Stellar Properties of Active G and K Stars: Exploring the Connection Between Starspots and Chromospheric Activity

Abstract

We gathered high resolution spectra for an ensemble of 55 bright active and inactive stars using the ARC 3.5 m Telescope Echelle Spectrograph at Apache Point Observatory ($R\approx$31,500). We measured spectroscopic effective temperatures, surface gravities and metallicities for most stars in the sample with SME and MOOG. Our stellar property results are consistent with the photometric effective temperatures from the Gaia DR2 pipeline. We also measured their chromospheric $S$ and $\log R_{\mathrm{HK}}^{\prime }$ indices to classify the stars as active or inactive and study the connection between chromospheric activity and starspots. We then attempted to infer the starspot covering fractions on the active stars by modeling their spectra as a linear combination of hot and cool inactive stellar spectral templates. We find that it is critical to use precise colors of the stars to place stringent priors on the plausible spot covering fractions. The inferred spot covering fractions generally increase with the chromospheric activity indicator $\log R_{\mathrm{HK}}^{\prime }$, though we are largely insensitive to spot coverages smaller than $f_S\lesssim 20$%. We find a dearth of stars with small $\log R_{\mathrm{HK}}^{\prime }$ and significant spot coverages.

arXiv commentAccepted for publication in AJ

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