Evaporation of Close-in Sub-Neptunes by Cooling White Dwarfs

Abstract: Motivated by the recent surge in interest concerning white dwarf (WD) planets, this work presents the first numerical exploration of WD-driven atmospheric escape, whereby the high-energy radiation from a hot/young WD can trigger the outflow of the hydrogen-helium envelope for close-in planets. As a pilot investigation, we focus on two specific cases: a gas giant and a sub-Neptune-sized planet, both orbiting a rapidly cooling WD with mass $M_\ast$ = 0.6 \msun\ and separation $a$ = 0.02 AU. In both cases, the ensuing mass outflow rates exceed $10^{14}$ g sec$^{-1}$ for WD temperatures greater than $T_{\rm WD} \simeq$ 50,000 K. At $T_{\rm WD} \simeq$ 18,000 K [/22,000 K], the sub-Neptune [/gas giant] mass outflow rate approaches $10^{12}$ g sec$^{-1}$, i.e., comparable to the strongest outflows expected from close-in planets around late main-sequence stars. Whereas the gas giant remains virtually unaffected from an evolutionary standpoint, atmospheric escape may have sizable effects for the sub-Neptune, depending on its dynamical history, e.g., assuming that the hydrogen-helium envelope makes up 1 [/4] per cent of the planet mass, the entire envelope would be evaporated away so long as the planet reaches 0.02 AU within the first 230 [/130] Myr of the WD formation. We discuss how these results can be generalized to eccentric orbits with effective semi-major axis $a'=a/(1-e^2)^{1/4}$, which receive the same orbit-averaged irradiation. Extended to a much broader parameter space, this approach can be exploited to model the expected demographics of WD planets as a function of their initial mass, composition and migration history, as well as their potential for habitability.