ON THE PHYSICAL NATURE OF THE WILSON-BAPPU EFFECT: Revising the gravity and temperature dependence

The Wilson-Bappu effect (WBE; Wilson & Bappu, 1957) is a well-known empirical relation for cool and giants stars between the K emission line width of Ca II and the absolute visual magnitude. To date, however, the underlying physical relation of the WBE with stellar parameters, in particular gravity and effective temperature, has not been fully established, nor its physical nature. This motivated us to find a physical relation of the WBE by current means, using a sample of 32 well-known late-type stars, with moderate activity levels, which covers four orders of magnitude of surface gravity and a representative range of effective temperature. For each star we obtained high signal-to-noise ratio (S/N) spectra from the Telescopio Internacional de Guanajuato Robótico-Espectroscópico (TIGRE-HEROS, Schmitt et al. 2014) with a spectral resolving power of R ~ 20,000 and measured the Ca II K line widths of interest, W0 and W1. The main physical parameters were determined by means of ISPEC synthesis and Gaia EDR3 parallaxes (Gaia Collaboration et al. 2021). Mass estimates are based on matching to evolution models.