Differential rotation and meridional flow on the lower zero age main sequence: Reynolds stress versus baroclinic flow

摘要

We study the variation of surface differential rotation and meridional flowalong the lower part of the zero age main sequence (ZAMS). We first compute asequence of stellar models with masses from 0.3 to 1.5 solar masses. We thenconstruct mean field models of their outer convection zones and computedifferential rotation and meridional flows by solving the Reynolds equationwith transport coefficients from the second order correlation approximation.For a fixed rotation period of 2.5 d we find a strong dependence of the surfacedifferential rotation on the effective temperature with weak surface shear forM dwarfs and very large values for F stars. The increase with effectivetemperature is modest below 6000 K but very steep above 6000 K. The meridionalflow shows a similar variation with temperature but the increase withtemperature is not quite so steep. Both the surface rotation and the meridionalcirculation are solar-type over the entire temperature range. We also study thedependence of differential rotation and meridional flow on the rotation periodfor masses. from 0.3 to 1.1 solar masses. The variation of the differentialrotation with period is weak except for very rapid rotation. The meridionalflow shows a systematic increase of the flow speed with the rotation rate.Numerical experiments in which either the $\Lambda$ effect is dropped in theReynolds stress or the baroclinic term in the equation of motion is cancelledshow that for effective temperatures below 6000 K the Reynolds stress is thedominant driver of differential rotation.