Learning about the latitudinal distribution of starspots through the periodogram analysis of photometric data

TL;DR

This paper investigates how periodogram analysis of stellar light curves can reveal starspot latitudes and stellar inclination, advancing understanding of stellar magnetic activity and differential rotation.

Contribution

It demonstrates that periodogram features can estimate starspot latitudes and stellar inclination, and identifies factors affecting the detection of differential rotation signs.

Findings

Periodogram can estimate starspot latitudes under certain conditions.

The ratio of harmonic heights relates to spot visibility time.

Identifies sources of false positives/negatives in differential rotation detection.

Abstract

Starspots are cooler and darker than the stellar surface. Therefore, the emitted flux of a star changes when spots are visible on its surface. The presence of spots together with the stellar rotation leads to a periodic modulation on the light curve. By studying that modulation one can then learn about the stellar rotation and also magnetic activity. Recently, Reinhold & Arlt (2015) proposed a method based on the analysis of the Lomb Scargle periodogram of the light curve to identify the sign of the differential rotation, i.e. whether the equator rotates faster than the poles or the opposite. We have been studying in detail the spots' impact on the light curve and on the resulting periodogram. We find that, under some conditions, the periodogram can actually provide an estimate of the true spot latitudes and/or the stellar inclination angle. Moreover, we find that the impact of the spot on the ratio between the heights of the second and first harmonics of the main peaks in the periodogram can be described by a single parameter, the visibility time of the spot. Finally, we also identify possible sources of false positives/negatives for the sign of the differential rotation.