Starspots due to large-scale vortices in rotating turbulent convection

TL;DR

This study uses numerical simulations to show that large-scale vortices in rotating turbulent convection can produce starspots, with vortex type depending on rotation speed, offering a non-magnetic explanation for observed stellar spots.

Contribution

It demonstrates that large-scale vortices caused by rotating turbulent convection can explain starspots without magnetic fields, linking simulations to stellar observations.

Findings

Rapid rotation leads to large cyclonic vortices.

Cool and warm vortices coexist depending on rotation speed.

Vortices cause temperature contrasts similar to observed starspots.

Abstract

We study the generation of large-scale vortices in rotating turbulent convection by means of Cartesian direct numerical simulations. We find that for sufficiently rapid rotation, cyclonic structures on a scale large in comparison to that of the convective eddies, emerge, provided that the fluid Reynolds number exceeds a critical value. For slower rotation, cool cyclonic vortices are preferred, whereas for rapid rotation, warm anti-cyclonic vortices are favored. In some runs in the intermediate regime both types of cyclones co-exist for thousands of convective turnover times. The temperature contrast between the vortices and the surrounding atmosphere is of the order of five per cent. We relate the simulation results to observations of rapidly rotating late-type stars that are known to exhibit large high-latitude spots from Doppler imaging. In many cases, cool spots are accompanied with spotted regions with temperatures higher than the average. In this paper, we investigate a scenario according to which the spots observed in the temperature maps could have a non-magnetic origin due to large-scale vortices in the convection zones of the stars.