Evolution and colors of helium-core white dwarf stars with high-metallicity progenitors

TL;DR

This study models the evolution and colors of helium-core white dwarfs from high-metallicity progenitors, highlighting the impact of residual nuclear burning and chemical diffusion on their cooling times and observable properties.

Contribution

It provides a comprehensive set of evolutionary calculations and colors for He-core white dwarfs considering high metallicity, including effects of diffusion and residual nuclear burning.

Findings

Residual nuclear burning delays white dwarf cooling by several Gyr without diffusion.

Chemical diffusion reduces the impact of residual nuclear burning on evolution.

New color and magnitude predictions based on improved non-gray atmospheres.

Abstract

Motivated by the recent detection of single and binary He-core white dwarfs in metal-rich clusters, we present a full set of evolutionary calculations and colors appropriate for the study of such white dwarfs. The paper is also aimed at investigating whether stable hydrogen burning may constitute a main source of energy for massive He-core white dwarfs resulting from high-metallicity progenitors. White dwarf sequences are derived by taking into account the evolutionary history of progenitor stars with supersolar metallicities. We also incorporate a self-consistent, time-dependent treatment of gravitational settling and chemical diffusion, as well as of the residual nuclear burning. We find that the influence of residual nuclear burning during the late stages of white dwarf evolution is strongly dependent on the occurrence of chemical diffusion at the base of the hydrogen-rich envelope. When no diffusion is considered, residual hydrogen burning strongly influences the advanced stages of white dwarf cooling, introducing evolutionary delays of several Gyr. By contrast, when diffusion is taken into account the role of residual nuclear burning is strongly mitigated, and the evolution is dictated only by the thermal content stored in the ions. In addition, for all of our sequences, we provide accurate color and magnitudes on the basis of new and improved non gray model atmospheres which explicitly include Ly$\alpha$ quasi-molecular opacity.