high-redshift galaxy populations through observations of local damped Lyman alpha dwarf galaxies Regina E. Schulte-Ladbeck; 18. Chemical enrichment and feedback in low metallicity environments: constraints on galaxy formation Francesca Matteucci; 19. Effects of reionization on dwarf galaxy formation Massimo Ricotti; 20. The importance of following the evolution of the dust in galaxies on their SEDs A. Schurer, F. Calura, L. Silva, A. Pipino, G. L. Granato, F. Matteucci and R. Maiolino; 21. About the chemical evolution of dSphs (and the peculiar globular cluster ωCen) Andrea Marcolini and Annibale D'Ercole; 22. Young star clusters in the small Magellanic cloud: impact of local and global conditions on star formation Elena Sabbi, Linda J. Smith, Lynn R. Carlson, Antonella Nota, Monca Tosi, Michele Cignoni, Jay S. Gallagher III, Marco Sirianni and Margaret Meixner; 23. Modeling the ISM properties of metal-poor galaxies and gamma-ray burst hosts Emily M. Levesque, Lisa J. Kewley, Kirsten Larson and Leonie Snijders; 24. Dwarf galaxies and the magnetisation of the IGM Uli Klein; Session III. Explosive Events in Low-Metallicity Environments: 25. Supernovae and their evolution in a low metallicity ISM Roger A. Chevalier; 26. First stars - type Ib supernovae connection Ken'ichi Nomoto, Masaomi Tanaka, Yasuomi Kamiya, Nozomu Tominaga and Keiichi Maeda; 27. Supernova nucleosynthesis in the early universe Nozomu Tominaga, Hideyuki Umeda, Keiichi Maeda, Ken'ichi Nomoto and Nobuyuki Iwamoto; 28. Powerful explosions at Z = 0? Sylvia Ekström, Georges Meynet, Raphael Hirschi and André Maeder; 29. Wind anisotropy and stellar evolution Cyril Georgy, Georges Meynet and André Maeder; 30. Low-mass and metal-poor gamma-ray burst

  2. DISCOVERY OF MIRA VARIABLE STARS IN THE METAL-POOR SEXTANS DWARF SPHEROIDAL GALAXY

    SciTech Connect

    Sakamoto, Tsuyoshi; Matsunaga, Noriyuki; Nakada, Yoshikazu; Hasegawa, Takashi

    2012-12-10

    We report the discovery of two Mira variable stars (Miras) toward the Sextans dwarf spheroidal galaxy (dSph). We performed optical long-term monitoring observations for two red stars in the Sextans dSph. The light curves of both stars in the I{sub c} band show large-amplitude (3.7 and 0.9 mag) and long-period (326 {+-} 15 and 122 {+-} 5 days) variations, suggesting that they are Miras. We combine our own infrared data with previously published data to estimate the mean infrared magnitudes. The distances obtained from the period-luminosity relation of the Miras (75.3{sup +12.8}{sub -10.9} and 79.8{sup +11.5}{sub -9.9} kpc, respectively), together with the radial velocities available, support memberships of the Sextans dSph (90.0 {+-} 10.0 kpc). These are the first Miras found in a stellar system with a metallicity as low as [Fe/H] {approx} -1.9 than any other known system with Miras.

  3. Star formation in globular clusters and dwarf galaxies and implications for the early evolution of galaxies

    NASA Technical Reports Server (NTRS)

    Lin, Douglas N. C.; Murray, Stephen D.

    1991-01-01

    Based upon the observed properties of globular clusters and dwarf galaxies in the Local Group, we present important theoretical constraints on star formation in these systems. These constraints indicate that protoglobular cluster clouds had long dormant periods and a brief epoch of violent star formation. Collisions between protocluster clouds triggered fragmentation into individual stars. Most protocluster clouds dispersed into the Galactic halo during the star formation epoch. In contrast, the large spread in stellar metallicity in dwarf galaxies suggests that star formation in their pregenitors was self-regulated: we propose the protocluster clouds formed from thermal instability in the protogalactic clouds and show that a population of massive stars is needed to provide sufficient UV flux to prevent the collapsing protogalactic clouds from fragmenting into individual stars. Based upon these constraints, we propose a unified scenario to describe the early epochs of star formation in the Galactic halo as well as the thick and thin components of the Galactic disk.

  4. A novel multi-scale analysis to determine red giant branch metallicities of Milky Way dwarf spheroidal galaxies

    NASA Astrophysics Data System (ADS)

    Rodgers, Christopher Thomas

    Through the last century the color-magnitude diagram has given a huge wealth of information about resolved stellar populations. Objects ranging from sparse star associations and open clusters to the massive spiral and elliptical galaxies have been measured in a wide array of photometric filter systems to understand how galaxies formed into the structure that we as humans see them as today. With a basic knowledge of nuclear physics fused with stellar evolution we have measured the ages of these systems of stars, along with estimates of the chemical abundances. Our understanding has been that smaller systems like open and globular star clusters were formed as a single population of stars at roughly the same time. In contrast the larger systems like spiral and elliptical galaxies were formed by a combination of constant star formation along with mergers of smaller proto systems. In fact, these mergers are still happening in the current epoch of the universe. Over the last decade higher resolution studies paved by larger 8-10 meter telescopes, along with the orbiting Hubble Space Telescope, have shown the simplistic view of the formation of globular clusters and dwarf galaxies is no longer acceptable. Photometric and spectroscopic observations show that the globular clusters and dwarf spheroidal galaxies have multiple populations that vary with age, and/or metallicity (Geisler et al. 2007, Tolstoy et al. 2009). Two objects that show the extremes of each are the Carina dwarf spheroidal galaxy (Hurley-Keller et al. 1998) and the massive o Centauri globular cluster (Sollima et al. 2005). The more massive globular clusters show hints of multiple populations such as the NGC2808 globular cluster. It seems as though our understanding of the universe has only begun as we uncover more complexities with better tools to probe the universe. This dissertation thesis brings a new tool for stellar population studies when analyzing data from photometric systems. I have chosen theM I

  5. Star Formation Histories of Dwarf Galaxies: Keys to Galaxy Formation and Dark Matter Structure

    NASA Astrophysics Data System (ADS)

    Boylan-Kolchin, Michael

    2014-10-01

    We propose to perform a suite of hydrodynamical simulations in order to understand the connection between dwarf galaxy star formation histories, stellar content, and central dark matter densities. This will leverage one of HST's unique and enduring scientific contributions, deep and uniform photometry of nearby dwarf galaxies that enables reconstructions of their ancient {z 6-10} star formation histories, to shed light on some of the main problems in galaxy formation and cosmology.Analysis of HST observations has definitively shown that dwarfs exhibit a surprising variety of star formation histories: some dwarfs host nearly purely ancient populations, while others have formed 90% of their stars after redshift 1. This diversity is unexpected in current theoretical models, which predict primarily old stellar populations in low-mass objects. The topic of star formation histories of dwarfs has also received significant attention recently in the context of possible small-scale problems of the LCDM model: if episodic bursts of star formation inject energy into dwarfs' dark matter halos, it may provide a natural explanation of the observed low densities of dwarf galaxies within LCDM.Our simulations will adopt physically-motivated, explicit feedback prescriptions that are fixed by our knowledge of stellar evolution. We will choose halos having diverse mass assembly histories within a narrow range of mass and compare their star formation histories to HST observations of nearby isolated dwarf galaxies, thereby testing fundamental aspects of galaxy formation modeling and dark matter astrophysics. Data from our simulations will be publicly released via a dedicated website.

  6. 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).

  7. M dwarfs and the fraction of high carbon-to-oxygen stars in the solar neighbourhood

    NASA Astrophysics Data System (ADS)

    Gizis, John E.; Marks, Zachary; Hauschildt, Peter H.

    2016-02-01

    We investigate the frequency of high carbon-to-oxygen (C/O = 0.9) M dwarf stars in the solar neighbourhood. Using synthetic spectra, we find that such M dwarfs would have weaker TiO bands relative to hydride features. Similar weakening has already been detected in M-subdwarf (sdM) stars. By comparing to existing spectroscopic surveys of nearby stars, we show that less than one per cent of nearby stars have high carbon-to-oxygen ratios. This limit does not include stars with C/O = 0.9, [m/H] > 0.3, and [C/Fe] > 0.1, which we predict to have low-resolution optical spectra similar to solar metallicity M dwarfs.

  8. Stellar Populations and Star Formation History of the Metal-poor Dwarf Galaxy DDO 68

    NASA Astrophysics Data System (ADS)

    Sacchi, E.; Annibali, F.; Cignoni, M.; Aloisi, A.; Sohn, T.; Tosi, M.; van der Marel, R. P.; Grocholski, A. J.; James, B.

    2016-10-01

    We present the star formation history (SFH) of the extremely metal-poor dwarf galaxy DDO 68, based on our photometry with the Advanced Camera for Surveys. With a metallicity of only 12+{log}({{O}}/{{H}})=7.15 and a very isolated location, DDO 68 is one of the most metal-poor galaxies known. It has been argued that DDO 68 is a young system that started forming stars only ∼0.15 Gyr ago. Our data provide a deep and uncontaminated optical color–magnitude diagram (CMD) that allows us to disprove this hypothesis since we find a population of at least ∼1 Gyr old stars. The star formation activity has been fairly continuous over all the look-back time. The current rate is quite low, and the highest activity occurred between 10 and 100 Myr ago. The average star formation rate over the whole Hubble time is ≃0.01 M ⊙ yr‑1, corresponding to a total astrated mass of ≃1.3 × 108 M ⊙. Our photometry allows us to infer the distance from the tip of the red giant branch, D = 12.08 ± 0.67 Mpc; however, to let our synthetic CMD reproduce the observed ones, we need a slightly higher distance, D = 12.65 Mpc, or (m ‑ M)0 = 30.51, still inside the errors of the previous determination, and we adopt the latter. DDO 68 shows a very interesting and complex history, with its quite disturbed shape and a long tail, probably due to tidal interactions. The SFH of the tail differs from that of the main body mainly for enhanced activity at recent epochs likely triggered by the interaction. Based on observations obtained with the NASA/ESA Hubble Space Telescope at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy under NASA Contract NAS5-26555.

  9. ATMOSPHERIC CHEMISTRY IN GIANT PLANETS, BROWN DWARFS, AND LOW-MASS DWARF STARS. III. IRON, MAGNESIUM, AND SILICON

    SciTech Connect

    Visscher, Channon; Lodders, Katharina; Fegley, Bruce E-mail: lodders@wustl.ed

    2010-06-20

    We use thermochemical equilibrium calculations to model iron, magnesium, and silicon chemistry in the atmospheres of giant planets, brown dwarfs, extrasolar giant planets (EGPs), and low-mass stars. The behavior of individual Fe-, Mg-, and Si-bearing gases and condensates is determined as a function of temperature, pressure, and metallicity. Our equilibrium results are thus independent of any particular model atmosphere. The condensation of Fe metal strongly affects iron chemistry by efficiently removing Fe-bearing species from the gas phase. Monatomic Fe is the most abundant Fe-bearing gas throughout the atmospheres of EGPs and L dwarfs, and in the deep atmospheres of giant planets and T dwarfs. Mg- and Si-bearing gases are effectively removed from the atmosphere by forsterite (Mg{sub 2}SiO{sub 4}) and enstatite (MgSiO{sub 3}) cloud formation. Monatomic Mg is the dominant magnesium gas throughout the atmospheres of EGPs and L dwarfs and in the deep atmospheres of giant planets and T dwarfs. Silicon monoxide (SiO) is the most abundant Si-bearing gas in the deep atmospheres of brown dwarfs and EGPs, whereas SiH{sub 4} is dominant in the deep atmosphere of Jupiter and other gas giant planets. Several other Fe-, Mg-, and Si-bearing gases become increasingly important with decreasing effective temperature. In principle, a number of Fe, Mg, and Si gases are potential tracers of weather or diagnostic of temperature in substellar atmospheres.

  10. VARIABLE STARS IN THE ULTRA-FAINT DWARF SPHEROIDAL GALAXY URSA MAJOR I

    SciTech Connect

    Garofalo, Alessia; Moretti, Maria Ida; Cusano, Felice; Clementini, Gisella; Ripepi, Vincenzo; Dall'Ora, Massimo; Coppola, Giuseppina; Musella, Ilaria; Marconi, Marcella E-mail: fcusano@na.astro.it E-mail: ripepi@na.astro.it E-mail: imoretti@na.astro.it E-mail: ilaria@na.astro.it

    2013-04-10

    We have performed the first study of the variable star population of Ursa Major I (UMa I), an ultra-faint dwarf satellite recently discovered around the Milky Way (MW) by the Sloan Digital Sky Survey. Combining time series observations in the B and V bands from four different telescopes, we have identified seven RR Lyrae stars in UMa I, of which five are fundamental-mode (RRab) and two are first-overtone pulsators (RRc). Our V, B - V color-magnitude diagram of UMa I reaches V {approx} 23 mag (at a signal-to-noise ratio of {approx}6) and shows features typical of a single old stellar population. The mean pulsation period of the RRab stars (P{sub ab}) = 0.628, {sigma} = 0.071 days (or (P{sub ab}) = 0.599, {sigma} = 0.032 days, if V4, the longest period and brightest variable, is discarded) and the position on the period-amplitude diagram suggest an Oosterhoff-intermediate classification for the galaxy. The RR Lyrae stars trace the galaxy horizontal branch (HB) at an average apparent magnitude of (V(RR)) = 20.43 {+-} 0.02 mag (average on six stars and discarding V4), giving in turn a distance modulus for UMa I of (m - M){sub 0} = 19.94 {+-} 0.13 mag, distance d = 97.3{sup +6.0}{sub -5.7} kpc, in the scale where the distance modulus of the Large Magellanic Cloud is 18.5 {+-} 0.1 mag. Isodensity contours of UMa I red giants and HB stars (including the RR Lyrae stars identified in this study) show that the galaxy has an S-shaped structure, which is likely caused by the tidal interaction with the MW. Photometric metallicities were derived for six of the UMa I RR Lyrae stars from the parameters of the Fourier decomposition of the V-band light curves, leading to an average metal abundance of [Fe/H] = -2.29 dex ({sigma} = 0.06 dex, average on six stars) on the Carretta et al. metallicity scale.

  11. Gemini spectra of 12000 K white dwarf stars

    NASA Astrophysics Data System (ADS)

    Kepler, S. O.; Castanheira, B. G.; Costa, A. F. M.; Koester, D.

    2006-11-01

    We report signal-to-noise ratio (S/N) ~= 100 optical spectra for four DA white dwarf stars acquired with the GMOS spectrograph of the 8-m Gemini north telescope. These stars have 18 < g < 19 and are around Teff ~ 12000 K, where the hydrogen lines are close to maximum. Our purpose is to test if the effective temperatures and surface gravities derived from the relatively low-S/N ( ~ 21) optical spectra acquired by the Sloan Digital Sky Survey through model atmosphere fitting are trustworthy. Our spectra range from 3800 to 6000 Å, therefore including Hβ to H9. The H8 line was only marginally present in the SDSS spectra, but is crucial to determine the gravity. When we compare the values published by Kleinman et al. and Eisenstein et al. with our line-profile technique (LPT) fits, the average differences are: ΔTeff ~= 320 K, systematically lower in the SDSS, and Δ log g ~= 0.24 dex, systematically larger in the SDSS. The correlation between the gravity and the effective temperature can only be broken at wavelengths bluer than 3800 Å. The uncertainties in Teff are 60 per cent larger, and in log g larger by a factor of 4, than the internal uncertainties of Kleinman et al. and Eisenstein et al. Based on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the Particle Physics and Astronomy Research Council (United Kingdom), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), CNPq (Brazil) and CONICET (Argentina). E-mail: kepler@if.ufrgs.br

  12. Quantitative Spectral Morphology Analysis of Unusually Red and Blue L Dwarfs

    NASA Astrophysics Data System (ADS)

    Camnasio, Sara; Alam, Munazza Khalida; Rice, Emily L.; Cruz, Kelle L.; Faherty, Jacqueline K.; Mace, Gregory N.; Martin, Emily; Logsdon, Sarah E.; McLean, Ian S.; Brown Dwarfs in New York City (BDNYC)

    2016-01-01

    In an effort to constrain the properties of photometric color outliers, we present a quantitative spectral morphology analysis of medium-resolution NIRSPEC (R~2,000), SpeX cross-dispersed (R~2,000), Palomar TripleSpec (R~2600), and Magellan FIRE (R~6000) J-band spectra for a sample of unusually red and blue L dwarfs. Some red L dwarfs are low surface gravity, young objects whose spectra present weak Na I doublets and FeH absorption bands, but strong VO features (Cruz et al. 2009). Some blue L dwarfs are subdwarfs with low metallicity spectral features such as greater H2 absorption, stronger metal hydride bands, and enhanced TiO absorption (Burgasser et al 2008c). We fit 3rd order polynomials to the pseudo-continuum in order to provide a quantitative comparison of spectral morphology with other peculiar L dwarfs, field standards, young L dwarfs, and L subdwarf. The results indicated that the coefficients of the fit correlate with spectral type, but are independent of color. This newly found trend provides a parameter which can be utilized as an additional tool in characterizing quantifiable differences in the spectra of brown dwarfs. Furthermore, this method can be applied in studying the atmospheric properties of exoplanets, given their similarities with brown dwarfs in mass and photospheric properties.

  13. Dwarf spheroidal satellites of M31. I. Variable stars and stellar populations in Andromeda XIX

    SciTech Connect

    Cusano, Felice; Clementini, Gisella; Garofalo, Alessia; Federici, Luciana E-mail: gisella.clementini@oabo.inaf.it E-mail: alessia.garofalo@studio.unibo.it; and others

    2013-12-10

    We present B, V time-series photometry of Andromeda XIX (And XIX), the most extended (half-light radius of 6.'2) of Andromeda's dwarf spheroidal companions, which we observed with the Large Binocular Cameras at the Large Binocular Telescope. We surveyed a 23' × 23' area centered on And XIX and present the deepest color-magnitude diagram (CMD) ever obtained for this galaxy, reaching, at V ∼ 26.3 mag, about one magnitude below the horizontal branch (HB). The CMD shows a prominent and slightly widened red giant branch, along with a predominantly red HB, which extends to the blue to significantly populate the classical instability strip. We have identified 39 pulsating variable stars, of which 31 are of RR Lyrae type and 8 are Anomalous Cepheids (ACs). Twelve of the RR Lyrae variables and three of the ACs are located within And XIX's half light radius. The average period of the fundamental mode RR Lyrae stars ((P {sub ab}) = 0.62 days, σ = 0.03 days) and the period-amplitude diagram qualify And XIX as an Oosterhoff-Intermediate system. From the average luminosity of the RR Lyrae stars ((V(RR)) = 25.34 mag, σ = 0.10 mag), we determine a distance modulus of (m – M){sub 0} = 24.52 ± 0.23 mag in a scale where the distance to the Large Magellanic Cloud (LMC) is 18.5 ± 0.1 mag. The ACs follow a well-defined Period-Wesenheit (PW) relation that appears to be in very good agreement with the PW relationship defined by the ACs in the LMC.

  14. PROTOPLANETARY DISK MASSES FROM STARS TO BROWN DWARFS

    SciTech Connect

    Mohanty, Subhanjoy; Mortlock, Daniel; Greaves, Jane; Pascucci, Ilaria; Apai, Daniel; Scholz, Aleks; Thompson, Mark; Lodato, Giuseppe; Looper, Dagny

    2013-08-20

    We present SCUBA-2 850 {mu}m observations of seven very low mass stars (VLMS) and brown dwarfs (BDs). Three are in Taurus and four in the TW Hydrae Association (TWA), and all are classical T Tauri (cTT) analogs. We detect two of the three Taurus disks (one only marginally), but none of the TWA ones. For standard grains in cTT disks, our 3{sigma} limits correspond to a dust mass of 1.2 M{sub Circled-Plus} in Taurus and a mere 0.2 M{sub Circled-Plus} in the TWA (3-10 Multiplication-Sign deeper than previous work). We combine our data with other submillimeter/millimeter (sub-mm/mm) surveys of Taurus, {rho} Oph, and the TWA to investigate the trends in disk mass and grain growth during the cTT phase. Assuming a gas-to-dust mass ratio of 100:1 and fiducial surface density and temperature profiles guided by current data, we find the following. (1) The minimum disk outer radius required to explain the upper envelope of sub-mm/mm fluxes is {approx}100 AU for intermediate-mass stars, solar types, and VLMS, and {approx}20 AU for BDs. (2) While the upper envelope of apparent disk masses increases with M{sub *} from BDs to VLMS to solar-type stars, no such increase is observed from solar-type to intermediate-mass stars. We propose this is due to enhanced photoevaporation around intermediate stellar masses. (3) Many of the disks around Taurus and {rho} Oph intermediate-mass and solar-type stars evince an opacity index of {beta} {approx} 0-1, indicating significant grain growth. Of the only four VLMS/BDs in these regions with multi-wavelength measurements, three are consistent with considerable grain growth, though optically thick disks are not ruled out. (4) For the TWA VLMS (TWA 30A and B), combining our 850 {mu}m fluxes with the known accretion rates and ages suggests substantial grain growth by 10 Myr, comparable to that in the previously studied TWA cTTs Hen 3-600A and TW Hya. The degree of grain growth in the TWA BDs (2M1207A and SSPM1102) remains largely unknown. (5) A

  15. Suppression of cooling by strong magnetic fields in white dwarf stars.

    PubMed

    Valyavin, G; Shulyak, D; Wade, G A; Antonyuk, K; Zharikov, S V; Galazutdinov, G A; Plachinda, S; Bagnulo, S; Machado, L Fox; Alvarez, M; Clark, D M; Lopez, J M; Hiriart, D; Han, Inwoo; Jeon, Young-Beom; Zurita, C; Mujica, R; Burlakova, T; Szeifert, T; Burenkov, A

    2014-11-01

    Isolated cool white dwarf stars more often have strong magnetic fields than young, hotter white dwarfs, which has been a puzzle because magnetic fields are expected to decay with time but a cool surface suggests that the star is old. In addition, some white dwarfs with strong fields vary in brightness as they rotate, which has been variously attributed to surface brightness inhomogeneities similar to sunspots, chemical inhomogeneities and other magneto-optical effects. Here we describe optical observations of the brightness and magnetic field of the cool white dwarf WD 1953-011 taken over about eight years, and the results of an analysis of its surface temperature and magnetic field distribution. We find that the magnetic field suppresses atmospheric convection, leading to dark spots in the most magnetized areas. We also find that strong fields are sufficient to suppress convection over the entire surface in cool magnetic white dwarfs, which inhibits their cooling evolution relative to weakly magnetic and non-magnetic white dwarfs, making them appear younger than they truly are. This explains the long-standing mystery of why magnetic fields are more common amongst cool white dwarfs, and implies that the currently accepted ages of strongly magnetic white dwarfs are systematically too young. PMID:25327247

  16. Suppression of cooling by strong magnetic fields in white dwarf stars.

    PubMed

    Valyavin, G; Shulyak, D; Wade, G A; Antonyuk, K; Zharikov, S V; Galazutdinov, G A; Plachinda, S; Bagnulo, S; Machado, L Fox; Alvarez, M; Clark, D M; Lopez, J M; Hiriart, D; Han, Inwoo; Jeon, Young-Beom; Zurita, C; Mujica, R; Burlakova, T; Szeifert, T; Burenkov, A

    2014-11-01

    Isolated cool white dwarf stars more often have strong magnetic fields than young, hotter white dwarfs, which has been a puzzle because magnetic fields are expected to decay with time but a cool surface suggests that the star is old. In addition, some white dwarfs with strong fields vary in brightness as they rotate, which has been variously attributed to surface brightness inhomogeneities similar to sunspots, chemical inhomogeneities and other magneto-optical effects. Here we describe optical observations of the brightness and magnetic field of the cool white dwarf WD 1953-011 taken over about eight years, and the results of an analysis of its surface temperature and magnetic field distribution. We find that the magnetic field suppresses atmospheric convection, leading to dark spots in the most magnetized areas. We also find that strong fields are sufficient to suppress convection over the entire surface in cool magnetic white dwarfs, which inhibits their cooling evolution relative to weakly magnetic and non-magnetic white dwarfs, making them appear younger than they truly are. This explains the long-standing mystery of why magnetic fields are more common amongst cool white dwarfs, and implies that the currently accepted ages of strongly magnetic white dwarfs are systematically too young.

  17. Detection of a white dwarf companion to the Hyades stars HD 27483

    NASA Technical Reports Server (NTRS)

    Boehm-Vitense, Erika

    1993-01-01

    We observed with IUE a white dwarf (WD) companion to the Hyades F6 V binary stars HD 27483. This system is known to be a close binary of two nearly equal stars with an orbital period of 3.05 days. Our IUE observations revealed the presence of a third star, a white dwarf with an effective temperature of 23,000 +/- 1000 K and a mass of approximately 0.6 solar mass. Its presence in the Hyades cluster with a known age permits me to derive the mass of its progenitor, which must have been about 2.3 solar masses. The presence of the white dwarf in a binary system opens the possibility that some of the envelope material, which was expelled by the WD progenitor, may have been collected by the F6 stars. We may thus be able to study abundance anomalies of the WD progenitor with known mass on the surface of the F6 companions.

  18. Probing the Deep End of the Milky Way with Kepler: Asteroseismic Analysis of 854 Faint Red Giants Misclassified as Cool Dwarfs

    NASA Astrophysics Data System (ADS)

    Mathur, S.; García, R. A.; Huber, D.; Regulo, C.; Stello, D.; Beck, P. G.; Houmani, K.; Salabert, D.

    2016-08-01

    Asteroseismology has proven to be an excellent tool to determine not only global stellar properties with good precision, but also to infer the stellar structure, dynamics, and evolution for a large sample of Kepler stars. Prior to the launch of the mission, the properties of Kepler targets were inferred from broadband photometry, leading to the Kepler Input Catalog (KIC). The KIC was later revised in the Kepler Star Properties Catalog, based on literature values and an asteroseismic analysis of stars that were unclassified in the KIC. Here, we present an asteroseismic analysis of 45,400 stars that were classified as dwarfs in the Kepler Star Properties Catalog. We found that around 2% of the sample shows acoustic modes in the typical frequency range that put them in the red-giant category rather than the cool dwarf category. We analyze the asteroseismic properties of these stars, derive their surface gravities, masses, and radii, and present updated effective temperatures and distances. We show that the sample is significantly fainter than the previously known oscillating giants in the Kepler field, with the faintest stars reaching down to a Kepler magnitude of Kp ˜ 16. We demonstrate that 404 stars are at distances beyond 5 kpc and that the stars are significantly less massive than for the original Kepler red-giant sample, consistent with a population of distant halo giants. A comparison with a galactic population model shows that up to 40 stars might be genuine halo giants, which would increase the number of known asteroseismic halo stars by a factor of 4. The detections presented here will provide a valuable sample for galactic archeology studies.

  19. Carbon in Red Giants in Globular Clusters and Dwarf Spheroidal Galaxies

    NASA Astrophysics Data System (ADS)

    Kirby, Evan N.; Guo, Michelle; Zhang, Andrew J.; Deng, Michelle; Cohen, Judith G.; Guhathakurta, Puragra; Shetrone, Matthew D.; Lee, Young Sun; Rizzi, Luca

    2015-03-01

    We present carbon abundances of red giants in Milky Way (MW) globular clusters and dwarf spheroidal galaxies (dSphs). Our sample includes measurements of carbon abundances for 154 giants in the clusters NGC 2419, M68, and M15 and 398 giants in the dSphs Sculptor, Fornax, Ursa Minor, and Draco. This sample doubles the number of dSph stars with measurements of [C/Fe]. The [C/Fe] ratio in the clusters decreases with increasing luminosity above log (L/{{L}})≃ 1.6, which can be explained by deep mixing in evolved giants. The same decrease is observed in dSphs, but the initial [C/Fe] of the dSph giants is not uniform. Stars in dSphs at lower metallicities have larger [C/Fe] ratios. We hypothesize that [C/Fe] (corrected to the initial carbon abundance) declines with increasing [Fe/H] due to the metallicity dependence of the carbon yield of asymptotic giant branch stars and due to the increasing importance of SNe Ia at higher metallicities. We also identified 11 very carbon-rich giants (eight previously known) in three dSphs. However, our selection biases preclude a detailed comparison to the carbon-enhanced fraction of the MW stellar halo. Nonetheless, the stars with [C/Fe]\\lt +1 in dSphs follow a different [C/Fe] track with [Fe/H] than the halo stars. Specifically, [C/Fe] in dSphs begins to decline at lower [Fe/H] than in the halo. The difference in the metallicity of the [C/Fe] “knee” adds to the evidence from [α/Fe] distributions that the progenitors of the halo had a shorter timescale for chemical enrichment than the surviving dSphs. The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.

  20. Fundmental Parameters of Low-Mass Stars, Brown Dwarfs, and Planets

    NASA Astrophysics Data System (ADS)

    Montet, Benjamin; Johnson, John A.; Bowler, Brendan; Shkolnik, Evgenya

    2016-01-01

    Despite advances in evolutionary models of low-mass stars and brown dwarfs, these models remain poorly constrained by observations. In order to test these predictions directly, masses of individual stars must be measured and combined with broadband photometry and medium-resolution spectroscopy to probe stellar atmospheres. I will present results from an astrometric and spectroscopic survey of low-mass pre-main sequence binary stars to measure individual dynamical masses and compare to model predictions. This is the first systematic test of a large number of stellar systems of intermediate age between young star-forming regions and old field stars. Stars in our sample are members of the Tuc-Hor, AB Doradus, and beta Pictoris moving groups, the last of which includes GJ 3305 AB, the wide binary companion to the imaged exoplanet host 51 Eri. I will also present results of Spitzer observations of secondary eclipses of LHS 6343 C, a T dwarf transiting one member of an M+M binary in the Kepler field. By combining these data with Kepler photometry and radial velocity observations, we can measure the luminosity, mass, and radius of the brown dwarf. This is the first non-inflated brown dwarf for which all three of these parameters have been measured, providing the first benchmark to test model predictions of the masses and radii of field T dwarfs. I will discuss these results in the context of K2 and TESS, which will find additional benchmark transiting brown dwarfs over the course of their missions, including a description of the first planet catalog developed from K2 data and a program to search for transiting planets around mid-M dwarfs.

  1. On the age of Galactic bulge microlensed dwarf and subgiant stars

    NASA Astrophysics Data System (ADS)

    Valle, G.; Dell'Omodarme, M.; Prada Moroni, P. G.; Degl'Innocenti, S.

    2015-05-01

    Context. Recent results by Bensby and collaborators on the ages of microlensed dwarf and subgiant stars in the Galactic bulge have challenged the picture of an exclusively old stellar population, because ages significantly younger than 9 Gyr have been found. Aims: However, these age estimates have not been independently confirmed with different techniques and theoretical stellar models. One of the aims of this paper is to verify these results by means of a grid-based method. We also quantify the systematic biases that might be induced by some assumptions adopted to compute stellar models. In particular, we explore the impact of increasing the initial helium abundance, neglecting the element microscopic diffusion, and changing the mixing-length calibration in theoretical stellar track computations. Methods: We adopt the SCEPtER pipeline with a newly computed stellar model grid for metallicities [Fe/H] from - 2.00 dex to 0.55 dex, and masses in the range [0.60; 1.60] M⊙ from the zero-age main sequence to the helium flash at the red giant branch tip. By means of Monte Carlo simulations we show for the considered evolutionary phases that our technique provides unbiased age estimates. Results: Our age results are in good agreement with Bensby and collaborators findings and show 16 stars younger than 5 Gyr and 28 younger than 9 Gyr over a sample of 58. The effect of a helium enhancement as large as ΔY/ ΔZ = 5 is quite modest, resulting in a mean age increase of metal rich stars of 0.6 Gyr. Even simultaneously adopting a high helium content and the upper values of age estimates, there is evidence of 4 stars younger than 5 Gyr and 15 younger than 9 Gyr. For stars younger than 5 Gyr, the use of stellar models computed by neglecting microscopic diffusion or by assuming a super-solar mixing-length value leads to a mean increase in the age estimates of about 0.4 Gyr and 0.5 Gyr respectively. Even considering the upper values for the age estimates, there are four stars

  2. A Population Study of Wide-Separation Brown Dwarf Companions to Main Sequence Stars

    NASA Technical Reports Server (NTRS)

    Smith, Jeffrey J.

    2005-01-01

    Increased interest in infrared astronomy has opened the frontier to study cooler objects that shed significant light on the formation of planetary systems. Brown dwarf research provides a wealth of information useful for sorting through a myriad of proposed formation theories. Our study combines observational data from 2MASS with rigorous computer simulations to estimate the true population of long-range (greater than 1000 AU) brown dwarf companions in the solar neighborhood (less than 25 pc from Earth). Expanding on Gizis et al. (2001), we have found the margin of error in previous estimates to be significantly underestimated after we included orbit eccentricity, longitude of pericenter, angle of inclination, field star density, and primary and secondary luminosities as parameters influencing the companion systems in observational studies. We apply our simulation results to current L- and T-dwarf catalogs to provide updated estimates on the frequency of wide-separation brown dwarf companions to main sequence stars.

  3. Brown dwarfs: At last filling the gap between stars and planets

    PubMed Central

    Zuckerman, Ben

    2000-01-01

    Until the mid-1990s a person could not point to any celestial object and say with assurance that “here is a brown dwarf.” Now dozens are known, and the study of brown dwarfs has come of age, touching upon major issues in astrophysics, including the nature of dark matter, the properties of substellar objects, and the origin of binary stars and planetary systems. PMID:10655468

  4. Medium-resolution Analysis of Unusually Red and Blue L Dwarfs

    NASA Astrophysics Data System (ADS)

    Camnasio, Sara; Khalida Alam, Munazza; Rice, Emily L.; Cruz, Kelle L.; Faherty, Jacqueline K.; Mace, Gregory N.; Martin, Emily; Logsdon, Sarah E.; McLean, Ian S.; Bdnyc

    2015-01-01

    We present an analysis of medium-resolution NIRSPEC (R~25,000) and SpeX cross-dispersed (R~2,000) J band spectra for a sample of unusually red and blue L dwarfs. Some red L dwarfs are low surface gravity, young objects whose spectra present weak Na I doublets and FeH absorption bands, but strong VO features (Cruz et al. 2009). Some blue L dwarfs are subdwarfs with low metallicity spectral features such as greater H2 absorptions, stronger metal hydride bands, and enhanced TiO absorption (Burgasser et al. 2008c). Our sample of interest is composed of L dwarfs that present J-Ks color deviations that are greater than 0.4 mag and that lack the spectral features usually accompanying these extreme colors. To analyze the cause of the unusual color and compare them, we have been performing multi-resolution analysis across our sample. We measured equivalent widths and full width at half maximum of KI lines at 1.1773 µm, 1.1776 µm, 1.2436 µm, and 1.2525 µm for objects with high-resolution NIRSPEC data. These spectral line measurements in high resolution have been implemented with medium resolution data fitting. The coefficients from these polynomial fits to the pseudo-continuum have been employed in investigating the differences between standard L dwarf subtypes and unusually red and blue L dwarfs. We expect a trend in the values of the coefficients for the 3rd and 4th order polynomial fits when plotted for each L subtype versus the average J-Ks colors. The result of this analysis will allow us to further probe into the underlying physical properties of L dwarfs that cause the extreme J-Ks colors that we have been exploring.

  5. The late-M dwarfs

    SciTech Connect

    Bessell, M.S. )

    1991-02-01

    Far-red spectra and VRIJHK photometry have been obtained for a sample of late-M dwarfs selected on the basis of large reduced red magnitudes from the LHS Catalog. Half of the stars in the three faintest 1 mag bins are late-M stars, the other red stars are metallic-hydride subdwarfs. Relations between various colors for the late-M dwarfs are investigated. Of all the colors I - K most reliably correlates with spectral type. FeH bands near 9900 A are clearly seen in the spectra of all dwarf stars later than M5. Two stars cooler than VB10, and similar in temperature to LHS2924 have been identified; both have H-alpha in emission and appear variable in magnitude and R - I color; one is a flare star. The other stars are of earlier spectral type and resemble W359 and VB8. The observed MI, I - K main sequence is in good agreement with the IG theoretical main sequence of Stringfellow, and the faintest stars could be about 0.09 solar mass red dwarfs or lower mass brown dwarfs. 65 refs.

  6. Investigating Low-Mass Binary Stars And Brown Dwarfs with Near-Infrared Spectroscopy

    NASA Astrophysics Data System (ADS)

    Mace, Gregory Nathan

    The mass of a star at formation determines its subsequent evolution and demise. Low-mass stars are the most common products of star formation and their long main-sequence lifetimes cause them to accumulate over time. Star formation also produces many substellar-mass objects known as brown dwarfs, which emerge from their natal molecular clouds and continually cool as they age, pervading the Milky Way. Low-mass stars and brown dwarfs exhibit a wide range of physical characteristics and their abundance make them ideal subjects for testing formation and evolution models. I have examined a pair of pre-main sequence spectroscopic binaries and used radial velocity variations to determine orbital solutions and mass ratios. Additionally, I have employed synthetic spectra to estimate their effective temperatures and place them on theoretical Hertzsprung-Russell diagrams. From this analysis I discuss the formation and evolution of young binary systems and place bounds on absolute masses and radii. I have also studied the late-type T dwarfs revealed by the Wide-field Infrared Survey Explorer (WISE). This includes the exemplar T8 subdwarf Wolf 1130C, which has the lowest inferred metallicity in the literature and spectroscopic traits consistent with old age. Comparison to synthetic spectra implies that the dispersion in near-infrared colors of late-type T dwarfs is a result of age and/or thin sulfide clouds. With the updated census of the L, T, and Y dwarfs we can now study specific brown dwarf subpopulations. Finally, I present a number of future studies that would develop our understanding of the physical qualities of T dwarf color outliers and disentangle the tracers of age and atmospheric properties.

  7. Episodic Model For Star Formation History and Chemical Abundances in Giant and Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Debsarma, Suma; Chattopadhyay, Tanuka; Das, Sukanta; Pfenniger, Daniel

    2016-08-01

    In search for a synthetic understanding, a scenario for the evolution of the star formation rate and the chemical abundances in galaxies is proposed, combining gas infall from galactic halos, outflow of gas by supernova explosions, and an oscillatory star formation process. The oscillatory star formation model is a consequence of the modelling of the fractional masses changes of the hot, warm and cold components of the interstellar medium. The observed periods of oscillation vary in the range (0.1 - 3.0) × 107 yr depending on various parameters existing from giant to dwarf galaxies. The evolution of metallicity varies in giant and dwarf galaxies and depends on the outflow process. Observed abundances in dwarf galaxies can be reproduced under fast outflow together with slow evaporation of cold gases into hot gas whereas slow outflow and fast evaporation is preferred for giant galaxies. The variation of metallicities in dwarf galaxies supports the fact that low rate of SNII production in dwarf galaxies is responsible for variation in metallicity in dwarf galaxies of similar masses as suggested by various authors.

  8. Episodic model for star formation history and chemical abundances in giant and dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Debsarma, Suma; Chattopadhyay, Tanuka; Das, Sukanta; Pfenniger, Daniel

    2016-11-01

    In search for a synthetic understanding, a scenario for the evolution of the star formation rate and the chemical abundances in galaxies is proposed, combining gas infall from galactic haloes, outflow of gas by supernova explosions, and an oscillatory star formation process. The oscillatory star formation model is a consequence of the modelling of the fractional masses changes of the hot, warm and cold components of the interstellar medium. The derived periods of oscillation vary in the range (0.1-3.0) × 107 yr depending on various parameters existing from giant to dwarf galaxies. The evolution of metallicity varies in giant and dwarf galaxies and depends on the outflow process. Observed abundances in dwarf galaxies can be reproduced under fast outflow together with slow evaporation of cold gases into hot gas whereas slow outflow and fast evaporation is preferred for giant galaxies. The variation of metallicities in dwarf galaxies supports the fact that low rate of SNII production in dwarf galaxies is responsible for variation in metallicity in dwarf galaxies of similar masses as suggested by various authors.

  9. The Herschel Virgo Cluster Survey. XVIII. Star-forming dwarf galaxies in a cluster environment

    NASA Astrophysics Data System (ADS)

    Grossi, M.; Hunt, L. K.; Madden, S. C.; Hughes, T. M.; Auld, R.; Baes, M.; Bendo, G. J.; Bianchi, S.; Bizzocchi, L.; Boquien, M.; Boselli, A.; Clemens, M.; Corbelli, E.; Cortese, L.; Davies, J.; De Looze, I.; di Serego Alighieri, S.; Fritz, J.; Pappalardo, C.; Pierini, D.; Rémy-Ruyer, A.; Smith, M. W. L.; Verstappen, J.; Viaene, S.; Vlahakis, C.

    2015-02-01

    To assess the effects of the cluster environment on the different components of the interstellar medium, we analyse the far-infrared (FIR) and submillimetre (submm) properties of a sample of star-forming dwarf galaxies detected by the Herschel Virgo Cluster Survey (HeViCS). We determine dust masses and dust temperatures by fitting a modified black body function to the spectral energy distributions (SEDs). Stellar and gas masses, star formation rates (SFRs), and metallicities are obtained from the analysis of a set of ancillary data. Dust is detected in 49 out of a total 140 optically identified dwarfs covered by the HeViCS field; considering only dwarfs brighter than mB = 18 mag, this gives a detection rate of 43%. After evaluating different emissivity indices, we find that the FIR-submm SEDs are best-fit by β = 1.5, with a median dust temperature Td = 22.4 K. Assuming β = 1.5, 67% of the 23 galaxies detected in all five Herschel bands show emission at 500 μm in excess of the modified black-body model. The fraction of galaxies with a submillimetre excess decreases for lower values of β, while a similarly high fraction (54%) is found if a β-free SED modelling is applied. The excess is inversely correlated with SFR and stellar masses. To study the variations in the global properties of our sample that come from environmental effects, we compare the Virgo dwarfs to other Herschel surveys,such as the Key Insights into Nearby Galaxies: Far-Infrared Survey with Herschel (KINGFISH), the Dwarf Galaxy Survey (DGS), and the HeViCS Bright Galaxy Catalogue (BGC). We explore the relations between stellar mass and Hi fraction, specific star formation rate, dust fraction, gas-to-dust ratio over a wide range of stellar masses (from 107 to 1011 M⊙) for both dwarfs and spirals. Highly Hi-deficient Virgo dwarf galaxies are mostly characterised by quenched star formation activity and lower dust fractions giving hints for dust stripping in cluster dwarfs. However, to explain the

  10. The Rose-red Glow of Star Formation

    NASA Astrophysics Data System (ADS)

    2011-03-01

    The vivid red cloud in this new image from ESO's Very Large Telescope is a region of glowing hydrogen surrounding the star cluster NGC 371. This stellar nursery lies in our neighbouring galaxy, the Small Magellanic Cloud. The object dominating this image may resemble a pool of spilled blood, but rather than being associated with death, such regions of ionised hydrogen - known as HII regions - are sites of creation with high rates of recent star birth. NGC 371 is an example of this; it is an open cluster surrounded by a nebula. The stars in open clusters all originate from the same diffuse HII region, and over time the majority of the hydrogen is used up by star formation, leaving behind a shell of hydrogen such as the one in this image, along with a cluster of hot young stars. The host galaxy to NGC 371, the Small Magellanic Cloud, is a dwarf galaxy a mere 200 000 light-years away, which makes it one of the closest galaxies to the Milky Way. In addition, the Small Magellanic Cloud contains stars at all stages of their evolution; from the highly luminous young stars found in NGC 371 to supernova remnants of dead stars. These energetic youngsters emit copious amounts of ultraviolet radiation causing surrounding gas, such as leftover hydrogen from their parent nebula, to light up with a colourful glow that extends for hundreds of light-years in every direction. The phenomenon is depicted beautifully in this image, taken using the FORS1 instrument on ESO's Very Large Telescope (VLT). Open clusters are by no means rare; there are numerous fine examples in our own Milky Way. However, NGC 371 is of particular interest due to the unexpectedly large number of variable stars it contains. These are stars that change in brightness over time. A particularly interesting type of variable star, known as slowly pulsating B stars, can also be used to study the interior of stars through asteroseismology [1], and several of these have been confirmed in this cluster. Variable stars

  11. Chemistry of Stars in the Sculptor Dwarf Galaxy from VLT-FLAMES

    NASA Astrophysics Data System (ADS)

    Venn, Kim A.; Hill, V.

    The chemical composition of 91 stars in the Sculptor dwarf spheroidal galaxy is presented as determined from spectra taken with the FLAMES multiobject spectrograph in the Medusa mode. The analysis methods are outlined. The [α/Fe] ratios are shown for Mg, Ca, and Ti, and compared with those of Galactic stars. Heavy element abundance ratios (Y, Ba, and Eu) are also presented. Since the Sculptor dwarf galaxy has had a significantly different star formation history and chemical evolution than the Galaxy, then comparison of Sculptor's metal-poor (old) stars to similar metallicity stars in the Galaxy can be used to discuss galaxy formation scenarios, as well as test some of our fundamental assumptions in stellar nucleosynthesis.

  12. A PHOTOMETRIC VARIABILITY SURVEY OF FIELD K AND M DWARF STARS WITH HATNet

    SciTech Connect

    Hartman, J. D.; Bakos, G. A.; Noyes, R. W.; Sipocz, B.; Pal, A.; Kovacs, G.; Mazeh, T.; Shporer, A.

    2011-05-15

    Using light curves from the HATNet survey for transiting extrasolar planets we investigate the optical broadband photometric variability of a sample of 27, 560 field K and M dwarfs selected by color and proper motion (V - K {approx}> 3.0, {mu} > 30 mas yr{sup -1}, plus additional cuts in J - H versus H - K{sub S} and on the reduced proper motion). We search the light curves for periodic variations and for large-amplitude, long-duration flare events. A total of 2120 stars exhibit potential variability, including 95 stars with eclipses and 60 stars with flares. Based on a visual inspection of these light curves and an automated blending classification, we select 1568 stars, including 78 eclipsing binaries (EBs), as secure variable star detections that are not obvious blends. We estimate that a further {approx}26% of these stars may be blends with fainter variables, though most of these blends are likely to be among the hotter stars in our sample. We find that only 38 of the 1568 stars, including five of the EBs, have previously been identified as variables or are blended with previously identified variables. One of the newly identified EBs is 1RXS J154727.5+450803, a known P = 3.55 day, late M-dwarf SB2 system, for which we derive preliminary estimates for the component masses and radii of M{sub 1} = M{sub 2} = 0.258 {+-} 0.008 M{sub sun} and R{sub 1} = R{sub 2} = 0.289 {+-} 0.007 R{sub sun}. The radii of the component stars are larger than theoretical expectations if the system is older than {approx}200 Myr. The majority of the variables are heavily spotted BY Dra-type stars for which we determine rotation periods. Using this sample, we investigate the relations between period, color, age, and activity measures, including optical flaring, for K and M dwarfs, finding that many of the well-established relations for F, G, and K dwarfs continue into the M dwarf regime. We find that the fraction of stars that is variable with peak-to-peak amplitudes greater than 0.01 mag

  13. Discovery of true, likely and possible symbiotic stars in the dwarf spheroidal NGC 205

    NASA Astrophysics Data System (ADS)

    Gonçalves, Denise R.; Magrini, Laura; de la Rosa, Ignacio G.; Akras, Stavros

    2015-02-01

    In this paper we discuss the photometric and spectroscopic observations of newly discovered (symbiotic) systems in the dwarf spheroidal galaxy NGC 205. The Gemini Multi-Object Spectrograph on-off band [O III] 5007 Å emission imaging highlighted several [O III] line emitters, for which optical spectra were then obtained. The detailed study of the spectra of three objects allows us to identify them as true, likely and possible symbiotic systems (SySts), the first ones discovered in this galaxy. SySt-1 is unambiguously classified as a symbiotic star, because of the presence of unique emission lines which belong only to symbiotic spectra, the well-known O VI Raman-scattered lines. SySt-2 is only possibly a SySt because the Ne VII Raman-scattered line at 4881 Å, recently identified in a well-studied Galactic symbiotic as another very conspicuous property of symbiotic, could as well be identified as N III or [Fe III]. Finally, SySt-3 is likely a symbiotic binary because in the red part of the spectrum it shows the continuum of a late giant, and forbidden lines of moderate to high ionization, like [Fe V] 4180 Å. The main source for scepticism on the symbiotic nature of the latter systems is their location in the planetary nebula region in the [O III]4363/Hγ versus [O III]5007/Hβ diagnostic diagram. It is worth mentioning that at least another two confirmed symbiotics, one of the Local Group dwarf spheroidal IC 10 and the other of the Galaxy, are also misplaced in this diagram.

  14. Evidence for extended chromospheres surrounding red giant stars

    NASA Technical Reports Server (NTRS)

    Stencel, R. E.

    1982-01-01

    Observational evidence and theoretical arguments are summarized which indicate that regions of partially ionized hydrogen extending several stellar radii are an important feature of red giant and supergiant stars. The implications of the existence of extended chromospheres are examined in terms of the nature of the other atmospheres of, and mass loss from cool stars.

  15. The influence of H2O line blanketing on the spectra of cool dwarf stars

    NASA Technical Reports Server (NTRS)

    Allard, F.; Hauschildt, P. H.; Miller, S.; Tennyson, J.

    1994-01-01

    We present our initial results of model atmosphere calculations for cool M dwarfs using an opacity sampling method and a new list of H2O lines. We obtain significantly improved fits to the infrared spectrum of the M dwarf VB10 when compared to earlier models. H2O is by far the dominant opacity source in cool stars. To illustrate this, we show the Rosseland mean of the total extinction under various assumptions. Our calculations demonstrate the importance of a good treatment of the water opacities in cool stars and the improvements possible by using up-to-date data for the water line absorption.

