Far-ultraviolet Flares and Variability of the Young M Dwarf AU Mic: A Nondetection of Planet C in Transit at Ly<i>α</i>

Abstract Atmospheric escape’s potential to shape the exoplanet population motivates detailed observations of systems actively undergoing escape. AU Mic is a young and active M dwarf hosting two close-in transiting sub- to Neptune-sized planets. Atmospheric escape was previously detected on the inner planet b, with radially blown neutral hydrogen producing ∼30% blueshifted absorption in Ly α . We obtained one Hubble Space Telescope/Space Telescope Imaging Spectrograph transit of the outer planet c, to search for the planet’s escaping atmosphere in transmission at Ly α and compare with AU Mic b. We detected six short-duration flares in Si iv and C iv , of which only one corresponded to a Ly α flare. We identified longer-duration stellar variability at the tens of percent level for lines less sensitive to stellar activity, including O i , C ii and Ly α , which inhibits detection of an exosphere. We do not report absorption associated with an exosphere containing neutral hydrogen or any metals detectable in the far-ultraviolet, and discuss the implications of the nondetection. This work highlights the importance of (1) careful consideration of stellar variability in atmospheric escape observations, and (2) the dual influence of photoionization and stellar wind when interpreting and modeling atmospheric escape.