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The inflated transiting hot Jupiter HD 209458b is one of the best studiedobjects since the beginning of exoplanet characterization. Transmissionobservations of this system between the mid infrared and the far ultraviolethave revealed the signature of atomic, molecular, and possibly aerosol speciesin the lower atmosphere of the planet, as well as escaping hydrogen and metalsin the upper atmosphere. From a re-analysis of near-ultraviolet (NUV)transmission observations of HD 209458b, we detect ionized iron (Fe II)absorption in a 100 A-wide range around 2370 A, lying beyond the planetaryRoche lobe. However, we do not detect absorption of equally strong Fe II linesexpected to be around 2600 A. Further, we find no evidence for absorption byneutral magnesium (Mg I), ionized magnesium (Mg II), nor neutral iron (Fe I).These results avoid the conflict with theoretical models previously found byVidal-Madjar et al. (2013), which detected Mg I but did not detect Mg II fromthis same data set. Our results indicate that hydrodynamic escape is strongenough to carry atoms as heavy as iron beyond the planetary Roche lobe, evenfor planets less irradiated than the extreme ultra-hot-Jupiters such asWASP-12b and KELT-9b. The detection of iron and non-detection of magnesium inthe upper atmosphere of HD 209458b can be explained by a model in which thelower atmosphere forms (hence, sequesters) primarily magnesium-bearingcondensates, rather than iron condensates. This is suggested by currentmicrophysical models. The inextricable synergy between upper- andlower-atmosphere properties highlights the value of combining observations thatprobe both regions.

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