On the Spectral Evolution of Hot White Dwarf Stars. I. A Detailed Model Atmosphere Analysis of Hot White Dwarfs from SDSS DR12

Abstract

As they evolve, white dwarfs undergo major changes in surface composition, a phenomenon known as spectral evolution. In particular, some stars enter the cooling sequence with helium atmospheres (type DO) but eventually develop hydrogen atmospheres (type DA), most likely through the upward diffusion of residual hydrogen. Our empirical knowledge of this process remains scarce: the fractions of white dwarfs that are born helium rich and that experience the DO-to-DA transformation are poorly constrained. We tackle this issue by performing a detailed model-atmosphere investigation of 1806 hot ( ≥ 30,000 K) white dwarfs observed spectroscopically by the Sloan Digital Sky Survey. We first introduce our new generations of model atmospheres and theoretical cooling tracks, both appropriate for hot white dwarfs. We then present our spectroscopic analysis, from which we determine the atmospheric and stellar parameters of our sample objects. We find that ∼24% of white dwarfs begin their degenerate life as DO stars, among which ∼2/3 later become DA stars. We also infer that the DO-to-DA transition occurs at substantially different temperatures (75,000 K > > 30,000 K) for different objects, implying a broad range of hydrogen content within the DO population. Furthermore, we identify 127 hybrid white dwarfs, including 31 showing evidence of chemical stratification, and we discuss how these stars fit in our understanding of the spectral evolution. Finally, we uncover significant problems in the spectroscopic mass scale of very hot ( > 60,000 K) white dwarfs.

Massive White Dwarfs in Young Star Clusters

“…Considering these restrictions, we primarily surveyed clusters from 10 Myr (turnoff mass ∼19.3 M e ) up to 500 Myr (turnoff mass 2.9 M e ) within 1 kpc of the Sun. A 1 M e hydrogen atmosphere WD that has been cooling for ∼250 Myr has an absolute G magnitude of ∼11.8 (Bédard et al 2020), corresponding to an apparent G magnitude of 20.3 at 500 pc. Since young clusters are near the Galactic plane, at this distance we can generally expect about 0.7 mag of extinction, placing the WD close to the Gaia limit of detection.…”

“…Candidate WDs were dereddened using associated cluster parameters, allowing their mass and cooling ages to be estimated using the python package WD_models. 7 WD masses 1.0 M e are modeled using the Montreal group CO cooling models (Bédard et al 2020), 8 while those > 1.0 M e use O/Ne core models (Camisassa et al 2019), 9 up to a maximum of 1.28 M e . Progenitor masses were estimated for each WD using the IFMR from Cummings et al (2018).…”

Magnetic dynamos in white dwarfs – II. Relating magnetism and pollution

“…We furthermore assumed a constant star formation rate, an age of the Galactic disk of 10 Gyr, and an initial mass function ∝ −2.3 . We used the the single star evolution code written by Hurley et al (2000) and the white dwarf cooling models of Bédard et al (2020).…”

“…Figure1. The onset and 80 per cent completed crystallization temperatures for thin (blue) and thick (black) hydrogen atmospheres taken fromBédard et al (2020). For thin atmospheres (DZ) crystallization starts at slightly higher temperatures than for thick atmospheres (DAZ).…”

Carbon-enhanced stars with short orbital and spin periods

“…Assuming that all of the binary companions are now white dwarfs, their cooling timescales can be estimated and provide a window over which tidal synchronisation could have occurred. Attributing all Galex ultraviolet flux in these systems to white dwarfs -and upper limits for non-detections -at the Gaia distances to the dC stars, and assuming a fixed white-dwarf surface gravity of log [g (cm s −2 )] = 8.0, cooling-age estimates and lower limits can be derived from models (Bédard et al 2020). The resulting cooling ages range from approximately 10 Myr (CBS 311) to over 1 Gyr, but in all cases are broadly consistent with the predicted synchronisation timescale.…”