A search for Hα absorption in the exosphere of the transiting extrasolar planet HD 209458b

There is evidence that the transiting planet HD 209458h has a large exosphere of neutral hydrogen, based on a 15% decrement in Lyman-α flux that was observed by Vidal-Madjar et al. during transits. Here we report upper limits on Hα absorption by the exosphere. The results are based on optical spectra of the parent star obtained with the Subaru High Dispersion Spectrograph. Comparison of the spectra taken inside and outside of transit reveals no exospheric Hα signal greater than 0.1% within a 5.1 Å band (chosen to have the same Δλ/λ as the 15% Lyα absorption). The corresponding limit on the column density of n = 2 neutral hydrogen is N₂ ≲ 10⁹cm^-2. This limit constrains proposed models involving a hot (∼ 10⁴K) and hydrodynamically escaping exosphere.