Observationally Constraining the Starspot Properties of Magnetically Active M67 Sub-subgiant S1063

Our understanding of the impact of magnetic activity on stellar evolution continues to unfold. This impact is seen in sub-subgiant stars, defined to be stars that sit below the subgiant branch and red of the main sequence in a cluster color-magnitude diagram. Here we focus on S1063, a prototypical sub-subgiant in open cluster M67. We use a novel technique combining a two-temperature spectral decomposition and light-curve analysis to constrain starspot properties over a multiyear time frame. Using a high-resolution near-infrared IGRINS spectrum and photometric data from K2 and ASAS-SN, we find a projected spot filling factor of 32% +/- 7% with a spot temperature of 4000 +/- 200 K. This value anchors the variability seen in the light curve, indicating the spot filling factor of S1063 ranged from 20% to 45% over a four-year time period with an average spot filling factor of 30%. These values are generally lower than those determined from photometric model comparisons but still indicate that S1063, and likely other sub-subgiants, are magnetically active spotted stars. We find observational and theoretical comparisons of spotted stars are nuanced due to the projected spot coverage impacting estimates of the surface-averaged effective temperature. The starspot properties found here are similar to those found in RS CVn systems, supporting classifying sub-subgiants as another type of active giant star binary system. This technique opens the possibility of characterizing the surface conditions of many more spotted stars than previous methods, allowing for larger future studies to test theoretical models of magnetically active stars.

Automated summary

Our understanding of the impact of magnetic activity on stellar evolution is expanding as we uncover new findings. In this study, we focused on S1063, a prototypical sub-subgiant in the open cluster M67, and used a novel technique to analyze starspots over a multiyear time frame. The results show that the spot filling factor of S1063 ranged from 20% to 45% over four years, with an average of 30%. These findings suggest that sub-subgiants, including S1063, are magnetically active spotted stars, similar to RS CVn systems.