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Sample records for red dwarf stars

  1. Photometric Parallaxes for Red Dwarf Stars

    NASA Astrophysics Data System (ADS)

    Humphrey, Nick; Robertson, T.

    2008-05-01

    The luminosity function of low luminosity red stars is import due to the high frequency of such stars and their substantial contribution to the mass of baryonic matter as determined by analysis of the numbers of such stars within a few parsecs of the Sun. Many such stars relatively close to the Sun have not been detected due to their low luminosity and to an inability to distinguish between red giant and dwarf stars. A sample of one hundred potential red dwarf stars was selected from 2MASS photometric data, and USNO-B photometric and astrometric data. Sample stars were observed using the SARA (Southeastern Association for Research in Astronomy) telescope using Kron-Cousins RI photometry and intermediate-band CaH photometry. All thirty two sample stars observed have luminosity classes consistent with red dwarfs. The photometric parallaxes range from 40 to 230 pc. A comparison of USNO-B R magnitudes and the observed CCD R magnitudes (observed - USNO) indicated no systematic difference (-0.03 with and standard error of the mean of 0.05). A comparison of the R-I color index showed a mean difference of -0.21 and standard error of the mean of 0.03. The selection criterion used seems to be quite efficient in identifying red dwarf stars. This study used data collected with the SARA Telescope and was funded by grants from the Indiana Space Grant Consortium and Ball State University.

  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. Planets of Red Dwarf Stars May Face Oxygen Loss in Habitable Zones

    NASA Video Gallery

    In this conceptual animation, X-ray and extreme ultraviolet light from a young red dwarf star cause ions to escape from an exoplanet’s atmosphere. Scientists have developed a model that estimates t...

  4. 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.

  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

    2006-08-03

    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. 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.

  7. 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.

  8. Living with an Old Red Dwarf: X-ray-UV Emissions of Kapteyn’s Star - Effects of X-UV radiation on Habitable Zone Planets hosted by old Red Dwarf Stars

    NASA Astrophysics Data System (ADS)

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

    2015-01-01

    Red dwarfs (dM) stars make up over 75% of the local stellar population and a significant fraction (~40-50%) are older than the Sun. Because of the high frequency of red dwarfs and their longevity (> 50 Gyr), there is a greater possibility of more advanced life in red dwarf-exoplanet systems. MEarths, UVES, SDSS-III, and the upcoming TESS mission are some surveys that are targeting red dwarfs in the search for hosted potentially habitalble planets. As part of Villanova's 'Living with a Red Dwarf' program, we have obtained HST-COS Ultraviolet spectra (1150-3000A) and Chandra X-ray observations of Kapteyn's star (GJ 191; M1 V, V = 8.85 mag , d = 12.76 +/- 0.05 ly). Kapyteyn's Star is important for the study of old red dwarfs because it is the nearest (Pop II) halo star with a radial velocity of +245.2 km/s and an estimated age of 11.2 +/-0.9 Gyrs. Recently Kapteyn's Star was found to host two super-Earth mass planets - one of these is orbiting inside the star's Habitable Zone (Anglada-Escude' 2014: MNRAS 443, L89). In our program, Kapteyn's star is the oldest red dwarf and as such serves as an anchor for our age, rotation, and activity relations. The spectra obtained from HST/COS provide one of the cleanest measurements of the important HI Lyman-alpha 1215.6 A emission flux for red dwarfs. This is due to the large Doppler shift from the high radial velocity, separating the stellar Ly-alpha emission from by the Ly-alpha ISM and local geo-coronal sources. These observations further provide calibrations at the old age/low rotation/low activity extremes for our relations. As the nearest and brightest old red dwarf star, Kapteyn's Star also provides insights into its magnetic properties to investigae coronal x-ray and UV emission for the large population of old, slowly rotating red dwarf stars. Kapteyn's star also serves as a proxy for the numerous metal-poor old disk - Pop II M dwarfs by providing information about X-UV emissions. This information is crucial for

  9. 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.

  10. 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

  11. 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

  12. Ground-based observation of emission lines from the corona of a red-dwarf star.

    PubMed

    Schmitt, J H; Wichmann, R

    2001-08-02

    All 'solar-like' stars are surrounded by coronae, which contain magnetically confined plasma at temperatures above 106 K. (Until now, only the Sun's corona could be observed in the optical-as a shimmering envelope during a total solar eclipse.) As the underlying stellar 'surfaces'-the photospheres-are much cooler, some non-radiative process must be responsible for heating the coronae. The heating mechanism is generally thought to be magnetic in origin, but is not yet understood even for the case of the Sun. Ultraviolet emission lines first led to the discovery of the enormous temperature of the Sun's corona, but thermal emission from the coronae of other stars has hitherto been detectable only from space, at X-ray wavelengths. Here we report the detection of emission from highly ionized iron (Fe XIII at 3,388.1 A) in the corona of the red-dwarf star CN Leonis, using a ground-based telescope. The X-ray flux inferred from our data is consistent with previously measured X-ray fluxes, and the non-thermal line width of 18.4 km s-1 indicates great similarities between solar and stellar coronal heating mechanisms. The accessibility and spectral resolution (45,000) of the ground-based instrument are much better than those of X-ray satellites, so a new window to the study of stellar coronae has been opened.

  13. The potential of planets orbiting red dwarf stars to support oxygenic photosynthesis and complex life

    NASA Astrophysics Data System (ADS)

    Gale, Joseph; Wandel, Amri

    2017-01-01

    We review the latest findings on extra-solar planets and their potential of having environmental conditions that could support Earth-like life. Focusing on planets orbiting red dwarf (RD) stars, the most abundant stellar type in the Milky Way, we show that including RDs as potential life supporting host stars could increase the probability of finding biotic planets by a factor of up to a thousand, and reduce the estimate of the distance to our nearest biotic neighbour by up to 10. We argue that binary and multiple star systems need to be taken into account when discussing habitability and the abundance of biotic exoplanets, in particular RDs in such systems. Early considerations indicated that conditions on RD planets would be inimical to life, as their habitable zones would be so close to the host star as to make planets tidally locked. This was thought to cause an erratic climate and expose life forms to flares of ionizing radiation. Recent calculations show that these negative factors are less severe than originally thought. It has also been argued that the lesser photon energy of the radiation of the relatively cool RDs would not suffice for oxygenic photosynthesis (OP) and other related energy expending reactions. Numerous authors suggest that OP on RD planets may evolve to utilize photons in the infrared. We however argue, by analogy to the evolution of OP and the environmental physiology and distribution of land-based vegetation on Earth, that the evolutionary pressure to utilize infrared radiation would be small. This is because vegetation on RD planets could enjoy continuous illumination of moderate intensity, containing a significant component of photosynthetic 400-700 nm radiation. We conclude that conditions for OP could exist on RD planets and consequently the evolution of complex life might be possible. Furthermore, the huge number and the long lifetime of RDs make it more likely to find planets with photosynthesis and life around RDs than around

  14. 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.

  15. 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.

  16. Abundance analyses of metal-poor stars. III - Red spectra of nine dwarfs

    NASA Astrophysics Data System (ADS)

    Peterson, R. C.

    1980-01-01

    Logarithmic iron abundances with respect to the sun are presented for nine cool, metal-poor dwarfs, and rederived for 15 hotter dwarfs and subgiants. Equivalent widths of lines as weak as 10 mA are used in this analysis, by invoking the goodness of the wavelength coincidence between observed and theoretical line positions to discriminate against noise features and line blends. For the stars discussed by Peterson (1978), the use of furnace gf-values and an independently determined value (-4.50) for the logarithm of the solar iron-to-hydrogen ratio produces abundances which are lower by 0.2 dex than those derived from a solar/stellar line-by-line comparison with vt = 1 km/sec.

  17. 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.

  18. LIVING WITH A RED DWARF: ROTATION AND X-RAY AND ULTRAVIOLET PROPERTIES OF THE HALO POPULATION KAPTEYN’S STAR

    SciTech Connect

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

    2016-04-20

    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{sup −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.

  19. 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

  20. 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

  1. Giants reveal what dwarfs conceal: Li abundance in lower red giant branch stars as diagnostic of the primordial Li

    NASA Astrophysics Data System (ADS)

    Mucciarelli, A.; Salaris, M.; Bonifacio, P.

    2012-01-01

    The discrepancy between cosmological Li abundance inferred from Population II dwarf stars and that derived from big bang nucleosynthesis calculations is still far from being satisfactorily solved. We investigated, as an alternative route, the use of Li abundances in Population II lower red giant branch stars as empirical diagnostic of the cosmological Li. Both theory and observations suggest that the surface Li abundance in metal-poor red giants after the completion of the first dredge-up and before the red giant branch bump is significantly less sensitive to the efficiency of atomic diffusion, compared with dwarf stars. The surface Li abundances in these objects - after the dilution caused by the first dredge-up - are predicted to be sensitive to the total Li content left in the star, i.e. they are affected only by the total amount of Li eventually burned during the previous main-sequence phase. Standard stellar models computed under different physical assumptions show that the inclusion of the atomic diffusion has an impact of about 0.07 dex in the determination of the primordial Li abundance - much smaller than the case of metal-poor main-sequence turnoff stars - and it is basically unaffected by reasonable variations of other parameters (overshooting, age, initial He abundance and mixing length). We have determined from spectroscopy the surface Li content of 17 halo lower red giant branch stars, in the metallicity range between [Fe/H] ˜- 3.4 and ˜- 1.4 dex, evolving before the extramixing episode that sets in at the red giant branch bump. The initial Li (customarily taken as estimate of the cosmological Li abundance A(Li)0) has then been inferred by accounting for the difference between initial and post-dredge-up Li abundances in the appropriate stellar models. It depends mainly on the Teff scale adopted in the spectroscopic analysis, and is only weakly sensitive to the efficiency of atomic diffusion in the models, so long as one neglects Li destruction

  2. X-ray and Hubble/COS UV Measures of Kapteyn's Star: A Crucial Proxy of X-UV Irradiances for Old Red Dwarf Stars that May Host Habitable Zone Planets

    NASA Astrophysics Data System (ADS)

    Durbin, Allyn J.; Guinan, E. F.; Engle, S. G.

    2014-01-01

    Red dwarfs (dM) stars make up over 80% of the local stellar population and a significant fraction of them are old (age > 4 Gyr). Because of the high frequency of red dwarfs and their longevity, there is a greater possibility of more advanced life in red dwarf planet systems. MEarths, UVES, SDSS-III, and the upcoming TESS mission are some surveys that are targeting these objects. As part of Villanova’s Living with a Red Dwarf program, we have obtained HST/COS spectra and Chandra X-ray observations of Kapteyn's star (M1V, V = 8.853, d = 12.76 +/- 0.05 ly, P_rot = 195 days). This star is crucial to the study of old red dwarfs as it is the nearest halo star with a radial velocity of +245.2 km/s and an estimated age of 10-12 Gyr. In our program, Kapteyn's star is the oldest red dwarf and as such serves as an anchor for our age, rotation, and activity relations. The spectra obtained from HST/COS provide one of the cleanest measurements of Lyman-alpha emission for red dwarfs. This is due to Doppler shift from the high radial velocity, separating the Lyman-alpha line from emission produced by the ISM and geocoronal sources. These observations further provide calibration at the old age/low rotation/low activity extremes for our relations. They also provide insights into the magnetic properties as investigating coronal x-ray and UV emission in very old, slowly rotating dM stars. Kapteyn’s star also serves as a proxy for metal-poor old disk/Pop II M dwarfs by providing information about X-UV emissions. This information is crucial for determining X-UV irradiances of possible habitable zone planets hosted by old red dwarfs. We gratefully acknowledge the support from NSF/RUI Grant AST-1009903, NASA/Chandra Grants GO1-12124X and GO2-13020X, and HST-GO-13020.

  3. 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.

  4. 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

  5. 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.

  6. A Study of Low-Metallicity Red Giant Stars in the Ursa Minor Dwarf Spheroidal Galaxy Using APOGEE Survey Data

    NASA Astrophysics Data System (ADS)

    Fu, Wanying; Simon, Joshua D.; APOGEE-2

    2017-01-01

    Studying the chemical evolution of stars in the Milky Way’s faint dwarf galaxy satellites can provide valuable insight into the formation of the Galaxy and its companions. Past chemical abundance studies of the Ursa Minor dwarf spheroidal galaxy contain a maximum of sixteen stars, but large surveys such as APOGEE (Apache Point Observatory Galactic Evolution Experiment), which perform high-resolution spectroscopy (R ˜ 22,500) for hundreds of stars at a time, have the potential to vastly expand the amount of available stellar chemical abundance data and provide a more comprehensive view of the dSph’s chemical evolution. However, the APOGEE reduction and analysis pipelines were designed for high S/N observations of bright stars, and have not been tested in the lower S/N regime of dSph stars. We evaluate the performance of the APOGEE pipeline for low S/N spectra taken from faint, low-metallicity stars in the galaxy. We compare APOGEE metallicities against those found in literature, and examine the spectra for elemental absorption lines. We also attempt to constrain the population of binary stars in the dSph.

  7. 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.

  8. Flare And Starspot-Induced Variabilities Of Red Dwarf Stars In The Open Cluster M37: Photometric Study On Stellar Magnetic Activity

    NASA Astrophysics Data System (ADS)

    Chang, Seo-Won; Byun, Yong-Ik; Hartman, Joel D.

    2016-07-01

    We investigate the statistical properties of flare and starspot-induced variabilities of red dwarf stars in the open cluster M37, particularly (1) understanding magnetic activity phenomena that are seen in groups of stars (with the same age and mass) and (2) the correlations among activity indicators. The use of both tracers is particularly useful for statistical studies since it can provide more homogeneous information about their activity behaviors. We recalibrate the archival imaging data of the M37 obtained by one-month observing run with MMT/Megacam camera, i.e., Deep, High-cadence and Long-term monitoring survey. To detect any significant variability from cool objects, forced photometry with our multi-aperture indexing technique is applied to the entire time-series images. In this contributed talk, we present an update on flare and rotational statistics of this cluster and further strong evidences that support the classical age-rotation-activity paradigm.

  9. 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.

  10. Tracking star formation in dwarf cluster galaxies

    NASA Astrophysics Data System (ADS)

    Rude, Cody Millard

    The evolution of galaxies in dense environments can be affected by close encounters with neighboring galaxies and interactions with the intracluster medium (ICM). Dwarf galaxies may be especially susceptible to these effects due to their low mass. The goal of my dissertation research is to look for signs of star formation in cluster dwarf galaxies by measuring and comparing the r- and u-band luminosity functions of 15 low redshift Abell galaxy clusters using archival data from the Canada-France-Hawaii Telescope (CFHT). Luminosity functions, dwarf-to-giant ratios, and blue fractions are measured in four cluster-centric annuli from stacked cluster data. To account for differences in cluster optical richness, each cluster is scaled according to r200, where r200 is the radius of a sphere, centered on the cluster, whose average density is 200 times the critical density of the universe. The outer region of the cluster sample shows an increase in the faint-end slope of the u-band luminosity function relative to the r-band, indicating star formation in dwarf galaxies. The blue fraction for dwarf galaxies steadily rises with increasing cluster-centric radii. The change in the blue fraction of giant galaxies also increases, but at a lower rate. Additionally, the inner regions of clusters ranging from 0.185 < z < 0.7 from the "Cluster Lensing and Supernova survey with Hubble (CLASH)" are used to generate blue- and red-band luminosity functions, dwarf-to-giant ratios, and blue fractions. Comparisons of the inner region of the CLASH and CFHT clusters show an increase in the blue fraction of dwarf galaxies with redshift that is not present in giant galaxies.

  11. Is PZ MON a Red Dwarf or a Red Giant?

    NASA Astrophysics Data System (ADS)

    Bondar', N. I.; Sergeev, S. G.

    Hipparcos data (ESA 1997) give for PZ Mon a parallax π = 0.71 +- 1.17 mas that is not in agreement with earlier estimations of distance to the star of 15-30 ps and with its status of K- dwarf. There are presented spectra of PZ Mon in the region 3750-6150 Α with resolution of 7 ≍ Α, which were carried out in the Nasmith focus of 2.6 m reflector in the Crimean Astrophysical observatory. Intensities of molecular absorptions in this spectral range are determined relative a nearby continuum and their relations between absolute magnitudes are considered. The calculated Mν =6.74 and r=29 ps correspond to an earlier classiffication of PZ Mon as a red dwarf.

  12. 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.

  13. 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)

  14. Are All Magnetic White Dwarf Stars Massive?

    NASA Astrophysics Data System (ADS)

    Nitta, A.; Kepler, S. O.; Kulebi, B.; Koester, D.; Kleinman, S. J.; Winget, D. E.; Castanheira, B. G.; Corsico, A. H.

    2017-03-01

    We obtained follow-up spectra on 25 white dwarf stars identified in our white dwarf catalog of Sloan Digital Sky Survey (SDSS) as massive or magnetic. We identified over 300 magnetic white dwarf stars from SDSS with some uncertainties due to the low S/N of the spectra. With much higher S/N Gemini data, our sample should be able to help us confirm accuracy of our determinations. We present here our results so far from the follow up observations.

  15. Innocent Bystanders and Smoking Guns: Dwarf Carbon Stars

    NASA Astrophysics Data System (ADS)

    Green, Paul J.

    2014-01-01

    As far as we know, most carbon throughout the Universe is created and dispersed by AGB stars. So it was at first surprising to find that the carbon stars most prevalent in the Galaxy are in fact dwarfs. We suspect that dC stars are most likely innocent bystanders in post-mass transfer binaries, and may be predominantly metal-poor. Among 1200 C stars found in the SDSS (Green 2013), we confirm 724 dCs, of which a dozen are DA/dC stars in composite spectrum binaries, quadrupling the total sample of these "smoking guns" for AGB binary mass transfer. The dCs likely span absolute magnitudes M_i from about 6.5 to 10.5. G-type dC stars with weak CN and relatively blue colors are probably the most massive dCs still cool enough to show C_2 bands. Eleven very red C stars with strong red CN bands appear to be N-type AGB stars at large Galactocentric distances, one likely a new discovery in the dIrr galaxy Le A. Two such stars within 30arcmin of each other may trace a previously unidentified dwarf galaxy or tidal stream at ~40 kpc. We describe follow-up projects to study the spatial, kinematic, and binary properties of these C-enriched dwarfs.

  16. Sizing Up Southern Red Dwarfs in the Solar Neighborhood

    NASA Astrophysics Data System (ADS)

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

    2017-01-01

    Red dwarfs comprise 75% of the stars in the Milky Way, yet determining one of their most fundamental characteristics --- age --- has proven difficult. Because no red dwarf has yet evolved beyond its main sequence life in the age of the Universe, as a class they are valuable touchstones fundamental to mapping the history of star and planet formation in the Milky Way, if their ages can be reliably determined. In the volume-limited RECONS 10 Parsec Sample census of 298 star systems, there are currently only two pre-main sequence young systems and three confirmed old subdwarf systems known. Four of these five extreme-age systems contain red dwarfs, but these are woefully insufficient to understand the complex star formation history of the Milky Way. Our all-sky study of ~1700 red dwarfs within 25 parsecs, supplemented with ~80 cool subdwarfs and ~90 young stars for comparison, will ultimately extend our calibration of red dwarf agesto populations that are statistically significant and robust, revealing for the first time the fractions of young and old systems out to 25 parsecs. Here we report our findings for the first ~600 stars south of Dec = 0.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 andbolometric fluxes for the red dwarfs. The full suites of our photometric and astrometric data (including hundreds of accurate new parallaxes by the RECONS team from 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. These radii allow us to identify young/old stars via their correspondingly large/small radii, revealing a snapshot of relative stellar ages in the solar neighborhood from which we can begin to disentangle

  17. Old stellar populations in star-forming dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Held, Enrico V.; Saviane, Ivo; Momany, Yazan; Rizzi, Luca; Bertelli, Gianpaolo

    We present deep VLT/FORS1 observations of the two distant, isolated Local Group dwarfs Phoenix and Antlia. Our results provide further evidence for the presence of old stars in these star-forming dwarf galaxies. Old stellar populations are known in all of the Local Group dwarf spheroidal galaxies and in some dwarf irregulars, implying that dwarf galaxies started forming stars at a sharply defined early epoch irrespective of their subsequent star formation histories (e.g., Held et al., 2000; Saviane et al., 2000; and references therein). The new color-magnitude diagrams of Phoenix confirm the presence of a spatially extended blue HB population, indicating a conspicuous old component (Held et al., 1999; Martínez-Delgado et al., 1999). A preliminary analysis of stellar variability has led to the discovery of several tens RR Lyrae variables, which can provide clue information on the earliest star formation episode (see, e.g., Siegel and Majewski, 2000). The young main sequence extends down to the limit of our photometry (V=25.5 mag), which suggests that Phoenix underwent nearly continuous star formation in the last 2 Gyr. Our deep color-magnitude diagrams of Antlia have been used to investigate the gradient in the stellar populations of this dwarf irregular/spheroidal galaxy. While the young stars appear to be concentrated in a round central region (Aparicio et al., 1997; Sarajedini et al., 1997), the spatial distribution of the red giant stars defines an extended flattened halo (or disk) 2-3 kpc across.

  18. Dwarf Star Erupts in Giant Flare

    NASA Technical Reports Server (NTRS)

    2005-01-01

    This movie taken by NASA'S Galaxy Evolution Explorer shows one of the largest flares, or star eruptions, ever recorded at ultraviolet wavelengths. The star, called GJ 3685A, just happened to be in the Galaxy Evolution Explorer's field of view while the telescope was busy observing galaxies. As the movie demonstrates, the seemingly serene star suddenly exploded once, then even more intensely a second time, pouring out in total about one million times more energy than a typical flare from our Sun. The second blast of light constituted an increase in brightness by a factor of at least 10,000.

    Flares are huge explosions of energy stemming from a single location on a star's surface. They are caused by the brief destruction of a star's magnetic fields. Many types of stars experience them, though old, small, rapidly rotating 'red dwarfs' like GJ 3685A tend to flare more frequently and dramatically. These stars, called flare stars, can experience powerful eruptions as often as every few hours. Younger stars, in general, also erupt more often. One of the reasons astronomers study flare stars is to gain a better picture and history of flare events taking place on the Sun.

    A preliminary analysis of the GJ 3685A flare shows that the mechanisms underlying stellar eruptions may be more complex than previously believed. Evidence for the two most popular flare theories was found.

    Though this movie has been sped up (the actual flare lasted about 20 minutes), time-resolved data exist for each one-hundredth of a second. These observations were taken at 2 p.m. Pacific time, April 24, 2004. In the still image, the time sequence starts in the upper left panel, continues in the upper right, then moves to the lower left and ends in the lower right.

    The circular and linear features that appear below and to the right of GJ 3685A during the flare event are detector artifacts caused by the extreme brightness of the flare.

  19. Search for Planets around Pulsating White Dwarf Stars

    NASA Astrophysics Data System (ADS)

    Mullally, F.; Winget, D. E.; Kepler, S. O.

    2005-12-01

    We present initial results from our search for planets around variable white dwarf stars. White dwarf stars are the end point of stellar evolution for 98% of main sequence stars. Theoretical calculations (Sackmann 1993; Duncan & Lissauer 1998) predict that planets further than 1 AU from their parent star will survive the red giant phase. When a hydrogen atmosphere white dwarf (DA) cools to about 12000K it becomes a variable star. A subset of these stars exhibit pulsational stability that rivals atomic clocks (˙ {P} ˜ 10-15; Kepler et al. 2005). The reflex orbital motion of the star around the center of mass of the system due to the presence of a planet changes the light travel time of these stable pulses and hence their observed arrival time on earth, providing a method to detect the planet. Because we are measuring change in distance to the star, planets in long period orbits are easier to detect, complementing the Doppler shift method. This work is supported by grant from the NASA Origins program, NAG5-13094 and performed in part under contract with JPL through the Michelson Fellowship Program.

  20. An overview of white dwarf stars

    NASA Astrophysics Data System (ADS)

    Fontaine, G.; Brassard, P.; Charpinet, S.; Randall, S. K.; Van Grootel, V.

    2013-03-01

    We present a brief summary of what is currently known about white dwarf stars, with an emphasis on their evolutionary and internal properties. As is well known, white dwarfs represent the end products of stellar evolution for the vast majority of stars and, as such, bear the signatures of past events (such as mass loss, mixing phases, loss and redistribution of angular momentum, and thermonuclear burning) that are of essential importance in the evolution of stars in general. In addition, white dwarf stars represent ideal testbeds for our understanding of matter under extreme conditions, and work on their constitutive physics (neutrino production rates, conductive and radiative opacities, interior liquid/solid equations of state, partially ionized and partially degenerate envelope equations of state, diffusion coefficients, line broadening mechanisms) is still being actively pursued. Given a set of constitutive physics, cooling white dwarfs can be used advantageously as cosmochronometers. Moreover, the field has been blessed by the existence of four distinct families of pulsating white dwarfs, each mapping a different evolutionary phase, and this allows the application of the asteroseismological method to probe and test their internal structure and evolutionary state. We set the stage for the reviews that follow on cooling white dwarfs as cosmochronometers and physics laboratories, as well as on the properties of pulsating white dwarfs and the asteroseismological results that can be inferred.

  1. 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.

  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. 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.

  4. Pulsating White Dwarf Stars and Precision Asteroseismology

    NASA Astrophysics Data System (ADS)

    Winget, D. E.; Kepler, S. O.

    2008-09-01

    Galactic history is written in the white dwarf stars. Their surface properties hint at interiors composed of matter under extreme conditions. In the forty years since their discovery, pulsating white dwarf stars have moved from side-show curiosities to center stage as important tools for unraveling the deep mysteries of the Universe. Innovative observational techniques and theoretical modeling tools have breathed life into precision asteroseismology. We are just learning to use this powerful tool, confronting theoretical models with observed frequencies and their time rate-of-change. With this tool, we calibrate white dwarf cosmochronology; we explore equations of state; we measure stellar masses, rotation rates, and nuclear reaction rates; we explore the physics of interior crystallization; we study the structure of the progenitors of Type Ia supernovae, and we test models of dark matter. The white dwarf pulsations are at once the heartbeat of galactic history and a window into unexplored and exotic physics.

  5. White Dwarf Stars: A Brief Overview

    NASA Astrophysics Data System (ADS)

    Fontaine, G.; Brassard, P.; Charpinet, S.; Randall, S. K.; Van Grootel, V.

    2013-12-01

    We present a brief summary of what is currently known about white dwarf stars, with an emphasis on their evolutionary and internal properties. As is well known, white dwarfs represent the end products of stellar evolution for the vast majority of stars and, as such, bear the signatures of past events (such as mass-loss, mixing phases, loss and redistribution of angular momentum, and thermonuclear burning) that are of essential importance in the evolution of stars in general. In addition, white dwarf stars represent ideal testbeds for our understanding of matter under extreme conditions, and work on their constitutive physics (neutrino production rates, conductive and radiative opacities, interior liquid and solid equations of state, partially ionized and partially degenerate envelope equations of state, diffusion coefficients, line broadening mechanisms) is still being actively pursued. Given a set of constitutive physics, cooling white dwarfs can be used advantageously as cosmochronometers. Moreover, the field has been blessed by the existence of four distinct families of pulsating white dwarfs, each mapping a different evolutionary phase, and this allows the application of the asteroseismological method to probe and test their internal structure and evolutionary state.

  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. Dwarf Discoveries from Serendipitous Field Star Observations

    NASA Astrophysics Data System (ADS)

    Green, Wayne

    2016-05-01

    For the past two years, The University of Colorado, in collaboration with Las Cumbres Observatory Global Telescope Network (LCOGTN) has been taking Sloans r' and i' images of approximately 200 galaxies during each new moon period to provide ground data in support of the approximately 1100 hours of warm Spitzer time awarded to Dr. Mansi Kasliwal's Caltech SPIRITS program. Currently there are over 6,000 images in this archive. Small telescope scientists routinely image the same fields, building similar archives brimming with science potential. This paper reports the technique to develop serendipitous observations of dwarf field stars. Answers to questions surrounding the dwarf's early life in proximity to non-hierarchal multiple star groups, about how dwarfs not only survive but are so numerous are well within the capabilities of small telescope scientists. The role of the small telescope scientist is of vital importance in these (re)discovery, confirmation, monitoring and reporting tasks.

  9. Massive Star Clusters in Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Larsen, Søren S.

    2017-03-01

    Dwarf galaxies can have very high globular cluster specific frequencies, and the GCs are in general significantly more metal-poor than the bulk of the field stars. In some dwarfs, such as Fornax, WLM, and IKN, the fraction of metal-poor stars that belong to GCs can be as high as 20%-25%, an order of magnitude higher than the 1%-2% typical of GCs in halos of larger galaxies. Given that chemical abundance anomalies appear to be present also in GCs in dwarf galaxies, this implies severe difficulties for self-enrichment scenarios that require GCs to have lost a large fraction of their initial masses. More generally, the number of metal-poor field stars in these galaxies is today less than what would originally have been present in the form of low-mass clusters if the initial cluster mass function was a power-law extending down to low masses. This may imply that the initial GC mass function in these dwarf galaxies was significantly more top-heavy than typically observed in present-day star forming environments.

  10. 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.

  11. Stellar model chromospheres. XIII - M dwarf stars

    NASA Technical Reports Server (NTRS)

    Giampapa, M. S.; Worden, S. P.; Linsky, J. L.

    1982-01-01

    Single-component, homogeneous model chromospheres that are consistent with high-resolution profiles of the Ca II K line calibrated in surface flux units for three dMe and 2 dM stars observed at quiescent times are constructed. The models reveal several systematic trends. Large values of the ratio of T(min) to T(eff) are derived, indicating a large amount of nonradiative heating present in the upper photospheres of M dwarf stars. It is also found that the lower chromospheric temperature gradient is similar for all the M dwarf stars. Since for the models here the chromospheric K line emission strength is most sensitive to the total amount of chromospheric material present within the approximate temperature range T(min)-6000 K, increasing the emission strength is not simply due to increasing chromospheric temperature gradients. It is also found that both the electron density and electron temperature at one thermalization length in the K line below the top of the chromospheres are greater in the dMe stars than in the dM stars. The M dwarf models here have microturbulent velocities between 1 and 2 km/sec, which are much smaller than for solar chromosphere models.

  12. 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.

  13. Stripped Red Giants - Helium Core White Dwarf Progenitors and their sdB Siblings

    NASA Astrophysics Data System (ADS)

    Heber, U.

    2017-03-01

    Some gaps in the mosaic of binary star evolution have recently been filled by the discoveries of helium-core white dwarf progenitors (often called extremely low mass (ELM) white dwarfs) as stripped cores of first-giant branch objects. Two varieties can be distinguished. One class is made up by SB1 binaries, companions being white dwarfs as well. Another class, the so-called EL CVn stars, are composite spectrum binaries, with A-Type companions. Pulsating stars are found among both classes. A riddle is posed by the apparently single objects. There is a one-to-one correspondence of the phenomena found for these new classes of star to those observed for sdB stars. In fact, standard evolutionary scenarios explain the origin of sdB stars as red giants that have been stripped close to the tip of first red giant branch. A subgroup of subluminous B stars can also be identified as stripped helium-cores of red giants. They form an extension of the ELM sequence to higher temperatures. Hence low mass white dwarfs of helium cores and sdB stars in binaries are close relatives in terms of stellar evolution.

  14. 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

  15. 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.

  16. The Star, the Dwarf and the Planet

    NASA Astrophysics Data System (ADS)

    2006-10-01

    Astronomers have detected a new faint companion to the star HD 3651, already known to host a planet. This companion, a brown dwarf, is the faintest known companion of an exoplanet host star imaged directly and one of the faintest T dwarfs detected in the Solar neighbourhood so far. The detection yields important information on the conditions under which planets form. "Such a system is an interesting example that might prove that planets and brown dwarfs can form around the same star", said Markus Mugrauer, lead author of the paper presenting the discovery. ESO PR Photo 39a/06 ESO PR Photo 39a/06 The Companion to HD 3651 HD 3651 is a star slightly less massive than the Sun, located 36 light-years away in the constellation Pisces (the "Fish"). For several years, it has been known to harbour a planet less massive than Saturn, sitting closer to its parent star than Mercury is from the Sun: the planet accomplishes a full orbit in 62 days. Mugrauer and his colleagues first spotted the faint companion in 2003 on images from the 3.8-m United Kingdom Infrared Telescope (UKIRT) in Hawaii. Observations in 2004 and 2006 using ESO's 3.6 m New Technology Telescope (NTT) at La Silla provided the crucial confirmation that the speck of light is not a spurious background star, but indeed a true companion. The newly found companion, HD 3651B, is 16 times further away from HD 3651 than Neptune is from the Sun. HD 3651B is the dimmest directly imaged companion of an exoplanet host star. Furthermore, as it is not detected on the photographic plates of the Palomar All Sky Survey, the companion must be even fainter in the visible spectral range than in the infrared, meaning it is a very cool low-mass sub-stellar object. Comparing its characteristics with theoretical models, the astronomers infer that the object has a mass between 20 and 60 Jupiter masses, and a temperature between 500 and 600 degrees Celsius. It is thus ten times colder and 300 000 less luminous than the Sun. These

  17. Identification of Nearby Dwarf Carbon Stars

    NASA Astrophysics Data System (ADS)

    Lowrance, P. J.; Kirkpatrick, J. D.; Reid, I. N.

    2004-12-01

    The comparison of optical and 2MASS near-infrared photometry for large samples of cataloged proper motion stars has the potential to discover previously unrecognized nearby objects of rare type. We have obtained classification spectra for carbon dwarf candidates which lie in a sparsely populated part of optical/near-IR color-color space within a cross-reference of the New Luytens Two-Tenths (NLTT) catalogue and 2MASS 2nd Release. We present the discovery of nine of the coolest and nearest carbon dwarfs, whose optical spectra, exhibiting absorptions by C2 and/or CN is displayed. The only known discriminator between carbon giants and dwarfs is luminosity, which can be gained through distance or inferred from proper motion. Therefore, we have also observed most known dwarfs and giants to fully explore spectroscopic diagnostics that can be used to differentiate between carbon dwarfs and giants including many published in the Sloan Digital Sky Survey papers. We also plan to continue to merge the NLTT with the 2MASS ALL-Sky Release and obtain classification spectra for candidates for the rest of the sky not covered in the first merge.

  18. Star formation in proto dwarf galaxies

    NASA Technical Reports Server (NTRS)

    Noriega-Crespo, A.; Bodenheimer, P.; Lin, D. N. C.; Tenorio-Tagle, G.

    1990-01-01

    The effects of the onset of star formation on the residual gas in primordial low-mass Local-Group dwarf spheroidal galaxies is studied by a series of hydrodynamical simulations. The models have concentrated on the effect of photoionization. The results indicate that photoionization in the presence of a moderate gas density gradient can eject most of the residual gas on a time scale of a few 10 to the 7th power years. High central gas density combined with inefficient star formation, however, may prevent mass ejection. The effect of supernova explosions is discussed briefly.

  19. 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.

  20. 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.

  1. GAS, STARS, AND STAR FORMATION IN ALFALFA DWARF GALAXIES

    SciTech Connect

    Huang Shan; Haynes, Martha P.; Giovanelli, Riccardo; Brinchmann, Jarle; Stierwalt, Sabrina; Neff, Susan G. E-mail: haynes@astro.cornell.edu E-mail: jarle@strw.leidenuniv.nl E-mail: susan.g.neff@nasa.gov

    2012-06-15

    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 <10{sup 7.7} M{sub Sun} and H I line widths <80 km s{sup -1}. Sloan Digital Sky Survey (SDSS) data are combined with photometric properties derived from Galaxy Evolution Explorer to derive stellar masses (M{sub *}) 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{sub *} 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{sub *} {approx}< 10{sup 8} M{sub Sun} 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{sub *} 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.

  2. 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.

  3. 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

  4. UBV photometry of hot white dwarf stars

    NASA Astrophysics Data System (ADS)

    Cheselka, Mathew; Holberg, J. B.; Watkins, Ron; Collins, James; Tweedy, R. W.

    1993-12-01

    Johnson UBV photometry has been obtained for a set of hot degenerate stars, primarily DA and DO white dwarfs from among those detected in the Palomar-Green survey of UV excess objects. Most of our program stars have estimated effective temperatures (Teff) in the range 22,000 to 80,000 K and have no previous photometry. Some objects selected are also x-ray and extreme ultraviolet sources from the ROSAT all sky survey. The importance of precise photometric measurements in the analysis of x-ray data is discussed. A discrepancy between the observed colors and predicted colors is noted, and possibly accounted for by difficulties in defining the atmospheric cutoff of the U band and a general lack of hot stars used to define the photometric transformation between theoretical and observed colors.

  5. The Gas in Virgo’s “Red and Dead” Dwarf Elliptical Galaxies

    NASA Astrophysics Data System (ADS)

    Hallenbeck, Gregory L.; Koopmann, Rebecca A.

    2017-01-01

    As star-forming dwarf irregulars and faint spirals fall onto a cluster, their gas content is easily and quickly removed by ram-pressure stripping or other cluster forces. Residual signs of star formation cease within 100 Myr, and only after approximately 1 Gyr do their optical features transition to elliptical.Despite this, ALFALFA has uncovered a population of three “red and dead” dwarf ellipticals in the Virgo Cluster which still have detectable reservoirs of HI. These dwarf ellipticals are extremely gas-rich—as gas-rich as the cluster’s star-forming dwarf irregulars (Hallenbeck et al. 2012). Where does this gas come from? We consider two possibilities. First, that the gas is recently acquired, and has not yet had time to form stars. Second, that the gas is primordial, and has been disrupted from being able to form stars during the current epoch.We present deep optical (using CFHT and KPNO) and HI (Arecibo and VLA) observations of this sample to demonstrate that this gas is primordial. These observations show that all three galaxies have exponentially decreasing profiles characteristic of dwarf ellipticals and that their rotation velocities are extremely low. However, like more massive elliptical galaxies with HI, these dwarf galaxies show irregular optical morphology. For one target, VCC 190, we additionally observe an HI tail consistent with a recent interaction with the massive spiral galaxy NGC 4224.

