Secondary ionisation in hot atmospheres and interactions between planetary and stellar winds

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

^★ Corresponding author; alexandre.gillet@cea.fr

Received: 10 October 2024
Accepted: 4 February 2025

Abstract

Context. The loss of close-in planetary atmospheres is influenced by various physical processes, such as photoionisation, which could potentially affect the atmosphere survivability on a secular timescale. The amount of stellar radiation converted into heat depends on the energy of the primary electrons produced by photoionisation and the local ionisation fraction. The H I Lyman-alpha (Lyα) line is an excellent probe for atmospheric escape, but the reason for the origin of the high velocities detected in this line is not yet fully understood.

Aims. We characterise the effect of secondary ionisation by photoelectrons on the ionisation and heating of the gas for different planet-star systems in a 2D geometry. We study the interaction between the planetary and stellar winds, the difference in the predicted mass-loss rates between 1D and 2D models, the Lyα signal, and the impact of stellar flares.

Methods. Using the PLUTO code, we performed 2D hydrodynamic simulations for four planets in the Neptune to Jupiter size range. We produced sunthetic Lyα profiles to determine the origin of the signal and in particular its high-velocity Doppler shift.

Results. We find a similar trend to that shown in the 1D models, with a decrease in the planetary mass-loss rate for all systems when secondary ionisation is taken into account. The mass-loss rates decrease by 48% for the least massive planet when secondary ionisation is accounted for. However, the decrease is less pronounced in 2D than in 1D. We observe differences in the Lyα profile between the different cases and significant asymmetries in all of them, especially for the lower-mass planets. Finally, we observe that stellar flares do not affect the mass-loss rate because they act, in general, on a timescale that is too short.

Conclusions. We find and confirm in our extended 2D model that photoelectrons affect mass-loss rates by factors that are potentially important for planetary evolution theories, and that they also affect the Lyα profile. We find velocities in the escaping atmosphere of up to 100 km/s, with the gas moving away from the star, which could be the result of the interaction with the stellar wind. We find that stellar flares generally occur on a timescale that is too short to have a visible impact on the mass-loss rate of the atmosphere.