Convection in Oblate Solar-Type Stars

TL;DR

This paper presents the first 3D simulations of thermal convection in oblate, rapidly rotating solar-type stars, revealing how oblateness influences heat flux distribution, convection patterns, and differential rotation.

Contribution

It introduces novel 3D simulations of convection in oblate stars, demonstrating the effects of oblateness on heat flux, convection, and differential rotation.

Findings

Heat flux is enhanced in polar and equatorial regions.

Oblateness limits differential rotation energy to 61%.

High-latitude convection forms elongated circulation cells.

Abstract

We present the first global 3D simulations of thermal convection in the oblate envelopes of rapidly-rotating solar-type stars. This has been achieved by exploiting the capabilities of the new Compressible High-ORder Unstructured Spectral difference (CHORUS) code. We consider rotation rates up to 85\% of the critical (breakup) rotation rate, which yields an equatorial radius that is up to 17\% larger than the polar radius. This substantial oblateness enhances the disparity between polar and equatorial modes of convection. We find that the convection redistributes the heat flux emitted from the outer surface, leading to an enhancement of the heat flux in the polar and equatorial regions. This finding implies that lower-mass stars with convective envelopes may not have darker equators as predicted by classical gravity darkening arguments. The vigorous high-latitude convection also establishes elongated axisymmetric circulation cells and zonal jets in the polar regions. Though the overall amplitude of the surface differential rotation, $\Delta \Omega$, is insensitive to the oblateness, the oblateness does limit the fractional kinetic energy contained in the differential rotation to no more than 61\%. Furthermore, we argue that this level of differential rotation is not enough to have a significant impact on the oblateness of the star.