  6. Star Formation in Dwarf Irregular Galaxies

    NASA Astrophysics Data System (ADS)

    Dohm-Palmer, Robbie Christopher

    I have explored the star formation histories of the dwarf irregular galaxies Sextans A and GR 8. I measured photometry of individual stars from images taken by the Wide Field and Planetary Camera 2 aboard the Hubble Space Telescope. With the photometry I constructed color-magnitude diagrams (CMDs) in the B, V, and I. I investigated the errors in the photometry extraction, and conducted artificial star tests to measure the photometric limits. The high resolution of the Hubble Space Telescope allowed photometric measurements that were far more accurate than ground-based observations. For galaxies at these distances (1-2 Mpc), the accuracy of stellar photometry from ground-based observations is limited by crowding of stellar images. The high accuracy photometry showed a clear separation of the main sequence from the massive, blue, core He-burning stars (HeB). These are stars in the bluest extent of the so-called 'blue-loop' phase of stellar evolution. This is the first time this phase of evolution has been clearly identified in a low metallicity system. The distributions of stars in the CMDs agreed very well with stellar evolution model predictions. I have used the CMDs to calculate the recent star formation histories of both galaxies. The main sequence luminosity function provided the star formation rate (SFR) over the past ~50 Myr. I developed a new technique for calculating the SFR from the blue HeB luminosity function. Furthermore, the blue HeB evolutionary phase has a one-to-one relation between age and magnitude. This allowed me to calculate the position, as well as the strength of star formation over the past ~500 Myr. The star formation was found in concentrated regions. These regions are of order 100 pc across and last of order 100 Myr. The regions were found near the highest density HI gas. I estimated the gas-to-star conversion efficiency to be 5-10%. The results from GR 8 suggest that the star forming gas clouds may be self-gravitating, and that each cloud

  7. Very Low-Mass Stars and Brown Dwarfs

    NASA Astrophysics Data System (ADS)

    Rebolo, Rafael; Rosa Zapatero-Osorio, Maria

    2001-02-01

    Part I. Searches in Clusters, Stellar Associations and the Field: 1. Open clusters after HIPPARCOS J. S. Mermilliod; 2. Proper motions of very low mass stars and brown dwarfs in open clusters N. C. Hambly; 3. Parallaxes for brown dwarfs in clusters C. G. Tinney; 4. Very low mass stars and brown dwarfs in the Belt of Orion S. J. Wolk and F. M. Walter; 5. Photometric surveys in open clusters M. R. Zapatero Osorio; 6. The mass function of the Pleiades R. F. Jameson et al.; 7. Brown dwarfs and the low-mass initial mass function in young clusters K. L. Luhman; 8. Very low mass stars in globular clusters I. R. King and G. Piotto; 9. The DENIS very low mass star and brown dwarf results X. Delfosse and T. Forveille; 10. Preliminary results from the 2MASS core project J. Liebert et al.; Part II. Spectroscopic Properties, Fundamental Parameters and Modelling: 11. Properties of M dwarfs in clusters and the field S. L. Hawley et al.; 12. Spectroscopy of very low mass stars and brown dwarfs in young clusters E. L. Martin; 13. High resolution spectra of L type stars and brown dwarfs G. Basri et al.; 14. Modelling very low mass stars and brown dwarf atmospheres F. Allard; 15. Dust in very cool dwarfs T. Tsuji; 16. On the interpretation of the optical spectra of very cool dwarfs Ya. V. Pavlenko; 17. Absolute dimensions for M type dwarfs A. Gimenez; 18. Theory of very low mass stars and brown dwarfs I. Baraffe; Part III. Convection, Rotation and Acitivity: 19. Convection in low mass stars F. D'Antona; 20. Rotation law and magnetic field in M dwarf models G. Rudiger and M. Kuker; 21. Doppler imaging of cool dwarf stars K. G. Strassmeier; 22. X-ray Emission from cool dwarfs in clusters S. Randich; 23. X-ray variability for dM stars G. Micela and A. Marino; 24. The coronae of AD Leo and EV Lac S. Sciortino et al.; 25. Prospects of vuture X-ray missions for low mass stars and cluster stars R. Pallavicini.

  8. 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.

  9. A search for a new class of pulsating DA white dwarf stars in the DB gap

    NASA Astrophysics Data System (ADS)

    Kurtz, D. W.; Shibahashi, H.; Dhillon, V. S.; Marsh, T. R.; Littlefair, S. P.

    2008-10-01

    While white dwarf stars are classified into many subgroups based on the appearance of hydrogen, helium, carbon, oxygen and other spectral lines - or even pure continuum with no lines in the case of the DC stars - the vast majority fall into two major subgroups: those with hydrogen atmospheres (the DA white dwarfs), and those with helium atmospheres (the DO and DB white dwarfs). Remarkably, in the range 45000 >= Teff >= 30000K there are only a few white dwarfs with helium atmospheres to be found - the vast majority are DAs in this temperature range - although white dwarfs with helium atmospheres are found at both hotter (DO) and cooler (DB) effective temperatures. This dearth of helium atmosphere white dwarfs in this temperature range is known as the `DB gap' and is understood in terms convective mixing of the outer atmospheres at the hot and cool ends of the gap, while radiative stability allows the lighter hydrogen to float to the top in the DB gap, so the stars are seen to be DA hydrogen atmosphere white dwarfs. Asteroseismology is an important tool for probing stellar interiors, and white dwarf stars are the most successfully studied group using this technique. In a stability analysis of the stars in the DB gap, Shibahashi has recently predicted the existence of a new class of pulsating white dwarf stars. He finds from models that DA white dwarfs near the red edge of the DB gap have convectively stable outer atmospheres because of a steep mean molecular weight gradient, yet nevertheless have a superadiabatic layer that renders them pulsationally unstable due to radiative heat exchange. There have been very few observational tests for pulsation among stars of this type. We have initiated a survey to search for the predicted pulsators and report here our first observations of five stars with the South African Astronomical Observatory 1.9-m telescope and University of Cape Town CCD photometer, and two stars with the William Herschel Telescope 4.2-m telescope and

  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. Luminosity Classification of Potential M Dwarf Stars Selected using 2MASS and Tycho2 Data

    NASA Astrophysics Data System (ADS)

    Mason, Justin R.; Humphrey, N.; Briggs, A.; Parrell, A.; Robertson, T.

    2008-03-01

    A sample of possible red dwarf stars selected using photometric criteria from the 2MASS catalog and proper motion data from the TYCHO2 catalog has been investigated to evaluate the effectiveness of the selection process. Program stars have been identified using the J-H and H-K two color diagram to identify potential red dwarf stars. Those stars having Tycho2 proper motions in excess of 0.050 arcsec/yr were included in the sample. Intermediate-band CaH photometry and Kron-Cousins R and I photometry have been obtained using the SARA (Southeastern Association for Research in Astronomy) Telescope. The SARA photometric data were used to determine luminosity classes for program stars. The addition of proper motion data in program star selection has significantly improved the selection of red dwarf stars using 2MASS data. This study used data collected with the SARA Telescope and was funded by grants from the Indiana Space Grant Consortium and Ball State University.

  13. 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.

  14. Accretion phenomena in nearby star-forming dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Annibali, F.; Tosi, M.; Aloisi, A.; Bellazzini, M.; Buzzoni, A.; Cignoni, M.; Ciotti, L.; Cusano, F.; Nipoti, C.; Sacchi, E.; Paris, D.; Romano, D.

    2017-03-01

    We present two pilot studies for the search and characterization of accretion events in star-forming dwarf galaxies. Our strategy consists of two complementary approaches: i) the direct search for stellar substructures around dwarf galaxies through deep wide-field imaging, and ii) the characterization of the chemical properties in these systems up to large galacto-centric distances. We show our results for two star-forming dwarf galaxies, the starburst irregular NGC 4449, and the extremely metal-poor dwarf DDO 68.

  15. A radio-pulsing white dwarf binary star.

    PubMed

    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

    2016-09-15

    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 10(7)-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.

  16. 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.

  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. 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.

  19. Luminosities and temperatures of M dwarf stars from infrared photometry

    NASA Technical Reports Server (NTRS)

    Veeder, G. J.

    1974-01-01

    Bolometric magnitudes for a large number of M type dwarf stars, obtained by broadband infrared photometry at 1.65, 2.2, and 3.5 microns, are reviewed. The data obtained indicate that one parameter is sufficient to describe the blanketing in all of the UBVRI bands for all types of M dwarfs. In general, late M dwarfs seem to have lower effective temperatures than are predicted by theoretical models.

  20. Direct detection of brown dwarf companions of nearby stars

    NASA Astrophysics Data System (ADS)

    Oppenheimer, Ben R.

    This thesis presents the first direct detection of a substellar companion of a star other than the Sun. This object, a brown dwarf called Gliese 229B, presented a unique opportunity to characterize low-temperature brown dwarfs for the first time. The discovery and initial spectrum of Gliese 229B show that the object must be substellar based on its intrinsic luminosity of 6.4×10-6Lsolar and its cool surface temperature, 900 K. Detailed study of Gliese 229B includes extensive photometric measurements from 0.5 to 12 μm, high signal-to-noise ratio spectroscopy from 0.84 to 5.0 μm and the detection of 0'' t; yr-1 of orbital motion. These results are presented in Chapters 2 and 3. A detailed review of brown dwarf science leads to a complete and scientifically meaningful definition of the classes ``planet'' and ``brown dwarf''' in Chapter 1. After the discovery of Gliese 229B, which was found in a survey for companions of young stars, we began an extensive search for brown dwarf companions in orbit about all known stars within 8 pc of the Sun and with δ > -35°. The search includes optical coronagraphic and infrared direct imaging of these stars, conducted on the Palomar 60' and 200' telescopes respectively. The search was designed to find companions of each star without color bias. While the search revealed no other brown dwarf companions of these stars, it did uncover 6 new stellar companions. The sensitivity limits of the survey permit the detection of brown dwarfs up to four magnitudes fainter than Gliese 229B around 90% of the stars. The sensitivity is, however, not uniform spatially or from star to star. This limits our ability to make strong statements about the prevalence of brown dwarf companions of nearby stars. The survey does have sensitivity to all stellar companions between 3 and 30' from the survey stars, however. Chapter 5 describes related work on very low-mass stars in the Pleiades star cluster. This optical spectroscopy involved trying to find a

  1. 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.

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

    PubMed

    Hodgkin; Oppenheimer; Hambly; Jameson; Smartt; Steele

    2000-01-06

    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.

  3. 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.

  4. 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

  5. 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.

  6. 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.

  7. 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.

  8. 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.

  9. A Search for Planets Around Red Stars

    NASA Astrophysics Data System (ADS)

    Gettel, Sara

    2012-12-01

    Our knowledge of planets around other stars has expanded drastically in recent years, from a handful Jupiter-mass planets orbiting Sun-like stars, to encompass a wide range of planet masses and stellar host types. In this thesis, I review the development of radial velocity planet searches and present results from projects focusing on the detection of planets around two classes of red stars. The first project is part of the Penn State - Torun Planet Search (PTPS) for substellar companions to K giant stars using the Hobby-Eberly Telescope (HET). The results of this work include the discovery of planetary systems around five evolved stars. These systems illustrate several of the differences between planet detection around giants and Solar-type stars, including increased masses and a lack of short period planets. One planet has a nearly six year orbit, the longest announced to date around a giant star, with an amplitude approaching the limits of detectability due to stellar "jitter". Two more of these systems also show long-term radial velocity trends which are likely caused by the presence of an additional, more distant binary companion. The remaining two systems show increased radial velocity noise, typical of giant systems. Finally I show that, if the stellar jitter is caused by p-mode oscillations, the amplitude of this noise is anti-correlated with metallicity. The second project focuses on the expansion of the current radial velocity calibration methods to a new wavelength regime. The absorption cell technique is modified to use the telluric O2 and water vapor bands found between ∼6000-9000 A. These features have been found to be stable to ∼10 m s-1 and allow access to the increased red flux of low-mass and evolved stars. I carry out a mock planet search of six early M dwarfs that are known to be radial velocity stable, providing a recoverable null result. Measurements are also made of several telluric standards, to improve the characterization of the

  10. 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.

  11. Chemical abundances in metal-poor stars in Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Venn, Kim; Norris, John; Shetrone, Matthew

    2015-08-01

    Stars in low-mass dwarf galaxies show a larger range in their chemical properties than those in the Milky Way halo. Not only are alpha-poor stars found at lower metallicities, but also r-process challenged stars, and a disparate fraction of carbon-enhanced metal-poor stars. A more pristine and chemically inhomogeneous interstellar medium, combined with stoichastic star formation in a metal-poor environment, is thought to cause these detectable differences in the early SN II contributions. We are also now finding stars in dwarf galaxies that appear to be iron-enhanced, i.e., stars that have formed in pockets of SN Ia enriched gas. A comparison of their chemical abundances with individual SN Ia models can provide unique constraints on the SN Ia progenitors.

  12. 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.

  13. 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.

    2016-12-01

    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 survey in the Southern hemisphere and the Large Sky Area Multi-Object Fiber Spectroscopic Telescope 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.

  14. A VLT/FORS2 spectroscopic survey of individual stars in a transforming dwarf galaxy

    NASA Astrophysics Data System (ADS)

    Battaglia, G.; Kacharov, N.; Rejkuba, M.

    2017-03-01

    Understanding the properties of dwarf galaxies is important not only to put them in their proper cosmological context, but also to understand the formation and evolution of the most common type of galaxies. Dwarf galaxies are divided into two main classes, dwarf irregulars (dIrrs) and dwarf spheroidals (dSphs), which differ from each other mainly because the former are gas-rich objects currently forming stars, while the latter are gas-deficient with no on-going star formation. Transition types (dT) are thought to represent dIs in the process of losing their gas, and can therefore shed light into the possible process of dwarf irregulars (dIrrs) becoming gas-deficient, passively evolving galaxies. Here we present preliminary results from our wide-area VLT/FORS2 MXU spectroscopic survey of the Phoenix dT, from which we obtained line-of-sight velocities and metallicities from the nIR Ca II triplet lines for a large sample of individual Red Giant Branch stars.

  15. 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).

  16. 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.

  17. 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

  18. 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.

  19. The narrow ultraviolet emission lines of the red dwarf Au Microscopii (dM1.6e)

    NASA Technical Reports Server (NTRS)

    Ayres, T. R.; Eriksson, K.; Linsky, J. L.; Stencel, R. E.

    1983-01-01

    It is pointed out that the red dwarfs are the smallest, coolest, faintest, least massive, but most common of normal main-sequence stars. The dMe (H-alpha emission) subclass of the red dwarfs exhibits the largest median soft X-ray to bolometric luminosity ratio of any group of late-type stars. In connection with the present investigation, attention is given to the first high-dispersion spectra of the chromospheric (6000 K) and higher temperature (up to 100,000 K) emissions of a dMe star, AU Microscopii in the far-ultraviolet (1150-2000 A) and middle-ultraviolet (2000-3000 A) bands accessible to the International Ultraviolet Explorer (IUE). AU Mic is one of the most luminous of lower main-sequence stars in C IV and soft X-ray emission.

  20. s-Process Abundances in Binary Stars With White Dwarfs

    NASA Astrophysics Data System (ADS)

    Merle, T.; Jorissen, A.; Van Eck, S.; Masseron, T.; van Winckel, H.

    2015-12-01

    The enrichment of barium stars in s-process elements is known to be due to pollution by mass transfer from an asymptotic giant branch (AGB) companion star, now an extincted C-O white-dwarf (McClure et al. [4]; Gray et al. [1]). We investigate the relationship between the level of enrichment in s-process elements in the barium star and the mass of its white dwarf (WD) companion. It is expected that helium WDs, which have masses smaller than about 0.5 M⊙ and whose progenitor never reached the AGB phase, should not pollute with s-process elements their giant companion. That companion should thus never turn into a barium star. Our results conform to the expectation that binary systems with WD companions less massive than 0.5 M⊙ do not host barium stars.

  1. On the Central Helium-burning Variable Stars of the LeoI Dwarf Spheroidal Galaxy

    NASA Astrophysics Data System (ADS)

    Fiorentino, G.; Stetson, P. B.; Monelli, M.; Bono, G.; Bernard, E. J.; Pietrinferni, A.

    2012-11-01

    We present a study of short-period, central helium-burning variable stars in the Local Group dwarf spheroidal galaxy LeoI, including 106 RR Lyrae stars and 51 Cepheids. So far, this is the largest sample of Cepheids and the largest Cepheids to RR Lyrae ratio found in such a kind of galaxy. Comparison with other Local Group dwarf spheroidals, Carina and Fornax, shows that the period distribution of RR Lyrae stars is quite similar, suggesting similar properties of the parent populations, whereas the Cepheid period distribution in LeoI peaks at longer periods (P ~ 1.26 days instead of ~0.5 days) and spans over a broader range, from 0.5 to 1.78 days. Evolutionary and pulsation predictions indicate, assuming a mean metallicity peaked within -1.5 <~ [Fe/H] <~ -1.3, that the current sample of LeoI Cepheids traces a unique mix of anomalous Cepheids (blue extent of the red-clump, partially electron-degenerate central helium-burning stars) and short-period classical Cepheids (blue-loop, quiescent central helium-burning stars). Current evolutionary prescriptions also indicate that the transition mass between the two different groups of stars is M HeF ~ 2.1 M ⊙, and it is constant for stars metal-poorer than [Fe/H] ~ -0.7. Finally, we briefly outline the different implications of the current findings on the star formation history of LeoI.

  2. 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.

  3. 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

  4. Search for brown-dwarf companions of stars

    NASA Astrophysics Data System (ADS)

    Sahlmann, J.; Ségransan, D.; Queloz, D.; Udry, S.; Santos, N. C.; Marmier, M.; Mayor, M.; Naef, D.; Pepe, F.; Zucker, S.

    2011-01-01

    Context. The frequency of brown-dwarf companions in close orbit around Sun-like stars is low compared to the frequency of planetary and stellar companions. There is presently no comprehensive explanation of this lack of brown-dwarf companions. Aims: By combining the orbital solutions obtained from stellar radial-velocity curves and Hipparcos astrometric measurements, we attempt to determine the orbit inclinations and therefore the masses of the orbiting companions. By determining the masses of potential brown-dwarf companions, we improve our knowledge of the companion mass-function. Methods: The radial-velocity solutions revealing potential brown-dwarf companions are obtained for stars from the CORALIE and HARPS planet-search surveys or from the literature. The best Keplerian fit to our radial-velocity measurements is found using the Levenberg-Marquardt method. The spectroscopic elements of the radial-velocity solution constrain the fit to the intermediate astrometric data of the new Hipparcos reduction. The astrometric solution and the orbit inclination are found using non-linear χ2-minimisation on a two-parameter search grid. The statistical confidence of the adopted orbital solution is evaluated based on the distribution-free permutation test. Results: The discovery of nine new brown-dwarf candidates orbiting stars in the CORALIE and HARPS radial-velocity surveys is reported. New CORALIE radial velocities yielding accurate orbits of six previously-known hosts of potential brown-dwarf companions are presented. Including the literature targets, 33 hosts of potential brown-dwarf companions are examined. Employing innovative methods, we use the new reduction of the Hipparcos data to fully characterise the astrometric orbits of six objects, revealing M-dwarf companions of masses between 90 MJ and 0.52 M_⊙. In addition, the masses of two companions can be restricted to the stellar domain. The companion to HD 137510 is found to be a brown dwarf. At 95% confidence

  5. Radial Velocity Variability of mid-F Dwarf Stars

    NASA Astrophysics Data System (ADS)

    Cochran, W. D.; Hatzes, A. P.

    1994-05-01

    The McDonald Observatory Planetary Search program has been obtaining high precision (sigma ~ 10ms(-1) ) observations of a sample of 36 slowly rotating F, G, and K dwarfs since the fall of 1987. The primary purpose of this survey has been to detect sub-stellar companions to these stars. Most of the survey stars seem to have a level of intrinsic stellar radial velocity variability of about 20ms(-1) or less. However, the four stars of type F6 and earlier all seem to show significantly larger intrinsic variability of 40ms(-1) or more. These stars are pi (3) Orionis (F6V), alpha Canis Minoris A (F5IV-V), theta Ursae Majoris, and gamma Serpens. Stars of spectral type F8 and later do not seem to show this large level of radial velocity variability. One star of type F7V has recently been added to the survey, but we do not yet have sufficient data on it to determine its level of intrinsic variability. We present the observational data on these variable mid-F dwarfs, along with statistical analysis of the data in order to determine possible periodicities within the data. It is possible that these stars may represent the extreme ``tail end'' of the cool delta Scuti stars, or they may be a new class of variable stars in their own right.

  6. 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.

  7. MWA targeted campaign of nearby, flaring M dwarf stars

    NASA Astrophysics Data System (ADS)

    Lynch, C.; Murphy, T.; Kaplan, D. L.

    2017-01-01

    Flaring activity is a common characteristic of magnetically active stellar systems. Flare events produce emission throughout the electromagnetic spectrum, implying a range of physical processes. Early 100 - 200 MHz observations of M dwarf flare stars detected bright (>100 mJy) flares with occurrence rates between 0.06 - 0.8 flares per hour. These rates imply that observing 100 - 200 MHz flares from M dwarf stars is fairly easy with many detections expected for modern low-frequency telescopes. However, long observational campaigns using these modern telescopes have not reproduced these early detections. This could be because the rates are over estimated and contaminated by radio frequency interference. Recently Lynch et al. (submitted) detected four flares from UV Ceti at 154 MHz using the Murchison Widefield Array. The flares have flux densities between 10-65 mJy -- a factor of 100 fainter than most flares in the literature at these frequencies -- and are only detected in circular polarization. The flare rates for these newly detected flares are roughly consistent with earlier rates however the uncertainties are large. Building off this result we propose a 102 hour survey of the closet six M dwarf stars with observed magnetic activity traced in X-rays and 100 - 200 MHz emission. The rates measured from this survey would inform the duration required for future blind surveys for flares from M dwarf stars.

  8. Microwave Observations of Red Dwarf Flare Stars

    NASA Astrophysics Data System (ADS)

    Kundu, M. R.; Jackson, P. D.; White, S. M.

    Observations of AD Leo, EQ Peg AB, L726-8AB (B = UV Ceti), Wolf 630AB, YY Gem and YZ CMi were made on March 22, 1985 in the 6 and 20 cm wavelength bands using the Very Large Array with hybrid A/B configuration.

  9. High-Frequency Properties of Ultracool Dwarf Star Radio Transients, or The Little Dwarfs that Could

    NASA Astrophysics Data System (ADS)

    Ravi, Vikram; Hobbs, George; Keith, Michael; Champion, David; Ferrario, Lilia; Wickramasinghe, Dayal

    2009-07-01

    Radio transients are among the most intriguing phenomena in astronomy. Numerous flaring events, some periodic, have lately surfaced, with only few identified with known objects such as magnetic stars. Periodic, non-thermal, highly circularly-polarised pulses and unusually strong quiescence have been recently detected from three late-type quickly-rotating (~2hr periods) ultracool dwarf stars (>M7) at centimetric wavelengths. This violates empirical relations and quantifiers of dwarf-star surface activity. Measurements of dwarf-star kiloGauss magnetic fields have led to emission models based on dipole fields and incoherent gyrosynchrotron or coherent electron-cyclotron maser mechanisms. We propose to observe two such similar objects at 1cm and 7mm (LP944-20 and DENIS1048-3956) that are known to flare but without detected periodicities. No observations of high-frequency emission from any magnetic star have been published. The broadband capabilities of CABB will provide extraordinary frequency-synthesised sensitivity in a search for periodicity. The obtained spectral indices, along with possible high-frequency spectral cut-offs, will greatly help constrain emission models of magnetic stars. This is the first attempt to characterise the high-frequency transient radio sky, a key science project for future telescopes such as ASKAP and the SKA.

  10. 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.

  11. 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

  12. 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.

  13. 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.

    2017-01-01

    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 (HST/ACS) 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.

  14. 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.

  15. Discovery of five new massive pulsating white dwarf stars

    NASA Astrophysics Data System (ADS)

    Castanheira, B. G.; Kepler, S. O.; Kleinman, S. J.; Nitta, A.; Fraga, L.

    2013-03-01

    Using the SOuthern Astrophysical Research telescope (SOAR) Optical Imager at the SOAR 4.1 m telescope, we report on the discovery of five new massive pulsating white dwarf stars. Our results represent an increase of about 20 per cent in the number of massive pulsators. We have detected both short and long periods, low and high amplitude pulsation modes, covering the whole range of the ZZ Ceti instability strip. In this paper, we present a first seismological study of the new massive pulsators based on the few frequencies detected. Our analysis indicates that these stars have masses higher than average, in agreement with the spectroscopic determinations. In addition, we study for the first time the ensemble properties of the pulsating white dwarf stars with masses above 0.8 M⊙. We found a bimodal distribution of the main pulsation period with the effective temperature for the massive DAVs, which indicates mode selection mechanisms.

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

    SciTech Connect

    Hoessel, J.G.; Abbott, M.J.; Saha, A.; Mossman, A.E.; Danielson, G.E. Space Telescope Science Institute, Baltimore, MD Palomar Observatory, Pasadena, CA )

    1990-10-01

    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. 25 refs.

  17. 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.

  18. About K Dwarfs - Investigating the Goldilocks Stars of Exobiology

    NASA Astrophysics Data System (ADS)

    Cuntz, Manfred; Guinan, Edward F.

    2017-01-01

    In this study, we argue that stars between spectral type late-G and mid-K (with a maximum at early-K), i.e., orange dwarfs, are expected to provide the best conditions for the development and sustainability of life, including advanced life forms. Though our study is mostly theoretical, observational data are considered as fit. Our analysis considers a variety of stellar properties, including (1) the frequency of the various types of stars, (2) the speed of stellar evolution 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 (super-)flares; both (4) and (5) greatly reduce the suitability of M-type dwarfs to host life-bearing planets. M-type dwarfs are numerous, having long lifetimes, but their narrow CLI-HZs and hazards from magnetic activity make them less suitable for hosting exolife. Therefore, we argue that K-dwarfs should be rightfully considered "Goldilocks" stars, thus deserving heightened attention in future observational and theoretical studies.

  19. Completing the Census of Isolated Dwarf Galaxy Star Formation Histories

    NASA Astrophysics Data System (ADS)

    Weisz, Daniel

    2014-10-01

    We propose to complete our census of the ancient star formation histories (SFHs) of isolated dwarf galaxies by obtaining deep ACS/WFC optical imaging of WLM and Pegasus Dwarf Irregular Galaxy (PegDIG). They are the only two systems without previous deep HST imaging that are isolated yet close enough to guarantee that their oldest main sequence turnoffs are accessible with HST. We will measure their lifetime SFHs with an age resolution of < 1 Gyr at all epochs to address questions about growth of stellar mass, the effects of reionization, radial population gradients, and variable star populations in WLM and PegDIG. This program is a concerted effort between theorists and observers to obtain the best possible observational constraints on the early epochs of star formation in isolated low-mass galaxies, which are essential to the next generation of galaxy simulations. With these new observations we will have completed our efforts to collect precise lifetime SFHs of all nearby isolated dwarfs that are accessible with HST. In combination with archival data, we will create a legacy sample isolated dwarfs with identically derived SFHs, that will be serve as the baseline for the community's understanding of how low-mass galaxies form and evolve over a Hubble time and in the absence of environmental effects of a massive host (e.g., tides, ram pressure).

  20. Magnetic white dwarf stars in the Sloan Digital Sky Survey

    NASA Astrophysics Data System (ADS)

    Kepler, S. O.; Pelisoli, I.; Jordan, S.; Kleinman, S. J.; Koester, D.; Külebi, B.; Peçanha, V.; Castanheira, B. G.; Nitta, A.; Costa, J. E. S.; Winget, D. E.; Kanaan, A.; Fraga, L.

    2013-03-01

    To obtain better statistics on the occurrence of magnetism among white dwarfs, we searched the spectra of the hydrogen atmosphere white dwarf stars (DAs) in the Data Release 7 of the Sloan Digital Sky Survey (SDSS) for Zeeman splittings and estimated the magnetic fields. We found 521 DAs with detectable Zeeman splittings, with fields in the range from around 1 to 733 MG, which amounts to 4 per cent of all DAs observed. As the SDSS spectra have low signal-to-noise ratios, we carefully investigated by simulations with theoretical spectra how reliable our detection of magnetic field was.

  1. Contrasting Accreting White Dwarf Pulsators with the ZZ Ceti Stars

    NASA Astrophysics Data System (ADS)

    Mukadam, A. S.; Szkody, P.; Gänsicke, B. T.; Pala, A.

    2017-03-01

    Understanding the similarities and differences between the accreting white dwarf pulsators and their non-interacting counterparts, the ZZ Ceti stars, will eventually help us deduce how accretion affects pulsations. ZZ Ceti stars pulsate in a narrow instability strip in the range 10800–12300 K due to H ionization in their pure H envelopes; their pulsation characteristics depend on their temperature and stellar mass. Models of accreting white dwarfs are found to be pulsationally unstable due to the H/HeI ionization zone, and even show a second instability strip around 15000 K due to HeII ionization. Both these strips are expected to merge for a He abundance higher than 0.48 to form a broad instability strip, which is consistent with the empirical determination of 10500–16000 K. Accreting pulsators undergo outbursts, during which the white dwarf is heated to temperatures well beyond the instability strip and is observed to cease pulsations. The white dwarf then cools to quiescence in a few years as its outer layers cool more than a million times faster than the evolutionary rate. This provides us with an exceptional opportunity to track the evolution of pulsations from the blue edge to quiescence in a few years, while ZZ Ceti stars evolve on Myr timescales. Some accreting pulsators have also been observed to cease pulsations without any apparent evidence of an outburst. This is a distinct difference between this class of pulsators and the non-interacting ZZ Ceti stars. While the ZZ Ceti instability strip is well sampled, the strip for the accreting white dwarfs is sparsely sampled and we hereby add two new potential discoveries to improve the statistics.

  2. 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

  3. Resolving the Tip of the Red Giant Branch of Two New Candidate Local Group Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Tollerud, Erik

    2014-10-01

    We propose to use ACS/WFC to observe two faint dwarf galaxies recently discovered via their HI emission. Based on a blind HI search of 40 HI clumps from 7500 square degrees of the GALFA-HI survey, these two candidates are the only objects with optical counterparts. They show HI and Halpha emission consistent with nearby galaxies, and have blue stars that are barely resolved in ground-based optical imaging with good seeing. These resolved stars are consistent with the galaxies being at Local Group distances. If they are in the Local Group, these galaxies are both less luminous and more compact than the recently-discovered Leo P, also found first with HI observations. They may then also be the faintest known star-forming galaxies. The ground-based imaging leaves large distance uncertainty, however, because the tip of the red giant branch cannot be resolved. We propose one orbit per galaxy of ACS/WFC imaging in F606W and F814W to measure accurate TRGB distances and determine if they truly are Local Group galaxies. If so, these galaxies provide tests on both the efficacy of Lambda CDM in predicting the properties of dwarf galaxies in low density environments, and the lowest-luminosity data points on models of galaxy star formation.

  4. 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.

  5. 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.

  6. Model Atmospheres for Irradiated Red Giant Stars with Winds

    NASA Astrophysics Data System (ADS)

    Aufdenberg, J. P.; Barman, T. S.

    2002-12-01

    We will present exploratory model atmosphere calculations applicable to symbiotic binary systems, where a hot white dwarf illuminates the extended atmosphere of a red giant. While sophisticated non-LTE photoionization models exist for these systems (e.g. Proga et al. 1998), detailed models for the ionized-to-neutral transition region in the red giant wind have lacked molecular line opacities. To make improvements in this area, we employ a new version of the PHOENIX stellar atmosphere and planetary radiative transfer code which combines the stellar wind module of Aufdenberg et al. (2002), now modified to treat the winds of cool stars, with the external illumination module of Barman, Hauschildt, & Allard (2001). Our present calculations include illuminated spherically symmetric models, with conditions similar to those found in EG And, that include non-LTE line blanketing, molecular opacity, and a realistic description of the incident white dwarf flux. Our goals include the prediction of changes in the red giant absorption-line spectrum with orbital phase, the prediction of emission-line strengths from the coolest, densest portions of the recombination region, and the detailing modeling of eclipse mapping observations. JPA is supported by a Harvard-Smithsonian Center for Astrophysics Postdoctoral Fellowship. Some of the calculations for this work were computed on the IBM SP ``Blue Horizon'' of the San Diego Supercomputer Center (SDSC), with support from the National Science Foundation, and on the IBM SP of the NERSC with support from the DOE.

  7. 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.

  8. 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.

  9. CARMENES: Looking for Blue Planets Orbiting Red Dwarfs

    NASA Astrophysics Data System (ADS)

    Quirrenbach, A.

    2014-03-01

    CARMENES (Calar Alto high-Resolution search for M dwarfs with Exo-earths with Near-infrared and optical Echelle Spectrographs) is a next-generation instrument being built for the 3.5m telescope at the Calar Alto Observatory by a consortium of eleven Spanish and German institutions. CARMENES will conduct a 600-night exoplanet survey targeting ~300 M dwarfs. The CARMENES instrument consists of two separate echelle spectrographs covering the wavelength range from 0.55 to 1.7 µm at a spectral resolution of R = 82,000, fed by fibers from the Cassegrain focus of the telescope. For late-M spectral types, the wavelength range around 1.0 µm (Y band) is the most important wavelength region for radial velocity work. Therefore, the efficiency of CARMENES will be optimized in this range. The main scientific objective of the CARMENES project is to carry out a survey of late-type main sequence stars with the goal of detecting low-mass planets in their HZs (see also Quirrenbach et al. 2010, 2012). In the focus of the project are very cool stars later than spectral type M4 and moderately active stars. In particular, we aim at being able to detect a 2M. planet in the HZ of an M5 star. A long-term radial velocity precision of 1ms-1 per measurement will permit to attain such goals. For stars later than M4 (M < 0.25M.), such precision will yield detections of super- Earths of 5M. and smaller inside the entire width of the HZ.

  10. 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.

  11. Accretion phenomena onto star-forming dwarf-galaxies.

    NASA Astrophysics Data System (ADS)

    Annibali, Francesca

    2017-01-01

    I will present our recent discovery (Annibali et al. 2016, ApJL, 826 L27), based on the combination of deep wide-field LBT imaging from the ground and HST data, of a stellar stream and substructures associated to the very metal-poor star-forming dwarf galaxy DDO 68, located in a Void at ~12.7 Mpc from us. DDO 68 is very light (only 108 Msun in stars), yet it shows evidence for the accretion of at least two smaller satellites. DDO 68 is one of the very few cases where the hierarchical formation process is caught in action at such small galactic scales. This study is part of a large ongoing project based on an approved 2-year strategic program with LBT to search for stellar streams around a sample of ~50 nearby star-forming dwarf galaxies. Our result demonstrates the high potential of wide-field instrumentation at 8-10 m telescopes in combination with HST (and with JWST in the near future) for the study of accretion phenomena onto dwarf-galaxies.

  12. 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.

  13. 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

  14. 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

  15. 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

  16. 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.

  17. 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.

  18. 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.

  19. 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.

  20. Synthetic activity indicators for M-type dwarf stars

    NASA Astrophysics Data System (ADS)

    Wedemeyer, Sven; Ludwig, Hans-Günter; Hauschildt, Peter; De Gennaro Aquino, Ivan

    2015-08-01

    Our understanding of the Sun has been substantially progressed owing to the advances in high-resolution observations during the last decades. These observations guided the development of numerical simulation codes for stellar atmospheres towards unprecedented levels of realism and complexity. Such 3D radiation magnetohydrodynamic (RMHD) codes can be applied and adapted to cooler stars. Here, we present a set of time-dependent 3D RMHD simulations for dwarf stars of spectral type M (representative of AD Leo). "M-dwarfs" are the most abundant stars in our galaxy and known to exhibit mega-flares. Comparisons of M-dwarf models with the Sun as fundamental reference case reveal differences and similarities, which lead to important insights into the structure and dynamics of quiescent "background" atmospheres. The models, which extend from the upper convection zone into the chromosphere, have different initial magnetic field strengths (up to 500G) and topologies, representing regions with different activity levels. The 3D model atmospheres are characterized by a very dynamic and intermittent structure on small spatial and temporal scales, final field strengths reaching a few kG and a wealth of physical processes, which by nature cannot be described by means of 1D static model atmospheres.Synthetic observables, i.e. spectra and intensity images, are calculated by using these models as input for detailed radiative transfer calculations and can be combined into synthetic full stellar disks, thus simulating spatially unresolved observations of M-dwarfs. The considered diagnostics, like, e.g., Halpha, Ca II lines, or the continuum intensity from UV to millimeter wavelengths, sample various properties of the dynamics, thermal and magnetic structure of the photosphere and the chromosphere and thus provide measures of stellar activity, which can be compared to observations. The complicated magnetic field structure and its imprint in synthetic diagnostics may have important

  1. Infrared Opacities in Dense Atmospheres of Cool White Dwarf Stars

    NASA Astrophysics Data System (ADS)

    Kowalski, P. M.; Blouin, S.; Dufour, P.

