4,234 research outputs found
Pure-hydrogen 3D model atmospheres of cool white dwarfs
A sequence of pure-hydrogen CO5BOLD 3D model atmospheres of DA white dwarfs
is presented for a surface gravity of log g = 8 and effective temperatures from
6000 to 13,000 K. We show that convective properties, such as flow velocities,
characteristic granulation size and intensity contrast of the granulation
patterns, change significantly over this range. We demonstrate that these 3D
simulations are not sensitive to numerical parameters unlike the 1D structures
that considerably depend on the mixing-length parameters. We conclude that 3D
spectra can be used directly in the spectroscopic analyses of DA white dwarfs.
We confirm the result of an earlier preliminary study that 3D model spectra
provide a much better characterization of the mass distribution of white dwarfs
and that shortcomings of the 1D mixing-length theory are responsible for the
spurious high-log g determinations of cool white dwarfs. In particular, the 1D
theory is unable to account for the cooling effect of the convective overshoot
in the upper atmospheres.Comment: 14 pages, 17 figures, accepted for publication in Astronomy and
Astrophysic
Cool White Dwarfs Found in the UKIRT Infrared Deep Sky Survey
We present the results of a search for cool white dwarfs in the United
Kingdom InfraRed Telescope (UKIRT) Infrared Deep Sky Survey (UKIDSS) Large Area
Survey (LAS). The UKIDSS LAS photometry was paired with the Sloan Digital Sky
Survey (SDSS) to identify cool hydrogen-rich white dwarf candidates by their
neutral optical colors and blue near-infrared colors, as well as faint Reduced
Proper Motion magnitudes. Optical spectroscopy was obtained at Gemini
Observatory, and showed the majority of the candidates to be newly identified
cool degenerates, with a small number of G- to K-type (sub)dwarf contaminants.
Our initial search of 280 deg2 of sky resulted in seven new white dwarfs with
effective temperature T_eff ~ 6000 K. The current followup of 1400 deg2 of sky
has produced thirteen new white dwarfs. Model fits to the photometry show that
seven of the newly identified white dwarfs have 4120 K <= T_eff <= 4480 K, and
cooling ages between 7.3 Gyr and 8.7 Gyr; they have 40 km/s <= v_tan <= 85 km/s
and are likely to be thick disk 10-11 Gyr-old objects. The other half of the
sample has 4610 K <= T_eff <= 5260 K, cooling ages between 4.3 Gyr and 6.9 Gyr,
and 60 km/s <= v_tan <= 100 km/s. These are either thin disk remnants with
unusually high velocities, or lower-mass remnants of thick disk or halo late-F
or G stars.Comment: To appear in ApJ, accepted April 18 2011. 34 pages include 11 Figures
and 5 Table
Spectroscopic analysis of DA white dwarfs with 3D model atmospheres
We present the first grid of mean three-dimensional (3D) spectra for
pure-hydrogen (DA) white dwarfs based on 3D model atmospheres. We use CO5BOLD
radiation-hydrodynamics 3D simulations instead of the mixing-length theory for
the treatment of convection. The simulations cover the effective temperature
range of 6000 < Teff (K) < 15,000 and the surface gravity range of 7 < log g <
9 where the large majority of DAs with a convective atmosphere are located. We
rely on horizontally averaged 3D structures (over constant Rosseland optical
depth) to compute spectra. It is demonstrated that our spectra can be
smoothly connected to their 1D counterparts at higher and lower Teff where the
3D effects are small. Analytical functions are provided in order to convert
spectroscopically determined 1D effective temperatures and surface gravities to
3D atmospheric parameters. We apply our improved models to well studied
spectroscopic data sets from the Sloan Digital Sky Survey and the White Dwarf
Catalog. We confirm that the so-called high-log g problem is not present when
employing spectra and that the issue was caused by inaccuracies in the 1D
mixing-length approach. The white dwarfs with a radiative and a convective
atmosphere have derived mean masses that are the same within ~0.01 Msun, in
much better agreement with our understanding of stellar evolution. Furthermore,
the 3D atmospheric parameters are in better agreement with independent Teff and
log g values from photometric and parallax measurements.Comment: 15 pages, 18 figures, 10 pages online appendix, accepted for
publication in Astronomy and Astrophysic
Granulation properties of giants, dwarfs, and white dwarfs from the CIFIST 3D model atmosphere grid
3D model atmospheres for giants, dwarfs, and white dwarfs, computed with the
CO5BOLD code and part of the CIFIST grid, have been used for spectroscopic and
asteroseismic studies. Unlike existing plane-parallel 1D structures, these
simulations predict the spatially and temporally resolved emergent intensity so
that granulation can be analysed, which provides insights on how convective
energy transfer operates in stars. The wide range of atmospheric parameters of
the CIFIST 3D simulations (3600 < Teff (K) < 13,000 and 1 < log g < 9) allows
the comparison of convective processes in significantly different environments.
