208 research outputs found
Ab initio Stellar Astrophysics: Reliable Modeling of Cool White Dwarf Atmospheres
Over the last decade {\it ab initio} modeling of material properties has
become widespread in diverse fields of research. It has proved to be a powerful
tool for predicting various properties of matter under extreme conditions. We
apply modern computational chemistry and materials science methods, including
density functional theory (DFT), to solve lingering problems in the modeling of
the dense atmospheres of cool white dwarfs (). Our
work on the revision and improvements of the absorption mechanisms in the
hydrogen and helium dominated atmospheres resulted in a new set of atmosphere
models. By inclusion of the Ly- red wing opacity we successfully
fitted the entire spectral energy distributions of known cool DA stars. In the
subsequent work we fitted the majority of the coolest stars with hydrogen-rich
models. This finding challenges our understanding of the spectral evolution of
cool white dwarfs. We discuss a few examples, including the cool companion to
the pulsar PSR J0437-4715. The two problems important for the understanding of
cool white dwarfs are the behavior of negative hydrogen ion and molecular
carbon in a fluid-like, helium dominated medium. Using {\it ab initio} methods
we investigate the stability and opacity of these two species in dense helium.
Our investigation of indicates that the absorption features observed
in the ``peculiar'' DQp white dwarfs resemble the absorption of perturbed in dense helium.Comment: 6 pages, 4 figures, submitted to proceedings of 17th European White
Dwarf Workshop, Tuebingen, Germany 201
Moving Objects in the Hubble Ultra Deep Field
We identify proper motion objects in the Hubble Ultra Deep Field (UDF) using
the optical data from the original UDF program in 2004 and the near-infrared
data from the 128-orbit UDF 2012 campaign. There are 12 sources brighter than
I=27 mag that display >3sigma significant proper motions. We do not find any
proper motion objects fainter than this magnitude limit. Combining optical and
near-infrared photometry, we model the spectral energy distribution of each
point-source using stellar templates and state-of-the-art white dwarf models.
For I<27 mag, we identify 23 stars with K0-M6 spectral types and two faint blue
objects that are clearly old, thick disk white dwarfs. We measure a thick disk
white dwarf space density of 0.1-1.7 E-3 per cubic parsec from these two
objects. There are no halo white dwarfs in the UDF down to I=27 mag. Combining
the Hubble Deep Field North, South, and the UDF data, we do not see any
evidence for dark matter in the form of faint halo white dwarfs, and the
observed population of white dwarfs can be explained with the standard Galactic
models.Comment: ApJ, in pres
The kinematics of Galactic disc white dwarfs in Gaia DR2
We present an analysis of the kinematics of Galactic disc white dwarf stars
in the Solar neighbourhood using data from Gaia Data Release 2. Selection of
white dwarfs based on parallax provides the first large, kinematically unbiased
sample of Solar neighbourhood white dwarfs to date. Various classical
properties of the Solar neighbourhood kinematics have been detected for the
first time in the WD population.
The disc white dwarf population exhibits a correlation between absolute
magnitude and mean age, which we exploit to obtain an independent estimate of
the Solar motion with respect to the Local Standard of Rest. This is found to
be kms. The
components agree with studies based on main sequence stars, however the
component differs and may be affected by systematics arising from metallicity
gradients in the disc. The velocity ellipsoid is shown to vary strongly with
magnitude, and exhibits a significant vertex deviation in the plane of
around 15 degrees, due to the non-axisymmetric Galactic potential.
The results of this study provide an important input to proper motion surveys
for white dwarfs, which require knowledge of the velocity distribution in order
to correct for missing low velocity stars that are culled from the sample to
reduce subdwarf contamination.Comment: Accepted for publication in MNRA
Faint Blue Objects in the Hubble Deep Field–South Revealed: White Dwarfs, Subdwarfs, and Quasars
We explore the nature of the faint blue objects in the Hubble Deep Field–South. We have derived proper motions for the point sources in the Hubble Deep Field–South using a 3 yr baseline. Combining our proper-motion measurements with spectral energy distribution fitting enabled us to identify four quasars and 42 stars, including three white dwarf candidates. Two of these white dwarf candidates, HDF-S 1444 and 895, are found to display significant proper motion, 21:1±7:9 and 34:9 ± 8:0 mas yr -1, and are consistent with being thick-disk or halo white dwarfs located at ̴2 kpc. The other faint blue objects analyzed by Mendez & Minniti do not show any significant proper motion and are inconsistent with being halo white dwarfs; they do not contribute to the Galactic dark matter. The observed population of stars and white dwarfs is consistent with standard Galactic models
The First Mid-Infrared Spectra of Cool White Dwarfs
We present the first mid-infrared spectra of two cool white dwarfs obtained with the Spitzer Space Telescope. We also present 3.5–8 μ m photometry for 19 cool white dwarfs with 5000 K ≤Teff ≤ 9000 K. We perform a detailed model atmosphere analysis of these white dwarfs by fitting their UBVRIJHK and Spitzer photometry with state-of-the-art model atmospheres, and demonstrate that the optical and infrared spectral energy distributions of cool white dwarfs are well reproduced by our grid of models. Our mid-infrared photometry and 7.5-14.5 μm spectrum of WD 0018–267 are consistent with a Teff = 5720 K, pure hydrogen white dwarf model atmosphere. On the other hand, LHS 1126 remains peculiar, with significant mid-infrared flux deficits in all IRAC bands and a featureless spectrum in the 5.2-7.5 μm range. Even though this deficit is attributed to collision-induced absorption (CIA) due to molecular hydrogen, the shape of the deficit cannot be explained with current CIA opacity calculations. The infrared portion of the LHS 1126 spectral energy distribution is best fit with a power-law index of –1.99, identical to a Rayleigh-Jeans spectrum. This argues that the deficit may be due to an unrecognized graylike opacity source in the infrared
The Mystery Deepens: Spitzer Observations of Cool White Dwarfs
We present 4.5 and 8 µm photometric observations of 18 cool white dwarfs obtained with the Spitzer Space Telescope. Our observations demonstrate that four white dwarfs with Teᶠᶠ\u3c 6000 K show slightly depressed mid-infrared fluxes relative to white dwarf models. In addition, another white dwarf with a peculiar optical and near-infrared spectral energy distribution (LHS 1126) is found to display significant flux deficits in Spitzer observations. These mid-infrared flux deficits are not predicted by the current white dwarf models including collision-induced absorption due to molecular hydrogen. We postulate that either the collision-induced absorption calculations are incomplete or there are other unrecognized physical processes occurring in cool white dwarf atmospheres. The spectral energy distribution of LHS 1126 surprisingly fits a Rayleigh-Jeans spectrum in the infrared, mimicking a hot white dwarf with effective temperature well in excess of 105 K. This implies that the source of this flux deficit is probably not molecular absorption but some other process
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