    2017-03-01

    Dense, He-rich atmospheres of cool white dwarfs represent a challenge to the modeling. This is because these atmospheres are constituted of a dense fluid in which strong multi-atomic interactions determine their physics and chemistry. Therefore, the ideal-gas-based description of absorption is no longer adequate, which makes the opacities of these atmospheres difficult to model. This is illustrated with severe problems in fitting the spectra of cool, He-rich stars. Good description of the infrared (IR) opacity is essential for proper assignment of the atmospheric parameters of these stars. Using methods of computational quantum chemistry we simulate the IR absorption of dense He/H media. We found a significant IR absorption from He atoms (He-He-He CIA opacity) and a strong pressure distortion of the H2-He collision-induced absorption (CIA). We discuss the implication of these results for the interpretation of the spectra of cool stars.

  2. 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.

  3. Assimilation of planets by red giant stars

    NASA Astrophysics Data System (ADS)

    Carlberg, Joleen Karen

    The typical red giant star rotates slowly. This characteristic is expected from the conservation of angular momentum as these stars expand during their evolution. Nevertheless, a small percentage of red giant stars are rapidly rotating. One possible source of these stars' excess angular momenta is the orbital angular momentum of a planetary companion. The transfer of orbital angular momentum to the stellar envelope decays the planet's orbit, ultimately leading to the rapid in-spiral of the planet into the star. Using the known sample of exoplanets around main sequence host stars, I simulated both the future evolution of these stars and the expected interactions with their planets and found that Jupiter-mass planets residing at inner solar system distances---relatively common in exoplanetary systems---can contribute enough angular momentum to cause rapid rotation in their host stars during the red giant phase. Gas giant planets are also massive enough to alter the chemical composition of their host stars' envelopes when they are accreted. The central experiment of this thesis is to search for abundance anomalies in the rapid rotators that could be indicative of planet accretion. Hypothetical anomalies include the replenishment of light elements that are diluted by giant stars during first dredge-up (such as the stellar surface abundance of lithium), changes in isotopic abundance ratios that were altered by nucleosynthesis (such as increasing the stellar surface 12C/13C), and the preferential enhancement of refractory elements (indicative of the accretion of chemically fractionated material such as a planet). To increase the total number of known rapid rotators, I measured rotational velocities in a large database of spectra collected for the Grid Giant Star Survey developed for NASA's Space Interferometry Mission's astrometric grid. The 28 new rapid rotators discovered in this sample were combined with rapid rotators from the literature and a control sample of slow

  4. Multiplicity among Young Brown Dwarfs and Very Low Mass Stars

    NASA Astrophysics Data System (ADS)

    Ahmic, Mirza; Jayawardhana, Ray; Brandeker, Alexis; Scholz, Alexander; van Kerkwijk, Marten H.; Delgado-Donate, Eduardo; Froebrich, Dirk

    2007-12-01

    We report on a near-infrared adaptive optics imaging survey of 31 young brown dwarfs and very low mass (VLM) stars, 28 of which are in the Chamaeleon I star-forming region, using the ESO Very Large Telescope. We resolve the suspected 0.16'' (~26 AU) binary Cha Hα 2 and present two new binaries, Hn 13 and CHXR 15, with separations of 0.13'' (~20 AU) and 0.30'' (~50 AU), respectively; the latter is one of the widest VLM systems known. We find a binary frequency of 11+9-6%, thus confirming the trend for a lower binary frequency with decreasing mass. By combining our work with previous surveys, we arrive at the largest sample of young VLM objects (72) with high angular resolution imaging to date. Its multiplicity fraction is in statistical agreement with that for VLM objects in the field. Furthermore, we note that many field stellar binaries with lower binding energies and/or wider cross sections have survived dynamical evolution and that statistical models suggest tidal disruption by passing stars is unlikely to affect the binary properties of our systems. Thus, we argue that there is no significant evolution of multiplicity with age among brown dwarfs and VLM stars in OB and T associations between a few megayears to several gigayears. Instead, the observations so far suggest that VLM objects are either less likely to be born in fragile multiple systems than solar-mass stars or such systems are disrupted very early. We dedicate this paper to the memory of our coauthor, Eduardo Delgado-Donate, who died in a hiking accident in Tenerife earlier this year.

  5. Applications of CaH Photometry to Red Stars

    NASA Astrophysics Data System (ADS)

    Robertson, Thomas H.

    2011-01-01

    A photometric system using Kron-Cousins R and I magnitudes with an intermediate-band CaH filter has been in use for a number of years. This system was designed to produce luminosity classes and photometric parallaxes for red dwarf stars with R-I > 0.7. Observations have been made on three different telescopes equipped with four different CCD cameras, two different CaH filters and three different Kron-Cousins filter sets. The system has remained consistent and provides for relatively easy transformation from one set in instrumental/standard magnitudes to another. Data collected using these various hardware systems have been transformed to a uniform system and the numerical parameters for luminosity classification are provided.

  6. Dust Content of Virgo Star-Forming Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Grossi, M.; Hunt, L. K.; Madden, S.; Vlahakis, C.; Bomans, D. J.; Baes, M.; Bendo, G. J.; Bianchi, S.; Boselli, A.; Clemens, M.; Corbelli, E.; Cortese, L.; Dariush, A.; Davies, J. I.; De Looze, I.; di Serego Alighieri, S.; Fadda, D.; Fritz, J.; Garcia-Appadoo, D. A.; Gavazzi, G.; Giovanardi, C.; Hughes, T. M.; Jones, A. P.; Pierini, D.; Pohlen, M.; Sabatini, S.; Smith, M. W. L.; Verstappen, J.; Xilouris, E. M.; Zibetti, S.

    We investigate the dust properties of a small sample of Virgo cluster dwarf galaxies drawn from the science demonstration phase data set of the Herschel Virgo Cluster Survey (HeViCS). These galaxies have low metallicities (7.8 < 12 + log(O/H) < 8.3) and star formation rates ≲ 0.1 M⊙ yr-1. We measure the spectral energy distribution (SED) from 100 to 500 μm and derive dust temperatures and masses. The SEDs are fitted by a cool component with T ≲ 20 K, implying dust masses around 105 M⊙ and dust-to-gas ratios (D) within the range 10-3-10-2.

  7. 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.

  8. 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.

  9. Dwarf Galaxy Formation with H2-regulated Star Formation

    NASA Astrophysics Data System (ADS)

    Kuhlen, Michael; Krumholz, Mark R.; Madau, Piero; Smith, Britton D.; Wise, John

    2012-04-01

    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 H2-regulated prescription leads to a suppression of star formation in low-mass halos (Mh <~ 1010 M ⊙) at z > 4, alleviating some of the dwarf galaxy problems faced by theoretical galaxy formation models. H2 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 H2 abundance in our most refined grid cells (76 proper parsec in size at z = 4) by applying the model of Krumholz, McKee, & Tumlinson, which is based on idealized one-dimensional radiative transfer calculations of H2 formation-dissociation balance in ~100 pc atomic-molecular complexes. Our H2-regulated simulations are able to reproduce the empirical (albeit lower z) Kennicutt-Schmidt relation, including the low Σgas 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.

  10. Fundamental Physics from Observations of White Dwarf Stars

    NASA Astrophysics Data System (ADS)

    Bainbridge, M. B.; Barstow, M. A.; Reindl, N.; Barrow, J. D.; Webb, J. K.; Hu, J.; Preval, S. P.; Holberg, J. B.; Nave, G.; Tchang-Brillet, L.; Ayres, T. R.

    2017-03-01

    Variation in fundamental constants provide an important test of theories of grand unification. Potentially, white dwarf spectra allow us to directly observe variation in fundamental constants at locations of high gravitational potential. We study hot, metal polluted white dwarf stars, combining far-UV spectroscopic observations, atomic physics, atmospheric modelling and fundamental physics, in the search for variation in the fine structure constant. This registers as small but measurable shifts in the observed wavelengths of highly ionized Fe and Ni lines when compared to laboratory wavelengths. Measurements of these shifts were performed by Berengut et al (2013) using high-resolution STIS spectra of G191-B2B, demonstrating the validity of the method. We have extended this work by; (a) using new (high precision) laboratory wavelengths, (b) refining the analysis methodology (incorporating robust techniques from previous studies towards quasars), and (c) enlarging the sample of white dwarf spectra. A successful detection would be the first direct measurement of a gravitational field effect on a bare constant of nature. We describe our approach and present preliminary results.

  11. 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.

  12. 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.

  13. 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.

  14. 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.

  15. 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.

  16. 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.

  17. 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.

  18. 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.

  19. 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.

  20. The Chromospheric Activity-Age Relation for M Dwarf Stars in Wide Binary Systems

    NASA Astrophysics Data System (ADS)

    Silvestri, N. M.

    2002-12-01

    We present new chromospheric activity-age relations for M dwarf stars in wide binary systems with white dwarf companions. This study is unique in that we use the cooling age of the white dwarf to determine the age of the M dwarf star in the binary system. Assuming that the members of the gravitationally bound system are coeval, the age of the white dwarf is therefore the age of the M dwarf companion. The colors and magnitudes at which chromospheric activity becomes pervasive (at the ``Hα limit") in M stars have been shown to correlate linearly with log(age) in young (<= 4 Gyr) cluster M dwarfs. We find that M dwarfs in wide binaries older than 4 Gyr depart from this linear relation and are found to have activity at colors and magnitudes both bluer and brighter than predicted by M dwarf cluster relations. Also, activity is present in nearly all cluster M dwarfs above the ``Hα limit", whereas not all binary M dwarfs are found to be active above this limit. These relations differ considerably from the rotationally driven dynamo relation for F, G, and K stars that suggests a different magnetic heating mechanism for M dwarf stars. The new relations extend to ages beyond the oldest ages provided by cluster M dwarf activity-age estimates. However, more work is necessary to decrease the uncertainties in these new relations and extend them to later (>= M5.5) spectral types. This work was supported by the NASA Graduate Researchers Program Grant NGT 200415; A Grant-in-Aid of Research from the National Academy of Sciences administered by Sigma Xi, The Scientific Research Society; NASA Grant Y701296; and NSF Grant AST 0206115.

  1. Kinematics and chemistry of faint high latitude dwarf carbon stars

    NASA Astrophysics Data System (ADS)

    Yoon, Jinmi; Beers, Timothy C.; Dietz, Sarah; Lee, Young Sun; Placco, Vinicius M.

    2017-01-01

    The diffuse halo system of the Milky Way is complex, and has been shown to comprise at least two main components: a near-zero net rotation inner-halo and a more rapidly rotating outer-halo component. Studies of the ancient, very metal-poor stars in the Galactic halo system are crucial for understanding its early formation history. The so-called carbon-enhanced metal-poor (CEMP) stars are an important subset of the stars in the halo system, which exhibit distinctive kinematic and chemical signatures that can be used to constrain the star-formation histories and assembly of the various Galactic components.We have examined the sample of main-sequence dwarf and other faint high Galactic latitude carbon-enhanced stars from the Sloan Digital Sky Survey studied by Green (2013). As noted by Green, many of these starsexhibit high proper motions, which have been later claimed to be related to possible binary ejection models Plant et al. (2016). By use of the CEMP sub-classification approach of Yoon et al. (2016), we investigate whether the kinematics of these stars might instead result from their membership in the inner/outer halo populations of the Galaxy.ReferencesGreen, P. 2013, ApJ, 765, 12Plant, K. et al. 2016, AAS 227.34115Yoon, J. et al. 2016, ApJ, in pressAcknowledgementThis work was supported in part by the National Science Foundation under Grant No. PHY-1430152 (JINA Center for the Evolution of the Elements).

  2. 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.

  3. 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.

  4. 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.

  5. 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.

  6. 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.

  7. Environmental effects on star formation in dwarf galaxies and star clusters

    NASA Astrophysics Data System (ADS)

    Pasetto, S.; Cropper, M.; Fujita, Y.; Chiosi, C.; Grebel, E. K.

    2015-01-01

    Context. The role of the environment in the formation of a stellar population is a difficult problem in astrophysics. The reason is that similar properties of a stellar population are found in star systems embedded in different environments or, vice versa, similar environments contain stellar systems with stellar populations having different properties. Aims: In this paper, we develop a simple analytical criterion to investigate the role of the environment on the onset of star formation. We will consider the main external agents that influence star formation (i.e. ram pressure, tidal interaction, Rayleigh-Taylor and Kelvin-Helmholtz instabilities) in a spherical galaxy moving through an external environment. The theoretical framework developed here has direct applications to the cases of dwarf galaxies in galaxy clusters and dwarf galaxies orbiting our Milky Way system, as well as any primordial gas-rich cluster of stars orbiting within its host galaxy. Methods: We develop an analytic formalism to solve the fluid dynamics equations in a non-inertial reference frame mapped with spherical coordinates. The two-fluids instability at the interface between a stellar system and its surrounding hotter and less dense environment is related to the star formation processes through a set of differential equations. The solution presented here is quite general, allowing us to investigate most kinds of orbits allowed in a gravitationally bound system of stars in interaction with a major massive companion. Results: We present an analytical criterion to elucidate the dependence of star formation in a spherical stellar system (as a dwarf galaxy or a globular cluster) on its surrounding environment useful in theoretical interpretations of numerical results as well as observational applications. We show how spherical coordinates naturally enlighten the interpretation of two-fluids instability in a geometry that directly applies to an astrophysical case. This criterion predicts the

  8. Strange stars, strange dwarfs, and planetary-like strange-matter objects

    SciTech Connect

    Weber, F.; Schaab, C.; Weigel, M.K.; Glendenning, N.K.

    1995-05-01

    This paper gives an overview of the properties of all possible equilibrium sequences of compact strange-matter stars with nuclear crusts, which range from strange stars to strange dwarfs. In contrast to their non-strange counterparts--neutron stars and white dwarfs--their properties are determined by two (rather than one) parameters, the central star density and the density at the base of the nuclear crust. This leads to stellar strange-matter configurations whose properties are much more complex than those of the conventional sequence. As an example, two generically different categories of stable strange dwarfs are found, which could be the observed white dwarfs. Furthermore the authors find very-low-mass strange stellar objects, with masses as small as those of Jupiter or even lighter planets. Such objects, if abundant enough, should be seen by the presently performed gravitational microlensing searches.

  9. Abundance ratios of red giants in low-mass ultra-faint dwarf spheroidal galaxies

    NASA Astrophysics Data System (ADS)

    François, P.; Monaco, L.; Bonifacio, P.; Moni Bidin, C.; Geisler, D.; Sbordone, L.

    2016-04-01

    Context. Low-mass dwarf spheroidal galaxies are key objects for our understanding of the chemical evolution of the pristine Universe and the Local Group of galaxies. Abundance ratios in stars of these objects can be used to better understand their star formation and chemical evolution. Aims: We report on the analysis of a sample of 11 stars belonging to five different ultra-faint dwarf spheroidal galaxies (UfDSph) that is based on X-Shooter spectra obtained at the VLT. Methods: Medium-resolution spectra have been used to determine the detailed chemical composition of their atmosphere. We performed a standard 1D LTE analysis to compute the abundances. Results: Considering all the stars as representative of the same population of low-mass galaxies, we found that the [α/Fe] ratios vs.s [Fe/H] decreases as the metallicity of the star increases in a way similar to that which is found for the population of stars that belong to dwarf spheroidal galaxies. The main difference is that the solar [α/Fe] is reached at a much lower metallicity for the UfDSph than for the dwarf spheroidal galaxies. We report for the first time the abundance of strontium in CVn II. The star we analyzed in this galaxy has a very high [Sr/Fe] and a very low upper limit of barium which makes it a star with an exceptionally high [Sr/Ba] ratio.

  10. 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

  11. 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.

  12. Model Atmospheres From Very Low Mass Stars to Brown Dwarfs

    NASA Astrophysics Data System (ADS)

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

    2011-12-01

    Since the discovery of brown dwarfs in 1994, and the discovery of dust cloud formation in the latest Very Low Mass Stars (VLMs) and Brown Dwarfs (BDs) in 1996, the most important challenge in modeling their atmospheres as become the understanding of cloud formation and advective mixing. For this purpose, we have developed radiation hydrodynamic 2D model atmosphere simulations to study the formation of forsterite dust in presence of advection, condensation, and sedimentation across the M-L-T VLMs to BDs sequence (Teff = 2800 K to 900 K, Freytag et al. 2010). We discovered the formation of gravity waves as a driving mechanism for the formation of clouds in these atmospheres, and derived a rule for the velocity field versus atmospheric depth and Teff, which is relatively insensitive to gravity. This rule has been used in the construction of the new model atmosphere grid, BT-Settl, to determine the micro-turbulence velocity, the diffusion coefficient, and the advective mixing of molecules as a function of depth. This new model grid of atmospheres and synthetic spectra has been computed for 100,000 K > Teff > 400 K, 5.5 > logg > -0.5, and [M/H]= +0.5 to -1.5, and the reference solar abundances of Asplund et al. (2009). We found that the new solar abundances allow an improved (close to perfect) reproduction of the photometric and spectroscopic VLMs properties, and, for the first time, a smooth transition between stellar and substellar regimes -- unlike the transition between the NextGen models from Hauschildt et al. 1999a,b, and the AMES-Dusty models from Allard et al. 2001. In the BDs regime, the BT-Settl models propose an improved explanation for the M-L-T spectral transition. In this paper, we therefore present the new BT-Settl model atmosphere grid, which explains the entire transition from the stellar to planetary mass regimes.

  13. The episodic star formation history of the Carina dwarf spheroidal galaxy

    NASA Astrophysics Data System (ADS)

    de Boer, T. J. L.; Tolstoy, E.; Lemasle, B.; Saha, A.; Olszewski, E. W.; Mateo, M.; Irwin, M. J.; Battaglia, G.

    2014-12-01

    We present deep photometry of the Carina dwarf spheroidal galaxy in the B and V filters from CTIO/MOSAIC out to and beyond the tidal radius of rell ≈ 0.48 degrees. The accurately calibrated photometry is combined with spectroscopic metallicity distributions of red giant branch (RGB) stars to determine the detailed star formation and chemical evolution history of Carina. The star formation history (SFH) confirms the episodic formation history of Carina and quantifies the duration and strength of each episode in great detail as a function of radius from the centre. Two main episodes of star formation occurred at old (>8 Gyr) and intermediate (2-8 Gyr) ages, both enriching stars starting from low metallicities ([Fe/H] < - 2 dex). By dividing the SFH into two components, we determine that 60 ± 9 percent of the total number of stars formed within the intermediate-age episode. Furthermore, within the tidal radius (0.48 degrees or 888 pc) a total mass in stars of 1.07 ± 0.08 × 106 M⊙ was formed, giving Carina a stellar mass-to-light ratio of 1.8 ± 0.8. By combining the detailed SFH with spectroscopic observations of RGB stars, we determined the detailed age-metallicity relation of each episode and the timescale of α-element evolution of Carina from individual stars. The oldest episode displays a tight age-metallicity relation during ≈6 Gyr with steadily declining α-element abundances and a possible α-element "knee" visible at [Fe/H] ≈ - 2.5 dex. The intermediate-age sequence displays a more complex age-metallicity relation starting from low metallicity and a sequence in α-element abundances with a slope much steeper than observed in the old episode, starting from [Fe/H] = -1.8 dex and [Mg/Fe] ≈ 0.4 dex and declining to Mg-poor values ([Mg/Fe] ≤ - 0.5 dex). This clearly indicates that the two episodes of star formation formed from gas with different abundance patterns, which is inconsistent with simple evolution in an isolated system. Tables 1-3 are

  14. Ghostly Halos in Dwarf Galaxies: a probe of star formation in the Early Universe

    NASA Astrophysics Data System (ADS)

    Kang, Hoyoung; Ricotti, Massimo

    2016-01-01

    We carry out numerical simulations to characterize the size, stellar mass, and stellar mass surface density of extended stellar halos in dwarf galaxies as a function of dark matter halo mass. We expect that for galaxies smaller than a critical value, these ghostly halos will not exist because the smaller galactic subunits that build it up, do not form any stars. The detection of ghostly halos around isolated dwarf galaxies is a sensitive test of the efficiency of star formation in the first galaxies and of whether ultra-faint dwarf satellites of the Milky Way are fossils of the first galaxies.

  15. 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).

  16. 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.

  17. Abundances in Globular Cluster Red Giant Stars

    NASA Astrophysics Data System (ADS)

    Cavallo, R. M.

    1997-12-01

    Observations of globular cluster red giant branch (RGB) stars have shown star-to-star variations in the abundances of C, N, O, Na, Mg, and Al, contrary to predictions of standard stellar evolutionary theory. I have modeled the variations in the abundance profiles around the hydrogen-burning shell (H shell) of metal-poor red giant stars by combining four RGB stellar evolutionary sequences of different metallicities with a detailed nuclear reaction network. This approach has significant advantages over previous research: (1) it allows for the variation in the temperature and density around the H shell; (2) it follows the effects of the changing H-shell structure as the sequence evolves; (3) it accounts for the effect of the metallicity on the abundance profiles; (4) it allows the reaction rates to be varied so that their uncertainties may be explored. The results are in good qualitative agreement with the observations. All the models show a region above the H shell in which first C, then O, is depleted in the CN and ON nuclear burning cycles. Within the C-depleted region, the (12) C/(13) C ratio is reduced to its equilibrium value. Just above the O-depleted region, Na is enhanced from proton captures on (22) Ne. In brighter models, Na becomes greatly enhanced within the O-depleted region as the NeNa cycle converts (20) Ne into (23) Na before attaining equilibrium inside the H shell. The more metal-poor models also show Al being increased around the H shell, first from (25,26) Mg, then from (24) Mg in the MgAl cycle. Despite the diminution (24) Mg suffers in synthesizing Al, the models show its abundance is increased due to the NeNa-cycle breakout reaction, (23) Na(p,γ)(24) Mg. This latter result is at odds with observations that show (24) Mg is depleted in a sample of M 13 and NGC 6752 giants (Shetrone 1996, 1997).

  18. 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.

  19. The star formation and chemical evolution history of the Fornax dwarf spheroidal galaxy

    NASA Astrophysics Data System (ADS)

    de Boer, T. J. L.; Tolstoy, E.; Hill, V.; Saha, A.; Olszewski, E. W.; Mateo, M.; Starkenburg, E.; Battaglia, G.; Walker, M. G.

    2012-08-01

    We present deep photometry in the B, V and I filters from CTIO/MOSAIC for about 270 000 stars in the Fornax dwarf spheroidal galaxy, out to a radius of rell ≈ 0.8 degrees. By combining the accurately calibrated photometry with the spectroscopic metallicity distributions of individual red giant branch stars we obtain the detailed star formation and chemical evolution history of Fornax. Fornax is dominated by intermediate age (1-10 Gyr) stellar populations, but also includes ancient (10-14 Gyr), and young (≤1 Gyr) stars. We show that Fornax displays a radial age gradient, with younger, more metal-rich populations dominating the central region. This confirms results from previous works. Within an elliptical radius of 0.8 degrees, or 1.9 kpc from the centre, a total mass in stars of 4.3 × 107 M⊙ was formed, from the earliest times until 250 Myr ago. Using the detailed star formation history, age estimates are determined for individual stars on the upper RGB, for which spectroscopic abundances are available, giving an age-metallicity relation of the Fornax dSph from individual stars. This shows that the average metallicity of Fornax went up rapidly from [Fe/H] ≤ -2.5 dex to [Fe/H] = -1.5 dex between 8-12 Gyr ago, after which a more gradual enrichment resulted in a narrow, well-defined sequence which reaches [Fe/H] ≈ -0.8 dex, ≈3 Gyr ago. These ages also allow us to measure the build-up of chemical elements as a function of time, and thus determine detailed timescales for the evolution of individual chemical elements. A rapid decrease in [Mg/Fe] is seen for the stars with [Fe/H] ≥ -1.5 dex, with a clear trend in age. Table 1 is only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/544/A73

  20. 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.

  1. Dark matter in dwarf galaxies: Correcting inferred galaxy masses for the orbital motion of binary stars

    NASA Astrophysics Data System (ADS)

    Minor, Quinn Eliot

    We introduce a Bayesian methodology for determining the velocity dispersions of dwarf galaxies which takes into account both binarity and contamination by nonmember stars in a self-consistent way. This method can be readily extended to determine masses and related quantities such as the dark matter annihilation cross-section of dwarf galaxies. In addition we show that measured velocity dispersions of dwarf spheroidal galaxies from about 4 to 10 km/s are unlikely to be inflated by more than 30% due to the orbital motion of binary stars, and demonstrate that the intrinsic velocity dispersions can be determined to within a few percent accuracy using multi-epoch observations with 1-2 years as the optimal time interval. This methodology also constrains properties of binary populations (e.g. binary fraction, period distribution) from multi-epoch velocity measurements, and can be applied to both dwarf galaxies as well as star clusters.

  2. Linking dwarf galaxies to halo building blocks with the most metal-poor star in Sculptor.

    PubMed

    Frebel, Anna; Kirby, Evan N; Simon, Joshua D

    2010-03-04

    Current cosmological models indicate that the Milky Way's stellar halo was assembled from many smaller systems. On the basis of the apparent absence of the most metal-poor stars in present-day dwarf galaxies, recent studies claimed that the true Galactic building blocks must have been vastly different from the surviving dwarfs. The discovery of an extremely iron-poor star (S1020549) in the Sculptor dwarf galaxy based on a medium-resolution spectrum cast some doubt on this conclusion. Verification of the iron-deficiency, however, and measurements of additional elements, such as the alpha-element Mg, are necessary to demonstrate that the same type of stars produced the metals found in dwarf galaxies and the Galactic halo. Only then can dwarf galaxy stars be conclusively linked to early stellar halo assembly. Here we report high-resolution spectroscopic abundances for 11 elements in S1020549, confirming its iron abundance of less than 1/4,000th that of the Sun, and showing that the overall abundance pattern follows that seen in low-metallicity halo stars, including the alpha-elements. Such chemical similarity indicates that the systems destroyed to form the halo billions of years ago were not fundamentally different from the progenitors of present-day dwarfs, and suggests that the early chemical enrichment of all galaxies may be nearly identical.

  3. 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.

  4. Searching For Infrared Excesses Around White Dwarf Stars

    NASA Astrophysics Data System (ADS)

    Deeb Wilson, Elin; Rebull, Luisa M.; Debes, John H.; Stark, Chris

    2017-01-01

    Many WDs have been found to be “polluted,” meaning they contain heavier elements in their atmospheres. Either an active process that counters gravitational settling is taking place, or an external mechanism is the cause. One proposed external mechanism for atmospheric pollution of WDs is the disintegration and accretion of rocky bodies, which would result in a circumstellar (CS) disk. As CS disks are heated, they emit excess infrared (IR) emission. WDs with IR excesses indicative of a CS disk are known as dusty WDs. Statistical studies are still needed to determine how numerous dusty, polluted WDs are, along with trends and correlations regarding rate of planetary accretion, the lifetimes of CS disks, and the structure and evolution of CS disks. These findings will allow for a better understanding of the fates of planets along with potential habitability of surviving planets.In this work, we are trying to confirm IR excesses around a sample of 69 WD stars selected as part of the WISE InfraRed Excesses around Degenerates (WIRED) Survey (Debes et al. 2011). We have archival data from WISE, Spitzer, 2MASS, DENIS, and SDSS. The targets were initially selected from the Sloan Digital Sky Survey (SDSS), and identified as containing IR excesses based on WISE data. We also have data from the Four Star Infrared Camera array, which is part of Carnegie Institution’s Magellan 6.5 meter Baade Telescope located at Las Campanas Observatory in Chile. These Four Star data are much higher spatial resolution than the WISE data that were used to determine if each WD has an IR excess. There are often not many bands delineating the IR excess portion of the SED; therefore, we are using the Four Star data to check if there is another source in the WISE beam affecting the IR excess.

  5. CARBON ABUNDANCES FOR RED GIANTS IN THE DRACO DWARF SPHEROIDAL GALAXY

    SciTech Connect

    Shetrone, Matthew D.; Stanford, Laura M.; Smith, Graeme H.; Siegel, Michael H.; Bond, Howard E. E-mail: graeme@ucolick.org E-mail: bond@stsci.edu

    2013-05-15

    Measurements of [C/Fe], [Ca/H], and [Fe/H] have been derived from Keck I LRISb spectra of 35 giants in the Draco dwarf spheroidal galaxy. The iron abundances are derived by a spectrum synthesis modeling of the wavelength region from 4850 to 5375 A, while calcium and carbon abundances are obtained by fitting the Ca II H and K lines and the CH G band, respectively. A range in metallicity of -2.9 {<=} [Fe/H] {<=} -1.6 is found within the giants sampled, with a good correlation between [Fe/H] and [Ca/H]. The great majority of stars in the sample would be classified as having weak absorption in the {lambda}3883 CN band, with only a small scatter in band strengths at a given luminosity on the red giant branch. In this sense the behavior of CN among the Draco giants is consistent with the predominantly weak CN bands found among red giants in globular clusters of metallicity [Fe/H] < -1.8. Over half of the giants in the Draco sample have [Fe/H] > -2.25, and among these there is a trend for the [C/Fe] abundance to decrease with increasing luminosity on the red giant branch. This is a phenomenon that is also seen among both field and globular cluster giants of the Galactic halo, where it has been interpreted as a consequence of deep mixing of material between the base of the convective envelope and the outer limits of the hydrogen-burning shell. However, among the six Draco giants observed that turn out to have metallicities -2.65 < [Fe/H] < -2.25 there is no such trend seen in the carbon abundance. This may be due to small sample statistics or primordial inhomogeneities in carbon abundance among the most metal-poor Draco stars. We identify a potential carbon-rich extremely metal-poor star in our sample. This candidate will require follow-up observations for confirmation.

  6. Constraining the Stellar Populations and Star Formation Histories of Blue Compact Dwarf Galaxies with SED Fits

    NASA Astrophysics Data System (ADS)

    Janowiecki, Steven; Salzer, John J.; van Zee, Liese; Rosenberg, Jessica L.; Skillman, Evan

    2017-02-01

    We discuss and test possible evolutionary connections between blue compact dwarf galaxies (BCDs) and other types of dwarf galaxies. BCDs provide ideal laboratories to study intense star formation episodes in low-mass dwarf galaxies, and have sometimes been considered a short-lived evolutionary stage between types of dwarf galaxies. To test these connections, we consider a sample of BCDs as well as a comparison sample of nearby galaxies from the Local Volume Legacy (LVL) survey for context. We fit the multi-wavelength spectral energy distributions (SED, far-ultra-violet to far-infrared) of each galaxy with a grid of theoretical models to determine their stellar masses and star formation properties. We compare our results for BCDs with the LVL galaxies to put BCDs in the context of normal galaxy evolution. The SED fits demonstrate that the star formation events currently underway in BCDs are at the extreme of the continuum of normal dwarf galaxies, both in terms of the relative mass involved and in the relative increase over previous star formation rates. Today’s BCDs are distinctive objects in a state of extreme star formation that is rapidly transforming them. This study also suggests ways to identify former BCDs whose star formation episodes have since faded.

  7. 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

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

    NASA Astrophysics Data System (ADS)

    Valyavin, G.; Shulyak, D.; Wade, G. A.; Antonyuk, K.; Zharikov, S. V.; Galazutdinov, G. A.; Plachinda, S.; Bagnulo, S.; Fox Machado, L.; 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.

  9. 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

    2014-11-06

    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.

  10. White Dwarf WD-1145+17 "Zombie Star" Consumes Asteroid

    NASA Astrophysics Data System (ADS)

    Kaye, Thomas G.; Gary, Bruce L.; Rappaport, Saul A.; Foote, Jerry, Benni, Paul

    2016-05-01

    It has long been suspected that white dwarfs accrete asteroid debris as evidenced by heavy metals in many white dwarf spectra. WD1145 was initially detected in Kepler data as an exoplanet candidate with a repeating 1.3% dip over the course of the Jul-Sep 2014 observing season. Follow-up ground based observations were conducted with professional telescopes during March through May of 2015, and these showed that the Kepler dip must likely consist of deeper and shorter dips which come and go with an uncertain pattern. It was hypothesized that the observations were due to an asteroid in a 4.5 hour orbit. In anticipation of its return to nighttime visibility, major observatories scheduled time starting in 2016 Feb. A pro/am collaboration was formed in late 2015 for amateur observations prior to the 2016 Feb professional observations in order to determine an ephemeris for fade activity for the purpose of scheduling relatively short observations with professional telescopes. The amateur observations began in 2015 Nov, sooner than requested, and they showed that the fade activity level had exploded, becoming 20 times the level measured by Kepler. As many as 13 different fades per 4.5-hour orbit were measured, and these varied in depth from night to night. The amateur project turned into a full assault on the star with as many as 4 amateur telescopes observing on the same night. Continuous monitoring mysteriously showed that the clouds drifted in phase with respect to the dominant period i.e., they have a shorter period than measured by Kepler; this would imply that the orbiting dust clouds were located inside the orbit of the parent planetesimal. The best model indicated that the parent planetesimal was releasing fragments from inside its Hill sphere at the L1 Lagrange point, causing them to fall into an inner orbit. New astrophysics was described for the first time when the team used the diameter of the planetesimal orbit, and the diameter of the drift fragment orbit, to

  11. 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.

  12. THE KINEMATICS AND CHEMISTRY OF RED HORIZONTAL BRANCH STARS IN THE SAGITTARIUS STREAMS

    SciTech Connect

    Shi, W. B.; Chen, Y. Q.; Carrell, K.; Zhao, G. E-mail: cyq@bao.ac.cn E-mail: gzhao@nao.cas.cn

    2012-06-01

    We have selected 556 red horizontal branch stars along the streams of the Sagittarius (Sgr) dwarf galaxy from Sloan Digital Sky Survey DR7 spectroscopic data using a theoretical model. The metallicity and {alpha}-element distributions are investigated for stars in the Sgr streams and for Galactic stars at the same locations. We find that the Sgr stars have two peaks in the metallicity distribution while the Galactic stars have a more prominent metal-poor peak. Meanwhile, [{alpha}/Fe] ratios of the Sgr stars are lower than those of the Galactic stars. Among the Sgr stars, we find a difference in the metallicity distribution between the leading and trailing arms of the Sgr tidal tails. The metallicity and [{alpha}/Fe] distribution of the leading arm is similar to that of the Galaxy. The trailing arm is composed mainly of a metal-rich component and [{alpha}/Fe] is obviously lower than that of the Galactic stars. The metallicity gradient is -(1.8 {+-} 0.3) Multiplication-Sign 10{sup -3} dex deg{sup -1} in the first wrap of the trailing arm and -(1.5 {+-} 0.4) Multiplication-Sign 10{sup -3} dex deg{sup -1} in the first wrap of the leading arm. No significant gradient exists along the second wraps of the leading or trailing arms. It seems that the Sgr dwarf galaxy initially lost the metal-poor component in the second wrap (older) arms due to the tidal force of our Galaxy and then the metal-rich component is disrupted in the first wrap (younger) arms. Finally, we found that the velocity dispersion of the trailing arm from 88 Degree-Sign < {Lambda}{sub Sun} < 112 Degree-Sign is {sigma} = 9.808 {+-} 1.0 km s{sup -1}, which is consistent with previous work in the literature.

  13. An r-process enhanced star in the dwarf galaxy Tucana III

    DOE PAGES

    Hansen, T. T.; Simon, J. D.; Marshall, J. L.; ...

    2017-03-20

    Chemically peculiar stars in dwarf galaxies provide a window for exploring the birth environment of stars with varying chemical enrichment. We present a chemical abundance analysis of the brightest star in the newly discovered ultra-faint dwarf galaxy candidate Tucana III. Because it is particularly bright for a star in an ultra-faint Milky Way (MW) satellite, we are able to measure the abundance of 28 elements, including 13 neutron-capture species. This star, DES J235532.66–593114.9 (DES J235532), shows a mild enhancement in neutron-capture elements associated with the r-process and can be classified as an r-I star. DES J235532 is the first r-Imore » star to be discovered in an ultra-faint satellite, and Tuc III is the second extremely low-luminosity system found to contain r-process enriched material, after Reticulum II. Comparison of the abundance pattern of DES J235532 with r-I and r-II stars found in other dwarf galaxies and in the MW halo suggests a common astrophysical origin for the neutron-capture elements seen in all r-process enhanced stars. Furthermore, we explore both internal and external scenarios for the r-process enrichment of Tuc III and show that with abundance patterns for additional stars, it should be possible to distinguish between them.« less

  14. An r-process Enhanced Star in the Dwarf Galaxy Tucana III

    NASA Astrophysics Data System (ADS)

    Hansen, T. T.; Simon, J. D.; Marshall, J. L.; Li, T. S.; Carollo, D.; DePoy, D. L.; Nagasawa, D. Q.; Bernstein, R. A.; Drlica-Wagner, A.; Abdalla, F. B.; Allam, S.; Annis, J.; Bechtol, K.; Benoit-Lévy, A.; Brooks, D.; Buckley-Geer, E.; Carnero Rosell, A.; Carrasco Kind, M.; Carretero, J.; Cunha, C. E.; da Costa, L. N.; Desai, S.; Eifler, T. F.; Fausti Neto, A.; Flaugher, B.; Frieman, J.; García-Bellido, J.; Gaztanaga, E.; Gerdes, D. W.; Gruen, D.; Gruendl, R. A.; Gschwend, J.; Gutierrez, G.; James, D. J.; Krause, E.; Kuehn, K.; Kuropatkin, N.; Lahav, O.; Miquel, R.; Plazas, A. A.; Romer, A. K.; Sanchez, E.; Santiago, B.; Scarpine, V.; Smith, R. C.; Soares-Santos, M.; Sobreira, F.; Suchyta, E.; Swanson, M. E. C.; Tarle, G.; Walker, A. R.; DES Collaboration

    2017-03-01

    Chemically peculiar stars in dwarf galaxies provide a window for exploring the birth environment of stars with varying chemical enrichment. We present a chemical abundance analysis of the brightest star in the newly discovered ultra-faint dwarf galaxy candidate Tucana III. Because it is particularly bright for a star in an ultra-faint Milky Way (MW) satellite, we are able to measure the abundance of 28 elements, including 13 neutron-capture species. This star, DES J235532.66‑593114.9 (DES J235532), shows a mild enhancement in neutron-capture elements associated with the r-process and can be classified as an r-I star. DES J235532 is the first r-I star to be discovered in an ultra-faint satellite, and Tuc III is the second extremely low-luminosity system found to contain r-process enriched material, after Reticulum II. Comparison of the abundance pattern of DES J235532 with r-I and r-II stars found in other dwarf galaxies and in the MW halo suggests a common astrophysical origin for the neutron-capture elements seen in all r-process enhanced stars. We explore both internal and external scenarios for the r-process enrichment of Tuc III and show that with abundance patterns for additional stars, it should be possible to distinguish between them. This paper includes data gathered with the 6.5 meter Magellan Telescopes located at Las Campanas Observatory, Chile.

