Four open questions in massive star evolution

TL;DR

This paper explores four key unresolved questions in the evolution of massive stars, including their rotation, pulsation, supernova outcomes, and progenitor detection challenges.

Contribution

It offers hypotheses and discussions addressing four major open questions in massive star evolution, integrating observational and theoretical perspectives.

Findings

Proposes that nitrogen-rich, slowly rotating stars may originate from initially fast rotators that have been braked.

Suggests alpha-Cygni variables could be post red supergiants despite surface abundance indications of pre red supergiant status.

Highlights the uncertainty in supernova types produced by stars of 18-30 solar masses and the scarcity of detected Ibc supernova progenitors.

Abstract

We discuss four questions dealing with massive star evolution. The first one is about the origin of slowly rotating, non-evolved, nitrogen rich stars. We propose that these stars may originate from initially fast rotating stars whose surface has been braked down. The second question is about the evolutionary status of alpha-Cygni variables. According to their pulsation properties, these stars should be post red supergiant stars. However, some stars at least present surface abundances indicating that they should be pre red supergiant stars. How to reconcile these two contradictory requirements? The third one concerns the various supernova types which are the end point of the evolution of stars with initial masses between 18 and 30 Msun, i.e. the most massive stars which go through a red supergiant phase during their lifetime. Do they produce types IIP, IIL, IIn, IIb or Ib supernovae or do they end without producing any SN event? Finally, we shall discuss reasons why so few progenitors of type Ibc supernovae have yet been detected?