Effects of Charge Exchange on the Evaporative Wind of HD 209458b

TL;DR

This study uses 3D hydrodynamic simulations to investigate whether charge exchange processes between stellar and planetary winds can explain high-velocity absorption features in the evaporative wind of HD 209458b, concluding they cannot within the examined parameters.

Contribution

The paper provides the first self-consistent 3D hydrodynamic simulation of charge exchange effects in exoplanetary winds, specifically for HD 209458b, and compares results with analytic predictions.

Findings

Charge exchange does not account for high-velocity absorption in the simulated conditions.

Simulations confirm analytic limits on ENA production via charge exchange.

Stellar wind parameters examined are insufficient to produce observed absorption velocities.

Abstract

The role of charge exchange in shaping exoplanet photoevaporation remains a topic of contention. Exchange of electrons between stellar wind protons from the exoplanet's host star and neutral hydrogen from the planet's wind has been proposed as a mechanism to create "energetic neutral atoms" (ENAs), which could explain the high absorption line velocities observed in systems where mass loss is occurring. In this paper we present results from 3D hydrodynamic simulations of the mass loss of a planet similar to HD 209458b. We self-consistently launch a planetary wind by calculating the ionization and heating resulting from incident high-energy radiation, inject a stellar wind into the simulation, and allow electron exchange between the stellar and planetary winds. We predict the potential production of ENAs by the wind-wind interaction analytically, then present the results of our simulations, which confirm the analytic limits. Within the limits of our hydrodynamic simulation, we find that charge exchange with the stellar wind properties examined here is unable to explain the absorption observed at high Doppler velocities.