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 Teff ~ 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 Teff ~ 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.

Accepted for publication in The Astrophysical Journal (34 pages, 11 figures, 1 table)

References in corpus (6)

• Likely detection of water-rich asteroid debris in a metal-polluted white dwarf
• The Ratio of Helium- to Hydrogen-Atmosphere White Dwarfs: Direct Evidence for Convective Mixing
• Hydrogen delivery onto white dwarfs from remnant exo-Oort cloud comets
• Hot DB White Dwarfs from the Sloan Digital Sky Survey
• Convective Overshoot and Macroscopic Diffusion in Pure-Hydrogen Atmosphere White Dwarfs
• Calibration of the mixing length theory for structures of helium-dominated atmosphere white dwarfs

Cited by in corpus (16)

• On the Spectral Evolution of Hot White Dwarf Stars. I. A Detailed Model-Atmosphere Analysis of Hot White Dwarfs from SDSS DR12
• Current challenges in the physics of white dwarf stars
• White dwarf pollution by hydrated planetary remnants: Hydrogen and Metals in WD J204713.76-125908.9
• Discovery of Beryllium in White Dwarfs Polluted by Planetesimal Accretion
• A Spectro-photometric Analysis of Cool White Dwarfs in the Gaia and Pan-STARRS Footprint
• 3D spectroscopic analysis of helium-line white dwarfs
• On the Spectral Evolution of Hot White Dwarf Stars. II. Time-dependent Simulations of Element Transport in Evolving White Dwarfs with STELUM
• The spectral evolution of white dwarfs: where do we stand?
• New chondritic bodies identified in eight oxygen-bearing white dwarfs
• GD424 -- a helium-atmosphere white dwarf with a large amount of trace hydrogen in the process of digesting a rocky planetesimal
• Carbon dredge-up required to explain the Gaia white dwarf colour-magnitude bifurcation
• On the Spectral Evolution of Hot White Dwarf Stars. IV. The Diffusion and Mixing of Residual Hydrogen in Helium-rich White Dwarfs
• The ubiquity of carbon dredge-up in hydrogen-deficient white dwarfs as revealed by GALEX
• Atmospheric Temperature Inversions and He I 5876 Core Profile Structure in White Dwarfs
• Gaia white dwarfs with infrared excess I. The 100 pc catalogue
• Double-faced white dwarfs and the magnetic inhibition of convection