The origin of hydrogen around HD 209458b

TL;DR

This paper demonstrates that radiation pressure alone can account for the high-velocity hydrogen around HD 209458b, challenging previous claims that a stellar wind is necessary, and highlights the importance of atmospheric escape in the ENA model.

Contribution

It shows that radiation pressure alone can explain observed hydrogen velocities, revising previous models that required stellar wind influence.

Findings

Radiation pressure can produce high-velocity hydrogen atoms.

Stellar wind may not be necessary to explain observations.

Atmospheric escape is significant for the ENA model.

Abstract

Using numerical simulation, Holmstrom et al. (2008) proposed a plausible alternative explanation of the observed Lyman-alpha absorption that was seen during the transit of HD 209458b (Vidal-Madjar et al. 2003). They conclude that radiation pressure alone cannot explain the observations and that a peculiar stellar wind is needed. Here we show that radiation pressure alone can in fact produce the observed high-velocity hydrogen atoms. We also emphasize that even if the stellar wind is responsible for the observed hydrogen, to have a sufficient number of atoms for charge exchange with stellar wind, the energetic neutral atom (ENA) model also needs a significant escape from the planet atmosphere of similar amplitude as quoted in Vidal-Madjar et al.(2003).