An Explosive End to Intermediate‐Mass Zero‐Metallicity Stars and Early Universe Nucleosynthesis

Abstract

We use the Cambridge stellar evolution code STARS to model the evolution of 5 M⊙ and 7 M⊙ zero‐metallicity stars. With enhanced resolution at the hydrogen and helium burning shell in the AGB phases, we are able to model the entire thermally pulsing asymptotic giant branch (TP‐AGB) phase. The helium luminosities of the thermal pulses are significantly lower than in higher metallicity stars so there is no third dredge‐up. The envelope is enriched in nitrogen by hot‐bottom burning of carbon that was previously mixed in during second dredge‐up. There is no s‐process enrichment owing to the lack of third dredge up. The thermal pulses grow weaker as the core mass increases and they eventually cease. From then on the star enters a quiescent burning phase which lasts until carbon ignites at the centre of the star when the CO core mass is 1.36 M⊙. With such a high degeneracy and a core mass so close to the Chandrasekhar mass, we expect these stars to explode as type 1.5 supernovae, very similar to Type Ia supernovae but inside a hydrogen rich envelope.