Heating efficiency in hydrogen-dominated upper atmospheres

TL;DR

This study models the heating efficiency of stellar XUV radiation in hydrogen-rich exoplanet atmospheres, revealing it does not exceed 20%, which impacts estimates of atmospheric escape rates.

Contribution

It provides a detailed kinetic model of energy deposition and photoelectron impact processes, offering more accurate heating efficiency estimates for hydrogen-dominated exoplanet atmospheres.

Findings

Heating efficiency does not exceed 20% when photoelectron impact is included.

Previous models assuming >= 20% may overestimate atmospheric escape.

More realistic escape rates are obtained with efficiencies below 20%.

Abstract

Context. The heating efficiency is defined as the ratio of the net local gas-heating rate to the rate of stellar radiative energy absorption. It plays an important role in thermal-escape processes from the upper atmospheres of planets that are exposed to stellar soft X-rays and extreme ultraviolet radiation (XUV). Aims. We model the thermal-escape-related heating efficiency of the stellar XUV radiation in the hydrogen-dominated upper atmosphere of the extrasolar gas giant HD 209458b. The model result is then compared with previous thermal-hydrogen-escape studies which assumed heating efficiency values between 10-100%. Methods. The photolytic and electron impact processes in the thermosphere were studied by solving the kinetic Boltzmann equation and applying a Direct Simulation Monte Carlo model. We calculated the energy deposition rates of the stellar XUV flux and that of the accompanying primary photoelectrons that are caused by electron impact processes in the H2 to H transition region in the upper atmosphere. Results. The heating by XUV radiation of hydrogen-dominated upper atmospheres does not reach higher than 20% above the main thermosphere altitude, if the participation of photoelectron impact processes is included. Conclusions. Hydrogen-escape studies from exoplanets that assume heating efficiency values that are >= 20 % probably overestimate the thermal escape or mass-loss rates, while those who assumed values that are < 20% probably produce more realistic atmospheric-escape rates.