* Astronomy

Title: Cool Companions to White Dwarf Stars from the Two Micron All Sky Survey All Sky Data Release
Authors: D. W. Hoard (1), S. Wachter (1), Laura K. Sturch (1,2), Allison M. Widhalm (1,3,4), Kevin P. Weiler (1,5,6), Magaretha L. Pretorius (1,7), Joseph W. Wellhouse (1,2,4), Maxsim Gibiansky (1,2) ((1) Spitzer Science Center, California Institute of Technology, (2) Harvey Mudd College, (3) University of Southern California, (4) New Mexico State University, (5) Marquette University, (6) DePaul University, (7) University of Southampton)

We present the culmination of our near-infrared survey of the optically spectroscopically identified white dwarf stars from the McCook & Sion catalogue, conducted using photometric data from the Two Micron All Sky Survey final All Sky Data Release. The colour-selection technique, which identifies candidate binaries containing a white dwarf and a low mass stellar (or sub-stellar) companion via their distinctive locus in the near-infrared colour-colour diagram, is demonstrated to be simple to apply and to yield candidates with a high rate of subsequent confirmation. We recover 105 confirmed binaries, and identify 28 firm candidates (20 of which are new to this work) and 21 tentative candidates (17 of which are new to this work) from the 2MASS data. Only a small number of candidates from our survey have likely companion spectral types later than M5, none of which is an obvious L type (i.e., potential brown dwarf) companion. Only one previously known WD + brown dwarf binary is detected. This result is discussed in the context of the 2MASS detection limits, as well as other recent observational surveys that suggest a very low rate of formation (or survival) for binary stars with extreme mass ratios.

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Title: The Lowest Mass White Dwarf
Authors: Mukremin Kilic, Carlos Allende Prieto, Warren R. Brown, D. Koester
(revised v2)

Extremely low mass white dwarfs are very rare objects likely formed in compact binary systems. We present MMT optical spectroscopy of 42 low mass white dwarf candidates serendipitously discovered in a survey for hypervelocity B-type stars. One of these objects, SDSS J0917+46, has Teff= 11,288 ± 72 K and log g = 5.48 ± 0.03; with an estimated mass of 0.17 M_sun, it is the lowest gravity/mass white dwarf currently known. However, 40 of the low mass candidates are normal DA white dwarfs with apparently inaccurate SDSS g magnitudes. We revisit the identification of low mass white dwarf candidates previously found in the SDSS, and conclude that four objects have M < 0.2 M_sun. None of these white dwarfs show excess emission from a binary companion, and radial velocity searches will be necessary to constrain the nature of the unseen companions.

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Title: A Dusty Disk Around WD1150-153: Explaining the Metals in White Dwarfs by Accretion from the Interstellar Medium versus Debris Disks
Authors: Mukremin Kilic (Ohio State), Seth Redfield (Texas)

We report the discovery of excess K-band radiation from a metal-rich DAV white dwarf star, WD1150-153. Our near infrared spectroscopic observations show that the excess radiation cannot be explained by a (sub)stellar companion, and is likely to be caused by a debris disk similar to the other DAZ white dwarfs with circumstellar debris disks. We find that the fraction of DAZ white dwarfs with detectable debris disks is at least 14%. We also revisit the problem of explaining the metals in white dwarf photospheres by accretion from the interstellar medium (ISM). We use the observed interstellar column densities toward stars in close angular proximity and similar distance as DAZ white dwarfs to constrain the contribution of accretion from the ISM. We find no correlation between the accretion density required to supply metals observed in DAZs with the densities observed in their interstellar environment, indicating that ISM accretion alone cannot explain the presence of metals in nearby DAZ white dwarfs. Although ISM accretion will certainly contribute, our analysis indicates that it is not the dominant source of metals for most DAZ white dwarfs. Instead, the growing number of circumstellar debris disks around DAZs suggests that circumstellar material may play a more dominant role in polluting the white dwarf atmospheres.

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Title: A Spitzer White Dwarf Infrared Survey
Authors: F. Mullally, Mukremin Kilic, William T. Reach, Marc J. Kuchner, Ted von Hippel, Adam Burrows, D. E. Winget

We present mid-infrared photometry of 124 white dwarf stars with Spitzer Space Telescope. Objects were observed simultaneously at 4.5 and 8.0um with sensitivities better than 1 mJy. This data can be used to test models of white dwarf atmospheres in a new wavelength regime, as well as to search for planetary companions and debris disks.

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