Self-consistent simulation of photoelectrons in exoplanet winds: Faster ionisation and weaker mass-loss rates

Abstract

Context. Close-in exoplanets undergo extreme irradiation levels leading to hydrodynamic atmospheric escape and the formation of planetary winds. The planetary mass-loss is governed by several physical mechanisms, including photoionisation that may impact the evolution of the atmosphere. The stellar radiation energy deposited as heat strongly depends on the energy of the primary electrons following photoionisation and on the local fractional ionisation. All these factors affect the model-estimated atmospheric mass-loss rates and other characteristics of the outflow in ways that have not been clearly elucidated. Moreover, the shape of the XUV stellar spectra strongly influences the photoionisation and heating deposition in the atmosphere. Substantial changes are to be expected in the planetary mass-loss rate. Aims. We study the effect of secondary ionisation by photoelectrons on the ionisation and heating of the gas for different planet-star systems. We focus on the local and planet-wise effects, to clearly demonstrate the significance of these interactions. Methods. Using the PLUTO code, we performed 1D hydrodynamics simulations for a variety of planets and stellar types. We included planets in the size range from Neptune to Jupiter, and stars from M dwarfs to Sun-like. Results. Our results indicate a significant decrease in the planetary mass-loss rate for all planetary systems when secondary ionisation is taken into account. The mass-loss rate is found to decrease by 43% for the more massive exoplanets to 54% for the less massive exoplanets orbiting Sun-like stars, and up to 52% for a Jovian-like planet orbiting an M-type star. Our results also indicate much faster ionisation of the atmosphere due to photoelectrons. Conclusions. We built a self-consistent model including secondary ionisation by photoelectrons to evaluate its impact on mass-loss rates. We find that photoelectrons affect the mass-loss rates by factors that are potentially important for planetary evolution theories. We also find that enhanced ionisation occurs at altitudes that are often probed with specific atomic lines in transmission spectroscopy. Future modelling of these processes should include the role of photoelectrons. For this purpose, we make a simple but accurate parameterisation for atomic hydrogen atmospheres available.