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

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 3 S) 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(2 3 S). 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(2 3 S) 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(2 3 S) 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 H 2 remains abundant to high altitudes. We will explore additional, previously ignored processes in future work.