Space Weather-driven Variations in Ly$alpha$ Absorption Signatures of Exoplanet Atmospheric Escape: MHD Simulations and the Case of AU Mic b. (arXiv:2205.08900v2 [astro-ph.EP] UPDATED)
<a href="http://arxiv.org/find/astro-ph/1/au:+Cohen_O/0/1/0/all/0/1">O. Cohen</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Alvarado_Gomez_J/0/1/0/all/0/1">J.D. Alvarado-Gomez</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Drake_J/0/1/0/all/0/1">J.J. Drake</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Harbach_L/0/1/0/all/0/1">L.M. Harbach</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Garraffo_C/0/1/0/all/0/1">C. Garraffo</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Fraschetti_F/0/1/0/all/0/1">F. Fraschetti</a>
We simulate the space environment around AU Microscopii b and the interaction
between the magnetized stellar wind with a planetary atmospheric outflow for
ambient stellar wind conditions and Coronal Mass Ejection (CME) conditions. We
also calculate synthetic Ly$alpha$ absorption due to neutral hydrogen in the
ambient and the escaping planetary atmosphere affected by this interaction. We
find that the Ly$alpha$ absorption is highly variable due to the
highly-varying stellar wind conditions. A strong Doppler blue-shift component
is observed in the Ly$alpha$ profile, in contradiction to the actual escape
velocity observed in the simulations themselves. This result suggest that the
strong Doppler blue-shift 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 of the atmosphere being neutral. This coupling
should be considered when interpreting Ly$alpha$observations in the context of
exoplanets atmospheric escape.
We simulate the space environment around AU Microscopii b and the interaction
between the magnetized stellar wind with a planetary atmospheric outflow for
ambient stellar wind conditions and Coronal Mass Ejection (CME) conditions. We
also calculate synthetic Ly$alpha$ absorption due to neutral hydrogen in the
ambient and the escaping planetary atmosphere affected by this interaction. We
find that the Ly$alpha$ absorption is highly variable due to the
highly-varying stellar wind conditions. A strong Doppler blue-shift component
is observed in the Ly$alpha$ profile, in contradiction to the actual escape
velocity observed in the simulations themselves. This result suggest that the
strong Doppler blue-shift 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 of the atmosphere being neutral. This coupling
should be considered when interpreting Ly$alpha$observations in the context of
exoplanets atmospheric escape.
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