Irradiation-driven escape of primordial planetary atmospheres

Making use of the publicly available 1D photoionization hydrodynamics code ATES we set out to investigate combined effects planetary gravitational potential energy ($\phi_p\equiv GM_p/R_p$) and stellar X-ray Extreme Ultraviolet (XUV) irradiation ($F_{\rm XUV}$) on evaporation efficiency ($\eta$) moderately-to-highly irradiated gaseous planets, from sub-Neptunes through hot Jupiters. We show that (known) existence a threshold above which energy-limited escape (i.e., $\eta\simeq 1$) is unattainable can be inferred analytically. For $\log \phi_p\gtrsim \log \phi_p^{\rm thr}\approx [12.9-13.2]$ (in cgs units), most absorption occurs where average kinetic acquired by ions photo-electron collisions insufficient for escape. This causes plummet with increasing $\phi_p$,. Whether or not planets $\phi_p\lesssim thr}$ exhibit outflows regulated primarily level. Specifically, low-gravity $F_{\rm XUV}\simeq 10^{4-5}$ erg cm$^{-2}$s$^{-1}$ Ly$\alpha$ losses overtake adiabatic advective cooling drops below approximation, albeit remaining largely independent $\phi_p$Further, whereas $\eta$ increases as XUV}$ $\phi^{\rm thr}_p$, opposite true planets. behavior understood examining relative fractional contributions radiative function atmospheric temperature. novel framework enables reliable, physically motivated prediction expected given system; an analytical approximation best-fitting in appendix.