Far-ultraviolet Flares and Variability of the Young M dwarf AU Mic: a Non-detection of Planet c in Transit at Lyman-alpha

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.

Keighley E. Rockcliffe, Elisabeth R. Newton, Allison Youngblood, Girish M. Duvvuri, Emily A. Gilbert, Peter Plavchan, Peter Gao, Hans-R. Müller, Adina D. Feinstein, Thomas Barclay, Eric D. Lopez

Comments: 22 pages, 12 figures, 5 tables
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2505.17197 [astro-ph.EP] (or arXiv:2505.17197v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2505.17197
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Journal reference: AJ 169 321 (2025)
Related DOI:
https://doi.org/10.3847/1538-3881/adccc7
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Submission history
From: Keighley Rockcliffe
[v1] Thu, 22 May 2025 18:05:44 UTC (4,896 KB)
https://arxiv.org/abs/2505.17197
Astrobiology