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

TL;DR

This study uses self-consistent 1D hydrodynamics simulations to show that secondary ionisation by photoelectrons significantly accelerates atmospheric ionisation and reduces planetary mass loss rates, impacting exoplanet evolution models.

Contribution

It introduces a self-consistent model including photoelectron effects, revealing faster ionisation and lower mass loss rates across various exoplanetary systems, which was not previously quantified.

Findings

Mass loss rates decrease by up to 54% when secondary ionisation is included.

Photoelectrons cause faster atmospheric ionisation at observable altitudes.

A parameterisation for atomic hydrogen atmospheres is provided.

Abstract

Planetary mass loss is governed by several physical mechanisms, including photoionisation that may impact the evolution of the atmosphere. Stellar radiation energy deposited as heat depends strongly 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. The shape of the XUV stellar spectra influences strongly the photoionisation and heating deposition on the atmosphere. We elaborate on the local and planet-wise effects, to clearly demonstrate the significance of such interactions. Using the PLUTO code, we performed 1D hydrodynamics simulations from Neptune to Jupiter size planets and stars from M dwarfs to Sun-like. Our results indicate a significant decrease of 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 exoplanet to 54$\%$ for the less massive exoplanet orbiting solar-like stars, and up to 52$\%$ for a Jupiter-like planet orbiting a M type star. Our results also indicate much faster ionisation of the atmosphere due to photoelectrons. We built a self-consistent model including secondary ionisation by photoelectron 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. Finally, we make available a simple yet accurate parameterisation for atomic hydrogen atmospheres.