In the Trenches of the Solar-Stellar Connection. VI. Total EclipSS

The Ecliptic poles Stellar Survey (EclipSS; 2018-2019) collected Hubble Space Telescope/Cosmic Origins Spectrograph (COS) 1150-1420 angstrom spectra of 49 FGK dwarfs at high ecliptic latitudes. These regions are favored by the scanning programs of the Transiting Exoplanet Survey Satellite (TESS; high-precision optical light curves) and Extended Roentgen Survey with an Imaging Telescope Array (eROSITA; repeated X-ray visits over half a decade), which can provide vital ancillary support in the exploration of high-energy "stellar activity." The present study is a follow-on, focused on new Chandra/High Resolution Camera 0.1-2.4 keV X-ray pointings, COS 1340-1720 angstrom spectra, and TESS photometry of the 10 most active EclipSS members, but also including archival X-ray and far-ultraviolet (FUV) observations of 13 representative solar neighborhood FGK dwarfs, together with high-energy irradiances of the Sun, for context. The EclipSS dK+dM binary HD 41004 proved exceptional. The unexpectedly short TESS period (1.3 days) appears to be associated with the red dwarf secondary, possibly about to engulf a close-orbiting brown-dwarf companion. The current study describes the emission-line profiles of a high-resolution subset of the full sample, finding the non-Gaussian line shapes (sharper peaks, broader wings) and redshifts of T similar to 10(5) K features to be remarkably independent of activity; quantifies X-ray and FUV "variability bias" based on several multiple-epoch campaigns; builds enhanced flux-flux diagrams (e.g., X-rays versus C iv); and constructs an "XUV" proxy flux, representing the total coronal radiative loss, to test models of the convective turnover time, tau (c). The associated Rossby number, Ro, is closely aligned to terrestrial and cosmic magnetic dynamo theories, and offers the prospect of a one-parameter description of stellar high-energy activity (relevant, for example, to host-star ionizing fluxes impacting exoplanets).