Differential Rotation and Meridional Flow for Fast‐rotating Solar‐Type Stars

Abstract

Observations indicate that normalized surface differential rotation decreases for fast-rotating stars, that is, |ΔΩ|/Ω ∝ Ω-0.3. An increase of |ΔΩ|/Ω is provided, however, by the current Reynolds stress theory of differential rotation in stellar convection zones, without the inclusion of meridional flow. We compute both the pole-equator difference of the surface angular velocity and the meridional drift for various Taylor numbers to demonstrate that the inclusion of meridional flow in the computations for fast rotation yields a systematic reduction of the resulting differential rotation. Our model's adiabatic and density-stratified convection zone, with stress-free surfaces and a thickness of 0.3 stellar radii, yields the relation |ΔΩ|/Ω∝ Ω-(0.15...0.30) for stars with faster rotation than the Sun, in agreement with previous observations. If the Coriolis number rather than the Taylor number is varied, we find a maximum differential rotation of 20%. For stars with fast rotation, exponents of up to n′ ≃ 0.4 are found. All rotation laws exhibit superrotating equators. © 1998. The American Astronomical Society. All rights reserved.