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 Å 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 Å 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 ∼ 105 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, τ 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).