Escape of Water- and Metal-enriched Atmospheres from compact Hot mini-Neptunes with CHAIN

Abstract

Recent observations reveal that atmospheric compositions of close-in sub-Neptunes are diverse and can differ strongly from pure H/He-dominated. We assess the possibility of modelling metal-rich and water-rich atmospheres with CHAIN model. We evaluate the major differences between the upper atmosphere photochemistry of such atmospheres compared to H/He, and the impact on atmospheric mass loss rates. We employ CHAIN to model upper atmospheres of two warm and hot sub-Neptune-like planets which were suggested to host possibly water-/metal-rich atmospheres: GJ 9827 d and TOI-238 b. For each planet, we consider a range of compositions between H/He atmospheres with solar metallicities to highly metal- and/or water-rich atmospheres and evaluate how our predictions change with increasing metal/water fractions. We find that for considered sub-Neptunes (1) an increase in water/metal abundance leads both to the increase in atmospheric cooling rates (due to the metal line cooling and molecular cooling processes) and the increase in heating (metal line heating in dense atmospheric layers and metals' ions); (2) due to the increase in cooling and the mean particle weight of the atmosphere, the atmospheric mass loss drops significantly at high water/metal fractions (water mass fractions of >=50% or metal enrichment over 100 times solar), while at low enrichment levels mass loss rates are similar to those of H/He atmospheres with solar abundances or slightly higher; (3) for the same atmospheric mean particle weight, the escape from water-rich atmospheres is generally lower. In the context of atmospheric evolution, it implies that the atmospheres with initially high water fraction in the atmosphere are more stable. Furthermore, due to the preferential escape of H, the atmospheric metal/H ratio is expected to increase significantly with time, leading to the earlier cessation of the hydrodynamic escape.