  16. Temperate Earth-sized planets transiting a nearby ultracool dwarf star.

    PubMed

    Gillon, Michaël; Jehin, Emmanuël; Lederer, Susan M; Delrez, Laetitia; de Wit, Julien; Burdanov, Artem; Van Grootel, Valérie; Burgasser, Adam J; Triaud, Amaury H M J; Opitom, Cyrielle; Demory, Brice-Olivier; Sahu, Devendra K; Bardalez Gagliuffi, Daniella; Magain, Pierre; Queloz, Didier

    2016-05-12

    Star-like objects with effective temperatures of less than 2,700 kelvin are referred to as 'ultracool dwarfs'. This heterogeneous group includes stars of extremely low mass as well as brown dwarfs (substellar objects not massive enough to sustain hydrogen fusion), and represents about 15 per cent of the population of astronomical objects near the Sun. Core-accretion theory predicts that, given the small masses of these ultracool dwarfs, and the small sizes of their protoplanetary disks, there should be a large but hitherto undetected population of terrestrial planets orbiting them--ranging from metal-rich Mercury-sized planets to more hospitable volatile-rich Earth-sized planets. Here we report observations of three short-period Earth-sized planets transiting an ultracool dwarf star only 12 parsecs away. The inner two planets receive four times and two times the irradiation of Earth, respectively, placing them close to the inner edge of the habitable zone of the star. Our data suggest that 11 orbits remain possible for the third planet, the most likely resulting in irradiation significantly less than that received by Earth. The infrared brightness of the host star, combined with its Jupiter-like size, offers the possibility of thoroughly characterizing the components of this nearby planetary system. PMID:27135924

  17. Temperate Earth-sized planets transiting a nearby ultracool dwarf star.

    PubMed

    Gillon, Michaël; Jehin, Emmanuël; Lederer, Susan M; Delrez, Laetitia; de Wit, Julien; Burdanov, Artem; Van Grootel, Valérie; Burgasser, Adam J; Triaud, Amaury H M J; Opitom, Cyrielle; Demory, Brice-Olivier; Sahu, Devendra K; Bardalez Gagliuffi, Daniella; Magain, Pierre; Queloz, Didier

    2016-05-02

    Star-like objects with effective temperatures of less than 2,700 kelvin are referred to as 'ultracool dwarfs'. This heterogeneous group includes stars of extremely low mass as well as brown dwarfs (substellar objects not massive enough to sustain hydrogen fusion), and represents about 15 per cent of the population of astronomical objects near the Sun. Core-accretion theory predicts that, given the small masses of these ultracool dwarfs, and the small sizes of their protoplanetary disks, there should be a large but hitherto undetected population of terrestrial planets orbiting them--ranging from metal-rich Mercury-sized planets to more hospitable volatile-rich Earth-sized planets. Here we report observations of three short-period Earth-sized planets transiting an ultracool dwarf star only 12 parsecs away. The inner two planets receive four times and two times the irradiation of Earth, respectively, placing them close to the inner edge of the habitable zone of the star. Our data suggest that 11 orbits remain possible for the third planet, the most likely resulting in irradiation significantly less than that received by Earth. The infrared brightness of the host star, combined with its Jupiter-like size, offers the possibility of thoroughly characterizing the components of this nearby planetary system.

  18. How the First Stars Shaped the Faintest Gas-dominated Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Verbeke, R.; Vandenbroucke, B.; De Rijcke, S.

    2015-12-01

    Low-mass dwarf galaxies are very sensitive test-beds for theories of cosmic structure formation since their weak gravitational fields allow the effects of the relevant physical processes to clearly stand out. Up to now, no unified account has existed of the sometimes seemingly conflicting properties of the faintest isolated dwarfs in and around the Local Group, such as Leo T and the recently discovered Leo P and Pisces A systems. Using new numerical simulations, we show that this serious challenge to our understanding of galaxy formation can be effectively resolved by taking into account the regulating influence of the ultraviolet radiation of the first population of stars on a dwarf's star formation rate while otherwise staying within the standard cosmological paradigm for structure formation. These simulations produce faint, gas-dominated, star-forming dwarf galaxies that lie on the baryonic Tully-Fisher relation and that successfully reproduce a broad range of chemical, kinematical, and structural observables of real late-type dwarf galaxies. Furthermore, we stress the importance of obtaining properties of simulated galaxies in a manner as close as possible to the typically employed observational techniques.

  19. ANCIENT PLANETARY SYSTEMS ARE ORBITING A LARGE FRACTION OF WHITE DWARF STARS

    SciTech Connect

    Zuckerman, B.; Melis, C.; Klein, B.; Jura, M.; Koester, D. E-mail: cmelis@ucsd.ed E-mail: jura@astro.ucla.ed

    2010-10-10

    Infrared studies have revealed debris likely related to planet formation in orbit around {approx}30% of youthful, intermediate mass, main-sequence stars. We present evidence, based on atmospheric pollution by various elements heavier than helium, that a comparable fraction of the white dwarf descendants of such main-sequence stars are orbited by planetary systems. These systems have survived, at least in part, through all stages of stellar evolution that precede the white dwarf. During the time interval ({approx}200 million years) that a typical polluted white dwarf in our sample has been cooling it has accreted from its planetary system the mass of one of the largest asteroids in our solar system (e.g., Vesta or Ceres). Usually, this accreted mass will be only a fraction of the total mass of rocky material that orbits these white dwarfs; for plausible planetary system configurations we estimate that this total mass is likely to be at least equal to that of the Sun's asteroid belt, and perhaps much larger. We report abundances of a suite of eight elements detected in the little studied star G241-6 that we find to be among the most heavily polluted of all moderately bright white dwarfs.

  20. A Search for Kilogauss Magnetic Fields in White Dwarfs and Hot Subdwarf Stars

    NASA Astrophysics Data System (ADS)

    Valyavin, G.; Bagnulo, S.; Fabrika, S.; Reisenegger, A.; Wade, G. A.; Han, Inwoo; Monin, D.

    2006-09-01

    We present new results of a survey for weak magnetic fields among DA white dwarfs, including some brighter hot subdwarf stars. We have detected variable circular polarization in the Hα line of the hot subdwarf star Feige 34 (spectroscopic type: sdO). From these data, we estimate that the longitudinal magnetic field of this star varies from -1.1+/-3.2 to +9.6+/-2.6 kG, with a mean of about +5 kG and a period longer than 2 hr. In this study, we also confirm the magnetic nature of white dwarf WD 1105-048, found earlier in a study by Aznar Cuadrado and coworkers, and present upper limits of kilogauss longitudinal magnetic fields of the five brightest DA white dwarfs. Our data support the finding of Aznar Cuadrado and coworkers that ~25% of white dwarfs have kilogauss magnetic fields. This frequency also confirms results of early estimates obtained using the magnetic field function of white dwarfs (Fabrika & Valyavin).

  1. Extreme Stellar Populations in the Universe: Backsplash Dwarf Galaxies and Wandering Stars

    NASA Astrophysics Data System (ADS)

    Teyssier, Maureen

    that Tucana, Cetus, NGC3109, SextansA, SextansB, Antlia, NGC6822, Phoenix, LeoT, and NGC185 have passed through the Milky Way. Indeed, several of these galaxies---especially those with lower masses---contain signatures in their morphology, star formation history and/or gas content indicative of evolution seen in simulations of satellite/parent galactic interactions. Our results offer strong support for scenarios in which dwarfs of different types form a sequence in morphology and gas content, with evolution along the sequence being driven by interaction history. We use the Via Lactea II cosmological N-body simulation of the formation of Milky Way and M31 Analogues, to explore the expected properties of intergalactic light (light found beyond the virial radii of galaxies) in poor groups and around isolated galaxies. We find that the luminosity fraction of intergalactic light is ~1%. This is similar to observational measurements of intergalactic light in poor groups. We expect this result to be observationally verifiable through observations of supernovae Ia by blind, repeated surveys like Pan-STARRS and LSST. We find the major contributors to the intergalactic light are the largest mass satellite haloes due to the low stellar fraction expected in smaller mass haloes. The intergalactic light produced by the most massive satellites has a much smaller spatial extent than that produced by lower mass satellites, meaning that baryon prescriptions designed to supress star formation in low mass satellites also shrink the spatial extent of intergalactic light. It may be possible to use observations of the large quantity of intergalactic red giants, that we expect in the Local Group, to define the spatial extent of the intergalactic light, and thereby place limits on the total star formation in progenitor satellites in the Local Group.

  2. The nature of the F str lambda 4077 stars. 3: Spectroscopy of the barium dwarfs and other CP stars

    NASA Technical Reports Server (NTRS)

    North, P.; Berthet, S.; Lanz, T.

    1994-01-01

    The abundances of C, O, Al, Ca, iron-peak and s-process elements have been derived from high-resolution spectra for a sample of stars classified as F str lambda 4077 by Bidelman. Among the 20 stars mentioned by Bidelman, we have discovered 8 barium dwarfs (or CH subgiants, according to Bond's terminology), while a 9th star, HD 182274, was already known as a CH subgiant. In addition, we have analyzed three barium stars taken from the list of Lu et al. (1983) which are probably dwarfs rather than giants, and three CH subgiants. The other 11 F str lambda 4077 stars resemble either the delta Delphini stars, since their iron abundance is enhanced while Ca is normal, or are probably spectrum composites. A few Am, Ap, lambda Bootis and normal stars have been analyzed for comparison. In particular, we have included three lambda Boo candidates, selected from their photometric properties, and their iron deficiency is confirmed. The spectroscopic, photometric and statistical evidences concerning the Ba dwarfs, support the idea that these stars may be the main sequence counterparts, and possibly the progenitors of the Ba giants. The C/O ratio varies in these stars from normal values to a maximum of 1.5, but mostly within 0.6 and 1.2. Some of these objects may therefore be considered, in this sense, as carbon stars. On the other hand, the abundances of carbon and s-process elements relative to iron are inversely correlated with metallicity, and may even exceed significantly those of typical, solar-metallicity carbon stars. Metal-deficient C stars must therefore have (C/Fe) greater than or approximately equal to 1 and (s/Fe) greater than or approximately equal to 1.5 as soon as (Fe/H) less than or approximately equal to -1. The neutron exposure is shown to increase when the metallicity decreases, which is compatible with the C-13 (alpha, n) O-16 neutron source, but not with the Ne-22 (alpha, n) Mg-25 one. The evolutionary state (within the main sequence) of the Ba dwarfs, is

  3. CHARACTERIZING THE BROWN DWARF FORMATION CHANNELS FROM THE INITIAL MASS FUNCTION AND BINARY-STAR DYNAMICS

    SciTech Connect

    Thies, Ingo; Pflamm-Altenburg, Jan; Kroupa, Pavel; Marks, Michael

    2015-02-10

    The stellar initial mass function (IMF) is a key property of stellar populations. There is growing evidence that the classical star-formation mechanism by the direct cloud fragmentation process has difficulties reproducing the observed abundance and binary properties of brown dwarfs and very-low-mass stars. In particular, recent analytical derivations of the stellar IMF exhibit a deficit of brown dwarfs compared to observational data. Here we derive the residual mass function of brown dwarfs as an empirical measure of the brown dwarf deficiency in recent star-formation models with respect to observations and show that it is compatible with the substellar part of the Thies-Kroupa IMF and the mass function obtained by numerical simulations. We conclude that the existing models may be further improved by including a substellar correction term that accounts for additional formation channels like disk or filament fragmentation. The term ''peripheral fragmentation'' is introduced here for such additional formation channels. In addition, we present an updated analytical model of stellar and substellar binarity. The resulting binary fraction and the dynamically evolved companion mass-ratio distribution are in good agreement with observational data on stellar and very-low-mass binaries in the Galactic field, in clusters, and in dynamically unprocessed groups of stars if all stars form as binaries with stellar companions. Cautionary notes are given on the proper analysis of mass functions and the companion mass-ratio distribution and the interpretation of the results. The existence of accretion disks around young brown dwarfs does not imply that these form just like stars in direct fragmentation.

  4. Lifestyles of the Stars.

    ERIC Educational Resources Information Center

    National Aeronautics and Space Administration, Cocoa Beach, FL. John F. Kennedy Space Center.

    Some general information on stars is provided in this National Aeronautics and Space Administration pamphlet. Topic areas briefly discussed are: (1) the birth of a star; (2) main sequence stars; (3) red giants; (4) white dwarfs; (5) neutron stars; (6) supernovae; (7) pulsars; and (8) black holes. (JN)

  5. Homogeneous Photometry VI: Variable Stars in the Leo I Dwarf Spheroidal Galaxy

    NASA Astrophysics Data System (ADS)

    Stetson, Peter B.; Fiorentino, Giuliana; Bono, Giuseppe; Bernard, Edouard J.; Monelli, Matteo; Iannicola, Giacinto; Gallart, Carme; Ferraro, Ivan

    2014-07-01

    From archival ground-based images of the Leo I dwarf spheroidal galaxy, we have identified and characterized the pulsation properties of 164 candidate RR Lyrae variables and 55 candidate anomalous and/or short-period Cepheids. We have also identified 19 candidate long-period variable stars and 13 other candidate variables whose physical nature is unclear, but due to the limitations of our observational material we are unable to estimate reliable periods for them. On the basis of its RR Lyrae star population, Leo I is confirmed to be an Oosterhoff-intermediate type galaxy, like several other dwarf spheroidals. From the RR Lyrae stars we have derived a range of possible distance moduli for Leo I : 22.06 ± 0.08 lsim μ0 lsim 22.25 ± 0.07 mag depending on the metallicity assumed for the old population ([Fe/H] from -1.43 to -2.15). This is in agreement with previous independent estimates. We show that in their pulsation properties, the RR Lyrae stars—representing the oldest stellar population in the galaxy—are not significantly different from those of five other nearby, isolated dwarf spheroidal galaxies. A similar result is obtained when comparing them to RR Lyrae stars in recently discovered ultra-faint dwarf galaxies. We are able to compare the period distributions and period-amplitude relations for a statistically significant sample of ab-type RR Lyrae stars in dwarf galaxies (~1300 stars) with those in the Galactic halo field (~14,000 stars) and globular clusters (~1000 stars). Field RRLs show a significant change in their period distribution when moving from the inner (dG lsim 14 kpc) to the outer (dG gsim 14 kpc) halo regions. This suggests that the halo formed from (at least) two dissimilar progenitors or types of progenitor. Considered together, the RR Lyrae stars in classical dwarf spheroidal and ultra-faint dwarf galaxies—as observed today—do not appear to follow the well defined pulsation properties shown by those in either the inner or the outer

  6. Exclusion of a luminous red giant as a companion star to the progenitor of supernova SN 2011fe.

    PubMed

    Li, Weidong; Bloom, Joshua S; Podsiadlowski, Philipp; Miller, Adam A; Cenko, S Bradley; Jha, Saurabh W; Sullivan, Mark; Howell, D Andrew; Nugent, Peter E; Butler, Nathaniel R; Ofek, Eran O; Kasliwal, Mansi M; Richards, Joseph W; Stockton, Alan; Shih, Hsin-Yi; Bildsten, Lars; Shara, Michael M; Bibby, Joanne; Filippenko, Alexei V; Ganeshalingam, Mohan; Silverman, Jeffrey M; Kulkarni, S R; Law, Nicholas M; Poznanski, Dovi; Quimby, Robert M; McCully, Curtis; Patel, Brandon; Maguire, Kate; Shen, Ken J

    2011-12-15

    Type Ia supernovae are thought to result from a thermonuclear explosion of an accreting white dwarf in a binary system, but little is known of the precise nature of the companion star and the physical properties of the progenitor system. There are two classes of models: double-degenerate (involving two white dwarfs in a close binary system) and single-degenerate models. In the latter, the primary white dwarf accretes material from a secondary companion until conditions are such that carbon ignites, at a mass of 1.38 times the mass of the Sun. The type Ia supernova SN 2011fe was recently detected in a nearby galaxy. Here we report an analysis of archival images of the location of SN 2011fe. The luminosity of the progenitor system (especially the companion star) is 10-100 times fainter than previous limits on other type Ia supernova progenitor systems, allowing us to rule out luminous red giants and almost all helium stars as the mass-donating companion to the exploding white dwarf. PMID:22170681

  7. Exclusion of a luminous red giant as a companion star to the progenitor of supernova SN 2011fe.

    PubMed

    Li, Weidong; Bloom, Joshua S; Podsiadlowski, Philipp; Miller, Adam A; Cenko, S Bradley; Jha, Saurabh W; Sullivan, Mark; Howell, D Andrew; Nugent, Peter E; Butler, Nathaniel R; Ofek, Eran O; Kasliwal, Mansi M; Richards, Joseph W; Stockton, Alan; Shih, Hsin-Yi; Bildsten, Lars; Shara, Michael M; Bibby, Joanne; Filippenko, Alexei V; Ganeshalingam, Mohan; Silverman, Jeffrey M; Kulkarni, S R; Law, Nicholas M; Poznanski, Dovi; Quimby, Robert M; McCully, Curtis; Patel, Brandon; Maguire, Kate; Shen, Ken J

    2011-12-14

    Type Ia supernovae are thought to result from a thermonuclear explosion of an accreting white dwarf in a binary system, but little is known of the precise nature of the companion star and the physical properties of the progenitor system. There are two classes of models: double-degenerate (involving two white dwarfs in a close binary system) and single-degenerate models. In the latter, the primary white dwarf accretes material from a secondary companion until conditions are such that carbon ignites, at a mass of 1.38 times the mass of the Sun. The type Ia supernova SN 2011fe was recently detected in a nearby galaxy. Here we report an analysis of archival images of the location of SN 2011fe. The luminosity of the progenitor system (especially the companion star) is 10-100 times fainter than previous limits on other type Ia supernova progenitor systems, allowing us to rule out luminous red giants and almost all helium stars as the mass-donating companion to the exploding white dwarf.

  8. REVERSAL OF FORTUNE: INCREASED STAR FORMATION EFFICIENCIES IN THE EARLY HISTORIES OF DWARF GALAXIES?

    SciTech Connect

    Madau, Piero; Weisz, Daniel R.; Conroy, Charlie

    2014-08-01

    On dwarf galaxy scales, the different shapes of the galaxy stellar mass function and the dark halo mass function require a star-formation efficiency (SFE) in these systems that is currently more than 1 dex lower than that of Milky Way-size halos. Here, we argue that this trend may actually be reversed at high redshift. Specifically, by combining the resolved star-formation histories of nearby isolated dwarfs with the simulated mass-growth rates of dark matter halos, we show that the assembly of these systems occurs in two phases: (1) an early, fast halo accretion phase with a rapidly deepening potential well, characterized by a high SFE; and (2) a late, slow halo accretion phase where, perhaps as a consequence of reionization, the SFE is low. Nearby dwarfs have more old stars than predicted by assuming a constant or decreasing SFE with redshift, a behavior that appears to deviate qualitatively from the trends seen among more massive systems. Taken at face value, the data suggest that at sufficiently early epochs, dwarf galaxy halos above the atomic cooling mass limit can be among the most efficient sites of star formation in the universe.

  9. The white dwarf companion of the B a 2 star zeta Cap

    NASA Technical Reports Server (NTRS)

    Boehm-Vitense, E.

    1981-01-01

    The Ba II star zeta Cap has a white dwarf companion. Its T (sub eff) is determined to be 22000 K, its mass is approximately one solar mass. The importance of this finding for the explanation of abundance peculiarities is discussed.

  10. On the absence of young white dwarf companions to five technetium stars

    NASA Technical Reports Server (NTRS)

    Smith, Verne V.; Lambert, David L.

    1987-01-01

    A search for hot companions to five stars of type MS and S has been carried out using the IUE satellite. No hot companions were detected for the MS stars HR 85, 4647, 6702, and 8062, and the S star HR 8714. Limits on the luminosities of possible white dwarf companions provide lower limits of 2-5x10 to the 8th yr to the ages of any degenerate companions. All five stars exhibit strong Tc I lines, and the presence of technetium, with a half-life of 2.1x10 to the 5th yr, signifies recent nucleosynthesis. The limits on the ages of possible white dwarf companions that are equal to or greater than 1000 half-lives of Tc exclude the possibility that the s-process elemental enhancement seen in these MS and S stars resulted from mass transfer from a more highly evolved companion (as is probably the mechanism by which barium stars are created). These MS and S stars represent a sample of true thermally pulsing asymptotic giant-branch stars.

  11. A brown dwarf desert for intermediate mass stars in Scorpius OB2?

    NASA Astrophysics Data System (ADS)

    Kouwenhoven, M. B. N.; Brown, A. G. A.; Kaper, L.

    2007-03-01

    We present JHKS observations of 22 intermediate-mass stars in the Scorpius-Centaurus OB association, obtained with the NAOS/CONICA system at the ESO Very Large Telescope. This survey was performed to determine the status of (sub)stellar candidate companions of Sco OB2 member stars of spectral type A and late-B. The distinction between companions and background stars is made on the basis of a comparison to isochrones and additional statistical arguments. We are sensitive to companions with an angular separation of 0.1''-11'' (13-1430 AU) and the detection limit is K_S=17 mag. We detect 62 stellar components of which 18 turn out to be physical companions, 11 candidate companions, and 33 background stars. Three of the 18 confirmed companions were previously undocumented as such. The companion masses are in the range 0.03 {M}_⊙ ≤ M ≤ 1.19 {M}_⊙, corresponding to mass ratios 0.06 ≤ q ≤ 0.55. We include in our sample a subset of 9 targets with multi-color ADONIS observations from Kouwenhoven et al. (2005, A&A, 430, 137). In the ADONIS survey secondaries with KS < 12 mag were classified as companions; those with KS > 12 mag as background stars. The multi-color analysis in this paper demonstrates that the simple K_S=12 mag criterion correctly classifies the secondaries in 80% of the cases. We reanalyse the total sample (i.e. NAOS/CONICA and ADONIS) and conclude that of the 176 secondaries, 25 are physical companions, 55 are candidate companions, and 96 are background stars. Although we are sensitive (and complete) to brown dwarf companions as faint as K_S=14 mag in the semi-major axis range 130-520 AU, we detect only one, corresponding to a brown dwarf companion fraction of 0.5 ± 0.5% (M ⪆ 30 {M_J}). However, the number of brown dwarfs is consistent with an extrapolation of the (stellar) companion mass distribution into the brown dwarf regime. This indicates that the physical mechanism for the formation of brown dwarf companions around intermediate mass

  12. What does an erupting nova do to its red dwarf companion

    SciTech Connect

    Kovetz, A.; Prialnik, D.; Shara, M.M.

    1988-02-01

    During nova eruptions and for decades afterward, the red dwards in cataclysmic binaries are irradiated with hundreds of times more luminosity than they themselves produce. Simulations of the time-dependent irradiation of three red dwarf models (0.25, 0.50, and 0.75 solar mass) are presented. The mass transfer rates forced by irradiation after nova eruption are found to be enhanced by two orders of magnitude because of the irradiation. The time scale for irradiation to become unimportant is that of the white dwarf cooling time scale, a few centuries. These two results support the hibernation scenario of novae, which suggests that novae remain bright for a few centuries after eruption because of irradiation-induced mass transfer. After irradiation decreases mass transfer slows, and some very old novae may then become extremely faint. 26 references.

  13. Habitable Worlds Around M Dwarf Stars: The CAPSCam Astrometric Planet Search

    NASA Astrophysics Data System (ADS)

    Boss, Alan P.; Weinberger, Alycia J.; Anglada-Escudé, Guillem; Thompson, Ian B.; Brahm, Rafael

    2014-04-01

    M dwarf stars are attractive targets in the search for habitable worlds as a result of their relative abundance and proximity, making them likely targets for future direct detection efforts. Hot super-Earths as well as gas giants have already been detected around a number of early M dwarfs, and the former appear to be the high-mass end of the population of rocky, terrestrial exoplanets. The Carnegie Astrometric Planet Search (CAPS) program has been underway since March 2007, searching ~ 100 nearby late M, L, and T dwarfs for gas giant planets on orbits wide enough for habitable worlds to orbit interior to them. The CAPSCam-N camera on the 2.5-m du Pont telescope at the Las Campanas Observatory has demonstrated the ability to detect planets as low in mass as Saturn orbiting at several AU around late M dwarfs within 15 pc. Over the next decade, the CAPS program will provide new constraints on the planetary census around late M dwarf stars, and hence on the suitability of these nearby planetary systems for supporting life.

  14. Temperate Earth-sized planets transiting a nearby ultracool dwarf star

    NASA Astrophysics Data System (ADS)

    Gillon, Michaël; Jehin, Emmanuël; Lederer, Susan M.; Delrez, Laetitia; de Wit, Julien; Burdanov, Artem; Van Grootel, Valérie; Burgasser, Adam J.; Triaud, Amaury H. M. J.; Opitom, Cyrielle; Demory, Brice-Olivier; Sahu, Devendra K.; Bardalez Gagliuffi, Daniella; Magain, Pierre; Queloz, Didier

    2016-05-01

    Star-like objects with effective temperatures of less than 2,700 kelvin are referred to as ‘ultracool dwarfs’. This heterogeneous group includes stars of extremely low mass as well as brown dwarfs (substellar objects not massive enough to sustain hydrogen fusion), and represents about 15 per cent of the population of astronomical objects near the Sun. Core-accretion theory predicts that, given the small masses of these ultracool dwarfs, and the small sizes of their protoplanetary disks, there should be a large but hitherto undetected population of terrestrial planets orbiting them—ranging from metal-rich Mercury-sized planets to more hospitable volatile-rich Earth-sized planets. Here we report observations of three short-period Earth-sized planets transiting an ultracool dwarf star only 12 parsecs away. The inner two planets receive four times and two times the irradiation of Earth, respectively, placing them close to the inner edge of the habitable zone of the star. Our data suggest that 11 orbits remain possible for the third planet, the most likely resulting in irradiation significantly less than that received by Earth. The infrared brightness of the host star, combined with its Jupiter-like size, offers the possibility of thoroughly characterizing the components of this nearby planetary system.

  15. Discovery of a brown dwarf companion to the A3V star β Circini

    NASA Astrophysics Data System (ADS)

    Smith, L. C.; Lucas, P. W.; Contreras Peña, C.; Kurtev, R.; Marocco, F.; Jones, H. R. A.; Beamin, J. C.; Napiwotzki, R.; Borissova, J.; Burningham, B.; Faherty, J.; Pinfield, D. J.; Gromadzki, M.; Ivanov, V. D.; Minniti, D.; Stimson, W.; Villanueva, V.

    2015-12-01

    We report the discovery of an L dwarf companion to the A3V star β Circini. VVV J151721.49-585131.5, or β Cir B, was identified in a proper motion and parallax catalogue of the VISTA Variables in the Vía Láctea survey as having near-infrared luminosity and colour indicative of an early L dwarf, and a proper motion and parallax consistent with that of β Cir. The projected separation of ˜3.6 arcmin corresponds to 6656 au, which is unusually wide. The most recent published estimate of the age of the primary combined with our own estimate based on newer isochrones yields an age of 370-500 Myr. The system therefore serves as a useful benchmark at an age greater than that of the Pleiades brown dwarfs and most other young L dwarf benchmarks. We have obtained a medium resolution echelle spectrum of the companion which indicates a spectral type of L1.0 ± 0.5 and lacks the typical signatures of low-surface gravity seen in younger brown dwarfs. This suggests that signs of low-surface gravity disappear from the spectra of early L dwarfs by an age of ˜370-500 Myr, as expected from theoretical isochrones. The mass of β Cir B is estimated from the BHAC15 isochrones as 0.056 ± 0.007 M⊙.

  16. Carbon-to-oxygen Ratios in M Dwarfs and Solar-type Stars

    NASA Astrophysics Data System (ADS)

    Nakajima, Tadashi; Sorahana, Satoko

    2016-10-01

    It has been suggested that high C/O ratios (>0.8) in circumstellar disks lead to the formation of carbon-dominated planets. Based on the expectation that elemental abundances in the stellar photospheres give the initial abundances in the circumstellar disks, the frequency distributions of C/O ratios of solar-type stars have been obtained by several groups. The results of these investigations are mixed. Some find C/O > 0.8 in more than 20% of stars, and C/O > 1.0 in more than 6%. Others find C/O > 0.8 in none of the sample stars. These works on solar-type stars are all differential abundance analyses with respect to the Sun and depend on the adopted C/O ratio in the Sun. Recently, a method of molecular line spectroscopy of M dwarfs, in which carbon and oxygen abundances are derived respectively from CO and H2O lines in the K band, has been developed. The resolution of the K-band spectrum is 20,000. Carbon and oxygen abundances of 46 M dwarfs have been obtained by this nondifferential abundance analysis. Carbon-to-oxygen ratios in M dwarfs derived by this method are more robust than those in solar-type stars derived from neutral carbon and oxygen lines in the visible spectra because of the difficulty in the treatment of oxygen lines. We have compared the frequency distribution of C/O distributions in M dwarfs with those of solar-type stars and have found that the low frequency of high-C/O ratios is preferred.

  17. THE NEAR-ULTRAVIOLET LUMINOSITY FUNCTION OF YOUNG, EARLY M-TYPE DWARF STARS

    SciTech Connect

    Ansdell, Megan; Baranec, Christoph; Gaidos, Eric; Mann, Andrew W.; Lépine, Sebastien; James, David; Buccino, Andrea; Mauas, Pablo; Petrucci, Romina; Law, Nicholas M.; Riddle, Reed

    2015-01-01

    Planets orbiting within the close-in habitable zones of M dwarf stars will be exposed to elevated high-energy radiation driven by strong magnetohydrodynamic dynamos during stellar youth. Near-ultraviolet (NUV) irradiation can erode and alter the chemistry of planetary atmospheres, and a quantitative description of the evolution of NUV emission from M dwarfs is needed when modeling these effects. We investigated the NUV luminosity evolution of early M-type dwarfs by cross-correlating the Lépine and Gaidos catalog of bright M dwarfs with the Galaxy Evolution Explorer (GALEX) catalog of NUV (1771-2831 Å) sources. Of the 4805 sources with GALEX counterparts, 797 have NUV emission significantly (>2.5σ) in excess of an empirical basal level. We inspected these candidate active stars using visible-wavelength spectra, high-resolution adaptive optics imaging, time-series photometry, and literature searches to identify cases where the elevated NUV emission is due to unresolved background sources or stellar companions; we estimated the overall occurrence of these ''false positives'' (FPs) as ∼16%. We constructed an NUV luminosity function that accounted for FPs, detection biases of the source catalogs, and GALEX upper limits. We found the NUV luminosity function to be inconsistent with predictions from a constant star-formation rate and simplified age-activity relation defined by a two-parameter power law.

  18. Panchromatic observations of dwarf starburst galaxies: Infant super star clusters and a low-luminosity AGN

    NASA Astrophysics Data System (ADS)

    Reines, Amy Ellen

    2011-01-01

    Globular star clusters and supermassive black holes are fundamental components of today's massive galaxies, with origins dating back to the very early universe. Both globular clusters and the seeds of supermassive black holes are believed to have formed in the progenitors of modern massive galaxies, although the details are poorly understood. Direct observations of these low-mass, distant, and hence faint systems are unobtainable with current capabilities. However, gas-rich dwarf starburst galaxies in the local universe, analogous in many ways to protogalaxies at high-redshift, can provide critical insight into the early stages of galaxy evolution including the formation of globular clusters and massive black holes. This thesis presents a panchromatic study of nearby dwarf starburst galaxies harboring nascent globular clusters still embedded in their birth material. Infant clusters are identified via their production of thermal radio emission at centimeter wavelengths, which comes from dense gas ionized by young massive stars. By combining radio observations with complementary data at ultraviolet, optical and infrared wavelengths, we obtain a comprehensive view of massive clusters emerging from their gaseous and dusty birth cocoons. This thesis also presents the first example of a nearby dwarf starburst galaxy hosting an actively accreting massive central black hole. The black hole in this dwarf galaxy is unusual in that it is not associated with a bulge, a nuclear star cluster, or any other well-defined nucleus, likely reflecting an early phase of black hole and galaxy evolution that has not been previously observed.

  19. HOW THE FIRST STARS SHAPED THE FAINTEST GAS-DOMINATED DWARF GALAXIES

    SciTech Connect

    Verbeke, R.; Vandenbroucke, B.; Rijcke, S. De

    2015-12-20

    Low-mass dwarf galaxies are very sensitive test-beds for theories of cosmic structure formation since their weak gravitational fields allow the effects of the relevant physical processes to clearly stand out. Up to now, no unified account has existed of the sometimes seemingly conflicting properties of the faintest isolated dwarfs in and around the Local Group, such as Leo T and the recently discovered Leo P and Pisces A systems. Using new numerical simulations, we show that this serious challenge to our understanding of galaxy formation can be effectively resolved by taking into account the regulating influence of the ultraviolet radiation of the first population of stars on a dwarf’s star formation rate while otherwise staying within the standard cosmological paradigm for structure formation. These simulations produce faint, gas-dominated, star-forming dwarf galaxies that lie on the baryonic Tully–Fisher relation and that successfully reproduce a broad range of chemical, kinematical, and structural observables of real late-type dwarf galaxies. Furthermore, we stress the importance of obtaining properties of simulated galaxies in a manner as close as possible to the typically employed observational techniques.

  20. Models for various aspects of dwarf novae and nova-like stars

    NASA Technical Reports Server (NTRS)

    Ladous, Constanze

    1993-01-01

    The first attempts to explain the nature of dwarf novae were based on the assumption of single-star phenomena, in which emission lines were assumed to be caused by circumstellar gas shells. The outburst behavior was tentatively ascribed to the kind of (also not understood) mechanism leading to nova outbursts. The realization that some, and possibly all, dwarf novae and nova-like stars (and novae) are binaries eventually led to models which bore more and more similarities to the modern interpretation on the basis of the Roche model. Not all cataclysmic variables are known binaries. In fact, with respect to the entire number of known objects, the proven binaries are still the minority, but all the brightest variables are in fact known to binaries. Not a single system is known which exhibits the usual characteristics of a cataclysmic variable and at the same time can be declared with certainty to be a single star. Two systems are known, the dwarf nova EY Cyg and the recurrent nova V1017 Sgr, in which, in spite of intensive search, no radial velocity variations have been found; but they still exhibit composite spectra consisting of a bright continuum, an emission spectrum, and a cool absorption spectrum. If the Roche model is correct, it is to be expected that a small percentage of objects is viewed pole-on, so orbital motions do not make themselves felt as Doppler shifts of spectral lines. So even these two systems support the hypothesis that all cataclysmic variables (with the possible exception of symbiotic stars) are binaries. In cataclysmic variables, it seems that the brightness changes observed in dwarf novae and nova-like stars in the optical and the UV are due directly to changes in the accretion disks. The study and understanding of accretion disks in these systems can bear potentially valuable consequences for many other fields in astronomy. The observed spectra of dwarf novae and nova-like stars comprise a fairly large range: pure emission spectra, pure

  1. Models for various aspects of dwarf novae and nova-like stars

    NASA Astrophysics Data System (ADS)

    La Dous, Constanze

    1993-09-01

    The first attempts to explain the nature of dwarf novae were based on the assumption of single-star phenomena, in which emission lines were assumed to be caused by circumstellar gas shells. The outburst behavior was tentatively ascribed to the kind of (also not understood) mechanism leading to nova outbursts. The realization that some, and possibly all, dwarf novae and nova-like stars (and novae) are binaries eventually led to models which bore more and more similarities to the modern interpretation on the basis of the Roche model. Not all cataclysmic variables are known binaries. In fact, with respect to the entire number of known objects, the proven binaries are still the minority, but all the brightest variables are in fact known to binaries. Not a single system is known which exhibits the usual characteristics of a cataclysmic variable and at the same time can be declared with certainty to be a single star. Two systems are known, the dwarf nova EY Cyg and the recurrent nova V1017 Sgr, in which, in spite of intensive search, no radial velocity variations have been found; but they still exhibit composite spectra consisting of a bright continuum, an emission spectrum, and a cool absorption spectrum. If the Roche model is correct, it is to be expected that a small percentage of objects is viewed pole-on, so orbital motions do not make themselves felt as Doppler shifts of spectral lines. So even these two systems support the hypothesis that all cataclysmic variables (with the possible exception of symbiotic stars) are binaries. In cataclysmic variables, it seems that the brightness changes observed in dwarf novae and nova-like stars in the optical and the UV are due directly to changes in the accretion disks. The study and understanding of accretion disks in these systems can bear potentially valuable consequences for many other fields in astronomy. The observed spectra of dwarf novae and nova-like stars comprise a fairly large range: pure emission spectra, pure

  2. Small-scale hero: Massive-star enrichment in the Hercules dwarf spheroidal

    NASA Astrophysics Data System (ADS)

    Koch, Andreas; Matteucci, Francesca; Feltzing, Sofia

    2012-09-01

    Dwarf spheroidal galaxies are often conjectured to be the sites of the first stars. The best current contenders for finding the chemical imprints from the enrichment by those massive objects are the ``ultrafaint dwarfs'' (UFDs). Here we present evidence for remarkably low heavy element abundances in the metal poor Hercules UFD. Combined with other peculiar abundance patterns this indicates that Hercules was likely only influenced by very few, massive explosive events - thus bearing the traces of an early, localized chemical enrichment with only very little other contributions from other sources at later times.

  3. Probing Galactic Structure with the Spatial Correlation Function of SEGUE G-dwarf Stars

    NASA Astrophysics Data System (ADS)

    Mao, Q.; Berlind, A. A.; Holley-Bockelmann, K.; Schlesinger, K. J.; Johnson, J. A.; Rockosi, C. M.

    2014-03-01

    We measure the 3-D two-point correlation function statistic of G-dwarf stars in the Milky Way. The G-dwarf sample is constructed from SDSS SEGUE data by Schlesinger et al. (2012). We find that the shapes of the correlation functions along individual SEGUE lines of sight depend sensitively on both the stellar density gradients and the survey geometry. We compare these SEGUE measurements with mock measurements from smooth disk galaxy models to obtain strong constraints on the thin and thick disk components of the Milky Way.

  4. Searching for star-forming dwarf galaxies in the Antlia cluster

    NASA Astrophysics Data System (ADS)

    Vaduvescu, O.; Kehrig, C.; Bassino, L. P.; Smith Castelli, A. V.; Calderón, J. P.

    2014-03-01

    Context. The formation and evolution of dwarf galaxies in clusters need to be understood, and this requires large aperture telescopes. Aims: In this sense, we selected the Antlia cluster to continue our previous work in the Virgo, Fornax, and Hydra clusters and in the Local Volume (LV). Because of the scarce available literature data, we selected a small sample of five blue compact dwarf (BCD) candidates in Antlia for observation. Methods: Using the Gemini South and GMOS camera, we acquired the Hα imaging needed to detect star-forming regions in this sample. With the long-slit spectroscopic data of the brightest seven knots detected in three BCD candidates, we derived their basic chemical properties. Using archival VISTA VHS survey images, we derived KS magnitudes and surface brightness profile fits for the whole sample to assess basic physical properties. Results: FS90-98, FS90-106, and FS90-147 are confirmed as BCDs and cluster members, based on their morphology, KS surface photometry, oxygen abundance, and velocity redshift. FS90-155 and FS90-319 did not show any Hα emission, and they could not be confirmed as dwarf cluster star-forming galaxies. Based on our data, we studied some fundamental relations to compare star forming dwarfs (BCDs and dIs) in the LV and in the Virgo, Fornax, Hydra, and Antlia clusters. Conclusions: Star-forming dwarfs in nearby clusters appear to follow same fundamental relations in the near infrared with similar objects in the LV, specifically the size-luminosity and the metallicity-luminosity, while other more fundamental relations could not be checked in Antlia due to lack of data. Based on observations acquired at Gemini South (GS-2010A-Q-51 and GS-2012A-Q-59) and ESO VISTA Hemisphere Survey (VHS).

  5. Bursts of star formation in computer simulations of dwarf galaxies

    SciTech Connect

    Comins, N.F.

    1984-09-01

    A three-dimensional Stochastic Self-Propagating Star Formation (SSPSF) model of compact galacies is presented. Two phases of gas, active and inactive, are present, and permanent depletion of gas in the form of long lived, low mass stars and remnants occurs. Similarly, global infall of gas from a galactic halo or through galactic cannibalism is permitted. We base our parameters on the observed properties of the compact blue galaxy I Zw 36. Our results are that bursts of star formation occur much more frequently in these runs than continuous nonbursting star formation, suggesting that the blue compact galaxies are probably undergoing bursts rather than continuous, nonbursting low-level star formation activity.

  6. Mapping out the Connections between Star Clusters, Ultra-compact Dwarfs, and Galactic Nuclei

    NASA Astrophysics Data System (ADS)

    Romanowsky, Aaron

    2015-08-01

    I will present photometric and spectroscopic observations and modeling of ultra-compact dwarfs (UCDs) and other compact stellar systems around nearby galaxies, both spirals and ellipticals, in order to map out the transitions between different classes of star clusters and galaxies, and to probe their origins. The results include extensions of UCD parameter space to join up with both extended star clusters and compact elliptical galaxies; discoveries of the new record-holders for densest galaxy and densest star cluster; spectroscopic analysis of ages and chemical abundances with comparisons to nuclear star clusters; dynamical studies of mass-to-light ratio variations; adaptive optics integral-field spectroscopy to search for supermassive black holes; orbital properties within the host galaxy halos; inferences about in-situ vs. ex-situ formation; and novel observations of UCDs and massive star clusters caught in the act of formation.

  7. First detection of nonflare microwave emissions from the coronae of single late-type dwarf stars

    NASA Technical Reports Server (NTRS)

    Gary, D. E.; Linsky, J. L.

    1981-01-01

    Results are presented of a search for nonflare microwave radiation from the coronae of nearby late-type dwarf stars comparable to the sun: single stars without evidence for either a large wind or circumstellar envelope. The observing program consisted of flux measurements of six stars over a 24-h period with the VLA in the C configuration at a wavelength of 6 cm with 50 MHz bandwidth. Positive detections at 6 cm were made for Chi 1 Ori (0.6 mJy) and the flare star UV Cet (1.55 mJy), and upper limits were obtained for the stars Pi 1 UMa, Xi Boo A, 70 Oph A and Epsilon Eri. It is suggested that Chi 1 Ori, and possibly UV Cet, represent the first detected members of a new class of radio sources which are driven by gyroresonance emission, i.e. cyclotron emission from nonrelativistic Maxwellian electrons.

  8. A brown dwarf companion to the intermediate-mass star HR 6037

    NASA Astrophysics Data System (ADS)

    Huélamo, N.; Nürnberger, D. E. A.; Ivanov, V. D.; Chauvin, G.; Carraro, G.; Sterzik, M. F.; Melo, C. H. F.; Bonnefoy, M.; Hartung, M.; Haubois, X.; Foellmi, C.

    2010-10-01

    Context. The frequency of brown dwarf and planetary-mass companions around intermediate-mass stars is still unknown. Imaging and radial velocity surveys have revealed a small number of substellar companions to these stars. Aims: In the course of an imaging survey we detected a visual companion to the intermediate-mass star HR 6037. We here confirm it as a co-moving substellar object. Methods: We present two epoch observations of HR 6037, an A6-type star with a companion candidate at 6farcs67 and position angle of 294 degrees. We also analyze near-infrared spectroscopy of the companion. Results: Two epoch observations of HR 6037 have allowed us to confirm HR 6037 B as a co-moving companion. Its J and H band spectra suggest that the object has a spectral type of M9, with a surface gravity that is intermediate between a 10 Myr dwarf and a field dwarf of the same spectral type. The comparison of its Ks-band photometry with evolutionary tracks allows us to derive a mass, effective temperature, and surface gravity of 62±20 MJup, Teff = 2330±200 K, and log g = 5.1±0.2, respectively. The low binary mass ratio, q 0.03, and its long orbital period, 5000 yr, make HR 6037 a rare and uncommon binary system. Based on observations collected at the Paranal Observatory under programs 272.D-5068(A), 77.D-0147(A), and 285.C-5008(A).

  9. Two new pulsating low-mass pre-white dwarfs or SX Phoenicis stars?

    NASA Astrophysics Data System (ADS)

    Corti, M. A.; Kanaan, A.; Córsico, A. H.; Kepler, S. O.; Althaus, L. G.; Koester, D.; Sánchez Arias, J. P.

    2016-03-01

    Context. The discovery of pulsations in low-mass stars opens an opportunity to probe their interiors and determine their evolution by employing the tools of asteroseismology. Aims: We aim to analyse high-speed photometry of SDSS J145847.02+070754.46 and SDSS J173001.94+070600.25 and discover brightness variabilities. In order to locate these stars in the Teff - log g diagram, we fit optical spectra (SDSS) with synthetic non-magnetic spectra derived from model atmospheres. Methods: To carry out this study, we used the photometric data we obtained for these stars with the 2.15 m telescope at CASLEO, Argentina. We analysed their light curves and applied the discrete Fourier transform (FT) to determine the pulsation frequencies. Finally, we compare both stars in the Teff - log g diagram, with two known pre-white dwarfs and seven pulsating pre-ELM white dwarf stars, δ Scuti, and SX Phe stars Results: We report the discovery of pulsations in SDSS J145847.02+070754.46 and SDSS J173001.94+070600.25. We determine their effective temperature and surface gravity to be Teff = 7972 ± 200 K, log g = 4.25 ± 0.5 and Teff = 7925 ± 200 K, log g = 4.25 ± 0.5, respectively. With these parameters, these new pulsating low-mass stars can be identified with either ELM white dwarfs (with ~0.17 M⊙) or more massive SX Phe stars. We identified pulsation periods of 3278.7 and 1633.9 s for SDSS J145847.02+070754.46 and a pulsation period of 3367.1 s for SDSS J173001.94+070600.25. These two new objects, together with those of Maxted et al. (2013, 2014), indicate the possible existence of a new instability domain towards the late stages of evolution of low-mass white dwarf stars, although their identification with SX Phe stars cannot be discarded. Visiting Astronomer, Complejo Astronómico El Leoncito operated under agreement between the Consejo Nacional de Investigaciones Científicas y Técnicas de la República Argentina and the National Universities of La Plata, Córdoba, and San Juan.

  10. Suppression of star formation in dwarf galaxies by photoelectric grain heating feedback

    NASA Astrophysics Data System (ADS)

    Forbes, John C.; Krumholz, Mark R.; Goldbaum, Nathan J.; Dekel, Avishai

    2016-07-01

    Photoelectric heating—heating of dust grains by far-ultraviolet photons—has long been recognized as the primary source of heating for the neutral interstellar medium. Simulations of spiral galaxies have shown some indication that photoelectric heating could suppress star formation; however, simulations that include photoelectric heating have typically shown that it has little effect on the rate of star formation in either spiral galaxies or dwarf galaxies, which suggests that supernovae are responsible for setting the gas depletion time in galaxies. This result is in contrast with recent work indicating that a star formation law that depends on galaxy metallicity—as is expected with photoelectric heating, but not with supernovae—reproduces the present-day galaxy population better than does a metallicity-independent one. Here we report a series of simulations of dwarf galaxies, the class of galaxy in which the effects of both photoelectric heating and supernovae are expected to be strongest. We simultaneously include space- and time-dependent photoelectric heating in our simulations, and we resolve the energy-conserving phase of every supernova blast wave, which allows us to directly measure the relative importance of feedback by supernovae and photoelectric heating in suppressing star formation. We find that supernovae are unable to account for the observed large gas depletion times in dwarf galaxies. Instead, photoelectric heating is the dominant means by which dwarf galaxies regulate their star formation rate at any given time, suppressing the rate by more than an order of magnitude relative to simulations with only supernovae.

  11. Suppression of star formation in dwarf galaxies by photoelectric grain heating feedback.

    PubMed

    Forbes, John C; Krumholz, Mark R; Goldbaum, Nathan J; Dekel, Avishai

    2016-07-28

    Photoelectric heating--heating of dust grains by far-ultraviolet photons--has long been recognized as the primary source of heating for the neutral interstellar medium. Simulations of spiral galaxies have shown some indication that photoelectric heating could suppress star formation; however, simulations that include photoelectric heating have typically shown that it has little effect on the rate of star formation in either spiral galaxies or dwarf galaxies, which suggests that supernovae are responsible for setting the gas depletion time in galaxies. This result is in contrast with recent work indicating that a star formation law that depends on galaxy metallicity--as is expected with photoelectric heating,but not with supernovae--reproduces the present-day galaxy population better than does a metallicity-independent one. Here we report a series of simulations of dwarf galaxies, the class of galaxy in which the effects of both photoelectric heating and supernovae are expected to be strongest. We simultaneously include space and time-dependent photoelectric heating in our simulations, and we resolve the energy-conserving phase of every supernova blast wave, which allows us to directly measure the relative importance of feedback by supernovae and photoelectric heating in suppressing star formation. We find that supernovae are unable to account for the observed large gas depletion times in dwarf galaxies. Instead, photoelectric heating is the dominant means by which dwarf galaxies regulate their star formation rate at any given time,suppressing the rate by more than an order of magnitude relative to simulations with only supernovae.

