Modeling helium in exoplanet atmospheres. A revised network with photoelectron-driven processes

Abstract

Context. The He I line at 1.08 μm is a valuable tracer of atmospheric escape in exoplanet atmospheres.Aims. We expand past networks used to predict the absorbing He(2^{3S) by including, firstly, processes that involve H_{2 and some molecular ions, and secondly, the interaction of photoelectrons with the atmosphere.Methods. We survey the literature on the chemical-collisional-radiative processes that govern the production-loss of He(23S). We simulate the atmospheric outflow from the Neptune-sized GJ 436 b by coupling a hydrodynamic model that solves the bulk properties of the gas and a Monte Carlo model that tracks the energy degradation of the photoelectrons.Results. We identify Penning ionization of H as a key He(23S) loss process at GJ 436 b and update its rate coefficient to a value consistent with the most recent available cross sections. The update significantly affects the predicted strength of the He I line. For GJ 436 b, photoelectron-driven processes (mainly ionization and excitation) modify the He(23S) population in layers too deep to affect the in-transit spectrum. The situation might be different for other atmospheres though. The spectral energy distribution of the host star GJ 436 has a strong effect on the predicted in-transit signal. The published nondetections of the He I line for GJ 436 b are reasonably consistent with our model predictions for a solar-metallicity atmosphere when the model adopts a recently proposed spectral energy distribution for the star.Conclusions. The interpretation of the He I line at 1.08 μm is model dependent. Our revised network provides a general framework for extracting more robust conclusions from measurements of this line, especially in atmospheres where H2 remains abundant to high altitudes. We will explore additional, previously ignored processes in future work.}}