Heating And Ionization By Non-thermal Electrons In The Upper Atmospheres Of Water-rich Exoplanets

By Keith Cowing
Status Report
August 14, 2023
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Heating And Ionization By Non-thermal Electrons In The Upper Atmospheres Of Water-rich Exoplanets
Some calculated properties from the slowing down of photoelectrons. Top: Fraction of the photoelectron initial energy E0 that is transferred to the gas in elastic collisions. Bottom: Number of secondary ions created by a photoelectron of initial energy E0, normalized by E0. The three color-style combinations refer to pure H (black, long dashed), pure O (black, dotted), and an O-H mixture with fOn/Hn=1/2 (blue, solid). For each mixture, the four curves that are displayed (distinguishable by their thickness) refer to fractional ionizations log10(xe)=−1 (thickest), −2, −3, and −4 (thinnest). Although not shown for the sake of clarity, the Eelas/E0 ratio for E0≳100 eV for pure H becomes essentially independent of fractional ionization at xe<10−4 . In contrast, Eelas/E0 continues to drop with xe for the other O-H mixtures. For example, for E0≳200 eV and xe∼10−6 , Eelas/E0∼0.05, and 0.07 for pure O and the O-H mixture with fOn/Hn=1/2, respectively. Calculations were done with the LG90 set of cross sections. -- astro-ph.EP

Context. The long-term evolution of an atmosphere and the remote detectability of its chemical constituents are susceptible to how the atmospheric gas responds to stellar irradiation. The response remains poorly characterized for water and its dissociation products, however, this knowledge is relevant to our understanding of hypothetical water-rich exoplanets.

Aims: Our work investigates the effect of photoelectrons, namely, the non-thermal electrons produced by photoionizing stellar radiation on the heating and ionization of extended atmospheres dominated by the dissociation products of water. Methods: We used a Monte Carlo model and up-to-date collision cross sections to simulate the slowing down of photoelectrons in O-H mixtures for a range of fractional ionizations and photoelectron energies.

Results: We find that that the fraction of energy of a photoelectron that goes into heating is similar in a pure H gas and in O-H mixtures, except for very low fractional ionizations, whereby the O atom remains an efficient sink of energy. The O-H mixtures will go on to produce more electrons because the O atom is particularly susceptible to ionization. We quantified all that information and present it in a way that can be easily incorporated into photochemical-hydrodynamical models.

Conclusions: Neglecting the role of photoelectrons in models of water-rich atmospheres will result in overestimations of the atmospheric heating and, foreseeably, the mass-loss rates as well. It will also underestimate the rate at which the atmospheric gas becomes ionized, which may have implications for the detection of extended atmospheres with Lyman-{\alpha} transmission spectroscopy. Our simulations for the small exoplanets {\pi} Men c and TRAPPIST-1 b reveal that they respond very differently to irradiation from their host stars, with water remaining in molecular form at lower pressures in the latter case.

A. García Muñoz

Comments: Published
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2308.06026 [astro-ph.EP] (or arXiv:2308.06026v1 [astro-ph.EP] for this version)
Journal reference: Astronomy & Astrophysics, Volume 672, id.A77, 12 pp., 2023
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Submission history
From: Antonio García Muñoz
[v1] Fri, 11 Aug 2023 09:12:45 UTC (739 KB)

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