Differential rotation of main sequence F stars

Abstract

The differential rotation of a 1.2 $M_\odot$ zero age MS star (spectral type F8) is computed and the results are compared with those from a similar model of the Sun. The rotation pattern is determined by solving the Reynolds equation including the convective energy transport. The latter is anisotropic due to the Coriolis force causing a horizontal temperature gradient of ~ 7 K between the poles and the equator. Comparison of the transport mechanisms of angular momentum (the eddy viscosity, the $\Lambda$-effect and the meridional flow) shows that for the F star the $\Lambda$-effect is the most powerful transporter for rotation periods of 7 d or less. In the limit of very fast rotation the $\Lambda$-effect is balanced by the meridional flow alone and the rotation is nearly rigid. The rotation pattern found for the F star is very similar to the solar rotation law, but the horizontal shear is about twice the solar value. As a function of the rotation period, the total equator-pole difference of the angular velocity has a (slight) maximum at a period of 7 d and (slowly) vanishes in both the limiting cases of very fast and very slow rotation. A comparison of the solar models with those for the F-type star shows a much stronger dependence of the differential surface rotation on the stellar luminosity rather than on the rotation rate.