Massive Oe/Be stars at low metallicity: Candidate progenitors of long GRBs?

TL;DR

This study investigates low metallicity Be/Oe stars in the SMC, finding they are more numerous and potentially serve as progenitors for long gamma-ray bursts, with implications for understanding stellar evolution and GRB rates.

Contribution

It provides observational evidence linking low metallicity Be/Oe stars to long GRB progenitors and estimates the GRB rates based on these stars.

Findings

Be star occurrence in SMC is 3-5 times higher than in the Galaxy.

Low metallicity Be/Oe stars are plausible long GRB progenitors.

Calculated GRB rates align well with observed rates.

Abstract

At low metallicity the B-type stars rotate faster than at higher metallicity, typically in the SMC. As a consequence, it was expected a larger number of fast rotators in the SMC than in the Galaxy, in particular more Be/Oe stars. With the ESO-WFI in its slitless mode, the SMC open clusters were examined and an occurence of Be stars 3 to 5 times larger than in the Galaxy was found. The evolution of the angular rotational velocity seems to be the main key on the understanding of the specific behaviour and of the stellar evolution of such stars at different metallicities. With the results of this WFI study and using observational clues on the SMC WR stars and massive stars, as well as the theoretical indications of long gamma-ray burst progenitors, we identify the low metallicity massive Be and Oe stars as potential LGRB progenitors. Therefore the expected rates and numbers of LGRB are calculated and compared to the observed ones, leading to a good probability that low metallicity Be/Oe stars are actually LGRB progenitors.