Magnetic Webs in Stellar Radiative Zones

Abstract

Rotational evolution of stellar radiative zones is an old puzzle. We argue that angular momentum transport by turbulent processes induced by differential rotation is insufficient, and propose that a key role is played by ``magnetic webs." We define magnetic webs as stable magnetic configurations that enforce corotation of their coupled mass shells, and discuss their resistance to differential torques that occur in stars. Magnetic webs are naturally expected in parts of radiative zones that were formerly convective, retaining memory of extinguished dynamos. For instance, red giants with moderate masses $M\gtrsim 1.3M_\odot$ likely contain a magnetic web deposited on the main sequence during the retreat of the central convective zone. The web couples the helium core to the hydrogen envelope of the evolving red giant and thus reduces spin-up of the contracting core. The magnetic field and the resulting slower rotation of the core are both consistent with asteroseismic observations, as we illustrate with a stellar evolution model with mass $1.6M_\odot$. Evolved massive stars host more complicated patterns of convective zones that may leave behind many webs, transporting angular momentum towards the surface. Efficient web formation likely results in most massive stars dying with magnetized and slowly rotating cores.