Differential Rotation in a 3D Simulation of Oxygen Shell Burning

TL;DR

This study uses 3D simulations to explore differential rotation in the oxygen shell of a massive star, revealing complex angular velocity patterns that challenge current models and impact supernova and neutron star formation.

Contribution

It provides the first detailed 3D simulation of differential rotation in a late-stage stellar oxygen shell, showing non-uniform angular velocity profiles not captured by existing models.

Findings

Oxygen shell develops a positive angular velocity gradient.

Angular momentum transport is not well described by diffusive models.

Stable large-scale meridional flows and entropy patterns are observed.

Abstract

We study differential rotation in late-stage shell convection in a 3D hydrodynamic simulation of a rapidly rotating $16M_\odot$ helium star with a particular focus on the convective oxygen shell. We find that the oxygen shell develops a quasi-stationary pattern of differential rotation that is neither described by uniform angular velocity as assumed in current stellar evolution models of supernova progenitors, nor by uniform specific angular momentum. Instead, the oxygen shell develops a positive angular velocity gradient with faster rotation at the equator than at the pole by tens of percent. We show that the angular momentum transport inside the convection zone is not adequately captured by a diffusive mixing-length flux proportional to the angular velocity or angular momentum gradient. Zonal flow averages reveal stable large-scale meridional flow and an entropy deficit near the equator that mirrors the patterns in the angular velocity. The structure of the flow is reminiscent of simulations of stellar surface convection zones and the differential rotation of the Sun, suggesting that similar effects are involved; future simulations will need to address in more detail how the interplay of buoyancy, inertial forces, and turbulent stresses shapes differential rotation during late-stage convection in massive stars. If convective regions develop positive angular velocity gradients, angular momentum could be shuffled out of the core region more efficiently, potentially making the formation of millisecond magnetars more difficult. Our findings have implications for neutron star birth spin periods and supernova explosion scenarios that involve rapid core rotation.