Abstract

Cosmochronology in Action: Determination of the Mean Core Composition of the White Dwarfs in M67

Amélie Simon (Université de Montréal)

Gilles Fontaine (Université de Montréal) and Pierre Brassard (Université de Montréal).

Since stellar helium thermonuclear burning rate is still not well constrained, the exact proportion of carbon and oxygen in the core of white dwarfs remains largely unknown. Given that the cooling time of these objects depends on their composition, it is thus crucial to constrain the latter in order to use white dwarfs as cosmochronometers. To address this, we compared theoretical luminosity functions built from a wide range of core compositions to the observed luminosity function of the coeval white dwarf population of the M67 cluster. Theoretical luminosity functions were determined for 300 state-of-the-art evolutionary sequences, combined with models for the stellar formation rate, initial mass function and initial-to-final mass relationship. Since the age of M67 is known to a good precision, this comparison allowed us to determine the mean carbon-to-oxygen ratio of its white dwarf population. Assuming that white dwarfs in M67 are 80% DAs and 20% DBs with standard "thick" envelopes, we find that their core composition is dominated by oxygen. We discuss how the envelope thickness, stellar formation rate and initial-to-final mass relationship affects these results.

Mode of presentation: poster