The Role of the Magnetorotational Instability in Massive Stars

TL;DR

This study investigates how the magnetorotational instability (MRI) influences the evolution, core structure, and potential supernova progenitors of massive stars, revealing its role in mixing processes and core rotation rates.

Contribution

It demonstrates that MRI can be active in late-stage massive stars, affecting core composition, rotation, and surface properties, which was not fully understood before.

Findings

MRI can be active in late-stage massive stars, causing mixing.

MRI influences the transition between core types, affecting final evolution.

MRI can slow core rotation, aligning with pulsar observations.

Abstract

The magnetorotational instability (MRI) is key physics in accretion disks and is widely considered to play some role in massive-star core collapse. Models of rotating massive stars naturally develop very strong shear at composition boundaries, a necessary condition for MRI instability, and the MRI is subject to triply-diffusive destabilizing effects in radiative regions. We have used the MESA stellar evolution code to compute magnetic effects due to the Spruit-Taylor mechanism and the MRI, separately and together, in a sample of massive star models. We find that the MRI can be active in the later stages of massive star evolution, leading to mixing effects that are not captured in models that neglect the MRI. The MRI and related magneto-rotational effects can move models of given ZAMS mass across "boundaries" from degenerate CO cores to degenerate O/Ne/Mg cores and from degenerate O/Ne/Mg cores to iron cores, thus affecting the final evolution and the physics of core collapse. The MRI acting alone can slow the rotation of the inner core in general agreement with the observed "initial" rotation rates of pulsars. The MRI analysis suggests that localized fields ~10^{12} G may exist at the boundary of the iron core. With both the ST and MRI mechanisms active in the 20 solar mass model, we find that the helium shell mixes entirely out into the envelope. Enhanced mixing could yield a population of yellow or even blue supergiant supernova progenitors that would not be standard SN IIP.