Deuterium Escape on Photoevaporating Sub-Neptunes

• Published in: Astrophysical journal. Letters Vol. 953; no. 2; p. L27
• Main Authors: Gu, Pin-Gao, Chen, Howard
• Format: Journal Article
• Language: English
• Published: Austin The American Astronomical Society 01-08-2023; IOP Publishing
• Subjects: Deuterium; Energy limitation; Exoplanet atmospheric composition; Exoplanet atmospheric evolution; Exoplanets; Extrasolar planets; Fractionation; Helium; Hydrogen; Mini Neptunes; Neptune; Planetary evolution; Planets; Target detection
• ISSN: 2041-8205; 2041-8213
• DOI: 10.3847/2041-8213/acee01

Abstract
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.