Irradiation-driven escape of primordial planetary atmospheres

Abstract

Intense X-ray and ultraviolet stellar irradiation can heat and inflate the atmospheres of closely orbiting exoplanets, driving mass outflows that may be significant enough to evaporate a sizable fraction of the planet atmosphere over the system lifetime. The recent surge in the number of known exoplanets, together with the imminent deployment of new ground and space-based facilities for exoplanet discovery and characterization, requires a prompt and efficient assessment of the most promising targets for intensive spectroscopic follow-ups. For this purpose, we developed ATmospheric EScape (ATES), a new hydrodynamics code that is specifically designed to compute the temperature, density, velocity, and ionization fraction profiles of highly irradiated planetary atmospheres, along with the current, steady-state mass loss rate. ATES solves the one-dimensional Euler, mass, and energy conservation equations in radial coordinates through a finite-volume scheme. The hydrodynamics module is paired with a photoionization equilibrium solver that includes cooling via bremsstrahlung, recombination, and collisional excitation and ionization for the case of a primordial atmosphere entirely composed of atomic hydrogen and helium, whilst also accounting for advection of the different ion species. Compared against the results of 14 moderately to highly irradiated planets simulated with The PLUTO-CLOUDY Interface (TPCI), which couples two sophisticated and computationally expensive hydrodynamics and radiation codes of much broader astrophysical applicability, ATES yields remarkably good agreement at a significantly smaller fraction of the time. A convergence study shows that ATES recovers stable, steady-state hydrodynamic solutions for systems with log(−Φp)≲12.9 + 0.17 log FXUV, where Φp and FXUV are the planet gravitational potential and stellar flux (in cgs units). Incidentally, atmospheres of systems above this threshold are generally thought to be undergoing Jeans escape. The code, which also features a user-friendly graphic interface, is available publicly as an online repository.

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

“…In this Paper, we make use of the ATES 1D photo-ionization hydrodynamics code (Caldiroli et al 2021) to carry out a systematic assessment of the combined effects of planetary gravity and stellar irradiation upon atmospheric escape of primordial (hydrogen-helium) atmospheres, with a particular focus on the limits of validity of the so called energy-limited approximation (Equation 1). To investigate as broad as possible a parameter space, we simulated a set of 15 known nearby sub-Neptunes and hot-Jupiters (Table 1) over a broad range of irradiation levels.…”

“…In each band, the spectrum is normalized to the (user-provided) input EUV and X-ray luminosity values. As discussed in Section 6 of Caldiroli et al (2021), this choice is motivated by the fact that the steady-state mass loss rate depends mainly on the total number of photoionizing photons, rather than the chosen stellar spectral shape.…”

“…We draw from a sample of 25 nearby ( < ∼ 100 pc), moderately to highly irradiated gaseous planets, from sub-Neptunes 3 through hot Jupiters, examined by Spinelli et al (in preparation) as part of Paper III of this series. Compared to this parent sample, we limit our investigation to 16 systems with (i) R p > 0.15R J (where the subscript J indicates Jovian units), to justify the assumption of a large ( > ∼ 1%) hydrogen-helium envelope (Lopez & Fortney 2014); (ii) X-ray detected host stars; and (iii) irradiation and specific gravitational potential values within the converge range of ATES (ATmospheric EScape; a publicly available photo-ionization hydrodynamics code that was presented in Paper I of this series; Caldiroli et al 2021). For each target Spinelli et al present revised stellar and orbital parameters based on Gaia DR2 parallactic distances (Gaia Collaboration et al 2018), perform a re-analysis of all the archival X-ray (Chandra and XMM-Newton) data for the purpose of uniformly estimating F XUV (using the scaling relations of King et al 2018), and make use of ATES to compute the outflow temperature, ionization and density profiles, and steady-state mass loss rate.…”

“…We argue below that this slope inversion can be understood in terms of the varying fractional contributions of abiabatic vs. radiative cooling, and specifically Lyα, to the overall energy budget. The inversion is ultimately driven by the exponential dependence of the Lyα cooling coefficient on atmospheric tempera-ture; Λ Lyα ∝ exp(−T 0 /T ) s −1 , where T 0 =118,348 K (the complete functional form Λ Lyα is given in Appendix A of Paper I of this series; (Caldiroli et al 2021)). To illustrate exactly how this works, we start by discussing the behaviour of low-gravity planets, and focus on GJ 3470 b (φ p = 12.30 erg g −1 ) as our case study.…”

“…This experiment reveals a sharp transition from energy-limited escape, which is valid up to specific gravitational potential energies as high as data adopting a homogeneous data reduction, and fluxes at the planet orbital distance were calculated adopting the X-ray to XUV scaling relations by King et al (2018). The ensuing mass outflow rates are derived using ATES (Caldiroli et al 2021). From Spinelli et al, in prep. (Paper III of this series).…”