# Effects of Radiation Pressure on the Evaporative Wind of HD 209458b

## TL;DR

This study uses high-resolution 3-D hydrodynamic simulations to assess whether radiation pressure influences the photoevaporative wind of HD 209458b, concluding it is unlikely to be a significant factor within expected flux limits.

## Contribution

The paper provides the first detailed 3-D hydrodynamic simulation analysis of radiation pressure effects on HD 209458b's wind, confirming its limited role in driving high-velocity atmospheric escape.

## Key findings

- Radiation pressure is unlikely to significantly influence the wind at expected Lyman-$\alpha$ fluxes.
- Simulations confirm analytic limits on radiation pressure effects.
- High velocities in observed winds are not explained by radiation pressure alone.

## Abstract

The role of radiation pressure in shaping exoplanet photoevaporation remains a topic of contention. Radiation pressure from the exoplanet's host star has been proposed as a mechanism to drive the escaping atmosphere into a "cometary" tail and explain the high velocities observed in systems where mass loss is occurring. In this paper we present results from high-resolution 3-D hydrodynamic simulations of a planet similar to HD 209458b. We self-consistently launch a wind flowing outward from the planet by calculating the ionization and heating resulting from incident high-energy radiation, and account for radiation pressure. We first present a simplified calculation, setting a limit on the Lyman-$\alpha$ flux required to drive the photo-evaporated planetary material to larger radii and line-of-sight velocities. We then present the results of our simulations, which confirm the limits determined by our analytic calculation. We thus demonstrate that, within the limits of our hydrodynamic simulation and for the Lyman-$\alpha$ fluxes expected for HD 209458, radiation pressure is unlikely to significantly affect photoevaporative winds or to explain the high velocities at which wind material is observed, though further possibilities remain to be investigated.