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

TL;DR

This study models how surface differential rotation and meridional flow vary with stellar mass, temperature, and rotation period on the lower zero age main sequence, highlighting the dominant role of Reynolds stress below 6000 K.

Contribution

It provides a detailed analysis of differential rotation and meridional flow across a range of stellar masses and temperatures, emphasizing the influence of Reynolds stress versus baroclinic flow.

Findings

Surface differential rotation strongly depends on effective temperature.

Meridional flow increases with rotation rate.

Reynolds stress dominates driving below 6000 K.

Abstract

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