Exoplanetology: Exoplanets & Exomoons

Physically Motivated Analytic Model of Energy Efficiency for EUV-driven Atmospheric Escape of Close-in Exoplanets

By Keith Cowing
Status Report
astro-ph.EP
February 13, 2025
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Physically Motivated Analytic Model of Energy Efficiency for EUV-driven Atmospheric Escape of Close-in Exoplanets
Estimated efficiencies (left panel) and mass-loss rates (right panel) for fixed planetary mass Mp = 0.7 MJ but different EUV fluxes from FEUV = 100 erg/s/cm2 (red) to FEUV = 106 erg/s/cm2 (orange). The efficiency of the traditional recombination-limited approach is shown in dashed curves. In the intense EUV flux case, our model predictions are almost the same as the traditional approach (orange and purple). The fitted efficiencies from 1D detailed simulations by Caldiroli et al. (2021) are shown as points. Some points of the strong-gravity case (Mp > 20 MJ) can not be seen in this panel due to the low efficiency. — astro-ph.EP

Extreme Ultraviolet (EUV) driven atmospheric escape is a key process in the atmospheric evolution of close-in exoplanets.

In many evolutionary models, the energy-limited mass-loss rate with a constant efficiency (typically ∼10%) is assumed for calculating the mass-loss rate. However, hydrodynamic simulations have demonstrated that this efficiency depends on various stellar and planetary parameters. Comprehending the underlying physics of the efficiency is essential for understanding planetary atmospheric evolution and recent observations of the upper atmosphere of close-in exoplanets.

We introduce relevant temperatures and timescales derived from physical principles to elucidate the mass-loss process. Our analytical mass-loss model is based on phenomenology and consistent across a range of planetary parameters. We compare our mass-loss efficiency and the radiation hydrodynamic simulations. The model can predict efficiency in both energy-limited and recombination-limited regimes.

We further apply our model to exoplanets observed with hydrogen absorption (Lyα and Hα). Our findings suggest that Lyα absorption is detectable in planets subjected to intermediate EUV flux; under these conditions, the escaping outflow is insufficient in low-EUV environments, while the photoionization timescale remains short in high-EUV ranges.

Conversely, Hα absorption is detectable under high EUV flux conditions, facilitated by the intense Lyα flux exciting hydrogen atoms. According to our model, the non-detection of neutral hydrogen can be explained by a low mass-loss rate and is not necessarily due to stellar wind confinement or the absence of a hydrogen-dominated atmosphere in many cases. This model assists in identifying future observational targets and explicates the unusual absorption detection/non-detection patterns observed in recent studies.

Hiroto Mitani, Riouhei Nakatani, Rolf Kuiper

Comments: 12 pages, 8 figures, 2 tables, accepted for publication in A&A
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2502.08398 [astro-ph.EP] (or arXiv:2502.08398v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2502.08398
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Submission history
From: Hiroto Mitani
[v1] Wed, 12 Feb 2025 13:41:40 UTC (744 KB)
https://arxiv.org/abs/2502.08398
Astrobiology,

Keith Cowing

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) 🖖🏻

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