Non-LTE effects in atmospheres of warm and hot Neptunes: implications for the atmospheric mass loss

The warm- and hot-Neptune-type planets are known to experience significant atmospheric mass loss and are easier to observe and characterize in comparison to terrestrial-like planets. They represent, therefore, interesting targets in the context of planetary atmospheric evolution and star-planet interaction. The majority of studies, however, employ hydrodynamic simulations relying on simplified approaches in describing photoionization processes, or, on the other hand, use high-level photoionization solvers not accounting for dynamical effects.
In this study, we aim to outline for which types of planets in the Neptune-like mass range the non-LTE effects become important (in particular - in terms of atmospheric mass loss) and to which extent the hydrodynamic effects (as adiabatic expansion) influence the theoretically predicted transmission spectra.
To achieve this goal, we combine our 1D hydrodynamic upper atmosphere model (Kubyshkina et al., 2018) with the latest version of the non-LTE photoionization and radiative transfer code Cloudy (Ferland et al., 2017), which accounts for ionization and dissociation, atomic level transitions and chemical reactions for the lightest chemical elements up to zink. Thus, the former is responsible for resolving the hydrodynamic outflow and the latter solves the realistic photoionization and heating of the planetary atmosphere. We verify this scheme by comparing our results for a dozen of the high profile planets with the predictions of Salz et al., 2016, which use a similar approach.
We apply this hybrid framework to model the upper atmospheres of a range of Neptune-like and terrestrial-like planets with masses between 1 and 50 Mearth, changing systematically the orbital parameters (and thus, equilibrium temperature), and the irradiation level from the host star. We find, that for the majority of warm and hot Neptunes, accounting for realistic non-LTE ionization/heating affects significantly the basic parameters of planetary atmospheres, predicting cooler, slower, and denser outflow compared to the predictions of the pure hydrodynamic model. The predictions of the hydrodynamic and hybrid models become closer at the highest levels of stellar radiation.
In turn, accounting for hydrodynamic effects, in particular for expansion cooling, considerably affects the predictions of Cloudy models, and is therefore important for the interpretation of observations.