  12. Suppression of star formation in dwarf galaxies by photoelectric grain heating feedback.

    PubMed

    Forbes, John C; Krumholz, Mark R; Goldbaum, Nathan J; Dekel, Avishai

    2016-07-28

    Photoelectric heating--heating of dust grains by far-ultraviolet photons--has long been recognized as the primary source of heating for the neutral interstellar medium. Simulations of spiral galaxies have shown some indication that photoelectric heating could suppress star formation; however, simulations that include photoelectric heating have typically shown that it has little effect on the rate of star formation in either spiral galaxies or dwarf galaxies, which suggests that supernovae are responsible for setting the gas depletion time in galaxies. This result is in contrast with recent work indicating that a star formation law that depends on galaxy metallicity--as is expected with photoelectric heating,but not with supernovae--reproduces the present-day galaxy population better than does a metallicity-independent one. Here we report a series of simulations of dwarf galaxies, the class of galaxy in which the effects of both photoelectric heating and supernovae are expected to be strongest. We simultaneously include space and time-dependent photoelectric heating in our simulations, and we resolve the energy-conserving phase of every supernova blast wave, which allows us to directly measure the relative importance of feedback by supernovae and photoelectric heating in suppressing star formation. We find that supernovae are unable to account for the observed large gas depletion times in dwarf galaxies. Instead, photoelectric heating is the dominant means by which dwarf galaxies regulate their star formation rate at any given time,suppressing the rate by more than an order of magnitude relative to simulations with only supernovae. PMID:27350244

  13. They are small worlds after all: revised properties of Kepler M dwarf stars and their planets

    NASA Astrophysics Data System (ADS)

    Gaidos, E.; Mann, A. W.; Kraus, A. L.; Ireland, M.

    2016-04-01

    We classified the reddest (r - J > 2.2) stars observed by the NASA Kepler mission into main-sequence dwarf or evolved giant stars and determined the properties of 4216 M dwarfs based on a comparison of available photometry with that of nearby calibrator stars, as well as available proper motions and spectra. We revised the properties of candidate transiting planets using the stellar parameters, high-resolution imaging to identify companion stars, and, in the case of binaries, fitting light curves to identify the likely planet host. In 49 of 54 systems, we validated the primary as the host star. We inferred the intrinsic distribution of M dwarf planets using the method of iterative Monte Carlo simulation. We compared several models of planet orbital geometry and clustering and found that one where planets are exponentially distributed and almost precisely coplanar best describes the distribution of multiplanet systems. We determined that Kepler M dwarfs host an average of 2.2 ± 0.3 planets with radii of 1-4 R⊕ and orbital periods of 1.5-180 d. The radius distribution peaks at ˜1.2 R⊕ and is essentially zero at 4 R⊕, although we identify three giant planet candidates other than the previously confirmed Kepler-45b. There is suggestive but not significant evidence that the radius distribution varies with orbital period. The distribution with logarithmic orbital period is flat except for a decline for orbits less than a few days. 12 candidate planets, including two Jupiter-size objects, experience an irradiance below the threshold level for a runaway greenhouse on an Earth-like planet and are thus in a `habitable zone'.

  14. Hiding in plain sight - red supergiant imposters? Super-AGB stars - bridging the divide between low/intermediate-mass and high-mass stars

    NASA Astrophysics Data System (ADS)

    Doherty, Carolyn Louise; Gil-Pons, Pilar; Lattanzio, John; Siess, Lionel

    2015-08-01

    Super Asymptotic Giant Branch (Super-AGB) stars reside in the mass range ~ 6.5-10 M⊙ and bridge the divide between low/intermediate-mass and massive stars. They are characterised by off-centre carbon ignition prior to a thermally pulsing phase which can consist of many tens to even thousands of thermal pulses. With their high luminosities and very large, cool, red stellar envelopes, these stars appear seemingly identical to their slightly more massive red supergiant counterparts. Due to their similarities, super-AGB stars may therefore act as stellar imposters and contaminate red supergiant surveys. Super-AGB stars undergo relatively extreme nucleosynthetic conditions, with very efficient proton-capture nucleosynthesis occurring at the base of the convective envelope and also heavy element (s-process) production during the thermal pulse to be later mixed to the surface during third dredge-up events. The surface enrichment from these two processes may result in a clear nucleosynthetic signature to differentiate these two classes of star.The final fate of super-AGB stars is also quite uncertain and depends primarily on the competition between the core growth and mass-loss rates. If the stellar envelope is removed prior to the core reaching the Chandrasekhar mass, an O-Ne white dwarf will remain, otherwise the star will undergo an electron-capture supernova leaving behind a neutron star. We describe the factors which influence these different final fate channels, such as the efficiency of convection, the mass-loss rates, the third dredge-up efficiency and the Fe-peak opacity instability which may lead to expulsion of the entire remaining stellar envelope. We determine the relative fraction of super-AGB stars that end life as either an O-Ne white dwarf or as a neutron star, and provide a mass limit for the lowest mass supernova over a broad range of metallicities from the earliest time (Z=0) right through until today (Z~0.04).

  15. A Very High Proper Motion Star and the First L Dwarf in the Kepler Field

    NASA Astrophysics Data System (ADS)

    Gizis, John E.; Troup, Nicholas W.; Burgasser, Adam J.

    2011-08-01

    We report two nearby high proper motion dwarfs of special interest identified using the Preliminary Data Release of the Wide-field Infrared Survey Explorer (WISE) and the Two Micron All Sky Survey. WISEP J191239.91-361516.4 has a motion of 2.1 arcsec yr-1. Photometry identifies it as a mid-M dwarf. WISEP J190648.47+401106.8 is a spectroscopically confirmed L1 dwarf in the Kepler Mission field with a motion of 0.48 arcsec yr-1. The estimated distance is 17 pc. Both lie at relatively low galactic latitudes and demonstrate the possibility of discovering proper motion stars independent of the historic photographic sky surveys.

  16. Young Star Clusters in the Outer Disks of LITTLE THINGS Dwarf Irregular Galaxies

    NASA Astrophysics Data System (ADS)

    Hunter, Deidre A.; Elmegreen, Bruce G.; Gehret, Elizabeth

    2016-06-01

    We examine FUV images of the LITTLE THINGS sample of nearby dwarf irregular (dIrr) and Blue Compact Dwarf galaxies to identify distinct young regions in their far outer disks. We use these data, obtained with the Galaxy Evolution Explorer satellite, to determine the furthest radius at which in situ star formation can currently be identified. The FUV knots are found at distances from the center of the galaxies of 1-8 disk scale lengths and have ages of ≤slant 20 Myr and masses of 20 M{}⊙ to 1 × 105M{}⊙ . The presence of young clusters and OB associations in the outer disks of dwarf galaxies shows that dIrrs do have star formation taking place there in spite of the extreme nature of the environment. Most regions are found where the H i surface density is ˜1 M{}⊙ pc-2, though both the H i and dispersed old stars go out much further. This limiting density suggests a cutoff in the ability to form distinct OB associations and perhaps even stars. We compare the star formation rates in the FUV regions to the average rates expected at their radii and beyond from the observed gas, using the conventional correlation for gas-rich regions. The localized rates are typically 10% of the expected average rates for the outer disks. Either star formation in dIrrs at surface densities \\lt 1 {M}⊙ pc-2 occurs without forming distinct associations, or the Kennicutt-Schmidt relation over-predicts the rate beyond this point. In the latter case, the stellar disks in the far-outer parts of dIrrs result from scattering of stars from the inner disk.

  17. Meridional circulation in rotating stars. V. Cooling white dwarfs

    SciTech Connect

    Tassoul, M.; Tassoul, J.

    1983-04-01

    In this paper we discuss the large-scale meridonal circulation and concomitant differential rotation in a cooling white dwarf that does not greatly depart from spherical symmetry. It is found that the circulation velocities are utterly negligible in the degenerate interior as well as in the thin nondegenerate envelope, where the presence of a (turbulent) viscous boundary layer allows the velocities to satisfy all of the surface boundary conditions. There are no 1/rho singularities in the meridional flow. The back reaction of the circulatory currents on the overall rotation rate is not very large either, thus implying at most a mild differential rotation when departure from sphericity is small.

  18. Lattice Structure in Astrophysics: A reconsideration of White Dwarfs, Variables, and Wolf-Rayet Stars

    NASA Astrophysics Data System (ADS)

    Robitaille, Pierre-Marie

    2016-03-01

    Stars of the main sequence display a mass-luminosity relation which indicates that they share a common building block (hydrogen) and lattice structure (hexagonal planar) with the solar photosphere. White dwarfs however display very low luminosity in spite of their elevated color temperature. Rather than postulate that these stars represent degenerate matter, as Eddington and Chandrasekhar were forced to assume given their gaseous models, within the context of a Liquid Metallic Hydrogen Solar Model white dwarfs might simply be thought as possessing a different lattice structure (e.g. body centered cubic) and hence a lowered emissivity. They do not need to possess exceeding densities, reduced radii, and degeneracy in order to account for their lowered emissivity. Similarly, variable stars might well be oscillating between lattices types wherein the energy differences involved in the transformations are small. Other stars, such as Wolf-Rayet stars, which lack photospheric emission, might be too hot to enable a discrete lattice to form. Though condensed, the photosphere in that case would have a lattice which is so poorly organized that its emissivity is trivial. Nonetheless, the broad emission lines of Wolf-Rayet stars indicates that these objects are not breaking apart but rather, are important sites of condensation.

  19. WISE and Brown Dwarfs

    NASA Astrophysics Data System (ADS)

    Kirkpatrick, J. Davy; WISE Team

    2009-05-01

    The search for the nearest and coolest brown dwarfs will use WISE's two short-wavelength channels (W1 and W2), which are optimized for brown dwarf detection. W1 samples the methane fundamental absorption band at 3.3 microns, and W2 measures the relatively opacity-free portion of the brown dwarf atmosphere near 4.7 microns. Cool brown dwarfs will thus have very red [W1]-[W2] colors, maximizing our chances of identifying them. Extrapolating preferred mass functions to very low masses and assuming that the star formation rate has been constant over the last 10 Gyr, we can predict the number of brown dwarfs WISE is expected to image. At spectral types later than T7 (Teff > 850K), WISE is expected to find 500 brown dwarfs, which makes WISE uniquely suited among future surveys to measure the low-mass limit of star formation for the first time. This sample will also show whether a new spectral class beyond T, dubbed "Y", is needed at the lowest temperatures. Although the primary six-month WISE mission will cover the entire sky once, WISE should have sufficient cryogen to perform a second, complete pass of the sky. In this case, the identification of nearby brown dwarfs need not rely solely on color selection. Kinematics (proper motion) and geometry (parallax) can also be used to distinguish our closest brown dwarf neighbors, one of which may lie less distant than Proxima Centauri or even fall within our own Oort Cloud.

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

    SciTech Connect

    Tayar, Jamie; Pinsonneault, Marc H.

    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 the 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.

  1. Implications of Rapid Core Rotation in Red Giants for Internal Angular Momentum Transport in Stars

    NASA Astrophysics Data System (ADS)

    Tayar, Jamie; Pinsonneault, Marc H.

    2013-09-01

    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 ("Otto") 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 the 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.

  2. Identification and characterization of low mass stars and brown dwarfs using Virtual Observatory tools

    NASA Astrophysics Data System (ADS)

    Aberasturi, Miriam

    2015-11-01

    Context: Two thirds of the stars in our galactic neighborhood (d < 10 pc) are M-dwarfs which also constitute the most common stellar objects in the Milky Way. This property, combined with their small stellar masses and radii, increases the likelihood of detecting terrestrial planets through radial velocity and transit techniques, making them very adequate targets for the exoplanet hunting projects. Nevertheless, M dwarfs have associated different observational difficulties. They are cool objects whose emission radiation peaks at infrared wavelengths and, thus, with a low surface brightness in the optical range. Also, the photometric variability as well as the significant chromospheric activity hinder the radial velocity and transit determinations. It is necessary, therefore, to carry out a detailed characterization of M-dwarfs before building a shortlist with the best possible candidates for exoplanet searches. Brown dwarfs (BDs) are self-gravitating objects that do not get enough mass to maintain a sufficiently high temperature in their core for stable hydrogen fusion. They represent the link between low-mass stars and giant planets. Due to their low temperatures, BDs emit significant flux at mid-infrared wavelength which makes this range very adequate to look for this type of objects. The Virtual Observatory (VO) is an international initiative designed to help the astronomical community in the exploitation of the multi-wavelength information that resides in data archives. In the last years the Spanish Virtual Observatory is conducting a number of projects focused on the study of substellar objects taking advantage of Virtual Observatory tools for an easy data access and analysis of large area surveys. This is the framework where this thesis has been carried out. This dissertation addresses three problems in the framework of low-mass stars and brown dwarfs, namely, the search for brown dwarf candidates crossmatching catalogues (Chapter 4), the search for nearby

  3. Identification and characterization of low mass stars and brown dwarfs using Virtual Observatory tools

    NASA Astrophysics Data System (ADS)

    Aberasturi, Miriam

    2015-11-01

    Context: Two thirds of the stars in our galactic neighborhood (d < 10 pc) are M-dwarfs which also constitute the most common stellar objects in the Milky Way. This property, combined with their small stellar masses and radii, increases the likelihood of detecting terrestrial planets through radial velocity and transit techniques, making them very adequate targets for the exoplanet hunting projects. Nevertheless, M dwarfs have associated different observational difficulties. They are cool objects whose emission radiation peaks at infrared wavelengths and, thus, with a low surface brightness in the optical range. Also, the photometric variability as well as the significant chromospheric activity hinder the radial velocity and transit determinations. It is necessary, therefore, to carry out a detailed characterization of M-dwarfs before building a shortlist with the best possible candidates for exoplanet searches. Brown dwarfs (BDs) are self-gravitating objects that do not get enough mass to maintain a sufficiently high temperature in their core for stable hydrogen fusion. They represent the link between low-mass stars and giant planets. Due to their low temperatures, BDs emit significant flux at mid-infrared wavelength which makes this range very adequate to look for this type of objects. The Virtual Observatory (VO) is an international initiative designed to help the astronomical community in the exploitation of the multi-wavelength information that resides in data archives. In the last years the Spanish Virtual Observatory is conducting a number of projects focused on the study of substellar objects taking advantage of Virtual Observatory tools for an easy data access and analysis of large area surveys. This is the framework where this thesis has been carried out. This dissertation addresses three problems in the framework of low-mass stars and brown dwarfs, namely, the search for brown dwarf candidates crossmatching catalogues (Chapter 4), the search for nearby

  4. Neutral interstellar medium phases and star formation tracers in dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Cigan, Phillip Johnathan

    Dwarf galaxies present interesting observational challenges for the studies of various galaxy properties: despite their abundance and proximity to the Milky Way, they typically have very low surface brightnesses and small physical sizes. Until now, only the extreme variety of dwarfs --- those undergoing strong bouts of star formation --- have been observed in the FIR, due to observational difficulties. However, this population does not represent the majority of dwarfs, which have only moderate star formation rates and extremely low metallicity (the fraction of heavy elements to hydrogen). The advent of the Herschel Space Telescope, with its superior resolution and sensitivity over previous generations of telescopes, has made it possible to measure FIR spectral lines and broadband continuum in normal dwarf galaxies, expanding the scope of studies beyond the brighter, but more extreme, varieties. The general goal of my research was to study the conditions in the interstellar media (ISM) of typical dwarf galaxies. The LITTLE THINGS (Local Irregulars That Trace Luminosity Extremes, TheHI Nearby Galaxy Survey) project aims to unravel many mysteries of nearby dwarfs using a suite of multi-wavelength data, and the new additions from Herschel help provide insight into the physics of these systems. I reduced and analyzed FIR fine-structure spectral line data for the LITTLE THINGS sample to study the different phases of the ISM, as well as FIR photometry data to access the dust properties and infrared continuum emission in these systems. The FIR spectral lines are diagnostics for the conditions in the ISM of galaxies, telling us about heating efficiency, the fraction of gas that resides in photodissociation regions (PDRs), abundance of highly ionized gas from massive stars, and other physical descriptions. The photometric continuum observations enable the modeling of interstellar dust properties -- dust plays an important role in shielding and cooling molecular clouds which

  5. Metallicity Distribution Functions of Dwarf Galaxies: A Probe of Star Formation History and Baryonic Physics

    NASA Astrophysics Data System (ADS)

    Escala, Ivanna; Kirby, Evan N.; Wetzel, Andrew R.; Hopkins, Philip F.

    2016-06-01

    We examine the metallicity distribution functions (MDFs) of simulated, isolated dwarf galaxies (M_{star} = 4 × 10^{4} - 3 × 10^{8} M_{⊙}) from the Feedback in Realistic Environments (FIRE) project to quantify the impact of star formation history (SFH) and baryonic physics. These high-resolution cosmological simulations include realistic treatments of stellar evolution and complex gas dynamics and do not require the usual approximations (e.g., instantaneous recycling and instantaneous mixing) of analytic chemical evolution models. The evolution of the MDF with redshift informs which processes drive the dominant contributions to the distribution at z = 0, thus enabling a reconstruction of the SFH and gas loss/accretion history. We then compare the theoretical MDFs to the observed MDFs of Local Group dwarf galaxies to infer plausible SFHs for each matched galaxy.

  6. How the first stars shaped the faintest gas-dominated dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Verbeke, Robbert; Vandenbroucke, Bert; de Rijcke, Sven

    2016-08-01

    Cosmological simulations predict that dark matter halos with circular velocities lower than 30 km/s should have lost most of their neutral gas by heating of the ultra-violet background. This is in stark contrast with gas-rich galaxies such as e.g. Leo T, Leo P and Pisces A, which all have circular velocities of ~15 km/s (Ryan-Weber et al. 2008, Bernstein-Cooper et al. 2014, Tollerud et al. 2015). We show that when we include feedback from the first stars into our models, simulated dwarfs have very different properties at redshift 0 than when this form of feedback is not included. Including this Population-III feedback leads to galaxies that lie on the baryonic Tully-Fisher relation over the entire mass range of star forming dwarf galaxies, as well as reproducing a broad range of other observational properties.

  7. INVESTIGATION OF THE PUZZLING ABUNDANCE PATTERN IN THE STARS OF THE FORNAX DWARF SPHEROIDAL GALAXY

    SciTech Connect

    Li Hongjie; Cui Wenyuan; Zhang Bo

    2013-09-20

    Many works have found unusual characteristics of elemental abundances in nearby dwarf galaxies. This implies that there is a key factor of galactic evolution that is different from that of the Milky Way (MW). The chemical abundances of the stars in the Fornax dwarf spheroidal galaxy (Fornax dSph) provide excellent information for setting constraints on the models of galactic chemical evolution. In this work, adopting the five-component approach, we fit the abundances of the Fornax dSph stars, including {alpha} elements, iron group elements, and neutron-capture elements. For most sample stars, the relative contributions from the various processes to the elemental abundances are not usually in the MW proportions. We find that the contributions from massive stars to the primary {alpha} elements and iron group elements increase monotonically with increasing [Fe/H]. This means that the effect of the galactic wind is not strong enough to halt star formation and the contributions from the massive stars to {alpha} elements did not halt for [Fe/H] {approx}< -0.5. The average contribution ratios of various processes between the dSph stars and the MW stars monotonically decrease with increasing progenitor mass. This is important evidence of a bottom-heavy initial mass function (IMF) for the Fornax dSph, compared to the MW. Considering a bottom-heavy IMF for the dSph, the observed relations of [{alpha}/Fe] versus [Fe/H], [iron group/Fe] versus [Fe/H], and [neutron-capture/Fe] versus [Fe/H] for the dSph stars can be explained.

  8. A fossil origin for the magnetic field in A stars and white dwarfs.

    PubMed

    Braithwaite, Jonathan; Spruit, Hendrik C

    2004-10-14

    Some main-sequence stars of spectral type A are observed to have a strong (0.03-3 tesla), static, large-scale magnetic field, of a chiefly dipolar shape: they are known as 'Ap stars', such as Alioth, the fifth star in the Big Dipper. Following the discovery of these fields, it was proposed that they are remnants of the star's formation, a 'fossil' field. An alternative suggestion is that they could be generated by a dynamo process in the star's convective core. The dynamo hypothesis, however, has difficulty explaining high field strengths and the observed lack of a correlation with rotation. The weakness of the fossil-field theory has been the absence of field configurations stable enough to survive in a star over its lifetime. Here we report numerical simulations that show that stable magnetic field configurations, with properties agreeing with those observed, can develop through evolution from arbitrary, unstable initial fields. The results are applicable equally to Ap stars, magnetic white dwarfs and some highly magnetized neutron stars known as magnetars. This establishes fossil fields as the natural, unifying explanation for the magnetism of all these stars.

  9. Self-consistent photometric and spectroscopic Star Formation Histories in Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    García-Benito, R.; Pérez, E.; Pérez-Montero, E.; González Delgado, R.; Vílchez, J. M.

    2016-06-01

    This project aims to unify the spectroscopic and stellar photometric views by performing a comprehensive study of a sample of the nearest Blue Compact Dwarf Galaxies (BCDs). We plan to derive Star Formation Histories (SFH) both by means of Color-Magnitude Diagrams (CMDs) from extant Hubble Space Telescope (HST) optical imaging and with spectral fitting methods techniques using MUSE, allowing us to obtain state-of-the-art 2D stellar properties and abundances of the gas in BCDs.

  10. The BT-Settl Model Atmospheres for Stars, Brown Dwarfs and Planets

    NASA Astrophysics Data System (ADS)

    Allard, F.

    2014-01-01

    We present a grid of stellar and substellar atmosphere models covering the range from solar-mass stars to the latest-type T and Y dwarfs with a single setup. For the first time our synthetic spectra and photometry reproduce the formation of clouds and in particular their clearing at the L/T transition. The BT-Settl models also naturally explain the dustier infrared properties of planets as an effect of low surface gravity.

  11. DETAILED ABUNDANCES OF TWO VERY METAL-POOR STARS IN DWARF GALAXIES

    SciTech Connect

    Kirby, Evan N.; Cohen, Judith G.

    2012-12-01

    The most metal-poor stars in dwarf spheroidal galaxies (dSphs) can show the nucleosynthetic patterns of one or a few supernovae (SNe). These SNe could have zero metallicity, making metal-poor dSph stars the closest surviving links to Population III stars. Metal-poor dSph stars also help to reveal the formation mechanism of the Milky Way (MW) halo. We present the detailed abundances from Keck/HIRES spectroscopy for two very metal-poor stars in two MW dSphs. One star, in the Sculptor dSph, has [Fe I/H] = -2.40. The other star, in the Ursa Minor dSph, has [Fe I/H] = -3.16. Both stars fall in the previously discovered low-metallicity, high-[{alpha}/Fe] plateau. Most abundance ratios of very metal-poor stars in these two dSphs are largely consistent with very metal-poor halo stars. However, the abundances of Na and some r-process elements lie at the lower end of the envelope defined by inner halo stars of similar metallicity. We propose that the metallicity dependence of SN yields is the cause. The earliest SNe in low-mass dSphs have less gas to pollute than the earliest SNe in massive halo progenitors. As a result, dSph stars at -3 < [Fe/H] < -2 sample SNe with [Fe/H] << -3, whereas halo stars in the same metallicity range sample SNe with [Fe/H] {approx} -3. Consequently, enhancements in [Na/Fe] and [r/Fe] were deferred to higher metallicity in dSphs than in the progenitors of the inner halo.

  12. The temperatures, abundances and gravities of F dwarf stars.

    NASA Technical Reports Server (NTRS)

    Bell, R. A.

    1971-01-01

    Theoretical colors computed from laboratory line data and from model stellar atmospheres have been used to interpret the colors of about 150 F and early G dwarfs. Effective temperatures have been derived from the H-beta index and from R-I, abundances have been obtained from m(sub 1) and from b-y, and gravities have been obtained from c(sub 1) and from b-y. The effective temperatures and gravities are in good agreement with values obtained from spectral scans. Absolute magnitudes have been obtained from the effective temperatures and gravities, the latter being used with assumed stellar masses to yield radii. The present results provide theoretical justification of the empirical formulas given by Crawford and by Stroemgren for the determination of absolute magnitudes and abundances from uvby photometry.

  13. Instability of g-mode oscillations in white dwarf stars

    NASA Technical Reports Server (NTRS)

    Keeley, D. A.

    1979-01-01

    A white dwarf model with M = 6 solar masses, Te = 12,000 K, and L = 1.2 x 10 to the 31st erg/sec provided by Cox has been tested for linear stability of radial oscillations. The radial mode instability first reported for this model by Cox, et al. (1979) has been confirmed. The growth rates obtained are comparable to the rates found by Cox. A sequence of l = 2 g-modes has also been found to be unstable. The e-folding times range from around 10 to the 11th periods for a 137 second mode (1 radial node) to less than 100 periods for a 629 second mode (17 nodes). It is likely that the latter rate is too high because the eigenfunction has been forced to vanish at the non-zero inner radius of the model, at which the Brunt-Vaisala frequency is barely less than the mode frequency.

  14. Quasars and a dwarf star break the rules in tucson.

    PubMed

    Travis, J

    1995-01-27

    Earlier this month, more than 2000 astronomers convened in Tucson, Arizona, for the American Astronomical Society's largest meeting ever. Even the war drums beaten by a few local Apaches and their supporters, protesting a University of Arizona telescope project, could not drown out lecture-hall and corridor discussions of topics such as naked quasars, flaring stars, and planetary searches.

  15. Quasars and a Dwarf Star Break the Rules in Tucson

    NASA Astrophysics Data System (ADS)

    Travis, John

    1995-01-01

    Earlier this month, more than 2000 astronomers convened in Tucson, Arizona, for the American Astronomical Society's largest meeting ever. Even the war drums beaten by a few local Apaches and their supporters, protesting a University of Arizona telescope project, could not drown out lecture-hall and corridor discussions of topics such as naked quasars, flaring stars, and planetary searches.

  16. Discovery of temperate Earth-sized planets transiting a nearby ultracool dwarf star

    NASA Astrophysics Data System (ADS)

    Jehin, Emmanuel; Gillon, Michael; Lederer, Susan M.; Delrez, Laetitia; de Wit, Julien; Burdanov, Artem; Van Grootel, Valérie; Burgasser, Adam; Triaud, Amaury; Demory, Brice-Olivier; Queloz, Didier

    2016-10-01

    We report the discovery of three short-period Earth-sized planets transiting a nearby ultracool dwarf star using data collected by the Liège TRAPPIST telescope, located in la Silla (Chile). TRAPPIST-1 is an isolated M8.0±0.5-type dwarf star at a distance of 12.0±0.4 parsecs as measured by its trigonometric parallax, with an age constrained to be > 500 Myr, and with a luminosity, mass, and radius of 0.05%, 8% and 11.5% those of the Sun, respectively. The small size of the host star, only slightly larger than Jupiter, translates into Earth-like radii for the three discovered planets, as deduced from their transit depths. The inner two planets receive four and two times the irradiation of Earth, respectively, placing them close to the inner edge of the habitable zone of the star. Several orbits remain possible for the third planet based on our current data. The infrared brightness of the host star combined with its Jupiter-like size offer the possibility of thoroughly characterizing the components of this nearby planetary system.

  17. Discovery of Temperate Earth-Sized Planets Transiting a Nearby Ultracool Dwarf Star

    NASA Technical Reports Server (NTRS)

    Jehin, Emmanuel; Gillon, Michael; Lederer, Susan M.; Delrez, Laetitia; De Wit, Julien; Burdanov, Artem; Van Grootel, Valerie; Burgasser, Adam; Triaud, Amaury; Demory, Brice-Olivier; Queloz, Didier

    2016-01-01

    We report the discovery of three short-period Earth-sized planets transiting a nearby ultracool dwarf star using data collected by the Liège TRAPPIST telescope, located in la Silla (Chile). TRAPPIST-1 is an isolated M8.0+/-0.5-type dwarf star at a distance of 12.0+/-0.4 parsecs as measured by its trigonometric parallax, with an age constrained to be > 500 Myr, and with a luminosity, mass, and radius of 0.05%, 8% and 11.5% those of the Sun, respectively. The small size of the host star, only slightly larger than Jupiter, translates into Earth-like radii for the three discovered planets, as deduced from their transit depths. The inner two planets receive four and two times the irradiation of Earth, respectively, placing them close to the inner edge of the habitable zone of the star. Several orbits remain possible for the third planet based on our current data. The infrared brightness of the host star combined with its Jupiter-like size offer the possibility of thoroughly characterizing the components of this nearby planetary system.

  18. Variable Stars in the Field of the Hydra II Ultra-Faint Dwarf Galaxy

    NASA Astrophysics Data System (ADS)

    Vivas, Anna Katherina; Olsen, Knut A.; Blum, Robert D.; Nidever, David L.; Walker, Alistair R.; Martin, Nicolas; Besla, Gurtina; Gallart, Carme; Van Der Marel, Roeland P.; Majewski, Steven R.; Munoz, Ricardo; Kaleida, Catherine C.; Saha, Abhijit; Conn, Blair; Jin, Shoko

    2016-06-01

    We searched for variable stars in Hydra II, one of the recently discovered ultra-faint dwarf satellites of the Milky Way, using gri time-series obtained with the Dark Energy Camera (DECam) at Cerro Tololo Inter-American Observatory, Chile. We discovered one RR Lyrae star in the galaxy which was used to derive a distance of 154±8 kpc to this system and to re-calculate its absolute magnitude and half-light radius.A comparison with other RR Lyrae stars in ultra-faint systems indicates similar pulsational properties among them, which are different to those found among halo field stars and those in the largest of the Milky Way satellites. We also report the discovery of 31 additional short period variables in the field of view (RR Lyrae, SX Phe, eclipsing binaries, and a likely anomalous cepheid) which are likely not related with Hydra II.

  19. ANCIENT STARS BEYOND THE LOCAL GROUP: RR LYRAE VARIABLES AND BLUE HORIZONTAL BRANCH STARS IN SCULPTOR GROUP DWARF GALAXIES

    SciTech Connect

    Da Costa, G. S.; Jerjen, H.; Rejkuba, M.; Grebel, E. K.

    2010-01-10

    We have used Hubble Space Telescope Advanced Camera for Surveys images to generate color-magnitude diagrams that reach below the magnitude of the horizontal branch in the Sculptor Group dwarf galaxies ESO294-010 and ESO410-005. In both diagrams, blue horizontal branch stars are unambiguously present, a signature of the existence of an ancient stellar population whose age is comparable to that of the Galactic halo globular clusters. The result is reinforced by the discovery of numerous RR Lyrae variables in both galaxies. The occurrence of these stars is the first direct confirmation of the existence of ancient stellar populations beyond the Local Group and indicates that star formation can occur at the earliest epochs even in low-density environments.

  20. Identification of dusty massive stars in star-forming dwarf irregular galaxies in the Local Group with mid-IR photometry

    NASA Astrophysics Data System (ADS)

    Britavskiy, N. E.; Bonanos, A. Z.; Mehner, A.; Boyer, M. L.; McQuinn, K. B. W.

    2015-12-01

    Context. Increasing the statistics of spectroscopically confirmed evolved massive stars in the Local Group enables the investigation of the mass loss phenomena that occur in these stars in the late stages of their evolution. Aims: We aim to complete the census of luminous mid-IR sources in star-forming dwarf irregular (dIrr) galaxies of the Local Group. To achieve this we employed mid-IR photometric selection criteria to identify evolved massive stars, such as red supergiants (RSGs) and luminous blue variables (LBVs), by using the fact that these types of stars have infrared excess due to dust. Methods: The method is based on 3.6 μm and 4.5 μm photometry from archival Spitzer Space Telescope images of nearby galaxies. We applied our criteria to four dIrr galaxies: Pegasus, Phoenix, Sextans A, and WLM, selecting 79 point sources that we observed with the VLT/FORS2 spectrograph in multi-object spectroscopy mode. Results: We identified 13 RSGs, of which 6 are new discoveries, as well as two new emission line stars, and one candidate yellow supergiant. Among the other observed objects we identified carbon stars, foreground giants, and background objects, such as a quasar and an early-type galaxy that contaminate our survey. We use the results of our spectroscopic survey to revise the mid-IR and optical selection criteria for identifying RSGs from photometric measurements. The optical selection criteria are more efficient in separating extragalactic RSGs from foreground giants than mid-IR selection criteria, but the mid-IR selection criteria are useful for identifying dusty stars in the Local Group. This work serves as a basis for further investigation of the newly discovered dusty massive stars and their host galaxies. Based on observations made with ESO Telescopes at the La Silla Paranal Observatory under programme IDs 090.D-0009 and 091.D-0010.Appendix A is available in electronic form at http://www.aanda.org

  1. EX-111 Thermal Emission from Hot White Dwarfs: The Suggested He Abundance-Temperature Correlation. EX-112: The Unique Emission Line White Dwarf Star GD 356

    NASA Technical Reports Server (NTRS)

    Shipman, H. L.

    1986-01-01

    Progress in the EXOSAT data analysis program is reported. EXOSAT observations for four white dwarfs (WD1031-115, WD0004+330, WD1615-154, and WD0109-264) were obtained. Counting rates were unexpectedly low, indicating that these objects have a substantial amount of x-ray absorbing matter in their photosheres. In addition, soft x-ray pulsations characterized by a 9.25 minute cycle were discovered in the DA white dwarf V471 Tauri. A residual x-ray flux from the K dwarf companion can be seen during the white dwarf eclipse at orbital phase 0.0. Pronounced dips in the soft x-ray light curve occur at orbital phases 0.15, 0.18, and 0.85. The dips may be correlated with the triangular Lagrangian points of the binary orbit. Smaller dips at phases near the eclipse may be associated with cool loops in the K star corona. Data for the white dwarf H1504+65 was also analyzed. This object is particularly unusual in that its photoshere is devoid of hydrogen and helium. Finally, existing data on the white dwarf Sirius B were analyzed to see what constraints from other data can be placed on the properties of this star. Interrelationships between radius, rotational velocity, and effective temperature were derived.

  2. PRIMUS: OBSCURED STAR FORMATION ON THE RED SEQUENCE

    SciTech Connect

    Zhu Guangtun; Blanton, Michael R.; Burles, Scott M.; Coil, Alison L.; Moustakas, John; Aird, James; Cool, Richard J.; Eisenstein, Daniel J.; Wong, Kenneth C.

    2011-01-10

    We quantify the fraction of galaxies at moderate redshifts (0.1 < z < 0.5) that appear red-and-dead in the optical, but in fact contain obscured star formation detectable in the infrared (IR), with the PRIsm MUlti-object Survey (PRIMUS). PRIMUS has measured {approx}120,000 robust redshifts with a precision of {sigma}{sub z}/(1 + z) {approx} 0.5% over 9.1 deg{sup 2} of the sky to the depth of i {approx} 23 (AB), up to redshift z {approx} 1. We specifically targeted 6.7 deg{sup 2} fields with existing deep IR imaging from the Spitzer Space Telescope from the SWIRE and S-COSMOS surveys. We select in these fields an i-band flux-limited sample (i < 20 mag in the SWIRE fields and i < 21 mag in the S-COSMOS field) of 3310 red-sequence galaxies at 0.1 < z < 0.5 for which we can reliably classify obscured star-forming (SF) and quiescent galaxies using IR color. Our sample constitutes the largest galaxy sample at intermediate redshift to study obscured star formation on the red sequence, and we present the first quantitative analysis of the fraction of obscured SF galaxies as a function of luminosity. We find that on average, at L {approx} L*, about 15% of red-sequence galaxies have IR colors consistent with SF galaxies. The percentage of obscured SF galaxies increases by {approx}8% per mag with decreasing luminosity from the highest luminosities to L {approx} 0.2 L*. Our results suggest that a significant fraction of red-sequence galaxies have ongoing star formation and that galaxy evolution studies based on optical color therefore need to account for this complication.

  3. Probability of CME Impact on Exoplanets Orbiting M Dwarfs and Solar-like Stars

    NASA Astrophysics Data System (ADS)

    Kay, C.; Opher, M.; Kornbleuth, M.

    2016-08-01

    Solar coronal mass ejections (CMEs) produce adverse space weather effects at Earth. Planets in the close habitable zone of magnetically active M dwarfs may experience more extreme space weather than at Earth, including frequent CME impacts leading to atmospheric erosion and leaving the surface exposed to extreme flare activity. Similar erosion may occur for hot Jupiters with close orbits around solar-like stars. We have developed a model, Forecasting a CME's Altered Trajectory (ForeCAT), which predicts a CME's deflection. We adapt ForeCAT to simulate CME deflections for the mid-type M dwarf V374 Peg and hot Jupiters with solar-type hosts. V374 Peg's strong magnetic fields can trap CMEs at the M dwarfs's Astrospheric Current Sheet, that is, the location of the minimum in the background magnetic field. Solar-type CMEs behave similarly, but have much smaller deflections and do not become trapped at the Astrospheric Current Sheet. The probability of planetary impact decreases with increasing inclination of the planetary orbit with respect to the Astrospheric Current Sheet: 0.5–5 CME impacts per day for M dwarf exoplanets, 0.05–0.5 CME impacts per day for solar-type hot Jupiters. We determine the minimum planetary magnetic field necessary to shield a planet's atmosphere from CME impacts. M dwarf exoplanets require values between tens and hundreds of Gauss. Hot Jupiters around a solar-type star, however, require a more reasonable <30 G. These values exceed the magnitude required to shield a planet from the stellar wind, suggesting that CMEs may be the key driver of atmospheric losses.

  4. Probability of CME Impact on Exoplanets Orbiting M Dwarfs and Solar-like Stars

    NASA Astrophysics Data System (ADS)

    Kay, C.; Opher, M.; Kornbleuth, M.

    2016-08-01

    Solar coronal mass ejections (CMEs) produce adverse space weather effects at Earth. Planets in the close habitable zone of magnetically active M dwarfs may experience more extreme space weather than at Earth, including frequent CME impacts leading to atmospheric erosion and leaving the surface exposed to extreme flare activity. Similar erosion may occur for hot Jupiters with close orbits around solar-like stars. We have developed a model, Forecasting a CME's Altered Trajectory (ForeCAT), which predicts a CME's deflection. We adapt ForeCAT to simulate CME deflections for the mid-type M dwarf V374 Peg and hot Jupiters with solar-type hosts. V374 Peg's strong magnetic fields can trap CMEs at the M dwarfs's Astrospheric Current Sheet, that is, the location of the minimum in the background magnetic field. Solar-type CMEs behave similarly, but have much smaller deflections and do not become trapped at the Astrospheric Current Sheet. The probability of planetary impact decreases with increasing inclination of the planetary orbit with respect to the Astrospheric Current Sheet: 0.5-5 CME impacts per day for M dwarf exoplanets, 0.05-0.5 CME impacts per day for solar-type hot Jupiters. We determine the minimum planetary magnetic field necessary to shield a planet's atmosphere from CME impacts. M dwarf exoplanets require values between tens and hundreds of Gauss. Hot Jupiters around a solar-type star, however, require a more reasonable <30 G. These values exceed the magnitude required to shield a planet from the stellar wind, suggesting that CMEs may be the key driver of atmospheric losses.

  5. The atomic and molecular content of disks around very low-mass stars and brown dwarfs

    SciTech Connect

    Pascucci, I.; Herczeg, G.; Carr, J. S.; Bruderer, S.

    2013-12-20

    There is growing observational evidence that disk evolution is stellar-mass-dependent. Here, we show that these dependencies extend to the atomic and molecular content of disk atmospheres. We analyze a unique dataset of high-resolution Spitzer/IRS spectra from eight very low mass star and brown dwarf disks. We report the first detections of Ne{sup +}, H{sub 2}, CO{sub 2}, and tentative detections of H{sub 2}O toward these faint and low-mass disks. Two of our [Ne II] 12.81 μm emission lines likely trace the hot (≥5000 K) disk surface irradiated by X-ray photons from the central stellar/sub-stellar object. The H{sub 2} S(2) and S(1) fluxes are consistent with arising below the fully or partially ionized surface traced by the [Ne II] emission in gas at ∼600 K. We confirm the higher C{sub 2}H{sub 2}/HCN flux and column density ratio in brown dwarf disks previously noted from low-resolution IRS spectra. Our high-resolution spectra also show that the HCN/H{sub 2}O fluxes of brown dwarf disks are on average higher than those of T Tauri disks. Our LTE modeling hints that this difference extends to column density ratios if H{sub 2}O lines trace warm ≥600 K disk gas. These trends suggest that the inner regions of brown dwarf disks have a lower O/C ratio than those of T Tauri disks, which may result from a more efficient formation of non-migrating icy planetesimals. An O/C = 1, as inferred from our analysis, would have profound implications on the bulk composition of rocky planets that can form around very low mass stars and brown dwarfs.

  6. The Effect of Feedback and Reionization on Star Formation in Low-mass Dwarf Galaxy Halos

    NASA Astrophysics Data System (ADS)

    Simpson, Christine M.; Bryan, G.; Johnston, K. V.; Smith, B. D.; Mac Low, M.; Sharma, S.; Tumlinson, J.

    2013-01-01

    I will present a set of high resolution simulations of a 109 M⊙ dark matter halo in a cosmological setting done with an adaptive-mesh refinement code as a mass analogue to local low-luminosity dwarf spheroidal galaxies. The primary goal of our simulations is to investigate the roles of reionization and supernova feedback in determining the star formation histories of low mass dwarf galaxies. We include a wide range of physical effects, including metal cooling, molecular hydrogen formation and cooling, photoionization and photodissociation from a metagalactic (but not local) background, a simple prescription for self-shielding, star formation, and a simple model for supernova driven energetic feedback. We find that reionization is primarily responsible for expelling most of the gas in our simulations, but that supernova feedback is required to disperse the dense, cold gas in the core of the halo. Moreover, we show that the timing of reionization can produce an order of magnitude difference in the final stellar mass of the system. For our full physics run with reionization at z=9, we find a stellar mass of about 105 M⊙ at z=0, and a mass-to-light ratio within the half-light radius of approximately 130 M⊙/L⊙, consistent with observed low-luminosity dwarfs. However, the resulting median stellar metallicity is 0.06 Z⊙, considerably larger than observed systems. In addition, we find star formation is truncated between redshifts 4 and 7, at odds with the observed late time star formation in isolated dwarf systems but in agreement with Milky Way ultrafaint dwarf spheroidals. We investigate the efficacy of energetic feedback in our simple thermal-energy driven feedback scheme, and suggest that it may still suffer from excessive radiative losses, despite reaching stellar particle masses of about 100 M⊙, and a comoving spatial resolution of 11 pc. This has led us to pursue improvements in our supernova feedback model to include kinetic as well as thermal energy in

  7. Additional red and reddened stars in Cyg OB2 association

    NASA Technical Reports Server (NTRS)

    Parthasarathy, M.; Jain, S. K.

    1989-01-01

    Several new red and reddened stars are detected in the most heavily reddened associations Cyg OB2. About 47 IRAS sources are detected in Cyg OB2. Their flux distributions, and colors, suggest that they are young stellar objects embedded in dust envelopes or disks (some of them may be proto stars) and are most likely members of the Cyg OB2 association. The large values of the flux ratio L sub IR/L sub VIS suggests that the central objects are obscured because of very large extinction.

  8. INSIGHTS INTO PRE-ENRICHMENT OF STAR CLUSTERS AND SELF-ENRICHMENT OF DWARF GALAXIES FROM THEIR INTRINSIC METALLICITY DISPERSIONS

    SciTech Connect

    Leaman, Ryan

    2012-12-01

    Star clusters are known to have smaller intrinsic metallicity spreads than dwarf galaxies due to their shorter star formation timescales. Here we use individual spectroscopic [Fe/H] measurements of stars in 19 Local Group dwarf galaxies, 13 Galactic open clusters, and 49 globular clusters to show that star cluster and dwarf galaxy linear metallicity distributions are binomial in form, with all objects showing strong correlations between their mean linear metallicity Z-bar and intrinsic spread in metallicity {sigma}(Z){sup 2}. A plot of {sigma}(Z){sup 2} versus Z-bar shows that the correlated relationships are offset for the dwarf galaxies from the star clusters. The common binomial nature of these linear metallicity distributions can be explained with a simple inhomogeneous chemical evolution model, where the star cluster and dwarf galaxy behavior in the {sigma}(Z){sup 2}- Z-bar diagram is reproduced in terms of the number of enrichment events, covering fraction, and intrinsic size of the enriched regions. The inhomogeneity of the self-enrichment sets the slope for the observed dwarf galaxy {sigma}(Z){sup 2}- Z-bar correlation. The offset of the star cluster sequence from that of the dwarf galaxies is due to pre-enrichment, and the slope of the star cluster sequence represents the remnant signature of the self-enriched history of their host galaxies. The offset can be used to separate star clusters from dwarf galaxies without a priori knowledge of their luminosity or dynamical mass. The application of the inhomogeneous model to the {sigma}(Z){sup 2}- Z-bar relationship provides a numerical formalism to connect the self-enrichment and pre-enrichment between star clusters and dwarf galaxies using physically motivated chemical enrichment parameters. Therefore we suggest that the {sigma}(Z){sup 2}- Z-bar relationship can provide insight into what drives the efficiency of star formation and chemical evolution in galaxies, and is an important prediction for galaxy

  9. First axion bounds from a pulsating helium-rich white dwarf star

    NASA Astrophysics Data System (ADS)

    Battich, T.; Córsico, A. H.; Althaus, L. G.; Miller Bertolami, M. M.

    2016-08-01

    The Peccei-Quinn mechanism proposed to solve the CP problem of Quantum Chromodynamics has as consequence the existence of axions, hypothetical weakly interacting particles whose mass is constrained to be on the sub-eV range. If these particles exist and interact with electrons, they would be emitted from the dense interior of white dwarfs, becoming an important energy sink for the star. Due to their well known physics, white dwarfs are good laboratories to study the properties of fundamental particles such as the axions. We study the general effect of axion emission on the evolution of helium-rich white dwarfs and on their pulsational properties. To this aim, we calculate evolutionary helium-rich white dwarf models with axion emission, and assess the pulsational properties of this models. Our results indicate that the rates of change of pulsation periods are significantly affected by the existence of axions. We are able for the first time to independently constrain the mass of the axion from the study of pulsating helium-rich white dwarfs. To do this, we use an estimation of the rate of change of period of the pulsating white dwarf PG 1351+489 corresponding to the dominant pulsation period. From an asteroseismological model of PG 1351+489 we obtain gae < 3.3 × 10‑13 for the axion-electron coupling constant, or macos2β lesssim 11.5 meV for the axion mass. This constraint is relaxed to gae < 5.5 × 10‑13 (macos2β lesssim 19.5 meV), when no detailed asteroseismological model is adopted for the comparison with observations.

  10. First axion bounds from a pulsating helium-rich white dwarf star

    NASA Astrophysics Data System (ADS)

    Battich, T.; Córsico, A. H.; Althaus, L. G.; Miller Bertolami, M. M.

    2016-08-01

    The Peccei-Quinn mechanism proposed to solve the CP problem of Quantum Chromodynamics has as consequence the existence of axions, hypothetical weakly interacting particles whose mass is constrained to be on the sub-eV range. If these particles exist and interact with electrons, they would be emitted from the dense interior of white dwarfs, becoming an important energy sink for the star. Due to their well known physics, white dwarfs are good laboratories to study the properties of fundamental particles such as the axions. We study the general effect of axion emission on the evolution of helium-rich white dwarfs and on their pulsational properties. To this aim, we calculate evolutionary helium-rich white dwarf models with axion emission, and assess the pulsational properties of this models. Our results indicate that the rates of change of pulsation periods are significantly affected by the existence of axions. We are able for the first time to independently constrain the mass of the axion from the study of pulsating helium-rich white dwarfs. To do this, we use an estimation of the rate of change of period of the pulsating white dwarf PG 1351+489 corresponding to the dominant pulsation period. From an asteroseismological model of PG 1351+489 we obtain gae < 3.3 × 10-13 for the axion-electron coupling constant, or macos2β lesssim 11.5 meV for the axion mass. This constraint is relaxed to gae < 5.5 × 10-13 (macos2β lesssim 19.5 meV), when no detailed asteroseismological model is adopted for the comparison with observations.

  11. THE PROPERTIES OF THE 500 K DWARF UGPS J072227.51-054031.2 AND A STUDY OF THE FAR-RED FLUX OF COLD BROWN DWARFS

    SciTech Connect

    Leggett, S. K.; Saumon, D.; Marley, M. S.; Lodders, K.; Fegley, B.; Canty, J.; Lucas, P.; Burningham, Ben; Jones, H. R. A.; Marocco, F.; Pinfield, D. J.; Smart, R. L.; Homeier, D.; Allard, F.; Day-Jones, A.; Ishii, Miki; Tamura, M.

    2012-04-01

    We present i and z photometry for 25 T dwarfs and 1 L dwarf. Combined with published photometry, the data show that the i - z, z - Y, and z - J colors of T dwarfs are very red, and continue to increase through to the late-type T dwarfs, with a hint of a saturation for the latest types with T{sub eff} Almost-Equal-To 600 K. We present new 0.7-1.0 {mu}m and 2.8-4.2 {mu}m spectra for the very late type T dwarf UGPS J072227.51-054031.2, as well as improved astrometry for this dwarf. Examination of the spectral energy distribution using new and published data, with Saumon and Marley models, shows that the dwarf has T{sub eff} = 505 {+-} 10 K, a mass of 3-11 M{sub Jupiter}, and an age between 60 Myr and 1 Gyr. This young age is consistent with the thin disk kinematics of the dwarf. The mass range overlaps with that usually considered to be planetary, despite this being an unbound object discovered in the field near the Sun. This apparently young rapid rotator is also undergoing vigorous atmospheric mixing, as determined by the IRAC and WISE 4.5 {mu}m photometry and the Saumon and Marley models. The optical spectrum for this 500 K object shows clearly detected lines of the neutral alkalis Cs and Rb, which are emitted from deep atmospheric layers with temperatures of 900-1200 K.

