Characterizing magnetic activity through the lens of sub-subgiant stars

As our knowledge of stellar evolution unfolds the complexities introduced by magnetic stellar activity become more apparent. One example is seen in sub-subgiants, magnetically active stars with starspots that sit below the subgiant branch and red of the main sequence on a cluster color-magnitude diagram. Sub-subgiants exhibit strong variable light curves as starspots rotate in and out of view. There is much to learn from one such system: S1063, a prototypical sub-subgiant in M67 with a rotation period of 23.5 days. A two-temperature spectral decomposition technique allows us to constrain the surface conditions of S1063, including the spot covering fraction and spot and ambient photosphere temperatures. We link these surface condition constraints to a multi-year light curve, finding the spot covering fraction varies between 20% to 45% over four years. This analysis brings to light some important nuances when considering observational and theoretical comparisons of spotted stars. We are currently expanding this study by observing field sub-subgiants using IGRINS with contemporaneous or near-contemporaneous TESS coverage. This technique opens the possibility of characterizing the surface conditions of many more spotted stars than previous methods, allowing for larger studies capable of testing theoretical models of magnetically active stars.