[Paper Review] The spin-up of contracting red supergiants

Alexander Heger, N. Langer|arXiv (Cornell University)|Mar 2, 1998

Stellar, planetary, and galactic studies1 references3 citations

TL;DR

This paper proposes a mechanism for the spin-up of contracting red supergiants due to angular momentum conservation during mass loss from their convective envelopes. As the star sheds mass from its outer layers while retaining angular momentum, the specific angular momentum increases, leading to rapid surface rotation—demonstrated in a 12 M☉ star that reaches critical rotation after leaving the red supergiant branch.

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.

Motivation & Objective

• To investigate the physical mechanism behind the spin-up of rotating red supergiants during post-main-sequence evolution.
• To understand how mass loss from convective envelopes affects angular momentum redistribution in massive stars.
• To explore the implications of enhanced surface rotation for the evolution and circumstellar environments of post-AGB stars, B[e] stars, and SN 1987A progenitors.
• To quantify the spin-up effect in a stellar evolution model of a 12 M☉ star.
• To determine whether mass loss can drive a star to critical rotation after leaving the red supergiant phase.

Proposed method

• Modeling rigidly rotating, convective stellar envelopes to analyze angular momentum transport during mass loss.
• Applying conservation of angular momentum in a spherically symmetric framework to track changes in specific angular momentum as mass is lost.
• Using a stellar evolution code to simulate the evolution of a 12 M☉ rotating star through the red supergiant phase.
• Analyzing the behavior of the convection zone and surface layers during contraction and mass loss to determine spin-up rates.
• Comparing theoretical predictions with observational constraints on rotation in post-AGB stars and SN 1987A progenitor.
• Employing post-processing analysis of angular momentum evolution in the stellar envelope to quantify spin-up.

Experimental results

Research questions

• RQ1 Can mass loss from the outer layers of a contracting red supergiant lead to significant surface spin-up beyond what is expected from contraction alone?
• RQ2 How does angular momentum conservation in a convective envelope influence the specific angular momentum during mass loss?
• RQ3 To what extent can mass loss drive a 12 M☉ star to critical rotation after leaving the red supergiant branch?
• RQ4 What are the implications of this spin-up mechanism for the circumstellar environments of B[e] stars and post-AGB stars?
• RQ5 Does this mechanism provide a viable explanation for the observed rapid rotation in the progenitor of Supernova 1987A?

Key findings

• Mass loss from the outer layers of a contracting red supergiant can lead to significant spin-up due to conservation of angular momentum in the convective envelope.
• The specific angular momentum increases in the convection zone as mass is lost, resulting in enhanced surface rotation.
• In a 12 M☉ stellar evolution model, the star is spun up to critical rotation after leaving the red supergiant branch.
• The spin-up effect is stronger than that expected from contraction alone, due to the retention of angular momentum during mass loss.
• The mechanism provides a plausible explanation for rapid rotation in post-AGB stars and B[e] stars.
• The findings suggest that the progenitor of Supernova 1987A may have been spun up via this mechanism during its red supergiant phase.