Presupernova Evolution of Differentially Rotating Massive Stars Including Magnetic Fields

As a massive star evolves through multiple stages of nuclear burning on itsway to becoming a supernova, a complex, differentially rotating structure isset up. Angular momentum is transported by a variety of classic instabilities,and also by magnetic torques from fields generated by the differentialrotation. We present the first stellar evolution calculations to follow theevolution of rotating massive stars including, at least approximately, allthese effects, magnetic and non-magnetic, from the zero-age main sequence untilthe onset of iron-core collapse. The evolution and action of the magneticfields is as described by Spruit 2002 and a range of uncertain parameters isexplored. In general, we find that magnetic torques decrease the final rotationrate of the collapsing iron core by about a factor of 30 to 50 when comparedwith the non-magnetic counterparts. Angular momentum in that part of thepresupernova star destined to become a neutron star is an increasing functionof main sequence mass. That is, pulsars derived from more massive stars willrotate faster and rotation will play a more dominant role in the star'sexplosion. The final angular momentum of the core is determined - to within afactor of two - by the time the star ignites carbon burning. For the lighterstars studied, around 15 solar masses, we predict pulsar periods at birth near15 ms, though a factor of two range is easily tolerated by the uncertainties.Several mechanisms for additional braking in a young neutron star, especiallyby fall back, are also explored.Comment: 32 pages, 3 figures (8 eps files), submitted to Ap