The spin-up of contracting red supergiants

TL;DR

This paper identifies a mechanism that significantly spins up contracting red supergiants, potentially reaching critical rotation, with implications for circumstellar environments and stellar evolution.

Contribution

It introduces a new spin-up mechanism for red supergiants involving angular momentum retention in retreating convective envelopes, supported by stellar evolution models.

Findings

Red supergiants can reach critical rotation after leaving the RSG phase.

The spin-up is driven by angular momentum retention in retreating convective envelopes.

Implications for circumstellar matter around various stellar objects.

Abstract

We report on a mechanism which may lead to a spin-up of the surface of a rotating single star leaving the Hayashi line, which is much stronger than the spin-up expected from the mere contraction of the star. By analyzing rigidly rotating, convective stellar envelopes, we qualitatively work out the mechanism through which these envelopes may be spun up or down by mass loss through their lower or upper boundary, respectively. We find that the first case describes the situation in retreating convective envelopes, which tend to retain most of the angular momentum while becoming less massive, thereby increasing the specific angular momentum in the convection zone and thus in the layers close to the stellar surface. We explore the spin-up mechanism quantitatively in a stellar evolution calculation of a rotating 12 M_sun star, which is found to be spun up to critical rotation after leaving the red supergiant branch. We discuss implications of this spin-up for the circumstellar matter around several types of stars, i.e., post-AGB stars, B[e] stars, pre-main sequence stars, and, in particular, the progenitor of Supernova 1987A.