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

Abstract

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.

The velocity distribution of white dwarfs in Gaia EDR3

“…Undeniably the bifurcation in the CMD is well-described by a difference in the atmospheric composition of WDs (Bédard et al 2021). It is however difficult to reconcile with the observed difference in the kinematics that we display here which requires significantly different dynamical heating histories between the two The top row shows the distribution for the whole WD sequence and the red and blue sequences corresponding to samples all_500, red_500, and blue_500 in Table 1.…”

Seismic Cartography of White-Dwarf Interiors From the Toulouse-Montréal Optimal-Design Approach

“…Finally, once all components (parameters) defining the white dwarf structure have been specified, stellar structure equations are solved to determine the corresponding equilibrium state. This is done with the Montreal stellar structure and evolution code STELUM (Bédard et al, 2021; see also Section 4.2), which is the most recent evolution of our white-dwarf and hot-subdwarf modeling tool. The code is based on a Galerkin finite element solver and implements current state-of-the-art physics (EOS, opacities) to describe the star interior.…”

“…A full description of STELUM is beyond the scope of the present paper. We refer the interested reader to Bédard et al (2021; see also Brassard and Fontaine 2015) for a detailed presentation of this numerical instrument and the historical background behind it. Yet, it is worth mentioning that one of the most important and defining particularity of STELUM is that it uses the so-called Galerkin finite-element scheme to solve the stellar structure equations, unlike most other stellar evolution codes that rely on finite-difference methods.…”

Accurate Diffusion Coefficients for Dense White Dwarf Plasma Mixtures

“…The current state of the art for diffusion coefficients in mixtures for stellar evolution is represented by Paquette et al (1986) and Stanton & Murillo (2016), e.g. see implementation of diffusion in the stellar evolution codes MESA (Paxton et al 2015(Paxton et al , 2018, STELUM (Bédard et al 2021), and LPCODE (Althaus et al 2001). 1 These coefficients rely on fits to numerical collision integrals for binary interactions between ions with a screened Coulomb potential, which cannot fully account for many-body dynamics at strong coupling.…”