Presupernova Evolution of Differentially Rotating Massive Stars Including Magnetic Fields

TLDR

Massive stars develop complex, differentially rotating structures during nuclear burning, with angular momentum redistributed by hydrodynamic instabilities and magnetic torques. The study presents the first stellar evolution calculations that follow rotating massive stars, incorporating both magnetic and non‑magnetic angular momentum transport, from the zero‑age main sequence to iron‑core collapse. The models employ the Spruit 2002 dynamo prescription for magnetic fields, explore a range of uncertain parameters, and examine additional braking mechanisms such as fallback in young neutron stars. Magnetic torques reduce the iron‑core rotation rate by a factor of 30–50 relative to non‑magnetic models, the core angular momentum rises with main‑sequence mass—implying faster‑rotating pulsars from more massive stars—and the core’s final spin is largely fixed by the onset of carbon burning, yielding predicted birth periods near 15 ms for 15 M⊙ progenitors.

Abstract

As a massive star evolves through multiple stages of nuclear burning on its way to becoming a supernova, a complex, differentially rotating structure is set up. Angular momentum is transported by a variety of classic instabilities, and also by magnetic torques from fields generated by the differential rotation. We present the first stellar evolution calculations to follow the evolution of rotating massive stars including, at least approximately, all these effects, magnetic and non-magnetic, from the zero-age main sequence until the onset of iron-core collapse. The evolution and action of the magnetic fields is as described by Spruit 2002 and a range of uncertain parameters is explored. In general, we find that magnetic torques decrease the final rotation rate of the collapsing iron core by about a factor of 30 to 50 when compared with the non-magnetic counterparts. Angular momentum in that part of the presupernova star destined to become a neutron star is an increasing function of main sequence mass. That is, pulsars derived from more massive stars will rotate faster and rotation will play a more dominant role in the star's explosion. The final angular momentum of the core is determined - to within a factor of two - by the time the star ignites carbon burning. For the lighter stars studied, around 15 solar masses, we predict pulsar periods at birth near 15 ms, though a factor of two range is easily tolerated by the uncertainties. Several mechanisms for additional braking in a young neutron star, especially by fall back, are also explored.