  15. VizieR Online Data Catalog: Lowell GR* red stars (Giclas+ 1972-1978)

    NASA Astrophysics Data System (ADS)

    Giclas, H. L.; Burnham, R.; Thomas, N. G.

    2009-10-01

    This file shows accurate coordinates for the ~500 red stars found by Giclas et al. near the south galactic pole. This was part of a general survey of large proper motion stars. In 2002 August I sent Gerard Jasniewicz corrections for the first batch of 47 stars, mainly just to see for myself what sorts of stars are involved. These 47 stars were observed photoelectrically by Warren (1976MNRAS.176..667W), who obtained UBV photometry. Although every star was a dwarf, they were not extraordinarily red, but merely garden-variety late-K and early-M dwarfs. Gerard corrected all those entries in SIMBAD, and the UBV data are linked. I have now gone through the entire list and show improved positions below. For most stars UCAC3 positions were adopted. In this part of the sky UCAC3 draws from both the Schmidt plate-scans and the Yale SPM series as well as the UCAC astrograph series. The multiple epochs mean the positions and proper motions for the fainter stars are usually the best ones. There are still a few where the motion is forced to zero or is blank, and UCAC2, 2MASS, or other source is used, as specified in the column 's' following the position. I show V magnitudes for all the stars. For stars brighter than between V = 14.5 and 15.0, the ASAS-3 is preferred. For the fainter stars this is merely average of the GSC-2.3 blue and red magnitudes. Where there is overlap, the naive b+r/2 from the Schmidt plates matches ASAS-3 to within 0.1 mag usually, and only sometimes differs by as much as 0.3 mag. Stars with motion >0".15/year are flagged 'lg pm' (large proper motion). In identifying the stars, I also found a substantial number of common-motion pairs, which have all been sent to Brian Mason (USNO) for possible inclusion in the WDS. I noticed a few stars whose 2MASS J-K colors are >0.9mag; these must be late-K/early-M giants --- interesting in themselves, since at Vmag 15 they are ~10 kpc out in the halo. Those I noticed are flagged in the remarks, but there could be more

  16. 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.

  17. LIMITS ON UNRESOLVED PLANETARY COMPANIONS TO WHITE DWARF REMNANTS OF 14 INTERMEDIATE-MASS STARS

    SciTech Connect

    Kilic, Mukremin; Gould, Andrew; Koester, Detlev

    2009-11-10

    We present Spitzer IRAC photometry of white dwarf remnants of 14 stars with M = 3-5 M{sub sun}. We do not detect mid-infrared excess around any of our targets. By demanding a 3sigma photometric excess at 4.5 mum for unresolved companions, we rule out planetary mass companions down to 5, 7, or 10 M {sub J} for 13 of our targets based on the Burrows et al. substellar cooling models. Combined with previous IRAC observations of white dwarf remnants of intermediate-mass stars, we rule out >=10M {sub J} companions around 40 white dwarfs and >=5M {sub J} companions around 10 white dwarfs.

  18. Outer atmospheres of cool stars. IX - A survey of ultraviolet emission from F-K dwarfs and giants with IUE

    NASA Technical Reports Server (NTRS)

    Ayres, T. R.; Marstad, N. C.; Linsky, J. L.

    1981-01-01

    Low-dispersion ultraviolet spectra (1150-2000 A) of a representative sample of cool stars, including dwarfs and giants of spectral types F-K, obtained with the IUE, are examined. The observation and the absolute calibration procedures are described. Correlation diagrams are constructed that compare chromospheric and transition-region emission line strengths and broadband coronal soft X-ray fluxes. The transition-region and coronal emission in the G-K dwarfs and G giants is well correlated with the Mg II (wavelength 2800) doublet emission strength, which is symptomatic of chromospheric energy losses. The power-law slopes are steeper than unity, particularly for soft X-rays. The implications of the correlations are discussed with respect to the weakening or disappearance of transition regions and hot coronae in the cool half of the red-giant branch and possible chromospheric and coronal heating mechanisms. It is proposed that the weakness of outer atmospheres in the red giants compared with the yellow giants can be understood as a consequence of stellar evolution, since it is possible that stars of slightly different spectral type in the giant branch have very different main-sequence progenitors.

  19. Mapping the Abyss: A Breakthrough in Mass Determinations for Stars and Brown Dwarfs using HST and RECONS Astrometry

    NASA Astrophysics Data System (ADS)

    Sevrinsky, Raymond Andrew; Henry, Todd J.; Jao, Wei-Chun; RECONS Team

    2016-01-01

    We present astrometric results for 7 close binary systems from the ongoing RECONS (REsearch Consortium On Nearby Stars, www.recons.org) astrometry program on the CTIO/SMARTS 0.9m telescope. The systems consist of red and brown dwarf components with masses of 0.05-0.30solar masses that straddle the transition region between stars and substellar objects. We report trigonometric parallaxes with errors less than 3 milliarcseconds that place the objects at distances between 10 and 33 parsecs.Measurements of the long-term perturbations in the systems' photocenters over 5-13 years allows us to derive orbital periods that are on the order of one decade for all seven systems. Followup analysis is underway using measurements from HST-WFC3 to measure the optical fluxes, separations, and position angles of the individual components in these systems. These new resolved astrometric data, coupled with the long-term ground-based work, will be used to convert the photocentric orbits into relative orbits to provide critical mass ratios and mass measurements for both components in each system. The 14 carefully characterized objects will comprise a fundamental set of standards that will stress-test theoretical models of the smallest stars and brown dwarfs for years to come. The results will be combined with our previous mass-luminosity relation work for stars with masses 0.08-0.60 Msun to extend our understanding into the realm of brown dwarfs. We will then have a detailed map covering a factor of more than 10 in mass for the most common objects in the Galaxy.This effort has been supported by the NSF through grants AST-0908402, AST-1109445, and AST-1412026, STScI grant HST-GO-13724.001-A, and via observations made possible by the SMARTS Consortium.

  20. 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

  1. 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.

  2. 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.

  3. Weighing the local dark matter with RAVE red clump stars

    NASA Astrophysics Data System (ADS)

    Bienaymé, O.; Famaey, B.; Siebert, A.; Freeman, K. C.; Gibson, B. K.; Gilmore, G.; Grebel, E. K.; Bland-Hawthorn, J.; Kordopatis, G.; Munari, U.; Navarro, J. F.; Parker, Q.; Reid, W.; Seabroke, G. M.; Siviero, A.; Steinmetz, M.; Watson, F.; Wyse, R. F. G.; Zwitter, T.

    2014-11-01

    We determine the Galactic potential in the solar neigbourhood from RAVE observations. We select red clump stars for which accurate distances, radial velocities, and metallicities have been measured. Combined with data from the 2MASS and UCAC catalogues, we build a sample of ~4600 red clump stars within a cylinder of 500 pc radius oriented in the direction of the South Galactic Pole, in the range of 200 pc to 2000 pc distances. We deduce the vertical force and the total mass density distribution up to 2 kpc away from the Galactic plane by fitting a distribution function depending explicitly on three isolating integrals of the motion in a separable potential locally representing the Galactic one with four free parameters. Because of the deep extension of our sample, we can determine nearly independently the dark matter mass density and the baryonic disc surface mass density. We find (i) at 1 kpc Kz/ (2πG) = 68.5 ± 1.0 M⊙ pc-2; and (ii) at 2 kpc Kz/ (2πG) = 96.9 ± 2.2 M⊙ pc-2. Assuming the solar Galactic radius at R0 = 8.5 kpc, we deduce the local dark matter density ρDM(z = 0) = 0.0143 ± 0.0011 M⊙pc-3 = 0.542 ± 0.042 Gev cm-3 and the baryonic surface mass density Σbar = 44.4 ± 4.1 M⊙pc-2. Our results are in agreement with previously published Kz determinations up to 1 kpc, while the extension to 2 kpc shows some evidence for an unexpectedly large amount of dark matter. A flattening of the dark halo of order 0.8 can produce such a high local density in combination with a circular velocity of 240 km s-1. It could also be consistent with a spherical cored dark matter profile whose density does not drop sharply with radius. Another explanation, allowing for a lower circular velocity, could be the presence of a secondary dark component, a very thick disc resulting either from the deposit of dark matter from the accretion of multiple small dwarf galaxies, or from the presence of an effective "phantom" thick disc in the context of effective galactic

  4. The late-M dwarfs

    NASA Astrophysics Data System (ADS)

    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.

  5. 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

  6. Star Formation in NGC4532/DDO 137'S Tidal Dwarf Galaxies and 500 KPC HI Stream

    NASA Astrophysics Data System (ADS)

    Higdon, Sarah

    Mergers and close-passages between gas rich galaxies can result in the formation of long HI/stellar streams. The tidally induced star formation and gas concentrations can result in the creation of tidal dwarf galaxies (TDGs). TDGs may contribute significantly to the dwarf galaxy population, by far the most common galaxy type in the current epoch. We have discovered one of the longest known tidal streams (500 kpc) in the NGC 4535/DDO 137 system. We propose 3 ksec FUV/NUV images centered on the stream and its five TDGs. We will readily detect faint/low mass star forming regions (~2E-17 erg s-1 cm-2 A-1) to 5-sigma. The GALEX observations are a unique opportunity to undertake a sensitive and comprehensive study of tidally induced star formation, dwarf galaxy formation and inter-galactic enrichment in this system.

  7. From Accretion to Explosion and Beyond: Transforming White Dwarfs to Neutron Stars and Black Holes

    NASA Astrophysics Data System (ADS)

    Di Stefano, Rosanne; Harris, R.

    2010-03-01

    White dwarfs accreting at high rates can grow in mass, exhibiting episodes of supersoft-source activity. Some can achieve the Chandrasekhar mass and will either become Type Ia supernovae or else will collapse, becoming neutron stars. We consider white dwarfs with giant donors, computing the rates of both supernovae and collapses. For the collapses, we follow each system to the end of accretion. Some of these systems will appear as ultraluminous x-ray sources and some will go on to become low-mass black holes. This scenario should be fairly common in young stellar populations and links a wide range of astrophysical phenomena. Indeed, it is a veritable cornucopia for the high-energy astrophysicist, offering accreting white dwarfs, neutron stars, and black holes, Type Ia supernovae, gamma-ray bursts, supersoft sources, ultraluminous sources, and neutron star and black hole binaries in globular clusters.

  8. Environmental effects on stellar populations of star clusters and dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Pasetto, Stefano; Cropper, Mark; Fujita, Yutaka; Chiosi, Cesare; Grebel, Eva K.

    2017-03-01

    We investigate the competitive role of the different dissipative phenomena acting on the onset of star formation of gravitationally bound systems in an external environment. Ram pressure, Kelvin-Helmholtz and Rayleigh-Taylor instabilities, and tidal forces are accounted for separately in an analytical framework and compared in their role in influencing the star forming regions. We present an analytical criterion to elucidate the dependence of star formation in a spherical stellar system on its surrounding environment. We consider the different signatures of these phenomena in synthetically realized colour-magnitude diagrams (CMDs) of the orbiting system thus investigating the detectability limits of these different effects for future observational projects and their relevance. The developed theoretical framework has direct applications to the cases of massive star clusters, dwarf galaxies in galaxy clusters and dwarf galaxies orbiting our Milky Way system, as well as any primordial gas-rich cluster of stars orbiting within its host galaxy.

  9. 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

  10. Comparison between high and low star forming sides of dwarf irregular galaxies with asymmetrical distributions of star formation.

    NASA Astrophysics Data System (ADS)

    Gallardo, Samavarti; Hunter, Deidre Ann; LEGUS Team

    2017-01-01

    Dwarf irregular galaxies DDO 187 and NGC 3738, in the LITTLE THINGS sample of nearby dwarfs, share the similar characteristic of having more star formation on one side of the galaxy than the other. I compared characteristics of the galaxies, such as pressure, HI surface density, and stellar mass surface density, measured on the high star formation half with those measured on the low star formation half. Comparing the galaxies, we see that the ratios of galactic properties from the high star formation side to the low star formation side are similar in both galaxies. We also see that the high star formation halves of the galaxies have higher pressure, higher stellar mass density, and higher gas mass density. Both galaxies also have peculiar gas kinematics. Looking at the young star clusters in NGC 3738 from the LEGUS survey, we see that there are younger and more clusters in the high star formation region. The cause of having such an asymmetrical distribution of star formation in these galaxies remains unknown.SG appreciates the funding to Northern Arizona University for the Research Experiences for Undergraduates program in the form of grant AST-1461200 from NSF. DAH is grateful for grant HST-GO-13364.022-A for participation in LEGUS.

  11. 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

  12. 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.

  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. Relations Between He I λ10830 Absorption Strength and Stellar Activity Amongst Dwarf Stars

    NASA Astrophysics Data System (ADS)

    Smith, Graeme H.

    2016-11-01

    Correlations are identified between the strength of the λ10830 He I triplet line and the following tracers of stellar activity amongst FGK dwarfs with colours of (B - V) > 0.47: coronal soft X-ray emission, emission in the λ1549 C IV and λ1335 C II lines originating from the transition region, and Ca II H and K emission from the chromosphere. No such correlations are present amongst dwarfs with spectral type earlier than F6. In addition, G and K dwarfs with strong triplet lines show evidence of excess flux in the GALEX FUV band compared to weak-triplet-line dwarfs. The X-ray spectra of late-F, G, and K dwarfs with He I triplets stronger than 160 mÅ have greater values of the ROSAT hardness ratio HR1 than are typical of weak-triplet dwarfs in the same range of spectral type. In other words, dwarfs later than F7V with strong He I triplet lines tend towards harder 0.1-2.0 keV X-ray spectra than weak-triplet dwarfs, although values of HR1 -0.2 to +0.1 can still be encountered amongst a minority of weak-He-triplet stars. As regards, FGK main sequence stars the observational data on the λ10830 triplet line remains sparse. Progress could be made through spectroscopy of high resolution for samples of hundreds of stars, selected on the basis of having other measures of chromospheric and coronal activity available.

  15. Properties and Star Formation Histories of Intermediate Redshift Dwarf Low-Mass Star-Forming Galaxies

    NASA Astrophysics Data System (ADS)

    Rodríguez-Muñoz, L.; Gallego, J.; Pacifici, C.; Tresse, L.; Charlot, S.; Gil de Paz, A.; Barro, G.; Villar, V.

    2017-03-01

    The epoch when low-mass star-forming galaxies (LMSFGs) form the bulk of their stellar mass is uncertain. While some models predict an early formation, others favor a delayed scenario until later ages of the Universe. We present improved constraints on the physical properties and star formation histories (SFHs) of a sample of intermediate redshift LMSFGs selected by their stellar mass or blue-compact-dwarf-like properties. Our work takes advantage of the deep UV-to-FIR photometric coverage available on the Extended-Chandra Deep Field South and our own dedicated deep VLT/VIMOS optical spectroscopy programs. On the one hand, we estimate the stellar mass (M_{*}), star formation rate (SFR), and SFH of each galaxy modeling its spectral energy distribution. We use a novel approach by Pacifici et al. 2012, that (1) consistently combines photometric (broad-band) and spectroscopic (emission line fluxes and equivalent widths) data, and (2) uses physically-motivated SFHs with non-uniform variations of the SFR as a function of time. On the other hand, we characterize the properties of their interstellar medium by analyzing the emission line features visible in the VIMOS spectroscopy. The final sample includes 91 spectroscopically confirmed LMSFGs (7.3 ≤ logM_{*}/M_{⊙} ≤ 9.5) at 0.3 star forming galaxies over 2 dex in stellar mass, and high specific-SFR. Furthermore, they are characterized by strong emission lines, low metallicity, and an enhanced level of excitation. Our selection criterion based on mass gathers galaxies within a wide range of properties, and possibly, different evolutionary stages. Despite the individual differences, the average SFH that we obtain suggests a late and fast (˜2 Gyr prior their observation) assembly scenario for this type of system.

  16. Dwarf

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Alfalfa dwarf occurs rarely in alfalfa fields. Dwarf has been identified only in California, where it is found at a low frequency. Plants with symptoms of dwarf were reported in the 1950s in Mississippi, Georgia, and Rhode Island, but experimental confirmation of the disease in those States was no...

  17. 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.

  18. Characterization of High-Energy Emissions of GKM Stars using Wide Binaries with White Dwarfs

    NASA Astrophysics Data System (ADS)

    Catalán, S.; Garcés, A.; Ribas, I.

    2011-12-01

    The definition of an age calibration for main-sequence late-type stars has multiple applications, e.g., in the fields of galactic evolution, stellar dynamos, theories of angular momentum loss and planetary atmospheres. In the latter, the characterization of the time-evolution of stellar high-energy emissions can help us understand the influence on planetary atmospheres and their potential habitability. A key element for this characterization is a reliable age determination. For this purpose we have studied a sample of late G, K, and M stars. To cover the age window up to 0.7 Gyr we have used stars belonging to open clusters, while for ages above this limit we use wide binaries containing white dwarfs. Since the evolution of white dwarfs can be understood as a cooling process, which is relatively well known at the moment, we can use them as age calibrators. Wide binary members are supposed to have been born simultaneously and with the same chemical composition. Since they are well separated (100-1000 AU aprox.) we can assume that no interaction has occurred between them in the past and they have evolved as single stars. So, from the white dwarf age we can infer the age of the system. We present our current results based in a sample of 30 binaries from the NLTT catalogue comprised by a DA white dwarf (showing only H absorption lines) and a G, K or M star.

  19. 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.

  20. Star Formation at Low Rates: How a Lack of Massive Stars Impacts the Evolution of Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Hensler, Gerhard

    2017-01-01

    In recent years dedicated observations have uncovered star formation at extremely low rates in dwarf galaxies, tidal tails, ram-pressure stripped gas clouds, and the outskirts of galactic disks. At the same time, numerical simulations of galaxy evolution have advanced to higher spatial and mass resolutions, but have yet to account for the underfilling of the uppermost mass bins of stellar initial mass function (IMF) at low star-formation rates. In such situations, simulations may simply scale down the IMF, without realizing that this unrealistically results infractions of massive stars, along with fractions of massive star feedback energy (e.g., radiation and SNII explosions). Not properlyaccounting for such parameters has consequences for the self-regulation of star formation, the energetics of galaxies, as well as for the evolution of chemical abundances.Here we present numerical simulations of dwarf galaxies with low star-formation rates allowing for two extreme cases of the IMF: a "filled" case with fractional massive stars vs. a truncated IMF, at which the IMF is built bottom-up until the gas reservoir allows the formation of a last single star at an uppermost mass. The aim of the study is to demonstrate the different effects on galaxy evolution with respect to self-regulation, feedback, and chemistry. The case of a stochastic sampled IMF is situated somewhere in between these extremes.

  1. Radial Velocity of the Phoenix Dwarf Galaxy: Linking Stars and H I Gas

    NASA Astrophysics Data System (ADS)

    Gallart, C.; Martínez-Delgado, D.; Gómez-Flechoso, M. A.; Mateo, M.

    2001-05-01

    We present the first radial velocity measurement of the stellar component of the Local Group dwarf galaxy Phoenix, using the FORS1 instrument at the VLT's Unit Telescope 1 (Antu). From the spectra of 31 red giant branch stars, we derive a heliocentric optical radial velocity for Phoenix of Vsolar=-52+/-6 km s-1. On the basis of this velocity, and taking into account the results of a series of semianalytical and numerical simulations, we discuss the possible association of the H I clouds observed in the Phoenix vicinity. We conclude that the characteristics of the H I cloud with heliocentric velocity -23 km s-1 are consistent with this gas having been associated with Phoenix in the past and being lost by the galaxy after the last event of star formation in the galaxy, about 100 Myr ago. Two possible scenarios are discussed: the ejection of the gas by the energy released by the supernovae (SNe) produced in that last event of star formation and a ram pressure stripping scenario. We derive that the kinetic energy necessary to eject the gas is ESNe~2×1051 ergs and that the number of SNe necessary to transfer this amount of kinetic energy to the gas cloud is ~20. This is consistent with the number of SNe expected for the last event of star formation in Phoenix, according to the star formation history derived by Martínez-Delgado, Gallart, & Aparicio. The drawback of this scenario is the regular appearance of the H I cloud and its anisotropic distribution with respect to the stellar component. Another possibility is that the H I gas was stripped as a consequence of ram pressure by the intergalactic medium. In our simulations, the structure of the gas remains quite smooth as it is stripped from Phoenix, keeping a distribution similar to that of the observed H I cloud. Both in the SNe ejection case and in the ram pressure sweeping scenario, the distances and relative velocities imply that the H I cloud is not gravitationally bound to Phoenix, since this would require a

  2. The properties of red giant stars along the Sagittarius tidal tails

    NASA Astrophysics Data System (ADS)

    Zhang, X.; Shi, W. B.; Chen, Y. Q.; Zhao, G.; Carrell, K.; Zhao, J. K.; Ruan, G. P.; Liang, Y. C.; Zhou, L.; Ren, H. B.; Zhang, Y.; Hou, Y. H.; Wang, Y. F.

    2017-01-01

    Aims: We aim to measure the metallicity distribution and velocity distribution of red giant branch (RGB) stars along the Sagittarius dwarf galaxy (Sgr) streams. Thanks to the large number of stars of the Sloan Digital Sky Survey (SDSS) sample, we can study the properties of streams as a function of the Λ⊙ from the Sgr core. Methods: Using the 22 000 RGB stars from the ninth data release of SDSS, we selected 1100 RGB stars belonging to the streams of the Sgr. As compared with red horizontal branch stars (Shi et al. 2012, ApJ, 751, 130) the RGB stars constitute a large sample size and extend to a metal-poor component of [Fe/H] -3.0 dex. In particular, this RGB sample has a significant number of stars in the second wrap of the leading stream of the Sgr (leading arm 2), and thus provides a good opportunity to understand the properties of the leading stream. Results: We derive a metallicity gradient of -(2.3 ± 0.5) × 10-3 dex deg-1 in leading arm 2 for the first time, of -(1.6 ± 0.4) × 10-6 dex deg-1 for the leading arm 1, and of -(1.3 ± 0.3) × 10-3 dex deg-1 for the trailing arm 1. We check the distribution of Sgr stars in phase space and find a velocity dispersion of 21.5 km s-1 for leading arm 1. Finally, we identify a possible new branch in leading arm 1. Full Table A.1 is 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/597/A54

  3. 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

  4. 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.

  5. I -Love- Q relations for white dwarf stars

    NASA Astrophysics Data System (ADS)

    Boshkayev, K.; Quevedo, H.; Zhami, B.

    2017-02-01

    We investigate the equilibrium configurations of uniformly rotating white dwarfs, using Chandrasekhar and Salpeter equations of state in the framework of Newtonian physics. The Hartle formalism is applied to integrate the field equation together with the hydrostatic equilibrium condition. We consider the equations of structure up to the second order in the angular velocity, and compute all basic parameters of rotating white dwarfs to test the so-called moment of inertia, rotational Love number, and quadrupole moment (I-Love-Q) relations. We found that the I-Love-Q relations are also valid for white dwarfs regardless of the equation of state and nuclear composition. In addition, we show that the moment of inertia, quadrupole moment, and eccentricity (I-Q-e) relations are valid as well.

  6. Spectroscopic Reductions of White Dwarf Stars to Support Dark Energy Survey Calibrations

    NASA Astrophysics Data System (ADS)

    Gulledge, Deborah Jean; Robertson, Jacob M.; Tucker, Douglas Lee; Smith, J. Allyn; Wester, William; Tremblay, Pier-Emmanuel; Fix, Mees B.

    2017-01-01

    The Dark Energy Survey is an imaging survey that covers 5000 square degrees in the Southern hemisphere to map galaxies and gather information on dark energy. Science requirements for the survey require a 0.5% uncertainty in color, driven by supernova science. The Dark Energy Survey relies a calibration technique that uses white dwarf stars to set zero points. These white dwarf spectra are fit to models which are used to generate synthetic photometry. These values are compared to the measured values from the survey to verify that the zero points are correct. We present results to date of the spectroscopic reductions of these white dwarf stars in support of the calibrations for the Dark Energy Survey.

  7. The IMF of Low-Mass Stars and Brown Dwarfs in Taurus

    NASA Astrophysics Data System (ADS)

    Luhman, K.

    2001-05-01

    By combining deep optical imaging and infrared spectroscopy with data from the Two-Micron All-Sky Survey (2MASS) and from previous studies, 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 Msun. Relative to the similarly derived IMF for the Trapezium Cluster (Luhman et al.), the IMF for Taurus exhibits a modest deficit of stars above one solar mass (i.e., steeper slope), the same turnover mass (0.8 Msun), 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 Msun) are expected in these Taurus fields where only one brown dwarf candidate is found. These results are used to test theories of the IMF.

  8. Optical-to-UV correlations and particle fluxes for M dwarf exoplanet host stars

    NASA Astrophysics Data System (ADS)

    Youngblood, Allison

    2017-01-01

    UV stellar radiation can significantly impact planetary atmospheres through heating and photochemistry, even regulating production of potential biomarkers. M dwarfs emit the majority of their UV radiation in the form of emission lines, and the incident UV radiation on habitable-zone planets is significant owing to their small orbital radii. Only recently have the UV spectral energy distributions (SEDs) of average M dwarfs been explored (e.g., the MUSCLES Treasury Survey). Emission lines tracing hot plasma in the stellar chromosphere and transition region dominate the far-UV spectra, even for optically inactive M dwarfs (i.e., those displaying Hα absorption spectra). Lyα (1216 Å) is the strongest of the UV emission lines, but resonant scattering from the interstellar medium makes direct observations of the intrinsic Lyα emission of even nearby stars challenging. I reconstruct the intrinsic Lyα profiles using an MCMC technique and use them to estimate the extreme-UV SED.Monitoring the long-term (years-to-decades) UV activity of M dwarfs will be important for assessing the potential habitability of short-period planets, but will only be feasible from the ground via optical proxies. Therefore, I also quantify correlations between UV and optical emission lines of the MUSCLES stars and other M dwarfs, for use when direct UV observations of M dwarf exoplanet host stars are not available. Recent habitability studies of M dwarf exoplanets have sought to address the impact of frequent flaring and are just beginning to include the damaging impact of stellar energetic particles that are typically associated with large flares. Working under the necessary assumption of solar-like particle production, I present a new technique for estimating >10 MeV proton flux during far-UV flares, and analyze a sample of the flares observed in the MUSCLES Treasury Survey.

  9. 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)

  10. 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.

  11. 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.

  12. Three Red Variable Stars in SDSS Stripe 82

    NASA Astrophysics Data System (ADS)

    Olinger, Kyle; Lutz, Julie H.

    2016-06-01

    We examined light curves of stars with g-r values greater than 0.6 in the LSST project's re-reduction of the SDSS Stripe 82 photometric data. A few stars have interesting light curves in which we chose for spectroscopic follow-up with DIS on the Apache Point 3.5-m telescope. In this poster we will report our findings on three of the most interesting red variable stars. One of the stars (GI Cet) has a period of 219.86 days, as determined from the Stripe 82 light curve. Spectra of GI Ceti taken at 3 epochs will be discussed. A second variable, TY Aqr, does not have a period. We will present the light curve and spectra taken in 2013 and 2014. The third variable is very red and a-periodic. The spectrum obtained in 2013 is that of a carbon star.

  13. Red giants in the vicinity of open clusters. Field stars

    NASA Astrophysics Data System (ADS)

    Pakhomov, Yu. V.; Antipova, L. I.; Boyarchuk, A. A.; Zhao, G.; Liang, Ya.

    2009-08-01

    We present a comparative analysis of the atmospheric abundances of red giants in the vicinity of open clusters. The atmospheric parameters, atmospheric abundances, masses, ages, Galactic velocities, and elements of the Galactic orbits are derived for all the studied stars. We have discovered high metal abundances (close to 0.3dex) for five stars, which we classify as super-metal-rich stars. Several stars have lower [Na/Fe] than normal red giants with similar atmospheric parameters. The kinematic characteristics of these stars are somewhat different from those for objects in the Galactic thin disk. We suggest that the observed effect can be explained by inhomogeneity of the chemical composition of gas-dust clouds, which could be due to different rates of SNe II supernovae in different regions of the Galaxy.

  14. Cool carbon stars in the halo and in dwarf galaxies: Hα, colours, and variability

    NASA Astrophysics Data System (ADS)

    Mauron, N.; Gigoyan, K. S.; Berlioz-Arthaud, P.; Klotz, A.

    2014-02-01

    The population of cool carbon (C) stars located far from the galactic plane is probably made of debris of small galaxies such as the Sagittarius dwarf spheroidal galaxy (Sgr), which are disrupted by the gravitational field of the Galaxy. We aim to know this population better through spectroscopy, 2MASS photometric colours, and variability data. When possible, we compared the halo results to C star populations in the Fornax dwarf spheroidal galaxy, Sgr, and the solar neighbourhood. We first present a few new discoveries of C stars in the halo and in Fornax. The number of spectra of halo C stars is now 125. Forty percent show Hα in emission. The narrow location in the JHK diagram of the halo C stars is found to differ from that of similar C stars in the above galaxies. The light curves of the Catalina and LINEAR variability databases were exploited to derive the pulsation periods of 66 halo C stars. A few supplementary periods were obtained with the TAROT telescopes. We confirm that the period distribution of the halo strongly resembles that of Fornax, and we found that it is very different from the C stars in the solar neighbourhood. There is a larger proportion of short-period Mira/SRa variables in the halo than in Sgr, but the survey for C stars in this dwarf galaxy is not complete, and the study of their variability needs to be continued to investigate the link between Sgr and the cool halo C stars. Based on observations made with the NTT and 3.6 m telescope at the European Southern Observatory (La Silla, Chile; programs 084.D-0302 and 070.D-0203), with the TAROT telescopes at La Silla and at Observatoire de la Côte d'Azur (France), and on the exploitation of the Catalina Sky Survey and the LINEAR variability databases.Appendix A is available in electronic form at http://www.aanda.org

  15. 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.

  16. The evolution of white dwarfs resulting from helium-enhanced, low-metallicity progenitor stars

    NASA Astrophysics Data System (ADS)

    Althaus, Leandro G.; De Gerónimo, Francisco; Córsico, Alejandro; Torres, Santiago; García-Berro, Enrique

    2017-01-01

    Context. Some globular clusters host multiple stellar populations with different chemical abundance patterns. This is particularly true for ω Centauri, which shows clear evidence of a helium-enriched subpopulation characterized by a helium abundance as high as Y = 0.4 Aims: We present a whole and consistent set of evolutionary tracks from the ZAMS to the white dwarf stage that is appropriate for the study of the formation and evolution of white dwarfs resulting from the evolution of helium-rich progenitors. Methods: We derived white dwarf sequences from progenitors with stellar mass ranging from 0.60 to 2.0 M⊙ and for an initial helium abundance of Y = 0.4. We adopted two values of metallicity: Z = 0.001 and Z = 0.0005. Results: We explored different issues of white dwarf evolution and their helium-rich progenitors. In particular, the final mass of the remnants, the role of overshooting during the thermally pulsing phase, and the cooling of the resulting white dwarfs differ markedly from the evolutionary predictions of progenitor stars with the standard initial helium abundance. Finally, the pulsational properties of the resulting white dwarfs are also explored. Conclusions: We find that, for the range of initial masses explored in this paper, the final mass of the helium-rich progenitors is markedly higher than the final mass expected from progenitors with the usual helium abundance. We also find that progenitors with initial mass lower than M ≃ 0.65 M⊙ evolve directly into helium-core white dwarfs in less than 14 Gyr, and that, for larger progenitor masses, the evolution of the resulting low-mass carbon-oxygen white dwarfs is dominated by residual nuclear burning. For helium-core white dwarfs, we find that they evolve markedly faster than their counterparts coming from standard progenitors. Also, in contrast with what occurs for white dwarfs resulting from progenitors with the standard helium abundance, the impact of residual burning on the cooling time of

  17. 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.

  18. 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.

  19. 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.

  20. Microwave emission from the coronae of late-type dwarf stars

    NASA Technical Reports Server (NTRS)

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

    1983-01-01

    VLA microwave observations of 14 late-type dwarf and subgiant stars and binary systems are examined. In this extensive set of observations, four sources at 6 cm (Chi-1 Ori, UV Cet, YY Gem, and Wolf 630AB) were detected and low upper limits for the remaining stars were found. The microwave luminosities of the nondetected F-K dwarfs are as small as 0.01 those of the dMe stars. The detected emission is slowly variable in all cases and is consistent with gyroresonant emission from thermal electrons spiraling in magnetic fields of about 300 gauss if the source sizes are as large as R/R(asterisk) = 3-4. This would correspond to magnetic fields that are probably in the range 0.001-0.0001 gauss at the photospheric level. An alternative mechanism is gyrosynchrotron emission from a relatively small number of electrons with effective temperature.

  1. Probing the Histories of Local Group Dwarf Galaxies with Pulsating Variable Stars

    NASA Astrophysics Data System (ADS)

    Ordoñez, Antonio J.; Sarajedini, Ata

    2017-01-01

    I have identified and characterized the Cepheid and RR Lyrae variables in several Local Group dwarf galaxies using archival Hubble Space Telescope imaging. Template light curve fitting routines have been applied to the observations in order to accurately characterize the properties of these variable stars. The pulsation properties of these stars help to constrain their masses and ages, which in turn shed light on the evolution of their respective host systems. I will summarize what this work has yielded in the context of dwarf galaxy evolution and the accretion history of the Milky Way halo. I will also discuss simulated observations on artificial light curves which we have used to characterize different observing strategies and analysis techniques for studies of pulsating variable stars.

  2. Stellar Evolution in NGC 6791: Mass Loss on the Red Giant Branch and the Formation of Low-Mass White Dwarfs

    NASA Astrophysics Data System (ADS)

    Kalirai, Jasonjot S.; Bergeron, P.; Hansen, Brad M. S.; Kelson, Daniel D.; Reitzel, David B.; Rich, R. Michael; Richer, Harvey B.

    2007-12-01

    We present the first detailed study of the properties (temperatures, gravities, and masses) of the NGC 6791 white dwarf population. This unique stellar system is both one of the oldest (8 Gyr) and most metal-rich ([Fe/H]~+0.4) open clusters in our Galaxy and has a color-magnitude diagram (CMD) that exhibits both a red giant clump and a much hotter extreme horizontal branch. Fitting the Balmer lines of the white dwarfs in the cluster using Keck/LRIS spectra suggests that most of these stars are undermassive, =0.43+/-0.06 Msolar, and therefore could not have formed from canonical stellar evolution involving the helium flash at the tip of the red giant branch. We show that at least 40% of NGC 6791's evolved stars must have lost enough mass on the red giant branch to avoid the flash and therefore did not convert helium into carbon-oxygen in their core. Such increased mass loss in the evolution of the progenitors of these stars is consistent with the presence of the extreme horizontal branch in the CMD. This unique stellar evolutionary channel also naturally explains the recent finding of a very young age (2.4 Gyr) for NGC 6791 from white dwarf cooling theory; helium-core white dwarfs in this cluster will cool ~3 times slower than carbon-oxygen-core stars, and therefore the corrected white dwarf cooling age is in fact >~7 Gyr, consistent with the well-measured main-sequence turnoff age. These results provide direct empirical evidence that mass loss is much more efficient in high-metallicity environments and therefore may be critical in interpreting the ultraviolet upturn in elliptical galaxies. 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. Based on observations obtained at the

  3. 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.

  4. 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.

  5. 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.

  6. 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

  7. 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.

  8. The seismic properties of low-mass He-core white dwarf stars

    NASA Astrophysics Data System (ADS)

    Córsico, A. H.; Romero, A. D.; Althaus, L. G.; Hermes, J. J.

