Helium Depletion in Escaping Atmospheres of Sub-Neptunes: A Signature of Primary-to-Secondary Transition

Abstract

Short-period sub-Neptunes are common in extrasolar systems. These sub-Neptunes are generally thought to have primary atmospheres of protoplanetary-disk gas origin. However, atmospheric escape followed by degassing from their interiors can lead to the transition to secondary atmospheres depleted in gases less-soluble to magma, such as helium. These primary and secondary atmospheres can potentially be distinguished from observations of escaping hydrogen and helium. This study aims to elucidate the impact of the primary-secondary transition on atmospheric compositions of short-period sub-Neptunes. We simulate their evolution with atmospheric escape driven by stellar X-ray and extreme ultraviolet irradiation and degassing of hydrogen, helium, and water from their rocky interiors, with a one-dimensional structure model. We show that the transition takes place for low-mass, close-in planets which experience extensive atmospheric escape. These planets show the depletion of helium and enrichment of water in their atmospheres, because of their low and high abundances in the planetary interiors, respectively. A compilation of our parameter survey (the orbital period, planetary mass, envelope mass, and mantle FeO content) shows a correlation between the planet radius and the helium escape rate. We suggest that the transition from primary to secondary atmospheres may serve an explanation for helium non-detection for relatively-small ($\lesssim 2.5\ R_\oplus$) exoplanets.