The VLT-FLAMES survey of massive stars: constraints on stellar evolution from the chemical compositions of rapidly rotating Galactic and Magellanic Cloud B-type stars

TL;DR

This study investigates nitrogen surface abundances in massive B-type stars across different metallicities to understand stellar evolution and mixing processes, revealing complex patterns influenced by age and binarity.

Contribution

It extends previous analyses to Galactic and SMC stars, providing new insights into nitrogen enrichment and challenging existing stellar evolution models.

Findings

Slowly rotating nitrogen-rich stars found in SMC and LMC.

No significant nitrogen enrichment in Galactic core-hydrogen burning stars.

Highly enriched supergiants likely experienced a red supergiant phase.

Abstract

We have previously analysed the spectra of 135 early B-type stars in the LMC and found several groups of stars that have chemical compositions that conflict with the theory of rotational mixing. Here we extend this study to Galactic and SMC metallicities with the analysis of ~50 Galactic and ~100 SMC early B-type stars with rotational velocities up to ~300km/s. The surface nitrogen abundances are utilised as a probe of the mixing process. In the SMC, we find a population of slowly rotating nitrogen-rich stars amongst the early B type core-hydrogen burning stars, similar to the LMC. In the Galactic sample we find no significant enrichment amongst the core hydrogen-burning stars, which appears to be in contrast with the expectation from both rotating single-star and close binary evolution models. However, only a small number of the rapidly rotating stars have evolved enough to produce a significant nitrogen enrichment, and these may be analogous to the non-enriched rapid rotators previously found in the LMC sample. Finally, in each metallicity regime, a population of highly enriched supergiants is observed, which cannot be the immediate descendants of core-hydrogen burning stars. Their abundances are, however, compatible with them having gone through a previous red supergiant phase. Together, these observations paint a complex picture of the nitrogen enrichment in massive main sequence and supergiant stellar atmospheres, where age and binarity cause crucial effects. Whether rotational mixing is required to understand our results remains an open question at this time, but could be answered by identifying the true binary fraction in those groups of stars that do not agree with single-star evolutionary models (abridged).