A Convective Dredge-up Model as the Origin of Hydrogen in DBA White Dwarfs

Abstract

We revisit the problem of the formation of DB white dwarfs, as well as the origin of hydrogen in DBA stars, using a new set of envelope model calculations with stratified and mixed hydrogen/helium compositions. We first describe an approximate model to simulate the so-called convective dilution process, where a thin, superficial hydrogen radiative layer is gradually eroded by the underlying and more massive convective helium envelope, thus transforming a DA white dwarf into a DB star. We show that this convective dilution process is able to account for the large increase in the number of DB white dwarfs below T eff ∼ 20, 000 K, but that the residual hydrogen abundances expected from this process are still orders of magnitude lower than those observed in DBA white dwarfs. Scenarios involving the accretion of hydrogen from the interstellar medium or other external bodies have often been invoked to explain these overabundances of hydrogen. In this paper, we describe a new paradigm where hydrogen, initially diluted within the thick stellar envelope, is still present and slowly diffusing upward in the deeper layers of a T eff ∼ 20, 000 K white dwarf. When the convective dilution process occurs, the bottom of the mixed H/He convection zone sinks deep into the star, resulting in large amounts of hydrogen being dredged-up to the stellar surface, a phenomenon similar to that invoked in the context of DQ white dwarfs.

On the Spectral Evolution of Hot White Dwarf Stars. II. Time-dependent Simulations of Element Transport in Evolving White Dwarfs with STELUM

“…At this stage of the evolution, our DB white dwarf has a superficial convection zone associated with the recombination of helium, which grows deeper with further cooling (Rolland et al 2018;Cukanovaite et al 2019). The location of the base of the convective region is shown by a black dashed line in Figure 1.…”

“…Such calculations were also performed by Althaus et al (2005aAlthaus et al ( , 2020, who however placed little emphasis on the chemical transformation itself and focused instead on its implications for stellar pulsations. The convective mixing of the hydrogen and helium layers at low effective temperature has an even longer modeling history, comprising the works of Baglin & Vauclair (1973), Koester (1976), D'Antona & Mazzitelli (1989), MacDonald & Vennes (1991, Althaus & Benvenuto (1998), Chen &Hansen (2011), andRolland et al (2018). It is beyond the scope of the present paper to review all of the aforementioned studies and their very diverse levels of sophistication.…”

“…The main difference here is that we are dealing with a runaway process. At a given effective temperature, a helium-rich convection zone is much deeper than a hydrogen-rich convection zone (Rolland et al 2018). Consequently, the increase in helium abundance results in a more extended convective region, which in turn brings more helium in the outer layers, and so on.…”

“…As the effective temperature decreases, the convective region grows and erodes the superficial hydrogen layer from below, until the latter is completely diluted into the helium envelope. The observational outcome is a helium-dominated atmosphere with a minute trace of hydrogen, namely, a DBA white dwarf (Mac-Donald & Vennes 1991;Rolland et al 2018Rolland et al , 2020.…”

“…Although still detectable immediately after the mixing episode, the hydrogen quickly becomes invisible as the effective temperature decreases. Consequently, the white dwarf evolves briefly into a heliumrich DA star, and then into a DC star (MacDonald & Vennes 1991;Chen & Hansen 2011;Rolland et al 2018). An analogous phenomenon is thought to be responsible for the existence of carbon-polluted DQ white dwarfs below T eff ∼ 10, 000 K: these objects descend from DB stars in which settling carbon (a relic of their PG 1159 past) is dredged-up from the deep envelope to the surface by the growing helium convection zone (Pelletier et al 1986;Dufour et al 2005;Camisassa et al 2017).…”