F. Wesemael

Photometric Calibration of Hydrogen- and Helium-Rich White Dwarf Models

Bolometric corrections and color indices on various photometric systems are provided for an extensive grid of hydrogen- and helium-rich white dwarf model atmospheres. Absolute visual magnitudes, masses, and ages are also obtained for each model from detailed evolutionary cooling sequences appropriate for these stars. The results of our calculations are briefly compared with published observational material. These calculations can easily be extended to any given photometric system.

New model atmospheres for very cool white dwarfs with mixed H/He and pure He compositions

New model atmosphere calculations for very cool white dwarfs with mixed H/He and pure He compositions are presented. The hydrogen-rich models incorporate improved cross section calculations of the collision-induced absorption by molecular hydrogen due to collisions with H2, H, and He. The effects associated with variations in the effective temperature (T(sub eff) greater than or equal to 4000 and less than or equal to 10,000 K), the surface gravity (log g greater than or equal to 7.5 and less than or equal to 9.5), and the chemical composition (N(He)/N(H) greater than or equal to O and less than or equal to 100) are investigated. Results from earlier calculations are confirmed qualitatively, but a more detailed comparison reveals large quantitative deviations. Cool white dwarfs with mixed H/He chemical compositions are shown to be easily recognizable from their predicted strong infrared flux deficiency. Pure helium model calculations are described as well. These include a modified version of the recently developed equation of state of D. Saumon and G. Chabrier. Nonideal effects brought about by various equations of state are explored in detail. For the purpose of this analysis, a model of pressure ionization based on an accurate description of the interactions in a mostly atomic helium fluid is developed. The effects of pressure ionization are shown to be the most important issue in the model calculations. A critical discussion of previous generations of pure helium model calculations is presented. Finally, broadband color indices are provided for the complete model grid.

Spectroscopic Studies of DB White Dwarfs: The Instability Strip of the Pulsating DB (V777 Herculis) Stars

We have secured optical spectra for the eight currently known variable DB, or V777 Her, stars. With the help of a new generation of synthetic spectra, spectroscopic effective temperatures are derived for these objects, as well as for 15 other DB or DBA stars above 20,000 K. We find that the location of the boundaries of the instability strip is sensitive to the atmospheric hydrogen abundance assumed for DB stars: the strip covers the range 22,400-27,800 K if atmospheres of pure helium are used and the range 21,800-24,700 K if undetectable traces of hydrogen are allowed for in the DB models. These determinations provide independent constraints for current seismological analyses of the V777 Her stars. More sensitive searches for weak hydrogen features in hot DB stars should help decide between the two temperature scales.

The White Dwarf Distance to the Globular Cluster 47 Tucanae and its Age

We present a new determination of the distance (and age) of the Galactic globular cluster 47 Tucanae (NGC 104) based on the fit of its white dwarf (WD) cooling sequence with the empirical fiducial sequence of local WD with known trigonometric parallax, following the method described by Renzini and collaborators. Both the cluster and the local WDs were imaged with HST WFPC2 using the same instrument setup. We obtain an apparent distance modulus of (m-M)V = 13.27 ± 0.14, consistent with previous ground-based determinations and shorter than that found using Hipparcos subdwarfs. Coupling our distance determination with a new measure of the apparent magnitude of the main-sequence turnoff based on our HST data, we derive an age of 13 ± 2.5 Gyr.

Adiabatic properties of pulsating DA white dwarfs. II - Mode trapping in compositionally stratified models

As part of a series of investigations aimed at understanding the adiabatic gravity-mode period structure of models of pulsating DA white dwarfs, we examine qualitatively and quantitatively the phenomenon of mode trapping caused by compositional layering in these stars. This is motivated by the real possibility that the pulsation modes detected in ZZ Ceti stars are actually trapped in the outer hydrogen layer. We first discuss the various manifestations of compositional layering on the pulsational properties of a representative ZZ Ceti star model

Carbon pollution in helium-rich white dwarf atmospheres: time-dependent calculations of the dredge-up process

On presente des calculs montrant que la diffusion du carbone a partir du cœur peut contaminer les atmospheres de naines blanches riches en He se refroidissant

Adiabatic properties of pulsating DA white dwarfs. IV - an extensive survey of the period structure of evolutionary models

The results of a detailed adiabatic survey of the pulsation properties of evolutionary models of pulsating DA white dwarfs are presented. Pulsation periods, kinetic energies, and first-order rotation coefficients were calculated for all g-modes with l ≤ 3 in the period window 0-1000 s. The survey was carried out with the help of a new, high-performance adiabatic pulsation code based on the Galerkin finite-element method of weighted residuals. A very large volume of parameter space was explored in order to obtain a complete picture of the fundamental asteroseismological properties of DA white dwarfs

PROCYON B: OUTSIDE THE IRON BOX

Procyon B is one of a select group of white dwarf stars in visual binaries that are used to put stringent constraints on the fundamental physics of stellar degeneracy. We present Space Telescope Imaging Spectrograph observations of this elusive white dwarf. Our spectra cover 1800-10000 A and contain carbon Swan bands, Mg II, and numerous iron features. Procyon B is therefore a rare DQZ white dwarf. Our analysis of the spectrum yields an effective temperature of 7740 ± 50 K and a radius of 0.01234 ± 0.00032 R☉. We discuss the implications of these results on observational support for the white dwarf mass-radius relation.

A study of metal abundance patterns in cool white dwarfs. II: Simulations of accretion episodes

Time-dependent simulations of episodic accretion of metals onto cool white dwarfs are presented. Calculations are made for Mg, Si, Ca, and Fe in H-rich and He-rich white dwarf models having a mass of 0.6 M ○ . in the range 25,000 K≥T eff ≥5000 K. It is first shown that under the assumption of continuous accretion the metal distributions rapidly reach a steady state in the outer layers of the models. The surface steady-state abundance of a metal directly scales on the assumed accretion rate. Using estimates of accretion rates based on a simplified understanding of the interstellar medium, a two-phase accretion model is next developed

A study of metal abundance patterns in cool white dwarfs. III: Comparison of the predictions of the two-phase accretion model with the observations

We compare the predictions of our two-phase accretion model with all the observations currently available on the metal-line phenomenon in cool white dwarfs. The model is based on a simplified picture of the interstellar medium, but makes specific predictions as to the presence of metals in the photospheres of these stars. We find that the gross observed metal abundance patterns are very well accounted for within the framework of the model. For many individual stars, it is also possible to infer directly the accretion rates necessary to maintain the photospheric metal abundances. These rates agree remarkably well with the estimates of the accretion rates from denser patches of the interstellar medium which we used in our idealized numerical simulations, thus providing an important self-consistency check