Evolution and colours of helium-core white dwarf stars: the case of low-metallicity progenitors

The present work is designed to explore the evolution of helium-core white dwarf (He WD) stars for the case of metallicities much lower than the solar metallicity (Z= 0.001 and 0.0002). Evolution is followed in a self-consistent way with the predictions of detailed and new non-grey model atmospheres, time-dependent element diffusion and the history of the white dwarf progenitor. Reliable initial models for low-mass He WDs are obtained by applying mass-loss rates to a 1-M_solar stellar model in such a way that the stellar radius remains close to the Roche lobe radius. The loss of angular momentum caused by gravitational wave emission and magnetic stellar wind braking are considered. Model atmospheres, based on a detailed treatment of the microphysics entering the WD atmosphere (such as the formalism of Hummer-Mihalas to deal with non-ideal effects) and hydrogen line and pseudo-continuum opacities, enable us to provide accurate colours and magnitudes at both early and advanced evolutionary stages. We find that most of our evolutionary sequences experience several episodes of hydrogen thermonuclear flashes. In particular, the lower the metallicity, the larger the minimum stellar mass for the occurrence of flashes induced by CNO cycle reactions. The existence of a mass threshold for the occurrence of diffusion-induced CNO flashes leads to a marked dichotomy in the age of our models. Another finding of this study is that our He WD models experience unstable hydrogen burning via PP nuclear reactions at late cooling stages as a result of hydrogen chemically diffusing inwards. Such PP flashes take place in models with very low metal content. We also find that models experiencing CNO flashes exhibit a pronounced turn-off in most of their colours at M_V~ 16. Finally, colour-magnitude diagrams for our models are presented and compared with recent observational data of He WD candidates in the globular clusters NGC 6397 and 47 Tucanae.