Paper detail

Inflection point in the power spectrum of stellar brightness variations. I. The model

Context. Considerable effort has been put into using light curves observed by space telescopes such as CoRoT, Kepler and TESS for determining stellar rotation periods. While rotation periods of active stars can be reliably determined, the light curves of many older and less active stars (e.g. stars similar to the Sun) are quite irregular, which hampers determination of their periods. Aims. We examine the factors causing the irregularities in stellar brightness variations and develop a method for determining rotation periods of low activity stars with irregular light curves. Methods. We extend the Spectral And Total Irradiance Reconstruction (SATIRE) approach for modelling solar brightness variations to Sun-like stars. We calculate the power spectra of stellar brightness variations for various combinations of parameters defining the surface configuration and evolution of stellar magnetic features. Results. The short lifetime of spots in comparison to the stellar rotation period as well as the interplay between spot and facular contributions to brightness variations of stars with near solar activity cause irregularities in their light curves. The power spectra of such stars often lack a peak associated with the rotation period. Nevertheless, the rotation period can still be determined by measuring the period where the concavity of the power spectrum plotted in the log-log scale changes sign, i.e. by identifying the position of the inflection point. Conclusions. The inflection point of the (log-log) power spectrum is found to be a new diagnostic for stellar rotation periods that is shown to work even in cases where the power spectrum shows no peak at the rotation rate.