The impact of Coulomb diffusion of ions on the pulsational properties of DA white dwarfs

TL;DR

This study investigates how Coulomb ion diffusion affects the internal structure and pulsation periods of white dwarfs, revealing significant impacts on ultra-massive stars' asteroseismology and minor effects on less massive ones.

Contribution

It introduces a comprehensive model incorporating Coulomb ion separation in white dwarf evolution, highlighting its significant influence on ultra-massive white dwarf pulsations.

Findings

Coulomb sedimentation alters chemical profiles in ultra-massive white dwarfs.

Period changes up to 15% in ultra-massive ZZ Ceti stars due to Coulomb effects.

Less impact on less-massive white dwarfs, with period changes below observational uncertainties.

Abstract

Element diffusion is a key physical process that substantially impacts the superficial abundances, internal structure, pulsation properties, and evolution of white dwarfs. We study the effect of Coulomb separation of ions in the cooling times of evolving white dwarfs, their chemical profiles, the Brunt-V\"ais\"al\"a (buoyancy) frequency, and the pulsational periods at the ZZ Ceti instability strip. We follow the full evolution of white-dwarf models derived from their progenitor history on the basis of a time-dependent element diffusion scheme that incorporates the effect of gravitational settling of ions due to Coulomb interactions at high densities. We find that Coulomb sedimentation profoundly alters the chemical profiles of ultra-massive ($M_*> 1 M_{sun}$) white dwarfs along their evolution, preventing helium from diffusing inward toward the core, and thus leading to much narrower chemical transition zones. As a result, significant changes in the $g$-mode pulsation periods as high as $15 \%$ are expected for ultra-massive ZZ Ceti stars. This should be taken into account in detailed asteroseismological analyses of such stars. For less-massive white dwarfs, the impact of Coulomb separation is much less noticeable, inflicting period changes in ZZ Ceti stars that are below the period changes that result from uncertainties in progenitor evolution, albeit larger than typical uncertainties of observed periods.