Inflection point in the power spectrum of stellar brightness variations III: Faculae vs. Spot dominance on stars with known rotation periods

TL;DR

This paper introduces the GPS method for accurately determining rotation periods of Sun-like stars, especially less active ones, and reveals that facular areas decrease with increasing stellar rotation rate.

Contribution

The paper presents a new power spectrum gradient method for stellar rotation period detection, effective for low activity stars like the Sun, and links facular dominance to stellar rotation.

Findings

GPS method yields precise rotation periods for Sun-like stars

Facular area decreases as stellar rotation rate increases

Method works on stars with irregular light curves

Abstract

Stellar rotation periods can be determined by observing brightness variations caused by active magnetic regions transiting visible stellar disk as the star rotates. The successful stellar photometric surveys stemming from the Kepler and TESS observations led to the determination of rotation periods in tens of thousands of young and active stars. However, there is still a lack of information about rotation periods of older and less active stars, like the Sun. The irregular temporal profiles of light curves caused by the decay times of active regions, which are comparable to or even shorter than stellar rotation periods, combine with the random emergence of active regions to make period determination for such stars very difficult. We tested the performance of the new method for the determination of stellar rotation periods against stars with previously determined rotation periods. The method is based on calculating the gradient of the power spectrum (GPS) and identifying the position of the inflection point (i.e. point with the highest gradient). The GPS method is specifically aimed at determining rotation periods of low activity stars like the Sun. We applied the GPS method to 1047 Sun-like stars observed by the Kepler telescope. We show that the GPS method returns precise values of stellar rotation periods. Furthermore, it allows us to constrain the ratio between facular and spot areas of active regions at the moment of their emergence. We show that relative facular area decreases with stellar rotation rate. Our results suggest that the GPS method can be successfully applied to retrieve periods of stars with both regular and non-regular light curves.