  12. Variable Stars in the Field of the Hydra II Ultra-faint Dwarf Galaxy

    NASA Astrophysics Data System (ADS)

    Vivas, A. Katherina; Olsen, Knut; Blum, Robert; Nidever, David L.; Walker, Alistair R.; Martin, Nicolas F.; Besla, Gurtina; Gallart, Carme; van der Marel, Roeland P.; Majewski, Steven R.; Kaleida, Catherine C.; Muñoz, Ricardo R.; Saha, Abhijit; Conn, Blair C.; Jin, Shoko

    2016-05-01

    We report the discovery of one RR Lyrae star in the ultra-faint satellite galaxy Hydra II based on time series photometry in the g, r and i bands obtained with the Dark Energy Camera at Cerro Tololo Inter-American Observatory, Chile. The association of the RR Lyrae star discovered here with Hydra II is clear because is located at 42\\prime\\prime from the center of the dwarf, well within its half-light radius of 102\\prime\\prime . The RR Lyrae star has a mean magnitude of i=21.30+/- 0.04 which is too faint to be a field halo star. This magnitude translates to a heliocentric distance of 151 ± 8 kpc for Hydra II; this value is ∼ 13% larger than the estimate from the discovery paper based on the average magnitude of several blue horizontal branch star candidates. The new distance implies a slightly larger half-light radius of {76}-10+12 pc and a brighter absolute magnitude of {M}V=-5.1+/- 0.3, which keeps this object within the realm of the dwarf galaxies. A comparison with other RR Lyrae stars in ultra-faint systems indicates similar pulsational properties among them, which are different to those found among halo field stars and those in the largest of the Milky Way satellites. We also report the discovery of 31 additional short period variables in the field of view (RR Lyrae, SX Phe, eclipsing binaries, and a likely anomalous cepheid) which are likely not related with Hydra II.

  13. STAR FORMATION MODELS FOR THE DWARF GALAXIES NGC 2915 AND NGC 1705

    SciTech Connect

    Elson, E. C.; De Blok, W. J. G.; Kraan-Korteweg, R. C.

    2012-01-15

    Crucial to a quantitative understanding of galaxy evolution are the properties of the interstellar medium that regulate galactic-scale star formation activity. We present here the results of a suite of star formation models applied to the nearby blue compact dwarf galaxies NGC 2915 and NGC 1705. Each of these galaxies has a stellar disk embedded in a much larger, essentially starless H I disk. These atypical stellar morphologies allow for rigorous tests of star formation models that examine the effects on star formation of the H I, stellar, and dark matter mass components, as well as the kinematics of the gaseous and stellar disks. We use far-ultraviolet and 24 {mu}m images from the Galaxy Evolution Explorer and the Spitzer Infrared Nearby Galaxies Survey, respectively, to map the spatial distribution of the total star formation rate surface density within each galaxy. New high-resolution H I line observations obtained with the Australia Telescope Compact Array are used to study the distribution and dynamics of each galaxy's neutral interstellar medium. The standard Toomre Q parameter is unable to distinguish between active and non-active star-forming regions, predicting the H I disks of the dwarfs to be sub-critical. Two-fluid instability models incorporating the stellar and dark matter components of each galaxy, in addition to the gaseous component, yield unstable portions of the inner disk. Finally, a formalization in which the H I kinematics are characterized by the rotational shear of the gas produces models that very accurately match the observations. This suggests the time available for perturbations to collapse in the presence of rotational shear to be an important factor governing galactic-scale star formation.

  14. Star Formation Models for the Dwarf Galaxies NGC 2915 and NGC 1705

    NASA Astrophysics Data System (ADS)

    Elson, E. C.; de Blok, W. J. G.; Kraan-Korteweg, R. C.

    2012-01-01

    Crucial to a quantitative understanding of galaxy evolution are the properties of the interstellar medium that regulate galactic-scale star formation activity. We present here the results of a suite of star formation models applied to the nearby blue compact dwarf galaxies NGC 2915 and NGC 1705. Each of these galaxies has a stellar disk embedded in a much larger, essentially starless H I disk. These atypical stellar morphologies allow for rigorous tests of star formation models that examine the effects on star formation of the H I, stellar, and dark matter mass components, as well as the kinematics of the gaseous and stellar disks. We use far-ultraviolet and 24 μm images from the Galaxy Evolution Explorer and the Spitzer Infrared Nearby Galaxies Survey, respectively, to map the spatial distribution of the total star formation rate surface density within each galaxy. New high-resolution H I line observations obtained with the Australia Telescope Compact Array are used to study the distribution and dynamics of each galaxy's neutral interstellar medium. The standard Toomre Q parameter is unable to distinguish between active and non-active star-forming regions, predicting the H I disks of the dwarfs to be sub-critical. Two-fluid instability models incorporating the stellar and dark matter components of each galaxy, in addition to the gaseous component, yield unstable portions of the inner disk. Finally, a formalization in which the H I kinematics are characterized by the rotational shear of the gas produces models that very accurately match the observations. This suggests the time available for perturbations to collapse in the presence of rotational shear to be an important factor governing galactic-scale star formation.

  15. Star formation and molecular hydrogen in dwarf galaxies: a non-equilibrium view

    NASA Astrophysics Data System (ADS)

    Hu, Chia-Yu; Naab, Thorsten; Walch, Stefanie; Glover, Simon C. O.; Clark, Paul C.

    2016-06-01

    We study the connection of star formation to atomic (H I) and molecular hydrogen (H2) in isolated, low-metallicity dwarf galaxies with high-resolution (mgas = 4 M⊙, Nngb = 100) smoothed particle hydrodynamics simulations. The model includes self-gravity, non-equilibrium cooling, shielding from a uniform and constant interstellar radiation field, the chemistry of H2 formation, H2-independent star formation, supernova feedback and metal enrichment. We find that the H2 mass fraction is sensitive to the adopted dust-to-gas ratio and the strength of the interstellar radiation field, while the star formation rate is not. Star formation is regulated by stellar feedback, keeping the gas out of thermal equilibrium for densities n < 1 cm-3. Because of the long chemical time-scales, the H2 mass remains out of chemical equilibrium throughout the simulation. Star formation is well correlated with cold (T ≤ 100 K) gas, but this dense and cold gas - the reservoir for star formation - is dominated by H I, not H2. In addition, a significant fraction of H2 resides in a diffuse, warm phase, which is not star-forming. The interstellar medium is dominated by warm gas (100 K < T ≤ 3 × 104 K) both in mass and in volume. The scaleheight of the gaseous disc increases with radius while the cold gas is always confined to a thin layer in the mid-plane. The cold gas fraction is regulated by feedback at small radii and by the assumed radiation field at large radii. The decreasing cold gas fractions result in a rapid increase in depletion time (up to 100 Gyr) for total gas surface densities Σ _{H I+H_2} ≲ 10 M⊙ pc-2, in agreement with observations of dwarf galaxies in the Kennicutt-Schmidt plane.

  16. A Survey of Nearby Cool Dwarf Star Spectra Obtained by FUSE

    NASA Astrophysics Data System (ADS)

    Redfield, S.; Linsky, J. L.; Dupree, A. K.; Young, P. R.; Lobel, A.; Ake, T. B.; FUSE Cool Star Team

    2001-12-01

    We present the cool star dwarf spectra obtained by the FUSE Science Team using the Far Ultraviolet Spectroscopic Explorer (FUSE). The survey includes the stars α Aql, α Cen A, α Cen B, ɛ Eri, AU Mic, and AB Dor. The spectral range extends from 900 to 1200 ~Å, at a spectral resolution of λ /Δ λ ~ 20,000. Numerous emission lines are present and identified. Lines of particular interest, such as C III (977 Å), O VI (1032 and 1038 Å), and the Lyman series, are present in all stars and studied in detail. The forbidden coronal Fe XVIII (975 Å) line is present in some spectra and is useful in studying the dynamics of the 107 K coronal plasma. Absorption by warm Local Interstellar Medium (LISM) gas is visible in several lines, such as C III (977 Å) and C II (1037 Å). Combined with LISM absorption studies of the same stars at longer wavelengths (ie. using spectrographs aboard the Hubble Space Telescope (HST)), the FUSE LISM absorption can provide important insights into the structure of the nearby warm ISM. This atlas of nearby cool stars in the far-ultraviolet provides important information pertaining to the atmospheres of dwarf stars and will be a useful tool in planning future observations of cool stars. This work is based on data obtained for the Guaranteed Time Team by the NASA-CNES-CSA FUSE mission operated by the Johns Hopkins Observatory. Financial support to U.S. participants has been provided by NASA contract NAS5-32985.

  17. A COMPREHENSIVE, WIDE-FIELD STUDY OF PULSATING STARS IN THE CARINA DWARF SPHEROIDAL GALAXY

    SciTech Connect

    Vivas, A. Katherina; Mateo, Mario E-mail: mmateo@umich.edu

    2013-12-01

    We report the detection of 388 pulsating variable stars (and some additional miscellaneous variables) in the Carina dwarf spheroidal galaxy over an area covering the full visible extent of the galaxy and extending a few times beyond its photometric (King) tidal radius along the direction of its major axis. Included in this total are 340 newly discovered dwarf Cepheids (DCs), which are mostly located ∼2.5 mag below the horizontal branch and have very short periods (<0.1 days), typical of their class and consistent with their location on the upper part of the extended main sequence of the younger populations of the galaxy. Several extra-tidal DCs were found in our survey up to a distance of ∼1° from the center of Carina. Our sample also includes RR Lyrae stars and anomalous Cepheids, some of which were found outside the galaxy's tidal radius as well. This supports past works that suggest that Carina is undergoing tidal disruption. We use the period-luminosity relationship for DCs to estimate a distance modulus of μ{sub 0} = 20.17 ± 0.10 mag, in very good agreement with the estimate from RR Lyrae stars. We find some important differences in the properties of the DCs of Carina and those in Fornax and the LMC, the only extragalactic samples of DCs currently known. These differences may reflect a metallicity spread, depth along the line of sight, and/or different evolutionary paths of the DC stars.

  18. Hubble Space Telescope observations of cool white dwarf stars: Detection of new species of heavy elements

    NASA Technical Reports Server (NTRS)

    Shipman, Harry; Barnhill, Maurice; Provencal, Judi; Roby, Scott; Bues, Irmela; Cordova, France; Hammond, Gordon; Hintzen, Paul; Koester, Detlev; Liebert, James

    1995-01-01

    Observations of cool white dwarf stars with the Hubble Space Telescope (HST) has uncovered a number of spectral features from previouslly unobserved species. In this paper we present the data on four cool white dwarfs. We present identifications, equivalent width measurements, and brief summaries of the significance of our findings. The four stars observed are GD 40 (DBZ3, G 74-7 (DAZ), L 745-46A (DZ), and LDS 749B (DBA). Many additional species of heavey elements were detected in GD 40 and G 74-7. In L 745-46A, while the detections are limited to Fe 1, Fe II, and Mg II, the quality of the Mg II h and K line profiles should permit a test of the line broadening theories, which are so crucial to abundance determinations. The clear detection of Mg II h and k in LDS 749 B should, once an abundance determination is made, provide a clear test of the hypothesis that the DBA stars are the result of accretion from the interstellar medium. This star contains no other clear features other than a tantalizing hint of C II 1335 with a P Cygni profile, and some expected He 1 lines.

  19. Orbital motion of the binary brown dwarf companions HD 130948 BC around their host star

    NASA Astrophysics Data System (ADS)

    Ginski, C.; Neuhäuser, R.; Mugrauer, M.; Schmidt, T. O. B.; Adam, C.

    2013-09-01

    Evolutionary models and mass estimates for brown dwarfs remain uncertain, hence determining the masses of brown dwarfs by model-independent methods is important to test and constrain such theories. Following the orbital motion of brown dwarf companions around their primaries gives us the opportunity to dynamically calculate the masses of these systems. In addition, detecting curvature (acceleration or deceleration) in the orbit would confirm that the companion is physically associated with its primary, thus eliminating the possibility of a by-chance alignment of the primary's and the companion's proper motions and positions. Furthermore, the orbit parameters can be important indicators for the formation process of such wide, massive substellar companions. The binary brown dwarf companions to HD 130948 were discovered by Potter et al. We present various observations of this triple system over the course of 7 yr. With these data points we can show that HD 130948 BC are indeed comoving with HD 130948 A with higher significance than before (˜32.4σ), and also for the first time that the BC pair shows differential motion relative to A (˜2.2σ). We introduce an orbit fitting approach and constrain the orbit parameters for the orbit of the BC binary around their host star.

  20. Metallicities of Low Mass Inefficient Star Forming Dwarfs in S4G: Testing the Closed Box Paradigm

    NASA Astrophysics Data System (ADS)

    McKay, Myles; Stirewalt, Sabrina; Sheth, Kartik; de Swardt, Bonita; Walter, Donald

    2015-03-01

    Low mass dwarf galaxies are the most numerous extragalactic population in the Local Universe. Many gas-rich dwarfs appear to be forming stars less efficiently than normal, massive disk galaxies and are therefore important laboratories for the study of star formation. Here we present new observations using the Palomar Double Spectrograph for 19 dwarf galaxies from the S4G Survey with the lowest stellar to HI mass ratios. Preliminary analysis of the data indicate a wide range of metallicities which vary by as much as 0.5 dex in a single galaxy in different star forming regions. Such a dispersion in metallicities favors an open box model and the results suggest a varied star formation history, possibly induced via minor mergers and accretion. The National Radio Astronomy Observatory(NRAO), National Science Foundation(NSF), and the National Astronomy Consortium (NAC) Cville Cohort. Additional support was provided by NSF Awards AST-0750814 and AST-1358913 to South Carolina State University.

  1. New PARSEC evolutionary tracks of massive stars at low metallicity: testing canonical stellar evolution in nearby star-forming dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Tang, Jing; Bressan, Alessandro; Rosenfield, Philip; Slemer, Alessandra; Marigo, Paola; Girardi, Léo; Bianchi, Luciana

    2014-12-01

    We extend the PARSEC library of stellar evolutionary tracks by computing new models of massive stars, from 14 to 350 M⊙. The input physics is the same used in the PARSEC V1.1 version, but for the mass-loss rate from considering the most recent updates in the literature. We focus on low metallicity, Z = 0.001 and Z = 0.004, for which the metal-poor dwarf irregular star-forming galaxies, Sextans A, the Wolf-Lundmark-Melotte galaxy and NGC 6822, provide simple but powerful workbenches. The models reproduce fairly well the observed colour-magnitude diagrams (CMDs) but the stellar colour distributions indicate that the predicted blue loop is not hot enough in models with a canonical extent of overshooting. In the framework of a mild extended mixing during central hydrogen burning, the only way to reconcile the discrepancy is to enhance the overshooting at the base of the convective envelope (EO) during the first dredge-up. The mixing scales required to reproduce the observed loops, EO = 2HP or EO = 4HP, are definitely larger than those derived from, e.g. the observed location of the red-giant-branch bump in low mass stars. This effect, if confirmed, would imply a strong dependence of the mixing scale below the formal Schwarzschild border, on the stellar mass or luminosity. Reproducing the features of the observed CMDs with standard values of envelope overshooting would require a metallicity significantly lower than the values measured in these galaxies. Other quantities, such as the star formation rate and the initial mass function, are only slightly sensitive to this effect. Future investigations will consider other metallicities and different mixing schemes.

  2. The Initial Mass Function of Low-Mass Stars and Brown Dwarfs in Taurus

    NASA Astrophysics Data System (ADS)

    Luhman, K. L.

    2000-12-01

    By combining deep optical imaging and infrared spectroscopy with data from the Two-Micron All-Sky Survey (2MASS) and from previous studies (e.g., Briceño et al.), I have measured the initial mass function (IMF) for a reddening-limited sample in four fields in the Taurus star-forming region. This IMF is representative of the young populations within these fields for masses above 0.02 Msolar. Relative to the similarly derived IMF for the Trapezium Cluster (Luhman et al.), the IMF for Taurus exhibits a modest deficit of stars above 1 solar mass (i.e., steeper slope), the same turnover mass (~0.8 Msolar), and a significant deficit of brown dwarfs. If the IMF in Taurus were the same as that in the Trapezium, 12.8+/-1.8 brown dwarfs (>0.02 Msolar) are expected in these Taurus fields where only one brown dwarf candidate is found. These results are used to test theories of the IMF. Visiting Astronomer, Kitt Peak National Observatory, National Optical Astronomy Observatories, which is operated by the Association of Universities for Research in Astronomy, Inc. (AURA), under cooperative agreement with the National Science Foundation.

  3. Magnetospheric accretion & outflows in stars & brown dwarfs: theories and observational constraints

    NASA Astrophysics Data System (ADS)

    Mohanty, S.

    2010-11-01

    The manner in which young classical T Tauri stars (cTTs) and brown dwarfs accrete gas from their surrounding disks and simultaneously drive jets and outflows is central to star and planet formation and angular momentum evolution, but remains an ill-understood and hotly debated subject. One of the central concerns is the stellar field geometry: while analytic theories assume an idealized stellar dipole, T Tauri fields are observed to be complex multipolar beasts. I present an analytic generalization of the X-wind theory to include such fields. Independent of the precise field geometry, the generalized model makes a unique prediction about the relationship between various cTTs observables. I show that this prediction is supported by observations of accretion rate, hot spot size, stellar rotation and field strength from stellar to brown dwarf masses, including recent detailed spectropolarimetric measurements. I also discuss the unique insights offered by recent magnetic field measurements on accreting brown dwarfs: while they agree with the accretion theory above, they also pose a puzzle for magnetic field generation theory. Resolving this conundrum promises to illuminate our general picture of accretion and angular momentum transport in fully convective objects.

  4. The incidence of magnetism among white dwarfs: The first stars below 100 kilogauss

    NASA Technical Reports Server (NTRS)

    Schmidt, Gary D.; Smith, Paul S.

    1994-01-01

    A survey for magnetic fields among a magnitude-limited sample of DA white dwarfs has identified two stars with weak circular polarization features across the profiles of H(alpha) and H(beta) WD 1350-090 (LP 907-037) was found to have a disk-averaged longitudinal field component B(sub e) = +85 +/- 9 kG at one epoch, while WD 0009+501 (G 217-037) has been measured on several occasions at values between B(sub e) approximately 0 and nearly -100 kG. The latter results imply an oblique rotator with a period between 2 and 20 hr. Magnetism on white dwarfs has now been detected over more than four orders of magnitude in strength. Assuming flux conservation, the new discoveries imply organized field patterns near the end of the main-sequence phase of only approximately 10 G. However, the overall incidence of magnetism among white dwarfs remains low, with more than 90% of stars having fields below approximately 10 kG. There is tentative evidence from line profile analysis that WD 1350-090 is a high-mass object (M greater than 1 solar mass), but an accurate parallax and more thorough spectroscopic study are required.

  5. Southern Very Low Mass Stars and Brown Dwarfs in Wide Binary and Multiple Systems

    NASA Astrophysics Data System (ADS)

    Caballero, José Antonio

    2007-09-01

    The results of the Königstuhl survey in the Southern Hemisphere are presented. I have searched for common proper motion companions to 173 field very low mass stars and brown dwarfs with spectral types >M5.0 V and magnitudes J<~14.5 mag. I have measured for the first time the common proper motion of two new wide systems containing very low mass components, Königstuhl 2 AB and 3 A-BC. Together with Königstuhl 1 AB and 2M 0126-50 AB, they are among the widest systems in their respective classes (r=450-11,900 AU). I have determined the minimum frequency of field wide multiples (r>100 AU) with late-type components at 5.0%+/-1.8% and the frequency of field wide late-type binaries with mass ratios q>0.5 at 1.2%+/-0.9%. These values represent a key diagnostic of evolution history and low-mass star and brown dwarf formation scenarios. In addition, the proper motions of 62 field very low mass dwarfs are measured here for the first time.

  6. Are passive red spirals truly passive?. The current star formation activity of optically red disc galaxies

    NASA Astrophysics Data System (ADS)

    Cortese, L.

    2012-07-01

    We used GALEX ultraviolet and WISE 22 μm observations to investigate the current star formation activity of the optically red spirals recently identified as part of the Galaxy Zoo project. These galaxies were accurately selected from the Sloan Digital Sky Survey as pure discs with low or no current star formation activity, representing one of the best optically selected samples of candidate passive spirals. However, we show that these galaxies are not only still forming stars at a significant rate (≳1 M⊙ yr-1) but, more importantly, their star formation activity is not different from that of normal star-forming discs of the same stellar mass (M∗ ≳ 1010.2 M⊙). Indeed, these systems lie on the UV-optical blue sequence, even without any corrections for internal dust attenuation, and they follow the same specific star formation rate vs. stellar mass relation of star-forming galaxies. Our findings clearly show that at high stellar masses, optical colours do not allow to distinguish between actively star-forming and truly quiescent systems.

  7. DISTANCE TO THE SAGITTARIUS DWARF GALAXY USING MACHO PROJECT RR LYRAE STARS

    SciTech Connect

    Kunder, Andrea; Chaboyer, Brian E-mail: brian.chaboyer@dartmouth.edu

    2009-05-15

    We derive the distance to the northern extension of the Sagittarius (Sgr) dwarf spheroidal galaxy from 203 Sgr RR0 Lyrae stars found in the MACHO database. Their distances are determined differentially with respect to 288 Galactic bulge RR0 Lyrae stars also found in the MACHO data. We find a distance modulus difference of 2.41 mag at l = 5{sup 0} and b = -8{sup 0} and that the extension of the Sgr galaxy toward the galactic plane is inclined toward us. Assuming R {sub GC} = 8 kpc, this implies the distance to these stars is (m - M){sub 0} = 16.97 {+-} 0.07 mag, which corresponds to D = 24.8 {+-} 0.8 kpc. Although this estimate is smaller than previous determinations for this galaxy and agrees with previous suggestions that Sgr's body is truly closer to us, this estimate is larger than studies at comparable galactic latitudes.

  8. THE ACS NEARBY GALAXY SURVEY TREASURY. VII. THE NGC 4214 STARBURST AND THE EFFECTS OF STAR FORMATION HISTORY ON DWARF MORPHOLOGY

    SciTech Connect

    Williams, Benjamin F.; Dalcanton, Julianne J.; Gilbert, Karoline M.; Weisz, Daniel R.; Seth, Anil C.; Skillman, Evan D.; Dolphin, Andrew E. E-mail: jd@astro.washington.edu E-mail: dweisz@astro.washington.edu E-mail: skillman@astro.umn.edu

    2011-07-01

    We present deep Hubble Space Telescope WFPC2 optical observations obtained as part of the ACS Nearby Galaxy Survey Treasury as well as early release Wide Field Camera 3 ultraviolet and infrared observations of the nearby dwarf starbursting galaxy NGC 4214. Our data provide a detailed example of how covering such a broad range in wavelength provides a powerful tool for constraining the physical properties of stellar populations. The deepest data reach the ancient red clump at M{sub F814W} {approx} - 0.2. All of the optical data reach the main-sequence turnoff for stars younger than {approx}300 Myr and the blue He-burning sequence for stars younger than 500 Myr. The full color-magnitude diagram (CMD) fitting analysis shows that all three fields in our data set are consistent with {approx}75% of the stellar mass being older than 8 Gyr, in spite of showing a wide range in star formation rates at present. Thus, our results suggest that the scale length of NGC 4214 has remained relatively constant for many gigayears. As previously noted by others, we also find the galaxy has recently ramped up production consistent with its bright UV luminosity and its population of UV-bright massive stars. In the central field we find UV point sources with F336W magnitudes as bright as -9.9. These are as bright as stars with masses of at least 52-56 M{sub sun} and ages near 4 Myr in stellar evolution models. Assuming a standard initial mass function, our CMD is well fitted by an increase in star formation rate beginning 100 Myr ago. The stellar populations of this late-type dwarf are compared with those of NGC 404, an early-type dwarf that is also the most massive galaxy in its local environment. The late-type dwarf appears to have a similar high fraction of ancient stars, suggesting that these dominant galaxies may form at early epochs even if they have low total mass and very different present-day morphologies.

  9. Planets around Low-mass Stars (PALMS). IV. The Outer Architecture of M Dwarf Planetary Systems

    NASA Astrophysics Data System (ADS)

    Bowler, Brendan P.; Liu, Michael C.; Shkolnik, Evgenya L.; Tamura, Motohide

    2015-01-01

    We present results from a high-contrast adaptive optics imaging search for giant planets and brown dwarfs (gsim1 M Jup) around 122 newly identified nearby (lsim40 pc) young M dwarfs. Half of our targets are younger than 135 Myr and 90% are younger than the Hyades (620 Myr). After removing 44 close stellar binaries (implying a stellar companion fraction of >35.4% ± 4.3% within 100 AU), 27 of which are new or spatially resolved for the first time, our remaining sample of 78 single M dwarfs makes this the largest imaging search for planets around young low-mass stars (0.1-0.6 M ⊙) to date. Our H- and K-band coronagraphic observations with Keck/NIRC2 and Subaru/HiCIAO achieve typical contrasts of 12-14 mag and 9-13 mag at 1'', respectively, which correspond to limiting planet masses of 0.5-10 M Jup at 5-33 AU for 85% of our sample. We discovered four young brown dwarf companions: 1RXS J235133.3+312720 B (32 ± 6 M Jup; L0+2-1; 120 ± 20 AU), GJ 3629 B (64+30-23 M Jup; M7.5 ± 0.5; 6.5 ± 0.5 AU), 1RXS J034231.8+121622 B (35 ± 8 M Jup; L0 ± 1; 19.8 ± 0.9 AU), and 2MASS J15594729+4403595 B (43 ± 9 M Jup; M8.0 ± 0.5; 190 ± 20 AU). Over 150 candidate planets were identified; we obtained follow-up imaging for 56% of these but all are consistent with background stars. Our null detection of planets enables strong statistical constraints on the occurrence rate of long-period giant planets around single M dwarfs. We infer an upper limit (at the 95% confidence level) of 10.3% and 16.0% for 1-13 M Jup planets between 10-100 AU for hot-start and cold-start (Fortney) evolutionary models, respectively. Fewer than 6.0% (9.9%) of M dwarfs harbor massive gas giants in the 5-13 M Jup range like those orbiting HR 8799 and β Pictoris between 10-100 AU for a hot-start (cold-start) formation scenario. The frequency of brown dwarf (13-75 M Jup) companions to single M dwarfs between 10-100 AU is 2.8+2.4-1.5%. Altogether we find that giant planets, especially massive ones, are rare

  10. PLANETS AROUND LOW-MASS STARS (PALMS). IV. THE OUTER ARCHITECTURE OF M DWARF PLANETARY SYSTEMS

    SciTech Connect

    Bowler, Brendan P.; Liu, Michael C.; Shkolnik, Evgenya L.; Tamura, Motohide

    2015-01-01

    We present results from a high-contrast adaptive optics imaging search for giant planets and brown dwarfs (≳1 M {sub Jup}) around 122 newly identified nearby (≲40 pc) young M dwarfs. Half of our targets are younger than 135 Myr and 90% are younger than the Hyades (620 Myr). After removing 44 close stellar binaries (implying a stellar companion fraction of >35.4% ± 4.3% within 100 AU), 27 of which are new or spatially resolved for the first time, our remaining sample of 78 single M dwarfs makes this the largest imaging search for planets around young low-mass stars (0.1-0.6 M {sub ☉}) to date. Our H- and K-band coronagraphic observations with Keck/NIRC2 and Subaru/HiCIAO achieve typical contrasts of 12-14 mag and 9-13 mag at 1'', respectively, which correspond to limiting planet masses of 0.5-10 M {sub Jup} at 5-33 AU for 85% of our sample. We discovered four young brown dwarf companions: 1RXS J235133.3+312720 B (32 ± 6 M {sub Jup}; L0{sub −1}{sup +2}; 120 ± 20 AU), GJ 3629 B (64{sub −23}{sup +30} M {sub Jup}; M7.5 ± 0.5; 6.5 ± 0.5 AU), 1RXS J034231.8+121622 B (35 ± 8 M {sub Jup}; L0 ± 1; 19.8 ± 0.9 AU), and 2MASS J15594729+4403595 B (43 ± 9 M {sub Jup}; M8.0 ± 0.5; 190 ± 20 AU). Over 150 candidate planets were identified; we obtained follow-up imaging for 56% of these but all are consistent with background stars. Our null detection of planets enables strong statistical constraints on the occurrence rate of long-period giant planets around single M dwarfs. We infer an upper limit (at the 95% confidence level) of 10.3% and 16.0% for 1-13 M {sub Jup} planets between 10-100 AU for hot-start and cold-start (Fortney) evolutionary models, respectively. Fewer than 6.0% (9.9%) of M dwarfs harbor massive gas giants in the 5-13 M {sub Jup} range like those orbiting HR 8799 and β Pictoris between 10-100 AU for a hot-start (cold-start) formation scenario. The frequency of brown dwarf (13-75 M {sub Jup}) companions to single

  11. Dwarf Nova V1040 Centauri and Variable Stars in its Vicinity

    NASA Astrophysics Data System (ADS)

    Rutkowski, A.; Pietrukowicz, P.; Olech, A.; Ak, T.; Złoczewski, K.; Poleski, R.; Tappert, C.; Eker, Z.

    2011-12-01

    We present the results of a photometric campaign of the dwarf nova V1040 Cen. The light curve shows two normal outbursts with recurrence time ≍40 days and amplitude ≍2.5 mag. Quiescence data show oscillations with periods in the range ≍0.1 days (2.4 h) to ≍0.5 days (12 h) of unknown origin. We measured the orbital period of V1040 Cen to be Porb=0.060458(80) days (1.451±0.002 h). Based on the MV-Porb relation we found the distance of V1040 Cen to be 137±31 pc. In this paper we also report the detection of eleven new variable stars in the field of the monitored dwarf nova.

  12. Constraints on First-Stars Models From Observations of Local Low-Mass Dwarf Galaxies and Galactic Metal-Poor Halo Stars

    NASA Astrophysics Data System (ADS)

    Yung, Long Yan; Venkatesan, A.

    2014-01-01

    The first metal-free stars in the universe had hard ionizing photon spectra and unique element yields from their supernovae, leaving signatures in the reionization of the intergalactic medium and in the metal enrichment of gas in the early universe. Here, we examine the metal abundances in a variety of systems in the local universe, from very metal-poor Galactic halo stars to ultra-faint dwarf spheroidal galaxies, and compare them with the latest theoretical models of massive stars with and without rotation. We confirm the similar abundance patterns found in the ultra-faint dwarfs and metal-poor halo stars by recent studies, and find new trends of interest in a variety of individual elements spanning metallicity values of [Fe/H] from about -2 to -5. We also compare our results with the abundances found in the very metal-deficient nearby dwarf irregular galaxy Leo P, which was recently discovered in the Arecibo ALFALFA survey. We comment on the similarities and differences between abundance trends in gas-rich dwarf galaxy systems like Leo P versus gas-poor ones like the ultra-faint dwarf spheroidals, and on the possibility of such systems hosting populations of the first stars. This work has been supported by NSF grant AST-1211005 and by Research Corporation through the Cottrell College Science Award.

  13. The Solar Neighborhood. XXVIII. The Multiplicity Fraction of Nearby Stars from 5 to 70 AU and the Brown Dwarf Desert around M Dwarfs

    NASA Astrophysics Data System (ADS)

    Dieterich, Sergio B.; Henry, Todd J.; Golimowski, David A.; Krist, John E.; Tanner, Angelle M.

    2012-08-01

    We report on our analysis of Hubble Space Telescope/NICMOS snapshot high-resolution images of 255 stars in 201 systems within ~10 pc of the Sun. Photometry was obtained through filters F110W, F180M, F207M, and F222M using NICMOS Camera 2. These filters were selected to permit clear identification of cool brown dwarfs through methane contrast imaging. With a plate scale of 76 mas pixel-1, NICMOS can easily resolve binaries with subarcsecond separations in the 19farcs5×19farcs5 field of view. We previously reported five companions to nearby M and L dwarfs from this search. No new companions were discovered during the second phase of data analysis presented here, confirming that stellar/substellar binaries are rare. We establish magnitude and separation limits for which companions can be ruled out for each star in the sample, and then perform a comprehensive sensitivity and completeness analysis for the subsample of 138 M dwarfs in 126 systems. We calculate a multiplicity fraction of 0.0+3.5 -0.0% for L companions to M dwarfs in the separation range of 5-70 AU, and 2.3+5.0 -0.7% for L and T companions to M dwarfs in the separation range of 10-70 AU. We also discuss trends in the color-magnitude diagrams using various color combinations and present astrometry for 19 multiple systems in our sample. Considering these results and results from several other studies, we argue that the so-called brown dwarf desert extends to binary systems with low-mass primaries and is largely independent of primary mass, mass ratio, and separations. While focusing on companion properties, we discuss how the qualitative agreement between observed companion mass functions and initial mass functions suggests that the paucity of brown dwarfs in either population may be due to a common cause and not due to binary formation mechanisms.

  14. WEAK GALACTIC HALO-DWARF SPHEROIDAL CONNECTION FROM RR LYRAE STARS

    SciTech Connect

    Fiorentino, Giuliana; Bono, Giuseppe; Monelli, Matteo; Gallart, Carme; Martínez-Vásquez, Clara E.; Tolstoy, Eline; Salaris, Maurizio; Bernard, Edouard J.

    2015-01-01

    We discuss the role that dwarf galaxies may have played in the formation of the Galactic halo (Halo) using RR Lyrae stars (RRL) as tracers of their ancient stellar component. The comparison is performed using two observables (periods, luminosity amplitudes) that are reddening and distance independent. Fundamental mode RRL in 6 dwarf spheroidals (dSphs) and 11 ultra faint dwarf galaxies (∼1300) show a Gaussian period distribution well peaked around a mean period of (Pab) = 0.610 ± 0.001 days (σ = 0.03). The Halo RRL (∼15,000) are characterized by a broader period distribution. The fundamental mode RRL in all the dSphs apart from Sagittarius are completely lacking in High Amplitude Short Period (HASP) variables, defined as those having P ≲ 0.48 days and A{sub V} ≥ 0.75 mag. Such variables are not uncommon in the Halo and among the globular clusters and massive dwarf irregulars. To further interpret this evidence, we considered 18 globulars covering a broad range in metallicity (–2.3 ≲ [Fe/H] ≲ –1.1) and hosting more than 35 RRL each. The metallicity turns out to be the main parameter, since only globulars more metal-rich than [Fe/H] ∼ –1.5 host RRL in the HASP region. This finding suggests that dSphs similar to the surviving ones do not appear to be the major building-blocks of the Halo. Leading physical arguments suggest an extreme upper limit of ∼50% to their contribution. On the other hand, massive dwarfs hosting an old population with a broad metallicity distribution (Large Magellanic Cloud, Sagittarius) may have played a primary role in the formation of the Halo.

  15. An extended star formation history in an ultra-compact dwarf

    NASA Astrophysics Data System (ADS)

    Norris, Mark A.; Escudero, Carlos G.; Faifer, Favio R.; Kannappan, Sheila J.; Forte, Juan Carlos; van den Bosch, Remco C. E.

    2015-08-01

    There has been significant controversy over the mechanisms responsible for forming compact stellar systems like ultra-compact dwarfs (UCDs), with suggestions that UCDs are simply the high-mass extension of the globular cluster population, or alternatively, the liberated nuclei of galaxies tidally stripped by larger companions. Definitive examples of UCDs formed by either route have been difficult to find, with only a handful of persuasive examples of stripped-nucleus-type UCDs being known. In this paper, we present very deep Gemini/GMOS spectroscopic observations of the suspected stripped-nucleus UCD NGC 4546-UCD1 taken in good seeing conditions (<0.7 arcsec). With these data we examine the spatially resolved kinematics and star formation history of this unusual object. We find no evidence of a rise in the central velocity dispersion of the UCD, suggesting that this UCD lacks a massive central black hole like those found in some other compact stellar systems, a conclusion confirmed by detailed dynamical modelling. Finally, we are able to use our extremely high signal-to-noise spectrum to detect a temporally extended star formation history for this UCD. We find that the UCD was forming stars since the earliest epochs until at least 1-2 Gyr ago. Taken together these observations confirm that NGC 4546-UCD1 is the remnant nucleus of a nucleated dwarf galaxy that was tidally destroyed by NGC 4546 within the last 1-2 Gyr.

  16. STAR-TO-STAR IRON ABUNDANCE VARIATIONS IN RED GIANT BRANCH STARS IN THE GALACTIC GLOBULAR CLUSTER NGC 3201

    SciTech Connect

    Simmerer, Jennifer; Ivans, Inese I.; Filler, Dan; Francois, Patrick; Charbonnel, Corinne; Monier, Richard; James, Gaeel E-mail: iii@physics.utah.edu E-mail: patrick.francois@obspm.fr E-mail: richard.monier@unice.fr

    2013-02-10

    We present the metallicity as traced by the abundance of iron in the retrograde globular cluster NGC 3201, measured from high-resolution, high signal-to-noise spectra of 24 red giant branch stars. A spectroscopic analysis reveals a spread in [Fe/H] in the cluster stars at least as large as 0.4 dex. Star-to-star metallicity variations are supported both through photometry and through a detailed examination of spectra. We find no correlation between iron abundance and distance from the cluster core, as might be inferred from recent photometric studies. NGC 3201 is the lowest mass halo cluster to date to contain stars with significantly different [Fe/H] values.

  17. Kinematics of Tycho-2 red giant clump stars

    NASA Astrophysics Data System (ADS)

    Bobylev, V. V.; Stepanishchev, A. S.; Bajkova, A. T.; Gontcharov, G. A.

    2009-12-01

    Based on the Ogorodnikov-Milne model, we analyze the proper motions of 95,633 red giant clump (RGC) stars from the Tycho-2 Catalogue. The following Oort constants have been found: A = 15.9 ± 0.2 km s-1 kpc-1 and B = -12.0±0.2 km s-1 kpc-1. Using 3632 RGC stars with known proper motions, radial velocities, and photometric distances, we show that, apart from the star centroid velocity components relative to the Sun, only the model parameters that describe the stellar motions in the XY plane differ significantly from zero. We have studied the contraction (a negative K effect) of the system of RGC stars as a function of their heliocentric distance and elevation above the Galactic plane. For a sample of distant (500-1000 pc) RGC stars located near the Galactic plane (|z| < 200 pc) with an average distance of d = 0.7 kpc, the contraction velocity is shown to be Kd = -3.5 ±0.9 km s-1; a noticeable vertex deviation, lxy = 9.1° ± 0.5°, is also observed for them. For stars located well above the Galactic plane (|z| ≥200 pc), these effects are less pronounced, Kd = -1.7 ± 0.5 km s-1 and lxy = 4.9° ± 0.6°. Using RGC stars, we have found a rotation around the Galactic X axis directed toward the Galactic center with an angular velocity of -2.5 ± 0.3 km s-1 kpc-1, which we associate with the warp of the Galactic stellar-gaseous disk.

  18. Star-forming dwarf galaxies: the correlation between far-infrared and radio fluxes

    NASA Astrophysics Data System (ADS)

    Schleicher, Dominik R. G.; Beck, Rainer

    2016-09-01

    The correlation between far-infrared and radio fluxes connects star formation and magnetic fields in galaxies and has been confirmed over a wide range in luminosities in the far-infrared to radio domain, both in the local Universe and even at redshifts of z ~ 2. Recent investigations have indicated that it may even hold in the regime of local dwarf galaxies, and we therefore explore here the expected behavior in the regime of star formation surface densities below 0.1 M⊙ kpc-2 yr-1. We derive two conditions that can be particularly relevant for inducing a change in the expected correlation: a critical star formation surface density to maintain the correlation between star formation rate and the magnetic field, and a critical star formation surface density below which cosmic-ray diffusion losses dominate their injection through supernova explosions. For rotation periods shorter than 1.5 × 107(H/ kpc)2 yr, with H the scale height of the disk, the first correlation will break down before diffusion losses are relevant because higher star formation rates are required to maintain the correlation between star formation rate and magnetic field strength. For high star formation surface densities ΣSFR, we derive a characteristic scaling of the nonthermal radio to the far-infrared and infrared emission with ΣSFR1/3 , corresponding to a scaling of the nonthermal radio luminosity Ls with the infrared luminosity Lth as Lth4/3 . The latter is expected to change when the above processes are no longer steadily maintained. In the regime of long rotation periods, we expect a transition toward a steeper scaling with ΣSFR2/3, implying Ls ∝ Lth5/3 , while the regime of fast rotation is expected to show a considerably enhanced scatter because a well-defined relation between star formation and magnetic field strength is not maintained. The scaling relations above explain the increasing thermal fraction of the radio emission observed within local dwarfs and can be tested with

  19. GIANO Y-band spectroscopy of dwarf stars: Phosphorus, sulphur, and strontium abundances

    NASA Astrophysics Data System (ADS)

    Caffau, E.; Andrievsky, S.; Korotin, S.; Origlia, L.; Oliva, E.; Sanna, N.; Ludwig, H.-G.; Bonifacio, P.

    2016-01-01

    Context. In recent years a number of poorly studied chemical elements, such as phosphorus, sulphur, and strontium, have received special attention as important tracers of the Galactic chemical evolution. Aims: By exploiting the capabilities of the infrared echelle spectrograph GIANO mounted at the Telescopio Nazionale Galileo, we acquired high resolution spectra of four Galactic dwarf stars spanning the metallicity range between about one-third and twice the solar value. We performed a detailed feasibility study about the effectiveness of the P, S, and Sr line diagnostics in the Y band between 1.03 and 1.10 μm. Methods: Accurate chemical abundances have been derived using one-dimensional model atmospheres computed in local thermodynamic equilibrium (LTE). We computed the line formation assuming LTE for P, while we performed non-LTE analysis to derive S and Sr abundances. Results: We were able to derive phosphorus abundance for three stars and an upper limit for one star, while we obtained the abundance of sulphur and strontium for all of the stars. We find [P/Fe] and [S/Fe] abundance ratios consistent with solar-scaled or slightly depleted values, while the [Sr/Fe] abundance ratios are more scattered (by ±0.2 dex) around the solar-scaled value. This is fully consistent with previous studies using both optical and infrared spectroscopy. Conclusions: We verified that high-resolution, Y-band spectroscopy as provided by GIANO is a powerful tool to study the chemical evolution of P, S, and Sr in dwarf stars. Based on observations obtained with GIANO.

  20. Halpha Emission Line Stars in M31, M33 and Seven Local Group Dwarfs

    NASA Astrophysics Data System (ADS)

    McNeill, Reagin T.; Massey, P.; Olsen, K. A.; Hodge, P. W.; Jacoby, G. H.; Blaha, C.; Smith, R. C.; Holmes, S. B.

    2006-12-01

    While there are many ideas as to how differing galactic environments affect the formation and evolution of massive stars, the numbers of stars with known physical properties outside the Milky Way are too scarce to provide much insight. For instance, we have a very poor idea as to the actual number of Luminous Blue Variables (LBVs) in nearby galaxies, since these have mostly been found on the basis of strong photometric variability over a span of a few decades. But, we know that the Galactic LBVs P Cygni and Eta Car had their last major photometric upsets centuries ago. Were these stars located in a nearby galaxy, we might well be unaware of them. In order to help rectify this situation, we have used the recent UBVRI catalogs of M31 and M33 (Massey et al. 2006) and seven dwarf galaxies in the Local Group (Massey et al. 2007) in conjunction with data from narrowband filters centered on Halpha, [SII] and [OIII] to select Halpha emission sources with similar characteristics to the known LBVs. This search uncovered over 300 potential Halpha emission sources in M31 and M33, and 41 potential Halpha emission sources in the dwarf galaxies. Many of the most promising objects in M31 and M33 were observed spectroscopically at WIYN in September, revealing a wealth of new LBVs and Wolf-Rayet stars. In our poster we will outline our selection method and show some of the newly found emission-lined stars. Funding provided by the NSF through grant number AST-0453611.

  1. Narrow-band, slowly varying decimetric radiation from the dwarf M flare star YZ Canis Minoris

    NASA Technical Reports Server (NTRS)

    Lang, K. R.; Willson, R. F.

    1986-01-01

    Observations of slowly varying radiation from the dwarf M star YZ Canis Minoris with a maximum flux density of 20 mJy and narrow-band frequency structure at frequencies near 1465 MHz are presented. Possible explanations for this radiation are examined. Thermal gyroresonant radiation would require impossibly large coronal loops and magnetic field strengths. The narrow-band structure cannot be explained by continuum emission processes such as thermal bremsstrahlung, thermal gyroresonant radiation, or nonthermal gyrosynchrotron radiation. Coherent burst mechanisms seem to be required.

  2. Topics in solid-state astrophysics: Magnetized neutron star crusts and multicomponent crusts/white dwarfs

    NASA Astrophysics Data System (ADS)

    Engstrom, Tyler A.

    Two research endeavors are described in this dissertation; both undertake problems in solid-state astrophysics, which is a branch of solid-state physics concerning the extreme conditions found within white dwarfs and the solid crusts of neutron stars. As much of our knowledge about these compact objects comes from observation of astrophysical phenomena, Chapter 1 is devoted to the phenomena, and how they can be exploited as material property probes. Several of the most interesting phenomena involve the enormous magnetic fields (B ≥ 1012 gauss) harbored by many neutron stars, and the interaction between these fields and the charged particles within the solid crust. Accordingly, Chapter 2 reviews some theory of strongly-magnetized electrons, which both sets the stage for Chapter 3, and (hopefully) serves as a useful reference for future research. Let it now be made clear that this dissertation focuses exclusively on the "outer crusts," of neutron stars, where no free neutrons are present (rho < 4x1011 g/cc), and the similarly-composed interiors of white dwarfs, which have central densities ˜ 107 g/cc. For the most part we specialize to even lower densities. In Chapter 3, static and dynamic properties of low density (rho ≥ 106 g/cc) outer envelopes of neutron stars are calculated within the nonlinear magnetic Thomas-Fermi model, assuming degenerate electrons. A novel domain decomposition enables proper description of lattice symmetry and may be seen as a prototype for the general class of problems involving nonlinear charge screening of periodic, quasi-low-dimensionality structures, e.g. liquid crystals. We describe a scalable implementation of the method using Hypre. Over the density range considered, the effective shear modulus appears to be a factor of ≈ 20 larger than in the linearlyscreened Coulomb crystal model, which could have implications for observables related to astroseismology as well as low temperature phonon-mediated thermal conductivity. Other

  3. Ultraviolet and Visible Analysis of Star-Forming Regions in Several Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Zernow, Lea; Hunter, D. A.

    2007-12-01

    As some of the smallest and most numerous galaxies in the universe, dwarf irregular (dIm) galaxies give many opportunities for developing an understanding of the manner in which the universe operates. In particular, these galaxies offer challenges to standard models of star formation. The concentration of gas is sufficiently small that the standard models cannot account for the presence of stars in the galaxies. In order to examine this paradox, we used ultraviolet images obtained with the GALEX satellite and optical ground-based imaging to examine the properties of star-forming regions in three dIm galaxies: DDO 50, IC 1613, and WLM. We identified candidate young star-forming regions on near ultraviolet (NUV, 2270 Angstroms) images. We then measured the brightness of the regions in the NUV, far ultraviolet (FUV, 1520 Angstroms), and UBV images. Colors were compared to cluster evolutionary models to estimate ages of the regions, which enabled an estimate of the visual magnitude at a fiducial age of ten million years. We divided the galaxies into three broad annuli in order to compare properties of the star-forming regions as a function of radius. We discuss the properties of star-forming regions in the three galaxies in the context of current star formation models that predict star formation on the basis of large-scale gravitational instability and local-scale gas compression. LZ extends gratitude for funding from the National Science Foundation's Research Experiences for Undergraduates (REU) program at Northern Arizona University through grant AST-0453611. DAH gratefully acknowledges funding for this research from the NASA GALEX program through grant NNX07AJ36G.

  4. White Dwarf Critical Tests for Modified Gravity.

    PubMed

    Jain, Rajeev Kumar; Kouvaris, Chris; Nielsen, Niklas Grønlund

    2016-04-15

    Scalar-tensor theories of gravity can lead to modifications of the gravitational force inside astrophysical objects. We exhibit that compact stars such as white dwarfs provide a unique setup to test beyond Horndeski theories of G^{3} type. We obtain stringent and independent constraints on the parameter ϒ characterizing the deviations from Newtonian gravity using the mass-radius relation, the Chandrasekhar mass limit, and the maximal rotational frequency of white dwarfs. We find that white dwarfs impose stronger constraints on ϒ than red and brown dwarfs. PMID:27127952

  5. White Dwarf Critical Tests for Modified Gravity

    NASA Astrophysics Data System (ADS)

    Jain, Rajeev Kumar; Kouvaris, Chris; Nielsen, Niklas Grønlund

    2016-04-01

    Scalar-tensor theories of gravity can lead to modifications of the gravitational force inside astrophysical objects. We exhibit that compact stars such as white dwarfs provide a unique setup to test beyond Horndeski theories of G3 type. We obtain stringent and independent constraints on the parameter ϒ characterizing the deviations from Newtonian gravity using the mass-radius relation, the Chandrasekhar mass limit, and the maximal rotational frequency of white dwarfs. We find that white dwarfs impose stronger constraints on ϒ than red and brown dwarfs.

  6. White Dwarf Critical Tests for Modified Gravity.

    PubMed

    Jain, Rajeev Kumar; Kouvaris, Chris; Nielsen, Niklas Grønlund

    2016-04-15

    Scalar-tensor theories of gravity can lead to modifications of the gravitational force inside astrophysical objects. We exhibit that compact stars such as white dwarfs provide a unique setup to test beyond Horndeski theories of G^{3} type. We obtain stringent and independent constraints on the parameter ϒ characterizing the deviations from Newtonian gravity using the mass-radius relation, the Chandrasekhar mass limit, and the maximal rotational frequency of white dwarfs. We find that white dwarfs impose stronger constraints on ϒ than red and brown dwarfs.

