Investigating variation of latitudinal stellar spot rotation and its relation to the real stellar surface rotation

TL;DR

This study uses dynamo models to analyze how stellar spot rotation varies with latitude and how it relates to actual stellar surface rotation, highlighting the influence of magnetic field scale on spot motion.

Contribution

It demonstrates that large scale dynamo fields do not reliably trace stellar surface rotation, while small scale fields do, impacting how stellar rotation is inferred from starspots.

Findings

Large scale dynamo spots do not consistently trace surface rotation.

Small scale magnetic fields produce spots that follow the surface flow patterns.

Surface differential rotation variability is observed over activity cycles.

Abstract

In this work the latitude dependent stellar spot rotation is investigated based on dynamo models. The maps of the magnetic pressure at the surface from the dynamo calculations are treated similarly to the temperature maps obtained using Doppler imaging techniques. A series of snapshots from the dynamo models are cross-correlated to obtain the shift of the magnetic patterns at each latitude and time point. The surface differential rotation patterns obtained from the snapshots of the dynamo calculations show in all studied cases variability over the activity cycle. In the models using only the large scale dynamo field the measured rotation patterns are only at times similar to the input rotation law. This is due to the spot motion being mainly determined by the geometric properties of the large scale dynamo field. In the models with additional small scale magnetic field the surface differential rotation measured from the model follows well the input rotation law. The results imply that the stellar spots caused by the large scale dynamo field are not necessarily tracing the stellar differential rotation, whereas the spots formed from small scale fields trace well the surface flow patterns. It can be questioned whether the large spots observed in active stars could be caused by small scale fields. Therefore, it is not clear that the true stellar surface rotation can be recovered using measurements of large starspots, which are currently the only ones that can be observed.