In the Trenches of the Solar–Stellar Connection. VII. Wilson–Bappu 2022

Abstract

The Wilson–Bappu effect (WBE) is a remarkable correlation between the width of the chromospheric emission core of the Ca ii 3934 Å K line and stellar absolute visual magnitude over an astonishing range, from diminutive red dwarfs to bloated yellow supergiants. The WBE signifies a deep connection between the nonclassical, mechanically heated chromosphere and fundamental stellar properties. The present study considers the parallel WBE in the analogous Mg ii k line at 2796 Å, utilizing near-ultraviolet spectra from Hubble Space Telescope (HST), and stellar parameters mainly derived from Gaia. The final sample is 300 stars (301 including the Sun). The high cosmic abundance of magnesium allowed Mg ii k emissions to be recovered in the warmer F-types, at the edge of convection and chromospheric activity, and in low-activity Sunlike dwarfs—situations in which Ca ii K might be too weak to measure. Nevertheless, the derived slope of the Mg ii WBE was similar to previous reports based on the K line; as well as other Mg ii efforts with smaller samples of pre-HST spectra. Further, a decomposition of the Mg ii WBE in temperature, gravity, activity, and metallicity yielded similar results to previous Ca ii work, but with the greater clarity afforded by the large, diverse HST sample. Parametric dependences of the k-line WBE match, tolerably well, predictions based on the “ionization-valve” theory of chromospheric structure. In the end, the WBE is not merely an astrophysical curiosity, but rather informs the question of how chromospheres—arguably the central bastion of nonclassical heating—have come to be.