    2012-11-01

    Context. In recent years, many low-mass (≲ 0.45 M⊙) white dwarf stars expected to harbor He cores have been detected in the field of the Milky Way and in several galactic globular and open clusters. Until recently, no objects of this kind showed pulsations. This situation has changed recently with the exciting discovery of SDSS J184037.78+642312.3, the first pulsating low-mass white dwarf star. Aims: Motivated by this extremely important finding, and in view of the very valuable asteroseismological potential of these objects, we present here a detailed pulsational study applied to low-mass He-core white dwarfs with masses ranging from 0.17 to 0.46 M⊙, based on full evolutionary models representative of these objects. This study is aimed to provide a theoretical basis from which to interpret future observations of variable low-mass white dwarfs. Methods: The background stellar models on which our pulsational analysis was carried out were derived by taking into account the complete evolutionary history of the progenitor stars, with special emphasis on the diffusion processes acting during the white dwarf cooling phase. We computed nonradial g-modes to assess the dependence of the pulsational properties of these objects with stellar parameters such as the stellar mass and the effective temperature, and also with element diffusion processes. We also performed a g- and p-mode pulsational stability analysis on our models and found well-defined blue edges of the instability domain, where these stars should start to exhibit pulsations. Results: We found substantial differences in the seismic properties of white dwarfs with M∗ ≳ 0.20 M⊙ and the extremely low-mass (ELM) white dwarfs (M∗ ≲ 0.20 M⊙). Specifically, g-mode pulsation modes in ELM white dwarfs mainly probe the core regions and are not dramatically affected by mode-trapping effects by the He/H interface, whereas the opposite is true for more massive He-core white dwarfs. We found that element

  9. 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.

  10. Outer atmospheres of cool stars. XII - A survey of IUE ultraviolet emission line spectra of cool dwarf stars

    NASA Technical Reports Server (NTRS)

    Linsky, J. L.; Bornmann, P. L.; Carpenter, K. G.; Hege, E. K.; Wing, R. F.; Giampapa, M. S.; Worden, S. P.

    1982-01-01

    Quantitative information is obtained on the chromospheres and transition regions of M dwarf stars, in order to determine how the outer atmospheres of dMe stars differ from dM stars and how they compare with the outer atmospheres of quiet and active G and K type dwarfs. IUE spectra of six dMe and four dM stars, together with ground-based photometry and spectroscopy of the Balmer and Ca II H and K lines, show no evidence of flares. It is concluded, regarding the quiescent behavior of these stars, that emission-line spectra resemble that of the sun and contain emission lines formed in regions with 4000-20,000 K temperatures that are presumably analogous to the solar chromosphere, as well as regions with temperatures of 20,000-200,000 K that are presumably analogous to the solar transition region. Emission-line surface fluxes are proportional to the emission measure over the range of temperatures at which the lines are formed.

  11. Astrometric Binaries: White Dwarfs?

    NASA Astrophysics Data System (ADS)

    Oliversen, Nancy A.

    We propose to observe a selection of astrometric or spectroscopicastrometric binaries nearer than about 20 pc with unseen low mass companions. Systems of this type are important for determining the luminosity function of low mass stars (white dwarfs and very late main sequence M stars), and their contribution to the total mass of the galaxy. Systems of this type are also important because the low mass, invisible companions are potential candidates in the search for planets. Our target list is selected primarily from the list of 31 astrometric binaries near the sun by Lippincott (1978, Space Sci. Rev., 22, 153), with additional candidates from recent observations by Kamper. The elimination of stars with previous IUE observations, red companions resolved by infrared speckle interferometry, or primaries later than M1 (because if white dwarf companions are present they should have been detected in the visible region) reduces the list to 5 targets which need further information. IUE SWP low dispersion observations of these targets will show clearly whether the remaining unseen companions are white dwarfs, thus eliminating very cool main sequence stars or planets. This is also important in providing complete statistical information about the nearest stars. The discovery of a white dwarf in such a nearby system would provide important additional information about the masses of white dwarfs. Recent results by Greenstein (1986, A. J., 92, 859) from binary systems containing white dwarfs imply that 80% of such systems are as yet undetected. The preference of binaries for companions of approximately equal mass makes the Lippincott-Kamper list of A through K primaries with unseen companions a good one to use to search for white dwarfs. The mass and light dominance of the current primary over the white dwarf in the visible makes ultraviolet observations essential to obtain an accurate census of white dwarf binaries.

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

    PubMed Central

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

    2017-01-01

    Stellar-like objects with effective temperatures of 2700K and below are referred to as “ultracool dwarfs”1. This heterogeneous group includes both extremely low-mass stars and brown dwarfs (substellar objects not massive enough to sustain hydrogen fusion), and represents about 15% of the stellar-like objects in the vicinity of the Sun2. Based on the small masses and sizes of their protoplanetary disks3,4, core-accretion theory for ultracool dwarfs predicts a large, but heretofore undetected population of close-in terrestrial planets5, ranging from metal-rich Mercury-sized planets6 to more hospitable volatile-rich Earth-sized planets7. Here we report the discovery of three short-period Earth-sized planets transiting an ultracool dwarf star 12 parsecs away using data collected by the TRAPPIST8 telescope as part of an ongoing prototype transit survey9. 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 star10. Eleven orbits remain possible for the third planet based on our data, the most likely resulting in an irradiation significantly smaller than Earth's. 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. PMID:27135924

  13. 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.

  14. 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.

  15. 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'.

  16. 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.

  17. The Puzzling Atmospheres of Low-mass Stars, Brown Dwarfs and Exoplanets Revealed by the Discovery Channel Telescope

    NASA Astrophysics Data System (ADS)

    Muirhead, Philip Steven; Croll, Bryce; Dalba, Paul A.; Veyette, Mark; Han, Eunkyu; Kesseli, Aurora; Healy, Brian

    2017-01-01

    The Large Monolithic Imager (LMI) on the Discovery Channel Telescope (DCT) enables high-precision photometry with a scriptable interface and rapid cycling between photometric bands, all while guiding off-axis. Using LMI, scientists at Boston University have undertaken a number of investigations into low-mass stars, brown dwarfs and extrasolar planets. We will report on recent results from these investigations, including (1) measurements of transiting asteroids orbiting a white dwarf, (2) refined ephemerides for long-period transiting exoplanets, (3) investigations revealing biases in space-based exoplanet light curves, (4) investigations of the nature of activity in low-mass stars and brown dwarfs and (5) investigations of low-mass eclipsing binary stars. We will also propose future studies of low-mass stars, brown dwarfs and exoplanets using current and future DCT instrumentation.

  18. Quantitative spectroscopy of OB stars: from dwarfs to supergiants

    NASA Astrophysics Data System (ADS)

    Przybilla, N.; Nieva, M. F.; Firnstein, M.; Butler, K.

    2013-02-01

    We discuss recent progress made in the spectral modelling of OB stars from the main sequence to evolved phases as BA-type supergiants. Non-LTE line-formation computations can now reproduce observed spectra over the entire optical and near-IR wavelength range with high confidence, facilitating stellar atmospheric parameters and elemental abundances to be determined at high accuracy and precision. An overview is given how the fundamental stellar parameters of single stars determined in our new approach compare to high-precision data derived from detached eclipsing massive binary stars. Finally, the observational constraints for a sample of Galactic objects are put in context with state-of-the-art evolution models for rotating massive stars.

  19. MINERVA-Red: A telescope dedicated to the discovery of planets orbiting the nearest low-mass stars

    NASA Astrophysics Data System (ADS)

    Sliski, David; Blake, Cullen; Johnson, John A.; Plavchan, Peter; Wittenmyer, Robert A.; Eastman, Jason D.; Barnes, Stuart; Baker, Ashley

    2017-01-01

    Results from Kepler and ground-based exoplanet surveys suggest that M-dwarfs host numerous small sized planets. Additionally, the discovery of the Earth-sized exoplanets orbiting Proxima Centauri and Trappist 1 demonstrate that these stars can host terrestrial planets in their habitable zones. Since low-mass stars are intrinsically faint at optical wavelengths, obtaining 1 m/s Doppler resolution to detect their planetary companions remains a challenge for instruments designed for sun-like stars. We describe a novel, high-cadence approach aimed at detecting and characterizing planets orbiting the closest low-mass stars to the Sun. MINERVA-Red is an echelle spectrograph optimized for the 'deep red', between 800 nm and 900 nm, where M-dwarfs are brightest. The spectrograph will be temperature controlled at 20C +/- 10mk and in a vacuum chamber which maintains a pressure below 0.01 mbar while using a Fabry-Perot etalon and U/Ne lamp for wavelength calibration. The spectrometer will operate with a robotic, 0.7-meter telescope at Mt. Hopkins, Arizona. We expect first light in 2017.

  20. Elucidating the True Binary Fraction of VLM Stars and Brown Dwarfs with Spectral Binaries

    NASA Astrophysics Data System (ADS)

    Bardalez Gagliuffi, Daniella; Burgasser, Adam J.; Gelino, Christopher R.; SAHLMANN, JOHANNES; Schmidt, Sarah J.; Gagne, Jonathan; Skrzypek, Nathalie

    2017-01-01

    The very lowest-mass (VLM) stars and brown dwarfs are found in abundance in nearly all Galactic environments, yet their formation mechanism(s) remain an open question. One means of testing current formation theories is to use multiplicity statistics. The majority of VLM binaries have been discovered through direct imaging, and current angular resolution limits (0.05”-0.1") are coincident with the 1-4 AU peak in the projected separation distribution of known systems, suggesting an observational bias. I have developed a separation-independent method to detect T dwarf companions to late-M/early-L dwarfs by identifying methane absorption in their unresolved, low-resolution, near-infrared spectra using spectral indices and template fitting. Over 60 spectral binary candidates have been identified with this and comparable methods. I discuss follow-up observations, including laser-guide star adaptive optics imaging with Keck/NIRC2, which have confirmed 9 systems; and radial velocity and astrometric monitoring observations that have confirmed 7 others. The direct imaging results indicate a resolved binary fraction of 18%, coincident with current estimates of the VLM binary fraction; however, our sample contained 5 previously confirmed binaries, raising its true binary fraction to 47%. To more accurately measure the true VLM binary fraction, I describe the construction of an unbiased, volume-limited, near-infrared spectral sample of M7-L5 dwarfs within 25 pc, of which 4 (1%) are found to be spectral binary candidates. I model the complex selection biases of this method through a population simulation, set constraints on the true binary fraction as traced by these systems, and compare to the predictions of current formation theories. I also describe how this method may be applied to conduct a separation-unbiased search for giant exoplanets orbiting young VLM stars and brown dwarfs.

  1. Analysis of the DA white dwarf HZ 43 A and its companion star

    NASA Astrophysics Data System (ADS)

    Napiwotzki, R.; Barstow, M. A.; Fleming, T.; Holweger, H.; Jordan, S.; Werner, K.

    1993-11-01

    The DA white dwarf HZ 43 A (WD 1314+293) is analyzed based on a newly obtained optical spectrogram. We demonstrate that the derived parameters Teff = 49.000 K and log g = 7.7 are in agreement with the observed Ly-alpha line, the slope of the UV continuum, and the measured trigonometric parallax. The EXOSAT spectrograms of Paerels et al. (1986) are used to obtain upper limits for the atmospheric abundance of helium, carbon, nitrogen, and oxygen of HZ 43 A by applying the new parameters and up-to-date Non Local Thermodynamic Equilibrium (NLTE) model atmospheres. The result is discussed within the framework of diffusion calculations. It turns out that the resulting abundances of the CNO elements are below the predictions of theory making HZ 43 A an unique object. A red/near-infrared spectrum of the companion star HZ 43 B is used to reclassify it and to estimate temperature and metallicity. We calculate EUV fluxes from models with the derived stellar parameters and use them to check the flux calibrations of EXOSAT and ROSAT. The agreement between predicted and measured count rates is reasonable for the ROSAT-Wide Field Camera (WFC) filters. Most EXOSAT photometric filters exhibit deviations. These are marginally consistent with our error limits for the LX 3000, LX 4000, and the PPL filters. The Al/P calibration is in error. Discrepant results are obtained for the EXOSAT spectrograph and the ROSAT Position Sensitive Proportional Counter (PSPC). These inconsistencies may cause systematic errors, if for instance PSPC measurements are combined with WFC data for an analysis.

  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. High-field magnetic white dwarfs as the progeny of early-type stars?

    NASA Astrophysics Data System (ADS)

    Dobbie, P. D.; Külebi, B.; Casewell, S. L.; Burleigh, M. R.; Parker, Q. A.; Baxter, R.; Lawrie, K. A.; Jordan, S.; Koester, D.

    2013-01-01

    We present an analysis of the newly resolved components of two hot, double-degenerate systems, SDSS J074853.07+302543.5 + J074852.95+302543.4 and SDSS J150813.24+394504.9 + J150813.31+394505.6 (CBS 229). We confirm that each system has widely separated components (a > 100 au) consisting of a H-rich, non-magnetic white dwarf and a H-rich, high-field magnetic white dwarf (HFMWD). The masses of the non-magnetic degenerates are found to be larger than typical of field white dwarfs. We use these components to estimate the total ages of the binaries and demonstrate that both magnetic white dwarfs are the progeny of stars with Minit > 2 M⊙. We briefly discuss the traits of all known hot, wide, magnetic + non-magnetic double degenerates in the context of HFMWD formation theories. These are broadly consistent (chance probability, P ≈ 0.065) with HFMWDs forming primarily from early-type stars and, in the most succinct interpretation, link their magnetism to the fields of their progenitors. Our results do not, however, rule out that HFMWDs can form through close binary interactions and studies of more young, wide double degenerates are required to reach firm conclusions on these formation pathways.

  4. New class of Well behaved exact solutions of relativistic charged white-dwarf star with perfect fluid

    NASA Astrophysics Data System (ADS)

    Pant, Neeraj

    2011-08-01

    The paper presents a class of interior solutions of Einstein-Maxwell field equations of general relativity for a static, spherically symmetric distribution of the charged fluid. This class of solutions describes well behaved charged fluid balls. The class of solutions gives us wide range of parameter K (0.3277≤ K≤0.49), for which the solution is well behaved hence, suitable for modeling of super dense star. For this solution the mass of a star is maximized with all degree of suitability and by assuming the surface density ρ b =2×1014 g/cm3. Corresponding to K=0.3277 with X=-0.15, the maximum mass of the star comes out to be M=0.92 M Θ with radius r b ≈17.15 km and the surface red shift Z b ≈0.087187. It has been observed that under well behaved conditions this class of solutions gives us the mass of super dense object within the range of white-dwarf.

  5. Stellar model chromospheres. IX - Chromospheric activity in dwarf stars

    NASA Technical Reports Server (NTRS)

    Kelch, W. L.; Worden, S. P.; Linsky, J. L.

    1979-01-01

    High-resolution Ca II K line profiles are used to model the upper photospheres and lower chromospheres of eight main-sequence stars ranging in spectral type from F0 to M0 and exhibiting different degrees of chromospheric activity. The model chromospheres are studied as a function of spectral type and activity for stars of similar spectral type in order to obtain evidence of enhanced nonradiative heating in the upper-photospheric models and in the ratio of minimum temperature at the base of the chromosphere to effective temperature, a correlation between activity and temperature in the lower chromospheres, and a correlation of the width at the base of the K-line emission core and at the K2 features with activity. Chromospheric radiative losses are estimated for the modelled stars and other previously analyzed main-sequence stars. The results obtained strengthen the argument that dMe flare stars exhibit fundamentally solar-type activity but on an increased scale.

  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. 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.

  8. Many-body treatment of white dwarf and neutron stars on the brane

    SciTech Connect

    Azam, Mofazzal; Sami, M.

    2005-07-15

    Brane-world models suggest modification of Newton's law of gravity on the 3-brane at submillimeter scales. The brane-world induced corrections are in higher powers of inverse distance and appear as additional terms with the Newtonian potential. The average interparticle distance in white dwarf and neutron stars is 10{sup -10} cms and 10{sup -13} cms, respectively, and therefore, the effect of submillimeter corrections needs to be investigated. We show, by carrying out simple many-body calculations, that the mass and mass-radius relationship of the white dwarf and neutron stars are not effected by submillimeter corrections. However, our analysis shows that the correction terms in the effective theory give rise to force akin to surface tension in normal liquids.

  9. Chromospherically Active Stars in the RAVE Survey. II. Young Dwarfs in the Solar Neighborhood

    NASA Astrophysics Data System (ADS)

    Žerjal, M.; Zwitter, T.; Matijevič, G.; Grebel, E. K.; Kordopatis, G.; Munari, U.; Seabroke, G.; Steinmetz, M.; Wojno, J.; Bienaymé, O.; Bland-Hawthorn, J.; Conrad, C.; Freeman, K. C.; Gibson, B. K.; Gilmore, G.; Kunder, A.; Navarro, J.; Parker, Q. A.; Reid, W.; Siviero, A.; Watson, F. G.; Wyse, R. F. G.

    2017-01-01

    A large sample of over 38,000 chromospherically active candidate solar-like stars and cooler dwarfs from the RAVE survey is addressed in this paper. An improved activity identification with respect to the previous study was introduced to build a catalog of field stars in the solar neighborhood with an excess emission flux in the calcium infrared triplet wavelength region. The central result of this work is the calibration of the age–activity relation for main-sequence dwarfs in a range from a few 10 {Myr} up to a few Gyr. It enabled an order of magnitude age estimation of the entire active sample. Almost 15,000 stars are shown to be younger than 1 {Gyr} and ∼2000 younger than 100 {Myr}. The young age of the most active stars is confirmed by their position off the main sequence in the J ‑ K versus {N}{UV}-V diagram showing strong ultraviolet excess, mid-infrared excess in the J ‑ K versus {W}1-{W}2 diagram, and very cool temperatures (J-K> 0.7). They overlap with the reference pre-main-sequence RAVE stars often displaying X-ray emission. The activity level increasing with the color reveals their different nature from the solar-like stars and probably represents an underlying dynamo-generating magnetic fields in cool stars. Of the RAVE objects from DR5, 50% are found in the TGAS catalog and supplemented with accurate parallaxes and proper motions by Gaia. This makes the database of a large number of young stars in a combination with RAVE’s radial velocities directly useful as a tracer of the very recent large-scale star formation history in the solar neighborhood. The data are available online in the Vizier database.

  10. 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.

  11. 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.

  12. 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.

  13. 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.

  14. Predicting the fate of binary red giants using the observed sequence E star population: binary planetary nebula nuclei and post-RGB stars

    NASA Astrophysics Data System (ADS)

    Nie, J. D.; Wood, P. R.; Nicholls, C. P.

    2012-07-01

    Sequence E variables are close binary red giants that show ellipsoidal light variations. They are likely the immediate precursors of planetary nebulae (PNe) with close binary central stars as well as other binary post-asymptotic giant branch (post-AGB) and binary post-red giant branch (post-RGB) stars. We have made a Monte Carlo simulation to determine the fraction of red giant binaries that go through a common envelope event leading to the production of a close binary system or a merged star. The novel aspect of this simulation is that we use the observed frequency of sequence E binaries in the Large Magellanic Cloud (LMC) to normalize our calculations. This normalization allows us to produce predictions that are relatively independent of model assumptions. In our standard model, and assuming that the relative numbers of PNe of various types are proportional to their birth rates, we find that in the LMC today the fraction of PNe with close binary central stars is 7-9 per cent, the fraction of PNe with intermediate period binary central stars having separations capable of influencing the nebula shape (orbital periods less than 500 yr) is 23-27 per cent, the fraction of PNe containing wide binaries that are unable to influence the nebula shape (orbital period greater than 500 yr) is 46-55 per cent, the fraction of PNe derived from single stars is 3-19 per cent, and 5-6 per cent of PNe are produced by previously merged stars. We also predict that the birth rate of post-RGB stars is ˜4 per cent of the total PN birth rate, equivalent to ˜50 per cent of the production rate of PNe with close binary central stars. These post-RGB stars most likely appear initially as luminous low-mass helium white dwarf binaries. The average lifetime of sequence E ellipsoidal variability with amplitude more than 0.02 mag is predicted to be ˜0.95 Myr. We use our model and the observed number of red giant stars in the top one magnitude of the RGB in the LMC to predict the number of PNe in

  15. 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.

  16. Detailed Abundances of Two Very Metal-poor Stars in Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    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-[α/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] Lt -3, whereas halo stars in the same metallicity range sample SNe with [Fe/H] ~ -3. Consequently, enhancements in [Na/Fe] and [r/Fe] were deferred to higher metallicity in dSphs than in the progenitors of the inner halo. Data 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 NASA. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.

  17. VizieR Online Data Catalog: Abundances of microlensed Bulge dwarf stars. V. (Bensby+, 2013)

    NASA Astrophysics Data System (ADS)

    Bensby, T.; Yee, J. C.; Feltzing, S.; Johnson, J. A.; Gould, A.; Cohen, J. G.; Asplund, M.; Melendez, J.; Lucatello, S.; Han, C.; Thompson, I.; Gal-Yam, A.; Udalski, A.; Benett, D. P.; Bond, I. A.; Kohei, W.; Sumi, T.; Suzuki, D.; Suzuki, K.; Takino, S.; Tristram, P.; Yamai, N.; Yonehara, A.

    2012-11-01

    For each spectral line we give the lower excitation potential, measured equivalent widths, and derived absolute abundances. We also give median abundances for each star, normalised to the Sun, and errors in the median abundances. These tables contain data for all so far 58 microlensed dwarf stars, and superseeds the tables in Bensby+ (2009, Cat. J/A+A/499/737), Bensby+ (2010, Cat. J/A+A/512/A41), and Bensby+ (2011, Cat. J/A+A/533/A134). (4 data files).

  18. 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.

  19. 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.

  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. White Dwarfs in Astrometric Binaries?

    NASA Astrophysics Data System (ADS)

    Oliversen, N. A.; Evans, N. R.; Feibelman, W. A.; Kamper, K. W.

    1993-12-01

    Lippincott (1978, Space Sci Rev, 22, 153) compiled a list of astrometric binaries with unseen companions typically within 20 pc of the sun. Red companions have been observed in a number of these systems (e.g. McCarthy, D. W. 1983, IAU Coll. # 76, p. 107). Unseen, low mass companions could also be white dwarfs. We have obtained IUE observations of stars on the list which have primaries with spectral types M1 or earlier (white dwarf companions of cooler primaries could be detected from the ground), and are brighter than 10 mag, which do not have known red companions. Preliminary reductions (comparison with standard stars of appropriate spectral types) indicate that there are no white dwarfs in the sample. Further processing is being done to determine limits on possible white dwarf temperatures.

  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. 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.

  5. The Variable Stars of the DRACO DWARF Spheroidal Glaxay: Revisited

    DTIC Science & Technology

    2008-11-01

    mag, those of Aparicio et al. (2001): 〈V (HB)〉 = 20.2±0.1 mag, and those of Bellazzini et al. (2002): 〈V (HB)〉 = 20.28± 0.10 mag, with a 2σ...254, 507 Aparicio , A., Carrera, R., & Martı́nez-Delgado, D. 2001, AJ, 122, 2524 No. 5, 2008 VARIABLE STARS IN DRACO 1939 Armandroff, T. E., Olszewski

  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. 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.

  8. 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.

  9. 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.

  10. 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.

  11. Characterizing Companions to Low-Mass Stars: A Large-Scale, Volume-Limited Survey of Local M-dwarfs

    NASA Astrophysics Data System (ADS)

    Ward-Duong, Kimberly; Patience, J.; De Rosa, R.; Rajan, A.

    2013-01-01

    M-dwarfs constitute the major fraction of stars within both the solar neighborhood and nearby star-forming regions. However, key M-dwarf companion characteristics - including multiplicity fraction, mass ratios, and separation distributions - are less certain for field stars, due to limited sample sizes and non-uniform selection criteria. Studies of star-forming regions often compare results to solar-type field stars due to the extensive population statistics available for G-dwarfs, but field M-dwarfs represent a more analogous population for comparison due to their prevalence. We present results on a stellar and substellar companion study covering separations from ~1 - 10,000 AU, based on a volume-limited survey of ~300 M-dwarfs within 15 pc. Our study constrains the frequency of binary companions and the shape of the companion separation and mass ratio distributions. Diffraction-limited, mid-to-near infrared archival data were obtained from the Very Large Telescope, Hubble Space Telescope, and Canada-France-Hawaii Telescope, to detect nearby companions to M-dwarfs from ~1 to 100 AU. To supplement the high-resolution data, wide-field archival plates were searched for companions with separations of 100 to 10,000 AU. The all-sky survey data include multiple epochs, and follow up observations at higher resolution will allow us to confirm or reject the new companion candidates detected during our analysis. These multi-epoch observations provide confirmation of common proper motions, thereby minimizing background contamination and providing comprehensive statistics for M-star binaries. Preliminary analysis of an initial subset of the sample suggests a lower limit to the multiplicity of 23 ± 7% within the restricted separation range. Characterizations of the binary frequency for M-dwarfs provide crucial insights into the low-mass star formation environment, and hold additional implications for the frequency and evolutionary histories of their associated disks and

  12. 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.

  13. 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.

  14. 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.

  15. The chemical compositions of two nitrogen-rich, metal-poor, halo dwarf stars

    NASA Astrophysics Data System (ADS)

    Beveridge, Renee C.; Sneden, Cristopher

    1994-07-01

    New high resolution, high signal-to-noise spectra have been obtained for HD 25329 and HD 74000, dwarf stars that are metal-poor but nitrogen-rich members of the galactic halo. An atmosphere parameter and chemical composition analysis confirms earlier assertions of both their metal poverty, (Fe/H) approximately equals -2, and their high gravity, log g greater than 4. The relative abundances of the alpha-capture and iron-peak elements are normal for metal-poor stars. Overabundances of sodium, and possibly aluminum as well, are derived, but there are no pronounced depletions of oxygen; thus these stars do not show the sodium/oxygen or nitrogen/oxygen anticorrelations seen in globular cluster giants. All very heavy elements synthesized through s-process neutron-capture nucleosynthesis are enhanced in these stars. It is likely that the enrichments of nitrogen, sodium, aluminum, and the very heavy elements in these stars originated in material dredged up from the helium-burning shells of former AGB stars, but there is no direct evidence for binary companions for these stars.

  16. Carbon enrichment of the evolved stars in the Sagittarius dwarf spheroidal

    NASA Astrophysics Data System (ADS)

    McDonald, I.; White, J. R.; Zijlstra, A. A.; Guzman Ramirez, L.; Szyszka, C.; van Loon, J. Th.; Lagadec, E.; Jones, O. C.

    2012-12-01

    We present spectra of 1142 colour-selected stars in the direction of the Sagittarius Dwarf Spheroidal (Sgr dSph) galaxy, of which 1058 were taken with VLT/FLAMES multi-object spectrograph and 84 were taken with the SAAO Radcliffe 1.9-m telescope grating spectrograph. Spectroscopic membership is confirmed (at >99 per cent confidence) for 592 stars on the basis of their radial velocity, and spectral types are given. Very slow rotation is marginally detected around the galaxy's major axis. We identify five S stars and 23 carbon stars, of which all but four carbon stars are newly determined and all but one (PQ Sgr) are likely Sgr dSph members. We examine the onset of carbon richness in this metal-poor galaxy in the context of stellar models. We compare the stellar death rate (one star per 1000-1700 yr) with the known planetary nebula dynamical ages and find that the bulk population produce the observed (carbon-rich) planetary nebulae. We compute average lifetimes of S and carbon stars as 60-250 and 130-500 kyr, compared to a total thermal-pulsing asymptotic giant branch lifetime of 530-1330 kyr. We conclude by discussing the return of carbon-rich material to the interstellar medium.

  17. 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.

  18. 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.

  19. The potential of asteroseismology for probing the core chemical stratification in white dwarf stars

    NASA Astrophysics Data System (ADS)

    Giammichele, N.; Charpinet, S.; Brassard, P.; Fontaine, G.

    2017-02-01

    Context. The details of the C/O core structure in white dwarf stars has mostly remained inaccessible to the technique of asteroseismology, despite several attempts carried out in the past. Aims: We aim to re-assess the potential of asteroseismology for probing the chemical stratification in white dwarf cores, in light of new highly efficient tools recently developed for that purpose. Methods: Using the forward modeling approach and a new parameterization for the core chemical stratification in ZZ Ceti stars, we tested several situations typical of the usually limited constraints available, such as small numbers of observed independent modes, to carry out asteroseismology of these stars. Results: We find that, even with a limited number of modes, the core chemical stratification (in particular, the location of the steep chemical transitions expected in the oxygen profile) can be determined quite precisely due to the significant sensitivity of some confined modes to partial reflexion (trapping) effects. These effects are similar to the well known trapping induced by the shallower chemical transitions at the edge of the core and at the bottom of the H-rich envelope. We also find that success to unravel the core structure depends on the information content of the available seismic data. In some cases, it may not be possible to isolate a unique, well-defined seismic solution and the problem remains degenerate. Conclusions: Our results establish that constraining the core chemical stratification in white dwarf stars based solely on asteroseismology is possible, an opportunity that we have begun to exploit.

  20. 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.

  1. Red-giant stars: Chemical clocks in the Milky Way

    NASA Astrophysics Data System (ADS)

    Lagarde, N.; Robin, A. C.; Reylé, C.; Nasello, G.

    2016-12-01

    A broad effort is ongoing with large spectroscopic surveys such as APOGEE, ESO-Gaia, RAVE from which stellar parameters, radial velocities and detailed chemical abundances can be measured for CoRoT, Kepler, and K2 targets. In addition, asteroseismic data of red-giants stars observed by the space missions CoRoT and Kepler allow determination of stellar masses, radii, and can be used to determine the position and ages of stars. This association between spectroscopic and asteroseismic constraints provide a new way to understand galactic and stellar evolutions. To exploit all potential of this combination it would be crucial to develop our approach of synthetic populations. We compute stellar populations synthesis with the Besançon Galactic model including the asteroseismic and chemical properties from stellar evolution models. These synthetic populations can be compared with sinificant large surveys as APOKASC (APOGEE+Kepler) or CoRoGEE (CoRoT+APOGEE). We focus here on the carbon and nitrogen surface abundances of Kepler red-giant stars. We underline the importance of transport processes occurring in red-giant stars as rotation and thermohaline instability to understand chemical properties of stellar populations in the Galaxy. The future for this area also starts taking shape with the launch of Gaia, futur spectroscopic surveys such as 4MOST and WEAVE, and the future space mission PLATO that will provide seismic data for more than 100 000 red-giants. Such synthetic population model is a key tool to investigate future observations and better understand the evolution of the Milky Way.

  2. Red horizontal-branch stars in the galactic disk

    NASA Astrophysics Data System (ADS)

    Rose, J. A.

    1985-05-01

    A class of red horizontal-branch (RHB) stars, similar to those in the "metal-rich" globular cluster M71, has been identified in the Galactic disk, using a quantitative three-dimensional spectral classification system developed earlier (Rose 1984) that uses 2.5-Å resolution spectra in the blue. A prototype for this class is the G5 III star HD 79452, which has been found by Helfer and Wallerstein (1968) to have [Fe/H] = -0.85 and MV = +1. The RHB stars are shown to be evolved stars on the basis of the strength of their Sr II λ4077 line, and are distinguished from post-main-sequence stars evolving through the same region of the HR diagram because of the unique appearance of their CN λ3883 and λ4216 bands. A preliminary estimate has been made of their space density, scale height perpendicular to the Galactic plane, and kinematics by surveying G5 - G7 stars in the Upgren (1962) North Galactic Pole survey.

  3. 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.

  4. 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

  5. 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

  6. The Lyman Continuum Escape Fraction of Dwarf, Star-Forming Galaxies at z~1

    NASA Astrophysics Data System (ADS)

    Rutkowski, Michael J.; Scarlata, Claudia; Teplitz, Harry I.; Hayes, Matthew; Salvato, Mara; Beck, Melanie; Mehta, Vihang; Pahl, Anthony

    2015-01-01

    The dominant astrophysical source(s) of Lyman Continuum (LyC, λ<912Å) photons which reionized neutral Hydrogen in the IGM at high (z > 6) redshift remains uncertain. Unfortunately, the direct detection of LyC photons escaping from the analogs of such sources --- i.e., star-forming galaxies --- in the low redshift (z<~1) universe has proven to be remarkably difficult with space-based observatories. Motivated by the few (~2) direct detections of LyC in the local Universe and the results of recent cosmological simulations of galaxy evolution which demonstrate that low-mass (M<~109M⊙) star-forming galaxies may be uniquely suited to contribute to the ionizing, UV background, we present results of recent work to study the LyC escape fraction in z~1 dwarf, star-forming galaxies. We present an independent re-reduction of the WFC3 IR grism data obtained as part of the 3DHST and AGHAST surveys, and identify and select star-forming galaxies at z=0.9-1.4 by their Hα emission. At this redshift range, GALEX FUV and NUV images can be used to cleanly measure the ratio of LyC to UV non-ionizing continuum (i.e., LyCesc,rel) photons. We join our line and redshift identifications with public photometric-redshift catalogs made available by the 3DHST team in order to select an ideal sample of star-forming galaxies which excludes likely contaminants (e.g., AGN, low-redshift interlopers, etc.). Stacking archival GALEX images of ~500 UV non-detected star-forming (SFR<~5M⊙yr-1 ) galaxies, we measure an upper limit to fLyCesc,relequal to ~5%. With these data we are also able to directly constrain fLyCesc,rel for a population of isolated, high equivalent width (EW>200Å), dwarf (M<109M⊙) star-forming galaxies, measuring an upper limit of fLyCesc,rel< ~20% from an analysis of stacked data. We will discuss the implications for reionization of these escape fractions measured from the stacking analysis, as well as possible UV detections from individual dwarf galaxies.

  7. Coronal heating of M dwarfs: The flare-energy distribution of fully convective stars

    NASA Astrophysics Data System (ADS)

    Feng, Ying; Poppenhaeger, K.; Goulding, A. D.; Bulbul, E.

    2014-01-01

    Stochastic flaring is an important mechanism for the coronal heating of the Sun and solar-like stars. The driver for these flares is a magnetic dynamo anchored at the boundary layer between the convective zone and the radiative core. Fully convective M dwarfs have been observed to produce powerful flares as well, but they lack a radiative core and must possess a different dynamo mechanism. How their flaring behavior differs from the solar case is not fully understood yet. We have analyzed X-ray flares of 22 M dwarfs, including both fully and partially convective ones, using archival XMM-Newton data. We extracted flares from the individual X-ray light curves and determined the amount of energy released by each flare in the observed X-ray band. We constructed flare-energy distributions of the targets to investigate the degree to which flares heat stellar coronae. We fitted the slopes of the flare-energy distributions for individual targets and for groups of targets bundled by spectral type. Depending on the value of the slope, the total energy released by flares, as extrapolated from the flare-energy distributions, could be sufficient to heat the entire corona. We find that the slopes of the flare-energy distributions are very similar to that of the Sun, for both partially and fully convective M dwarfs. The dynamo process at work in the fully convective stars of our sample needs to have a flare production efficiency which is very close to the solar case. Further observations will cover ultracool targets near the brown dwarfs boundary to test for which masses this solar analogy is valid. This work is supported in part by the NSF REU and DOD ASSURE programs under NSF grant no. 1262851 and by the Smithsonian Institution.

  8. 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.

  9. White Dwarf Stars From the Telescope to the Laboratory and Back Again: Exploring Extreme Physics

    NASA Astrophysics Data System (ADS)

    Winget, Donald E.

    2012-01-01

    Astronomy, in contrast with other sciences, has traditionally been considered an observational science; it has not been possible to perform experiments on the objects we observe. This situation has changed in a way that is transformational. Although laboratory astrophysics has long been an important part of astronomical research, what has changed is the ability to produce large enough chunks of a star that we can make measurements and perform experiments. We are now able to make macroscopic quantities of star stuff in the lab: plasmas created under conditions that are the same as the plasmas in stars. In a cosmic sense, as physicist Greg Rochau likes to point out, the conditions on Earth are far from normal, they could even be considered extreme or bizarre compared to the more cosmically normal conditions in stars. We can now examine, on Earth, matter under more cosmically "normal” conditions. I will describe how this came about, the technology behind it, and the results of our recent laboratory experiments done on Z at Sandia National Laboratories. We will discuss how this will change our understanding of white dwarf stars and, through this, what we know about the universe and its contents based on these stars. Finally, we will briefly examine other fundamental astrophysics being done on Z and focus on the tremendous potential of the Z platform for astrophysics experiments in the future.

  10. Gravity, Rotation, Ages, and Magnetism of Solar-like Stars and Red Giants observed by Kepler and K2

    NASA Astrophysics Data System (ADS)

    Mathur, Savita

    Scientific Objectives: Asteroseismology has proved to be a very powerful tool thanks to the high-precision data obtained by the space missions such as Kepler and CoRoT. Solar-like oscillations have been detected and reported for around 15,600 red giants and 540 main-sequence stars observed by the nominal Kepler mission. Hence, these stars have their surface gravities, masses, and radii obtained with seismology. However, according to the latest Kepler star properties catalog (Mathur et al., in prep.) more than 24,000 red giants, 127,000 FGK dwarfs, and 10,000 subgiants were targeted. K2 has been observing 90,000 red giants and dwarfs. Moreover, the continuous photometric data of 4 yrs (resp. 3 mo) collected by Kepler (resp. K2) contain the signature of other phenomena such as convection, rotation, and magnetism, which are very important to understand stellar evolution and can also be used to obtain precise fundamental stellar parameters even when pulsations are not detected. We propose to perform the largest homogeneous analysis to date of seismic oscillations, convection, and rotation/magnetic activity across the full range of stellar spectral types and evolutionary states present in the K2 and Kepler missions. We will use the longest publicly available time series to derive the most accurate surface gravities, rotation periods, evolutionary states, and magnetic activity levels to characterize rotation-age-magnetic activity relationships and oscillations-magnetism interaction. Relevance: The determination of the gravity, mass, radius, and age of planet host stars allow us to better characterize the planetary systems. By studying the stellar surface rotation periods, we can better understand the angular momentum transport involved during the stellar evolution and have more accurate rotation-age relationships. Finally, the study of the magnetic activity of a large number of stars will allow us to put the Sun in a broader context. This work will also have an impact on

  11. 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.

