Detection of Oxygen and Carbon in the Upper Atmosphere of the Extrasolar Planet Hd209458b

Four transits of the planet orbiting the star HD209458 were observed with the STIS spectrograph on board HST. The wavelength domain (1180-1710Å) includes H i as well as C i, C ii, C iv, Nv, O i, S i, Si ii, Si iii, Si iv and Fe ii lines. During the transits, absorptions are detected in H i, O i and C ii (5±2%, 10±3.5% and 6±3%, respectively). No absorptions are detected for other lines. The 5% mean absorption over the whole H i Lyman α line is consistent with the previous detection at higher resolution (Vidal-Madjar et al. 2003). The absorption depths in O i and C ii show that oxygen and carbon are present in the extended upper atmosphere of HD209458b. These species must be carried out up to the Roche lobe and beyond, most likely in a state of hydrodynamic escape. Subject headings: Star: individual (HD209458) – Stars: planetary systems