How Can We Constrain Stellar Coronal Mass Ejections? A Proof of Concept for Coronal Dimming Using Archival Far Ultraviolet Spectrophotometry of ε Eridani

Coronal mass ejections (CMEs), frequent partners to flares, are a regular fact of space weather in the solar system. CMEs may contribute significantly to solar/stellar spin down and, when they impact planets, they perturb atmospheric chemistry and strip mass. The regular flares of other stars strongly suggest regular CMEs, but direct evidence is sparse. On the Sun, a reliable indicator of CMEs is the drop in coronal emission that results directly from the ejection and loss of emitting material, termed “coronal irradiance dimming.” Generalizing this diagnostic for use with stellar observations can enable novel constraints on stellar CMEs. We present the results from a search for post-flare coronal dimming on the 400 Myr K2 dwarf ε Eridani using archival HST spectra. The analysis depends on coronal Fe XII (formed at ~1 MK) and Fe XXI (formed at ~10 MK) emission lines in far-ultraviolet spectra observed by the Cosmic Origins Spectrograph. From two upper limits and one inconclusive dimming event, we find that flare-associated CMEs with masses greater than a few 10¹⁵ g, which occur every few days on the Sun, are unlikely to occur more than 10x as frequently on the younger, more active ε Eridani. These results, based on less than a day of data, demonstrate the potential value of coronal dimming observations for characterizing the space weather environments of stars other than the Sun.