Evolution of long-lived globular cluster stars
IV. Initial helium content and white-dwarf properties
1 Department of Astronomy, University of Geneva, Chemin des Maillettes 51, 1290 Versoix, Switzerland
2 IRAP, UMR 5277 CNRS and Université de Toulouse, 14 Av. E. Belin, 31400 Toulouse, France
Received: 31 January 2017
Accepted: 3 April 2017
Context. Globular clusters host stars with chemical peculiarities. The associated helium enrichment is expected to affect the evolution of stars, in general, and of low-mass stars, and in particular the progenitors of white dwarfs (WDs).
Aims. We investigate the effects of different initial helium contents on the properties of WDs such as their masses, compositions, and the time since their formation.
Methods. We used the grid of stellar models that we presented in the first papers of this series, which were computed for low-mass, low-metallicity stars with different helium content at [Fe/H] = −1.75 up to the end of the thermally pulsing asymptotic giant branch (TP-AGB) phase. We determined an initial-to-final mass relation as a function of the initial helium mass fraction, where the final mass is determined at the end of the TP-AGB phase. We couple the results with different possible distributions of the initial helium content for low-mass stars in NGC 6752 to predict the properties of WDs in this cluster.
Results. In a globular cluster at a given age, the He enrichment implies lower initial masses for stars at a given phase. Thus it leads to a decrease of the masses of WDs reaching the cooling sequence. In addition the He enrichment increases the total mass and number of WDs and eventually allows the presence of He white dwarf from single progenitors.
Conclusions. The low He enrichment determined in most globular clusters with different methods results in negligible effects on the white dwarf properties. However, in the few globular clusters that display a high He enrichment, this may significantly affect the characteristics of the WDs. In NGC 2808 and ω Centauri the high He enrichment even leads to the formation of He WDs from single He-rich progenitors. Therefore investigating the white dwarf mass domain in globular clusters with a high He enrichment would provide an additional indirect way to measure and constrain the He enrichment degree.
Key words: stars: abundances / white dwarfs / stars: chemically peculiar / stars: evolution / stars: low-mass / globular clusters: general
© ESO, 2017