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

Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191, Gif-sur-Yvette, France

^★ Corresponding author: antonio.garciamunoz@cea.fr

Received: 13 April 2025
Accepted: 15 May 2025

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(2³S) by including, firstly, processes that involve H₂ 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.