Forward modelling of brightness variations in Sun-like stars

Abstract

Context. The amplitude and morphology of light curves of Sun-like stars change substantially with increasing rotation rate: brightness variations are amplified and become more regular. This has not been explained so far. Aims. We develop a modelling approach for calculating brightness variations of stars with various rotation rates and use it to explain the observed trends in stellar photometric variability. Methods. We combined numerical simulations of magnetic flux emergence and transport with a model for stellar brightness variability to calculate synthetic light curves of stars as observed by the Kepler telescope. We computed the distribution of the magnetic flux on the stellar surface for various rotation rates and degrees of active-region nesting (i.e. the tendency of active regions to emerge in the vicinity of recently emerged regions). Using the resulting maps of the magnetic flux, we computed the rotational variability of our simulated stellar light curves as a function of rotation rate and nesting of magnetic features and compared our calculations to Kepler observations. Results. We show that both the rotation rate and the degree of nesting have a strong impact on the amplitude and morphology of stellar light curves. In order to explain the variability of most of the Kepler targets with known rotation rates, we need to increase the degree of nesting to values that are much higher than the values on the Sun. Conclusions. The suggested increase in nesting with the rotation rate can provide clues about the flux emergence process for high levels of stellar activity.