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

TL;DR

This study models 5-7 solar mass zero-metallicity stars, revealing they end as type 1.5 supernovae due to core collapse, with unique nucleosynthesis signatures in the early universe.

Contribution

It provides detailed stellar evolution models of zero-metallicity stars, showing their explosive end as type 1.5 supernovae and their nucleosynthesis processes.

Findings

No third dredge-up occurs in these stars.

Stars end as type 1.5 supernovae near the Chandrasekhar mass.

Envelope enriched in nitrogen, no s-process enrichment.

Abstract

We use the Cambridge stellar evolution code STARS to model the evolution of 5-7 solar mass 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 solar mass. 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.