We show that the relative intensity contrast is correlated with both the Mach
and Peclet numbers in the photosphere. The horizontal size of granules varies
between 3 and 10 times the local pressure scale height, with a tight
correlation between the factor and the Mach number of the flow. Given that
convective giants, dwarfs, and white dwarfs cover the same range of Mach and
Peclet numbers, we conclude that photospheric convection operates in a very
similar way in those objects.Comment: 16 pages, 17 figures, 37 pages online appendix, accepted for
publication in Astronomy and Astrophysic
Infrared Photometric Analysis of White Dwarfs from The Two Micron All Sky Survey and the Spitzer Space Telescope
We review the available near- and mid-infrared photometry for white dwarfs
obtained from the Two Micron All-Sky Survey (2MASS) and by the Spitzer Space
Telescope. Both data sets have recently been used to seek white dwarfs with
infrared excesses due to the presence of unresolved companions or circumstellar
disks, and also to derive the atmospheric parameters of cool white dwarfs. We
first attempt to evaluate the reliability of the 2MASS photometry by comparing
it with an independent set of published JHK CIT magnitudes for 160 cool white
dwarf stars, and also by comparing the data with the predictions of detailed
model atmosphere calculations. The possibility of using 2MASS to identify
unresolved M dwarf companions or circumstellar disks is then discussed. We also
revisit the analysis of 46 binary candidates from Wachter et al. using the
synthetic flux method and confirm the large near-infrared excesses in most
objects. We perform a similar analysis by fitting Spitzer 4.5 and 8 micron
photometric observations of white dwarfs with our grid of model atmospheres,
and demonstrate the reliability of both the Spitzer data and the theoretical
calculations up to 8 micron. Finally, we search for massive disks resulting
from the merger of two white dwarfs in a 2MASS sample composed of 57 massive
degenerates, and show that massive disks are uncommon in such stars.Comment: 41 pages, 9 figures, accepted for publication in the Astrophysical
Journa
Spectroscopic and photometric studies of white dwarfs in the Hyades
The Hyades cluster is known to harbour ten so-called classical white dwarf
members. Numerous studies through the years have predicted that more than twice
this amount of degenerate stars should be associated with the cluster. Using
the PPMXL catalog of proper motions and positions, a recent study proposed 17
new white dwarf candidates. We review the membership of these candidates by
using published spectroscopic and photometric observations, as well as by
simulating the contamination from field white dwarfs. In addition to the ten
classical Hyades white dwarfs, we find six white dwarfs that may be of Hyades
origin and three more objects that have an uncertain membership status due to
their unknown or imprecise atmospheric parameters. Among those, two to three
are expected as field stars contamination. Accurate radial velocity
measurements will confirm or reject the candidates. One consequence is that the
longstanding problem that no white dwarf older than ~340 Myr appears to be
associated with the cluster remains unsolved.Comment: 14 pages, 9 figures, accepted for publication in the Astronomy and
Astrophysics journa
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