Space-weather-driven Variations in Lyα Absorption Signatures of Exoplanet Atmospheric Escape: MHD Simulations and the Case of AU Mic b

Abstract

We simulate the space environment around AU Microscopii b and the interaction between the magnetized stellar wind and a planetary atmospheric outflow for ambient stellar wind conditions and coronal mass ejection (CME) conditions. We also calculate synthetic Lyα absorption due to neutral hydrogen in the ambient and the escaping planetary atmosphere affected by this interaction. We find that the Lyα absorption is highly variable owing to the highly varying stellar wind conditions. A strong Doppler blueshift component is observed in the Lyα profile, in contradiction to the actual escape velocity observed in the simulations themselves. This result suggests that the strong Doppler blueshift is likely attributed to the stellar wind, not the escaping neutral atmosphere, either through its advection of neutral planetary gas or through the creation of a fast neutral flow via charge exchange between the stellar wind ions and the planetary neutrals. Indeed, our CME simulations indicate a strong stripping of magnetospheric material from the planet, including some of the neutral escaping atmosphere. Our simulations show that the pressure around close-in exoplanets is not much lower, and may be even higher, than the pressure at the top of the planetary atmosphere. Thus, the neutral atmosphere is hydrodynamically escaping with a very small velocity (<15 km s−1). Moreover, our simulations show that an MHD treatment is essential in order to properly capture the coupled magnetized stellar wind and the escaping atmosphere, despite the atmosphere being neutral. This coupling should be considered when interpreting Lyα observations in the context of exoplanets’ atmospheric escape.

The Variable Detection of Atmospheric Escape around the Young, Hot Neptune AU Mic b

“…The potential intermediate stellar wind strength argued above is at odds with the extreme stellar wind strength scenarios depicted in Carolan et al (2020) and Cohen et al (2022). Carolan et al (2020), motivated by the unknown stellar wind environment of exoplanets orbiting M dwarfs, simulated AU Mic b's outflow within different stellar wind strengths.…”

A 7 Day Multiwavelength Flare Campaign on AU Mic. I. High-time-resolution Light Curves and the Thermal Empirical Neupert Effect

“…AU Mic also has a confirmed planetary system consisting of one Neptune-sized planet, which has an orbital period of 8.46 days and is situated at a distance of 0.07 au (Plavchan et al 2020), and one slightly smaller planet with a period of 18.86 days (Martioli et al 2021;Gilbert et al 2022). The space weather conditions surrounding AU Mic may make the possibility of planetary atmospheres unlikely, however (Alvarado-Gómez et al 2022;Cohen et al 2022;Klein et al 2022).…”

Star-exoplanet interactions: A growing interdisciplinary field in heliophysics

“…(Cohen et al, 2014;Saur et al, 2013)]. Though some close-in planets on extremely short orbital periods may be moving fast enough to produce a bow shock ahead of the planet as its Keplerian speed exceeds the local sound speed [for example, WASP-12b (Vidotto et al, 2011)], still other close-in planets are expected to experience sub-Alfvénic interaction with their host star winds for some or most of their orbits [for example, TRAPPIST-1e (Harbach et al, 2021) or AU Mic b Cohen et al (2022)], even when the orbital speed of the planet is accounted for. It is therefore important to understand the interaction of planetary magnetospheres and atmospheres with their host star winds under a variety of conditions.…”