Carbon burning in intermediate-mass primordial stars

Abstract

The evolution of a zero metallicity 9 M⊙ star is computed, analyzed and compared with that of a solar metallicity star of identical ZAMS mass. Our computations range from the main sequence until the formation of a massive oxygen-neon white dwarf. Special attention has been payed to carbon burning in conditions of partial degeneracy as well as to the subsequent thermally pulsing Super-AGB phase. The latter develops in a fashion very similar to that of a solar metallicity 9 M⊙ star, as a consequence of the significant enrichment in metals of the stellar envelope that ensues due to the so-called third dredge-up episode. The abundances in mass of the main isotopes in the final ONe core resulting from the evolution are X(16O) ≈ 0.59, X(20Ne) ≈ 0.28 and X( 24Mg) ≈ 0.05. This core is surrounded by a 0.05 M ⊙ buffer mainly composed of carbon and oxygen, and on top of it a He envelope of mass ∼10-4 M⊙. © ESO 2005.