The Role of the Magnetorotational Instability in Massive Stars

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.

32 pages, 19 figures, accepted for publication in the Astrophysical Journal

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• The 12C(a,g)16O reaction and its implications for stellar helium burning
• Angular momentum transport by heat-driven g-modes in slowly pulsating B stars
• On Variations Of Pre-Supernova Model Properties
• Long-term evolution of a magnetic massive merger product
• The effects of surface fossil magnetic fields on massive star evolution: I. Magnetic field evolution, mass-loss quenching and magnetic braking
• The effects of surface fossil magnetic fields on massive star evolution: II. Implementation of magnetic braking in MESA and implications for the evolution of surface rotation in OB stars
• The Betelgeuse Project: Constraints from Rotation
• Stochastic Core Spin-Up in Massive Stars -- Implications of 3D Simulations of Oxygen Shell Burning
• Convective Differential Rotation in Stars and Planets I: Theory
• Quasi-two-dimensional nonlinear evolution of helical magnetorotational instability in a magnetized Taylor-Couette flow
• The rotational shear in pre-collapse cores of massive stars