Papers › Irradiation-driven escape of primordial planetary atmospheres II. Evaporation...
Irradiation-driven escape of primordial planetary atmospheres II. Evaporation efficiency of sub-Neptunes through hot Jupiters
Andrea Caldiroli, Francesco Haardt, Elena Gallo, Riccardo Spinelli, Isaac Malsky, Emily Rauscher
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Making use of the publicly available 1D photoionization hydrodynamics code ATES we set out to investigate the combined effects of planetary gravitational potential energy (ϕₚ≡GMₚ/Rₚ) and stellar X-ray and Extreme Ultraviolet (XUV) irradiation (F_(XUV)) on the evaporation efficiency (η) of moderately-to-highly irradiated gaseous planets, from sub-Neptunes through hot Jupiters. We show that the (known) existence of a threshold potential above which energy-limited escape (i.e., η≃1) is unattainable can be inferred analytically. For logϕₚ≳logϕₚᵗʰʳ≈[12.9-13.2] (in cgs units), most of the energy absorption occurs where the average kinetic energy acquired by the ions through photo-electron collisions is insufficient for escape. This causes the evaporation efficiency to plummet with increasing ϕₚ,. Whether or not planets with ϕₚ≲ϕₚᵗʰʳ exhibit energy-limited outflows is regulated primarily by the stellar irradiation level. Specifically, for low-gravity planets, above F_(XUV)≃10⁴⁻⁵ erg cm⁻²s⁻¹ Lyα losses overtake adiabatic and advective cooling and the evaporation efficiency of low-gravity planets drops below the energy-limited approximation, albeit remaining largely independent of ϕₚFurther, we show that whereas η increases as F_(XUV) increases for planets above ϕᵗʰʳₚ, the opposite is true for low-gravity planets. This behavior can be understood by examining the relative fractional contributions of advective and radiative losses as a function of atmospheric temperature. This novel framework enables a reliable, physically motivated prediction of the expected evaporation efficiency for a given planetary system; an analytical approximation of the best-fitting η is given in the appendix.
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