Spectral, Spatial, and Time Properties of the Hydrogen Nebula around Exoplanet HD 209458b

Abstract

All far-ultraviolet (FUV) observations of HD 209458 tend to support a scenario in which the inflated hydrogen atmosphere of its planetary companion strongly absorbs the stellar Lyα flux during transit. However, it is not clear how the transit absorption depends on the selected wavelength range in the stellar line profile, nor how the atomic hydrogen cloud is distributed spatially around HD 209458b. Here we report a sensitivity study of observed time and spectral variations of the stellar flux. In particular, the sensitivity of the absorption depth during transit to the assumed spectral range in the stellar line profile is shown to be very weak, leading to a transit depth in the range (8.4%-8.9%)+/-2.0% for all possible wavelength ranges, and thereby confirming our initially reported absorption rate. Taking the ratio of the line profile during transit to the unperturbed line profile, we also show that the spectral signature of the absorption by the exoplanetary hydrogen nebula is symmetric and typical of a Lorentzian, optically thick medium. Our results question the adequacy of models that require a huge absorption and/or a strong asymmetry between the blue and red sides of the absorption line during transit, as no such features could be detected in the HST FUV absorption profile. Finally, we show that standard atmospheric models of HD 209458b provide a good fit to the observed absorption profile during transit. Other hybrid models assuming a standard model with a thin layer of superthermal hydrogen on top remain possible.