  7. THE LEO IV DWARF SPHEROIDAL GALAXY: COLOR-MAGNITUDE DIAGRAM AND PULSATING STARS

    SciTech Connect

    Moretti, Maria Ida; Dall'Ora, Massimo; Ripepi, Vincenzo E-mail: dallora@na.astro.it

    2009-07-10

    We present the first V, B - V color-magnitude diagram of the Leo IV dwarf spheroidal galaxy, a faint Milky Way satellite recently discovered by the Sloan Digital Sky Survey. We have obtained B, V time-series photometry reaching about half a magnitude below the Leo IV turnoff, which we detect at V = 24.7 mag, and have performed the first study of the variable star population. We have identified three RR Lyrae stars (all fundamental-mode pulsators, RRab) and one SX Phoenicis variable in the galaxy. In the period-amplitude diagram the Leo IV RR Lyrae stars are located close to the loci of Oosterhoff type I systems and the evolved fundamental-mode RR Lyrae stars in the Galactic globular cluster M3. However, their mean pulsation period, (Pab) = 0.655 days, would suggest an Oosterhoff type II classification for this galaxy. The RR Lyrae stars trace very well the galaxy's horizontal branch, setting its average magnitude at (V {sub RR}) = 21.48 {+-} 0.03 mag (standard deviation of the mean). This leads to a distance modulus of {mu}{sub 0} = 20.94 {+-} 0.07 mag, corresponding to a distance of 154 {+-} 5 kpc, by adopting for the Leo IV dSph a reddening E(B - V) = 0.04 {+-} 0.01 mag and a metallicity of [Fe/H] = -2.31 {+-} 0.10.

  8. THE STELLAR POPULATION AND STAR FORMATION PROPERTIES OF BLUE COMPACT DWARF GALAXIES

    SciTech Connect

    Zhao Yinghe; Gao Yu; Gu Qiusheng E-mail: yugao@pmo.ac.cn

    2011-02-15

    We study stellar populations, star formation histories (SFHs), and star formation properties for a sample of blue compact dwarf galaxies (BCDs) selected by cross-correlating the Gil de Paz et al. sample with the Sloan Digital Sky Survey Data Release 6. The sample includes 31 BCDs, which span a large range of galactic parameters. Using a stellar population synthesis method, we derive stellar populations and reconstruct SFHs for these BCDs. Our studies confirm that BCDs are not young systems experiencing their first star formation, but old systems undergoing a starburst activity. The stellar mass-weighted ages can be up to 10 Gyr, while the luminosity-weighted ages might be up to approximately three orders of magnitude younger ({approx}10 Myr) for most galaxies. Based on multiwavelength data, we also study the integrated star formation properties. The star formation rate (SFR) for our sample galaxies spans nearly three orders of magnitude, from a few 10{sup -3} to {approx}1 M{sub sun} yr{sup -1}, with a median value of {approx}0.1 M{sub sun} yr{sup -1}. We find that about 90% of BCDs in our sample have their birthrate parameter (the ratio of the current SFR to the averaged past SFR) b>2-3. We further discuss correlations of the current SFR with the integrated galactic stellar mass and explore the connection between SFR and metallicity.

  9. Evidence for Neutron Star Formation from Accretion Induced Collapse of a White Dwarf

    NASA Technical Reports Server (NTRS)

    Paradijis, J. Van; VanDenHeuvel, E. P. J.; Kouveliotou, C.; Fishman, G. J.; Finger, M. H.; Lewin, W. H. G.

    1997-01-01

    The orbital parameters of the recently discovered transient burster/pulsar GRO J1744-28 indicate that this system is a low-mass X-ray binary in an advanced stage of its mass transfer, with several tenths of a solar mass already transferred from the donor to the compact star. All neutron stars known to have accreted such an amount have very weak magnetic fields, and this has led to the idea that the magnetic fields of neutron stars decay as a result of accretion. The observation of a strongly magnetized neutron star in GRO J1744-28 then suggests that this neutron star was formed recently as a result of the collapse of a white dwarf during an earlier stage of the current phase of mass transfer. It is shown that this model can consistently explain the observed characteristics of GRO J1744-28. Attractive progenitors for such an evolution are the luminous supersoft X-ray sources detected with ROSAT.

  10. The relation between atomic gas and star formation rate densities in faint dwarf irregular galaxies

    NASA Astrophysics Data System (ADS)

    Roychowdhury, Sambit; Chengalur, Jayaram N.; Kaisin, Serafim S.; Karachentsev, Igor D.

    2014-12-01

    We use data for faint (MB > -14.5) dwarf irregular galaxies drawn from the Faint Irregular Galaxy GMRT Survey to study the correlation between the surface densities of atomic gas (Σgas,atomic) and star formation rate (ΣSFR) in the galaxies. The estimated gas-phase metallicity of our sample galaxies is Z ˜ 0.1 Z⊙. Understanding star formation in such molecule-poor gas is of particular importance since it is likely to be of direct relevance to simulations of early galaxy formation. For about 20 per cent (9/43) of our sample galaxies, we find that the H I distribution is significantly disturbed, with little correspondence between the optical and H I distributions. We exclude these galaxies from the comparison. We also exclude galaxies with very low star formation rates, for which stochastic effects make it difficult to estimate the true star formation rates. For the remaining galaxies, we compute the Σgas,atomic and ΣSFR averaged over the entire star-forming disc of the galaxy. For these galaxies, we find a nearly linear relation between the star formation rate and the atomic gas density, namely {log Σ _{SFR} = 0.91^{+0.23}_{-0.25} log Σ _{gas,atomic} - 3.84^{+0.15}_{-0.19}}. The corresponding gas consumption time-scale is ˜10 Gyr, i.e. significantly smaller than the ˜100 Gyr estimated for the outer regions of spiral galaxies. We also estimate the gas consumption time-scale computed using the global gas content and the global star formation rate for all galaxies with a reliable measurement of the star formation rate, regardless of whether the H I distribution is disturbed or not. The mean gas consumption time-scale computed using this entire gas reservoir is ˜18 Gyr, i.e. still significantly smaller than that estimated for the outer parts of spirals. The gas consumption time-scale for dwarfs is intermediate between the values of ˜100 and ˜2 Gyr estimated for the outer molecule-poor and inner molecule-rich regions of spiral discs.

  11. Exploring masses and CNO surface abundances of red giant stars

    NASA Astrophysics Data System (ADS)

    Halabi, Ghina M.; Eid, Mounib El

    2015-08-01

    A grid of evolutionary sequences of stars in the mass range 1.2-7M⊙, with solar-like initial composition is presented. We focus on this mass range in order to estimate the masses and calculate the CNO surface abundances of a sample of observed red giants. The stellar models are calculated from the zero-age main sequence till the early asymptotic giant branch (AGB) phase. Stars of M ≤ 2.2M⊙ are evolved through the core helium flash. In this work, an approach is adopted that improves the mass determination of an observed sample of 21 RGB and early AGB stars. This approach is based on comparing the observationally derived effective temperatures and absolute magnitudes with the calculated values based on our evolutionary tracks in the Hertzsprung-Russell diagram. A more reliable determination of the stellar masses is achieved by using evolutionary tracks extended to the range of observation. In addition, the predicted CNO surface abundances are compared to the observationally inferred values in order to show how far standard evolutionary calculation can be used to interpret available observations and to illustrate the role of convective mixing. We find that extra mixing beyond the convective boundary determined by the Schwarzschild criterion is needed to explain the observational oxygen isotopic ratios in low-mass stars. The effect of recent determinations of proton capture reactions and their uncertainties on the 16O/17O and 14N/15N ratios is also shown. It is found that the 14N( p, γ)15O reaction is important for predicting the 14N/15N ratio in red giants.

  12. The SONYC survey: Towards a complete census of brown dwarfs in star forming regions

    NASA Astrophysics Data System (ADS)

    Muzic, K.; Scholz, A.; Jayawardhana, R.; Geers, V. C.; Dawson, P.; Ray, T. P.; Tamura, M.

    2014-10-01

    Deep surveys of star forming regions are the backbone of observational studies on the origin of stars and planets: On one side, they provide large and homogeneous object samples required to study disks, accretion, and multiplicity. On the other side, such surveys determine the shape and the low-mass limit of the Initial Mass Function (IMF), which are fundamental constraints on star formation theory. SONYC, short for ``Substellar Objects in Nearby Young Clusters", is an ongoing project to provide a census of the substellar population in nearby star forming regions. We have conducted deep optical and near-infrared photometry, combined with proper motions, and followed by extensive spectroscopic follow-up campaigns with Subaru and VLT, in which we have obtained more than 700 spectra of candidate objects in NGC1333, ρ Ophiuchi, Chamaeleon-I, Upper Sco, and Lupus-3. We have identified and characterized more than 60 new substellar objects, among them a handful of objects with masses close to, or below the Deuterium burning limit. Thanks to the SONYC survey and the efforts of other groups, the substellar IMF is now well characterized down to ˜ 5 - 10 M_{J}, and we find that the ratio of the number of stars with respect to brown dwarfs lies between 2 ad 6. Another important piece of information for the star formation theories is that, down to ˜ 5 M_J, the free-floating objects with planetary masses are 20-50 times less numerous than stars, so that their total contribution to the mass budget of the clusters can be neglected. In this contribution we will present the status of the SONYC survey, discuss its main results, and focus on the latest findings in NGC1333, Lupus-3 and Upper-Sco.

  13. An M Dwarf Companion to an F-type Star in a Young Main-sequence Binary

    NASA Astrophysics Data System (ADS)

    Eigmüller, Ph.; Eislöffel, J.; Csizmadia, Sz.; Lehmann, H.; Erikson, A.; Fridlund, M.; Hartmann, M.; Hatzes, A.; Pasternacki, Th.; Rauer, H.; Tkachenko, A.; Voss, H.

    2016-03-01

    Only a few well characterized very low-mass M dwarfs are known today. Our understanding of M dwarfs is vital as these are the most common stars in our solar neighborhood. We aim to characterize the properties of a rare F+dM stellar system for a better understanding of the low-mass end of the Hertzsprung-Russel diagram. We used photometric light curves and radial velocity follow-up measurements to study the binary. Spectroscopic analysis was used in combination with isochrone fitting to characterize the primary star. The primary star is an early F-type main-sequence star with a mass of (1.493 ± 0.073) M⊙ and a radius of (1.474 ± 0.040) R⊙. The companion is an M dwarf with a mass of (0.188 ± 0.014) M⊙ and a radius of (0.234 ± 0.009) R⊙. The orbital period is (1.35121 ± 0.00001) days. The secondary star is among the lowest-mass M dwarfs known to date. The binary has not reached a 1:1 spin-orbit synchronization. This indicates a young main-sequence binary with an age below ˜250 Myr. The mass-radius relation of both components are in agreement with this finding.

  14. ULTRA-FAINT DWARF GALAXIES AS A TEST OF EARLY ENRICHMENT AND METALLICITY-DEPENDENT STAR FORMATION

    SciTech Connect

    Tassis, Konstantinos; Gnedin, Nickolay Y.; Kravtsov, Andrey V.

    2012-01-20

    The close relation of star formation with molecular gas indicated by observations and assumed in recent models implies that the efficiency with which galaxies convert their gas into stars depends on gas metallicity. This is because abundance of molecular hydrogen is sensitive to abundance of dust, which catalyzes formation of H{sub 2} and helps to shield it from dissociating radiation. In this study, we point out that in the absence of significant pre-enrichment by Population III stars forming out of zero metallicity gas, such H{sub 2}-based star formation is expected to leave an imprint in the form of bi-modality in the metallicity distribution among dwarf galaxies and in the metallicity distribution of stars within individual galaxies. The bi-modality arises because when gas metallicity (and dust abundance) is low, formation of molecular gas is inefficient, the gas consumption timescale is long, and star formation and metal enrichment proceed slowly. When metallicity reaches a critical threshold value star formation and enrichment accelerate, which leads to rapid increase in both stellar mass and metallicity of galaxies. We demonstrate this process both using a simple analytical model and full cosmological simulations. In contrast, the observed metallicity distributions of dwarf galaxies or stars within them are not bi-modal. We argue that this discrepancy points to substantial early stochastic pre-enrichment by Population III stars to levels Z {approx} 10{sup -2} Z{sub Sun} in dense, star-forming regions of early galaxies.

  15. The Properties of the 500 K Dwarf UGPS J072227.51-054031.2 and a Study of the Far-red Flux of Cold Brown Dwarfs

    NASA Astrophysics Data System (ADS)

    Leggett, S. K.; Saumon, D.; Marley, M. S.; Lodders, K.; Canty, J.; Lucas, P.; Smart, R. L.; Tinney, C. G.; Homeier, D.; Allard, F.; Burningham, Ben; Day-Jones, A.; Fegley, B.; Ishii, Miki; Jones, H. R. A.; Marocco, F.; Pinfield, D. J.; Tamura, M.

    2012-04-01

    We present i and z photometry for 25 T dwarfs and 1 L dwarf. Combined with published photometry, the data show that the i - z, z - Y, and z - J colors of T dwarfs are very red, and continue to increase through to the late-type T dwarfs, with a hint of a saturation for the latest types with T eff ≈ 600 K. We present new 0.7-1.0 μm and 2.8-4.2 μm spectra for the very late type T dwarf UGPS J072227.51-054031.2, as well as improved astrometry for this dwarf. Examination of the spectral energy distribution using new and published data, with Saumon & Marley models, shows that the dwarf has T eff = 505 ± 10 K, a mass of 3-11 M Jupiter, and an age between 60 Myr and 1 Gyr. This young age is consistent with the thin disk kinematics of the dwarf. The mass range overlaps with that usually considered to be planetary, despite this being an unbound object discovered in the field near the Sun. This apparently young rapid rotator is also undergoing vigorous atmospheric mixing, as determined by the IRAC and WISE 4.5 μm photometry and the Saumon & Marley models. The optical spectrum for this 500 K object shows clearly detected lines of the neutral alkalis Cs and Rb, which are emitted from deep atmospheric layers with temperatures of 900-1200 K. Based on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the Science and Technology Facilities Council (United Kingdom), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), Ministério da Ciência e Tecnologia (Brazil), and Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina); also based on data collected at Subaru Telescope, which is operated by the National Astronomical Observatory of Japan; and also based on observations made at the UK Infrared Telescope

  16. The star formation histories of local group dwarf galaxies. II. Searching for signatures of reionization

    SciTech Connect

    Weisz, Daniel R.; Dolphin, Andrew E.; Skillman, Evan D.; Holtzman, Jon; Gilbert, Karoline M.; Dalcanton, Julianne J.; Williams, Benjamin F.

    2014-07-10

    We search for signatures of reionization in the star formation histories (SFHs) of 38 Local Group dwarf galaxies (10{sup 4} < M{sub *} < 10{sup 9} M{sub ☉}). The SFHs are derived from color-magnitude diagrams using archival Hubble Space Telescope/Wide Field Planetary Camera 2 imaging. Only five quenched galaxies (And V, And VI, And XIII, Leo IV, and Hercules) are consistent with forming the bulk of their stars before reionization, when full uncertainties are considered. Observations of 13 of the predicted 'true fossils' identified by Bovill and Ricotti show that only two (Hercules and Leo IV) indicate star formation quenched by reionization. However, both are within the virial radius of the Milky Way and evidence of tidal disturbance complicates this interpretation. We argue that the late-time gas capture scenario posited by Ricotti for the low mass, gas-rich, and star-forming fossil candidate Leo T is observationally indistinguishable from simple gas retention. Given the ambiguity between environmental effects and reionization, the best reionization fossil candidates are quenched low mass field galaxies (e.g., KKR 25).

  17. Gyrochronology of Low-mass Stars - Age-Rotation-Activity Relations for Young M Dwarfs

    NASA Astrophysics Data System (ADS)

    Kidder, Benjamin; Shkolnik, E.; Skiff, B.

    2014-01-01

    New rotation periods for 34 young <300 Myr), early-M dwarfs within 25 parsecs were measured using photometric data collected with telescopes at Lowell Observatory during 2012 and 2013. An additional 25 rotation periods for members of the same sample were found in the literature. Ages were derived from Hα and X-ray emission, lithium absorption, surface gravity, and kinematic association of members of known young moving groups (YMGs). We compared rotation periods with the estimated ages as well as indicators of magnetic activity, with the intention of strengthening age-rotation-activity relations and assessing the possible use of gyrochronology in young, low-mass stars. We compared ages and rotation periods of our target stars to cluster members spanning 1-600 Myr. Rotation periods at every age exhibit a large scatter, with values typically ranging from 0.2 to 15 days. This suggests that gyrochronology for individual field stars will not be possible without a better understanding of the underlying mechanisms that govern angular momentum evolution. Yet, on average, the data still support the predicted trends for spin-up during contraction and spin-down on the main sequence, with the turnover occurring at around 150 Myr for early Ms. This suggests that rotation period distributions can be helpful in evaluating the ages of coeval groups of stars. Many thanks to the National Science Foundation for their support through the Research Experience for Undergraduates Grant AST- 1004107.

  18. Macho project photometry of RR Lyrae stars in the Sagittarius dwarf galaxy

    SciTech Connect

    Alcock, C. |; Allsman, R.A. |; Alves, D.R. |; Axelrod, T.S. |; Becker, A.C. |; Bennett, D.P.; Cook, K.H. |; Freeman, K.C.; Griest, K. |; Guern, J.A.; Lehner, M.J.; Marshall, S.L.; Minniti, D.; Peterson, B.A.; Pratt, M.R.

    1997-01-01

    We report the discovery of 30 type a, b RR Lyrae (RRab) stars that are likely members of the Sagittarius dwarf galaxy (Sgr). Accurate positions, periods, amplitudes, and magnitudes are presented. Their distances are determined with respect to RRab stars in the Galactic bulge found also in the MACHO 1993 data. For R{sub {circle_dot}}=8kpc, the mean distance to these stars is D=22{plus_minus}1kpc, smaller than previous determinations for this galaxy. This indicates that Sgr has an elongated main body extending for more than 10 kpc, which is inclined along the line of sight, with its northern part (in Galactic coordinates) closer to us. The size and shape of Sgr give clues about the past history of this galaxy. If the shape of Sgr follows the direction of its orbit, the observed spatial orientation suggests that Sgr is moving away from the Galactic plane. Also, Sgr stars may be the sources of some of the microlensing events seen toward the bulge. {copyright} {ital 1997} {ital The American Astronomical Society}

  19. NEW BROWN DWARF COMPANIONS TO YOUNG STARS IN SCORPIUS-CENTAURUS

    SciTech Connect

    Janson, Markus; Jayawardhana, Ray; Bonavita, Mariangela; Girard, Julien H.; Lafreniere, David; Gizis, John; Brandeker, Alexis

    2012-10-10

    We present the discoveries of three faint companions to young stars in the Scorpius-Centaurus region, imaged with the NICI instrument on Gemini South. We have confirmed all three companions through common proper motion tests. Follow-up spectroscopy has confirmed two of them, HIP 65423 B and HIP 65517 B, to be brown dwarfs, while the third, HIP 72099 B, is more likely a very low mass star just above the hydrogen burning limit. The detection of wide companions in the mass range of {approx}40-100 M{sub jup} complements previous work in the same region, reporting detections of similarly wide companions with lower masses, in the range of {approx}10-30 M{sub jup}. Such low masses near the deuterium burning limit have raised the question of whether those objects formed like planets or stars. The existence of intermediate objects as reported here could represent a bridge between lower-mass companions and stellar companions, but in any case demonstrate that mass alone may not provide a clear-cut distinction for the formation of low-mass companions to stars.

  20. STAR FORMATION IN ULTRA-FAINT DWARFS: CONTINUOUS OR SINGLE-AGE BURSTS?

    SciTech Connect

    Webster, David; Bland-Hawthorn, Joss; Sutherland, Ralph

    2015-01-30

    We model the chemical evolution of six ultra-faint dwarfs (UFDs): Bootes I, Canes Venatici II, Coma Berenices, Hercules, Leo IV, and Ursa Major I based on their recently determined star formation histories. We show that two single-age bursts cannot explain the observed [α/Fe] versus [Fe/H] distribution in these galaxies and that some self-enrichment is required within the first burst. An alternative scenario is modeled, in which star formation is continuous except for short interruptions when one or more supernovae temporarily blow the dense gas out from the center of the system. This model allows for self-enrichment and can reproduce the chemical abundances of the UFDs in which the second burst is only a trace population. We conclude that the most likely star formation history is one or two extended periods of star formation, with the first burst lasting for at least 100 Myr. As found in earlier work, the observed properties of UFDs can be explained by formation at a low mass (M{sub vir}∼10{sup 7} M{sub ⊙}), rather than being stripped remnants of much larger systems.

  1. Star Formation in Ultrafaint Dwarfs: Continuous or Single-Age Bursts?

    NASA Astrophysics Data System (ADS)

    Webster, David; Bland-Hawthorn, Joss; Sutherland, Ralph

    2015-02-01

    We model the chemical evolution of six ultra-faint dwarfs (UFDs): Bootes I, Canes Venatici II, Coma Berenices, Hercules, Leo IV, and Ursa Major I based on their recently determined star formation histories. We show that two single-age bursts cannot explain the observed [α/Fe] versus [Fe/H] distribution in these galaxies and that some self-enrichment is required within the first burst. An alternative scenario is modeled, in which star formation is continuous except for short interruptions when one or more supernovae temporarily blow the dense gas out from the center of the system. This model allows for self-enrichment and can reproduce the chemical abundances of the UFDs in which the second burst is only a trace population. We conclude that the most likely star formation history is one or two extended periods of star formation, with the first burst lasting for at least 100 Myr. As found in earlier work, the observed properties of UFDs can be explained by formation at a low mass ({{M}vir}∼ {{10}7} M⊙), rather than being stripped remnants of much larger systems.

  2. New Brown Dwarf Companions to Young Stars in Scorpius-Centaurus

    NASA Astrophysics Data System (ADS)

    Janson, Markus; Jayawardhana, Ray; Girard, Julien H.; Lafrenière, David; Bonavita, Mariangela; Gizis, John; Brandeker, Alexis

    2012-10-01

    We present the discoveries of three faint companions to young stars in the Scorpius-Centaurus region, imaged with the NICI instrument on Gemini South. We have confirmed all three companions through common proper motion tests. Follow-up spectroscopy has confirmed two of them, HIP 65423 B and HIP 65517 B, to be brown dwarfs, while the third, HIP 72099 B, is more likely a very low mass star just above the hydrogen burning limit. The detection of wide companions in the mass range of ~40-100 M jup complements previous work in the same region, reporting detections of similarly wide companions with lower masses, in the range of ~10-30 M jup. Such low masses near the deuterium burning limit have raised the question of whether those objects formed like planets or stars. The existence of intermediate objects as reported here could represent a bridge between lower-mass companions and stellar companions, but in any case demonstrate that mass alone may not provide a clear-cut distinction for the formation of low-mass companions to stars. Based on Gemini observations from programs GS-2011A-Q-44, GS-2012A-Q-18, and GS-2012A-DD-6, and on ESO observations from program 089.C-0422(A).

  3. OUTSIDE-IN SHRINKING OF THE STAR-FORMING DISK OF DWARF IRREGULAR GALAXIES

    SciTech Connect

    Zhang Hongxin; Hunter, Deidre A.; Elmegreen, Bruce G.; Gao Yu; Schruba, Andreas E-mail: dah@lowell.edu E-mail: bge@us.ibm.com

    2012-02-15

    We have studied multi-band surface brightness profiles of a representative sample of 34 nearby dwarf irregular galaxies. Our data include Galaxy Evolution Explorer (GALEX) FUV/NUV, UBV, and H{alpha} and Spitzer 3.6 {mu}m images. These galaxies constitute the majority of the LITTLE THINGS survey (Local Irregulars That Trace Luminosity Extremes-The H I Nearby Galaxy Survey). By modeling the azimuthal averages of the spectral energy distributions with a complete library of star formation histories, we derived the stellar mass surface density distributions and the star formation rate averaged over three different timescales: the recent 0.1 Gyr, 1 Gyr, and a Hubble time. We find that, for {approx}80% (27 galaxies) of our sample galaxies, radial profiles (at least in the outer part) at shorter wavelengths, corresponding to younger stellar populations, have shorter disk scale lengths than those at longer wavelengths, corresponding to older stellar populations. This indicates that the star-forming disk has been shrinking. In addition, the radial distributions of the stellar mass surface density are well described as piece-wise exponential profiles, and {approx}80% of the galaxies have steeper mass profiles in the outer disk than in the inner region. The steep radial decline of the star formation rate in the outer parts compared to that in the inner disks gives a natural explanation for the down-bending stellar mass surface density profiles. Within the inner disks, our sample galaxies on average have constant ratios of recent star formation rate to stellar mass with radius. Nevertheless, {approx}35% (12 galaxies, among which 7 have baryonic mass {approx}<10{sup 8} M{sub Sun} ) of the sample exhibit negative slopes across the observed disk, which is in contrast with the so-called inside-out disk growth scenario suggested for luminous spiral galaxies. The tendency of star formation to become concentrated toward the inner disks in low-mass dwarf irregular galaxies is

  4. Outside-in Shrinking of the Star-forming Disk of Dwarf Irregular Galaxies

    NASA Astrophysics Data System (ADS)

    Zhang, Hong-Xin; Hunter, Deidre A.; Elmegreen, Bruce G.; Gao, Yu; Schruba, Andreas

    2012-02-01

    We have studied multi-band surface brightness profiles of a representative sample of 34 nearby dwarf irregular galaxies. Our data include Galaxy Evolution Explorer (GALEX) FUV/NUV, UBV, and Hα and Spitzer 3.6 μm images. These galaxies constitute the majority of the LITTLE THINGS survey (Local Irregulars That Trace Luminosity Extremes—The H I Nearby Galaxy Survey). By modeling the azimuthal averages of the spectral energy distributions with a complete library of star formation histories, we derived the stellar mass surface density distributions and the star formation rate averaged over three different timescales: the recent 0.1 Gyr, 1 Gyr, and a Hubble time. We find that, for ~80% (27 galaxies) of our sample galaxies, radial profiles (at least in the outer part) at shorter wavelengths, corresponding to younger stellar populations, have shorter disk scale lengths than those at longer wavelengths, corresponding to older stellar populations. This indicates that the star-forming disk has been shrinking. In addition, the radial distributions of the stellar mass surface density are well described as piece-wise exponential profiles, and ~80% of the galaxies have steeper mass profiles in the outer disk than in the inner region. The steep radial decline of the star formation rate in the outer parts compared to that in the inner disks gives a natural explanation for the down-bending stellar mass surface density profiles. Within the inner disks, our sample galaxies on average have constant ratios of recent star formation rate to stellar mass with radius. Nevertheless, ~35% (12 galaxies, among which 7 have baryonic mass lsim108 M ⊙) of the sample exhibit negative slopes across the observed disk, which is in contrast with the so-called inside-out disk growth scenario suggested for luminous spiral galaxies. The tendency of star formation to become concentrated toward the inner disks in low-mass dwarf irregular galaxies is interpreted as a result of their susceptibility to

  5. Multi-fibre optical spectroscopy of low-mass stars and brown dwarfs in Upper Scorpius

    NASA Astrophysics Data System (ADS)

    Lodieu, N.; Dobbie, P. D.; Hambly, N. C.

    2011-03-01

    Context. Knowledge of the mass function in open clusters constitutes one way to critically examine the formation mechanisms proposed to explain the existence of low-mass stars and brown dwarfs. Aims: The aim of the project is to determine as accurately as possible the shape of the mass function across the stellar/substellar boundary in the young (5 Myr) and nearby (d = 145 pc) Upper Sco association. Methods: We have obtained multi-fibre intermediate-resolution (R ~ 1100) optical (~5750-8800 Å) spectroscopy of 94 photometric and proper motion selected low-mass star and brown dwarf candidates in Upper Sco with the AAOmega spectrograph on the Anglo-Australian Telescope. Results: We have estimated the spectral types and measured the equivalent widths of youth (Hα) and gravity (Na I and K I) diagnostic features to confirm the spectroscopic membership of about 95% of the photometric and proper motion candidates extracted from 6.5 square degrees surveyed in Upper Sco by the UKIRT Infrared Deep Sky Survey (UKIDSS) Galactic Clusters Survey (GCS). We also detect lithium in the spectra with the highest signal-to-noise, consolidating our conclusions about their youth. Furthermore, we derive an estimate of the efficiency of the photometric and proper motion selections used in our earlier studies using spectroscopic data obtained for a large number of stars falling into the instrument's field-of-view. We have estimated the effective temperatures and masses for each new spectroscopic member using the latest evolutionary models available for low-mass stars and brown dwarfs. Combining the current optical spectroscopy presented here with near-infrared spectroscopy obtained for the faintest photometric candidates, we confirm the shape and slope of our earlier photometric mass function. The luminosity function drawn from the spectroscopic sample of 113 USco members peaks at around M6 and is flat at later spectral type. We may detect the presence of the M7/M8 gap in the luminosity

  6. Be stars with white dwarf companions: a new single degenerate binary channel to type Ia supernovae explosions

    NASA Astrophysics Data System (ADS)

    Orio, Marina; Luna, Gerardo; Zemko, Polina; Kotulla, Ralf; Gallagher, Jay; Harbeck, Daniel

    2016-07-01

    A handful of supersoft X-ray sources in the Magellanic Clouds that could not be identified with transient nova outbursts turned out to be mainly massive close binaries. 6 years ago we suggested that several such sources may exist in M31, because we found that a certain fraction of supersoft sources was located in star forming regions. Following that discovery, we clearly identified a Be binary in M31, and are currently collecting data for another candidate in that galaxy. Work is in progress to assess whether the compact object companion really is a hydrogen burning white dwarf (the alternative being a massive stellar-mass black hole). If we can demonstrate that Be+white dwarf interacting close binaries are common, and that hydrogen is often ignited on the white dwarf in these systems, we have discovered a new promising channel towards the explosion of supernovae of type Ia in star forming regions, without invoking double degenerate systems.

  7. Mass Modelling of Dwarf Spheroidal Galaxies: the Effect of Unbound Stars From Tidal Tails And the Milky Way

    SciTech Connect

    Klimentowski, Jaroslaw; Lokas, Ewa L.; Kazantzidis, Stelios; Prada, Francisco; Mayer, Lucio; Mamon, Gary A.; /Paris, Inst. Astrophys. /Meudon Observ.

    2006-11-14

    We study the origin and properties of the population of unbound stars in the kinematic samples of dwarf spheroidal galaxies. For this purpose we have run a high resolution N- body simulation of a two-component dwarf galaxy orbiting in a Milky Way potential. In agreement with the tidal stirring scenario of Mayer et al., the dwarf is placed on a highly eccentric orbit, its initial stellar component is in the form of an exponential disk and it has a NFW-like dark matter halo. After 10 Gyrs of evolution the dwarf produces a spheroidal stellar component and is strongly tidally stripped so that mass follows light and the stars are on almost isotropic orbits. From this final state, we create mock kinematic data sets for 200 stars by observing the dwarf in different directions.We find that when the dwarf is observed along the tidal tails the kinematic samples are strongly contaminated by unbound stars from the tails.We also study another source of possible contamination by adding stars from the Milky Way. We demonstrate that most of the unbound stars can be removed by the method of interloper rejection proposed by den Hartog & Katgert and recently tested on simulated dark matter haloes. We model the cleaned up kinematic samples using solutions of the Jeans equation with constant mass-to-light ratio and velocity anisotropy parameter. We show that even for such strongly stripped dwarf the Jeans analysis, when applied to cleaned samples, allows us to reproduce the mass and mass-to-light ratio of the dwarf with accuracy typically better than 25 percent and almost exactly in the case when the line of sight is perpendicular to the tidal tails. The analysis was applied to the new data for the Fornax dSph galaxy for which we find a mass-to-light ratio of 11 solar units and isotropic orbits. We demonstrate that most of the contamination in the kinematic sample of Fornax probably originates from the Milky Way.

  8. Mass modelling of dwarf spheroidal galaxies: the effect of unbound stars from tidal tails and the Milky Way

    NASA Astrophysics Data System (ADS)

    Klimentowski, Jarosław; Łokas, Ewa L.; Kazantzidis, Stelios; Prada, Francisco; Mayer, Lucio; Mamon, Gary A.

    2007-06-01

    We study the origin and properties of the population of unbound stars in the kinematic samples of dwarf spheroidal (dSph) galaxies. For this purpose we have run a high-resolution N-body simulation of a two-component dwarf galaxy orbiting in a Milky Way potential. In agreement with the tidal stirring scenario of Mayer et al., the dwarf is placed on a highly eccentric orbit, its initial stellar component is in the form of an exponential disc and it has a NFW-like dark matter (DM) halo. After 10 Gyr of evolution the dwarf produces a spheroidal stellar component and is strongly tidally stripped so that mass follows light and the stars are on almost isotropic orbits. From this final state, we create mock kinematic data sets for 200 stars by observing the dwarf in different directions. We find that when the dwarf is observed along the tidal tails the kinematic samples are strongly contaminated by unbound stars from the tails. We also study another source of possible contamination by adding stars from the Milky Way. We demonstrate that most of the unbound stars can be removed by the method of interloper rejection proposed by den Hartog & Katgert and recently tested on simulated DM haloes. We model the cleaned-up kinematic samples using solutions of the Jeans equation with constant mass-to-light ratio (M/L) and velocity anisotropy parameter. We show that even for such a strongly stripped dwarf the Jeans analysis, when applied to cleaned samples, allows us to reproduce the mass and M/L of the dwarf with accuracy typically better than 25 per cent and almost exactly in the case when the line of sight is perpendicular to the tidal tails. The analysis was applied to the new data for the Fornax dSph galaxy. We show that after careful removal of interlopers the velocity dispersion profile of Fornax can be reproduced by a model in which mass traces light with a M/L of 11 solar units and isotropic orbits. We demonstrate that most of the contamination in the kinematic sample of

  9. BOO-1137-AN EXTREMELY METAL-POOR STAR IN THE ULTRA-FAINT DWARF SPHEROIDAL GALAXY BOOeTES I

    SciTech Connect

    Norris, John E.; Yong, David; Gilmore, Gerard; Wyse, Rosemary F. G.

    2010-03-01

    We present high-resolution (R {approx} 40,000), high-signal-to-noise ratio (20-90) spectra of an extremely metal-poor giant star Boo-1137 in the 'ultra-faint' dwarf spheroidal galaxy (dSph) Booetes I, absolute magnitude M{sub V} {approx} -6.3. We derive an iron abundance of [Fe/H] = -3.7, making this the most metal-poor star as yet identified in an ultra-faint dSph. Our derived effective temperature and gravity are consistent with its identification as a red giant in Booetes I. Abundances for a further 15 elements have also been determined. Comparison of the relative abundances, [X/Fe], with those of the extremely metal-poor red giants of the Galactic halo shows that Boo-1137 is 'normal' with respect to C and N, the odd-Z elements Na and Al, the iron-peak elements, and the neutron-capture elements Sr and Ba, in comparison with the bulk of the Milky Way halo population having [Fe/H] {approx}<-3.0. The alpha-elements Mg, Si, Ca, and Ti are all higher by DELTA[X/Fe] {approx} 0.2 than the average halo values. Monte Carlo analysis indicates that DELTA[alpha/Fe] values this large are expected with a probability {approx}0.02. The elemental abundance pattern in Boo-1137 suggests inhomogeneous chemical evolution, consistent with the wide internal spread in iron abundances we previously reported. The similarity of most of the Boo-1137 relative abundances with respect to halo values, and the fact that the alpha-elements are all offset by a similar small amount from the halo averages, points to the same underlying galaxy-scale stellar initial mass function, but that Boo-1137 likely originated in a star-forming region where the abundances reflect either poor mixing of supernova (SN) ejecta, or poor sampling of the SN progenitor mass range, or both.

  10. Probing the brown dwarf population of the Chamaeleon I star forming region

    NASA Astrophysics Data System (ADS)

    Comerón, F.; Neuhäuser, R.; Kaas, A. A.

    2000-07-01

    We present observations of a sample of 13 very low mass stars and brown dwarfs in the central region of the Chamaeleon I star forming cloud. The observations include slitless spectroscopy around Hα to identify new members, low resolution long-slit visible and near-infrared spectroscopy, deep ROSAT PSPC X-ray observations, and ISOCAM mid-infrared observations. Our sample adds seven new objects to those discussed by Comerón, Rieke, and Neuhäuser (1999, A&A, 343, 477) and extends the range of spectral types up to M8. We study different narrow-band indices as a tool for detecting and classifying very late-type young stellar objects. As to K-band spectra, we find that the visible features are not appropriate to yield a spectral classification more accurate than a few subclasses at best beyond M6. None of our sources displays K-band excess emission, but four have excess at 6.7 mu m suggesting that, although circumstellar disks are common around young very low mass stars, their inner regions are in general not hot enough to radiate significantly in the K band. Mid-infrared emission loosely correlates with Hα emission: sources without mid-IR excesses are always weak Hα emitters, while mid-IR excess sources have a broad range of Hα equivalent widths. X-ray emission is detected for 7 objects with spectral type M6 or later, including one bona-fide brown dwarf and three objects near the border separating stars and brown dwarfs. X-ray to bolometric luminosity ratios are typical of low mass, fully convective stars. The non-detection of X-ray emission at comparable levels from more evolved brown dwarfs suggests that X-ray activity may be restricted to early stages of brown dwarf evolution. We discuss in detail the temperatures and luminosities of our objects based on their magnitudes and spectra, and use the derived values to estimate masses and ages according to two different sets of pre-main sequence evolutionary tracks. Both sets of models are in good agreement

  11. Massive stars at low metallicity. Evolution and surface abundances of O dwarfs in the SMC

    NASA Astrophysics Data System (ADS)

    Bouret, J.-C.; Lanz, T.; Martins, F.; Marcolino, W. L. F.; Hillier, D. J.; Depagne, E.; Hubeny, I.

    2013-07-01

    Aims: We aim to study the properties of massive stars at low metallicity, with an emphasis on their evolution, rotation, and surface abundances. We focus on O-type dwarfs in the Small Magellanic Cloud. These stars are expected to have weak winds that do not remove significant amounts of their initial angular momentum. Methods: We analyzed the UV and optical spectra of twenty-three objects using the NLTE stellar atmosphere code cmfgen and derived photospheric and wind properties. Results: The observed binary fraction of the sample is ≈26%, which is consistent with more systematic studies if one considers that the actual binary fraction is potentially larger owing to low-luminosity companions and that the sample was biased because it excluded obvious spectroscopic binaries. The location of the fastest rotators in the Hertzsprung-Russell (H-R) diagram built with fast-rotating evolutionary models and isochrones indicates that these could be several Myr old. The offset in the position of these fast rotators compared with the other stars confirms the predictions of evolutionary models that fast-rotating stars tend to evolve more vertically in the H-R diagram. Only one star of luminosity class Vz, expected to best characterize extreme youth, is located on the zero-age main sequence, the other two stars are more evolved. We found that the distribution of O and B stars in the ɛ(N) - vsin i diagram is the same, which suggests that the mechanisms responsible for the chemical enrichment of slowly rotating massive stars depend only weakly on the star's mass. We furthermore confirm that the group of slowly rotating N-rich stars is not reproduced by the evolutionary tracks. Even for more massive stars and faster rotators, our results call for stronger mixing in the models to explain the range of observed N abundances. All stars have an N/C ratio as a function of stellar luminosity that match the predictions of the stellar evolution models well. More massive stars have a higher

  12. FORMING HABITABLE PLANETS AROUND DWARF STARS: APPLICATION TO OGLE-06-109L

    SciTech Connect

    Wang Su; Zhou Jilin E-mail: zhoujl@nju.edu.cn

    2011-02-01

    Dwarf stars are believed to have a small protostar disk where planets may grow up. During the planet formation stage, embryos undergoing type I migration are expected to be stalled at an inner edge of the magnetically inactive disk (a{sub crit} {approx} 0.2-0.3 AU). This mechanism makes the location around a{sub crit} a 'sweet spot' for forming planets. In dwarf stars with masses {approx}0.5 M{sub sun}, a{sub crit} is roughly inside the habitable zone of the system. In this paper, we study the formation of habitable planets due to this mechanism using model system OGLE-06-109L, which has a 0.51 M{sub sun} dwarf star with two giant planets in 2.3 and 4.6 AU observed by microlensing. We model the embryos undergoing type I migration in the gas disk with a constant disk-accretion rate ( M-dot ). Giant planets in outside orbits affect the formation of habitable planets through secular perturbations at the early stage and secular resonance at the late stage. We find that the existence and the masses of the habitable planets in the OGLE-06-109L system depend on both M-dot and the speed of type I migration. If planets are formed earlier, so that M-dot is larger ({approx}10{sup -7} M{sub sun} yr{sup -1}), terrestrial planets cannot survive unless the type I migration rate is an order of magnitude less. If planets are formed later, so that M-dot is smaller ({approx}10{sup -8} M{sub sun} yr{sup -1}), single and high-mass terrestrial planets with high water contents ({approx}5%) will be formed by inward migration of outer planet cores. A slower-speed migration will result in several planets via collisions of embryos, and thus their water contents will be low ({approx}2%). Mean motion resonances or apsidal resonances among planets may be observed if multiple planets survive in the inner system.

  13. DWARF GALAXY FORMATION WITH H{sub 2}-REGULATED STAR FORMATION

    SciTech Connect

    Kuhlen, Michael; Krumholz, Mark R.; Madau, Piero; Smith, Britton D.; Wise, John

    2012-04-10

    We describe cosmological galaxy formation simulations with the adaptive mesh refinement code Enzo that incorporate a star formation prescription regulated by the local abundance of molecular hydrogen. We show that this H{sub 2}-regulated prescription leads to a suppression of star formation in low-mass halos (M{sub h} {approx}< 10{sup 10} M{sub Sun }) at z > 4, alleviating some of the dwarf galaxy problems faced by theoretical galaxy formation models. H{sub 2} regulation modifies the efficiency of star formation of cold gas directly, rather than indirectly reducing the cold gas content with 'supernova feedback'. We determine the local H{sub 2} abundance in our most refined grid cells (76 proper parsec in size at z = 4) by applying the model of Krumholz, McKee, and Tumlinson, which is based on idealized one-dimensional radiative transfer calculations of H{sub 2} formation-dissociation balance in {approx}100 pc atomic-molecular complexes. Our H{sub 2}-regulated simulations are able to reproduce the empirical (albeit lower z) Kennicutt-Schmidt relation, including the low {Sigma}{sub gas} cutoff due to the transition from atomic to molecular phase and the metallicity dependence thereof, without the use of an explicit density threshold in our star formation prescription. We compare the evolution of the luminosity function, stellar mass density, and star formation rate density from our simulations to recent observational determinations of the same at z = 4-8 and find reasonable agreement between the two.

  14. DO R CORONAE BOREALIS STARS FORM FROM DOUBLE WHITE DWARF MERGERS?

    SciTech Connect

    Staff, Jan. E.; Clayton, Geoffrey C.; Tohline, Joel E.; Menon, Athira; Herwig, Falk; Even, Wesley; Fryer, Chris L.; Motl, Patrick M.; Geballe, Tom; Pignatari, Marco

    2012-09-20

    A leading formation scenario for R Coronae Borealis (RCB) stars invokes the merger of degenerate He and CO white dwarfs (WDs) in a binary. The observed ratio of {sup 16}O/{sup 18}O for RCB stars is in the range of 0.3-20 much smaller than the solar value of {approx}500. In this paper, we investigate whether such a low ratio can be obtained in simulations of the merger of a CO and a He WD. We present the results of five three-dimensional hydrodynamic simulations of the merger of a double WD system where the total mass is 0.9 M{sub Sun} and the initial mass ratio (q) varies between 0.5 and 0.99. We identify in simulations with q {approx}< 0.7 a feature around the merged stars where the temperatures and densities are suitable for forming {sup 18}O. However, more {sup 16}O is being dredged up from the C- and O-rich accretor during the merger than the amount of {sup 18}O that is produced. Therefore, on the dynamical timescale over which our hydrodynamics simulation runs, an {sup 16}O/{sup 18}O ratio of {approx}2000 in the 'best' case is found. If the conditions found in the hydrodynamic simulations persist for 10{sup 6} s the oxygen ratio drops to 16 in one case studied, while in a hundred years it drops to {approx}4 in another case studied, consistent with the observed values in RCB stars. Therefore, the merger of two WDs remains a strong candidate for the formation of these enigmatic stars.

  15. A Far Ultraviolet Spectroscopic Explorer Survey of Late-Type Dwarf Stars

    NASA Astrophysics Data System (ADS)

    Redfield, Seth; Linsky, Jeffrey L.; Ake, Thomas B.; Ayres, Thomas R.; Dupree, A. K.; Robinson, Richard D.; Wood, Brian E.; Young, Peter R.

    2002-12-01

    We describe the 910-1180 Å spectra of seven late-type dwarf stars obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite. The stars include Altair (A7 IV), Procyon (F5 IV-V), α Cen A (G2 V), AB Dor (K1 V), α Cen B (K2 V), ɛ Eri (K2 V), and AU Mic (M0 V). We present line identifications, fluxes, Doppler shifts, and widths. Doppler shifts are measured with respect to heliocentric wavelength scales determined from interstellar absorption lines, and are compared with transition region line shifts seen in Hubble Space Telescope (HST) ultraviolet spectra. For the warmer stars the O VI lines extend the trend of increasing redshift with line formation temperature, but for the cooler stars the O VI line redshifts are essentially zero. The C III and O VI lines of most stars in the sample are best fit with two Gaussians, and we confirm the correlation of increasing importance of the broad component with increasing stellar activity. The nonthermal velocities of the narrow component are subsonic and exhibit a trend toward larger velocities with decreasing surface gravity, while the nonthermal velocities of the broad components show no obvious trend with stellar gravity. The C III and O VI lines of Altair show unique broad horned profiles. Two flares were observed on AU Mic. One shows increasing continuum flux to shorter wavelengths, which we interpret as free-free emission from hot plasma, and relatively narrow, redshifted C III and O VI emission. The other shows very broad line profiles.

  16. THE SOLAR NEIGHBORHOOD. XXVIII. THE MULTIPLICITY FRACTION OF NEARBY STARS FROM 5 TO 70 AU AND THE BROWN DWARF DESERT AROUND M DWARFS

    SciTech Connect

    Dieterich, Sergio B.; Henry, Todd J.; Golimowski, David A.; Krist, John E.; Tanner, Angelle M.

    2012-08-15

    We report on our analysis of Hubble Space Telescope/NICMOS snapshot high-resolution images of 255 stars in 201 systems within {approx}10 pc of the Sun. Photometry was obtained through filters F110W, F180M, F207M, and F222M using NICMOS Camera 2. These filters were selected to permit clear identification of cool brown dwarfs through methane contrast imaging. With a plate scale of 76 mas pixel{sup -1}, NICMOS can easily resolve binaries with subarcsecond separations in the 19.''5 Multiplication-Sign 19.''5 field of view. We previously reported five companions to nearby M and L dwarfs from this search. No new companions were discovered during the second phase of data analysis presented here, confirming that stellar/substellar binaries are rare. We establish magnitude and separation limits for which companions can be ruled out for each star in the sample, and then perform a comprehensive sensitivity and completeness analysis for the subsample of 138 M dwarfs in 126 systems. We calculate a multiplicity fraction of 0.0{sup +3.5}{sub -0.0}% for L companions to M dwarfs in the separation range of 5-70 AU, and 2.3{sup +5.0}{sub -0.7}% for L and T companions to M dwarfs in the separation range of 10-70 AU. We also discuss trends in the color-magnitude diagrams using various color combinations and present astrometry for 19 multiple systems in our sample. Considering these results and results from several other studies, we argue that the so-called brown dwarf desert extends to binary systems with low-mass primaries and is largely independent of primary mass, mass ratio, and separations. While focusing on companion properties, we discuss how the qualitative agreement between observed companion mass functions and initial mass functions suggests that the paucity of brown dwarfs in either population may be due to a common cause and not due to binary formation mechanisms.

  17. Hey There Edgar Snow, What Happened to the Red Star over Yan'an?

    ERIC Educational Resources Information Center

    Boshier, Roger; Huang, Yan

    2008-01-01

    Edgar Snow scored an extraordinary scoop in 1936 when he persuaded Mao Zedong to tell his story. The resulting book--"Red Star Over China"--was a best-seller in the West and translated editions caused a sensation in China. Adult education was the centrepiece of Communist revolution and featured prominently in Red Star. It is now the twenty-first…

  18. ON THE SURVIVAL OF BROWN DWARFS AND PLANETS ENGULFED BY THEIR GIANT HOST STAR

    SciTech Connect

    Passy, Jean-Claude; Mac Low, Mordecai-Mark; De Marco, Orsola

    2012-11-10

    The recent discovery of two Earth-mass planets in close orbits around an evolved star has raised questions as to whether substellar companions can survive encounters with their host stars. We consider whether these companions could have been stripped of significant amounts of mass during the phase when they orbited through the dense inner envelopes of the giant. We apply the criterion derived by Murray et al. for disruption of gravitationally bound objects by ram pressure to determine whether mass loss may have played a role in the histories of these and other recently discovered low-mass companions to evolved stars. We find that the brown dwarf and Jovian-mass objects circling WD 0137-349, SDSS J08205+0008, and HIP 13044 are most unlikely to have lost significant mass during the common envelope phase. However, the Earth-mass planets found around KIC 05807616 could well be the remnants of one or two Jovian-mass planets that lost extensive mass during the common envelope phase.

  19. Spectra of late type dwarf stars of known abundance for stellar population models

    NASA Technical Reports Server (NTRS)

    Oconnell, R. W.