  12. Evolutionary and pulsational properties of low-mass white dwarf stars with oxygen cores resulting from close binary evolution

    NASA Astrophysics Data System (ADS)

    Althaus, L. G.; Córsico, A. H.; Gautschy, A.; Han, Z.; Serenelli, A. M.; Panei, J. A.

    2004-01-01

    The present work is designed to explore the evolutionary and pulsational properties of low-mass white dwarfs with carbon/oxygen cores. In particular, we follow the evolution of a 0.33-Msolar white dwarf remnant in a self-consistent way with the predictions of nuclear burning, element diffusion and the history of the white dwarf progenitor. Attention is focused on the occurrence of hydrogen shell flashes induced by diffusion processes during cooling phases. The evolutionary stages prior to the white dwarf formation are also fully accounted for by computing the conservative binary evolution of an initially 2.5-Msolar Population I star with a 1.25-Msolar companion, and with period Pi= 3 d. Evolution is followed down to the domain of the ZZ Ceti stars on the white dwarf cooling branch. We find that chemical diffusion induces the occurrence of an additional hydrogen thermonuclear flash, which leads to stellar models with thin hydrogen envelopes. As a result, a fast cooling is encountered at advanced stages of evolution. In addition, we explore the adiabatic pulsational properties of the resulting white dwarf models. As compared with their helium-core counterparts, low-mass oxygen-core white dwarfs are characterized by a pulsational spectrum much more featured, an aspect which could eventually be used for distinguishing both types of stars, if low-mass white dwarfs were in fact found to pulsate as ZZ Ceti-type variables. Finally, we perform a non-adiabatic pulsational analysis on the resulting carbon/oxygen low-mass white dwarf models.

  13. 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.

  14. Optical Spectroscopy of Low-Mass Stars and Brown Dwarfs in Orion

    NASA Astrophysics Data System (ADS)

    Riddick, F. C.; Roche, P. F.; Lucas, P. W.

    2006-06-01

    Using multi-object optical spectroscopy from the AAT and Gemini-North, 35 low-mass stars and brown dwarfs in the Trapezium Cluster in Orion have been classified both by comparison with other previously classified young, low-mass sources in the Chamaeleon I star-forming region and by the use of spectral indices: narrowband indices which measure the strength of various highly temperature-sensitive molecular lines. The objects are all very likely cluster members, by analysis of the strength of the gravity-sensitive Na doublet, which is much weaker than in dwarfs for these very young objects. The spectral types obtained have been converted to effective temperatures using the temperature scale of Luhman et al. (2003b), which is intermediate between dwarf and giant scales and hence suitable for young pre-main sequence objects. In combination with the dereddened H band luminosities obtained from the photometry of Lucas & Roche (2000), the objects have been placed on an H-R diagram overlaid with the theoretical isochrones of Baraffe et al. (1998). The low-mass stars and the higher mass substellar objects are found to be clustered around the 1 Myr isochrone, while the lower mass substellar objects are located well above this isochrone, probably due to selection effects. The average age of 1 Myr for the majority of the objects is in agreement with other age estimates for the region, but the lack of any objects older than 5 Myr is in contrast to the results of Slesnick et al. (2004) which show in addition an older population at 10 Myr. Assuming coevality of the sources and an average age of 1 Myr, the masses of the objects have been estimated and 18 of the objects are found to have substellar masses.

  15. MASS AND RADIUS DETERMINATIONS FOR FIVE TRANSITING M-DWARF STARS

    SciTech Connect

    Fernandez, Jose M.; Latham, David W.; Torres, Guillermo; Charbonneau, David; Esquerdo, Gilbert A.; Stefanik, Robert P.; Everett, Mark E.; Mandushev, Georgi; O'Donovan, Francis T.; Alonso, Roi; Hergenrother, Carl W.

    2009-08-10

    We have derived masses and radii for both components in five short-period single-lined eclipsing binary stars discovered by the TrES wide-angle photometric survey for transiting planets. All these systems consist of a visible F-star primary and an unseen M-star secondary (M {sub A} {>=} 0.8 M {sub sun}, M {sub B} {<=} 0.45 M {sub sun}). The spectroscopic orbital solution combined with a high-precision transit light curve for each system gives sufficient information to calculate the density of the primary star and the surface gravity of the secondary. The masses of the primary stars were obtained using stellar evolution models, which requires accurate determinations of metallicities and effective temperatures. In our case, the uncertainty in the metallicity of the primary stars is the most important limiting factor in order to obtain accurate results for the masses and radii of the unseen M-dwarf secondaries. The solutions were compared with results obtained by calculating the radius of the primary stars under the assumption of rotational synchronization with the orbital period and alignment between their spin axis and the axis of the orbit, using the observed broadening of the spectral lines as an indicator of stellar rotation. Four systems show an acceptable match between the two sets of results when their metallicity is allowed to vary around solar values (-0.5 {<=} [Fe/H] {<=} +0.5), but one system shows a clear mismatch between the two solutions, which may indicate the absence of synchronization or a misalignment between the rotational and orbital axis. When compared to low-mass stellar evolution models, the derived masses and radii of the unseen M dwarfs are inconsistent (three only marginally) with the predicted values, with all of the radii being larger than expected for their masses. These results confirm the discrepancy shown in a previous work between the predicted and observed radii on low-mass binary stars. This work also shows that reliance on the

  16. Mass and Radius Determinations for Five Transiting M-Dwarf Stars

    NASA Astrophysics Data System (ADS)

    Fernandez, Jose M.; Latham, David W.; Torres, Guillermo; Everett, Mark E.; Mandushev, Georgi; Charbonneau, David; O'Donovan, Francis T.; Alonso, Roi; Esquerdo, Gilbert A.; Hergenrother, Carl W.; Stefanik, Robert P.

    2009-08-01

    We have derived masses and radii for both components in five short-period single-lined eclipsing binary stars discovered by the TrES wide-angle photometric survey for transiting planets. All these systems consist of a visible F-star primary and an unseen M-star secondary (M A >= 0.8 M sun, M B <= 0.45 M sun). The spectroscopic orbital solution combined with a high-precision transit light curve for each system gives sufficient information to calculate the density of the primary star and the surface gravity of the secondary. The masses of the primary stars were obtained using stellar evolution models, which requires accurate determinations of metallicities and effective temperatures. In our case, the uncertainty in the metallicity of the primary stars is the most important limiting factor in order to obtain accurate results for the masses and radii of the unseen M-dwarf secondaries. The solutions were compared with results obtained by calculating the radius of the primary stars under the assumption of rotational synchronization with the orbital period and alignment between their spin axis and the axis of the orbit, using the observed broadening of the spectral lines as an indicator of stellar rotation. Four systems show an acceptable match between the two sets of results when their metallicity is allowed to vary around solar values (-0.5 <= [Fe/H] <= +0.5), but one system shows a clear mismatch between the two solutions, which may indicate the absence of synchronization or a misalignment between the rotational and orbital axis. When compared to low-mass stellar evolution models, the derived masses and radii of the unseen M dwarfs are inconsistent (three only marginally) with the predicted values, with all of the radii being larger than expected for their masses. These results confirm the discrepancy shown in a previous work between the predicted and observed radii on low-mass binary stars. This work also shows that reliance on the assumption of synchronization to

  17. 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.

  18. 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.

  19. The Stellar Population and Star Formation Properties of Blue Compact Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Zhao, Yinghe; Gu, Qiusheng; Gao, Yu

    2011-02-01

    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 (~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 10-3 to ~1 M sun yr-1, with a median value of ~0.1 M sun yr-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.

  20. 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.

  1. Spatialy Resolved Star Formation History Movies of 9 Dwarf Irregular Galaxies In The M81 Group.

    NASA Astrophysics Data System (ADS)

    Weisz, Daniel R.; Skillman, E.; Cannon, J.; Dolphin, A.; Kennicutt, R.; Lee, J.; Walter, F.

    2007-12-01

    The role of 'feedback' i.e., supernovae, stellar winds, outflows, in the process of star formation (SF), interstellar medium (ISHM) structure alteration, and galaxy evolution remains an interesting and open question. One way to help define the role of feedback is through observational constraints. Recent HST/ACS observations of 9 dwarf irregular galaxies (dIrrs) in the M81 group will help characterize the spatial and temporal components of feedback in a set of diverse dIrrs, spanning a factor of 6 magnitudes in luminosity, 1000 in current star formation rate, and 0.5 dex in metallicity. Here, I present movies that trace the star formation activity in the recent ( 0.5 Gyrs) history of each M81 dI in our sample. I am able to see how star formation events relate to one another both in time and space. We are able to clearly see the SF duty cycle, periods of activity and quiescence, as well as how events may serve to trigger future events by their spatial and temporal proximity. The M81 dIrr SF movies provide us with unique insight into how stars form and how they impact the evolution of each galaxy. Support for this work is provided by NASA/HST grant GO-10605.01.

  2. Lithium abundances in samples of dwarf stars of population II and very old population I

    NASA Astrophysics Data System (ADS)

    Beckman, J.; Rebolo, R.; Molaro, P.

    The CCD camera and Intermediate Dispersion Spectrograph of the 2.5 m Issaac Newton Telescope are used to obtain high quality spectra of the 6708-A Li-7 resonance doublet in 22 dwarfs with metallicities not greater than -0.7. A mean constant value of Log N(Li) = 2.07 (+ or - 0.1) is found for highly metal deficient dwarfs (Fe/H abundance ratios of not greater than 1.5) with atmospheric temperatures T(eff) greater than 5500 K, and a larger spread with values of Log N(Li) up to 2.4 for the warmer dwarfs with metallicities between -0.7 and -1.2. The results agree with previous findings by Spite et al. (1982, 1984) showing a highly uniform Li abundance near the inception of the galaxy. Li depletion appears to set in at higher temperatures for the moderately metal deficient stars than for the extremely metal deficient, consistent with metallicity-dependent depletion rates (low metals, low depletion).

  3. Get Ready for Gaia: Cool White Dwarfs in Common Proper Motion with Tycho Stars

    NASA Astrophysics Data System (ADS)

    Hambly, N.; Rowell, N.; Lam, M.

    2017-03-01

    We discuss the Gaia Data Release 1 (September 2016) and preliminary work on maximising the benefit for cool white dwarf (WD) science in advance of the full parallax catalogue which will appear around one year later in DR2. The Tycho catalogue is used in conjunction with the all–sky ground based astrometric/ photometric SuperCOSMOS Sky Survey in order to identify candidate faint common proper motion objects to the Tycho stars. Gaia DR1 is supplemented by the Tycho–Gaia Astrometric Solution catalogue containing some 2 million parallaxes with Hipparcos–like precision for Tycho stars. While hotter, brighter WDs are present in Tycho, cooler examples are much rarer (if present at all) and CPM offers one method to infer precision distances for a statistically useful sample of these very faint WDs.

  4. Flare Activity and Polarization States of White Dwarfs in Binary Star Systems

    NASA Astrophysics Data System (ADS)

    Boneva, D.; Filipov, L.

    2017-03-01

    We investigate flare activity and emission properties of white dwarf binary stars. We apply the polarization as a mechanism to probe the flares and the released resulting radiation. The polarization could appear as patterns in these cases, as it depends mainly on the properties of radiation and geometry of the source. The observational data of MV Lyr and CH Cyg are analysed. A repeated variability in the brightness could affect the degree of polarization. Detectable variations in the polarization parameters of selected binaries for the flares activity period are shown in the result. The analysis may help us to establish more evidence of the close correlation between flares, flow structure transformation around the primary star and polarization parameter variability.

  5. Homogeneous abundance analysis of FGK dwarf, subgiant, and giant stars with and without giant planets

    NASA Astrophysics Data System (ADS)

    da Silva, Ronaldo; Milone, André de C.; Rocha-Pinto, Helio J.

    2015-08-01

    Aims: We have analyzed high-resolution and high signal-to-noise-ratio optical spectra of nearby FGK stars with and without detected giant planets in order to homogeneously measure their photospheric parameters, mass, age, and the abundances of volatile (C, N, and O) and refractory (Na, Mg, Si, Ca, Ti, V, Mn, Fe, Ni, Cu, and Ba) elements. Our sample contains 309 stars from the solar neighborhood (up to the distance of 100 pc), out of which 140 are dwarfs, 29 are subgiants, and 140 are giants. Methods: The photospheric parameters are derived from the equivalent widths of Fe i and Fe ii lines. Masses and ages come from the interpolation in evolutionary tracks and isochrones on the Hertzsprung-Russell diagram. The abundance determination is based on the equivalent widths of selected atomic lines of the refractory elements and on the spectral synthesis of C2, CN, C i, O i, and Na i features. We apply a set of statistical methods to analyze the abundances derived for the three subsamples. Results: Our results show that: i) giant stars systematically exhibit underabundance in [C/Fe] and overabundance in [N/Fe] and [Na/Fe] in comparison with dwarfs, a result that is normally attributed to evolution-induced mixing processes in the envelope of evolved stars; ii) for solar analogs alone, the abundance trends with the condensation temperature of the elements are correlated with age and anticorrelated with the surface gravity, which agrees with recent studies; iii) as in the case of [Fe/H], dwarf stars with giant planets are systematically enriched in [X/H] for all the analyzed elements, except for O and Ba (the former due to limitations of statistics), confirming previous findings in the literature that it is not only iron that has an important relation with the planetary formation; and iv) giant planet hosts are also significantly overabundant for the same metallicity when the elements from Mg to Cu are combined. Based on public data from the ELODIE archive (Moultaka et al

  6. Young stars and brown dwarfs surrounding Alnilam (ɛ Orionis) and Mintaka (δ Orionis)

    NASA Astrophysics Data System (ADS)

    Caballero, J. A.; Solano, E.

    2008-07-01

    Aims: We look for new regions to search for substellar objects. Methods: Two circular areas, 45 arcmin-radius each, centred on the young massive star systems Alnilam and Mintaka in the Orion Belt, were explored. The regions are very young (less than 10 Ma), have low extinction, and are neighbours to σ Orionis (~3 Ma), a young open cluster very rich in brown dwarfs and planetary-mass objects. We used Virtual Observatory tools, the astro-photometric Tycho-2, DENIS and 2MASS catalogues, 10 control fields at similar galactic latitudes, as well as X-ray, mid-infrared, and spectroscopic data from the literature. Results: We compiled exhaustive lists of known young stars and new candidate members in the Ori OB1b association and of fore- and background sources. A total of 136 stars display features of extreme youth, like early spectral types, lithium in absorption, or mid-infrared flux excess. Other two young brown dwarf and 289 star candidates have been identified from an optical/near-infrared colour-magnitude diagram. We list another 74 known objects that might belong to the association. This catalogue can serve as input for characterising the stellar and high-mass substellar populations in the Orion Belt. Finally, we investigated the surface densities and radial distributions of young objects surrounding Alnilam and Mintaka and compared them with those in the σ Orionis cluster. We report on a new open cluster centred on Mintaka. Conclusions: Both regions can be analogues to the σ Orionis cluster, but more massive, more extended, slightly older, and less radially concentrated. Tables A.1 to A.18 are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/485/931

  7. AN M DWARF COMPANION TO AN F-TYPE STAR IN A YOUNG MAIN-SEQUENCE BINARY

    SciTech Connect

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

    2016-03-15

    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{sub ⊙} and a radius of (1.474 ± 0.040) R{sub ⊙}. The companion is an M dwarf with a mass of (0.188 ± 0.014) M{sub ⊙} and a radius of (0.234 ± 0.009) R{sub ⊙}. 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.

  8. Merger of white dwarf-neutron star binaries: Prelude to hydrodynamic simulations in general relativity

    SciTech Connect

    Paschalidis, Vasileios; MacLeod, Morgan; Baumgarte, Thomas W.; Shapiro, Stuart L.

    2009-07-15

    White dwarf-neutron star binaries generate detectable gravitational radiation. We construct Newtonian equilibrium models of corotational white dwarf-neutron star (WDNS) binaries in circular orbit and find that these models terminate at the Roche limit. At this point the binary will undergo either stable mass transfer (SMT) and evolve on a secular time scale, or unstable mass transfer (UMT), which results in the tidal disruption of the WD. The path a given binary will follow depends primarily on its mass ratio. We analyze the fate of known WDNS binaries and use population synthesis results to estimate the number of LISA-resolved galactic binaries that will undergo either SMT or UMT. We model the quasistationary SMT epoch by solving a set of simple ordinary differential equations and compute the corresponding gravitational waveforms. Finally, we discuss in general terms the possible fate of binaries that undergo UMT and construct approximate Newtonian equilibrium configurations of merged WDNS remnants. We use these configurations to assess plausible outcomes of our future, fully relativistic simulations of these systems. If sufficient WD debris lands on the NS, the remnant may collapse, whereby the gravitational waves from the inspiral, merger, and collapse phases will sweep from LISA through LIGO frequency bands. If the debris forms a disk about the NS, it may fragment and form planets.

  9. THE ACS LCID PROJECT. II. FAINT VARIABLE STARS IN THE ISOLATED DWARF IRREGULAR GALAXY IC 1613

    SciTech Connect

    Bernard, Edouard J.; Monelli, Matteo; Gallart, Carme; Aparicio, Antonio; Drozdovsky, Igor; Hidalgo, Sebastian L.; Cassisi, Santi; Skillman, Evan D.; Stetson, Peter B. E-mail: monelli@iac.e E-mail: antapaj@iac.e E-mail: slhidalgo@iac.e E-mail: skillman@astro.umn.ed

    2010-04-01

    We present the results of a new search for variable stars in the Local Group (LG) isolated dwarf galaxy IC 1613, based on 24 orbits of F475W and F814W photometry from the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope. We detected 259 candidate variables in this field, of which only 13 (all of them bright Cepheids) were previously known. Out of the confirmed variables, we found 90 RR Lyrae stars, 49 classical Cepheids (including 36 new discoveries), and 38 eclipsing binary stars for which we could determine a period. The RR Lyrae include 61 fundamental (RRab) and 24 first-overtone (FO, RRc) pulsators, and five pulsating in both modes simultaneously (RRd). As for the majority of LG dwarfs, the mean periods of the RRab and RRc (0.611 and 0.334 days, respectively) as well as the fraction of overtone pulsators (f{sub c} = 0.28) place this galaxy in the intermediate regime between the Oosterhoff types. From their position on the period-luminosity diagram and light-curve morphology, we can unambiguously classify 25 and 14 Cepheids as fundamental and FO mode pulsators, respectively. Another two are clearly second-overtone Cepheids, the first ones to be discovered beyond the Magellanic Clouds. Among the remaining candidate variables, five were classified as delta-Scuti and five as long-period variables. Most of the others are located on the main sequence, the majority of them likely eclipsing binary systems, although some present variations similar to pulsating stars. We estimate the distance to IC 1613 using various methods based on the photometric and pulsational properties of the Cepheids and RR Lyrae stars. The values we find are in very good agreement with each other and with previous estimates based on independent methods. When corrected to a common reddening of E(B - V) = 0.025 and true Large Magellanic Cloud (LMC) distance modulus of (m-M){sub LMC,0} = 18.515 +- 0.085, we find that all the distance determinations from the literature converge

  10. Extrasolar planets and brown dwarfs around AF-type stars. IX. The HARPS southern sample

    NASA Astrophysics Data System (ADS)

    Borgniet, S.; Lagrange, A.-M.; Meunier, N.; Galland, F.

    2017-03-01

    Context. Massive, main-sequence (MS) AF-type stars have so far remained unexplored in past radial velocities (RV) surveys due to their small number of spectral lines and high rotational velocities that prevent the classic RV computation method. Aims: Our aim is to search for giant planets (GPs) around AF MS stars, to get primary statistical information on their occurrence rate and to compare the results with evolved stars and lower-mass MS stars. Methods: We used the HARPS spectrograph located on the 3.6 m telescope at ESO La Silla Observatory to observe 108 AF MS stars with B-V in the range -0.04 to 0.58 and masses in the range 1.1 to 3.6 M⊙. We used our SAFIR software developed to compute the RV and other spectroscopic observables of these early-type stars. We characterized the detected companions as well as the intrinsic stellar variability. We computed the detection limits and used them as well as the detected companions to derive the first estimates of the close-in brown dwarf (BD) and GP frequencies around AF stars. Results: We report the detection of a mpsini = 4.51MJup planetary companion with an 826-day period to the F6V dwarf HD 111998. We also present new data on the two-planet system around the F6IV-V dwarf HD 60532. We also report the detections of 14 binaries with long-term RV trends and/or high-amplitude RV variations combined to a flat RV-bisector span diagram. We constrain the minimal masses and semi-major axes of these companions and check that these constraints are compatible with the stellar companions previously detected by direct imaging or astrometry for six of these targets. We get detection limits deep into the planetary domain with 70% of our targets showing detection limits between 0.1 and 10 MJup at all orbital periods in the 1- to 103-day range. We derive BD (13 ≤mpsini ≤ 80 MJup) occurrence rates in the 1- to 103-day period range of 2-2+5% and 2.6-2.6+6.7% for stars with M⋆ in the ranges 1.1 to 1.5 and 1.5 to 3 M

  11. 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.

  12. 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).

  13. 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.

  14. 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.

  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. 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

  17. 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.

  18. 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

  19. 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.

  20. 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.

  1. 40 Eridani: The Vulcan Sun as a Benchmark for the Evolutionary Properties of White Dwarfs and Cool Stars

    NASA Astrophysics Data System (ADS)

    Ballouz, Ronald-Louis; Guinan, E. F.; Wasatonic, R.; Engle, S. G.

    2010-01-01

    40 Eridani (omicron-2 Eri) is a bright nearby (d=5.04+/-0.02 pc) triple star system. 40 Eri A is a 4.4-mag K1 V star and its two more distant ( 400 AU) companion stars, 40 Eri B and 40 Eri C (which form an astrometric binary system), are a 9th mag DA4 white dwarf & an 11th mag M4.5 star. 40 Eri A is well known (in science fiction circles) as the probable host star to the planet Vulcan; however, 40 Eri is also well known in astronomy as containing the first identified white dwarf , only a handful of which have well-determine dynamical masses. Utilizing archival X-ray, ultraviolet (IUE) data, and Ca II HK emission measures, we determined (or improved) the properties of the system members. Using calibrated age-rotation-activity relations developed by us and others, we determine an age of 5.2+/-1.2 Gyr for 40 Eri A (and thus the system). It appears that 40 Eri A has similar activity levels (and thus age) to Alpha Centauri B - a middle-age ( 5.5+/-0.5 Gyr) K1 V star. This age is in good agreement with the age estimated from white dwarf component of 5.0+/-1 Gyr [main-sequence + cooling time (0.1Gyr)]. The accurate age, evolution, and mass of the hot white dwarf star provide a firm benchmark for calibrating ages/cooling times of lower mass white dwarfs. The implications of this new age determination on the evolution & cooling times of low mass white dwarfs (like 40 Eri B) and cosmochronology are discussed. Furthermore, we are carrying out high precision photometry of 40 Eri A with the aim of determining its rotation period (from star spot modulations) and thus its age using rotation-age relations for dK stars. This work is partially supported with grants from NSF/RUI & NASA/FUSE programs.

  2. 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.

  3. The HARPS-N Red Dwarf Exoplanets Survey (HADES) - Time Resolved Spectroscopic Analysis of The Steady Chromosphere Of Low-Activity Early-M Dwarfs

    NASA Astrophysics Data System (ADS)

    Scandariato, Gaetano

    2016-06-01

    We analyze the spectra of the quiet early-M dwarfs that the HADES project monitored for 3.5 years. The wavelength rangecovered by the spectra allows us to analyze simultaneously the CaII H&K doublet and the Hα line, while the intensive follow up gives us a large number of spectra (up to 100) for each targeted star. We show how the CaII H&K and Hα fluxesare correlated at low activity levels and we infer the evolution timescales of chromospheric active regions.

  4. 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.

  5. Extreme Star Formation in Low-Metallicity Dusty Compact Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Thuan, Trinh; Hunt, Leslie; Izotov, Yuri

    2008-03-01

    Dusty star formation plays a key role in the early universe. ISOCAM, MIPS and submillimeter surveys have found significant populations of dusty luminous galaxies which dominate the cosmic infrared background at z >= 1. Some of these galaxies host starbursts with high star formation rates (SFRs) in such compact regions that they approach the maximum starburst intensity limit. Similar extreme conditions can occur in star clusters within nearby starburst galaxies, but these clusters generally do not dominate the light. We propose here to study extreme SF in the local universe in systems where most of the SF occurs in massive star clusters, the light of which does dominate the galaxy light. These systems are 23 low-metallicity dusty compact dwarf galaxies (DCDs) selected from the Sloan Digital Sky Survey to have a high SFR/area, potentially near the starburst intensity limit, and significant warm dust emission, a good indicator of extreme SF in massive star clusters. These DCDs will allow us to study the physics of extreme SF with a sensitivity and spatial and spectral resolution that high-z starbursts do not allow. We wish to use IRAC, IRS and MIPS to investigate how the spectral energy distributions of the DCDs, the dust properties, and the SFR/area change as a function of various physical parameters such as metallicity, the hardness of the ionizing radiation, and the compactness of the starburst region. Combining our proposed DCD observations with other data will allow us to quantify the amount of dust-enshrouded SF over a wide range of metallicities and dust properties and derive true SFRs, which are crucial for deriving the SFR as a function of cosmic epoch.

  6. 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.

  7. 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.

  8. 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.

  9. Angular Momentum Evolution of Young Very Low Mass Stars and Brown Dwarfs: The Orion Nebula Cluster

    NASA Astrophysics Data System (ADS)

    Rodríguez-Ledesma, M. V.; Mundt, R.; Eislöffel, J.; Herbst, W.

    2008-12-01

    The rotational periods of young late-type stars and brown dwarfs (BDs) can be derived from photometric light curves, due to the rotational brightness modulation by surface features (i.e. magnetic cool spots). These kind of studies give important constrains on certain aspects of the so-called angular momentum problem of star formation. We report the first results of an extensive rotational period study of young stellar objects (YSOs) down into the BD mass regime in the Orion Nebula Cluster (ONC, d=450pc, age ˜ 1Myr). Our results are based on an deep photometric monitoring campaign, using the Wide Field Imager (WFI) camera on the ESO/MPG 2.2 meter telescope in La Silla, Chile. We found that 487 objects show detectable periodic light modulations, 377 of which are new detections. In addition 124 are potential BDs. This is by far the most extensive and complete rotational periods data set in the very low mass (VLM) star and BD regime. The spatial distribution of the variable objects, their rotational periods as well as the amplitude of the brightness modulation have been analyzed clearly indicating different stellar properties inside and outside the half-mass cluster radius of the ONC (R_{cluster} = 6.7'). In addition, we studied the dependence of the periodic brightness modulation on the magnitude (mass) of the objects and performed a comparison of the found period distribution with those of higher-mass objects in the ONC ( te{H2002}).

  10. 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.

  11. 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.

  12. 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.

  13. NO NEUTRON STAR COMPANION TO THE LOWEST MASS SDSS WHITE DWARF

    SciTech Connect

    Agueeros, Marcel A.; Camilo, Fernando; Heinke, Craig; Kilic, Mukremin; Anderson, Scott F.; Silvestri, Nicole M.; Freire, Paulo; Kleinman, Scot J.; Liebert, James W.

    2009-08-01

    SDSS J091709.55+463821.8 (hereafter J0917+4638) is the lowest surface gravity white dwarf (WD) currently known, with log g = 5.55 {+-} 0.05 (M {approx} 0.17 M{sub sun}). Such low-mass white dwarfs (LMWDs) are believed to originate in binaries that evolve into WD/WD or WD/neutron star (NS) systems. An optical search for J0917+4638's companion showed that it must be a compact object with a mass {>=}0.28 M{sub sun}. Here we report on Green Bank Telescope 820 MHz and XMM-Newton X-ray observations of J0917+4638 intended to uncover a potential NS companion to the LMWD. No convincing pulsar signal is detected in our radio data. Our X-ray observation also failed to detect X-ray emission from J0917+4638's companion, while we would have detected any of the millisecond radio pulsars in 47 Tuc. We conclude that the companion is almost certainly another WD.

  14. 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.

  15. The Eating Habits of Milky Way Mass Halos: Destroyed Dwarf Satellites and the Metallicity Distribution of Accreted Stars

    SciTech Connect

    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), 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.

  16. THE EATING HABITS OF MILKY WAY-MASS HALOS: DESTROYED DWARF SATELLITES AND THE METALLICITY DISTRIBUTION OF ACCRETED STARS

    SciTech Connect

    Deason, Alis J.; Mao, Yao-Yuan; Wechsler, Risa H.

    2016-04-10

    We study the mass spectrum of destroyed dwarfs that contribute to the accreted stellar mass of Milky Way (MW)-mass (M{sub vir} ∼ 10{sup 12.1} M{sub ⊙}) 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 M{sub star} ∼ 10{sup 8}–10{sup 10}M{sub ⊙}. Halos with more quiescent accretion histories tend to have lower mass progenitors (10{sup 8}–10{sup 9} M{sub ⊙}), and lower overall accreted stellar masses. Ultra-faint mass (M{sub star} < 10{sup 5} M{sub ⊙}) 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 10{sup 5} < M{sub star}/M{sub ⊙} < 10{sup 8} provide a substantial amount of the very metal-poor stellar material (∼40%–80%), and even relatively metal-rich dwarfs with M{sub star} > 10{sup 8} M{sub ⊙} 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.

  17. 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).

  18. 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

    2016-12-01

    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 2. 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.

  19. Chemical evolution of dwarf spheroidal galaxies based on model calculations incorporating observed star formation histories

    NASA Astrophysics Data System (ADS)

    Homma, H.; Murayama, T.

    We investigate the chemical evolution model explaining the chemical composition and the star formation histories (SFHs) simultaneously for the dwarf spheroidal galaxies (dSphs). Recently, wide imaging photometry and multi-object spectroscopy give us a large number of data. Therefore, we start to develop the chemical evolution model based on an SFH given by photometric observations and estimates a metallicity distribution function (MDF) comparing with spectroscopic observations. With this new model we calculate the chemical evolution for 4 dSphs (Fornax, Sculptor, Leo II, Sextans), and then we found that the model of 0.1 Gyr for the delay time of type Ia SNe is too short to explain the observed [alpha /Fe] vs. [Fe/H] diagrams.

  20. 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…

  1. The Eating Habits of Milky Way Mass Halos: Destroyed Dwarf Satellites and the Metallicity Distribution of Accreted Stars

    DOE PAGES

    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

  2. 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.

  3. 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.

  4. On The Explosion Geometry of Red Supergiant Stars

    NASA Astrophysics Data System (ADS)

    Leonard, Douglas C.; Supernova Spectropolarimetry Project (SNSPOL)

    2016-06-01

    From progenitor studies, type II-Plateau supernovae (SNe II-P) have been decisively and uniquely determined to arise from isolated red supergiant (RSG) stars, establishing the most homogeneous --- and well understood --- progenitor class of any type of core-collapse supernova. The physical process by which these stars explode, however, remains a mystery. A fundamental clue to the nature of the explosion mechanism is explosion geometry: In short, are supernovae round? Because young supernova atmospheres are electron-scattering dominated, their net linear polarization provides a direct probe of early-time supernova geometry, with higher degrees of polarization generally indicating greater departures from spherical symmetry. This presentation will describe the ongoing work being carried out on RSG explosion geometry 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, 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.

  5. 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.

  6. Chemical gradients in the Milky Way from the RAVE data. I. Dwarf stars

    NASA Astrophysics Data System (ADS)

    Boeche, C.; Siebert, A.; Piffl, T.; Just, A.; Steinmetz, M.; Sharma, S.; Kordopatis, G.; Gilmore, G.; Chiappini, C.; Williams, M.; Grebel, E. K.; Bland-Hawthorn, J.; Gibson, B. K.; Munari, U.; Siviero, A.; Bienaymé, O.; Navarro, J. F.; Parker, Q. A.; Reid, W.; Seabroke, G. M.; Watson, F. G.; Wyse, R. F. G.; Zwitter, T.

    2013-11-01

    Aims: We aim at measuring the chemical gradients of the elements Mg, Al, Si, and Fe along the Galactic radius to provide new constraints on the chemical evolution models of the Galaxy and Galaxy models such as the Besançon model. Thanks to the large number of stars of our RAVE sample we can study how the gradients vary as function of the distance from the Galactic plane. Methods: We analysed three different samples selected from three independent datasets: a sample of 19 962 dwarf stars selected from the RAVE database, a sample of 10 616 dwarf stars selected from the Geneva-Copenhagen Survey (GCS) dataset, and a mock sample (equivalent to the RAVE sample) created by using the GALAXIA code, which is based on the Besançon model. The three samples were analysed by using the very same method for comparison purposes. We integrated the Galactic orbits and obtained the guiding radii (Rg) and the maximum distances from the Galactic plane reached by the stars along their orbits (Zmax). We measured the chemical gradients as functions of Rg at different Zmax. Results: We found that the chemical gradients of the RAVE and GCS samples are negative and show consistent trends, although they are not equal: at Zmax< 0.4 kpc and 4.5 stars towards lower Rg and metallicity which, together with the thin

  7. The ACS LCID Project. II. Faint Variable Stars in the Isolated Dwarf Irregular Galaxy IC 1613

    NASA Astrophysics Data System (ADS)

    Bernard, Edouard J.; Monelli, Matteo; Gallart, Carme; Aparicio, Antonio; Cassisi, Santi; Drozdovsky, Igor; Hidalgo, Sebastian L.; Skillman, Evan D.; Stetson, Peter B.

    2010-04-01

    We present the results of a new search for variable stars in the Local Group (LG) isolated dwarf galaxy IC 1613, based on 24 orbits of F475W and F814W photometry from the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope. We detected 259 candidate variables in this field, of which only 13 (all of them bright Cepheids) were previously known. Out of the confirmed variables, we found 90 RR Lyrae stars, 49 classical Cepheids (including 36 new discoveries), and 38 eclipsing binary stars for which we could determine a period. The RR Lyrae include 61 fundamental (RRab) and 24 first-overtone (FO, RRc) pulsators, and five pulsating in both modes simultaneously (RRd). As for the majority of LG dwarfs, the mean periods of the RRab and RRc (0.611 and 0.334 days, respectively) as well as the fraction of overtone pulsators (f c = 0.28) place this galaxy in the intermediate regime between the Oosterhoff types. From their position on the period-luminosity diagram and light-curve morphology, we can unambiguously classify 25 and 14 Cepheids as fundamental and FO mode pulsators, respectively. Another two are clearly second-overtone Cepheids, the first ones to be discovered beyond the Magellanic Clouds. Among the remaining candidate variables, five were classified as δ-Scuti and five as long-period variables. Most of the others are located on the main sequence, the majority of them likely eclipsing binary systems, although some present variations similar to pulsating stars. We estimate the distance to IC 1613 using various methods based on the photometric and pulsational properties of the Cepheids and RR Lyrae stars. The values we find are in very good agreement with each other and with previous estimates based on independent methods. When corrected to a common reddening of E(B - V) = 0.025 and true Large Magellanic Cloud (LMC) distance modulus of (m-M)LMC,0 = 18.515 ± 0.085, we find that all the distance determinations from the literature converge to a common

  8. 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

  9. 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.

  10. On The Upper IMF in Nearby Dwarf Galaxies: Modeling the Effects of Star Formation Histories

    NASA Astrophysics Data System (ADS)

    Weisz, Daniel R.; Johnson, B. D.; Johnson, L. C.; Skillman, E. D.; LVL Team

    2011-01-01

    Observations of lower than expected Hα-FUV flux ratios in nearby dwarf galaxies have led to suggestions that the upper stellar IMF may systematically vary with respect to environment. We investigate the influence of star formation histories (SFHs) on the Hα-FUV flux ratios in nearby galaxies to assess the plausibility that SFHs could account for the observed trends. We model a wide range of SFH parameters, including some that resemble those of measured SFHs from studies of resolved stellar populations in nearby galaxies. Assuming a fully populated Chabrier IMF, we generate model predictions of the Hα-FUV flux ratios, R-band surface brightness, and total stellar mass, and compare to observations of 127 nearby star forming galaxies from the LVL and SDSS samples. We find excellent agreement between the model SFH predications and the observational data, demonstrating that a systematically varying IMF is unnecessary to explain the observed trends. We also explore the how extinction corrections, sample completeness, and choice of independent physical parameter (e.g., stellar mass, R-band surface brightness) can all introduce unphysical biases into the data. Our findings do not rule out competing effects such as photon leakage or stochastic sampling of the IMF, and are consistent with a combination of these effects causing the observed trends.

  11. Galactic fountains and outflows in star forming dwarf galaxies: ISM expulsion and chemical enrichment

    NASA Astrophysics Data System (ADS)

    Melioli, Claudio

    2015-08-01

    We investigated the impact of supernova feedback in gas-rich dwarf galaxies experiencing a low-to-moderate star formation rate, typical of relatively quiescent phases between starbursts. We calculated the long term evolution of the ISM and the metal-rich SN ejecta using 3D hydrodynamical simulations, in which the feedback energy is deposited by SNII exploding in distinct OB associations. We found that a circulation flow similar to galactic fountains is generally estabilished, with some ISM lifted at heights of one to few kpc above the galactic plane. This gas forms an extra-planar layer, which falls back to the plane in about 108 yr, once the star formation stops. Very little or no ISM is expelled outside the galaxy system for the considered SFRs, even though in the most powerful model the SN energy is comparable to the gas binding energy. The metal-rich SN ejecta is instead more vulnerable to the feedback and we found that a significant fraction (25-80%) is vented in the intergalactic medium, even for low SN rate. About half of the metals retained by the galaxy are located far (z > 500 pc) from the galactic plane. Moreover, our models indicate that the circulation of the metal-rich gas out from and back to the galactic disk is not able to erase the chemical gradients imprinted by the (centrally concentrated) SN explosions.

