Near-ultraviolet Transmission Spectroscopy of HD 209458b: Evidence of Ionized Iron Beyond the Planetary Roche Lobe

The inflated transiting hot Jupiter HD 209458b is one of the best studied objects since the beginning of exoplanet characterization. Transmission observations of this system between the mid-infrared and the far-ultraviolet have revealed the signature of atomic, molecular, and possibly aerosol species in the lower atmosphere of the planet, as well as escaping hydrogen and metals in the upper atmosphere. From a re-analysis of near-ultraviolet transmission observations of HD 209458b we detect ionized iron (Fe⁺) absorption in a 100 Å-wide range around 2370 Å, lying beyond the planetary Roche lobe. However, we do not detect absorption of equally strong Fe⁺ lines expected to be around 2600 Å. Further, we find no evidence for absorption by neutral magnesium (Mg), ionized magnesium (Mg⁺), nor neutral iron (Fe). These results avoid the conflict with theoretical models previously found by Vidal-Madjar et al., which detected Mg but did not detect Mg⁺ from this same data set. Our results indicate that hydrodynamic escape is strong enough to carry atoms as heavy as iron beyond the planetary Roche lobe, even for planets less irradiated than the extreme ultra-hot Jupiters such as WASP-12 b and KELT-9 b. The detection of iron and nondetection of magnesium in the upper atmosphere of HD 209458b can be explained by a model in which the lower atmosphere forms (hence, sequesters) primarily magnesium-bearing condensates, rather than iron condensates. This is suggested by current microphysical models. The inextricable synergy between upper- and lower-atmosphere properties highlights the value of combining observations that probe both regions.