Coronal X-ray emission and planetary irradiation in HD 209458

HD 209458 is one of the benchmark objects in the study of hot Jupiter atmospheres and their evaporation through planetary winds. The expansion of the planetary atmosphere is thought to be driven by high-energy extreme ultraviolet (EUV) and X-ray irradiation. We obtained new Chandra High Resolution Camera (HRC-I) data, which unequivocally show that HD 209458 is an X-ray source. Combining these data with archival XMM-Newton observations, we find that the corona of HD 209458 is characterized by a temperature of about 1 MK and an emission measure of 7 × 10⁴⁹ cm^-3, yielding an X-ray luminosity of 1.6 × 10²⁷ erg s^-1 in the 0.124-2.48 keV band. HD 209458 is an inactive star that has a coronal temperature comparable to that of the inactive Sun but that has a larger emission measure. At this level of activity, the planetary high-energy emission is sufficient to support mass loss at a rate of a few times 10¹⁰ g s^-1.