  12. 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.

  13. The Brown Dwarf Eclipsing Binary 2M0535-05: A Case Study for Activity Effects on Physical Properties of Low-Mass Stars and Brown Dwarfs

    NASA Astrophysics Data System (ADS)

    Stassun, K. G.

    2013-02-01

    2M0535-05 is a one-of-a-kind eclipsing binary (EB) comprising two brown dwarfs (BDs), and is an important benchmark for understanding the fundamental properties of BDs and low-mass stars. Because 2M0535-05 presents a peculiar reversal of temperatures with mass (the higher mass, magnetically active BD in the system is cooler than the lower mass companion BD), 2M0535-05 is particularly important as a case study for the effects of magnetic activity on the properties of low-mass objects. Using a large number of low-mass M-dwarfs and EBs in the field, we have developed empirical relations for determining the amount by which the temperatures and radii-and therefore the estimated masses-of low-mass stars and BDs are altered due to chromospheric activity. The relations link the amount by which an active object's temperature is suppressed, and its radius inflated, to the strength of its Hα emission. These relations are found to approximately preserve bolometric luminosity. Applying these relations to 2M0535-05 brings the activity-corrected radii and temperatures of 2M0535-05 into precise agreement with theoretical isochrones for inactive stars. The relations that we present are applicable to BDs and low-mass stars with masses below 0.8 M⊙ and for which the activity, as measured by Hα, is in the range - 4.6 < log LHα/Lbol < -3.3. We discuss implications of this work for determinations of young cluster IMFs, and discuss competing ideas for the physical mechanism by which magnetic fields alter the temperatures and radii of low-mass stars.

  14. The initial stellar masses for the formation of white dwarfs, neutron stars and black holes

    NASA Astrophysics Data System (ADS)

    Meynet, Georges

    As is well known a star may end its nuclear lifetime as a white dwarf, neutron star, black hole or may, in certain circumstances, leave no remnant at all. The main question to be addressed in this review is the following: what are the progenitors of these different final stages? After a brief review of the major physical principles governing stellar evolution, we present the different evolutionary scenarios resulting from numerical calculations. Particular attention will be paid to the effect of mass loss on theoretical determinations of the mass limits M WD and MBH which are respectively the maximum initial mass leading to the formation of a white dwarf and the minimum initial mass for the formation of a black hole. We terminate this review by the presentation of some relevant observational results. The bulk of this paper is devoted to the discussion of the evolution of single Population I stars. Les étoiles terminent leur évolution soit comme naines blanches, étoiles à neutrons, ou trous noirs, il peut également arriver qu'aucun résidu ne subsiste, l'étoile étant complètement détruite dans ses phases ultimes. La question à laquelle nous allons essayer de répondre dans cet article est la suivante : quels sont les progéniteurs de ces états finaux? Après un bref rappel de quelques principes importants gouvernant l'évolution stellaire, les différents scénarios évolutifs, tels qu'ils sont proposés par les modèles numériques, sont présentés. Les valeurs de la masse initiale maximale pour la formation des naines blanches ainsi que celles de la masse initiale minimale pour la formation des trous noirs sont discutées tant du point de vue théorique, qu'observationnel. La majeure partie de cet article concerne l'évolution d'étoiles simples et de composition chimique solaire.

  15. 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.

  16. 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.

  17. 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

  18. Chemical Abundances of M-dwarfs from the APOGEE Survey. I. The Exoplanet Hosting Stars Kepler-138 and Kepler-186

    NASA Astrophysics Data System (ADS)

    Souto, D.; Cunha, K.; García-Hernández, D. A.; Zamora, O.; Allende Prieto, C.; Smith, V. V.; Mahadevan, S.; Blake, C.; Johnson, J. A.; Jönsson, H.; Pinsonneault, M.; Holtzman, J.; Majewski, S. R.; Shetrone, M.; Teske, J.; Nidever, D.; Schiavon, R.; Sobeck, J.; García Pérez, A. E.; Gómez Maqueo Chew, Y.; Stassun, K.

    2017-02-01

    We report the first detailed chemical abundance analysis of the exoplanet-hosting M-dwarf stars Kepler-138 and Kepler-186 from the analysis of high-resolution (R ∼ 22,500) H-band spectra from the SDSS-IV–APOGEE survey. Chemical abundances of 13 elements—C, O, Na, Mg, Al, Si, K, Ca, Ti, V, Cr, Mn, and Fe—are extracted from the APOGEE spectra of these early M-dwarfs via spectrum syntheses computed with an improved line list that takes into account H2O and FeH lines. This paper demonstrates that APOGEE spectra can be analyzed to determine detailed chemical compositions of M-dwarfs. Both exoplanet-hosting M-dwarfs display modest sub-solar metallicities: [Fe/H]Kepler-138 = ‑0.09 ± 0.09 dex and [Fe/H]Kepler-186 = ‑0.08 ± 0.10 dex. The measured metallicities resulting from this high-resolution analysis are found to be higher by ∼0.1–0.2 dex than previous estimates from lower-resolution spectra. The C/O ratios obtained for the two planet-hosting stars are near-solar, with values of 0.55 ± 0.10 for Kepler-138 and 0.52 ± 0.12 for Kepler-186. Kepler-186 exhibits a marginally enhanced [Si/Fe] ratio.

  19. 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.

  20. 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.

  1. The spectroscopic indistinguishability of red giant branch and red clump stars

    NASA Astrophysics Data System (ADS)

    Masseron, T.; Hawkins, K.

    2017-01-01

    Context. Stellar spectroscopy provides useful information on the physical properties of stars such as effective temperature, metallicity and surface gravity. However, those photospheric characteristics are often hampered by systematic uncertainties. The joint spectro-sismo project (APOGEE+Kepler, aka APOKASC) of field red giants has revealed a puzzling offset between the surface gravities (log g) determined spectroscopically and those determined using asteroseismology, which is largely dependent on the stellar evolutionary status. Aims: Therefore, in this letter, we aim to shed light on the spectroscopic source of the offset. Methods: We used the APOKASC sample to analyse the dependencies of the log g discrepancy as a function of stellar mass and stellar evolutionary status. We discuss and study the impact of some neglected abundances on spectral analysis of red giants, such as He and carbon isotopic ratio. Results: We first show that, for stars at the bottom of the red giant branch where the first dredge-up had occurred, the discrepancy between spectroscopic log g and asteroseismic log g depends on stellar mass. This seems to indicate that the log g discrepancy is related to CN cycling. Among the CN-cycled elements, we demonstrate that the carbon isotopic ratio (12C /13C) has the largest impact on stellar spectrum. In parallel, we observe that this log g discrepancy shows a similar trend as the 12C /13C ratios as expected by stellar evolution theory. Although we did not detect a direct spectroscopic signature of 13C, other corroborating evidences suggest that the discrepancy in log g is tightly correlated to the production of 13C in red giants. Moreover, by running the data-driven algorithm (the Cannon) on a synthetic grid trained on the APOGEE data, we try to evaluate more quantitatively the impact of various 12C /13C ratios. Conclusions: While we have demonstrated that 13C indeed impacts all parameters, the size of the impact is smaller than the observed offset

  2. 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).

  3. 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.

  4. 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

  5. From White Dwarf To Neutron Star To Black Hole: Accretion, Gamma-ray Bursts, And Their Aftermath

    NASA Astrophysics Data System (ADS)

    Di Stefano, Rosanne

    2010-01-01

    When white dwarfs with massive companions experience accretion-induced-collapse, the newborn neutron star may continue to accrete until its mass becomes larger than the maximum neutron-star mass. The resulting black hole may have special properties that allow it to be identified post-collapse. We present a set of such evolutions, punctuated by gamma-ray bursts, and assess the expected rates. An individual system may exhibit a remarkable range of high-energy states: supersoft source, ultraluminous x-ray source, hard x-ray binary, and gamma-ray bursts.

  6. 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.

  7. 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.

  8. 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

  9. Dynamics and Stellar Content of the Giant Southern Stream in M31. I. Keck Spectroscopy of Red Giant Stars

    NASA Astrophysics Data System (ADS)

    Guhathakurta, Puragra; Rich, R. Michael; Reitzel, David B.; Cooper, Michael C.; Gilbert, Karoline M.; Majewski, Steven R.; Ostheimer, James C.; Geha, Marla C.; Johnston, Kathryn V.; Patterson, Richard J.

    2006-05-01

    This paper presents the first results from a large spectroscopic survey of red giant branch (RGB) stars in M31 using DEIMOS on the Keck 10 m telescope. A photometric prescreening method, based on the intermediate-width DDO51 band centered on the Mg b/MgH absorption feature, was used to select spectroscopic targets. RGB candidates were targeted in a small section of M31's giant southern tidal stream at a projected distance of 31 kpc from the galaxy's center. We isolate a clean sample of 68 RGB stars by removing contaminants (foreground Milky Way dwarf stars and background galaxies) using a combination of spectroscopic, imaging, and photometric methods: radial velocity and the surface gravity-sensitive Na I doublet are particularly useful in this regard. About 65% of the M31 stars are found to be members of the giant southern stream, while the rest appear to be members of the general spheroid population. The mean (heliocentric) radial velocity of the stream in our field is -458 km s-1, blueshifted by -158 km s-1 relative to M31's systemic velocity, in good agreement with recent velocity measurements at other points along the stream. The intrinsic velocity dispersion of the stream is found to be 15+8-15 km s-1 (90% confidence limit). A companion paper by Font and coworkers discusses possible orbits, implications of the coldness of the stream, and properties of the progenitor satellite galaxy. The kinematics, and possibly the metallicity distribution, of the general spheroid (i.e., nonstream) population in this region of M31 indicate that it is significantly different from samples drawn from other parts of the M31 spheroid; this is probably an indication of substructure in the bulge and halo. The stream appears to have a higher mean metallicity than the general spheroid, <[Fe/H]>~-0.54 versus -0.74, and a smaller metallicity spread. The relatively high metallicity of the stream implies that its progenitor must have been a luminous dwarf galaxy. The Ca II triplet line

  10. A new population of cool stars and brown dwarfs in the Lupus clouds

    NASA Astrophysics Data System (ADS)

    Comerón, F.; Spezzi, L.; López Martí, B.

    2009-06-01

    Context: Most studies of the stellar and substellar populations of star-forming regions rely on using the signatures of accretion, outflows, disks, or activity characterizing the early stages of stellar evolution. However, these signatures rapidly decay with time. Aims: We present the results of a wide-area study of the stellar population of clouds in the Lupus star-forming region. When combined with 2MASS photometry, our data allow us to fit the spectral energy distributions of over 150 000 sources and identify possible new members based on their photospheric fluxes, independent of any display of the signposts of youth. Methods: We used the Wide Field Imager (WFI) at the La Silla 2.2 m telescope to image an area of more than 6 square degrees in the Lupus 1, 3 and 4 clouds in the R_C, I_C, and z_WFI bands, selected so as to overlap with the areas observed in the Spitzer Legacy Program “From molecular cores to planet-forming disks”. We complement our data with 2MASS photometry to sample the spectral energy distribution from 0.6 μm to 2.2 μm. We validate our method on the census of known members of the Lupus clouds, for which spectroscopic classification is available. The temperatures derived for cool objects are generally accurate, with most of the exceptions attributed to veiling, strong emission lines at short wavelengths, near-infrared excess, variability, or the presence of close companions. Results: Considering that the dereddened fluxes of most cool (T_eff < 3500 K) young stellar objects at the distance of Lupus occupy a gap between those typical both of field cool dwarfs and of background giants, we identify a new population of cool members of Lupus 1 and 3. The approximately 130 new members are only moderately concentrated toward the densest clouds, they appear to have ages in the same range as the known members, and very few show the infrared excess caused by warm disks. This population is absent in Lupus 4. Conclusions: This new population of Lupus

  11. 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.

  12. 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

  13. Magnetic activity in the HARPS M dwarf sample. The rotation-activity relationship for very low-mass stars through

    NASA Astrophysics Data System (ADS)

    Astudillo-Defru, N.; Delfosse, X.; Bonfils, X.; Forveille, T.; Lovis, C.; Rameau, J.

    2017-03-01

    Context. Atmospheric magnetic fields in stars with convective envelopes heat stellar chromospheres, and thus increase the observed flux in the Ca ii H and K doublet. Starting with the historical Mount Wilson monitoring program, these two spectral lines have been widely used to trace stellar magnetic activity, and as a proxy for rotation period (Prot) and consequently for stellar age. Monitoring stellar activity has also become essential in filtering out false-positives due to magnetic activity in extra-solar planet surveys. The Ca ii emission is traditionally quantified through the -index, which compares the chromospheric flux in the doublet to the overall bolometric flux of the star. Much work has been done to characterize this index for FGK-dwarfs, but M dwarfs - the most numerous stars of the Galaxy - were left out of these analyses and no calibration of their Ca ii H and K emission to an exists to date. Aims: We set out to characterize the magnetic activity of the low- and very-low-mass stars by providing a calibration of the -index that extends to the realm of M dwarfs, and by evaluating the relationship between and the rotation period. Methods: We calibrated the bolometric and photospheric factors for M dwarfs to properly transform the S-index (which compares the flux in the Ca ii H and K lines to a close spectral continuum) into the . We monitored magnetic activity through the Ca ii H and K emission lines in the HARPS M dwarf sample. Results: The index, like the fractional X-ray luminosity LX/Lbol, shows a saturated correlation with rotation, with saturation setting in around a ten days rotation period. Above that period, slower rotators show weaker Ca ii activity, as expected. Under that period, the index saturates to approximately 10-4. Stellar mass modulates the Ca ii activity, with showing a constant basal activity above 0.6 M⊙ and then decreasing with mass between 0.6 M⊙ and the fully-convective limit of 0.35 M⊙. Short-term variability of the

  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. NuSTAR and swift observations of the fast rotating magnetized white dwarf AE Aquarii

    SciTech Connect

    Kitaguchi, Takao; An, Hongjun; Kaspi, Victoria M.; Beloborodov, Andrei M.; Gotthelf, Eric V.; Hailey, Charles J.; Hayashi, Takayuki; Boggs, Steven E.; Craig, William W.; Christensen, Finn E.; Stern, Daniel; Zhang, Will W.

    2014-02-10

    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 (L {sub X} ∼ 10{sup 31} erg s{sup –1}). 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{sub −0.45}{sup +0.18}, 2.29{sub −0.82}{sup +0.96}, and 9.33{sub −2.18}{sup +6.07} keV, or an optically thin thermal plasma model with two temperatures of 1.00{sub −0.23}{sup +0.34} and 4.64{sub −0.84}{sup +1.58} keV plus a power-law component with photon index of 2.50{sub −0.23}{sup +0.17}. 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.

  17. Self-consistent evolution of accreting low-mass stars and brown dwarfs

    NASA Astrophysics Data System (ADS)

    Baraffe, I.; Elbakyan, V. G.; Vorobyov, E. I.; Chabrier, G.

    2017-01-01

    We present self-consistent calculations coupling numerical hydrodynamics simulations of collapsing pre-stellar cores and stellar evolution models of accreting objects. We analyse the main impact of consistent accretion history on the evolution and lithium depletion of young low-mass stars and brown dwarfs. These consistent models confirm the generation of a luminosity spread in Herzsprung-Russell diagrams at ages 1-10 Myr. They also confirm that early accretion can produce objects with abnormal Li depletion, as found in a previous study that was based on arbitrary accretion rates. The results strengthen that objects with anomalously high level of Li depletion in young clusters should be extremely rare. We also find that early phases of burst accretion can produce coeval models of similar mass with a range of different Li surface abundances, and in particular with Li-excess compared to the predictions of non-accreting counterparts. This result is due to a subtle competition between the effect of burst accretion and its impact on the central stellar temperature, the growth of the stellar radiative core and the accretion of fresh Li from the accretion disk. Only consistent models could reveal such a subtle combination of effects. This new result could explain the recent, puzzling observations of Li-excess of fast rotators in the young cluster NGC 2264. Present self-consistent accreting models are available in electronic form.

  18. 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.

  19. An Improved Red Spectrum of the Methane or T Dwarf SDSS 1624+0029: The Role of the Alkali Metals.

    PubMed

    Liebert; Reid; Burrows; Burgasser; Kirkpatrick; Gizis

    2000-04-20

    A Keck II low-resolution spectrum shortward of 1 µm is presented for SDSS 1624+0029, the first field methane or T dwarf discovered in the Sloan Digital Sky Survey. Significant flux is detected down to the spectrum's short-wavelength limit of 6200 Å. The spectrum exhibits a broad absorption feature centered at 7700 Å, which we interpret as the K i lambdalambda7665, 7699 resonance doublet. The observed flux declines shortward of 7000 Å, most likely owing to the red wing of the Na i doublet. Both Cs i doublet lines are detected more strongly than in an earlier red spectrum. Neither Li i absorption nor Halpha emission are detected. An exploratory model fit to the spectrum suggests that the shape of the red spectrum can be primarily accounted for by the broad wings of the K i and Na i doublets. This behavior is consistent with the argument proffered by Burrows, Marley, & Sharp that strong alkali absorption is principally responsible for depressing T dwarf spectra shortward of 1 µm. In particular, there seems no compelling reason at this time to introduce dust or an additional opacity source in the atmosphere of the Sloan object. The width of the K i and strengths of the Cs i lines also indicate that the Sloan object is warmer than Gl 229B.

  20. A new method for the asteroseismic determination of the evolutionary state of red-giant stars

    NASA Astrophysics Data System (ADS)

    Elsworth, Yvonne; Hekker, Saskia; Basu, Sarbani; Davies, Guy R.

    2017-04-01

    Determining the ages of red-giant stars is a key problem in stellar astrophysics. One of the difficulties in this determination is to know the evolutionary state of the individual stars - i.e. have they started to burn Helium in their cores? That is the topic of this paper. Asteroseismic data provide a route to achieving this information. What we present here is a highly autonomous way of determining the evolutionary state from an analysis of the power spectrum of the light curve. The method is fast and efficient and can provide results for a large number of stars. It uses the structure of the dipole-mode oscillations, which have a mixed character in red-giant stars, to determine some measures that are used in the categorization. It does not require that all the individual components of any given mode be separately characterized. Some 6604 red-giant stars have been classified. Of these, 3566 are determined to be on the red-giant branch, 2077 are red-clump and 439 are secondary-clump stars. We do not specifically identify the low-metallicity, horizontal-branch stars. The difference between red-clump and secondary-clump stars is dependent on the manner in which Helium burning is first initiated. We discuss that the way the boundary between these classifications is set may lead to mis-categorization in a small number of stars. The remaining 522 stars were not classified either because they lacked sufficient power in the dipole modes (so-called depressed dipole modes) or because of conflicting values in the parameters.

  1. 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.

  2. Tracing Recent Star Formation of Red Early-type Galaxies out to z ~ 1

    NASA Astrophysics Data System (ADS)

    Ko, Jongwan; Hwang, Ho Seong; Im, Myungshin; Le Borgne, Damien; Lee, Jong Chul; Elbaz, David

    2014-08-01

    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 star > 1010.5 M ⊙ 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 star > 1011.2 M ⊙, 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.

  3. 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.

  4. The Initial Mass Function of Low-Mass Stars and Brown Dwarfs in Young Clusters

    NASA Astrophysics Data System (ADS)

    Luhman, K. L.; Rieke, G. H.; Young, Erick T.; Cotera, Angela S.; Chen, H.; Rieke, Marcia J.; Schneider, Glenn; Thompson, Rodger I.

    2000-09-01

    We have obtained images of the Trapezium Cluster (140''×140'' 0.3 pc×0.3 pc) with the Hubble Space Telescope Near-Infrared Camera and Multi-Object Spectrometer (NICMOS). Combining these data with new ground-based K-band spectra (R=800) and existing spectral types and photometry, we have constructed an H-R diagram and used it and other arguments to infer masses and ages. To allow comparison with the results of our previous studies of IC 348 and ρ Oph, we first use the models of D'Antona & Mazzitelli. With these models, the distributions of ages of comparable samples of stars in the Trapezium, ρ Oph, and IC 348 indicate median ages of ~0.4 Myr for the first two regions and ~1-2 Myr for the latter. The low-mass initial mass functions (IMFs) in these sites of clustered star formation are similar over a wide range of stellar densities (ρ Oph, n=0.2-1×103 pc-3 IC 348, n=1×103 pc-3 Trapezium, n=1-5×104 pc-3) and other environmental conditions (e.g., presence or absence of OB stars). With current data, we cannot rule out modest variations in the substellar mass functions among these clusters. We then make the best estimate of the true form of the IMF in the Trapezium by using the evolutionary models of Baraffe et al. and an empirically adjusted temperature scale and compare this mass function to recent results for the Pleiades and the field. All of these data are consistent with an IMF that is flat or rises slowly from the substellar regime to about 0.6 Msolar and then rolls over into a power law that continues from about 1 Msolar to higher masses with a slope similar to or somewhat larger than the Salpeter value of 1.35. For the Trapezium, this behavior holds from our completeness limit of ~0.02 Msolar and probably, after a modest completeness correction, even from 0.01-0.02 Msolar. These data include ~50 likely brown dwarfs. We test the predictions of theories of the IMF against (1) the shape of the IMF, which is not log-normal, in clusters and the field, (2) the

  5. 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).

  6. On using occultation and microlensing to find white dwarf and planetary companions of main sequence stars in tranit searches.

    NASA Astrophysics Data System (ADS)

    Agol, E.; Farmer, A.; Mandel, K.

    2002-12-01

    We will prove a gravitational lensing theorem: the magnification of a large source of uniform brightness by a small foreground point-mass lens is M=1+(2RE2-R_L^2)/R_S^2, where RS is the radius of the source and RE and RL are the Einstein radius and size of the lens projected into the source plane; this provides an accurate approximation to the exact magnification for RL,R_E << RS. Remarkably, this result is independent of the shape of the source or position of the lens (except near the edges). We show that this formula can be generalized to include limb-darkening of a circular source by simply inserting the surface-brightness at the position of the foreground object (divided by the average surface-brightness of the star). We use this theorem to compute transit lightcurves in binaries for which the foreground star or planet has a size and Einstein radius much smaller than the background star. White dwarf stars in binaries with semi-major axes of 0.1 AU have a size comparable to their Einstein radii. Thus, white dwarfs orbiting main-sequence stars can show either brightening or eclipse. Since white dwarfs are similar in size to terrestrial planets, we predict how many white dwarf-main sequence binaries might be found in transit searches for terrestrial type planets such as Kepler and Eddington. We estimate that dozens of systems may be found, although the precise number will depend on the properties of low-level variability of the main-sequence stars in these binaries. We also apply these results to planetary transits (RE ~ 0), demonstrating that this simple approximation accurately describes the exact limb-darkened transit lightcurves. Using the HST lightcurve of HD 209458 from Brown et al. (2001), the ratio of the planet radius to stellar radius is measured to be Rp/R_*=0.1207+/- 0.0003 , marginalized over the limb-darkening.

  7. Could Ultracool Dwarfs Have Sun-Like Activity?

    NASA Astrophysics Data System (ADS)

    Kohler, Susanna

    2016-11-01

    emission primarily polarized in a single direction. The dwarfs flares in late 2013, however, all showed polarization in the opposite direction. Could this be an indication that J1047+21 has a stable, global dipolar field that flipped polarity in between the two sets of observations? If so, this could mean that the star has a magnetic cycle similar to the Suns.Artists impression showing the relative sizes and colors of the Sun, a red dwarf (M-dwarf), a hotter brown dwarf (L-dwarf), a cool brown dwarf (T-dwarf) similar to J1047+21, and the planet Jupiter [Credit: NASA/IPAC/R. Hurt (SSC)]Inspired by this possibility, Route conducted an investigation of the long-term magnetic behavior of all known radio-flaring ultracool dwarfs, a list of 14 stars. Using polarized radio emission measurements, he found that many of his targets exhibited similar polarity flips, which he argues is evidence that these dwarfs are undergoing magnetic field reversals on roughly decade-long timescales, analogous to those reversals that occur in the Sun.If this is indeed true, then we need to examine our models of how magnetic fields are generated in stars: the interface between the radiative and convective layers may not be necessary to produce large-scale magnetic fields. Understanding this process is certainly an important step in interpreting the potential habitability of planets around ultracool dwarfs.CitationMatthew Route 2016 ApJL 830 L27. doi:10.3847/2041-8205/830/2/L27

  8. Toward High-precision Seismic Studies of White Dwarf Stars: Parametrization of the Core and Tests of Accuracy

    NASA Astrophysics Data System (ADS)

    Giammichele, N.; Charpinet, S.; Fontaine, G.; Brassard, P.

    2017-01-01

    We present a prescription for parametrizing the chemical profile in the core of white dwarfs in light of the recent discovery that pulsation modes may sometimes be deeply confined in some cool pulsating white dwarfs. Such modes may be used as unique probes of the complicated chemical stratification that results from several processes that occurred in previous evolutionary phases of intermediate-mass stars. This effort is part of our ongoing quest for more credible and realistic seismic models of white dwarfs using static, parametrized equilibrium structures. Inspired by successful techniques developed in design optimization fields (such as aerodynamics), we exploit Akima splines for the tracing of the chemical profile of oxygen (carbon) in the core of a white dwarf model. A series of tests are then presented to better seize the precision and significance of the results that can be obtained in an asteroseismological context. We also show that the new parametrization passes an essential basic test, as it successfully reproduces the chemical stratification of a full evolutionary model.

  9. 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.

  10. NuSTAR Observations of the Dwarf Nova GK Persei in 2015: Comparison between Outburst and Quiescent Phases

    NASA Astrophysics Data System (ADS)

    Wada, Y.; Yuasa, T.; Nakazawa, K.; Makishima, K.; Hayashi, T.; Ishida, M.

    2017-03-01

    We report on NuSTAR observations of the intermediate polar GK Persei which also behaves as a dwarf nova. It exhibited a dwarf nova outburst in 2015 March-April. The object was observed in 3–79 keV X-rays with NuSTAR, once at the outburst peak, and again in 2015 September during quiescence. The 5-50 keV flux during the outburst was 26 times higher than that during the quiescence. With a multi-temperature emission model and a reflection model, we derived the post-shock temperature as 19.2 ± 0.7 keV in the outburst, and 38.5 +4.1–3.6 keV in the quiescence. This temperature difference is considered to reflect changes in the radius at which the accreting matter, forming an accretion disk, is captured by the magnetosphere of the white dwarf (WD). Assuming that this radius scales as the power of –2/7 of the mass accretion rate, and utilizing the two temperature measurements, as well as the standard mass-radius relation of WDs, we determined the WD mass in GK Persei as 0.90 ±0.06 solar masses. The magnetic field is estimated as 4 × 105 G.

  11. Atmospheric Chemistry in Giant Planets, Brown Dwarfs, and Low-Mass Dwarf Stars. II. Sulfur and Phosphorus

    NASA Astrophysics Data System (ADS)

    Visscher, Channon; Lodders, Katharina; Fegley, Bruce, Jr.

    2006-09-01

    Thermochemical equilibrium and kinetic calculations are used to model sulfur and phosphorus chemistry in giant planets, brown dwarfs, and extrasolar giant planets (EGPs). The chemical behavior of individual S- and P-bearing gases and condensates is determined as a function of pressure, temperature, and metallicity. The results are independent of particular model atmospheres, and in principle, the equilibrium composition along the pressure-temperature profile of any object can be determined. Hydrogen sulfide (H2S) is the dominant S-bearing gas throughout substellar atmospheres and approximately represents the atmospheric sulfur inventory. Silicon sulfide (SiS) is a potential tracer of weather in substellar atmospheres. Disequilibrium abundances of phosphine (PH3) approximately representative of the total atmospheric phosphorus inventory are expected to be mixed upward into the observable atmospheres of giant planets and T dwarfs. In hotter objects, several P-bearing gases (e.g., P2, PH3, PH 2, PH, and HCP) become increasingly important at high temperatures.

  12. 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.

  13. The ACS Nearby Galaxy Survey Treasury. VIII. The Global Star Formation Histories of 60 Dwarf Galaxies in the Local Volume

    NASA Astrophysics Data System (ADS)

    Weisz, Daniel R.; Dalcanton, Julianne J.; Williams, Benjamin F.; Gilbert, Karoline M.; Skillman, Evan D.; Seth, Anil C.; Dolphin, Andrew E.; McQuinn, Kristen B. W.; Gogarten, Stephanie M.; Holtzman, Jon; Rosema, Keith; Cole, Andrew; Karachentsev, Igor D.; Zaritsky, Dennis

    2011-09-01

    We present uniformly measured star formation histories (SFHs) of 60 nearby (D <~ 4 Mpc) dwarf galaxies based on color-magnitude diagrams of resolved stellar populations from images taken with the Hubble Space Telescope and analyzed as part of the ACS Nearby Galaxy Survey Treasury program (ANGST). This volume-limited sample contains 12 dwarf spheroidal (dSph)/dwarf elliptical (dE), 5 dwarf spiral, 28 dwarf irregular (dI), 12 dSph/dI (transition), and 3 tidal dwarf galaxies. The sample spans a range of ~10 mag in MB and covers a wide range of environments, from highly interacting to truly isolated. From the best-fit SFHs, we find three significant results for dwarf galaxies in the ANGST volume: (1) the majority of dwarf galaxies formed the bulk of their mass prior to z ~ 1, regardless of current morphological type; (2) the mean SFHs of dIs, transition dwarf galaxies (dTrans), and dSphs are similar over most of cosmic time, and only begin to diverge a few Gyr ago, with the clearest differences between the three appearing during the most recent 1 Gyr and (3) the SFHs are complex and the mean values are inconsistent with simple SFH models, e.g., single bursts, constant star formation rates (SFRs), or smooth, exponentially declining SFRs. The mean SFHs show clear divergence from the cosmic SFH at z <~ 0.7, which could be evidence that low-mass systems have experienced delayed star formation relative to more massive galaxies. The sample shows a strong density-morphology relationship, such that the dSphs in the sample are less isolated than the dIs. We find that the transition from a gas-rich to gas-poor galaxy cannot be solely due to internal mechanisms such as stellar feedback, and instead is likely the result of external mechanisms, e.g., ram pressure and tidal stripping and tidal forces. In terms of their environments, SFHs, and gas fractions, the majority of the dTrans appear to be low-mass dIs that simply lack Hα emission, similar to Local Group (LG) dTrans DDO 210

  14. Quasisoft X-Ray Sources: White Dwarfs? Neutron Stars? Black Holes?

    NASA Astrophysics Data System (ADS)

    Di Stefano, Rosanne

    Two of the most exciting areas of current research in astrophysics are black holes and Type Ia supernovae. We propose archival work that has the potential to shed light on both areas. The focus of our research is a newly-established class of x-ray sources called Quasisoft X-ray Sources (QSSs). Although they comprise a significant fraction of the x- ray sources in galaxies of all types, including M31, it has proved difficult to identify members of this class in the Milky Way or Magellanic Clouds. We have developed methods to find these sources, and have begun to meet with success in the application of our methods. The three-year project we propose will allow us to identify QSSs. We will then use the full range of archived data to determine which QSS candidates are highly luminous, and which are members of less luminous classes, such as quiescent low-mass x-ray binaries (qLMXBs), or even isolated neutron stars. Many will be nearby x-ray active stars, or else distant AGN, whose discovery will also be of interest to a range of researchers. In the end, we will have a subset of intriguing physical systems, some of which may be accreting black holes and some of which may be unusual states of neutron stars or even of nuclear-burning white dwarfs. The systems identified through this ADAP program will be targets of future observing programs, from space and from the ground. The information we derive from NASA archived data will provide insight into important astrophysical questions. Do intermediate-mass black holes (IMBHs) exist? It has only been during the past 15 years or so that accreting compact objects that were considered as black hole candidates have been promoted to black holes. This achievement required years of observations of candidates in the Milky Way and Magellanic Clouds. The discovery of ultraluminous X- ray source in external galaxies suggests that there are black holes with masses larger than the 10-30 solar masses typical of the known black holes. To

  15. 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.

  16. 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

    2017-01-01

    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 debris disc 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 submicron/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.

  17. 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

  18. 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

  19. Can Life Develop in the Expanded Habitable Zones around Red Giant Stars?

    NASA Astrophysics Data System (ADS)

    Lopez, Bruno; Schneider, Jean; Danchi, William C.

    2005-07-01

    We present some new ideas about the possibility of life developing around subgiant and red giant stars. Our study concerns the temporal evolution of the habitable zone. The distance between the star and the habitable zone, as well as its width, increases with time as a consequence of stellar evolution. The habitable zone moves outward after the star leaves the main sequence, sweeping a wider range of distances from the star until the star reaches the tip of the asymptotic giant branch. Currently there is no clear evidence as to when life actually formed on the Earth, but recent isotopic data suggest life existed at least as early as 7×108 yr after the Earth was formed. Thus, if life could form and evolve over time intervals from 5×108 to 109 yr, then there could be habitable planets with life around red giant stars. For a 1 Msolar star at the first stages of its post-main-sequence evolution, the temporal transit of the habitable zone is estimated to be several times 109 yr at 2 AU and around 108 yr at 9 AU. Under these circumstances life could develop at distances in the range 2-9 AU in the environment of subgiant or giant stars, and in the far distant future in the environment of our own solar system. After a star completes its first ascent along the red giant branch and the He flash takes place, there is an additional stable period of quiescent He core burning during which there is another opportunity for life to develop. For a 1 Msolar star there is an additional 109 yr with a stable habitable zone in the region from 7 to 22 AU. Space astronomy missions, such as proposed for the Terrestrial Planet Finder (TPF) and Darwin, that focus on searches for signatures of life on extrasolar planets, should also consider the environments of subgiants and red giant stars as potentially interesting sites for understanding the development of life. We performed a preliminary evaluation of the difficulty of interferometric observations of planets around red giant stars

  20. On the coronae of rapidly rotating stars. IV - Coronal activity in F dwarfs and implications for the onset of the dynamo

    NASA Technical Reports Server (NTRS)

    Walter, F. M.

    1983-01-01

    X-ray observations of 14 early F dwarfs are reported and these stars are used, together with a complete sample from the literature, to examine how the characteristics of coronal X-ray emission vary from dwarfs of spectral type A through G. Evidence for a rotation-activity relation in stars redder than B - V = 0.45 is found. Stellar duplicity and age, except insofar as they influence the rotation rate, do not appear to be important in determining the coronal X-ray flux level in F dwarfs. It is suggested that the appearance of a relation between rotation and activity at B - V = 0.45 indicates the turn-on of a solar-like dynamo. The high X-ray surface fluxes and small variance thereof for dwarfs with B - V = 0.3-0.45 are also discussed.

  1. CHANDRA DETECTION OF X-RAY EMISSION FROM ULTRACOMPACT DWARF GALAXIES AND EXTENDED STAR CLUSTERS

    SciTech Connect

    Hou, Meicun; Li, Zhiyuan

    2016-03-10

    We have conducted a systematic study of X-ray emission from ultracompact dwarf (UCD) galaxies and extended star clusters (ESCs), based on archival Chandra observations. Among a sample of 511 UCDs and ESCs complied from the literature, 17 X-ray counterparts with 0.5–8 keV luminosities above ∼5 × 10{sup 36} erg s{sup −1} are identified, which are distributed in eight early-type host galaxies. To facilitate comparison, we also identify X-ray counterparts of 360 globular clusters (GCs) distributed in four of the eight galaxies. The X-ray properties of the UCDs and ESCs are found to be broadly similar to those of the GCs. The incidence rate of X-ray-detected UCDs and ESCs, 3.3% ± 0.8%, while lower than that of the X-ray-detected GCs (7.0% ± 0.4%), is substantially higher than expected from the field populations of external galaxies. A stacking analysis of the individually undetected UCDs/ESCs further reveals significant X-ray signals, which corresponds to an equivalent 0.5–8 keV luminosity of ∼4 × 10{sup 35} erg s{sup −1} per source. Taken together, these provide strong evidence that the X-ray emission from UCDs and ESCs is dominated by low-mass X-ray binaries having formed from stellar dynamical interactions, consistent with the stellar populations in these dense systems being predominantly old. For the most massive UCDs, there remains the possibility that a putative central massive black hole gives rise to the observed X-ray emission.

  2. 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.

