Antisolar differential rotation of slowly rotating cool stars

Rotating stellar convection transports angular momentum towards the equator, generating the characteristic equatorial acceleration of the solar rotation while the radial flux of angular momentum is always inwards. New numerical box simulations for the meridional cross-correlation $\langle u_\theta u_\phi\rangle $, however, reveal the angular momentum transport towards the poles for slow rotation and towards the equator for fast rotation. The explanation is that for slow rotation a negative radial gradient of the angular velocity always appears, which in combination with a so-far neglected rotation-induced off-diagonal eddy viscosity term $\nu_\bot$ provides "antisolar rotation" laws with a decelerated equator. Similarly, the simulations provided positive values for the rotation-induced correlation $\langle u_r u_\theta\rangle $, which is relevant for the resulting latitudinal temperature profiles (cool or warm poles) for slow rotation and negative values for fast rotation. Observations of the differential rotation of slowly rotating stars will therefore lead to a better understanding of the actual stress-strain relation, the heat transport, and the underlying rotating convection.

G. Rüdiger, M. Küker, P. Käpylä, K.G. Strassmeier · 2019