Tepid Supergiants: Chemical Signatures of Stellar Evolution & The Extent of Blue Loops

TL;DR

This study analyzes chemical signatures in tepid supergiants to test and refine models of stellar evolution, revealing higher mixing efficiencies and extended blue loops than current models predict.

Contribution

It provides observational evidence that suggests the need to incorporate additional physical effects into stellar evolution models, such as magnetic fields and circulation.

Findings

Higher mixing efficiency than predicted by models

Blue loops extend to higher masses and temperatures

Chemical abundance patterns indicate enhanced transport processes

Abstract

Massive stars can develop into tepid supergiants at several stages of their post main-sequence evolution, prior to core He-burning, on a blue loop, or close to the final supernova explosion. We discuss observational constraints on models of massive star evolution obtained from the analysis of a sample of Galactic supergiants and put them in the context of the cosmic abundance standard as recently proposed from the study of their OB-type progenitors (Z=0.014 for stars in the solar neighbourhood). High-precision abundance analyses for He and CNO, with uncertainties as low as ~10-20%, trace the transport efficiency of nuclear-processed material to the stellar surface, either by rotational mixing or during the first dredge-up. A mixing efficiency higher by a factor ~2 than predicted by current evolution models for rotating stars is indicated, implying that additional effects need to be considered in evolutionary models like e.g. the interplay of circulation and magnetic fields. Blue loops are suggested to extend to higher masses and to higher Teff than predicted by the current generation of stellar evolution models.