White dwarf evolutionary sequences for low-metallicity progenitors: The impact of third dredge-up

TL;DR

This study provides new white dwarf evolutionary models for low-metallicity stars, highlighting how the absence of third dredge-up episodes leads to thick hydrogen envelopes and prolonged stable hydrogen burning, affecting cooling times.

Contribution

It introduces comprehensive evolutionary sequences considering full stellar history, emphasizing the impact of third dredge-up absence on white dwarf properties and evolution.

Findings

Thick hydrogen envelopes in low-metallicity white dwarfs enable stable hydrogen burning.

Stable hydrogen burning significantly influences white dwarf cooling times.

The results are independent of mass-loss rate assumptions during late stellar phases.

Abstract

We present new white dwarf evolutionary sequences for low-metallicity progenitors. White dwarf sequences have been derived from full evolutionary calculations that take into account the entire history of progenitor stars, including the thermally-pulsing and the post-asymptotic giant branch phases. We show that for progenitor metallicities in the range 0.00003--0.001, and in the absence of carbon enrichment due to the occurrence of a third dredge-up episode, the resulting H envelope of the low-mass white dwarfs is thick enough to make stable H burning the most important energy source even at low luminosities. This has a significant impact on white dwarf cooling times. This result is independent of the adopted mass-loss rate during the thermally-pulsing and post-AGB phases, and the planetary nebulae stage. We conclude that in the absence of third dredge-up episodes, a significant part of the evolution of low-mass white dwarfs resulting from low-metallicity progenitors is dominated by stable H burning. Our study opens the possibility of using the observed white dwarf luminosity function of low-metallicity globular clusters to constrain the efficiency of third dredge up episodes during the thermally-pulsing AGB phase of low-metallicity progenitors.