    1990-01-01

    The project consisted of two parts. The first was to obtain new low-dispersion, long-wavelength, high S/N IUE spectra of F-G-K dwarf stars with previously determined abundances, temperatures, and gravities. To insure high quality, the spectra are either trailed, or multiple exposures are taken within the large aperture. Second, the spectra are assembled into a library which combines the new data with existing IUE Archive data to yield mean spectral energy distributions for each important type of star. My principal responsibility is the construction and maintenance of this UV spectral library. It covers the spectral range 1200-3200A and is maintained in two parts: a version including complete wavelength coverage at the full spectral resolution of the Low Resolution cameras; and a selected bandpass version, consisting of the mean flux in pre-selected 20A bands. These bands are centered on spectral features or continuum regions of special utility - e.g. the C IV lambda 1550 or Mg II lambda 2800 feature. In the middle-UV region, special emphasis is given to those features (including continuum 'breaks') which are most useful in the study of F-G-K star spectra in the integrated light of old stellar populations.

  20. Black hole, neutron star and white dwarf candidates from microlensing with OGLE-III★

    NASA Astrophysics Data System (ADS)

    Wyrzykowski, Ł.; Kostrzewa-Rutkowska, Z.; Skowron, J.; Rybicki, K. A.; Mróz, P.; Kozłowski, S.; Udalski, A.; Szymański, M. K.; Pietrzyński, G.; Soszyński, I.; Ulaczyk, K.; Pietrukowicz, P.; Poleski, R.; Pawlak, M.; Iłkiewicz, K.; Rattenbury, N. J.

    2016-05-01

    Most stellar remnants so far have been found in binary systems, where they interact with matter from their companions. Isolated neutron stars and black holes are difficult to find as they are dark, yet they are predicted to exist in our Galaxy in vast numbers. We explored the OGLE-III data base of 150 million objects observed in years 2001-2009 and found 59 microlensing events exhibiting a parallax effect due to the Earth's motion around the Sun. Combining parallax and brightness measurements from microlensing light curves with expected proper motions in the Milky Way, we identified 13 microlensing events which are consistent with having a white dwarf, neutron star or a black hole lens and we estimated their masses and distances. The most massive of our black hole candidates has 9.3 M⊙ and is at a distance of 2.4 kpc. The distribution of masses of our candidates indicates a continuum in mass distribution with no mass gap between neutron stars and black holes. We also present predictions on how such events will be observed by the astrometric Gaia mission.

  1. Black hole, neutron star and white dwarf candidates from microlensing with OGLE-III

    NASA Astrophysics Data System (ADS)

    Wyrzykowski, Ł.; Kostrzewa-Rutkowska, Z.; Skowron, J.; Rybicki, K. A.; Mróz, P.; Kozłowski, S.; Udalski, A.; Szymański, M. K.; Pietrzyński, G.; Soszyński, I.; Ulaczyk, K.; Pietrukowicz, P.; Poleski, R.; Pawlak, M.; Iłkiewicz, K.; Rattenbury, N. J.

    2016-05-01

    Most stellar remnants so far have been found in binary systems, where they interact with matter from their companions. Isolated neutron stars and black holes are difficult to find as they are dark, yet they are predicted to exist in our Galaxy in vast numbers. We explored the OGLE-III data base of 150 million objects observed in years 2001-2009 and found 59 microlensing events exhibiting a parallax effect due to the Earth's motion around the Sun. Combining parallax and brightness measurements from microlensing light curves with expected proper motions in the Milky Way, we identified 13 microlensing events which are consistent with having a white dwarf, neutron star or a black hole lens and we estimated their masses and distances. The most massive of our black hole candidates has 9.3 M⊙ and is at a distance of 2.4 kpc. The distribution of masses of our candidates indicates a continuum in mass distribution with no mass gap between neutron stars and black holes. We also present predictions on how such events will be observed by the astrometric Gaia mission.

  2. A STRANGE STAR SCENARIO FOR THE FORMATION OF ECCENTRIC MILLISECOND PULSAR/HELIUM WHITE DWARF BINARIES

    SciTech Connect

    Jiang, Long; Li, Xiang-Dong; Dey, Jishnu; Dey, Mira

    2015-07-01

    According to the recycling scenario, millisecond pulsars (MSPs) have evolved from low-mass X-ray binaries (LMXBs). Their orbits are expected to be circular due to tidal interactions during binary evolution, as observed in most binary MSPs. There are some peculiar systems that do not fit this picture. Three recent examples are the PSRs J2234+06, J1946+3417, and J1950+2414, all of which are MSPs in eccentric orbits but with mass functions compatible with expected He white dwarf (WD) companions. It has been suggested these MSPs may have formed from delayed accretion-induced collapse of massive WDs, or the eccentricity may be induced by dynamical interaction between the binary and a circumbinary disk. Assuming that the core density of accreting neutron stars (NSs) in LMXBs may reach the density of quark deconfinement, which can lead to phase transition from NSs to strange quark stars, we show that the resultant MSPs are likely to have an eccentric orbit, due to the sudden loss of the gravitational mass of the NS during the transition. The eccentricities can be reproduced with a reasonable estimate of the mass loss. This scenario might also account for the formation of the youngest known X-ray binary Cir X–1, which also possesses a low-field compact star in an eccentric orbit.

  3. A planetary system and a highly eccentric brown dwarf around the giant stars HIP 67851 and HIP 97233

    NASA Astrophysics Data System (ADS)

    Jones, M. I.; Jenkins, J. S.; Rojo, P.; Melo, C. H. F.; Bluhm, P.

    2015-01-01

    Context. So far more than 60 substellar companions have been discovered around giant stars. These systems present physical and orbital properties that contrast with those detected orbiting less evolved stars. Aims: We are conducting a radial velocity survey of 166 bright giant stars in the southern hemisphere. The main goals of our project are to detect and characterize planets in close-in orbits around giant stars in order to study the effects of the host star evolution on their orbital and physical properties. Methods: We have obtained precision radial velocities for the giant stars HIP 67851 and HIP 97233 that have revealed periodic signals, which are most likely induced by the presence of substellar companions. Results: We present the discovery of a planetary system and an eccentric brown dwarf orbiting the giant stars HIP 67851 and HIP 97233, respectively. The inner planet around HIP 67851 has a period of 88.8 days, a projected mass of 1.4 MJ and an eccentricity of 0.09. HIP 67851 b is one the few known planets orbiting a giant star interior to 0.5 AU. Although the orbit of the outer object is not fully constrained, it is likely a super-Jupiter. The brown dwarf around HIP 97233 has an orbital period of 1058.8 days, a minimum mass of 20.0 MJ and an eccentricity of 0.61. This is the most eccentric known brown dwarf around a giant star. Based on observations collected at La Silla - Paranal Observatory under programs ID's 085.C-0557, 087.C.0476, 089.C-0524, 090.C-0345 and through the Chilean Telescope Time under programs ID's CN 12A-073, CN 12B-047 and CN 13A-111.

  4. A HYBRID SCENARIO FOR THE FORMATION OF BROWN DWARFS AND VERY LOW MASS STARS

    SciTech Connect

    Basu, Shantanu; Vorobyov, Eduard I. E-mail: eduard.vorobiev@univie.ac.at

    2012-05-01

    We present a calculation of protostellar disk formation and evolution in which gaseous clumps (essentially, the first Larson cores formed via disk fragmentation) are ejected from the disk during the early stage of evolution. This is a universal process related to the phenomenon of ejection in multiple systems of point masses. However, it occurs in our model entirely due to the interaction of compact, gravitationally bound gaseous clumps and is free from the smoothing-length uncertainty that is characteristic of models using sink particles. Clumps that survive ejection span a mass range of 0.08-0.35 M{sub Sun }, and have ejection velocities 0.8 {+-} 0.35 km s{sup -1}, which are several times greater than the escape speed. We suggest that, upon contraction, these clumps can form substellar or low-mass stellar objects with notable disks, or even close-separation very low mass binaries. In this hybrid scenario, allowing for ejection of clumps rather than finished protostars/proto-brown-dwarfs, disk formation and the low velocity dispersion of low-mass objects are naturally explained, while it is also consistent with the observation of isolated low-mass clumps that are ejection products. We conclude that clump ejection and the formation of isolated low-mass stellar and substellar objects is a common occurrence, with important implications for understanding the initial mass function, the brown dwarf desert, and the formation of stars in all environments and epochs.

  5. Importance of tides for periastron precession in eccentric neutron star-white dwarf binaries

    SciTech Connect

    Sravan, N.; Valsecchi, F.; Kalogera, V.; Althaus, L. G.

    2014-09-10

    Although not nearly as numerous as binaries with two white dwarfs, eccentric neutron star-white dwarf (NS-WD) binaries are important gravitational-wave (GW) sources for the next generation of space-based detectors sensitive to low frequency waves. Here we investigate periastron precession in these sources as a result of general relativistic, tidal, and rotational effects; such precession is expected to be detectable for at least some of the detected binaries of this type. Currently, two eccentric NS-WD binaries are known in the galactic field, PSR J1141–6545 and PSR B2303+46, both of which have orbits too wide to be relevant in their current state to GW observations. However, population synthesis studies predict the existence of a significant Galactic population of such systems. Though small in most of these systems, we find that tidally induced periastron precession becomes important when tides contribute to more than 3% of the total precession rate. For these systems, accounting for tides when analyzing periastron precession rate measurements can improve estimates of the inferred WD component mass and, in some cases, will prevent us from misclassifying the object. However, such systems are rare, due to rapid orbital decay. To aid the inclusion of tidal effects when using periastron precession as a mass measurement tool, we derive a function that relates the WD radius and periastron precession constant to the WD mass.

  6. VizieR Online Data Catalog: New white dwarf and subdwarf stars in SDSS DR12 (Kepler+, 2016)

    NASA Astrophysics Data System (ADS)

    Kepler, S. O.; Pelisoli, I.; Koester, D.; Ourique, G.; Romero, A. D.; Reindl, N.; Kleinman, S. J.; Eisenstein, D. J.; Valois, A. D. M.; Amaral, L. A.

    2016-07-01

    Our selection of white dwarf candidates among DR12 objects was similar to that reported for DR10 (Kepler et al., 2015, Cat. J/MNRAS/446/4078). We did not restrict our sample by magnitude, but by S/N>=3. In addition to the 762 targeted white dwarf candidates after DR10 by anc 42, we selected the spectra of any object classified by the elodie pipeline (Bolton et al., 2012AJ....144..144B) as a white dwarf, which returned 35708 spectra, an O, B or A star, which returned another 144471 spectra. Our general colour selection from Kleinman et al. (2013, Cat. J/ApJS/204/5), which takes into account that SDSS multicolour imaging separates hot white dwarf and subdwarf stars from the bulk of the stellar and quasar loci in colour-colour space (Harris et al., 2003, Cat. J/AJ/126/1023), returned 68836 new spectra, from which we identified another 2092 white dwarfs, 79 subdwarfs, 36 cataclysmic variables (CVs), and 3 PG 1159. Most of these spectra were overlapping with the elodie selections. (1 data file).

  7. Multifrequency study of star formation in the blue compact dwarf galaxy I Zw 36

    SciTech Connect

    Viallefond, F.; Thuan, T.X.

    1983-06-15

    We present radio, near-infrared, optical, and ultraviolet observations of the blue compact dwarf galaxy (BCG) I Zw 36equivalentMrk 209equivalentHaro 29. The H I distribution shows a core-halo structure. The core contains half of the mass and shows systematic motions. The halo is diffuse and contains several H I clumps. The visible star formation region is associated with the core but is slightly shifted with respect to the H I peak column density. The virial mass is 5 to 7 times the H I mass. I Zw 36 has about a tenth of the solar metallicity. Its dust-to-oxygen abundance ratio by mass is approx.3 times greater than the ratio in the solar neighborhood. The near-infrared observations indicate the presence of an old stellar population of G and K giants.

  8. Dwarf nova-type cataclysmic variable stars are significant radio emitters

    NASA Astrophysics Data System (ADS)

    Coppejans, Deanne L.; Körding, Elmar. G.; Miller-Jones, James C. A.; Rupen, Michael P.; Sivakoff, Gregory R.; Knigge, Christian; Groot, Paul J.; Woudt, Patrick A.; Waagen, Elizabeth O.; Templeton, Matthew

    2016-08-01

    We present 8-12 GHz radio light curves of five dwarf nova (DN) type Cataclysmic Variable stars (CVs) in outburst (RX And, U Gem and Z Cam), or superoutburst (SU UMa and YZ Cnc), increasing the number of radio-detected DN by a factor of two. The observed radio emission was variable on time-scales of minutes to days, and we argue that it is likely to be synchrotron emission. This sample shows no correlation between the radio luminosity and optical luminosity, orbital period, CV class, or outburst type; however higher-cadence observations are necessary to test this, as the measured luminosity is dependent on the timing of the observations in these variable objects. The observations show that the previously detected radio emission from SS Cyg is not unique in type, luminosity (in the plateau phase of the outburst), or variability time-scales. Our results prove that DN, as a class, are radio emitters in outburst.

  9. Is beryllium ultra-depletion in solar-type stars linked to the presence of a white dwarf companion?

    NASA Astrophysics Data System (ADS)

    Desidera, S.; D'Orazi, V.; Lugaro, M.

    2016-03-01

    Context. Abundance studies of solar-type stars revealed a small fraction of objects with extreme depletion of beryllium. Aims: We investigate the possible link between the beryllium depletion and the presence of companions. Methods: The classical methods (radial velocity, astrometry, imaging) used to search for binary companions were exploited. We also performed a chemical analysis to identify binaries by the alteration in abundances that is produced by the accretion of material lost by a former evolved companion. Results: We found that all the four previously investigated stars that were found to be ultra-depleted in Be are binaries. In two cases the companion is a white dwarf, and in the other two cases the companion might be a white dwarf or a main-sequence star. One new barium star was identified. Conclusions: We speculate that the interaction with the white dwarf progenitor caused an alteration in the abundance pattern of the star, which resulted in severe beryllium depletion. Possible mechanisms such as thermohaline mixing, episodic accretion, and rotational mixing are discussed. We also briefly discuss predictions for validating this scenario.

  10. The Eating Habits of Milky Way Mass Halos: Destroyed Dwarf Satellites and the Metallicity Distribution of Accreted Stars

    DOE PAGESBeta

    Deason, Alis J.; Mao, Yao-Yuan; Wechsler, Risa H.

    2016-04-01

    In this paper, we study the mass spectrum of destroyed dwarfs that contribute to the accreted stellar mass of Milky Way (MW)-mass (Mvir ~ 1012.1 M⊙) halos using a suite of 45 zoom-in dissipationless simulations. Empirical models are employed to relate (peak) subhalo mass to dwarf stellar mass, and we use constraints from z = 0 observations and hydrodynamical simulations to estimate the metallicity distribution of the accreted stellar material. The dominant contributors to the accreted stellar mass are relatively massive dwarfs with Mstar ~ 108–1010M⊙. Halos with more quiescent accretion histories tend to have lower mass progenitors (108–109 M⊙),more » and lower overall accreted stellar masses. Ultra-faint mass (Mstar < 105 M⊙) dwarfs contribute a negligible amount (<<1%) to the accreted stellar mass and, despite having low average metallicities, supply a small fraction (~2%–5%) of the very metal-poor stars with [Fe/H] < -2. Dwarfs with masses 105 < Mstar/M⊙ < 108 provide a substantial amount of the very metal-poor stellar material (~40%–80%), and even relatively metal-rich dwarfs with Mstar > 108 M⊙ can contribute a considerable fraction (~20%–60%) of metal-poor stars if their metallicity distributions have significant metal-poor tails. Finally, we find that the generic assumption of a quiescent assembly history for the MW halo seems to be in tension with the mass spectrum of its surviving dwarfs. In conclusion, we suggest that the MW could be a "transient fossil"; a quiescent halo with a recent accretion event(s) that disguises the preceding formation history of the halo.« less

  11. The Eating Habits of Milky Way-mass Halos: Destroyed Dwarf Satellites and the Metallicity Distribution of Accreted Stars

    NASA Astrophysics Data System (ADS)

    Deason, Alis J.; Mao, Yao-Yuan; Wechsler, Risa H.

    2016-04-01

    We study the mass spectrum of destroyed dwarfs that contribute to the accreted stellar mass of Milky Way (MW)-mass (Mvir ∼ 1012.1 M⊙) halos using a suite of 45 zoom-in dissipationless simulations. Empirical models are employed to relate (peak) subhalo mass to dwarf stellar mass, and we use constraints from z = 0 observations and hydrodynamical simulations to estimate the metallicity distribution of the accreted stellar material. The dominant contributors to the accreted stellar mass are relatively massive dwarfs with Mstar ∼ 108–1010M⊙. Halos with more quiescent accretion histories tend to have lower mass progenitors (108–109 M⊙), and lower overall accreted stellar masses. Ultra-faint mass (Mstar < 105 M⊙) dwarfs contribute a negligible amount (≪1%) to the accreted stellar mass and, despite having low average metallicities, supply a small fraction (∼2%–5%) of the very metal-poor stars with [Fe/H] < ‑2. Dwarfs with masses 105 < Mstar/M⊙ < 108 provide a substantial amount of the very metal-poor stellar material (∼40%–80%), and even relatively metal-rich dwarfs with Mstar > 108 M⊙ can contribute a considerable fraction (∼20%–60%) of metal-poor stars if their metallicity distributions have significant metal-poor tails. Finally, we find that the generic assumption of a quiescent assembly history for the MW halo seems to be in tension with the mass spectrum of its surviving dwarfs. We suggest that the MW could be a “transient fossil” a quiescent halo with a recent accretion event(s) that disguises the preceding formation history of the halo.

  12. The Eating Habits of Milky Way-mass Halos: Destroyed Dwarf Satellites and the Metallicity Distribution of Accreted Stars

    NASA Astrophysics Data System (ADS)

    Deason, Alis J.; Mao, Yao-Yuan; Wechsler, Risa H.

    2016-04-01

    We study the mass spectrum of destroyed dwarfs that contribute to the accreted stellar mass of Milky Way (MW)-mass (Mvir ˜ 1012.1 M⊙) halos using a suite of 45 zoom-in dissipationless simulations. Empirical models are employed to relate (peak) subhalo mass to dwarf stellar mass, and we use constraints from z = 0 observations and hydrodynamical simulations to estimate the metallicity distribution of the accreted stellar material. The dominant contributors to the accreted stellar mass are relatively massive dwarfs with Mstar ˜ 108-1010M⊙. Halos with more quiescent accretion histories tend to have lower mass progenitors (108-109 M⊙), and lower overall accreted stellar masses. Ultra-faint mass (Mstar < 105 M⊙) dwarfs contribute a negligible amount (≪1%) to the accreted stellar mass and, despite having low average metallicities, supply a small fraction (˜2%-5%) of the very metal-poor stars with [Fe/H] < -2. Dwarfs with masses 105 < Mstar/M⊙ < 108 provide a substantial amount of the very metal-poor stellar material (˜40%-80%), and even relatively metal-rich dwarfs with Mstar > 108 M⊙ can contribute a considerable fraction (˜20%-60%) of metal-poor stars if their metallicity distributions have significant metal-poor tails. Finally, we find that the generic assumption of a quiescent assembly history for the MW halo seems to be in tension with the mass spectrum of its surviving dwarfs. We suggest that the MW could be a “transient fossil” a quiescent halo with a recent accretion event(s) that disguises the preceding formation history of the halo.

  13. R Coronae Borealis Stars As The Result Of White Dwarf Mergers?

    NASA Astrophysics Data System (ADS)

    Staff, Jan E.; Menon, A.; Herwig, F.; Even, W.; Clayton, G.; Tohline, J.; Fryer, C. L.; Motl, P.; Geballe, T.

    2012-01-01

    R Coronae Borealis (RCB) stars have masses around a solar mass, are hydrogen-deficient variable stars that suddenly fade by several magnitudes at irregular intervals after which they gradually return to their original brightness over a period of some months. The fading is thought to be due to the formation of dust blocking light from the star. RCBs are often thought to be the result of the merger of a He and a CO white dwarfs. Here we present the results of 3 dimensional hydrodynamic simulations of the merger of double white dwarf systems where total mass is 0.9 solar mass and initial mass ratios ranging between q=0.5 and q=1. We use a zero-temperature plus ideal gas equation of state that allows for heating through shocks. These simulations allow us to follow the evolution of the system for 10-20 initial orbital periods (1000-2000 seconds) to a point after merger when the combined object has settled into a nearly steady-state like configuration. A hot shell forms around the merged core in low q simulations, but not in the high q simulations. The conditions found in the steady state like configuration is used as input to a nucleosynthesis code. We are particularly interested in seeing how much 18O is formed, as observations of RCB stars often show a very high ratio of 18O to 16O of order unity. In the very best case scenario, we find a ratio of 1/12 in the hot shell. This work has been supported, in part, by grant OIA-0963375 from the U.S. National Science Foundation and, in part, by NASA/ATP grants NNX10AC72G. This research also has been made possible by grants of high-performance computing time on the TeraGrid (TG-AST090104), at LSU, and across LONI (Louisiana Optical Network Initiative), especially awards loni_astro08 and loni_astro09).

  14. AN UNDERSTANDING OF THE SHOULDER OF GIANTS: JOVIAN PLANETS AROUND LATE K DWARF STARS AND THE TREND WITH STELLAR MASS

    SciTech Connect

    Gaidos, Eric; Fischer, Debra A.; Mann, Andrew W.; Howard, Andrew W.

    2013-07-01

    Analyses of exoplanet statistics suggest a trend of giant planet occurrence with host star mass, a clue to how planets like Jupiter form. One missing piece of the puzzle is the occurrence around late K dwarf stars (masses of 0.5-0.75 M{sub Sun} and effective temperatures of 3900-4800 K). We analyzed four years of Doppler radial velocity (RVs) data for 110 late K dwarfs, one of which hosts two previously reported giant planets. We estimate that 4.0% {+-} 2.3% of these stars have Saturn-mass or larger planets with orbital periods <245 days, depending on the planet mass distribution and RV variability of stars without giant planets. We also estimate that 0.7% {+-} 0.5% of similar stars observed by Kepler have giant planets. This Kepler rate is significantly (99% confidence) lower than that derived from our Doppler survey, but the difference vanishes if only the single Doppler system (HIP 57274) with completely resolved orbits is considered. The difference could also be explained by the exclusion of close binaries (without giant planets) from the Doppler but not Kepler surveys, the effect of long-period companions and stellar noise on the Doppler data, or an intrinsic difference between the two populations. Our estimates for late K dwarfs bridge those for solar-type stars and M dwarfs, and support a positive trend with stellar mass. Small sample size precludes statements about finer structure, e.g., a ''shoulder'' in the distribution of giant planets with stellar mass. Future surveys such as the Next Generation Transit Survey and the Transiting Exoplanet Satellite Survey will ameliorate this deficiency.

  15. Extreme Emission Line Galaxies in CANDELS: Broad-Band Selected, Star-Bursting Dwarf Galaxies at Z greater than 1

    NASA Technical Reports Server (NTRS)

    VanDerWel, A.; Straughn, A. N.; Rix, H.-W.; Finkelstein, S. L.; Koekemoer, A. M.; Weiner, B. J.; Wuyts, S.; Bell, E. F.; Faber, S. M.; Trump, J. R.; Koo, D.; Ferguson, H. C.; Scarlata, C.; Hathi, N. P.; Dunlop, J. S.; Newman, J. A.; Kocevski, D. D.; Lai, K.; Grogin, N. A.; Rodney, S. A.; Lee, K.-S.; Guo, Y.

    2011-01-01

    We identify an abundant population of extreme emission line galaxies at redshift z=1.6 - 1.8 in the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey (CANDELS) imaging from Hubble Space Telescope/Wide Field Camera 3 (HST/WFC3). 69 candidates are selected by the large contribution of exceptionally bright emission lines to their near-infrared, broad-band fluxes. Supported by spectroscopic confirmation of strong [OIII] emission lines - with equivalent widths approximately 1000A - in the four candidates that have HST/WFC3 grism observations, we conclude that these objects are dwarf galaxies with approximately 10(exp 8) solar mass in stellar mass, undergoing an enormous star-burst phase with M*/M* of only approximately 10 Myr. The star formation activity and the co-moving number density (3.7 x 10(exp -4) Mpc(exp -3)) imply that strong, short-lived bursts play a significant, perhaps even dominant role in the formation and evolution of dwarf galaxies at z greater than 1. The observed star formation activity can produce in less than 5 Gyr the same amount of stellar mass density as is presently contained in dwarf galaxies. Therefore, our observations provide a strong indication that the stellar populations of present-day dwarf galaxies formed mainly in strong, short-lived bursts, mostly at z greater than 1.

  16. Star Formation in Extreme Environments: The Case of the Prototypical Blue Compact Dwarf Galaxy II Zw 40

    NASA Astrophysics Data System (ADS)

    Kepley, Amanda; Leroy, Adam; Johnson, Kelsey; Sandstrom, Karin; Chen, Rosie

    2015-08-01

    With their high star formation rate surface densities and low metallicities, blue compact dwarf galaxies represent one of the most extreme environments for star formation in the local universe: one more akin to that found in high redshift galaxies than in local spirals. Until the advent of ALMA, however, the molecular gas fueling the prodigious star formation in blue compact dwarfs was difficult to observe because these galaxies generally have weak CO emission. In this talk, I present the first detailed study of the molecular gas content (as traced by CO) in the prototypical nearby blue compact dwarf galaxy II Zw 40. Using the extraordinary resolution and sensitivity of our ALMA Cycle 1 observations, we have separated the molecular gas emission into discrete GMC-sized clumps and measured their properties. Surprisingly, we find that -- aside from their low CO luminosities -- the giant molecular clouds in this extreme galaxy have similar properties to clouds in normal spiral galaxies. This discovery suggests that giant molecular clouds share a similar set of properties, despite the differences in their surrounding galactic environment. We suggest that the observed clouds include a range of evolutionary states providing us with important clues about the eventual fate of II Zw 40. Finally, we also report on some of the first observations of dense gas tracers in a Local Group blue compact dwarf, giving a first look at the internal structure of molecular gas in these extreme galaxies.

  17. On the Explosion Geometry of Red Supergiant Stars

    NASA Astrophysics Data System (ADS)

    Leonard, Douglas Christopher; Dessart, Luc; Pignata, Giuliano; Hillier, D. John; Williams, George Grant; Smith, Paul S.; Khandrika, Harish; Bilinski, Christopher; Duong, Nhieu; Flatland, Kelsi; Gonzalez, Luis; Hoffman, Jennifer L.; Horst, Chuck; Huk, Leah; Milne, Peter; Rachubo, Alisa A.; Smith, Nathan

    2015-08-01

    From progenitor studies, type II-Plateau supernovae (SNe II-P) have been decisively and uniquely determined to arise from isolated red supergiant (RSG) stars with initial masses ranging from 8 to 16 solar masses (Smartt 2009), establishing the most homogeneous -- and well understood -- progenitor class of any type of core-collapse supernova. However, we must admit a fundamental truth: We do not know how these stars explode. A basic discriminant among proposed explosion models is explosion geometry, since some models predict severe distortions from spherical symmetry. A primary method to gain such geometric information is through spectropolarimetry of the expanding (but, unresolved) atmosphere, with higher degrees of linear polarization generally demanding larger departures from spherical symmetry. Initially, as a class, SNe II-P were found to be only weakly polarized at the early epochs observed, suggesting a nearly spherical explosion for RSG stars. However, late-time observations of SN 2004dj captured a dramatic spike in polarization at just the moment the "inner core" of the ejecta was first revealed in this SN II-P (i.e., at the "drop" off of the photometric plateau; Leonard et al. 2006). This raised the possibility that the explosion of RSGs might be driven by a strongly non-spherical mechanism, with the evidence for the asphericity cloaked at early times by the massive, opaque, quasi-spherical hydrogen envelope. In this presentation we shall describe the continuing work on the explosion geometry of RSGs being carried out by the SuperNova SpectroPOLarimetry project (SNSPOL), with a particular focus on SN 2013ej -- an SN II-P that exhibited remarkably high polarization just days after the explosion (Leonard et al. 2013), and for which twelve epochs of spectropolarimetry trace an intriguing tale about its geometry deep into the nebular phase. We acknowledge support from NSF grants AST-1009571 and AST-1210311, under which part of this research was carried out.

  18. Trumpeting M dwarfs with CONCH-SHELL: a catalogue of nearby cool host-stars for habitable exoplanets and life

    NASA Astrophysics Data System (ADS)

    Gaidos, E.; Mann, A. W.; Lépine, S.; Buccino, A.; James, D.; Ansdell, M.; Petrucci, R.; Mauas, P.; Hilton, E. J.

    2014-09-01

    We present an all-sky catalogue of 2970 nearby (d ≲ 50 pc), bright (J < 9) M- or late K-type dwarf stars, 86 per cent of which have been confirmed by spectroscopy. This catalogue will be useful for searches for Earth-size and possibly Earth-like planets by future space-based transit missions and ground-based infrared Doppler radial velocity surveys. Stars were selected from the SUPERBLINK proper motion catalogue according to absolute magnitudes, spectra, or a combination of reduced proper motions and photometric colours. From our spectra, we determined gravity-sensitive indices, and identified and removed 0.2 per cent of these as interloping hotter or evolved stars. 13 per cent of the stars exhibit Hα emission, an indication of stellar magnetic activity and possible youth. The mean metallicity is [Fe/H] = -0.07 with a standard deviation of 0.22 dex, similar to nearby solar-type stars. We determined stellar effective temperatures by least-squares fitting of spectra to model predictions calibrated by fits to stars with established bolometric temperatures, and estimated radii, luminosities, and masses using empirical relations. Six per cent of stars with images from integral field spectra are resolved doubles. We inferred the planet population around M dwarfs using Kepler data and applied this to our catalogue to predict detections by future exoplanet surveys.

  19. A census of very-low-mass stars and brown dwarfs in the σ Orionis cluster

    NASA Astrophysics Data System (ADS)

    Lodieu, N.; Zapatero Osorio, M. R.; Rebolo, R.; Martín, E. L.; Hambly, N. C.

    2009-10-01

    Context: The knowledge of the initial mass function (IMF) in open clusters constitutes one way of constraining the formation of low-mass stars and brown dwarfs, along with the frequency of multiple systems and the properties of disks. Aims: The aim of the project is to determine the shape of the mass function in the low-mass and substellar regimes in the σ Orionis cluster (~3 Myr, ~352 pc, solar metallicity) as accurately as possible and compare it with the results in other clusters. Methods: We have analysed the near-infrared photometric data from the fourth data release (DR4) of the UKIRT Infrared Deep Sky Suvey (UKIDSS) Galactic clusters survey (GCS) to derive the cluster luminosity and mass functions, evaluate the extent of the cluster, and study the distribution and variability of low-mass stars and brown dwarfs down to the deuterium-burning limit. Results: We have recovered most of the previously published members and found a total of 287 candidate members within the central 30 arcmin in the 0.5-0.009 M⊙ mass range, including new objects not previously reported in the literature. This new catalogue represents a homogeneous dataset of brown dwarf member candidates over the central 30 arcmin of the cluster. The expected photometric contamination by field objects with similar magnitudes and colours to σ Orionis members is ~15%. We present evidence of variability at the 99.5% confidence level over ~yearly timescales in 10 member candidates that exhibit signs of youth and the presence of disks. The level of variability is low (≤0.3 mag) and does not impact the derivation of the cluster luminosity and mass functions. Furthermore, we find a possible dearth of brown dwarfs within the central five arcmin of the cluster, which is not caused by a lower level of photometric sensitivity around the massive, O-type multiple star σ Ori in the GCS survey. Using state-of-the-art theoretical models, we derived the luminosity and mass functions within the central 30

  20. Comparative multivariate analysis of biometric traits of West African Dwarf and Red Sokoto goats.

    PubMed

    Yakubu, Abdulmojeed; Salako, Adebowale E; Imumorin, Ikhide G

    2011-03-01

    The population structure of 302 randomly selected West African Dwarf (WAD) and Red Sokoto (RS) goats was examined using multivariate morphometric analyses. This was to make the case for conservation, rational management and genetic improvement of these two most important Nigerian goat breeds. Fifteen morphometric measurements were made on each individual animal. RS goats were superior (P<0.05) to the WAD for the body size and skeletal proportions investigated. The phenotypic variability between the two breeds was revealed by their mutual responses in the principal components. While four principal components were extracted for WAD goats, three components were obtained for their RS counterparts with variation in the loading traits of each component for each breed. The Mahalanobis distance of 72.28 indicated a high degree of spatial racial separation in morphology between the genotypes. The Ward's option of the cluster analysis consolidated the morphometric distinctness of the two breeds. Application of selective breeding to genetic improvement would benefit from the detected phenotypic differentiation. Other implications for management and conservation of the goats are highlighted. PMID:21080228

  1. Astrophysical false positives in direct imaging for exoplanets: a white dwarf close to a rejuvenated star

    NASA Astrophysics Data System (ADS)

    Zurlo, A.; Vigan, A.; Hagelberg, J.; Desidera, S.; Chauvin, G.; Almenara, J. M.; Biazzo, K.; Bonnefoy, M.; Carson, J. C.; Covino, E.; Delorme, P.; D'Orazi, V.; Gratton, R.; Mesa, D.; Messina, S.; Moutou, C.; Segransan, D.; Turatto, M.; Udry, S.; Wildi, F.

    2013-06-01

    Context. As is the case for all techniques involved in the research for exoplanets, direct imaging has to take into account the probability of so-called astrophysical false positives, which are phenomena that mimic the signature of the objects we are seeking. Aims: In this work we present the case of a false positive found during a direct-imaging survey conducted with VLT/NACO. A promising exoplanet candidate was detected around the K2-type star HD 8049 in July 2010. Its contrast of ΔH = 7.05 at 1.57 arcsec allowed us to assume a 35 MJup companion at 50 projected AU, for the nominal system age and heliocentric distance. Methods: To check whether it was gravitationally bound to the host star, as opposed to an unrelated background object, we re-observed the system one year later and concluded a high probability of a bound system. We also used radial velocity measurements of the host star, spanning a time range of ~30 yr, to constrain the companion's mass and orbital properties, as well as to probe the host star's spectral age indicators and general spectral energy distribution. We also obtained U-band imaging with EFOSC and near-infrared spectroscopy for the companion. Results: Combining all these information we conclude that the companion of HD 8049 is a white dwarf (WD) with temperature Teff = 18 800 ± 2100 K and mass MWD = 0.56 ± 0.08 M⊙. The significant radial velocity trend combined with the imaging data indicates that the most probable orbit has a semi-major axis of about 50 AU. The discrepancy between the age indicators speaks against a bona-fide young star. The moderately high level of chromospheric activity and fast rotation, mimicking the properties of a young star, might be induced by the exchange of mass with the progenitor of the WD. This example demonstrates some of the challenges in determining accurate age estimates and identifications of faint companions. Based on observations collected at La Silla and Paranal Observatory, ESO (Chile): Programs

  2. A search for p-mode pulsations in white dwarf stars using the Berkeley Visible Imaging Tube detector

    NASA Astrophysics Data System (ADS)

    Kilkenny, D.; Welsh, B. Y.; Koen, C.; Gulbis, A. A. S.; Kotze, M. M.

    2014-01-01

    We present high-speed photometry (resolution 0.1 s) obtained during the commissioning of the Berkely Visible Imaging Tube system on the Southern African Large Telescope (SALT). The observations were an attempt to search for very rapid p-mode oscillations in white dwarf stars and included three DA stars known to be g-mode pulsators (ZZ Cet, HK Cet and AF Pic), one other DA star (WD 1056-384) not known to be variable and one AM Cvn star (HP Lib). No evidence was found for any variations greater than about 1 mmag in amplitude (˜0.1 per cent) at frequencies in excess of 60 mHz (periods <17 s) in any of the target stars, though several previously known g-mode frequencies were recovered.

  3. Star Formation in Extreme Environments: The Case of the Prototypical Blue Compact Dwarf Galaxy II Zw 40

    NASA Astrophysics Data System (ADS)

    Kepley, Amanda A.; Leroy, Adam; Johnson, Kelsey E.; Sandstrom, Karin; Chen, C.-H. Rosie

    2016-01-01

    With their high star formation rate surface densities and low metallicities, blue compact dwarf galaxies represent one of the most extreme environments for star formation in the local universe: one more akin to that found in high redshift galaxies than in local spirals. Until the advent of ALMA, however, the molecular gas fueling the prodigious star formation in blue compact dwarfs was difficult to observe because these galaxies generally have weak CO emission. In this talk, I present the first detailed study of the dust and molecular gas content (as traced by CO) in the prototypical nearby blue compact dwarf galaxy II Zw 40. Using the extraordinary resolution and sensitivity of our ALMA Cycle 1 observations, we have separated the molecular gas emission into discrete GMC-sized clumps and measured their properties. The clouds within II Zw 40 have high linewidths for their size, reflecting the greater turbulence within II Zw40. However, despite their large linewidths, these clouds are largely still in virial equilibrium. Comparing the virial masses of the clouds to their CO luminosities, we find that the CO to molecular gas conversion factor within this galaxy is at least 5 times that of the Milky Way and possibly as high as 35 times. Even with the maximum CO-to-molecular gas conversion factor for this galaxy, we find that the star formation efficiency is still at least a factor of 3 higher than solar metallicity systems.

  4. IMAGING FAINT BROWN DWARF COMPANIONS CLOSE TO BRIGHT STARS WITH A SMALL, WELL-CORRECTED TELESCOPE APERTURE

    SciTech Connect

    Serabyn, E.; Mawet, D.; Bloemhof, E.; Haguenauer, P.; Mennesson, B.; Wallace, K.; Hickey, J.

    2009-05-01

    We have used our 1.6 m diameter off-axis well-corrected subaperture (WCS) on the Palomar Hale telescope in concert with a small inner-working-angle phase-mask coronagraph to image the immediate environs of a small number of nearby stars. Test cases included three stars (HD 130948, HD 49197, and HR7672) with known brown dwarf companions at small separations, all of which were detected. We also present the initial detection of a new object close to the nearby young G0V star HD171488. Follow-up observations are needed to determine if this object is a bona fide companion, but its flux is consistent with the flux of a young brown dwarf or low-mass M star at the same distance as the primary. Interestingly, at small angles our WCS coronagraph demonstrates a limiting detectable contrast comparable to that of extant Lyot coronagraphs on much larger telescopes corrected with current-generation adaptive optics (AO) systems. This suggests that small apertures corrected to extreme AO (ExAO) levels can be used to carry out initial surveys for close brown dwarf and stellar companions, leaving follow-up observations for larger telescopes.

  5. A NEW CHEMICAL EVOLUTION MODEL FOR DWARF SPHEROIDAL GALAXIES BASED ON OBSERVED LONG STAR FORMATION HISTORIES

    SciTech Connect

    Homma, Hidetomo; Murayama, Takashi; Kobayashi, Masakazu A. R.; Taniguchi, Yoshiaki

    2015-02-01

    We present a new chemical evolution model for dwarf spheroidal galaxies (dSphs) in the local universe. Our main aim is to explain both their observed star formation histories and metallicity distribution functions simultaneously. Applying our new model for the four local dSphs, that is, Fornax, Sculptor, Leo II, and Sextans, we find that our new model reproduces the observed chemical properties of the dSphs consistently. Our results show that the dSphs have evolved with both a low star formation efficiency and a large gas outflow efficiency compared with the Milky Way, as suggested by previous works. Comparing the observed [α/Fe]-[Fe/H] relation of the dSphs with the model predictions, we find that our model favors a longer onset time of Type Ia supernovae (i.e., 0.5 Gyr) than that suggested in previous studies (i.e., 0.1 Gyr). We discuss the origin of this discrepancy in detail.

  6. New directions in microscopic physics of neutron star crusts and white dwarfs

    NASA Astrophysics Data System (ADS)

    Engstrom, Tyler; Crespi, Vincent; Owen, Benjamin

    2015-04-01

    Several aspects of solid-state physics pertaining to compact objects will be discussed. First, we report on properties of strongly magnetized outer envelopes of neutron stars, investigated within the nonlinear magnetic Thomas-Fermi model. In particular, we address the question of phonon thermal conductivity and the degree to which it limits the overall transport anisotropy arising from strongly anisotropic electron conductivity. Second, we have implemented the linear response Thomas-Fermi model in a genetic crystal structure prediction code, and describe a global search of structure and composition for multi-component, accreted NS crusts and Fe-enriched C-O white dwarfs. As part of this work, we have developed a new, self-consistent method for combining the phase stability calculation with the stellar structure calculation. Time permitting, we will show results from grain growth simulation in neutron star crusts, using a phase field method with the linear response Thomas-Fermi model. Connections to astrophysical observables will be emphasized throughout.

  7. The red dwarf pair GJ65 AB: inflated, spinning twins of Proxima. Fundamental parameters from PIONIER, NACO, and UVES observations

    NASA Astrophysics Data System (ADS)

    Kervella, P.; Mérand, A.; Ledoux, C.; Demory, B.-O.; Le Bouquin, J.-B.

    2016-10-01

    The nearby red dwarf binary GJ65 AB (UV+BL Ceti, M5.5Ve+M6Ve) is a cornerstone system to probe the physics of very low-mass stars. The radii of the two stars are currently known only from indirect photometric estimates, however, and this prevents us from using GJ65 AB as calibrators for the mass-radius (M-R) relation. We present new interferometric measurements of the angular diameters of the two components of GJ65 with the VLTI/PIONIER instrument in the near-infrared H band: θUD(A) = 0.558 ± 0.008 ± 0.020 mas and θUD(B) = 0.539 ± 0.009 ± 0.020 mas. They translate into limb-darkened angular diameters of θLD(A) = 0.573 ± 0.021 mas and θLD(B) = 0.554 ± 0.022 mas. Based on the known parallax, the linear radii are R(A) = 0.165 ± 0.006 R⊙ and R(B) = 0.159 ± 0.006 R⊙ (σ(R) /R = 4%). We searched for the signature of flares and faint companions in the interferometric visibilities and closure phases, but we did not identify any significant signal. We also observed GJ65 with the VLT/NACO adaptive optics and refined the orbital parameters and infrared magnitudes of the system. We derived masses for the two components of m(A) = 0.1225 ± 0.0043 M⊙ and m(B) = 0.1195 ± 0.0043 M⊙ (σ(m) /m = 4%). To derive the radial and rotational velocities of the two stars as well as their relative metallicity with respect to Proxima, we also present new individual UVES high-resolution spectra of the two components. Placing GJ65 A and B in the mass-radius diagram shows that their radii exceed expectations from recent models by 14 ± 4% and 12 ± 4%, respectively. Following previous theories, we propose that this discrepancy is caused by the inhibition of convective energy transport by a strong internal magnetic field generated by dynamo effect in these two fast-rotating stars. A comparison with the almost identical twin Proxima, which is rotating slowly, strengthens this hypothesis because the radius of Proxima does not appear to be inflated compared to models. Based on

  8. Carbon Shell or Core Ignitions in White Dwarfs Accreting from Helium Stars

    NASA Astrophysics Data System (ADS)

    Brooks, Jared; Bildsten, Lars; Schwab, Josiah; Paxton, Bill

    2016-04-01

    White dwarfs accreting from helium stars can stably burn at the accreted rate and avoid the challenge of mass loss associated with unstable helium burning that is a concern for many SNe Ia scenarios. We study binaries with helium stars of mass 1.25{M}⊙ ≤slant {M}{{He}}≤slant 1.8{M}⊙ , which have lost their hydrogen rich envelopes in an earlier common envelope event and now orbit with periods ({P}{{orb}}) of several hours with non-rotating 0.84 and 1.0{M}⊙ C/O WDs. The helium stars fill their Roche lobes after exhaustion of central helium and donate helium on their thermal timescales (˜ {10}5 years). As shown by others, these mass transfer rates coincide with the steady helium burning range for WDs, and grow the WD core up to near the Chandrasekhar mass ({M}{{Ch}}) and a core carbon ignition. We show here, however, that many of these scenarios lead to an ignition of hot carbon ashes near the outer edge of the WD and an inward going carbon flame that does not cause an explosive outcome. For {P}{{orb}}=3 hr, 1.0{M}⊙ C/O WDs with donor masses {M}{{He}}≳ 1.8{M}⊙ experience a shell carbon ignition, while {M}{{He}}≲ 1.3{M}⊙ will fall below the steady helium burning range and undergo helium flashes before reaching core C ignition. Those with 1.3{M}⊙ ≲ {M}{{He}}≲ 1.7{M}⊙ will experience a core C ignition. We also calculate the retention fraction of accreted helium when the accretion rate leads to recurrent weak helium flashes.

  9. Spectroscopic study of extended star clusters in dwarf galaxy NGC 6822

    SciTech Connect

    Hwang, Narae; Kim, Sang Chul; Park, Hong Soo; Lee, Myung Gyoon; Lim, Sungsoon; Hodge, Paul W.; Weisz, Daniel; Miller, Bryan

    2014-03-01

    We present a spectroscopic study of the four extended star clusters (ESCs) in NGC 6822 based on the data obtained with the Gemini Multi-Object Spectrograph on the Gemini-South 8.1 m telescope. The radial velocities derived from the spectra range from –61.2 ± 20.4 km s{sup –1} (for C1) to –115.34 ± 57.9 km s{sup –1} (for C4) and, unlike the intermediate-age carbon stars, they do not display any sign of systematic rotation around NGC 6822. The ages and metallicities derived using the Lick indices show that the ESCs are old (≥8 Gyr) and metal poor ([Fe/H] ≲ –1.5). NGC 6822 is found to have both metal poor ([Fe/H] ≈–2.0) and metal rich ([Fe/H] ≈–0.9) star clusters within 15' (2 kpc) from the center, whereas only metal poor clusters are observed in the outer halo with r ≥ 20'(2.6 kpc). The kinematics, old ages, and low metallicities of ESCs suggest that ESCs may have accreted into the halo of NGC 6822. Based on the velocity distribution of ESCs, we have determined the total mass and the mass-to-light ratio of NGC 6822: M{sub N6822}=7.5{sub −0.1}{sup +4.5}×10{sup 9} M{sub ⊙} and (M/L){sub N6822}=75{sub −1}{sup +45}(M/L){sub ⊙}. It shows that NGC 6822 is one of the most dark matter dominated dwarf galaxies in the Local Group.

  10. The star formation histories of local group dwarf galaxies. I. Hubble space telescope/wide field planetary camera 2 observations

    SciTech Connect

    Weisz, Daniel R.; Dolphin, Andrew E.; Skillman, Evan D.; Holtzman, Jon; Gilbert, Karoline M.; Dalcanton, Julianne J.; Williams, Benjamin F.

    2014-07-10

    We present uniformly measured star formation histories (SFHs) of 40 Local Group (LG) dwarf galaxies based on color-magnitude diagram (CMD) analysis from archival Hubble Space Telescope imaging. We demonstrate that accurate SFHs can be recovered from CMDs that do not reach the oldest main sequence turn-off (MSTO), but emphasize that the oldest MSTO is critical for precisely constraining the earliest epochs of star formation. We find that: (1) the average lifetime SFHs of dwarf spheroidals (dSphs) can be approximated by an exponentially declining SFH with τ ∼ 5 Gyr; (2) lower luminosity dSphs are less likely to have extended SFHs than more luminous dSphs; (3) the average SFHs of dwarf irregulars (dIrrs), transition dwarfs, and dwarf ellipticals can be approximated by the combination of an exponentially declining SFH (τ ∼ 3-4 Gyr) for lookback ages >10-12 Gyr ago and a constant SFH thereafter; (4) the observed fraction of stellar mass formed prior to z = 2 ranges considerably (80% for galaxies with M < 10{sup 5} M{sub ☉} to 30% for galaxies with M > 10{sup 7} M{sub ☉}) and is largely explained by environment; (5) the distinction between 'ultra-faint' and 'classical' dSphs is arbitrary; (6) LG dIrrs formed a significantly higher fraction of stellar mass prior to z = 2 than the Sloan Digital Sky Survey galaxies from Leitner and the SFHs from the abundance matching models of Behroozi et al. This may indicate higher than expected star formation efficiencies at early times in low mass galaxies. Finally, we provide all the SFHs in tabulated electronic format for use by the community.

  11. The Star Formation Histories of Local Group Dwarf Galaxies. I. Hubble Space Telescope/Wide Field Planetary Camera 2 Observations

    NASA Astrophysics Data System (ADS)

    Weisz, Daniel R.; Dolphin, Andrew E.; Skillman, Evan D.; Holtzman, Jon; Gilbert, Karoline M.; Dalcanton, Julianne J.; Williams, Benjamin F.

    2014-07-01

    We present uniformly measured star formation histories (SFHs) of 40 Local Group (LG) dwarf galaxies based on color-magnitude diagram (CMD) analysis from archival Hubble Space Telescope imaging. We demonstrate that accurate SFHs can be recovered from CMDs that do not reach the oldest main sequence turn-off (MSTO), but emphasize that the oldest MSTO is critical for precisely constraining the earliest epochs of star formation. We find that: (1) the average lifetime SFHs of dwarf spheroidals (dSphs) can be approximated by an exponentially declining SFH with τ ~ 5 Gyr (2) lower luminosity dSphs are less likely to have extended SFHs than more luminous dSphs; (3) the average SFHs of dwarf irregulars (dIrrs), transition dwarfs, and dwarf ellipticals can be approximated by the combination of an exponentially declining SFH (τ ~ 3-4 Gyr) for lookback ages >10-12 Gyr ago and a constant SFH thereafter; (4) the observed fraction of stellar mass formed prior to z = 2 ranges considerably (80% for galaxies with M < 105 M ⊙ to 30% for galaxies with M > 107 M ⊙) and is largely explained by environment; (5) the distinction between "ultra-faint" and "classical" dSphs is arbitrary; (6) LG dIrrs formed a significantly higher fraction of stellar mass prior to z = 2 than the Sloan Digital Sky Survey galaxies from Leitner and the SFHs from the abundance matching models of Behroozi et al. This may indicate higher than expected star formation efficiencies at early times in low mass galaxies. Finally, we provide all the SFHs in tabulated electronic format for use by the community. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the Data Archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.

