Abstract
Context. Considerable effort has gone into using light curves observed by such space telescopes 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, such as stars that are similar to the Sun, are quite irregular. This hampers the determination of their rotation periods. Aims. We aim to examine the factors causing these irregularities in stellar brightness variations and to develop a method for determining rotation periods for low-activity stars with irregular light curves. Methods. We extended the Spectral And Total Irradiance Reconstruction approach for modeling solar brightness variations to Sun-like stars. We calculated the power spectra of stellar brightness variations for various combinations of parameters that define 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 its sign, that is, 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 which is shown to work even in cases where the power spectrum shows no peak at the rotation rate.
Highlights
The magnetic features on stellar surfaces lead to quasi-periodic variations in stellar brightness as stars rotate
We aim to examine the factors causing these irregularities in stellar brightness variations and to develop a method for determining rotation periods for low-activity stars with irregular light curves
The inflection point of the power spectrum is found to be a new diagnostic for stellar rotation periods which is shown to work even in cases where the power spectrum shows no peak at the rotation rate
Summary
The magnetic features on stellar surfaces lead to quasi-periodic variations in stellar brightness as stars rotate. Model description Strong concentrations of a magnetic field emerging on the stellar surface lead to the formation of active regions encompassing magnetic features, such as dark spots and bright faculae (see e.g., a review by Solanki et al 2006) The transits of these regions over the visible stellar disk as the star rotates, as well as their evolution, are dominant sources of brightness variations in Sun-like stars on timescales from about a day. As illustrated by Eq (1), the variability of the flux F(λ, t) is brought about by the time-dependence of the solid angles of active regions seen from the vantage point of observer, Ωi, and by the time-dependence of facular and spot fractions αi,F(t) and αi,S (t) The former is attributed to the evolution of magnetic features, as well as to the rotation of the star, and consequent change of the foreshortening factor. We note that the absolute size of spots does not play a role in the calculations presented since it affects only the amplitude of the brightness variations and has no effect on the profile of their power spectrum
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