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

doi:10.1051/0004-6361/202142763

paper

Irradiation-driven escape of primordial planetary atmospheres II. Evaporation efficiency of sub-Neptunes through hot Jupiters

arXiv:2112.00744 · doi:10.1051/0004-6361/202142763

Abstract

Making use of the publicly available 1D photoionization hydrodynamics code ATES we set out to investigate the combined effects of planetary gravitational potential energy ( $ϕ_{p} \equiv G M_{p} / R_{p}$ ) 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 $lo g ϕ_{p} ≳ lo g ϕ_{p}^{thr} \approx [12.9 - 13.2]$ (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 $ϕ_{p}$ ,. Whether or not planets with $ϕ_{p} ≲ ϕ_{p}^{thr}$ exhibit energy-limited outflows is regulated primarily by the stellar irradiation level. Specifically, for low-gravity planets, above $F_{XUV} ≃ 1 0^{4 - 5}$ erg cm $^{- 2}$ s $^{- 1}$ Ly $α$ 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 $ϕ_{p}$ Further, we show that whereas $η$ increases as $F_{XUV}$ increases for planets above $ϕ_{p}^{thr}$ , 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.

12 pages, 8 figures. Accepted for publication in A&A. In the new version, section 4 and 5 have been completely revised after the referee revision. Comments are welcome

Irradiation-driven escape of primordial planetary atmospheres II. Evaporation efficiency of sub-Neptunes through hot Jupiters

Abstract

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