  3. THE BIMODALITY OF ACCRETION IN T TAURI STARS AND BROWN DWARFS

    SciTech Connect

    Vorobyov, E. I.; Basu, Shantanu E-mail: basu@astro.uwo.c

    2009-09-20

    We present numerical solutions of the collapse of prestellar cores that lead to the formation and evolution of circumstellar disks. The disk evolution is then followed for up to three million years. A variety of models of different initial masses and rotation rates allow us to study disk accretion around brown dwarfs and low-mass T Tauri stars (TTSs), with central object mass M{sub *} < 0.2 M{sub sun}, as well as intermediate- and upper-mass TTSs (0.2 M{sub sun} < M{sub *} < 3.0 M{sub sun}). Our models include self-gravity and allow for nonaxisymmetric motions. In addition to the self-consistently generated gravitational torques, we introduce an effective turbulent alpha-viscosity with alpha = 0.01, which allows us particularly to model accretion in the low-mass regime where disk self-gravity is diminishing. A range of models with observationally motivated values of the initial ratio of rotational-to-gravitational energy yield a correlation between mass accretion rate M-dot and M{sub *} that is relatively steep, as observed. Additionally, our modeling reveals evidence for a bimodality in the M-dot - M{sub *} correlation, with a steeper slope at lower masses and a shallower slope at intermediate and upper masses, as also implied by observations. Furthermore, we show that the neglect of disk self-gravity leads to a much steeper M-dot - M{sub *} relation for intermediate- and upper-mass TTSs. This demonstrates that an accurate treatment of global self-gravity is essential to understanding observations of circumstellar disks.

  4. BANYAN. VIII. New Low-mass Stars and Brown Dwarfs with Candidate Circumstellar Disks

    NASA Astrophysics Data System (ADS)

    Boucher, Anne; Lafrenière, David; Gagné, Jonathan; Malo, Lison; Faherty, Jacqueline K.; Doyon, René; Chen, Christine H.

    2016-11-01

    We present the results of a search for new circumstellar disks around low-mass stars and brown dwarfs with spectral types >K5 that are confirmed or candidate members of nearby young moving groups. Our search input sample was drawn from the BANYAN surveys of Malo et al. and Gagné et al. Two Micron All-Sky Survey and Wide-field Infrared Survey Explorer data were used to detect near- to mid-infrared excesses that would reveal the presence of circumstellar disks. A total of 13 targets with convincing excesses were identified: 4 are new and 9 were already known in the literature. The new candidates are 2MASS J05010082-4337102 (M4.5), J08561384-1342242 ({{M}}8γ ), J12474428-3816464 ({{M}}9γ ), and J02265658-5327032 ({{L}}0δ ); they are candidate members of the TW Hya (˜ 10+/- 3 Myr), Columba (˜{42}-4+6 Myr), and Tucana-Horologium (˜ 45+/- 4 Myr) associations, with masses of 120 and 13-18 {M}{Jup}. The M8-L0 objects in Columba and Tucana-Horologium are potentially among the first substellar disk systems aged ˜40 Myr. Estimates of the new candidates’ mean disk temperatures and fractional luminosities are in the ranges ˜135{--}520 {{K}} and 0.021{--}0.15, respectively. New optical spectroscopy of J0501-4337 reveals strong Hα emission, possibly indicating ongoing accretion, provides a detection of lithium absorption, and shows a radial velocity measurement that is consistent with a membership to Columba. We also present a near-infrared spectrum of J0226-5327 that reveals Paschen β emission and shows signs of low surface gravity, consistent with accretion from a disk and a young age.

  5. Pushing the Limits, Episode 2: K2 Observations of Extragalactic RR Lyrae Stars in the Dwarf Galaxy Leo IV

    NASA Astrophysics Data System (ADS)

    Molnár, L.; Pál, A.; Plachy, E.; Ripepi, V.; Moretti, M. I.; Szabó, R.; Kiss, L. L.

    2015-10-01

    We present the first observations of extragalactic pulsating stars in the K2 ecliptic survey of the Kepler space telescope. The variability of all three RR Lyrae stars in the dwarf spheroidal galaxy Leo IV was successfully detected, at a brightness of Kp ≈ 21.5 mag, from data collected during Campaign 1. We identified one modulated star and another likely Blazhko candidate with periods of 29.8 ± 0.9 days and more than 80 days, respectively. EPIC 210282473 represents the first star beyond the Magellanic Clouds for which the Blazhko period and cycle-to-cycle variations in the modulation were unambiguously measured. The photometric [Fe/H] indices of the stars agree with earlier results that Leo IV is a very metal-poor galaxy. Two out of the three stars blend with brighter background galaxies in the K2 frames. We demonstrate that image subtraction can be reliably used to extract photometry from faint confused sources, which will be crucial not only for the K2 mission but also for future space photometric missions.

  6. 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.

  7. A subsynchronously rotating pulsating subdwarf B star in a short-period binary with a white dwarf companion

    NASA Astrophysics Data System (ADS)

    Baran, A. S.; Telting, J. H.; Németh, P.; Østensen, R. H.; Reed, M. D.; Kiaeerad, F.

    2016-01-01

    We present our analysis of KIC 7664467, an sdB pulsator that we have found to be residing in a 1.56-day binary system with a white-dwarf companion. This system was observed photometrically with the Kepler spacecraft and spectroscopically with ground-based telescopes. We analyzed the amplitude spectra detecting 61 periods, rotationally split multiplets, and an equally spaced sequence in period. These two features helped with the mode identification. We derived both the binary and rotation periods showing that this is another binary system with a subsynchronous sdB star. From our spectroscopy of the sdB star, we determined Teff = 27440 ± 120 K, log g = 5.38 ± 0.02 dex. The abundance pattern follows the general trend observed in sdB stars, where light metals are subsolar, while the Fe abundance is very close to the solar value. We found the N enrichment and low abundances of C and O that resemble the equilibrium abundances of the CNO cycle. We could also measure the Mg and Si abundances. Using the radial velocity amplitude K1 = 57(3) km s-1 and the Doppler boosting-dominated photometric signal at the orbital period, we constrained the companion to be a compact object, almost certainly a white dwarf.

  8. 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

  9. Three New Eclipsing White-dwarf-M-dwarf Binaries Discovered in a Search for Transiting Planets around M-dwarfs

    NASA Astrophysics Data System (ADS)

    Law, Nicholas M.; Kraus, Adam L.; Street, Rachel; Fulton, Benjamin J.; Hillenbrand, Lynne A.; Shporer, Avi; Lister, Tim; Baranec, Christoph; Bloom, Joshua S.; Bui, Khanh; Burse, Mahesh P.; Cenko, S. Bradley; Das, H. K.; Davis, Jack. T. C.; Dekany, Richard G.; Filippenko, Alexei V.; Kasliwal, Mansi M.; Kulkarni, S. R.; Nugent, Peter; Ofek, Eran O.; Poznanski, Dovi; Quimby, Robert M.; Ramaprakash, A. N.; Riddle, Reed; Silverman, Jeffrey M.; Sivanandam, Suresh; Tendulkar, Shriharsh P.

    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 × 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 ⊙ (0.01 AU). The M-dwarfs have masses of approximately 0.35 M ⊙, and the white dwarfs have hydrogen-rich atmospheres with temperatures of around 8000 K and have masses of approximately 0.5 M ⊙. 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 ⊙ (95% confidence) in one of the systems. Accounting for our detection efficiency and geometric factors, we estimate that 0.08%^{+0.10%}_{-0.05%} (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 relatively large orbital radii. Similar eclipsing

  10. M Dwarf Mysteries

    NASA Astrophysics Data System (ADS)

    Henry, Todd J.; Jao, Wei-Chun; Irwin, Jonathan; Dieterich, Sergio; Finch, Charlie T.; Riedel, Adric R.; Subasavage, John P.; Winters, Jennifer; RECONS Team

    2017-01-01

    During RECONS' 17-year (so far) astrometry/photometry program at the CTIO/SMARTS 0.9m, we have observed thousands of the ubiquitous red dwarfs in the solar neighborhood. During this reconnaissance, a few mysterious characters have emerged ...The Case of the Mercurial Stars: One M dwarf has been fading steadily for more than a decade, at last measure 6% fainter than when it was first observed. Another has grown brighter by 7% over 15 years. Are these brightness changes part of extremely long stellar cycles, or something else entirely?The Case of Identical Stellar Twins that Aren't: Two M dwarfs seem at first to be identical siblings traveling together through the Galaxy. They have virtually identical spectra at optical wavelengths and identical colors throughout the VRIJHK bands. Long-term astrometry indicates that they are, indeed, at the same distance via parallax measurements, and their proper motions match precisely. Yet, one of the twins is FOUR times brighter than the other. Followup work has revealed that the brighter component is a very close spectroscopic double, but no other stars are seen. So, the mystery may be half solved, but why do the close stars remain twice as bright as their widely-separated twin?The Case of the Great Kaboom!: After more than 1000 nights of observing on the reliable 0.9m telescope, with generally routine frames reading out upon the screen, one stellar system comprised of five red dwarfs flared in stunning fashion. Of the two distinct sources, the fainter one (an unresolved double) surpassed the brightness of the brighter one (an unresolved triple), increasing by more than three full magnitudes in the V filter. Which component actually flared? Is this magnificent outburst an unusual event, or in fact typical for this system and other M dwarfs?At the AAS meeting, we hope to probe the cognoscenti who study the Sun's smaller cousins to solve these intriguing M Dwarf Mysteries.This effort has been supported by the NSF through grants

  11. The brown dwarf kinematics project

    NASA Astrophysics Data System (ADS)

    Faherty, Jackie K.

    2010-10-01

    Brown dwarfs are a recent addition to the plethora of objects studied in Astronomy. With theoretical masses between 13 and 75 MJupiter , they lack sustained stable Hydrogen burning so they never join the stellar main sequence. They have physical properties similar to both planets and low-mass stars so studies of their population inform on both. The distances and kinematics of brown dwarfs provide key statistical constraints on their ages, moving group membership, absolute brightnesses, evolutionary trends, and multiplicity. Yet, until my thesis, fundamental measurements of parallax and proper motion were made for only a relatively small fraction of the known population. To address this deficiency, I initiated the Brown Dwarf Kinematics (BDKP). Over the past four years I have re-imaged the majority of spectroscopically confirmed field brown dwarfs (or ultracool dwarfs---UCDs) and created the largest proper motion catalog for ultracool dwarfs to date. Using new astrometric information I examined population characteristics such as ages calculated from velocity dispersions and correlations between kinematics and colors. Using proper motions, I identified several new wide co-moving companions and investigated binding energy (and hence formation) limitations as well as the frequency of hierarchical companions. Concurrently over the past four years I have been conducting a parallax survey of 84 UCDs including those showing spectral signatures of youth, metal-poor brown dwarfs, and those within 20 pc of the Sun. Using absolute magnitude relations in J,H, and K, I identified overluminous binary candidates and investigated known flux-reversal binaries. Using current evolutionary models, I compared the MK vs J-K color magnitude diagram to model predictions and found that the low-surface gravity dwarfs are significantly red-ward and underluminous of predictions and a handful of late-type T dwarfs may require thicker clouds to account for their scatter.

  12. 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

  13. 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.

  14. 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

  15. 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

  16. Circumstellar Environments of Southern M Dwarfs in the Solar Neighborhood

    NASA Astrophysics Data System (ADS)

    Silverstein, Michele L.; Henry, Todd J.; Jao, Wei-Chun; Winters, Jennifer G.; Recons Team

    2015-01-01

    We present the first results from SIRENS, the Search for InfraRed Excesses around Nearby Stars. Our goal is to characterize the circumstellar environments of the most common and closest stars in the Universe, the ubiquitous red dwarfs. In this phase of the study, we search 1404 southern M dwarfs within 25 parsecs of the Sun, as reported in Winters et. al 2014, using (Johnson-Kron-Cousins) optical, (2MASS) near-infrared, and (WISE) mid-infrared photometry for circumstellar disks and low-mass companions. Several studies have recently used WISE photometry to detect circumstellar disks and companions --- searches around members of the nearby young moving groups, objects with parallaxes from Hipparcos, and many northern M stars in the SDSS. However, no work has yet been done that focuses on the nearest red dwarfs, which account for at least 75% of all stars. This study, a volume-limited search around M dwarfs in the southern sky, includes statistical conclusions applicable to a majority of the stars in the universe, and opens potential gateways to a better understanding of star and planet formation.

  17. Large-scale environmental dependence of chemical abundances in dwarf galaxies and implications for connecting star formation history and halo mass

    NASA Astrophysics Data System (ADS)

    Douglass, Kelly; Vogeley, Michael S.

    2017-01-01

    We study how the cosmic environment affects galaxy evolution in the Universe by comparing the gas-phase metallicities and abundance ratios of dwarf galaxies in voids with dwarf galaxies in more dense regions. Using spectroscopic observations from Sloan Digital Sky Survey Data Release 7, we estimate the oxygen and nitrogen abundances of 1014 void dwarf galaxies and 787 dwarf galaxies in more dense regions. We develop an alternate calculation for the oxygen abundance that does not use the [OII] λ3727 doublet, permitting oxygen abundance estimates of SDSS dwarf galaxies with the Direct Te method at all redshifts. We find that void dwarf galaxies (Mr > -17) have 9% higher average oxygen abundances and 9% lower average nitrogen abundances than dwarf galaxies in more dense regions. There is a 23% difference in the relative abundances of nitrogen and oxygen in the dwarf galaxies between the two environments. We also find similar N/O abundance ratio shifts in a larger sample (2050 void galaxies and 3883 galaxies in dense regions) of somewhat brighter galaxies (-17 > Mr > -20). These abundance shifts in galaxies fainter than L* may indicate retarded star formation and larger dark matter halo mass to stellar mass ratios in void galaxies, as seen in high-resolution hydrodynamic simulations.

  18. Exploring the substellar IMF in the Taurus cloud. New brown dwarfs in the Taurus star forming region

    NASA Astrophysics Data System (ADS)

    Guieu, S.; Dougados, C.; Monin, J.-L.; Magnier, E.; Mart{í}n, E.

    Recent studies of the substellar population in the Taurus cloud have revealed a deficit of substellar objects by a factor ≃ 2 compared to the Trapezium cluster population (Briceno et al. 1998; Luhman 2000). If confirmed, the higher low-mass cutoff in Taurus could have strong implications on IMF and substellar formation models. However, all studies to date have concentrated on the highest stellar density regions of the Taurus cloud. Reipurth & Clarke (2001) have proposed that brown dwarfs are stellar embryos ejected from their birth site early in their evolution. In order to test this scenario and investigate a possible spatial segregation between stars and brown dwarfs, we have performed a large scale optical survey of the Taurus cloud covering a total area of ≃ 30 deg2 down to a mass detection limits of 15 M_Jup. We first present results from a spectroscopic follow-up of a sub-sample of substellar candidates that revealed 4 new Brown Dwarfs and one Very Low Mass Taurus member. We then discuss the selection of candidates from a larger scale optical survey and the implications for the substellar IMF in Taurus.

  19. 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.

  20. Using red clump stars to correct the Gaia DR1 parallaxes

    NASA Astrophysics Data System (ADS)

    Davies, Guy R.; Lund, Mikkel N.; Miglio, Andrea; Elsworth, Yvonne; Kuszlewicz, James S.; North, Thomas S. H.; Rendle, Ben; Chaplin, William J.; Rodrigues, Thaíse S.; Campante, Tiago L.; Girardi, Léo; Hale, Steven J.; Hall, Oliver; Jones, Caitlin D.; Kawaler, Steven D.; Roxburgh, Ian; Schofield, Mathew

    2017-01-01

    Recent results have suggested that there is tension between the Gaia DR1 TGAS distances and the distances obtained using luminosities determined by eclipsing binaries or asteroseismology on red giant stars. We use the Ks-band luminosities of red clump stars, identified and characterized by asteroseismology, to make independent distance estimates. Our results suggest that Gaia TGAS distances contain a systematic error that decreases with increasing distance. We propose a correction to mitigate this offset as a function of parallax that is valid for the Kepler field and values of parallax that are less than 1.6 mas. For parallaxes greater than this, we find agreement with previously published values. We note that the TGAS distances to the red clump stars of the open cluster M67 show a high level of disagreement that is difficult to correct for.

  1. The mass donor star and the accretion disc of the dwarf nova V2051 Ophiuchi in the infrared

    NASA Astrophysics Data System (ADS)

    Wojcikiewicz, Eduardo; Baptista, Raymundo; Ribeiro, Tiago

    2016-07-01

    We report the analysis of infrared JHK_s high speed photometry of the dwarf nova V2051 Oph in quiescence. We model the ellipsoidal variations in the light curve to measure the fluxes of the mass donor star. Its colors are consistent with an M8 ± 1 spectral type with an equivalent blackbody temperature of T_{bb}= (2700± 300) K, in agreement with spectroscopic measurements and with theoretical expectation for donor stars at the same orbital period. We use the mass donor star fluxes and the Barnes & Evans relation to find a photometric parallax distance of (102 ± 16) pc to the binary. At this distance the outbursts of V2051 Oph occur at disc temperatures everywhere lower than the minimum/critical temperature predicted by the disc instability model, underscoring previous suggestions that they are powered by mass transfer bursts. We subtract the contribution of the mass donor star and apply eclipse mapping techniques to the remaining light curve in order to investigate the structure and emission of its accretion disc. The infrared accretion disc is bright and 'blue' in the inner regions and becomes progressively fainter and redder with increasing radii, indicating that the disc temperature decreases with radius. Bulges in the eclipse shape, more prominent in the H and K_s bands, lead to asymmetric arcs in the eclipse maps reminiscent of the spiral arms found in disc maps of outbursting dwarf novae. The arcs show an azimuthal extent of ˜90^o, extend from the intermediate to the outer disc regions (0.3-0.4 R_{L1}, where R_{L1} is the distance from disc center to the inner lagrangian point), and account for ≃ 30 per cent of the total flux in the H and K_s bands.

  2. 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...

  3. A search for companions to brown dwarfs in the Taurus and Chamaeleon star-forming regions

    SciTech Connect

    Todorov, K. O.; Luhman, K. L.; Konopacky, Q. M.; McLeod, K. K.; Apai, D.; Pascucci, I.; Ghez, A. M.; Robberto, M.

    2014-06-10

    We have used WFPC2 on board the Hubble Space Telescope to obtain images of 47 members of the Taurus and Chamaeleon I star-forming regions that have spectral types of M6-L0 (M ∼ 0.01-0.1 M {sub ☉}). An additional late-type member of Taurus, FU Tau (M7.25+M9.25), was also observed with adaptive optics at Keck Observatory. In these images, we have identified promising candidate companions to 2MASS J04414489+2301513 (ρ = 0.''105/15 AU), 2MASS J04221332+1934392 (ρ = 0.''05/7 AU), and ISO 217 (ρ = 0.''03/5 AU). We reported the first candidate in a previous study, showing that it has a similar proper motion as the primary in images from WFPC2 and Gemini adaptive optics. We have collected an additional epoch of data with Gemini that further supports that result. By combining our survey with previous high-resolution imaging in Taurus, Chamaeleon I, and Upper Sco (τ ∼ 10 Myr), we measure binary fractions of 14/93 = 0.15{sub −0.03}{sup +0.05} for M4-M6 (M ∼ 0.1-0.3 M {sub ☉}) and 4/108 = 0.04{sub −0.01}{sup +0.03} for >M6 (M ≲ 0.1 M {sub ☉}) at separations of >10 AU. Given the youth and low density of these regions, the lower binary fraction at later types is probably primordial rather than due to dynamical interactions among association members. The widest low-mass binaries (>100 AU) also appear to be more common in Taurus and Chamaeleon I than in the field, which suggests that the widest low-mass binaries are disrupted by dynamical interactions at >10 Myr, or that field brown dwarfs have been born predominantly in denser clusters where wide systems are disrupted or inhibited from forming.

  4. A Search for Companions to Brown Dwarfs in the Taurus and Chamaeleon Star-Forming Regions

    NASA Astrophysics Data System (ADS)

    Todorov, K. O.; Luhman, K. L.; Konopacky, Q. M.; McLeod, K. K.; Apai, D.; Ghez, A. M.; Pascucci, I.; Robberto, M.

    2014-06-01

    We have used WFPC2 on board the Hubble Space Telescope to obtain images of 47 members of the Taurus and Chamaeleon I star-forming regions that have spectral types of M6-L0 (M ~ 0.01-0.1 M ⊙). An additional late-type member of Taurus, FU Tau (M7.25+M9.25), was also observed with adaptive optics at Keck Observatory. In these images, we have identified promising candidate companions to 2MASS J04414489+2301513 (ρ = 0.''105/15 AU), 2MASS J04221332+1934392 (ρ = 0.''05/7 AU), and ISO 217 (ρ = 0.''03/5 AU). We reported the first candidate in a previous study, showing that it has a similar proper motion as the primary in images from WFPC2 and Gemini adaptive optics. We have collected an additional epoch of data with Gemini that further supports that result. By combining our survey with previous high-resolution imaging in Taurus, Chamaeleon I, and Upper Sco (τ ~ 10 Myr), we measure binary fractions of 14/93 = 0.15^{+0.05}_{-0.03} for M4-M6 (M ~ 0.1-0.3 M ⊙) and 4/108 = 0.04^{+0.03}_{-0.01} for >M6 (M <~ 0.1 M ⊙) at separations of >10 AU. Given the youth and low density of these regions, the lower binary fraction at later types is probably primordial rather than due to dynamical interactions among association members. The widest low-mass binaries (>100 AU) also appear to be more common in Taurus and Chamaeleon I than in the field, which suggests that the widest low-mass binaries are disrupted by dynamical interactions at >10 Myr, or that field brown dwarfs have been born predominantly in denser clusters where wide systems are disrupted or inhibited from forming. Based on observations performed with the NASA/ESA Hubble Space Telescope, Gemini Observatory, and the W. M. Keck Observatory. The Hubble observations are associated with proposal IDs 11203, 11204, and 11983 and were 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.

  5. 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.

  6. The mass-ratio and eccentricity distributions of barium and S stars, and red giants in open clusters

    NASA Astrophysics Data System (ADS)

    Van der Swaelmen, M.; Boffin, H. M. J.; Jorissen, A.; Van Eck, S.

    2017-01-01

    Context. A complete set of orbital parameters for barium stars, including the longest orbits, has recently been obtained thanks to a radial-velocity monitoring with the HERMES spectrograph installed on the Flemish Mercator telescope. Barium stars are supposed to belong to post-mass-transfer systems. Aims: In order to identify diagnostics distinguishing between pre- and post-mass-transfer systems, the properties of barium stars (more precisely their mass-function distribution and their period-eccentricity (P-e) diagram) are compared to those of binary red giants in open clusters. As a side product, we aim to identify possible post-mass-transfer systems among the cluster giants from the presence of s-process overabundances. We investigate the relation between the s-process enrichment, the location in the (P-e) diagram, and the cluster metallicity and turn-off mass. Methods: To invert the mass-function distribution and derive the mass-ratio distribution, we used the method pioneered by Boffin et al. (1992) that relies on a Richardson-Lucy deconvolution algorithm. The derivation of s-process abundances in the open-cluster giants was performed through spectral synthesis with MARCS model atmospheres. Results: A fraction of 22% of post-mass-transfer systems is found among the cluster binary giants (with companion masses between 0.58 and 0.87 M⊙, typical for white dwarfs), and these systems occupy a wider area than barium stars in the (P-e) diagram. Barium stars have on average lower eccentricities at a given orbital period. When the sample of binary giant stars in clusters is restricted to the subsample of systems occupying the same locus as the barium stars in the (P-e) diagram, and with a mass function compatible with a WD companion, 33% (=4/12) show a chemical signature of mass transfer in the form of s-process overabundances (from rather moderate - about 0.3 dex - to more extreme - about 1 dex). The only strong barium star in our sample is found in the cluster with

  7. The Herschel Virgo Cluster Survey. V. Star-forming dwarf galaxies - dust in metal-poor environments

    NASA Astrophysics Data System (ADS)

    Grossi, M.; Hunt, L. K.; Madden, S.; Vlahakis, C.; Bomans, D. J.; Baes, M.; Bendo, G. J.; Bianchi, S.; Boselli, A.; Clemens, M.; Corbelli, E.; Cortese, L.; Dariush, A.; Davies, J. I.; De Looze, I.; di Serego Alighieri, S.; Fadda, D.; Fritz, J.; Garcia-Appadoo, D. A.; Gavazzi, G.; Giovanardi, C.; Hughes, T. M.; Jones, A. P.; Pierini, D.; Pohlen, M.; Sabatini, S.; Smith, M. W. L.; Verstappen, J.; Xilouris, E. M.; Zibetti, S.

    2010-07-01

    We present the dust properties of a small sample of Virgo cluster dwarf galaxies drawn from the science demonstration phase data set of the Herschel Virgo Cluster Survey. These galaxies have low metallicities (7.8 < 12 + log(O/H) < 8.3) and star-formation rates ≲10-1 M⊙ yr-1. We measure the spectral energy distribution (SED) from 100 to 500 μm and derive dust temperatures and dust masses. The SEDs are fitted by a cool component of temperature T ≲ 20 K, implying dust masses around 105 M⊙ and dust-to-gas ratios D within the range 10-3-10-2. The completion of the full survey will yield a larger set of galaxies, which will provide more stringent constraints on the dust content of star-forming dwarf galaxies. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.

  8. Limits on a gravitational field dependence of the proton-electron mass ratio from H2 in white dwarf stars.

    PubMed

    Bagdonaite, J; Salumbides, E J; Preval, S P; Barstow, M A; Barrow, J D; Murphy, M T; Ubachs, W

    2014-09-19

    Spectra of molecular hydrogen (H2) are employed to search for a possible proton-to-electron mass ratio (μ) dependence on gravity. The Lyman transitions of H2, observed with the Hubble Space Telescope towards white dwarf stars that underwent a gravitational collapse, are compared to accurate laboratory spectra taking into account the high temperature conditions (T∼13 000  K) of their photospheres. We derive sensitivity coefficients Ki which define how the individual H2 transitions shift due to μ dependence. The spectrum of white dwarf star GD133 yields a Δμ/μ constraint of (-2.7±4.7stat±0.2syst)×10(-5) for a local environment of a gravitational potential ϕ∼10(4) ϕEarth, while that of G29-38 yields Δμ/μ=(-5.8±3.8stat±0.3syst)×10(-5) for a potential of 2×10(4) ϕEarth.

  9. The type Ia supernova SNLS-03D3bb from a super-Chandrasekhar-mass white dwarf star.

    PubMed

    Howell, D Andrew; Sullivan, Mark; Nugent, Peter E; Ellis, Richard S; Conley, Alexander J; Le Borgne, Damien; Carlberg, Raymond G; Guy, Julien; Balam, David; Basa, Stephane; Fouchez, Dominique; Hook, Isobel M; Hsiao, Eric Y; Neill, James D; Pain, Reynald; Perrett, Kathryn M; Pritchet, Christopher J

    2006-09-21

    The accelerating expansion of the Universe, and the need for dark energy, were inferred from observations of type Ia supernovae. There is a consensus that type Ia supernovae are thermonuclear explosions that destroy carbon-oxygen white dwarf stars that have accreted matter from a companion star, although the nature of this companion remains uncertain. These supernovae are thought to be reliable distance indicators because they have a standard amount of fuel and a uniform trigger: they are predicted to explode when the mass of the white dwarf nears the Chandrasekhar mass of 1.4 solar masses (M(o)). Here we show that the high-redshift supernova SNLS-03D3bb has an exceptionally high luminosity and low kinetic energy that both imply a super-Chandrasekhar-mass progenitor. Super-Chandrasekhar-mass supernovae should occur preferentially in a young stellar population, so this may provide an explanation for the observed trend that overluminous type Ia supernovae occur only in 'young' environments. As this supernova does not obey the relations that allow type Ia supernovae to be calibrated as standard candles, and as no counterparts have been found at low redshift, future cosmology studies will have to consider possible contamination from such events.

  10. The type Ia supernova SNLS-03D3bb from a super-Chandrasekhar-masswhite dwarf star

    SciTech Connect

    Howell, D.Andrew; Sullivan, Mark; Nugent, Peter E.; Ellis,Richard S.; Conley, Alexander J.; Le Borgne, Damien; Carlberg, RaymondG.; Guy, Julien; Balam, David; Basa, Stephane; Fouchez, Dominique; Hook,Isobel M.; Hsiao, Eric Y.; Neill, James D.; Pain, Reynald; Perrett,Kathryn M.; Pritchet, Christopher J.

    2006-02-01

    The acceleration of the expansion of the universe, and theneed for Dark Energy, were inferred from the observations of Type Iasupernovae (SNe Ia) 1;2. There is consensus that SNeIa are thermonuclearexplosions that destroy carbon-oxygen white dwarf stars that accretematter from a companion star3, although the nature of this companionremains uncertain. SNe Ia are thought to be reliable distance indicatorsbecause they have a standard amount of fuel and a uniform trigger theyare predicted to explode when the mass of the white dwarf nears theChandrasekhar mass 4 - 1.4 solar masses. Here we show that the highredshift supernova SNLS-03D3bb has an exceptionally high luminosity andlow kinetic energy that both imply a super-Chandrasekhar mass progenitor.Super-Chandrasekhar mass SNeIa shouldpreferentially occur in a youngstellar population, so this may provide an explanation for the observedtrend that overluminous SNe Ia only occur in young environments5;6. Sincethis supernova does not obey the relations that allow them to becalibrated as standard candles, and since no counterparts have been foundat low redshift, future cosmology studies will have to considercontamination from such events.

  11. 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. C.; Ferguson, H. C.; Scarlata, C.; Hathi, N. P.; Dunlop, J. S.; Newman, J. A.; Dickinson, M.; Jahnke, K.; Salmon, B. W.; deMello, D. F.; Kkocevski, D. D.; Lai, K.; Grogin, N. A.; Rodney, S. A.; Guo, Yicheng

    2012-01-01

    We identify an abundant population of extreme emission line galaxies (EELGs) at redshift z approx. 1.7 in the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey (CANDELS) imaging from Hubble Space Telescope/Wide Field Camera 3 (HST/WFC3). 69 EELG candidates are selected by the large contribution of exceptionally bright emission lines to their near-infrared broad-band magnitudes. Supported by spectroscopic confirmation of strong [OIII] emission lines . with rest-frame equivalent widths approx. 1000A in the four candidates that have HST/WFC3 grism observations, we conclude that these objects are galaxies with approx.10(exp 8) Solar Mass in stellar mass, undergoing an enormous starburst phase with M*/M* of only approx. 15 Myr. These bursts may cause outflows that are strong enough to produce cored dark matter profiles in low-mass galaxies. The individual star formation rates and the co-moving number density (3.7x10(exp -4) Mpc(sup -3) can produce in approx.4 Gyr much of the stellar mass density that is presently contained in 10(exp 8) - 10(exp 9) Solar Mass dwarf galaxies. Therefore, our observations provide a strong indication that many or even most of the stars in present-day dwarf galaxies formed in strong, short-lived bursts, mostly at z > 1.

  12. PHYSICAL PROPERTIES OF YOUNG BROWN DWARFS AND VERY LOW MASS STARS INFERRED FROM HIGH-RESOLUTION MODEL SPECTRA

    SciTech Connect

    Rice, Emily L.; Mclean, Ian S.; Barman, T.; Prato, L.; Kirkpatrick, J. Davy

    2010-01-01

    By comparing near-infrared spectra with atmospheric models, we infer the effective temperature, surface gravity, projected rotational velocity, and radial velocity for 21 very low mass stars and brown dwarfs. The unique sample consists of two sequences in spectral type from M6-M9, one of 5-10 Myr objects and one of >1 Gyr field objects. A third sequence is comprised of only {approx}M6 objects with ages ranging from <1 Myr to >1 Gyr. Spectra were obtained in the J band at medium (R {approx} 2000) and high (R {approx} 20,000) resolutions with NIRSPEC on the Keck II telescope. Synthetic spectra were generated from atmospheric structures calculated with the PHOENIX model atmosphere code. Using multi-dimensional least-squares fitting and Monte Carlo routines we determine the best-fit model parameters for each observed spectrum and note which spectral regions provide consistent results. We identify successes in the reproduction of observed features by atmospheric models, including pressure-broadened K I lines, and investigate deficiencies in the models, particularly missing FeH opacity, that will need to be addressed in order to extend our analysis to cooler objects. The precision that can be obtained for each parameter using medium- and high-resolution near-infrared spectra is estimated and the implications for future studies of very low mass stars and brown dwarfs are discussed.

  13. The Convection of Close Red Supergiant Stars Observed With Near-Infrared Interferometry

    NASA Astrophysics Data System (ADS)

    Montargès, M.; Kervella, P.; Perrin, G.; Chiavassa, A.; Aurière, M.

    2015-12-01

    Our team has obtained observations of the photosphere of the two closest red supergiant stars Betelgeuse (α Ori) and Antares (α Sco) using near infrared interferometry. We have been monitoring the photosphere of Betelgeuse with the VLTI/PIONIER instrument for three years. On Antares, we obtained an unprecedented sampling of the visibility function. These data allow us to probe the convective photosphere of massive evolved stars.

  14. Is mass loss from red giant stars dust driven?

    NASA Astrophysics Data System (ADS)

    Yates, J. A.

    1992-12-01

    Long period variable stars on the Asymptotic Giant Branch are observed to be losing mass in the form of cool dusty molecular stellar winds at rates from 10-7 to 10-4 Msunyr-1. The driving force for this mass loss is thought to be radiation pressure on dust particles. The dust transfers its momentum to gas molecules via collisions. This paper discusses the existing evidence for this scenario. New results, from analysis of 22 GHz H2O maser observations made by Merlin, show that the cruical acceleration past the stellar escape velocity of the central star takes place in the inner circumstellar envelope around the central star. The analysis of the velocity fields of the circumstellar envelopes of VX Sgr and VY CMa using the model described by Chapman and Cohen (1986) are discussed.

  15. A study of two high-velocity red horizontal branch stars

    NASA Astrophysics Data System (ADS)

    Pereira, C. B.; Jilinski, E. G.; Drake, N. A.; Ortega, V. G.; Roig, F.

    2013-11-01

    Context. High-velocity halo stars provide important information about the properties of the extreme Galactic halo. The study of Population II unbound and bound stars enables us better estimate the mass of the halo. Aims: We carried out a detailed spectroscopic and kinematic study of two red horizontal branch stars, CD-41°15048 and HD 214362. Methods: The atmospheric parameters, chemical abundances, and kinematical properties were determined using high-resolution optical spectroscopy and employing the local thermodynamic equilibrium model atmospheres of Kurucz and the spectral analysis code moog. Results: We found that CD-41°15048 and HD 214362 are metal-poor red horizontal branch stars. Their abundance patterns are similar to other metal-poor halo stars already investigated. Our kinematic analysis of the 3D space motions shows that HD 214362 has a highly eccentric (e = 0.95) orbit and passes at 2 kpc from the Galactic center. CD-41°15048, for an adopted distance of 1.3 kpc, has an extreme retrograde motion and travels with very high velocity relative to the Galactocentric reference frame (VGRF = 583 km s-1). Conclusions: CD-41°15048 is a bound or an unbound star, depending on the adopted Galactic potential. We also show that the red horizontal branch star BD+09°3223 is another example of a hypervelocity star. Whether it is bound or unbound to the Galaxy depends on the assumed mass and adopted Galactic potential. Possible origins of these two high-velocity stars are briefly discussed. CD-41°15048 and BD+09°3223 are further examples of evolved stars to join the restricted group of hypervelocity stars. Finally, our results seem to contradict the idea that a passage of a star very close to the Galactic center is the only possible origin of hypervelocity stars. Based on observations made with the 2.2 m telescope at the European Southern Observatory (La Silla, Chile) under the agreement between ESO and Observatório Nacional (2007-2010).Figures 3-5, 8, 10

  16. Dwarf Galaxies Swimming in Tidal Tails

    NASA Technical Reports Server (NTRS)

    2005-01-01

    This false-color infrared image from NASA's Spitzer Space Telescope shows little 'dwarf galaxies' forming in the 'tails' of two larger galaxies that are colliding together. The big galaxies are at the center of the picture, while the dwarfs can be seen as red dots in the red streamers, or tidal tails. The two blue dots above the big galaxies are stars in the foreground.

    Galaxy mergers are common occurrences in the universe; for example, our own Milky Way galaxy will eventually smash into the nearby Andromeda galaxy. When two galaxies meet, they tend to rip each other apart, leaving a trail, called a tidal tail, of gas and dust in their wake. It is out of this galactic debris that new dwarf galaxies are born.

    The new Spitzer picture demonstrates that these particular dwarfs are actively forming stars. The red color indicates the presence of dust produced in star-forming regions, including organic molecules called polycyclic aromatic hydrocarbons. These carbon-containing molecules are also found on Earth, in car exhaust and on burnt toast, among other places. Here, the molecules are being heated up by the young stars, and, as a result, shine in infrared light.

    This image was taken by the infrared array camera on Spitzer. It is a 4-color composite of infrared light, showing emissions from wavelengths of 3.6 microns (blue), 4.5 microns (green), 5.8 microns (orange), and 8.0 microns (red). Starlight has been subtracted from the orange and red channels in order to enhance the dust features.

  17. A numerical study of the stability of radiative shocks. [in accretion flows onto white dwarf stars

    NASA Technical Reports Server (NTRS)

    Imamura, J. N.; Wolff, M. T.; Durisen, R. H.

    1984-01-01

    Attention is given to the oscillatory instability of optically thin radiative shocks in time-dependent numerical calculations of accretion flows onto degenerate dwarfs. The present nonlinear calculations yield good quantitative agreement with the linear results obtained for oscillation frequencies, damping rates, and critical alpha-values. The fundamental mode and the first overtone in the shock radius and luminosity variations can be clearly identified, and evidence is sometimes seen for the second overtone. Time-dependent calculations are also performed which include additional physics relevant to degenerate dwarf accretion, such as electron thermal conduction, unequal electron and ion temperatures, Compton cooling, and relativistic corrections to the bremsstrahlung cooling law. All oscillatory modes are found to be damped, and hence stable, in the case of a 1-solar mass white dwarf accreting in spherical symmetry.

  18. An expanded set of brown dwarf and very low mass star models

    <