Modeling Stellar Surface Features on a Subgiant Star with an M-dwarf Companion

Understanding magnetic activity on the surface of stars other than the Sun is important for exoplanet analyses to properly characterize an exoplanet's atmosphere and to further characterize stellar activity on a wide range of stars. Modeling stellar surface features of a variety of spectral types and rotation rates is key to understanding the magnetic activity of these stars. Using data from Kepler, we use the starspot modeling program STarSPot (STSP) to measure the position and size of spots for K01-340, which is an eclipsing binary consisting of a subgiant star (T-eff = 5593 +/- 27 K, R-* = 1.98 +/- 0.05 R-circle dot) with an M-dwarf companion (M-* = 0.214 +/- 0.006 M-circle dot). STSP uses a novel technique to measure the spot positions and radii by using the transiting secondary to study and model individual active regions on the stellar surface using high-precision photometry. We find that the average size of spot features on KOI-340's primary is similar to 10% the radius of the star, i.e., two times larger than the mean size of solar-maximum sunspots. The spots on KOI-340 are present at every longitude and show possible signs of differential rotation. The minimum fractional spotted area of KOI-340's primary is 2(-2)(+12)(%), while the spotted area of the Sun is at most 0.2%. One transit of K01-340 shows a signal in the transit consistent with a plage; this plage occurs right before a dark spot, indicating that the plage and spot might be colocated on the surface of the star.

Automated summary

Understanding the magnetic activity of stars other than the Sun is crucial for studying exoplanets' atmospheres and stellar activity. By modeling the surface features of various spectral types and rotation rates, we can gain insights into the magnetic activity of these stars.