A Flexible Model for Investigating the Properties of Starspots: Model Characterization

Starspots are sites of intense magnetic fields near a star’s optical surface that actively suppress convective flows causing the area to be dimmer and cooler than the surrounding ambient photosphere. It is typically assumed that starspots trap energy below a star’s superadiabatic layer, forcing the star to thermally restructure to maintain thermal equilibrium. For this reason, starspots have been proposed to explain noted discrepancies between stellar evolution model predictions and measured properties of young stars, whereby measurements find real stars appear cooler than model predictions at a fixed luminosity. However, there is little observational evidence to firmly support the assumption that starspots force stars to thermally restructure. To establish where within a star energy becomes trapped, and how a star responds to the trapped energy, we develop a flexible model to predict how starspots affect a star's observable properties. We find that different families of starspot models lead to unique color-magnitude diagram morphologies that roughly correlate with the location and duration of trapped energy. We present a study of three starspot model families to explore how stellar colors and magnitudes respond to varying starspot surface coverages and temperature contrasts. The three families include: (1) short duration starspots that do not cause thermal restructuring, (2) deep rooted starspots that lead to thermal restructuring, and (3) shallow rooted starspots where trapped energy is redistributed throughout the surrounding photosphere by radiation. When compared against measured properties of real stars, our model can be used to test whether starspots are responsible for the anomalous properties of young stars and potentially delineate between different starspot formation mechanisms.