  12. A bag of tricks: Using proper motions of Galactic stars to identify the Hercules ultra-faint dwarf galaxy members

    NASA Astrophysics Data System (ADS)

    Fabrizio, M.; Raimondo, G.; Brocato, E.; Bellini, A.; Libralato, M.; Testa, V.; Cantiello, M.; Musella, I.; Clementini, G.; Carini, R.; Marconi, M.; Piotto, G.; Ripepi, V.; Buonanno, R.; Sani, E.; Speziali, R.

    2014-10-01

    Context. Discovered in the last decade as overdensities of resolved stars, the ultra-faint dwarfs (UFDs) are among the least luminous, most dark-matter dominated, and most metal-poor galaxies known today. They appear as sparse, loose objects with high mass-to-light ratios. Hercules is the prototype of the UFD galaxies. To date, there are still no firm constraints on its total luminosity due to the difficulty of disentangling Hercules bona-fide stars from the severe Galactic field contamination. Aims: To better constrain Hercules properties, we aim at removing foreground and background contaminants in the galaxy field using the proper motions of the Milky Way stars and the colour-colour diagram. Methods: We have obtained images of Hercules in the rSloan , BBessel and Uspec bands with the Large Binocular Telescope (LBT) and LBC-BIN mode capabilities. The rSloan new dataset combined with data from the LBT archive span a time baseline of about 5 yr, allowing us to measure proper motions of stars in the Hercules direction for the first time. The Uspec data along with existing LBT photometry allowed us to use colour-colour diagram to further remove the field contamination. Results: Thanks to a highly-accurate procedure to derive the rSloan -filter geometric distortion solution for the LBC-red, we were able to measure stellar relative proper motions to a precision of better than 5 mas yr-1 down to rSloan≃ 22 mag and disentangle a significant fraction (>90%) of Milky Way contaminants. We ended up with a sample of 528 sources distributed over a large portion of the galaxy body (~0.12 deg2). Of these sources, 171 turned out to be background galaxies and additional foreground stars from the analysis of the Uspec - BBessel vs. BBessel - rSloan colour-colour diagram. This leaves us with a sample of 357 likely members of the Hercules UFD. We compared the cleaned colour-magnitude diagram (CMD) with evolutionary models and synthetic CMDs, confirming the presence in Hercules of

  13. Detection of a red supergiant progenitor star of a type II-plateau supernova.

    PubMed

    Smartt, Stephen J; Maund, Justyn R; Hendry, Margaret A; Tout, Christopher A; Gilmore, Gerard F; Mattila, Seppo; Benn, Chris R

    2004-01-23

    We present the discovery of a red supergiant star that exploded as supernova 2003gd in the nearby spiral galaxy M74. The Hubble Space Telescope (HST) and the Gemini Telescope imaged this galaxy 6 to 9 months before the supernova explosion, and subsequent HST images confirm the positional coincidence of the supernova with a single resolved star that is a red supergiant of 8(+4)(-2) solar masses. This confirms both stellar evolution models and supernova theories predicting that cool red supergiants are the immediate progenitor stars of type II-plateau supernovae.

  14. Detection of a red supergiant progenitor star of a type II-plateau supernova.

    PubMed

    Smartt, Stephen J; Maund, Justyn R; Hendry, Margaret A; Tout, Christopher A; Gilmore, Gerard F; Mattila, Seppo; Benn, Chris R

    2004-01-23

    We present the discovery of a red supergiant star that exploded as supernova 2003gd in the nearby spiral galaxy M74. The Hubble Space Telescope (HST) and the Gemini Telescope imaged this galaxy 6 to 9 months before the supernova explosion, and subsequent HST images confirm the positional coincidence of the supernova with a single resolved star that is a red supergiant of 8(+4)(-2) solar masses. This confirms both stellar evolution models and supernova theories predicting that cool red supergiants are the immediate progenitor stars of type II-plateau supernovae. PMID:14739452

  15. Follow-up spectroscopic observations of HD 107148 B: A new white dwarf companion of an exoplanet host star

    NASA Astrophysics Data System (ADS)

    Mugrauer, M.; Dinçel, B.

    2016-07-01

    We report on our follow-up spectroscopy of HD 1071478 B, a recently detected faint co-moving companion of the exoplanet host star HD 107148 A. The companion is separated from its primary star by about 35 arcsec (or 1790 AU of projected separation) and its optical and near infrared photometry is consistent with a white dwarf, located at the distance of HD 107148 A. In order to confirm the white dwarf nature of the co-moving companion, we obtained follow-up spectroscopic observations of HD 107148 B with CAFOS at the CAHA 2.2 m telescope. According to our CAFOS spectroscopy HD 107148 B is a DA white dwarf with an effective temperature in the range between 5900 and 6400 K. The properties of HD 107148 B can further be constrained with the derived effective temperature and the known visual and infrared photometry of the companion, using evolutionary models of DA white dwarfs. We obtain for HD 107148 B a mass of 0.56±0.05 M_⊙, a luminosity of (2.0±0.2)×10-4 L_⊙, log g [cm s-2])=7.95±0.09, and a cooling age of 2100±270 Myr. With its white dwarf companion the exoplanet host star HD 107148 A forms an evolved stellar system, which hosts at least one exoplanet. So far, only few of these evolved systems are known, which represent only about 5 % of all known exoplanet host multiple stellar systems. HD 107148 B is the second confirmed white dwarf companion of an exoplanet host star with a projected separation to its primary star of more than 1000 AU. Based on observations collected at the Centro Astronómico Hispano Alemán (CAHA) at Calar Alto, operated jointly by the Max-Planck Institut für Astronomie and the Instituto de Astrofísica de Andalucía (CSIC).

  16. Follow-up spectroscopic observations of HD 107148 B: A new white dwarf companion of an exoplanet host star

    NASA Astrophysics Data System (ADS)

    Mugrauer, M.; Dinçel, B.

    2016-07-01

    We report on our follow-up spectroscopy of HD 1071478 B, a recently detected faint co-moving companion of the exoplanet host star HD 107148 A. The companion is separated from its primary star by about 35 arcsec (or 1790 AU of projected separation) and its optical and near infrared photometry is consistent with a white dwarf, located at the distance of HD 107148 A. In order to confirm the white dwarf nature of the co-moving companion, we obtained follow-up spectroscopic observations of HD 107148 B with CAFOS at the CAHA 2.2 m telescope. According to our CAFOS spectroscopy HD 107148 B is a DA white dwarf with an effective temperature in the range between 5900 and 6400 K. The properties of HD 107148 B can further be constrained with the derived effective temperature and the known visual and infrared photometry of the companion, using evolutionary models of DA white dwarfs. We obtain for HD 107148 B a mass of 0.56±0.05 M_ȯ, a luminosity of (2.0±0.2)×10-4 L_ȯ, log g [cm s-2])=7.95±0.09, and a cooling age of 2100±270 Myr. With its white dwarf companion the exoplanet host star HD 107148 A forms an evolved stellar system, which hosts at least one exoplanet. So far, only few of these evolved systems are known, which represent only about 5 % of all known exoplanet host multiple stellar systems. HD 107148 B is the second confirmed white dwarf companion of an exoplanet host star with a projected separation to its primary star of more than 1000 AU. Based on observations collected at the Centro Astronómico Hispano Alemán (CAHA) at Calar Alto, operated jointly by the Max-Planck Institut für Astronomie and the Instituto de Astrofísica de Andalucía (CSIC).

  17. A HERSCHEL SURVEY OF COLD DUST IN DISKS AROUND BROWN DWARFS AND LOW-MASS STARS

    SciTech Connect

    Harvey, Paul M.; Evans, Neal J. II; Henning, Thomas; Liu Yao; Wolf, Sebastian; Menard, Francois; Pinte, Christophe; Pascucci, Ilaria E-mail: nje@astro.as.utexas.edu E-mail: wolf@astrophysik.uni-kiel.de E-mail: yliu@pmo.ac.cn E-mail: christophe.pinte@obs.ujf-grenoble.fr E-mail: pascucci@lpl.arizona.edu

    2012-08-10

    We report the complete photometric results from our Herschel study which is the first comprehensive program to search for far-infrared emission from cold dust around young brown dwarfs (BDs). We surveyed 50 fields containing 51 known or suspected BDs and very low mass stars that have evidence of circumstellar disks based on Spitzer photometry and/or spectroscopy. The objects with known spectral types range from M3 to M9.5. Four of the candidates were subsequently identified as extragalactic objects. Of the remaining 47 we have successfully detected 36 at 70 {mu}m and 14 at 160 {mu}m with signal-to-noise ratio (S/N) greater than 3, as well as several additional possible detections with low S/N. The objects exhibit a range of [24]-[70] {mu}m colors suggesting a range in mass and/or structure of the outer disk. We present modeling of the spectral energy distributions of the sample and discuss trends visible in the data. Using two Monte Carlo radiative transfer codes we investigate disk masses and geometry. We find a very wide range in modeled total disk masses from less than 10{sup -6} M{sub Sun} up to 10{sup -3} M{sub Sun} with a median disk mass of the order of 3 Multiplication-Sign 10{sup -5} M{sub Sun }, suggesting that the median ratio of disk mass to central object mass may be lower than for T Tauri stars. The disk scale heights and flaring angles, however, cover a range consistent with those seen around T Tauri stars. The host clouds in which the young BDs and low-mass stars are located span a range in estimated age from {approx}1-3 Myr to {approx}10 Myr and represent a variety of star-forming environments. No obvious dependence on cloud location or age is seen in the disk properties, though the statistical significance of this conclusion is not strong.

  18. BRIGHT ULTRAVIOLET REGIONS AND STAR FORMATION CHARACTERISTICS IN NEARBY DWARF GALAXIES

    SciTech Connect

    Melena, Nicholas W.; Hunter, Deidre A.; Zernow, Lea; Elmegreen, Bruce G. E-mail: dah@lowell.edu E-mail: bge@us.ibm.com

    2009-11-15

    We compare star formation in the inner and outer disks of 11 dwarf irregular galaxies (dIm) within 3.6 Mpc. The regions are identified on Galaxy Evolution Explorer near-UV images, and modeled with UV, optical, and near-IR colors to determine masses and ages. A few galaxies have made 10{sup 5}-10{sup 6} M {sub sun} complexes in a starburst phase, while others have not formed clusters in the last 50 Myr. The maximum region mass correlates with the number of regions as expected from the size-of-sample effect. We find no radial gradients in region masses and ages, even beyond the realm of H{alpha} emission, although there is an exponential decrease in the luminosity density and number density of the regions with radius. H{alpha} is apparently lacking in the outer parts only because nebular emission around massive stars is too faint to see. The outermost regions for the five galaxies with H I data formed at average gas surface densities of 1.9-5.9 M {sub sun} pc{sup -2}. These densities are at the low end of commonly considered thresholds for star formation and imply either that local gas densities are higher before star formation begins or subthreshold star formation is possible. The first case could be explained by supernovae triggering and other local processes, while the second case could be explained by gravitational instabilities with angular momentum loss in growing condensations. The distribution of regions on a log(mass) - log(age) plot is examined. The distribution is usually uniform along log(age) for equal intervals of log(mass) and this implies a region count that varies as 1/age. This variation results from either an individual region mass that varies as 1/age or a region disruption probability that varies as 1/age. A correlation between fading-corrected surface brightness and age suggests the former. The implied loss of mass is from fading of region envelopes below the surface brightness limit.

  19. NEW M, L, AND T DWARF COMPANIONS TO NEARBY STARS FROM THE WIDE-FIELD INFRARED SURVEY EXPLORER

    SciTech Connect

    Luhman, Kevin L.; Loutrel, Nicholas P.; McCurdy, Nicholas S.; Melso, Nicole D.; Star, Kimberly M.; Terrien, Ryan C.; Mace, Gregory N.; McLean, Ian S.; Young, Michael D.; Rhode, Katherine L.; Davy Kirkpatrick, J.

    2012-12-01

    We present 11 candidate late-type companions to nearby stars identified with data from the Wide-field Infrared Survey Explorer (WISE) and the Two Micron All Sky Survey (2MASS). Eight of the candidates are likely to be companions based on their common proper motions with the primaries. The remaining three objects are rejected as companions, one of which is a free-floating T7 dwarf. Spectral types are available for five of the companions, which consist of M2V, M8.5V, L5, T8, and T8. Based on their photometry, the unclassified companions are probably two mid-M dwarfs and one late-M/early-L dwarf. One of the T8 companions, WISE J142320.84+011638.0, has already been reported by Pinfield and coworkers. The other T8 companion, ULAS J095047.28+011734.3, was discovered by Burningham and coworkers through the United Kingdom Infrared Telescope Infrared Deep Sky Survey, but its companionship has not been previously recognized in the literature. The L5 companion, 2MASS J17430860+8526594, is a new member of a class of L dwarfs that exhibit unusually blue near-IR colors. Among the possible mechanisms that have been previously proposed for the peculiar colors of these L dwarfs, low metallicity does not appear to be a viable explanation for 2MASS J17430860+8526594 since our spectrum of the primary suggests that its metallicity is not significantly subsolar.

  20. Radial velocities of very low mass stars and candidate brown dwarf members of the Hyades and Pleiades

    NASA Technical Reports Server (NTRS)

    Stauffer, John R.; Liebert, James; Giampapa, Mark; Macintosh, Bruce; Reid, Neill; Hamilton, Donald

    1994-01-01

    We have determined H alpha equivalent widths and radial velocities with 1 sigma accuracies of approximately 5 km s(exp -1) for approximately 20 candidate very low mass members of the Hyades and Pleiades clusters. The radial velocities for the Hyades sample suggest that nearly all of these stars are indeed highly probable members of the Hyades. The faintest stars in the Hyades sample have masses of order 0.1 solar mass. We also obtained radial velocities for four candidate very low mass members of the Pleiades and two objects that are candidate BD Pleiads. All of these stars have apparent V magnitudes fainter than the Hyades stars we observed, and the resultant radial velocity accuracy is worse. We believe that the three brighter stars are indeed likely very low mass stellar members of the Pleiades, whereas the status of the two brown dwarf candidates is uncertain. The Hyades stars we have observed and the three Pleiades very low mass stars are the lowest mass members of any open cluster whose membership has been confirmed by radial velocities and whose chromospheric activity has been measured. We see no change in chromospheric activity at the boundary where stars are expected to become fully convective (M approximately equals 0.3 solar mass) in either cluster. In the Pleiades, however, there may be a decrease in chromospheric activity for stars with (V-I)(sub K) greater than 3.5 (M less than or equal to 0.1 solar mass).

  1. Numerical simulations of bubble-induced star formation in dwarf irregular galaxies with a novel stellar feedback scheme

    NASA Astrophysics Data System (ADS)

    Kawata, Daisuke; Gibson, Brad K.; Barnes, David J.; Grand, Robert J. J.; Rahimi, Awat

    2014-02-01

    To study the star formation and feedback mechanism, we simulate the evolution of an isolated dwarf irregular galaxy (dIrr) in a fixed dark matter halo, similar in size to Wolf-Lundmark-Melotte, using a new stellar feedback scheme. We use the new version of our original N-body/smoothed particle chemodynamics code, GCD+, which adopts improved hydrodynamics, metal diffusion between the gas particles and new modelling of star formation and stellar wind and supernovae feedback. Comparing the simulations with and without stellar feedback effects, we demonstrate that the collisions of bubbles produced by strong feedback can induce star formation in a more widely spread area. We also demonstrate that the metallicity in star-forming regions is kept low due to the mixing of the metal-rich bubbles and the metal-poor interstellar medium. Our simulations also suggest that the bubble-induced star formation leads to many counter-rotating stars. The bubble-induced star formation could be a dominant mechanism to maintain star formation in dIrrs, which is different from larger spiral galaxies where the non-axisymmetric structures, such as spiral arms, are a main driver of star formation.

  2. Direct measurements of the fundamental properties of low-mass stars and brown dwarfs

    NASA Astrophysics Data System (ADS)

    Dupuy, Trent J.

    2010-10-01

    Detailed theoretical models of stars, developed and observationally tested over the last century, now underlie most of modern astronomy. In contrast, models of lower temperature objects, brown dwarfs and gas giant planets, have only recently been developed and remained largely unconstrained by observations. Despite this lack of empirical validation, these models have become entrenched in many active areas of astronomical research, and thus rigorously testing them is imperative. Dynamical masses from visual binaries are central to this effort, but such measurements have been previously been impeded by observational limitations (ultracool binaries are faint, and their orbital separations are very small). This dissertation presents results from our program to test models using precise dynamical masses (as good as 2%) for ultracool binaries, based on infrared parallaxes, near-infrared spectroscopy, and Keck laser guide star adaptive optics astrometry for a sample of over 30 objects. In just the last 2 years, we have more than tripled the number of ultracool binaries with dynamical masses, extending these measurements to much lower temperatures in previously unexplored areas of parameter space. Our main results are summarized as follows: (1) For most field binaries, based on direct measurements of their luminosities and masses, we find that the temperatures predicted by evolutionary models are discrepant with those derived from fitting the observed spectra with model atmospheres, indicating systematic errors of [approximate]200 K in temperature (or 15%-20% in radius). We have also devised alternative model tests for these systems using their space motion and chromospheric activity to constrain their ages. (2) For the only field binary with an independent age determination from the solar-type primary in its hierarchical triple system (from age-activity-rotation relations), we find that evolutionary models systematically underpredict luminosities by a factor of

  3. Studies of Evolved Star Mass Loss: GRAMS Modeling of Red Supergiant and Asymptotic Giant Branch Stars in the Magellanic Clouds

    NASA Astrophysics Data System (ADS)

    Sargent, Benjamin A.; Srinivasan, S.; Riebel, D.; Boyer, M.; Meixner, M.

    2012-01-01

    As proposed in our NASA Astrophysics Data Analysis Program (ADAP) proposal, my colleagues and I are studying mass loss from evolved stars. Such stars lose their own mass in their dying stages, and in their expelled winds they form stardust. To model mass loss from these evolved stars, my colleagues and I have constructed GRAMS: the Grid of Red supergiant and Asymptotic giant branch star ModelS. These GRAMS radiative transfer models are fit to optical through mid-infrared photometry of red supergiant (RSG) stars and asymptotic giant branch (AGB) stars. I will discuss our current studies of mass loss from AGB and RSG stars in the Small Magellanic Cloud (SMC), fitting GRAMS models to the photometry of SMC evolved star candidates identified from the SAGE-SMC (PI: K. Gordon) Spitzer Space Telescope Legacy survey. This work will be briefly compared to similar work we have done for the LMC. I will also discuss Spitzer Infrared Spectrograph (IRS) studies of the dust produced by AGB and RSG stars in the LMC. BAS is grateful for support from the NASA-ADAP grant NNX11AB06G.

  4. X-ray survey of hot white dwarf stars - evidence for a m(He)/n(H) versus Teff correlation

    SciTech Connect

    Petre, R.; Shipman, H.L.; Canizares, C.R.

    1986-05-01

    Observations of 13 white dwarf and subdwarf stars using the Einstein Observatory High Resolution Image are reported. Included are stars of classes DA, DB, DAV, sDO, and sDB, with optically determined effective temperatures in the range 10,000-60,000 K. X-ray emission was detected from two of the 13: the very hot (55,000 K) DA1 star WD 2309 + 105 (= EG 233), with a count rate one-fifth that of HZ 43, and the relatively cool (26,000 K) DA3 star WD 1052 - 273 (=GD 125). The effective temperatures determined from ultraviolet and optical observations were used to place limits on the He content of the white dwarf photospheres, presuming that trace photospheric He is the missing opacity source which quenches the thermal X-rays in these stars. When presently obtained results were combined with those available from the literature evidence was found for a correlation between Teff and n(He)/n(H), in which HZ 43 is a conspicuous exception to the general trend. Both this correlation and the exceptional behavior of HZ 43 are qualitatively accounted for by a radiative acceleration model, in which the rate of upward movement of the He is a function of temperature and surface gravity 59 references.

  5. Significance of brown dwarfs

    NASA Technical Reports Server (NTRS)

    Black, D. C.

    1986-01-01

    The significance of brown dwarfs for resolving some major problems in astronomy is discussed. The importance of brown dwarfs for models of star formation by fragmentation of molecular clouds and for obtaining independent measurements of the ages of stars in binary systems is addressed. The relationship of brown dwarfs to planets is considered.

  6. 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.

  7. Double White Dwarfs as Probes of Single and Binary Star Evolution

    NASA Astrophysics Data System (ADS)

    Andrews, Jeffrey John

    2016-01-01

    As the endpoints of stars less massive than roughly eight solar masses, the population of Galactic white dwarfs (WD) contain information about complex stellar evolution processes. Associated pairs of WDs add an extra degree of leverage; both WDs must have formed and evolved together. The work presented in this dissertation uses various populations of double WDs (DWD) to constrain evolution of both single and binary stars. One example is the set of low-mass WDs with unseen WD companions, which are formed through a dynamically-unstable mass loss process called the common envelope. To work toward a quantitative understanding of the common envelope, we develop and apply a Bayesian statistical technique to identify the masses of the unseen WD companions. We provide results which can be compared to evolutionary models and hence a deeper understanding of how binary stars evolve through a common envelope. The statistical technique we develop can be applied to any population of single-line spectroscopic binaries. Binaries widely separated enough that they avoid any significant interaction independently evolve into separate WDs that can be identified in photometric and astrometric surveys. We discuss techniques for finding these objects, known as wide DWDs. We present a catalog of 142 candidate wide DWDs, combining both previously detected systems and systems we identify in our searches in the Sloan Digital Sky Survey. Having been born at the same time, the masses and cooling ages of the WDs in wide DWDs, obtained with our spectroscopic follow-up campaign can be used to constrain the initial-final mass relation, which relates a main sequence star to the mass of the WD into which it will evolve. We develop a novel Bayesian technique to interpret our data and present our resulting constraints on this relation which are particularly strong for initial masses between two and four solar masses. During this process, we identified one wide DWD, HS 2220+2146, that was peculiar since

  8. THE NATURE OF STARBURSTS. I. THE STAR FORMATION HISTORIES OF EIGHTEEN NEARBY STARBURST DWARF GALAXIES

    SciTech Connect

    McQuinn, Kristen B. W.; Skillman, Evan D.; Stark, David; Weisz, Daniel; Cannon, John M.; Dalcanton, Julianne; Williams, Benjamin; Dolphin, Andrew; Hidalgo-RodrIguez, Sebastian

    2010-09-20

    We use archival Hubble Space Telescope observations of resolved stellar populations to derive the star formation histories (SFHs) of 18 nearby starburst dwarf galaxies. In this first paper, we present the observations, color-magnitude diagrams (CMDs), and the SFHs of the 18 starburst galaxies, based on a homogeneous approach to the data reduction, differential extinction, and treatment of photometric completeness. We adopt a star formation rate (SFR) threshold normalized to the average SFR of the individual system as a metric for classifying starbursts in SFHs derived from resolved stellar populations. This choice facilitates finding not only the currently bursting galaxies but also 'fossil' bursts increasing the sample size of starburst galaxies in the nearby (D < 8 Mpc) universe. Thirteen of the eighteen galaxies are experiencing ongoing bursts and five galaxies show fossil bursts. From our reconstructed SFHs, it is evident that the elevated SFRs of a burst are sustained for hundreds of Myr with variations on small timescales. A long >100 Myr temporal baseline is thus fundamental to any starburst definition or identification method. The longer lived bursts rule out rapid 'self-quenching' of starbursts on global scales. The bursting galaxies' gas consumption timescales are shorter than the Hubble time for all but one galaxy confirming the short-lived nature of starbursts based on fuel limitations. Additionally, we find that the strength of the H{alpha} emission usually correlates with the CMD-based SFR during the last 4-10 Myr. However, in four cases, the H{alpha} emission is significantly less than what is expected for models of starbursts; the discrepancy is due to the SFR changing on timescales of a few Myr. The inherently short timescale of the H{alpha} emission limits identifying galaxies as starbursts based on the current characteristics which may or may not be representative of the recent SFH of a galaxy.

  9. Calibrating the Optical Luminosity of Red Clump Stars: An Archival Study of Star Clusters

    NASA Astrophysics Data System (ADS)

    Grocholski, Aaron

    2010-09-01

    The core helium burning stars of the red clump {RC} are a conspicuous feature in the color-magnitude diagram of many stellar populations. Its ease of identification, along with its relative brightness {M_I 0} make the RC a popular feature for HST studies of stellar populations in galaxies out to a few Mpc. Such studies generally interpret the data through comparison to theoretical isochrones. For accurate results, the theoretical predictions must be calibrated to match the RC properties of observed populations of known age and metallicity. However, no large scale studies of the luminosity of the RC currently exist in the optical bands. We propose to remedy this situation with an archival study of RC properties in star clusters in the Milky Way, LMC, and SMC. We will focus on HST images of globular clusters, but we will augment the sample with ground-based open cluster observations to extend the coverage of parameter space. The goal is to build a large and homogeneous database, through new analysis and incorporation of literature data, of cluster ages, abundances, distances, and RC photometry. This database will allow us to explore the variations in the RC luminosity as a function of age and [Fe/H] over the full range of parameter space where the RC exists, for both the V and I bands. The results will provide a fundamental calibration for all future HST studies of stellar populations and distances of nearby galaxies using the RC. They will also allow for verification or improvement of theoretical models for red giant phase evolution. This in turn will help many subjects, from stellar modeling to population synthesis and fitting of spectral energy distributions of distant galaxies.

  10. Lithium and Hα in stars and brown dwarfs of sigma Orionis.

    NASA Astrophysics Data System (ADS)

    Zapatero Osorio, M. R.; Béjar, V. J. S.; Pavlenko, Ya.; Rebolo, R.; Allende Prieto, C.; Martín, E. L.; García López, R. J.

    2002-03-01

    We present intermediate- and low-resolution optical spectra around Hα and Li I lambda 6708 Åfor a sample of 25 low mass stars and 2 brown dwarfs with confirmed membership in the pre-main sequence stellar sigma Orionis cluster. Our observations are intended to investigate the age of the cluster. The spectral types derived for our target sample are found to be in the range K6-M8.5, which corresponds to a mass interval of roughly 1.2-0.02 Msun on the basis of state-of-the-art evolutionary models. Radial velocities (except for one object) are found to be consistent with membership in the Orion complex. All cluster members show considerable Hα emission and the Li I resonance doublet in absorption, which is typical of very young ages. We find that our pseudo-equivalent widths of Hα and Li I (measured relative to the observed local pseudo-continuum formed by molecular absorptions) appear rather dispersed (and intense in the case of Hα ) for objects cooler than M3.5 spectral class, occurring at the approximate mass where low mass stars are expected to become fully convective. The least massive brown dwarf in our sample, S Ori 45 (M8.5, ~ 0.02 Msun), displays variable Hα emission and a radial velocity that differs from the cluster mean velocity. Tentative detection of forbidden lines in emission indicates that this brown dwarf may be accreting mass from a surrounding disk. We also present recent computations of Li I lambda 6708 Åcurves of growth for low gravities and for the temperature interval (about 4000-2600 K) of our sample. The comparison of our observations to these computations allows us to infer that no lithium depletion has yet taken place in sigma Orionis, and that the observed pseudo-equivalent widths are consistent with a cluster initial lithium abundance close to the cosmic value. Hence, the upper limit to the sigma Orionis cluster age can be set at 8 Myr, with a most likely value around 2-4 Myr. Based on observations made with the following telescopes

  11. Qatar-1b: a hot Jupiter orbiting a metal-rich K dwarf star

    NASA Astrophysics Data System (ADS)

    Alsubai, K. A.; Parley, N. R.; Bramich, D. M.; West, R. G.; Sorensen, P. M.; Collier Cameron, A.; Latham, D. W.; Horne, K.; Anderson, D. R.; Bakos, G. Á.; Brown, D. J. A.; Buchhave, L. A.; Esquerdo, G. A.; Everett, M. E.; Fżrész, G.; Hartman, J. D.; Hellier, C.; Miller, G. M.; Pollacco, D.; Quinn, S. N.; Smith, J. C.; Stefanik, R. P.; Szentgyorgyi, A.

    2011-10-01

    We report the discovery and initial characterization of Qatar-1b, a hot Jupiter-orbiting metal-rich K dwarf star, the first planet discovered by the Qatar Exoplanet Survey. We describe the strategy used to select candidate transiting planets from photometry generated by the Qatar Exoplanet Survey camera array. We examine the rate of astrophysical and other false positives found during the spectroscopic reconnaissance of the initial batch of candidates. A simultaneous fit to the follow-up radial velocities and photometry of Qatar-1b yields a planetary mass of 1.09 ± 0.08 MJ and a radius of 1.16 ± 0.05 RJ. The orbital period and separation are 1.420 033 ± 0.000 016 d and 0.023 43 ± 0.000 26 au for an orbit assumed to be circular. The stellar density, effective temperature and rotation rate indicate an age greater than 4 Gyr for the system.

  12. A pulsation search among young brown dwarfs and very-low-mass stars

    SciTech Connect

    Cody, Ann Marie; Hillenbrand, Lynne A.

    2014-12-01

    In 2005, Palla and Baraffe proposed that brown dwarfs (BDs) and very-low-mass stars (VLMSs; < 0.1 solar masses) may be unstable to radial oscillations during the pre-main-sequence deuterium burning phase. With associated periods of one to four hours, this potentially new class of pulsation offers unprecedented opportunities to probe the interiors and evolution of low-mass objects in the 1-15 million year age range. Following up on reports of short-period variability in young clusters, we designed a high-cadence photometric monitoring campaign to search for deuterium-burning pulsation among a sample of 348 BDs and VLMSs in the four young clusters σ Orionis, Chamaeleon I, IC 348, and Upper Scorpius. In the resulting light curves we achieved sensitivity to periodic signals of amplitude several millimagnitudes, on timescales from 15 minutes to two weeks. Despite the exquisite data quality, we failed to detect any periodicities below seven hours. We conclude that D-burning pulsations are not able to grow to observable amplitudes in the early pre-main sequence. In spite of the nondetection, we did uncover a rich set of variability behavior—both periodic and aperiodic—on day to week timescales. We present new compilations of variable sources from our sample, as well as three new candidate cluster members in Chamaeleon I.

  13. Confronting predictions of stellar evolution theory: the case of single field M dwarf stars

    NASA Astrophysics Data System (ADS)

    Feiden, Gregory A.; Mann, Andrew W.; Gaidos, Eric

    2015-01-01

    Using a homogenous sample of single field M dwarf stars from the CONCH-SHELL catalog, we confront the reliability of predictions from low mass stellar evolution models. Empirical values for the bolometric flux, effective temperature, and stellar radius are typically determined with better than 1%, 2%, and 5% precision, respectively. Coupled with precise [M/H] values, these observations place strong constraints on the accuracy of stellar models. A Markov Chain Monte Carlo (MCMC) formalism is used to establish the most likely stellar properties, with associated uncertainties, by interpolating within a dense grid of Dartmouth stellar evolution models with mass, age, metallicity, and distance as free parameters. The observed effective temperature and bolometric flux are adopted as independent observables in the MCMC likelihood function with the addition of the observed [M/H] and distance as informative Bayesian priors. Results are presented comparing model mass estimates to those from an empirical mass-luminosity calibration, and showing how well stellar models reproduce the observed radii, effective temperatures, and luminosities. Reliability of stellar models is then investigated as a function of mass, [M/H], equivalent width of H-alpha, and X-ray luminosity. Finally, we briefly discuss various physical mechanisms to explain the observed trends, particularly in the context of the hypothesis that magnetic activity is the source of model-observation discrepancies.

  14. NuSTAR AND SWIFT Observations of the Fast Rotating Magnetized White Dwarf AE Aquarii

    NASA Technical Reports Server (NTRS)

    Kitaguchi, Takao; An, Hongjun; Beloborodov, Andrei M.; Gotthelf, Eric V.; Hayashi, Takayuki; Kaspi, Victoria M.; Rana, Vikram R.; Boggs, Steven E.; Christensen, Finn E.; Craig, William W.; Hailey, Charles J.; Harrison, Fiona A.; Stern, Daniel; Zhang, Will W.

    2014-01-01

    AE Aquarii is a cataclysmic variable with the fastest known rotating magnetized white dwarf (P(sub spin) = 33.08 s). Compared to many intermediate polars, AE Aquarii shows a soft X-ray spectrum with a very low luminosity (LX (is) approximately 10(exp 31) erg per second). We have analyzed overlapping observations of this system with the NuSTAR and the Swift X-ray observatories in 2012 September. We find the 0.5-30 keV spectra to be well fitted by either an optically thin thermal plasma model with three temperatures of 0.75(+0.18 / -0.45), 2.29(+0.96 / -0.82), and 9.33 (+6.07 / -2.18) keV, or an optically thin thermal plasma model with two temperatures of 1.00 (+0.34 / -0.23) and 4.64 (+1.58 / -0.84) keV plus a power-law component with photon index of 2.50 (+0.17 / -0.23). The pulse profile in the 3-20 keV band is broad and approximately sinusoidal, with a pulsed fraction of 16.6% +/- 2.3%. We do not find any evidence for a previously reported sharp feature in the pulse profile.

  15. NuStar and Swift Observations of the Fast Rotating Magnetized White Dwarf AE Aquarii

    NASA Technical Reports Server (NTRS)

    Kitaguchi, Takao; An, Hongjun; Beloborodov, Andrei M.; Gotthelf, Eric V.; Hayashi, Takayuki; Kaspi, Victoria M.; Rana, Vikram R.; Boggs, Steven E.; Christensen, Finn E.; Craig, William W.; Hailey, Charles J.; Harrison, Fiona A.; Stern, Daniel; Zhang, Will W.

    2014-01-01

    AE Aquarii is a cataclysmic variable with the fastest known rotating magnetized white dwarf (P(sub spin) = 33.08 s). Compared to many intermediate polars, AE Aquarii shows a soft X-ray spectrum with a very low luminosity (LX (is) approximately 10(exp 31) erg per second). We have analyzed overlapping observations of this system with the NuSTAR and the Swift X-ray observatories in 2012 September. We find the 0.5-30 keV spectra to be well fitted by either an optically thin thermal plasma model with three temperatures of 0.75(+0.18 / -0.45), 2.29(+0.96 / -0.82), and 9.33 (+6.07 / -2.18) keV, or an optically thin thermal plasma model with two temperatures of 1.00 (+0.34 / -0.23) and 4.64 (+1.58 / -0.84) keV plus a power-law component with photon index of 2.50 (+0.17 / -0.23). The pulse profile in the 3-20 keV band is broad and approximately sinusoidal, with a pulsed fraction of 16.6% +/- 2.3%. We do not find any evidence for a previously reported sharp feature in the pulse profile.

  16. A Neutron Star-White Dwarf Binary Model for Repeating Fast Radio Burst 121102

    NASA Astrophysics Data System (ADS)

    Gu, Wei-Min; Dong, Yi-Ze; Liu, Tong; Ma, Renyi; Wang, Junfeng

    2016-06-01

    We propose a compact binary model for the fast radio burst (FRB) repeaters, where the system consists of a magnetic white dwarf (WD) and a neutron star (NS) with strong bipolar magnetic fields. When the WD fills its Roche lobe, mass transfer will occur from the WD to the NS through the inner Lagrange point. The accreted magnetized materials may trigger magnetic reconnection when they approach the NS surface, and therefore the electrons can be accelerated to an ultra-relativistic speed. In this scenario, the curvature radiation of the electrons moving along the NS magnetic field lines can account for the characteristic frequency and the timescale of an FRB. Owing to the conservation of angular momentum, the WD may be kicked away after a burst, and the next burst may appear when the system becomes semi-detached again through the gravitational radiation. By comparing our analyses with the observations, we show that such an intermittent Roche-lobe overflow mechanism can be responsible for the observed repeating behavior of FRB 121102.

  17. Tracing recent star formation of red early-type galaxies out to z ∼ 1

    SciTech Connect

    Ko, Jongwan; Lee, Jong Chul; Hwang, Ho Seong; Im, Myungshin; Le Borgne, Damien; Elbaz, David

    2014-08-20

    We study the mid-infrared (IR) excess emission of early-type galaxies (ETGs) on the red sequence at z < 1 using a spectroscopic sample of galaxies in the fields of Great Observatories Origins Deep Survey (GOODS). In the mass-limited sample of 1025 galaxies with M {sub star} > 10{sup 10.5} M {sub ☉} and 0.4 < z < 1.05, we identify 696 Spitzer 24 μm detected (above the 5σ) galaxies and find them to have a wide range of NUV-r and r-[12 μm] colors despite their red optical u – r colors. Even in the sample of very massive ETGs on the red sequence with M {sub star} > 10{sup 11.2} M {sub ☉}, more than 18% show excess emission over the photospheric emission in the mid-IR. The combination with the results of red ETGs in the local universe suggests that the recent star formation is not rare among quiescent, red ETGs at least out to z ∼ 1 if the mid-IR excess emission results from intermediate-age stars or/and from low-level ongoing star formation. Our color-color diagram including near-UV and mid-IR emissions are efficient not only for identifying ETGs with recent star formation, but also for distinguishing quiescent galaxies from dusty star-forming galaxies.

  18. Evolved Massive Stars in the Local Group. I. Identification of Red Supergiants in NGC 6822, M31, and M33

    NASA Astrophysics Data System (ADS)

    Massey, Philip

    1998-07-01

    Knowledge of the red supergiant (RSG) population of nearby galaxies allows us to probe massive star evolution as a function of metallicity; however, contamination by foreground Galactic dwarfs dominates surveys for red stars in Local Group galaxies beyond the Magellanic Clouds. Model atmospheres predict that low-gravity supergiants will have B-V values that are redder by several tenths of a magnitude than foreground dwarfs at a given V-R color, a result that is largely independent of reddening. We conduct a BVR survey of several fields in the Local Group galaxies NGC 6822, M33, and M31 as well as neighboring control fields and identify RSG candidates from CCD photometry. The survey is complete to V = 20.5, corresponding to MV = -4.5 or an Mbol of -6.3 for the reddest stars. Follow-up spectroscopy at the Ca II triplet of 130 stars is used to demonstrate that our photometric criterion for identifying RSGs is highly successful (96% for stars brighter than V = 19.5; 82% for V = 19.5-20.5). Classification spectra are also obtained for a number of stars in order to calibrate color with spectral type empirically. We find that there is a marked progression in the average (B-V)0 and (V-R)0 colors of RSGs in these three galaxies, with the higher metallicity systems having a later average spectral type, which is consistent with previous findings by Elias, Frogel, & Humphreys for the Milky Way and Magellanic Clouds. More significantly, we find that there is a clear progression with metallicity in the relative number of the highest luminosity RSGs, a trend that is apparent both in absolute visual magnitude and in bolometric luminosity. Thus any use of RSGs as distance indicators requires correction for the metallicity of the parent galaxy. Our findings are in accord with the predictions of the ``Conti scenario'' in which higher metallicities result in higher mass-loss rates, resulting in a star of a given luminosity spending an increasing fraction of its He-burning lifetime as

  19. THE CHEMICAL SIGNATURE OF A RELIC STAR CLUSTER IN THE SEXTANS DWARF SPHEROIDAL GALAXY-IMPLICATIONS FOR NEAR-FIELD COSMOLOGY

    SciTech Connect

    Karlsson, Torgny; Bland-Hawthorn, Joss; Freeman, Ken C.; Silk, Joe

    2012-11-10

    We present tentative evidence for the existence of a dissolved star cluster at [Fe/H] = -2.7 in the Sextans dwarf spheroidal galaxy. We use the technique of chemical tagging to identify stars that are highly clustered in a multi-dimensional chemical abundance space (C-space). In a sample of six stars, three, possibly four, stars are identified as potential cluster stars. The initial stellar mass of the parent cluster is estimated from two independent observations to be M{sub *,init}=1.9{sup +1.5}{sub -0.9}(1.6{sup +1.2}{sub -0.8}) Multiplication-Sign 10{sup 5} M{sub sun}, assuming a Salpeter (Kroupa) initial mass function. If corroborated by follow-up spectroscopy, this star cluster is the most metal-poor system identified to date. Chemical signatures of remnant clusters in dwarf galaxies like Sextans provide us with a very powerful probe to the high-redshift universe. From available observational data, we argue that the average star cluster mass in the majority of the newly discovered ultra-faint dwarf galaxies was notably lower than it is in the Galaxy today and possibly lower than in the more luminous, classical dwarf spheroidal galaxies. Furthermore, the mean cumulative metallicity function of the dwarf spheroidals falls below that of the ultra-faints, which increases with increasing metallicity as predicted from our stochastic chemical evolution model. These two findings, together with a possible difference in the ([Mg/Fe]) ratio suggest that the ultra-faint dwarf galaxy population, or a significant fraction thereof, and the dwarf spheroidal population were formed in different environments and would thus be distinct in origin.

  20. Spectroscopic Study of Extended Star Clusters in Dwarf Galaxy NGC 6822

    NASA Astrophysics Data System (ADS)

    Hwang, Narae; Park, Hong Soo; Lee, Myung Gyoon; Lim, Sungsoon; Hodge, Paul W.; Kim, Sang Chul; Miller, Bryan; Weisz, Daniel

    2014-03-01

    We present a spectroscopic study of the four extended star clusters (ESCs) in NGC 6822 based on the data obtained with the Gemini Multi-Object Spectrograph on the Gemini-South 8.1 m telescope. The radial velocities derived from the spectra range from -61.2 ± 20.4 km s-1 (for C1) to -115.34 ± 57.9 km s-1 (for C4) and, unlike the intermediate-age carbon stars, they do not display any sign of systematic rotation around NGC 6822. The ages and metallicities derived using the Lick indices show that the ESCs are old (>=8 Gyr) and metal poor ([Fe/H] <~ -1.5). NGC 6822 is found to have both metal poor ([Fe/H] ≈-2.0) and metal rich ([Fe/H] ≈-0.9) star clusters within 15' (2 kpc) from the center, whereas only metal poor clusters are observed in the outer halo with r >= 20'(2.6 kpc). The kinematics, old ages, and low metallicities of ESCs suggest that ESCs may have accreted into the halo of NGC 6822. Based on the velocity distribution of ESCs, we have determined the total mass and the mass-to-light ratio of NGC 6822: M_{N6822} = 7.5^{+4.5}_{-0.1} \\times 10^{9}\\ M_{\\odot } and (M/L)_{N6822} = 75^{+45}_{-1} (M/L)_{\\odot }. It shows that NGC 6822 is one of the most dark matter dominated dwarf galaxies in the Local Group. Based on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the Science and Technology Facilities Council (United Kingdom), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), Ministério da Ciência, Tecnologia e Inovação (Brazil), and Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina).

  1. AN ALUMINUM/CALCIUM-RICH, IRON-POOR, WHITE DWARF STAR: EVIDENCE FOR AN EXTRASOLAR PLANETARY LITHOSPHERE?

    SciTech Connect

    Zuckerman, B.; Klein, B.; Jura, M.; Koester, D.; Dufour, P.; Melis, Carl

    2011-10-01

    The presence of elements heavier than helium in white dwarf atmospheres is often a signpost for the existence of rocky objects that currently or previously orbited these stars. We have measured the abundances of various elements in the hydrogen-atmosphere white dwarfs G149-28 and NLTT 43806. In comparison with other white dwarfs with atmospheres polluted by heavy elements, NLTT 43806 is substantially enriched in aluminum but relatively poor in iron. We compare the relative abundances of Al and eight other heavy elements seen in NLTT 43806 with the elemental composition of bulk Earth, with simulated extrasolar rocky planets, with solar system meteorites, with the atmospheric compositions of other polluted white dwarfs, and with the outer layers of the Moon and Earth. The best agreement is found with a model that involves accretion of a mixture of terrestrial crust and upper mantle material onto NLTT 43806. The implication is that NLTT 43806 is orbited by a differentiated rocky planet, perhaps quite similar to Earth, that has suffered a collision that stripped away some of its outer layers.

  2. Luminous and Variable Stars in M31 and M33. III. The Yellow and Red Supergiants and Post-red Supergiant Evolution

    NASA Astrophysics Data System (ADS)

    Gordon, Michael S.; Humphreys, Roberta M.; Jones, Terry J.

    2016-07-01

    Recent supernova (SN) and transient surveys have revealed an increasing number of non-terminal stellar eruptions. Though the progenitor class of these eruptions includes the most luminous stars, little is known of the pre-SN mechanics of massive stars in their most evolved state, thus motivating a census of possible progenitors. From surveys of evolved and unstable luminous star populations in nearby galaxies, we select a sample of yellow and red supergiant (RSG) candidates in M31 and M33 for review of their spectral characteristics and spectral energy distributions (SEDs). Since the position of intermediate- and late-type supergiants on the color–magnitude diagram can be heavily contaminated by foreground dwarfs, we employ spectral classification and multi-band photometry from optical and near-infrared surveys to confirm membership. Based on spectroscopic evidence for mass loss and the presence of circumstellar (CS) dust in their SEDs, we find that 30%–40% of the yellow supergiants are likely in a post-RSG state. Comparison with evolutionary tracks shows that these mass-losing, post-RSGs have initial masses between 20 and 40 M ⊙. More than half of the observed RSGs in M31 and M33 are producing dusty CS ejecta. We also identify two new warm hypergiants in M31, J004621.05+421308.06 and J004051.59+403303.00, both of which are likely in a post-RSG state. Based on observations obtained with the Large Binocular Telescope (LBT), an international collaboration among institutions in the United States, Italy, and Germany. LBT Corporation partners are: The University of Arizona on behalf of the Arizona university system; Istituto Nazionale di Astrofisica, Italy; LBT Beteiligungsgesellschaft, Germany, representing the Max-Planck Society, the Astrophysical Institute Potsdam, and Heidelberg University; The Ohio State University, and The Research Corporation, on behalf of The University of Notre Dame, University of Minnesota, and University of Virginia.

  3. Luminous and Variable Stars in M31 and M33. III. The Yellow and Red Supergiants and Post-red Supergiant Evolution

    NASA Astrophysics Data System (ADS)

    Gordon, Michael S.; Humphreys, Roberta M.; Jones, Terry J.

    2016-07-01

    Recent supernova (SN) and transient surveys have revealed an increasing number of non-terminal stellar eruptions. Though the progenitor class of these eruptions includes the most luminous stars, little is known of the pre-SN mechanics of massive stars in their most evolved state, thus motivating a census of possible progenitors. From surveys of evolved and unstable luminous star populations in nearby galaxies, we select a sample of yellow and red supergiant (RSG) candidates in M31 and M33 for review of their spectral characteristics and spectral energy distributions (SEDs). Since the position of intermediate- and late-type supergiants on the color-magnitude diagram can be heavily contaminated by foreground dwarfs, we employ spectral classification and multi-band photometry from optical and near-infrared surveys to confirm membership. Based on spectroscopic evidence for mass loss and the presence of circumstellar (CS) dust in their SEDs, we find that 30%-40% of the yellow supergiants are likely in a post-RSG state. Comparison with evolutionary tracks shows that these mass-losing, post-RSGs have initial masses between 20 and 40 M ⊙. More than half of the observed RSGs in M31 and M33 are producing dusty CS ejecta. We also identify two new warm hypergiants in M31, J004621.05+421308.06 and J004051.59+403303.00, both of which are likely in a post-RSG state. Based on observations obtained with the Large Binocular Telescope (LBT), an international collaboration among institutions in the United States, Italy, and Germany. LBT Corporation partners are: The University of Arizona on behalf of the Arizona university system; Istituto Nazionale di Astrofisica, Italy; LBT Beteiligungsgesellschaft, Germany, representing the Max-Planck Society, the Astrophysical Institute Potsdam, and Heidelberg University; The Ohio State University, and The Research Corporation, on behalf of The University of Notre Dame, University of Minnesota, and University of Virginia.

  4. Testing the core of red-giant-branch stars using the period spacing of gravity modes

    NASA Astrophysics Data System (ADS)

    Lagarde, Nadège; Diego, Bossini; Miglio, Andrea

    2015-08-01

    The blooming of asteroseismology of red-giant stars with the CoRoT and Kepler space missions paves the way to a better understanding of the stellar structure and physical processes occurring in low-mass-giant stars.We investigate the effect of rotation on the asymptotic period spacing of gravity modes (DP) and on the coupling strength between acoustic and gravity modes. We focus on red-giant-branch stars (RGB) which ignite He in degenerate conditions (M<2.0Msun), and we compare standard models with those including the effects of rotation and overshooting beyond the convective core.We find that, in the case of red-giant stars below the RGB bump, additional transport processes of chemicals have an impact on DP, hence on the determination of the stellar mass when DP is used as a constraint. Moreover we show that the coupling strength gives a direct signature of rotation occuring in red-giant stars. Whether this signature can be inferred from current data needs however to be investigated further. Finally we show that, irrespective of additional transport processes occurring during the main sequence, the period spacing of red-giant stars brighter than the RGB bump is an accurate proxy for the stellar luminosity, due to the well known relation between MHecore and luminosity.

  5. SPATIAL DISTRIBUTION AND EVOLUTION OF THE STELLAR POPULATIONS AND CANDIDATE STAR CLUSTERS IN THE BLUE COMPACT DWARF I ZWICKY 18

    SciTech Connect

    Contreras Ramos, R.; Annibali, F.; Fiorentino, G.; Tosi, M.; Clementini, G.; Aloisi, A.; Van der Marel, R. P.; Marconi, M.; Musella, I.; Saha, A.

