Deuterium Escape on Photoevaporating Sub-Neptunes

We investigate the evolution of the deuterium-to-hydrogen (D/H) mass ratio driven by EUV photoevaporation of hydrogen-rich atmospheres of close-in sub-Neptunes around solar-type stars. For the first time, the diffusion-limited approach in conjunction with energy-limited photoevaporation is considere...

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Bibliographic Details
Published in:Astrophysical journal. Letters 2023-08, Vol.953 (2), p.L27
Main Authors: Gu, Pin-Gao, Chen, Howard
Format: Article
Language:English
Subjects:
Deuterium
Energy limitation
Exoplanet atmospheric composition
Exoplanet atmospheric evolution
Exoplanets
Extrasolar planets
Fractionation
Helium
Hydrogen
Mini Neptunes
Neptune
Planetary evolution
Planets
Target detection
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Summary:We investigate the evolution of the deuterium-to-hydrogen (D/H) mass ratio driven by EUV photoevaporation of hydrogen-rich atmospheres of close-in sub-Neptunes around solar-type stars. For the first time, the diffusion-limited approach in conjunction with energy-limited photoevaporation is considered in evaluating deuterium escape from evolving exoplanet H/He envelopes. We find that the planets with smaller initial gas envelopes and thus smaller sizes can lead to weaker atmospheric escape, which facilitates hydrogen–deuterium fractionation. Specifically, in our grid of simulations with a low envelope mass fraction of less than 0.005, a low-mass sub-Neptune (4–5 M ⊕ ) at about 0.25–0.4 au or a high-mass sub-Neptune (10–15 M ⊕ ) at about 0.1–0.25 au can increase the D/H values by greater than 20% over 7.5 Gyr. Akin to the helium-enhanced envelopes of sub-Neptunes due to photoevaporating escape, the planets along the upper boundary of the radius valley are the best targets to detect high D/H ratios. The ratio can rise by a factor of ≲1.65 within 7.5 Gyr in our grid of evolutionary calculations. The D/H ratio is expected to be higher in thinner envelopes as long as the planets do not become bare rocky cores.
ISSN:2041-8205
2041-8213
DOI:10.3847/2041-8213/acee01