Convection in Oblate Solar-Type Stars

Wednesday, July 13, 2016

We present the first global 3D simulations of thermal convection in the oblate envelopes of rapidly-rotating solar-type stars.

3D simulation image
Vertical velocity (normal to a geopotential; red upflow, blue downflow) in a convection simulation of a young a solar-like star rotating 120 times faster than the Sun (85% of the critical breakup rotation rate). The rapid rotation distorts the shape of the star such that the equatorial radius is 17% larger than the polar radius.

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.