Far-ultraviolet flares and variability of the young M dwarf AU Mic: a non-detection of planet c in transit at Lyman-alpha

TL;DR

This study used HST/STIS to observe the outer planet c of AU Mic, finding no evidence of an escaping atmosphere in Lyman-alpha, highlighting the challenges posed by stellar variability in detecting exospheres.

Contribution

First detailed Lyman-alpha transit observation of AU Mic c, emphasizing the impact of stellar activity on atmospheric escape detection and interpretation.

Findings

No neutral hydrogen exosphere detected around AU Mic c.

Stellar variability complicates the detection of planetary atmospheres.

Flares observed did not correlate with Lyman-alpha activity.

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% blue-shifted absorption in Lyman-alpha. We obtained one HST/STIS transit of the outer planet c, to search for the planet's escaping atmosphere in transmission at Lyman-alpha and compare with AU Mic b. We detected 6 short-duration flares in Si IV and C IV, of which only one corresponded to a Lyman-alpha 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 Lyman-alpha, 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 non-detection. 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.