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تسجيل مستخدم جديدA comprehensive spectroscopic and photometric analysis of DA and DB white dwarfs from SDSS and Gaia
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We present a detailed spectroscopic and photometric analysis of DA and DB white dwarfs drawn from the Sloan Digital Sky Survey with trigonometric parallax measurements available from the Gaia mission. The temperature and mass scales obtained from fits to $ugriz$ photometry appear reasonable for both DA and DB stars, with almost identical mean masses of $langle M rangle = 0.617~M_odot$ and $0.620~M_odot$, respectively. The comparison with similar results obtained from spectroscopy reveals several problems with our model spectra for both pure hydrogen and pure helium compositions. In particular, we find that the spectroscopic temperatures of DA stars exceed the photometric values by $sim$10% above $T_{rm eff}sim14,000$~K, while for DB white dwarfs, we observe large differences between photometric and spectroscopic masses below $T_{rm eff}sim16,000$~K. We attribute these discrepancies to the inaccurate treatment of Stark and van der Waals broadening in our model spectra, respectively. Despite these problems, the mean masses derived from spectroscopy --- $langle M rangle = 0.615~M_odot$ and $0.625~M_odot$ for the DA and DB stars, respectively --- agree extremely well with those obtained from photometry. Our analysis also reveals the presence of several unresolved double degenerate binaries, including DA+DA, DB+DB, DA+DB, and even DA+DC systems. We finally take advantage of the Gaia parallaxes to test the theoretical mass-radius relation for white dwarfs. We find that 65% of the white dwarfs are consistent within the 1$sigma$ confidence level with the predictions of the mass-radius relation, thus providing strong support to the theory of stellar degeneracy.
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We present a comprehensive analysis of DB white dwarfs drawn from the Sloan Digital Sky Survey, based on model fits to $ugriz$ photometry and medium resolution spectroscopy from the SDSS. We also take advantage of the exquisite trigonometric parallax
measurements recently obtained by the Gaia mission. Using the so-called photometric and spectroscopic techniques, we measure the atmospheric and physical parameters of each object in our sample ($T_{rm eff}$, $log g$, H/He, Ca/He, $R$, $M$), and compare the values obtained from both techniques in order to assess the precision and accuracy of each method. We then explore in great detail the surface gravity, stellar mass, and hydrogen abundance distributions of DB white dwarfs as a function of effective temperature. We present some clear evidence for a large population of unresolved double degenerate binaries composed of DB+DB and even DB+DA white dwarfs. In the light of our results, we finally discuss the spectral evolution of DB white dwarfs, in particular the evolution of the DB-to-DA ratio as a function of $T_{rm eff}$, and we revisit the question of the origin of hydrogen in DBA white dwarfs.
We report on a comparison of spectroscopic analyses for hydrogen (DA) and helium atmosphere (DB) white dwarfs with Gaia Data Release 2 (DR2) parallaxes and photometry. We assume a reddening law and a mass-radius relation to connect the effective temp
eratures (Teff) and surface gravities (log g) to masses and radii. This allows the comparison of two largely independent sets of fundamental parameters for 7039 DA and 521 DB stars with high-quality observations. This subset of the Gaia white dwarf sample is large enough to detect systematic trends in the derived parameters. We find that spectroscopic and photometric parameters generally agree within uncertainties when the expectation of a single star is verified. Gaia allows the identification of a small systematic offset in the temperature scale between the two techniques, as well as confirming a small residual high-mass bump in the DA mass distribution around 11,000-13,000 K. This assessment of the accuracy of white dwarf fundamental parameters derived from Gaia is a first step in understanding systematic effects in related astrophysical applications such as the derivation of the local stellar formation history, initial-to-final mass relation, and statistics of evolved planetary systems.
The main objective of the present work is to extend these investigations by computing the gravity and limb-darkening coefficients for white dwarf atmosphere models with hydrogen, helium, or mixed compositions (types DA, DB, and DBA). We computed gr
avity and limb-darkening coefficients for DA, DB, and DBA white dwarfs atmosphere models, covering the transmission curves of the Sloan, UBVRI, Kepler, TESS, and Gaia photometric systems. Specific calculations for the HiPERCAM instrument were also carried out. For all calculations of the limb-darkening coefficients we used the least-squares method. Concerning the effects of tidal and rotational distortions, we also computed for the first time the gravity-darkening coefficients $y(lambda)$ for white dwarfs using the same models of stellar atmospheres as in the case of limb-darkening. A more general differential equation was introduced to derive these quantities, including the partial derivative $left(partial{ln I_o(lambda)}/{partial{ln g}}right)_{T_{rm eff}}$. Six laws were adopted to describe the specific intensity distribution: linear, quadratic, square root, logarithmic, power-2, and a more general one with four coefficients. The computations are presented for the chemical compositions log[H/He] = $-$10.0 (DB), $-$2.0 (DBA) and He/H = 0 (DA), with log g varying between 5.0 and 9.5 and effective temperatures between 3750 K-100,000 K. For effective temperatures higher than 40,000 K, the models were also computed adopting nonlocal thermal equilibirum (DA). The adopted mixing-length parameters are ML2/$alpha = 0.8$ (DA case) and 1.25 (DB and DBA). The results are presented in the form of 112 tables. Additional calculations, such as for other photometric systems and/or different values of log[H/He], $log g,$ and T$_{rm eff}$ can be performed upon request.
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s for Experiments in Stellar Evolution, we create DB WDs, and simulate the element diffusion due to high gravitational fields and the metal-rich material accretion coming from the planet disrupted by the WD. In our models, the input parameters ($alpha_{rm MLT}$, $alpha_{rm th}$ and $Z$) have very weak effect on DB WD structures including interior temperatures, chemical profiles and convective zones.The mass-accretion rate and the effective temperature of DB WDs determine the abundances of heavy elements. The evolutionary sequence of Ca element for about 1500 observed DB or DBZ WDs cannot be explained by the model with a constant mass-accretion rate, but is consistent well with the model in which the mass-accretion rate decreases by one power law when $T_{rm eff}>10$ kK and slightly increases by another power law when $T_{rm eff}<10$ kK. The observed DB WD evolutionary sequence of heavy element abundances originates from WD cooling and the change of mass-accretion rate.
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he limb-darkening coefficients for 3D models were carried out using the least-squares method for these photometric systems. The beaming factor calculations, which use realistic models of stellar atmospheres, show that the black body approximation is not accurate, particularly for the filters $u$, $u$, $U$, $g$, $g$, and $B$. The black body approach is only valid for high effective temperatures and/or long effective wavelengths. Therefore, for more accurate analyses of light curves, we recommend the use of the beaming factors presented in this paper. Concerning limb-darkening, the distribution of specific intensities for 3D models indicates that, in general, these models are less bright toward the limb than their 1D counterparts, which implies steeper profiles. To describe these intensities better, we recommend the use of the four-term law (also for 1D models) given the level of precision that is being achieved with Earth-based instruments and space missions such as Kepler and TESS (and PLATO in the future).
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