Photoevaporation is a potential explanation for several features within exoplanet demographics. Atmospheric escape observed in young Neptune-sized exoplanets can provide insight into and characterize which mechanisms drive this evolution and at what times they dominate. AU Mic b is one such exoplanet, slightly larger than Neptune (4.19 R⊕). It closely orbits a 23 Myr pre-main-sequence M dwarf with an orbital period of 8.46 days. We obtained two visits of AU Mic b at Lyα with Hubble Space Telescope (HST)/Space Telescope Imaging Spectrograph. One flare within the first HST visit is characterized and removed from our search for a planetary transit. We present a nondetection in our first visit, followed by the detection of escaping neutral hydrogen ahead of the planet in our second visit. The outflow absorbed ∼30% of the star's Lyα blue wing 2.5 hr before the planet's white-light transit. We estimate that the highest-velocity escaping material has a column density of 1013.96 cm−2 and is moving 61.26 km s−1 away from the host star. AU Mic b's large high-energy irradiation could photoionize its escaping neutral hydrogen in 44 minutes, rendering it temporarily unobservable. Our time-variable Lyα transit ahead of AU Mic b could also be explained by an intermediate stellar wind strength from AU Mic that shapes the escaping material into a leading tail. Future Lyα observations of this system will confirm and characterize the unique variable nature of its Lyα transit, which, combined with modeling, will tune the importance of stellar wind and photoionization.
The Variable Detection of Atmospheric Escape around the Young, Hot Neptune AU Mic b
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1.
California Institute of Technology
Abstract
Copyright and License
© 2023. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Acknowledgement
We sincerely thank Dr. Hans R. Müller, Dr. Brian Chaboyer, and Dr. James Owen for their help and guidance throughout the progress of this work. The authors appreciate the support, care, and community provided by the graduate students within Dartmouth College's Department of Physics & Astronomy. We would like to express our sincere appreciation for Nova, Jacques, Margot, Charlie, Edmund, and Nessie, who have begrudgingly accepted our transition back to in-person work. We will miss you, Charlie. Welcome to the family Jonah and Theo!
This research is based on observations made with the NASA/ESA Hubble Space Telescope obtained from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 526555. These observations are associated with HST-GO-15836. Support for program HST-GO-15836 was provided by NASA through a grant from the STScI.
CHIANTI is a collaborative project involving George Mason University, the University of Michigan (USA), University of Cambridge (UK), and NASA Goddard Space Flight Center (USA).
Facilities
HST/STIS -
Software References
astropy (The Astropy Collaboration et al. 2018), batman (Kreidberg 2015), cos_flares (Feinstein et al. 2022), emcee (Foreman-Mackey et al. 2013), lightkurve (Lightkurve Collaboration et al. 2018), lyapy (Youngblood et al. 2016), matplotlib (Hunter 2007), numpy (Harris et al. 2020), scipy (Virtanen et al. 2020), stistools (https://github.com/spacetelescope/stistools)
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Additional details
- ISSN
- 1538-3881
- National Aeronautics and Space Administration
- NAS 526555
- National Aeronautics and Space Administration
- HST-GO-15836
- Caltech groups
- Infrared Processing and Analysis Center (IPAC)