    2011-10-01

    The evolutionary properties and spatial distribution of I Zwicky 18 (IZw18) stellar populations are analyzed by means of Hubble Space Telescope/Advanced Camera for Surveys deep and accurate photometry. A comparison of the resulting color-magnitude diagrams (CMDs) with stellar evolution models indicates that stars of all ages are present in all the system's components, including objects possibly up to 13 Gyr old, intermediate-age stars, and very young ones. The CMDs show evidence of thermally pulsing asymptotic giant branch and carbon stars. classical and ultra-long-period Cepheids as well as long-period variables have been measured. About 20 objects could be unresolved star clusters; these are mostly concentrated in the northwest (NW) portion of the main body (MB). If interpreted with simple stellar population models, these objects indicate a particularly active star formation over the past 100 Myr in IZw18. The stellar spatial distribution shows that the younger ones are more centrally concentrated, while old and intermediate-age stars are distributed homogeneously over the two bodies, although they are more easily detectable at the system's periphery. The oldest stars are most visible in the secondary body (SB) and in the southeast (SE) portion of the MB, where crowding is less severe, but are also present in the rest of the MB, where they are measured with larger uncertainties. The youngest stars are a few Myr old, are located predominantly in the MB, and are mostly concentrated in its NW portion. The SE portion of the MB appears to be in a similar, but not as young, evolutionary stage as the NW, while the SB stars are older than at least 10 Myr. There is then a sequence of decreasing age of the younger stars from the SB to the SE portion of the MB to the NW portion. All our results suggest that IZw18 is not atypical compared to other blue compact dwarfs.

  6. The circumstellar dust envelopes of red giant stars. I - M giant stars with the 10-micron silicate emission band

    NASA Technical Reports Server (NTRS)

    Hashimoto, O.; Nakada, Y.; Onaka, T.; Kamijo, F.; Tanabe, T.

    1990-01-01

    Spherical dust envelope models of red giant stars are constructed by solving the radiative transfer equations of the generalized two-stream Eddington approximation. The IRAS observations of M giant stars which show the 10-micron silicate emission band in IRAS LRS spectra are explained by the models with the dirty silicate grains with K proportional to lambda exp -1.5 for lambda greather than 28 microns. Under the assumption of steady mass flow in the envelope, this model analysis gives the following conclusions: (1) the strength of the silicate emission peak at 10 microns is a good indicator of the mass loss rate of the star, (2) no stars with the 10-microns silicate emission feature are observed in the range of mass loss rate smaller than 7 x 10 to the -8th solar mass/yr, and (3) the characteristic time of the mass loss process of M stars does not exceed a few 10,000 years.

  7. On the metallicity dependence of the winds from red supergiants and Asymptotic Giant Branch stars

    NASA Astrophysics Data System (ADS)

    van Loon, J. Th.

    2006-12-01

    Over much of the initial mass function, stars are destined to become luminous and cool red giants. They may then be able to produce dust in an atmosphere which has been elevated by strong radial pulsations, and hence drive a wind. The amount of mass that is lost in this way can be a very significant fraction of the stellar mass, and especially in the case of intermediate-mass stars it is highly enriched. The delay between a star's birth and its feedback into the environment varies from several million years for massive stars to almost the age of the Universe for the least massive red giants we see today. I here present a review on the metallicity dependence of red giant winds. I show that recent measurements not only confirm theoretical expectations, but also admonish of common misconceptions with implications for feedback at low initial metallicity.

  8. Asteroseismology of red-giant stars as a novel approach in the search for gravitational waves

    NASA Astrophysics Data System (ADS)

    Campante, Tiago L.; Lopes, Ilídio; Bossini, Diego; Miglio, Andrea; Chaplin, William J.

    2015-08-01

    Stars are massive resonators that may in principle be used as gravitational-wave (GW) detectors with an isotropic sensitivity. New insights on stellar physics have been made possible by asteroseismology, the study of stars by the observation of their natural, resonant oscillations. The continuous monitoring of oscillation modes in stars of different masses and sizes (e.g., as carried out by NASA’s Kepler space telescope) thus opens the possibility of surveying the local Universe for GW radiation. Red-giant stars are of particular interest in this regard. Since the mean separation between red giants in open clusters is small (of a few light years), this can in principle be used to look for the same GW imprint on the oscillation modes of different stars as a GW propagates across the cluster. Furthermore, the frequency range probed by oscillations in red giants overlaps with, and complements, the capabilities of the planned eLISA space interferometer. We propose asteroseismology of red-giant stars as a novel approach in the search for gravitational waves and assess to what extent oscillations in these stars can be excited by a passing, monochromatic GW.

  9. Red clump stars from the LAMOST data I: identification and distance

    NASA Astrophysics Data System (ADS)

    Wan, Jun-Chen; Liu, Chao; Deng, Li-Cai; Cui, Wen-Yuan; Zhang, Yong; Hou, Yong-Hui; Yang, Ming; Wu, Yue

    2015-08-01

    We present a sample of about 120 000 red clump candidates selected from the LAMOST DR2 catalog based on the empirical distribution model in the effective temperature vs. surface gravity plane. Although, in general, red clump stars are considered as standard candles, they do not exactly stay in a narrow range of absolute magnitude, but may have a range of more than one magnitude depending on their initial mass. Consequently, conventional oversimplified distance estimations with the assumption of a fixed luminosity may lead to systematic bias related to the initial mass or age, which can potentially affect the study of the evolution of the Galaxy with red clump stars. We therefore employ an isochrone-based method to estimate the absolute magnitude of red clump stars from their observed surface gravities, effective temperatures and metallicities. We verify that the estimation removes the systematics well and provides initial mass/age estimates that are independent of distance with accuracy better than 10%.

  10. Spectral energy distribution simulations of a possible ring structure around the young, red brown dwarf G 196-3 B

    NASA Astrophysics Data System (ADS)

    Zakhozhay, Olga V.; Zapatero Osorio, María Rosa; Béjar, Víctor J. S.; Boehler, Yann

    2016-09-01

    The origin of the very red optical and infrared colours of intermediate-age (˜10-500 Myr) L-type dwarfs remains unknown. It has been suggested that low-gravity atmospheres containing large amounts of dust may account for the observed reddish nature. We explored an alternative scenario by simulating protoplanetary and debris discs around G 196-3 B, which is an L3 young brown dwarf with a mass of ˜15 MJup and an age in the interval 20-300 Myr. The best-fit solution to G 196-3 B's photometric spectral energy distribution from optical wavelengths through 24 μm corresponds to the combination of an unreddened L3 atmosphere (Teff ≈ 1870 K) and a warm (≈ 1280 K), narrow (≈ 0.07-0.11 R⊙) debris disc located at very close distances (≈ 0.12-0.20 R⊙) from the central brown dwarf. This putative, optically thick, dusty belt, whose presence is compatible with the relatively young system age, would have a mass ≥7 × 10-10 M⊕ comprised of sub-micron/micron characteristic dusty particles with temperatures close to the sublimation threshold of silicates. Considering the derived global properties of the belt and the disc-to-brown dwarf mass ratio, the dusty ring around G 196-3 B may resemble the rings of Neptune and Jupiter, except for its high temperature and thick vertical height (≈6 × 103 km). Our inferred debris disc model is able to reproduce G 196-3 B's spectral energy distribution to a satisfactory level of achievement.

  11. The Palomar/MSU Nearby-Star Spectroscopic Survey. I. The Northern M Dwarfs -Bandstrengths and Kinematics

    NASA Astrophysics Data System (ADS)

    Reid, I. Neill; Hawley, Suzanne L.; Gizis, John E.

    1995-10-01

    The Third Catalogue of Nearby Stars (Gliese & Jahreiss Preliminary Version of the third Catalogue of Nearby Stars, 1991) includes over 1850 stars which lie north of δ 30° and are either identified as spectral type M, or are unclassified but with an absolute visual magnitude estimate MV>+8.O. Although there is no uniformity in selection criteria, and many of the stars lack basic data (radial velocities, spectral types, accurate photometry), the observational properties of these stars underlie most estimates of the fundamental characteristics of the Galactic Disk. We have obtained optical spectroscopy of 1746 of the 1876 stars-the remaining 130 are binary companions of brighter stars and inaccessible to our observations. These spectra allow us, first, to exclude 61 stars as either degenerates or as misclassified earlier-type (B - K) stars lying beyond the 25 pc limit; to establish radial velocities accurate to ±10 km s-1 for all stars confirmed as late-type dwarfs; to determine spectral types and absolute magnitudes from the TiO bandstrength, allowing more accurate distance estimates for stars with inaccurate (or no) trigonometric parallax measurements; and to identify stars with Ha emission (chromospherically active stars) and with strong CaH absorption (perhaps including some metal-poor disk subdwarfs). We have determined the nearby-star luminosity function from complete samples derived by applying both the distance limits defined by Wielen (1974) and by using limits derived from our own analysis. In both cases, we find good agreement with Wielen's results to MV ˜+11, but lower densities at the maximum (MV˜+12). The latter analysis results in a luminosity function, ΦCNS, which closely matches photometric parallax analyses for MV<+11 and MV>+14 -- we do not recover the apparent excess of low-luminosity stars inferred from analysis of the 5 pc sample. However, ΦCNS does lie below Φphot at the peak (MV˜12), and we suggest that this offset is caused by the

  12. PLANETS AROUND LOW-MASS STARS. III. A YOUNG DUSTY L DWARF COMPANION AT THE DEUTERIUM-BURNING LIMIT ,

    SciTech Connect

    Bowler, Brendan P.; Liu, Michael C.; Shkolnik, Evgenya L.; Dupuy, Trent J.

    2013-09-01

    We report the discovery of an L-type companion to the young M3.5V star 2MASS J01225093-2439505 at a projected separation of 1.''45 ( Almost-Equal-To 52 AU) as part of our adaptive optics imaging search for extrasolar giant planets around young low-mass stars. 2MASS 0122-2439 B has very red near-infrared colors similar to the HR 8799 planets and the reddest known young/dusty L dwarfs in the field. Moderate-resolution (R Almost-Equal-To 3800) 1.5-2.4 {mu}m spectroscopy reveals a near-infrared spectral type of L4-L6 and an angular H-band shape, confirming its cool temperature and young age. The kinematics of 2MASS 0122-2439 AB are marginally consistent with members of the {approx}120 Myr AB Dor young moving group based on the photometric distance to the primary (36 {+-} 4 pc) and our radial velocity measurement of 2MASS 0122-2439 A from Keck/HIRES. We adopt the AB Dor group age for the system, but the high energy emission, lack of Li I {lambda}6707 absorption, and spectral shape of 2MASS 0122-2439 B suggest a range of {approx}10-120 Myr is possible. The age and luminosity of 2MASS 0122-2439 B fall in a strip where ''hot-start'' evolutionary model mass tracks overlap as a result of deuterium burning. Several known substellar companions also fall in this region (2MASS J0103-5515 ABb, AB Pic b, {kappa} And b, G196-3 B, SDSS 2249+0044 B, LP 261-75 B, HD 203030 B, and HN Peg B), but their dual-valued mass predictions have largely been unrecognized. The implied mass of 2MASS 0122-2439 B is Almost-Equal-To 12-13 M{sub Jup} or Almost-Equal-To 22-27 M{sub Jup} if it is an AB Dor member, or possibly as low as 11 M{sub Jup} if the wider age range is adopted. Evolutionary models predict an effective temperature for 2MASS 0122-2439 B that corresponds to spectral types near the L/T transition ( Almost-Equal-To 1300-1500 K) for field objects. However, we find a mid-L near-infrared spectral type, indicating that 2MASS 0122-2439 B represents another case of photospheric dust being

  13. Planets around Low-mass Stars. III. A Young Dusty L Dwarf Companion at the Deuterium-burning Limit

    NASA Astrophysics Data System (ADS)

    Bowler, Brendan P.; Liu, Michael C.; Shkolnik, Evgenya L.; Dupuy, Trent J.

    2013-09-01

    We report the discovery of an L-type companion to the young M3.5V star 2MASS J01225093-2439505 at a projected separation of 1.''45 (≈52 AU) as part of our adaptive optics imaging search for extrasolar giant planets around young low-mass stars. 2MASS 0122-2439 B has very red near-infrared colors similar to the HR 8799 planets and the reddest known young/dusty L dwarfs in the field. Moderate-resolution (R ≈ 3800) 1.5-2.4 μm spectroscopy reveals a near-infrared spectral type of L4-L6 and an angular H-band shape, confirming its cool temperature and young age. The kinematics of 2MASS 0122-2439 AB are marginally consistent with members of the ~120 Myr AB Dor young moving group based on the photometric distance to the primary (36 ± 4 pc) and our radial velocity measurement of 2MASS 0122-2439 A from Keck/HIRES. We adopt the AB Dor group age for the system, but the high energy emission, lack of Li I λ6707 absorption, and spectral shape of 2MASS 0122-2439 B suggest a range of ~10-120 Myr is possible. The age and luminosity of 2MASS 0122-2439 B fall in a strip where "hot-start" evolutionary model mass tracks overlap as a result of deuterium burning. Several known substellar companions also fall in this region (2MASS J0103-5515 ABb, AB Pic b, κ And b, G196-3 B, SDSS 2249+0044 B, LP 261-75 B, HD 203030 B, and HN Peg B), but their dual-valued mass predictions have largely been unrecognized. The implied mass of 2MASS 0122-2439 B is ≈12-13 M Jup or ≈22-27 M Jup if it is an AB Dor member, or possibly as low as 11 M Jup if the wider age range is adopted. Evolutionary models predict an effective temperature for 2MASS 0122-2439 B that corresponds to spectral types near the L/T transition (≈1300-1500 K) for field objects. However, we find a mid-L near-infrared spectral type, indicating that 2MASS 0122-2439 B represents another case of photospheric dust being retained to cooler temperatures at low surface gravities, as seen in the spectra of young (8-30 Myr) planetary

  14. Star-planet interactions. II. Is planet engulfment the origin of fast rotating red giants?

    NASA Astrophysics Data System (ADS)

    Privitera, Giovanni; Meynet, Georges; Eggenberger, Patrick; Vidotto, Aline A.; Villaver, Eva; Bianda, Michele

    2016-10-01

    Context. Fast rotating red giants in the upper part of the red giant branch have surface velocities that cannot be explained by single star evolution. Aims: We check whether tides between a star and a planet followed by planet engulfment can indeed accelerate the surface rotation of red giants for a sufficiently long time to produce these fast rotating red giants. Methods: We studied how the surface rotation velocity at the stellar surface evolves using rotating stellar models, accounting for the redistribution of the angular momentum inside the star by different transport mechanisms, the exchanges of angular momentum between the planet orbit and the star before the engulfment, and for the deposition of angular momentum inside the star at the engulfment. We considered different situations with masses of stars in the range between 1.5 and 2.5 M⊙, masses of the planets between 1 and 15 MJ (Jupiter mass), and initial semimajor axis between 0.5 and 1.5 au. The metallicity Z for our stellar models is 0.02. Results: We show that the surface velocities reached at the end of the orbital decay due to tidal forces and planet engulfment can be similar to values observed for fast rotating red giants. This surface velocity then decreases when the star evolves along the red giant branch but at a sufficiently slow pace to allowing stars to be detected with such a high velocity. More quantitatively, star-planet interaction can produce a rapid acceleration of the surface of the star, above values equal to 8 km s-1, for periods lasting up to more than 30% the red giant branch phase. As found already by previous works, the changes of the surface carbon isotopic ratios produced by the dilution of the planetary material into the convective envelope is modest. The increase of the lithium abundance due to this effect might be much more important, however lithium may be affected by many different, still uncertain, processes. Thus any lithium measurement can hardly be taken as a support

  15. Magnetospheric structure and atmospheric Joule heating of habitable planets orbiting M-dwarf stars

    SciTech Connect

    Cohen, O.; Drake, J. J.; Garraffo, C.; Poppenhaeger, K.; Glocer, A.; Ridley, A. J.; Gombosi, T. I.

    2014-07-20

    We study the magnetospheric structure and the ionospheric Joule Heating of planets orbiting M-dwarf stars in the habitable zone using a set of magnetohydrodynamic models. The stellar wind solution is used to drive a model for the planetary magnetosphere, which is coupled with a model for the planetary ionosphere. Our simulations reveal that the space environment around close-in habitable planets is extreme, and the stellar wind plasma conditions change from sub- to super-Alfvénic along the planetary orbit. As a result, the magnetospheric structure changes dramatically with a bow shock forming in the super-Alfvénic sectors, while no bow shock forms in the sub-Alfvénic sectors. The planets reside most of the time in the sub-Alfvénic sectors with poor atmospheric protection. A significant amount of Joule Heating is provided at the top of the atmosphere as a result of the intense stellar wind. For the steady-state solution, the heating is about 0.1%-3% of the total incoming stellar irradiation, and it is enhanced by 50% for the time-dependent case. The significant Joule Heating obtained here should be considered in models for the atmospheres of habitable planets in terms of the thickness of the atmosphere, the top-side temperature and density, the boundary conditions for the atmospheric pressure, and particle radiation and transport. Here we assume constant ionospheric Pedersen conductance similar to that of the Earth. The conductance could be greater due to the intense EUV radiation leading to smaller heating rates. We plan to quantify the ionospheric conductance in future study.

  16. The Nature of Starbursts. I. The Star Formation Histories of Eighteen Nearby Starburst Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    McQuinn, Kristen B. W.; Skillman, Evan D.; Cannon, John M.; Dalcanton, Julianne; Dolphin, Andrew; Hidalgo-Rodríguez, Sebastian; Holtzman, Jon; Stark, David; Weisz, Daniel; Williams, Benjamin

    2010-09-01

    We use archival Hubble Space Telescope observations of resolved stellar populations to derive the star formation histories (SFHs) of 18 nearby starburst dwarf galaxies. In this first paper, we present the observations, color-magnitude diagrams (CMDs), and the SFHs of the 18 starburst galaxies, based on a homogeneous approach to the data reduction, differential extinction, and treatment of photometric completeness. We adopt a star formation rate (SFR) threshold normalized to the average SFR of the individual system as a metric for classifying starbursts in SFHs derived from resolved stellar populations. This choice facilitates finding not only the currently bursting galaxies but also "fossil" bursts increasing the sample size of starburst galaxies in the nearby (D < 8 Mpc) universe. Thirteen of the eighteen galaxies are experiencing ongoing bursts and five galaxies show fossil bursts. From our reconstructed SFHs, it is evident that the elevated SFRs of a burst are sustained for hundreds of Myr with variations on small timescales. A long >100 Myr temporal baseline is thus fundamental to any starburst definition or identification method. The longer lived bursts rule out rapid "self-quenching" of starbursts on global scales. The bursting galaxies' gas consumption timescales are shorter than the Hubble time for all but one galaxy confirming the short-lived nature of starbursts based on fuel limitations. Additionally, we find that the strength of the Hα emission usually correlates with the CMD-based SFR during the last 4-10 Myr. However, in four cases, the Hα emission is significantly less than what is expected for models of starbursts; the discrepancy is due to the SFR changing on timescales of a few Myr. The inherently short timescale of the Hα emission limits identifying galaxies as starbursts based on the current characteristics which may or may not be representative of the recent SFH of a galaxy. Based on observations made with the NASA/ESA Hubble Space Telescope

  17. Molecular gas and star formation in the tidal dwarf galaxy VCC 2062

    NASA Astrophysics Data System (ADS)

    Lisenfeld, U.; Braine, J.; Duc, P. A.; Boquien, M.; Brinks, E.; Bournaud, F.; Lelli, F.; Charmandaris, V.

    2016-05-01

    The physical mechanisms driving star formation (SF) in galaxies are still not fully understood. Tidal dwarf galaxies (TDGs), made of gas ejected during galaxy interactions, seem to be devoid of dark matter and have a near-solar metallicity. The latter makes it possible to study molecular gas and its link to SF using standard tracers (CO, dust) in a peculiar environment. We present a detailed study of a nearby TDG in the Virgo Cluster, VCC 2062, using new high-resolution CO(1-0) data from the Plateau de Bure, deep optical imaging from the Next Generation Virgo Cluster Survey (NGVS), and complementary multiwavelength data. Until now, there was some doubt whether VCC 2062 was a true TDG, but the new deep optical images from the NGVS reveal a stellar bridge between VCC 2062 and its parent galaxy, NGC 4694, which is clear proof of its tidal origin. Several high-resolution tracers (Hα, UV, 8 μm, and 24 μm) of the star formation rate (SFR) are compared to the molecular gas distribution as traced by the CO(1-0). Coupled with the SFR tracers, the NGVS data are used with the CIGALE code to model the stellar populations throughout VCC 2062, yielding a declining SFR in the recent past, consistent with the low Hα/UV ratio, and a high burst strength. HI emission covers VCC 2062, whereas the CO is concentrated near the HI maxima. The CO peaks correspond to two very distinct regions: one with moderate SF to the NE and one with only slightly weaker CO emission but with nearly no SF. Even where SF is clearly present, the SFR is below the value expected from the surface density of the molecular and the total gas as compared to spiral galaxies and other TDGs. After discussing different possible explanations, we conclude that the low surface brightness is a crucial parameter to understand the low SFR. The reduced data cubes 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/590/A92

  18. THE ACS NEARBY GALAXY SURVEY TREASURY. X. QUANTIFYING THE STAR CLUSTER FORMATION EFFICIENCY OF NEARBY DWARF GALAXIES

    SciTech Connect

    Cook, David O.; Dale, Daniel A.; Seth, Anil C.; Johnson, L. Clifton; Weisz, Daniel R.; Fouesneau, Morgan; Dalcanton, Julianne J.; Olsen, Knut A. G.; Engelbracht, Charles W.

    2012-06-01

    We study the relationship between the field star formation and cluster formation properties in a large sample of nearby dwarf galaxies. We use optical data from the Hubble Space Telescope and from ground-based telescopes to derive the ages and masses of the young (t{sub age} {approx}< 100 Myr) cluster sample. Our data provide the first constraints on two proposed relationships between the star formation rate (SFR) of galaxies and the properties of their cluster systems in the low SFR regime. The data show broad agreement with these relationships, but significant galaxy-to-galaxy scatter exists. In part, this scatter can be accounted for by simulating the small number of clusters detected from stochastically sampling the cluster mass function. However, this stochasticity does not fully account for the observed scatter in our data, suggesting that there may be true variations in the fraction of stars formed in clusters in dwarf galaxies. Comparison of the cluster formation and the brightest cluster in our sample galaxies also provide constraints on cluster destruction models.

  19. Star-forming dwarf galaxies in the Virgo cluster: the link between molecular gas, atomic gas, and dust

    NASA Astrophysics Data System (ADS)

    Grossi, M.; Corbelli, E.; Bizzocchi, L.; Giovanardi, C.; Bomans, D.; Coelho, B.; De Looze, I.; Gonçalves, T. S.; Hunt, L. K.; Leonardo, E.; Madden, S.; Menéndez-Delmestre, K.; Pappalardo, C.; Riguccini, L.

    2016-05-01

    We present 12CO(1-0) and 12CO(2-1) observations of a sample of 20 star-forming dwarfs selected from the Herschel Virgo Cluster Survey, with oxygen abundances ranging from 12 + log (O / H) ~ 8.1 to 8.8. CO emission is observed in ten galaxies and marginally detected in another one. CO fluxes correlate with the FIR 250 μm emission, and the dwarfs follow the same linear relation that holds for more massive spiral galaxies extended to a wider dynamical range. We compare different methods to estimate H2 molecular masses, namely a metallicity-dependent CO-to-H2 conversion factor and one dependent on H-band luminosity. The molecular-to-stellar mass ratio remains nearly constant at stellar masses ≲ 109 M⊙, contrary to the atomic hydrogen fraction, MHI/M∗, which increases inversely with M∗. The flattening of the MH2/M∗ ratio at low stellar masses does not seem to be related to the effects of the cluster environment because it occurs for both Hi-deficient and Hi-normal dwarfs. The molecular-to-atomic ratio is more tightly correlated with stellar surface density than metallicity, confirming that the interstellar gas pressure plays a key role in determining the balance between the two gaseous components of the interstellar medium. Virgo dwarfs follow the same linear trend between molecular gas mass and star formation rate as more massive spirals, but gas depletion timescales, τdep, are not constant and range between 100 Myr and 6 Gyr. The interaction with the Virgo cluster environment is removing the atomic gas and dust components of the dwarfs, but the molecular gas appears to be less affected at the current stage of evolution within the cluster. However, the correlation between Hi deficiency and the molecular gas depletion time suggests that the lack of gas replenishment from the outer regions of the disc is lowering the star formation activity. Based on observations carried out with the IRAM 30-m telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany

  20. Hotspots and a clumpy disc: variability of brown dwarfs and stars in the young σOri cluster

    NASA Astrophysics Data System (ADS)

    Scholz, A.; Xu, X.; Jayawardhana, R.; Wood, K.; Eislöffel, J.; Quinn, C.

    2009-09-01

    The properties of accretion discs around stars and brown dwarfs in the σOri cluster (age 3Myr) are studied based on near-infrared (IR) time series photometry supported by mid-IR spectral energy distributions (SEDs). We monitor ~30 young low-mass sources over eight nights in the J and K band using the duPont telescope at Las Campanas. We find three objects showing variability with J-band amplitudes >=0.5mag five additional objects exhibit low-level variations. All three highly variable sources have been previously identified as highly variable; thus, we establish the long-term nature of their flux changes. The light curves contain periodic components with time-scales of ~0.5-8d, but have additional irregular variations superimposed - the characteristic behaviour for classical T Tauri stars. Based on the colour variability, we conclude that hotspots are the dominant cause of variations in two objects (#19 and #33), including one likely brown dwarf, with spot temperatures in the range of 6000-7000K. For the third one (#2), a brown dwarf or very low-mass star, inhomogeneities at the inner edge of the disc are the likely origin of variability. Based on mid-IR data from Spitzer, we confirm that the three highly variable sources are surrounded by circum-(sub)-stellar discs. They show typical SEDs for T Tauri-like objects. Using SED models, we infer an enhanced scaleheight in the disc for the object #2, which favours the detection of disc inhomogeneities in light curves and is thus consistent with the information from variability. In the σOri cluster, about every fifth accreting low-mass object shows persistent high-level photometric variability. We demonstrate that estimates for fundamental parameters in such objects can be significantly improved by determining the extent and origin of the variations.

  1. Exploring the Role of Sub-micron-sized Dust Grains in the Atmospheres of Red L0–L6 Dwarfs

    NASA Astrophysics Data System (ADS)

    Hiranaka, Kay; Cruz, Kelle L.; Douglas, Stephanie T.; Marley, Mark S.; Baldassare, Vivienne F.

    2016-10-01

    We examine the hypothesis that the red near-infrared colors of some L dwarfs could be explained by a “dust haze” of small particles in their upper atmospheres. This dust haze would exist in conjunction with the clouds found in dwarfs with more typical colors. We developed a model that uses Mie theory and the Hansen particle size distributions to reproduce the extinction due to the proposed dust haze. We apply our method to 23 young L dwarfs and 23 red field L dwarfs. We constrain the properties of the dust haze including particle size distribution and column density using Markov Chain Monte Carlo methods. We find that sub-micron-range silicate grains reproduce the observed reddening. Current brown dwarf atmosphere models include large-grain (1–100 μm) dust clouds but not sub-micron dust grains. Our results provide a strong proof of concept and motivate a combination of large and small dust grains in brown dwarf atmosphere models.

  2. THREE NEW ECLIPSING WHITE-DWARF-M-DWARF BINARIES DISCOVERED IN A SEARCH FOR TRANSITING PLANETS AROUND M-DWARFS

    SciTech Connect

    Law, Nicholas M.; Kraus, Adam L.; Street, Rachel; Fulton, Benjamin J.; Shporer, Avi; Lister, Tim; Hillenbrand, Lynne A.; Baranec, Christoph; Bui, Khanh; Davis, Jack T. C.; Dekany, Richard G.; Kulkarni, S. R.; Ofek, Eran O.; Bloom, Joshua S.; Cenko, S. Bradley; Filippenko, Alexei V.; Burse, Mahesh P.; Das, H. K.; Kasliwal, Mansi M.; Nugent, Peter; and others

    2012-10-01

    We present three new eclipsing white-dwarf/M-dwarf binary systems discovered during a search for transiting planets around M-dwarfs. Unlike most known eclipsing systems of this type, the optical and infrared emission is dominated by the M-dwarf components, and the systems have optical colors and discovery light curves consistent with being Jupiter-radius transiting planets around early M-dwarfs. We detail the PTF/M-dwarf transiting planet survey, part of the Palomar Transient Factory (PTF). We present a graphics processing unit (GPU)-based box-least-squares search for transits that runs approximately 8 Multiplication-Sign faster than similar algorithms implemented on general purpose systems. For the discovered systems, we decompose low-resolution spectra of the systems into white-dwarf and M-dwarf components, and use radial velocity measurements and cooling models to estimate masses and radii for the white dwarfs. The systems are compact, with periods between 0.35 and 0.45 days and semimajor axes of approximately 2 R{sub Sun} (0.01 AU). The M-dwarfs have masses of approximately 0.35 M{sub Sun }, and the white dwarfs have hydrogen-rich atmospheres with temperatures of around 8000 K and have masses of approximately 0.5 M{sub Sun }. We use the Robo-AO laser guide star adaptive optics system to tentatively identify one of the objects as a triple system. We also use high-cadence photometry to put an upper limit on the white-dwarf radius of 0.025 R{sub Sun} (95% confidence) in one of the systems. Accounting for our detection efficiency and geometric factors, we estimate that 0.08%{sub -0.05%}{sup +0.10%} (90% confidence) of M-dwarfs are in these short-period, post-common-envelope white-dwarf/M-dwarf binaries where the optical light is dominated by the M-dwarf. The lack of detections at shorter periods, despite near-100% detection efficiency for such systems, suggests that binaries including these relatively low-temperature white dwarfs are preferentially found at

  3. Using red clump stars to decompose the galactic magnetic field with distance

    SciTech Connect

    Pavel, Michael D.

    2014-09-01

    A new method for measuring the large-scale structure of the Galactic magnetic field is presented. The Galactic magnetic field has been probed through the Galactic disk with near-infrared starlight polarimetry; however, the distance to each background star is unknown. Using red clump stars as near-infrared standard candles, this work presents the first attempt to decompose the line-of-sight structure of the sky-projected Galactic magnetic field. Two example lines of sight are decomposed: toward a field with many red clump stars and toward a field with few red clump stars. A continuous estimate of magnetic field orientation over several kiloparsecs of distance is possible in the field with many red clump stars, while only discrete estimates are possible in the sparse example. Toward the outer Galaxy, there is a continuous field orientation with distance that shows evidence of perturbation by the Galactic warp. Toward the inner Galaxy, evidence for a large-scale change in the magnetic field geometry is consistent with models of magnetic field reversals, independently derived from Faraday rotation studies. A photo-polarimetric method for identifying candidate intrinsically polarized stars is also presented. The future application of this method to large regions of the sky will begin the process of mapping the Galactic magnetic field in a way never before possible.

  4. A super lithium-rich red-clump star in the open cluster Trumpler 5

    NASA Astrophysics Data System (ADS)

    Monaco, L.; Boffin, H. M. J.; Bonifacio, P.; Villanova, S.; Carraro, G.; Caffau, E.; Steffen, M.; Ahumada, J. A.; Beletsky, Y.; Beccari, G.

    2014-04-01

    Context. The existence of lithium-rich low-mass red giant stars still represents a challenge for stellar evolution models. Stellar clusters are privileged environments for this kind of investigation. Aims: To investigate the chemical abundance pattern of the old open cluster Trumpler 5, we observed a sample of four red-clump stars with high-resolution optical spectrographs. One of them (#3416) reveals extremely strong lithium lines in its spectrum. Methods: One-dimensional, local thermodynamic equilibrium analysis was performed on the spectra of the observed stars. A 3D-NLTE analysis was performed to derive the lithium abundance of star #3416. Results: Star #3416 is super Li-rich with A(Li) = 3.75 dex. The lack of 6Li enrichment (6Li/7Li < 2%), the low carbon isotopic ratio (12C/13C = 14 ± 3), and the lack of evidence for radial velocity variation or enhanced rotational velocity (vsini = 2.8 km s-1) all suggest that lithium production has occurred in this star through the Cameron & Fowler mechanism. Conclusions: We identified a super Li-rich core helium-burning, red-clump star in an open cluster. Internal production is the most likely cause of the observed enrichment. Given the expected short duration of a star's Li-rich phase, enrichment is likely to have occurred at the red clump or in the immediately preceding phases, namely during the He-flash at the tip of the red giant branch (RGB) or while ascending the brightest portion of the RGB. Based on observations made with ESO Telescopes at the La Silla Paranal Observatory under program ID 088.D-0045(A).Appendix A is available in electronic form at http://www.aanda.org

  5. Dwarf novae

    NASA Technical Reports Server (NTRS)

    Ladous, Constanze

    1993-01-01

    Dwarf novae are defined on grounds of their semi-regular brightness variations of some two to five magnitudes on time scales of typically 10 to 100 days. Historically several different classification schemes have been used. Today, dwarf novae are divided into three sub-classes: the U Geminorum stars, the SU Ursae Majoris stars, and the Z Camelopardalis stars. Outbursts of dwarf novae occur at semi-periodic intervals of time, typically every 10 to 100 days; amplitudes range from typically 2 to 5 mag. Within certain limits values are characteristic for each object. Relations between the outburst amplitude, or the total energy released during outburst, and the recurrence time have been found, as well as relations between the orbital period and the outburst decay time, the absolute magnitude during outburst maximum, and the widths of long and short outbursts, respectively. Some dwarf novae are known to have suspended their normal outburst activity altogether for a while. They later resumed it without having undergone any observable changes. The optical colors of dwarf novae all are quite similar during outburst, considerably bluer than during the quiescent state. During the outburst cycle, characteristic loops in the two color diagram are performed. At a time resolution on the order of minutes, strictly periodic photometric changes due to orbital motion become visible in the light curves of dwarf novae. These are characteristic for each system. Remarkably little is known about orbital variations during the course of an outburst. On time-scales of minutes and seconds, further more or less periodic types of variability are seen in dwarf novae. Appreciable flux is emitted by dwarf novae at all wavelengths from the X-rays to the longest IR wavelengths, and in some cases even in the radio. Most dwarf novae exhibit strong emission line spectra in the optical and UV during quiescence, although some have only very weak emissions in the optical and/or weak absorptions at UV

  6. HATS-7b: A Hot Super Neptune Transiting a Quiet K Dwarf Star

    NASA Astrophysics Data System (ADS)

    Bakos, G. Á.; Penev, K.; Bayliss, D.; Hartman, J. D.; Zhou, G.; Brahm, R.; Mancini, L.; de Val-Borro, M.; Bhatti, W.; Jordán, A.; Rabus, M.; Espinoza, N.; Csubry, Z.; Howard, A. W.; Fulton, B. J.; Buchhave, L. A.; Ciceri, S.; Henning, T.; Schmidt, B.; Isaacson, H.; Noyes, R. W.; Marcy, G. W.; Suc, V.; Howe, A. R.; Burrows, A. S.; Lázár, J.; Papp, I.; Sári, P.

    2015-11-01

    We report the discovery by the HATSouth network of HATS-7b, a transiting Super-Neptune with a mass of 0.120 ± 0.012 {M}{{J}}, a radius of {0.563}-0.034+0.046 {R}{{J}}, and an orbital period of 3.1853 days. The host star is a moderately bright (V=13.340\\+/- 0.010 mag, {K}S=10.976\\+/- 0.026 mag) K dwarf star with a mass of 0.849 ± 0.027 {M}ȯ , a radius of {0.815}-0.035+0.049 {R}ȯ , and a metallicity of [{Fe}/{{H}}] =+0.250\\+/- 0.080. The star is photometrically quiet to within the precision of the HATSouth measurements, has low RV jitter, and shows no evidence for chromospheric activity in its spectrum. HATS-7b is the second smallest radius planet discovered by a wide-field ground-based transit survey, and one of only a handful of Neptune-size planets with mass and radius determined to 10% precision. Theoretical modeling of HATS-7b yields a hydrogen–helium fraction of 18 ± 4% (rock-iron core and H2–He envelope), or 9 ± 4% (ice core and H2–He envelope), i.e., it has a composition broadly similar to that of Uranus and Neptune, and very different from that of Saturn, which has 75% of its mass in H2–He. Based on a sample of transiting exoplanets with accurately (<20%) determined parameters, we establish approximate power-law relations for the envelopes of the mass–density distribution of exoplanets. HATS-7b, which, together with the recently discovered HATS-8b, is one of the first two transiting super-Neptunes discovered in the Southern sky, is a prime target for additional follow-up observations with Southern hemisphere facilities to characterize the atmospheres of Super-Neptunes (which we define as objects with mass greater than that of Neptune, and smaller than halfway between that of Neptune and Saturn, i.e., 0.054 {M}{{J}}\\lt {M}{{p}}\\lt 0.18 {M}{{J}}). The HATSouth network is operated by a collaboration consisting of Princeton University (PU), the Max Planck Institute für Astronomie (MPIA), the Australian National University (ANU), and the

  7. HATS-7b: A Hot Super Neptune Transiting a Quiet K Dwarf Star

    NASA Astrophysics Data System (ADS)

    Bakos, G. Á.; Penev, K.; Bayliss, D.; Hartman, J. D.; Zhou, G.; Brahm, R.; Mancini, L.; de Val-Borro, M.; Bhatti, W.; Jordán, A.; Rabus, M.; Espinoza, N.; Csubry, Z.; Howard, A. W.; Fulton, B. J.; Buchhave, L. A.; Ciceri, S.; Henning, T.; Schmidt, B.; Isaacson, H.; Noyes, R. W.; Marcy, G. W.; Suc, V.; Howe, A. R.; Burrows, A. S.; Lázár, J.; Papp, I.; Sári, P.

    2015-11-01

    We report the discovery by the HATSouth network of HATS-7b, a transiting Super-Neptune with a mass of 0.120 ± 0.012 {M}{{J}}, a radius of {0.563}-0.034+0.046 {R}{{J}}, and an orbital period of 3.1853 days. The host star is a moderately bright (V=13.340\\+/- 0.010 mag, {K}S=10.976\\+/- 0.026 mag) K dwarf star with a mass of 0.849 ± 0.027 {M}⊙ , a radius of {0.815}-0.035+0.049 {R}⊙ , and a metallicity of [{Fe}/{{H}}] =+0.250\\+/- 0.080. The star is photometrically quiet to within the precision of the HATSouth measurements, has low RV jitter, and shows no evidence for chromospheric activity in its spectrum. HATS-7b is the second smallest radius planet discovered by a wide-field ground-based transit survey, and one of only a handful of Neptune-size planets with mass and radius determined to 10% precision. Theoretical modeling of HATS-7b yields a hydrogen-helium fraction of 18 ± 4% (rock-iron core and H2-He envelope), or 9 ± 4% (ice core and H2-He envelope), i.e., it has a composition broadly similar to that of Uranus and Neptune, and very different from that of Saturn, which has 75% of its mass in H2-He. Based on a sample of transiting exoplanets with accurately (<20%) determined parameters, we establish approximate power-law relations for the envelopes of the mass-density distribution of exoplanets. HATS-7b, which, together with the recently discovered HATS-8b, is one of the first two transiting super-Neptunes discovered in the Southern sky, is a prime target for additional follow-up observations with Southern hemisphere facilities to characterize the atmospheres of Super-Neptunes (which we define as objects with mass greater than that of Neptune, and smaller than halfway between that of Neptune and Saturn, i.e., 0.054 {M}{{J}}\\lt {M}{{p}}\\lt 0.18 {M}{{J}}). The HATSouth network is operated by a collaboration consisting of Princeton University (PU), the Max Planck Institute für Astronomie (MPIA), the Australian National University (ANU), and the Pontificia

  8. Binary frequency of planet-host stars at wide separations. A new brown dwarf companion to a planet-host star

    NASA Astrophysics Data System (ADS)

    Lodieu, N.; Pérez-Garrido, A.; Béjar, V. J. S.; Gauza, B.; Ruiz, M. T.; Rebolo, R.; Pinfield, D. J.; Martín, E. L.

    2014-09-01

    Aims: The aim of the project is to improve our knowledge on the multiplicity of planet-host stars at wide physical separations. Methods: We cross-matched approximately 6200 square degree area of the southern sky imaged by the Visible Infrared Survey Telescope for Astronomy (VISTA) Hemisphere Survey (VHS) with the Two Micron All Sky Survey (2MASS) to look for wide common proper motion companions to known planet-host stars. We complemented our astrometric search with photometric criteria. Results: We confirmed spectroscopically the co-moving nature of seven sources out of 16 companion candidates and discarded eight, while the remaining one stays as a candidate. Among these new wide companions to planet-host stars, we discovered a T4.5 dwarf companion at 6.3 arcmin (~9000 au) from HIP 70849, a K7V star which hosts a 9 Jupiter mass planet with an eccentric orbit. We also report two new stellar M dwarf companions to one G and one metal-rich K star. We infer stellar and substellar binary frequencies for our complete sample of 37 targets of 5.4±3.8% and 2.7±2.7% (1σ confidence level), respectively, for projected physical separations larger than ~60-160 au assuming the range of distances of planet-host stars (24-75 pc). These values are comparable to the frequencies of non planet-host stars. We find that the period-eccentricity trend holds with a lack of multiple systems with planets at large eccentricities (e> 0.2) for periods less than 40 days. However, the lack of planets more massive than 2.5 Jupiter masses and short periods (<40 days) orbiting single stars is not so obvious due to recent discoveries by ground-based transit surveys and space missions. Appendix A is available in electronic form at http://www.aanda.orgSpectra 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/569/A120Based on observations collected at the European Organisation for Astronomical Research

  9. Cloud Atlas: Discovery of Patchy Clouds and High-amplitude Rotational Modulations in a Young, Extremely Red L-type Brown Dwarf

    NASA Astrophysics Data System (ADS)

    Lew, Ben W. P.; Apai, Daniel; Zhou, Yifan; Schneider, Glenn; Burgasser, Adam J.; Karalidi, Theodora; Yang, Hao; Marley, Mark S.; Cowan, Nicolas B.; Bedin, Luigi R.; Metchev, Stanimir A.; Radigan, Jacqueline; Lowrance, Patrick J.

    2016-10-01

    Condensate clouds fundamentally impact the atmospheric structure and spectra of exoplanets and brown dwarfs, but the connections between surface gravity, cloud structure, dust in the upper atmosphere, and the red colors of some brown dwarfs remain poorly understood. Rotational modulations enable the study of different clouds in the same atmosphere, thereby providing a method to isolate the effects of clouds. Here, we present the discovery of high peak-to-peak amplitude (8%) rotational modulations in a low-gravity, extremely red (J-K s = 2.55) L6 dwarf WISEP J004701.06+680352.1 (W0047). Using the Hubble Space Telescope (HST) time-resolved grism spectroscopy, we find a best-fit rotational period (13.20 ± 0.14 hr) with a larger amplitude at 1.1 μm than at 1.7 μm. This is the third-largest near-infrared variability amplitude measured in a brown dwarf, demonstrating that large-amplitude variations are not limited to the L/T transition but are present in some extremely red L-type dwarfs. We report a tentative trend between the wavelength dependence of relative amplitude, possibly proxy for small dust grains lofted in the upper atmosphere, and the likelihood of large-amplitude variability. By assuming forsterite as a haze particle, we successfully explain the wavelength-dependent amplitude with submicron-sized haze particle sizes of around 0.4 μm. W0047 links the earlier spectral and later spectral type brown dwarfs in which rotational modulations have been observed; the large amplitude variations in this object make this a benchmark brown dwarf for the study of cloud properties close to the L/T transition.

  10. Comparative Response of the West African Dwarf Goats to Experimental Infections with Red Sokoto and West African Dwarf Goat Isolates of Haemonchus contortus.

    PubMed

    Ngongeh, Lucas Atehmengo; Onyeabor, Amaechi

    2015-01-01

    Response of the West African Dwarf (WAD) goats to two different isolates of Haemonchus contortus, the Red Sokoto (RS) goat isolate (RSHc) and the WAD goat isolate (WADHc) (isolated from WAD goats), was studied by experimental infections of 4-6-month-old male WAD goat kids. Group 1 and Group 2 goats were each infected with 4500 infective larvae (L3) of RSHc and WADHc, respectively. Group 3 animals served as uninfected control. Prepatent period (PPP), faecal egg counts (FEC), worm burden (WB), body weight (BW), packed cell volume (PCV), and body condition score (BCS) were determined. WAD goats infected with RSHc isolate and the ones infected with WADHc isolate had mean PPP of 19.63 ± 0.26 and 19.50 ± 0.19, respectively. Goats infected with WADHc isolate had significantly higher FEC (P = 0.004) and WB (P = 0.001). BW were significantly higher (P = 0.004) both in the controls and in Group 2 goats infected with WADHc isolate than in Group 1 goats infected with the RSHc isolate. BCS of animals in both infected groups dropped significantly (P = 0.001). There was a significant drop in PCV (P = 0.004) of both infected groups in comparison. Both isolates of H. contortus were pathogenic to the host. PMID:26697224

  11. Comparative Response of the West African Dwarf Goats to Experimental Infections with Red Sokoto and West African Dwarf Goat Isolates of Haemonchus contortus.

    PubMed

    Ngongeh, Lucas Atehmengo; Onyeabor, Amaechi

    2015-01-01

    Response of the West African Dwarf (WAD) goats to two different isolates of Haemonchus contortus, the Red Sokoto (RS) goat isolate (RSHc) and the WAD goat isolate (WADHc) (isolated from WAD goats), was studied by experimental infections of 4-6-month-old male WAD goat kids. Group 1 and Group 2 goats were each infected with 4500 infective larvae (L3) of RSHc and WADHc, respectively. Group 3 animals served as uninfected control. Prepatent period (PPP), faecal egg counts (FEC), worm burden (WB), body weight (BW), packed cell volume (PCV), and body condition score (BCS) were determined. WAD goats infected with RSHc isolate and the ones infected with WADHc isolate had mean PPP of 19.63 ± 0.26 and 19.50 ± 0.19, respectively. Goats infected with WADHc isolate had significantly higher FEC (P = 0.004) and WB (P = 0.001). BW were significantly higher (P = 0.004) both in the controls and in Group 2 goats infected with WADHc isolate than in Group 1 goats infected with the RSHc isolate. BCS of animals in both infected groups dropped significantly (P = 0.001). There was a significant drop in PCV (P = 0.004) of both infected groups in comparison. Both isolates of H. contortus were pathogenic to the host.

  12. Comparative Response of the West African Dwarf Goats to Experimental Infections with Red Sokoto and West African Dwarf Goat Isolates of Haemonchus contortus

    PubMed Central

    Ngongeh, Lucas Atehmengo; Onyeabor, Amaechi

    2015-01-01

    Response of the West African Dwarf (WAD) goats to two different isolates of Haemonchus contortus, the Red Sokoto (RS) goat isolate (RSHc) and the WAD goat isolate (WADHc) (isolated from WAD goats), was studied by experimental infections of 4–6-month-old male WAD goat kids. Group 1 and Group 2 goats were each infected with 4500 infective larvae (L3) of RSHc and WADHc, respectively. Group 3 animals served as uninfected control. Prepatent period (PPP), faecal egg counts (FEC), worm burden (WB), body weight (BW), packed cell volume (PCV), and body condition score (BCS) were determined. WAD goats infected with RSHc isolate and the ones infected with WADHc isolate had mean PPP of 19.63 ± 0.26 and 19.50 ± 0.19, respectively. Goats infected with WADHc isolate had significantly higher FEC (P = 0.004) and WB (P = 0.001). BW were significantly higher (P = 0.004) both in the controls and in Group 2 goats infected with WADHc isolate than in Group 1 goats infected with the RSHc isolate. BCS of animals in both infected groups dropped significantly (P = 0.001). There was a significant drop in PCV (P = 0.004) of both infected groups in comparison. Both isolates of H. contortus were pathogenic to the host. PMID:26697224

  13. Effect of Raspberry bushy dwarf virus, Raspberry leaf mottle virus, and Raspberry latent virus on plant growth and fruit crumbliness in ‘Meeker’ red Raspberry

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Raspberry crumbly fruit in red raspberry (Rubus idaeus L.), widespread in the Pacific Northwest of the United States and British Columbia, Canada, is most commonly caused by a virus infection. Raspberry bushy dwarf virus (RBDV) has long been attributed as the causal agent of the disease. Recently, t...

  14. 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.

  15. Diagnostics of models and observations in the contexts of exoplanets, brown dwarfs, and very low-mass stars.

    NASA Astrophysics Data System (ADS)

    Kopytova, Taisiya

    2016-01-01

    When studying isolated brown dwarfs and directly imaged exoplanets with insignificant orbital motion,we have to rely on theoretical models to determine basic parameters such as mass, age, effective temperature, and surface gravity.While stellar and atmospheric models are rapidly evolving, we need a powerful tool to test and calibrate them.In my thesis, I focussed on comparing interior and atmospheric models with observational data, in the effort of taking into account various systematic effects that can significantly influence the data analysis.As a first step, about 460 candidate member os the Hyades were screened for companions using diffraction limited imaging observation (both our own data and archival data). As a result I could establish the single star sequence for the Hyades comprising about 250 stars (Kopytova et al. 2015, accepted to A&A). Open clusters contain many coeval objects of the same chemical composition and age, and spanning a range of masses. We compare the obtained sequence with a set of theoretical isochrones identifying systematic offsets and revealing probable issues in the models.However, there are many cases when it is impossible to test models before comparing them with observations.As a second step, we apply atmospheric models for constraining parameters of WISE 0855-07, the coolest known Y dwarf(Kopytova et al. 2014, ApJ 797, 3). We demonstrate the limits of constraining effective temperature and the presence/absence of water clouds.As a third step, we introduce a novel method to take into account the above-mentioned systematics. We construct a "systematics vector" that allows us to reveal problematic wavelength ranges when fitting atmospheric models to observed near-infrared spectraof brown dwarfs and exoplanets (Kopytova et al., in prep.). This approach plays a crucial role when retrieving abundances for these objects, in particularly, a C/O ratio. The latter parameter is an important key to formation scenarios of brown dwarf and

  16. The mass donor