The white dwarf mass–radius relation and its dependence on the hydrogen envelope.

Abstract

We present a study of the dependence of the mass-radius relation for DA white dwarf stars on the hydrogen envelope mass and the impact on the value of log g, and thus the determination of the stellar mass. We employ a set of full evolutionary carbonoxygen core white dwarf sequences with white dwarf mass between 0.493 and 1.05M ⊙ . Computations of the pre-white dwarf evolution uncovers an intrinsic dependence of the maximum mass of the hydrogen envelope with stellar mass, i.e., it decreases when the total mass increases. We find that a reduction of the hydrogen envelope mass can lead to a reduction in the radius of the model of up to ∼ 12%. This translates directly into an increase in log g for a fixed stellar mass, that can reach up to 0.11 dex, mainly overestimating the determinations of stellar mass from atmospheric parameters. Finally, we find a good agreement between the results from the theoretical mass-radius relation and observations from white dwarfs in binary systems. In particular, we find a thin hydrogen mass of M H ∼ 2 × 10 −8 M ⊙ , for 40 Eridani B, in agreement with previous determinations. For Sirius B, the spectroscopic mass is 4.3% lower than the dynamical mass. However, the values of mass and radius from gravitational redshift observations are compatible with the theoretical mass-radius relation for a thick hydrogen envelope of M H = 2 × 10 −6 M ⊙ .

A New Class of Roche Lobe–filling Hot Subdwarf Binaries

“…We derive a blackbody temperature of 63,000± 10,000 K for the WD, with a radius of 0.0148±0.0020 R e , which is 10%-50% larger than a fully degenerate WD. This is consistent with predictions for hot WDs as shown in Romero et al (2019), who predict 25% increased radius for a carbon-oxygen WD with T eff =60,000 K and a hydrogen layer of M=10 −4 M e . The spectral energy distribution (SED) in combination with the Gaia parallax allows for an independent estimate of the stellar parameters of the hot subdwarf.…”

From hydrogen to helium: the spectral evolution of white dwarfs as evidence for convective mixing

“…Our first hypothesis is that the observed change in Herich fraction is caused by convective mixing alone. As the helium envelope is typically orders of magnitude more massive (Iben & Renzini 1983;Romero et al 2019), the prediction is that this runaway process quickly leaves a trace amount of hydrogen in a predominantly helium atmosphere (Rolland et al 2018). Furthermore, convective motions in He-rich envelopes are many orders of magnitude faster (Fontaine & van Horn 1976) than any microscopic diffusion process that could separate helium and hydrogen (Koester 2009), hence we assume this transition to be permanent.…”

Pulsation in the white dwarf HE 1017−1352: confirmation of the class of hot DAV stars

“…These two determinations are in agreement within 0.2σ in T eff and 3.2σ in log g, but most importantly they agree that HE 1017−1352 is a hot DA white dwarf star. From the spectroscopic parameters, we computed the stellar mass using the white dwarf evolutionary sequences from Romero et al (2019a). For theoretical sequences with canonical hydrogen envelopes -those obtained from single stellar evolution computations -we obtained a stellar mass of 0.661 ± 0.030 M considering the spectroscopic values from Gianninas et al 2011and 0.566 ± 0.003 M for those obtained by Koester et al (2009).…”

“…This opens the possibility to study the interior of these objects through asteroseismology. In addition, the confirmation of pulsations in hot DA stars is another evidence for the existence of white dwarfs with very thin hydrogen envelopes (Romero et al 2012(Romero et al , 2019a, which are possibly a product of This paper has been typeset from a T E X/L A T E X file prepared by the author.…”