Hα absorption in transiting exoplanet atmospheres

Abstract

Recent observations by Jensen et al. of Hα absorption by the upper atmosphere of HD189733b have motivated the need for a theoretical understanding of the distribution of n=2 hydrogen within hot Jupiter atmospheres. With this in mind, we model the n=2 state of atomic hydrogen in a hydrostatic atmosphere in thermal and photoionization equilibrium. Both collisional and radiative transitions are included in the calculation of the n = 2 state level population. In our model, the Hα absorption is dominated by a τ ~ 1 shell composed of metastable 2s hydrogen located within the neutral atomic layer, with the contribution coming roughly uniformly throughout the layer instead of from a specific impact parameter. An ionization rate an order of magnitude over the expected value can reproduce the observed transit depth. Copyright © 2013, International Astronomical Union.