The evolution of helium white dwarfs: II. Thermal instabilities

TL;DR

This study models helium-core white dwarfs to analyze hydrogen-shell flashes, revealing their occurrence within specific mass ranges and their impact on stellar evolution and surface hydrogen content.

Contribution

It provides detailed evolutionary models showing the conditions for hydrogen-shell flashes and their effects on white dwarf cooling, extending previous research.

Findings

Hydrogen-shell flashes occur in 0.21-0.30 Msun white dwarfs.

Maximum luminosity during flashes reaches about 10^5 Lsun.

Hydrogen burning influences cooling down to 8000 K.

Abstract

We calculated a grid of evolutionary models for white dwarfs with helium cores (He-WDs) and investigated the occurrence of hydrogen-shell flashes due to unstable hydrogen burning via CNO cycling. Our calculations show that such thermal instabilities are restricted to a certain mass range (M=0.21...0.30Msun), consistent with earlier studies. Models within this mass range undergo the more hydrogen shell flashes the less massive they are. This is caused by the strong dependence of the envelope mass on the white dwarf core mass. The maximum luminosities from hydrogen burning during the flashes are of the order of 10^5 Lsun. Because of the development of a pulse-driven convection zone whose upper boundary temporarily reaches the surface layers, the envelope's hydrogen content decreases by Delta(X)=0.06 per flash. Our study further shows that an additional high mass-loss episode during a flash-driven Roche lobe overflow to the white dwarf's companion does not affect the final cooling behaviour of the models. Independent of hydrogen shell flashes the evolution along the final white dwarf cooling branch is determined by hydrogen burning via pp-reactions down to effective temperatures as low as 8000 K.