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Posts: 131433
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RE: White Dwarfs binaries
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Title: Astrophysics of white dwarf binaries
Authors: G. Nelemans

White dwarf binaries are the most common compact binaries in the Universe and are especially important for low-frequency gravitational wave detectors such as LISA. There are a number of open questions about binary evolution and the Galactic population of white dwarf binaries that can be solved using gravitational wave data and at the same time, our ever improving knowledge about these binaries will help to predict the signals that can be expected for LISA. In addition a number of white dwarf binaries will serve as verification sources for the instrument. I will discuss these issues and report recent, surprising, developments in this field. Finally I report calculations about the feasibility of complementary electro-magnetic observations which unfortunately cannot reproduce the optimistic results of Cooray et al. (2004).

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Posts: 131433
Date:
White Dwarf Stars
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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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Posts: 131433
Date:
RE: White Dwarfs
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Title: White Dwarfs in GALEX Survey
Authors: Adela Kawka, Stephane Vennes

We have cross-correlated the 2dF QSO Redshift Survey (2QZ) white dwarf catalogue with the GALEX 2nd Data Release and the Sloan Digital Sky Survey (SDSS) data release 5 to obtain ultraviolet photometry (FUV, NUV) for approximately 700 objects and optical photometry (ugriz) for approximately 800 objects. We have compared the optical-ultraviolet colours to synthetic white dwarf colours to obtain temperature estimates for approximately 250 of these objects. These white dwarfs have effective temperatures ranging from 10 000 K (cooling age of about 1Gyr) up to about 40 000 K (cooling age of about 3 Myrs), with a few that have even higher temperatures. We found that to distinguish white dwarfs from other stellar luminosity classes both optical and ultraviolet colours are necessary, in particular for the hotter objects where there is contamination from B and O main-sequence stars. Using this sample we build a luminosity function for the DA white dwarfs with M_V < 12 mag.

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Posts: 131433
Date:
Lowest Mass White Dwarf
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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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Posts: 131433
Date:
RE: White Dwarfs
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Title: The age and colours of massive white dwarf stars
Authors: L. G. Althaus, E. García-Berro, J. Isern, A. H. Córsico, R. D. Rohrmann

We present evolutionary calculations and colours for massive white dwarfs with oxygen-neon cores for masses between 1.06 and 1.28 Mo. The evolutionary stages computed cover the luminosity range from log(L/Lo) approx. 0.5 down to -5.2. Our cooling sequences are based on evolutionary calculations that take into account the chemical composition expected from massive white dwarf progenitors that burned carbon in partially degenerate conditions. The use of detailed non-grey model atmospheres provides us with accurate outer boundary conditions for our evolving models at low effective temperatures. We examine the cooling age, colours and magnitudes of our sequences. We find that massive white dwarfs are characterized by very short ages to such an extent that they reach the turn-off in their colours and become blue at ages well below 10 Gyr. Extensive tabulations for massive white dwarfs, accessible from our web site, are also presented.

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Posts: 131433
Date:
Dusty Disk Around WD1150-153
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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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Posts: 131433
Date:
R Coronae Borealis (RCB) stars
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Astronomers have announced the discovery of huge quantities of an unusual variety of oxygen in two very rare types of stars. The finding suggests that the origin of these oddball stars may lie in the physics behind the mergers of white dwarf star pairs.
The unusual stars are known as hydrogen-deficient (HdC) and R Coronae Borealis (RCB) stars. Both types have almost no hydrogen - an element that makes up about 90% of most stars. Surprisingly, they contain up to a thousand times more of the isotope oxygen-18 than normal stars like our Sun. The discovery of abnormal quantities of oxygen-18 is based on near-infrared spectroscopic observations from the Gemini Near-Infrared Spectrograph (GNIRS) on the 8-meter Gemini-South telescope in Chile.

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Posts: 131433
Date:
RE: White Dwarfs
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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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L

Posts: 131433
Date:
SDSS J0917+46
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Title: The Lowest Mass White Dwarf
Authors: Mukremin Kilic, Carlos Allende Prieto, Warren R. Brown, D. Koester

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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Posts: 131433
Date:
RE: White Dwarfs
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Title: Spitzer White Dwarf Planet Limits
Authors: F. Mullally, Ted von Hippel, D. E. Winget

We present preliminary limits on the presence of planets around white dwarf stars using the IRAC photometer on the Spitzer space telescope. Planets emit strongly in the mid-infrared which allows their presence to be detected as an excess at these wavelengths. We place limits of 5 M_J for 8 stars assuming ages of 1 Gyr, and 10 M_J for 23 stars. We describe our survey, present our results and comment on approaches to improve our methodology.

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