In the Trenches of the Solar–Stellar Connection. III. The HST/COS Ecliptic-poles Stellar Survey (EclipSS)

Abstract

Abstract The Ecliptic-poles Stellar Survey (EclipSS) collected far-ultraviolet (FUV: 1160–1420 Å) spectra of 49 nearby (d ≲ 100 pc) F3–K3 main-sequence stars, located at high ecliptic latitudes (north and south), using the Cosmic Origins Spectrograph of the Hubble Space Telescope. The ecliptic poles receive higher exposures from scanning missions like the Transiting Exoplanet Survey Satellite (high-precision optical photometry) and Extended Roentgen Survey with an Imaging Telescope Array (X-ray monitoring), which can deliver crucial contextual information, not otherwise easily secured. The objective was to support theoretical studies of stellar hot outer atmospheres—chromospheres (∼104 K) and coronae (≳1 MK)—which, among other things, can adversely impact exoplanets via host-star “space weather.” Flux–flux diagrams (e.g., C ii 1335 Å versus O i 1306 Å) were constructed for the EclipSS stars, solar Cycle 23/24 irradiances, and long-term FUV records of α Cen A (G2 V) and B (K1 V). The EclipSS cohort displays similar minimum (“basal”) fluxes to the Sun and solar twin α Cen A, in chromospheric O i 1306 Å. In hotter C ii 1335 Å, a downward slump of the basal fluxes—noted in previous, less controlled surveys—can now be explained as an effect of subsolar abundances. The consistent basal minima in chromospheric and higher temperature species at solar metallicity favor the idea that stellar analogs of the solar supergranulation network provide a baseline of high-energy emissions. The magnetic network is replenished by a “local dynamo” independently of the stellar spin. It can operate even when the starspot-spawning internal dynamo has ceased cycling, as during the Sun’s 17th century Maunder Minimum.