A Relationship between Stellar Age and Spot Coverage

Abstract

We investigate starspot distributions consistent with space-based photometry of F, G, and K stars in six stellar associations ranging in age from 10 Myr to 4 Gyr. We show that a simple light-curve statistic called the “smoothed amplitude” is proportional to stellar age as t −1/2 , following a Skumanich-like spin-down relation. We marginalize over the unknown stellar inclinations by forward modeling the ensemble of light curves for direct comparison with the Kepler , K2 , and TESS photometry. We sample the posterior distributions for spot coverage with approximate Bayesian computation. We find typical spot coverages in the range 1%–10%, which decrease with increasing stellar age. The spot coverage is proportional to t n where n  = −0.37 ± 0.16, also statistically consistent with a Skumanich-like t −1/2 decay of starspot coverage with age. We apply two techniques to estimate the spot coverage of young exoplanet-hosting stars likely to be targeted for transmission spectroscopy with the James Webb Space Telescope , and estimate the bias in exoplanet radius measurements due to varying starspot coverage.