Quiescent nuclear burning in low-metallicity white dwarfs

TL;DR

This study investigates how residual nuclear burning affects the cooling and luminosity of low-metallicity white dwarfs, revealing prolonged burning phases that influence their evolution and the stellar populations they belong to.

Contribution

It provides the first self-consistent evolutionary models of low-metallicity white dwarfs, highlighting the significant role of residual nuclear burning in their cooling processes.

Findings

Nuclear burning significantly prolongs white dwarf cooling times.

White dwarfs with masses below 0.6 M_sun have dominant nuclear energy sources at low luminosities.

Residual nuclear burning impacts the white dwarf luminosity function in old, low-metallicity populations.

Abstract

We discuss the impact of residual nuclear burning in the cooling sequences of hydrogen-rich DA white dwarfs with very low metallicity progenitors ($Z=0.0001$). These cooling sequences are appropriate for the study of very old stellar populations. The results presented here are the product of self-consistent, fully evolutionary calculations. Specifically, we follow the evolution of white dwarf progenitors from the zero-age main sequence through all the evolutionary phases, namely the core hydrogen-burning phase, the helium-burning phase, and the thermally pulsing asymptotic giant branch phase to the white dwarf stage. This is done for the most relevant range of main sequence masses, covering the most usual interval of white dwarf masses --- from $0.53\, M_{\sun}$ to $0.83\, M_{\sun}$. Due to the low metallicity of the progenitor stars, white dwarfs are born with thicker hydrogen envelopes, leading to more intense hydrogen burning shells as compared with their solar metallicity counterparts. We study the phase in which nuclear reactions are still important and find that nuclear energy sources play a key role during long periods of time, considerably increasing the cooling times from those predicted by standard white dwarf models. In particular, we find that for this metallicity and for white dwarf masses smaller than about $0.6\, M_{\sun}$, nuclear reactions are the main contributor to the stellar luminosity for luminosities as low as $\log(L/L_{\sun})\simeq -3.2$. This, in turn, should have a noticeable impact in the white dwarf luminosity function of low-metallicity stellar populations.