The variability of Sun-like stars: reproducing observed photometric trends

TL;DR

This study models the photometric variability of Sun-like stars by considering the effects of magnetic features like spots and faculae, explaining the transition from brightness increase to decrease with activity levels.

Contribution

The paper introduces a model that reproduces observed stellar brightness trends by extrapolating solar magnetic activity effects to other Sun-like stars.

Findings

Starspot contribution increases faster than faculae with activity.

Transition from faculae-dominated to spot-dominated variability explained.

Model accounts for viewing angle and active region distribution effects.

Abstract

The Sun and stars with low magnetic activity levels, become photometrically brighter when their activity increases. Magnetically more active stars display the opposite behaviour and get fainter when their activity increases. We reproduce the observed photometric trends in stellar variations with a model that treats stars as hypothetical Suns with coverage by magnetic features different from that of the Sun. The presented model attributes the variability of stellar spectra to the imbalance between the contributions from different components of the solar atmosphere, such as dark starspots and bright faculae. A stellar spectrum is calculated from spectra of the individual components, by weighting them with corresponding disc area coverages. The latter are obtained by extrapolating the solar dependences of spot and facular disc area coverages on chromospheric activity to stars with different levels of mean chromospheric activity. We have found that the contribution by starspots to the variability increases faster with chromospheric activity than the facular contribution. This causes the transition from faculae-dominated variability and direct activity--brightness correlation to spot-dominated variability and inverse activity--brightness correlation with increasing chromospheric activity level. We have shown that the regime of the variability also depends on the angle between the stellar rotation axis and the line-of-sight and on the latitudinal distribution of active regions on the stellar surface. Our model can be used as a tool to extrapolate the observed photometric variability of the Sun to Sun-like stars at different activity levels, which makes possible the direct comparison between solar and stellar irradiance data.