Differential Rotation in Magnetized and Non-magnetized Stars

TL;DR

This study compares magnetohydrodynamic and hydrodynamic models of stellar differential rotation, revealing how magnetic fields influence the transition between solar-type and anti-solar rotation profiles across different Rossby numbers.

Contribution

It demonstrates that magnetic fields modify the critical conditions and smoothness of the differential rotation transition, unifying the transition behavior in terms of Rossby number regardless of magnetic effects.

Findings

Transition occurs at Ro ≈ 1 in both MHD and HD models.

Magnetic fields suppress bistability near the transition.

The sharpness of the transition is similar in MHD and HD models.

Abstract

Effects of magnetic field on stellar differential rotation are studied by comparing magnetohydrodynamic (MHD) models and their hydrodynamic (HD) counterparts in the broad range of rotation rate and in varying initial rotation profile. Fully-compressible MHD simulations of rotating penetrative convection are performed in a full-spherical shell geometry. Critical conditions for the transition of the differential rotation between faster equator (solar-type) and slower equator (anti-solar type) are explored with focusing on the "Rossby number (${\rm Ro}$)" and the "convective Rossby number (${\rm Ro}_{\rm conv}$)". It is confirmed that the transition is more gradual and the critical value for it is higher in the MHD model than the HD model in the view of the ${\rm Ro}_{\rm conv}$-dependence. The rotation profile shows, as observed in earlier studies, the bistability near the transition in the HD model, while it disappears when allowing the growth of magnetic fields except for the model with taking anti-solar type solution as the initial condition. We find that the transition occurs at ${\rm Ro} \simeq 1$ both in the MHD and HD models independently of the hysteresis. Not only the critical value, the sharpness of the transition is also similar between the two models in the view of the ${\rm Ro}$-dependence. The influences of the dynamo-generated magnetic field and/or the hysteresis on convective motion are reflected in the ${\rm Ro}$. This would be the reason why the transition is unified in the view of the ${\rm Ro}$-dependence. We finally discuss the ${\rm Ro}$-dependence of magnetic dynamo activities with emphasis on its possible relation to the kinetic helicity profile.