Simulating energetic neutral hydrogen in exoplanet atmospheres

Far-ultraviolet observations of exoplanet atmospheres at the Lyman alpha line reveal the existence of hydrogen atoms out to several planetary radii. The observed transmission spectrum may be modeled as absorption by atoms traveling at speeds exceeding 100km/s, much faster than typical atoms in the planet’s atmosphere. We investigate the ability of hot stellar wind protons to transfer momentum to the atoms in the time before the atom is ionized by collisions with stellar wind electrons or stellar photons. To this end, we have carried out a Monte-Carlo simulation of the interaction of hydrogen atoms with a gas of hot protons and electrons, taking into account elastic collisions, radiation pressure forces and ionization. The resultant distribution of velocities is then used to create a line profile for absorption of stellar Lyman alpha by the atoms. We compare our model line profile to Hubble STIS measurements of HD 209458b.