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

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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.