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

• Published in: The Astrophysical journal Vol. 927; no. 1; pp. 128 - 144
• Main Authors: Bédard, A., Brassard, P., Bergeron, P., Blouin, S.
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 01.03.2022; IOP Publishing
• Subjects: ASTRONOMY AND ASTROPHYSICS; Astrophysics; Atomic properties; Carbon; Chemical transport; Convective mixing; Diffusion; Dwarf stars; Helium; Hydrogen; Late stellar evolution; Mathematical models; Numerical simulations; Oxygen; Simulation; Spectra; Stars; Stellar convection envelopes; Stellar diffusion; Stellar evolution; Stellar evolutionary models; Stellar winds; Time dependent analysis; White dwarf stars; White dwarfs
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/ac4497

White dwarf stars are subject to various element transport mechanisms that can cause their surface composition to change radically as they cool, a phenomenon known as spectral evolution. In this paper, we undertake a comprehensive theoretical investigation of the spectral evolution of white dwarfs. First, we introduce STELUM, a new implementation of the stellar evolutionary code developed at the Université de Montréal. We provide a thorough description of the physical content and numerical techniques of the code, covering the treatment of both stellar evolution and chemical transport. Then, we present two state-of-the-art numerical simulations of element transport in evolving white dwarfs. Atomic diffusion, convective mixing, and mass loss are considered simultaneously as time-dependent diffusive processes and are fully coupled to the cooling. We first model the PG 1159−DO−DB−DQ evolutionary channel: a helium-, carbon-, and oxygen-rich PG 1159 star transforms into a pure-helium DB white dwarf due to gravitational settling and then into a helium-dominated, carbon-polluted DQ white dwarf through convective dredge-up. We also compute for the first time the full DO−DA−DC evolutionary channel: a helium-rich DO white dwarf harboring residual hydrogen becomes a pure-hydrogen DA star through the float-up process and then a helium-dominated, hydrogen-bearing DC star due to convective mixing. We demonstrate that our results are in excellent agreement with available empirical constraints. In particular, our DO−DA−DC simulation perfectly reproduces the lower branch of the bifurcation observed in the Gaia color–magnitude diagram, which can therefore be interpreted as a signature of spectral evolution.