Hubble Space Telescope detection of oxygen in the atmosphere of exoplanet HD189733b

Detecting heavy atoms in the inflated atmospheres of giant exoplanets that orbit close to their parent stars is a key factor for understanding their bulk composition, their evolution, and the processes that drive their expansion and interaction with the impinging stellar wind. Unfortunately, very few detections have been made thus far. Here, we use archive data obtained with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope to report an absorption of similar to 6.4%+/-1.8% by neutral oxygen during the HD189733b transit. Using published results from a simple hydrodynamic model of HD189733b, and assuming a mean temperature of similar to(8-12) x 10(3) K for the upper atmosphere of the exoplanet, a mean vertical integrated OI density column of similar to 8x10(15) cm(-2) produces only a 3.5% attenuation transit. Much like the case of the hot-Jupiter HD209458b, super-solar abundances and/or super-thermal broadening of the absorption lines are required to fit the deep transit drop-off observed in most far-ultraviolet lines. We also report evidence of short-time variability in the measured stellar flux, a variability that we analyze using time series derived from the time-tagged exposures, which we then compare to solar flaring activity. In that frame, we find that non-statistical uncertainties in the measured fluxes are not negligible, which calls for caution when reporting transit absorptions. Despite cumulative uncertainties that originate from variability in the stellar and sky background signals and in the instrument response, we also show a possible detection for both a transit and early-ingress absorption in the ion C II 133.5 nm lines. If confirmed, this would be the second exoplanet for which an early ingress absorption is reported. In contrast, such an early ingress signature is not detected for neutral OI. Assuming the HD189733b magnetosphere to be at the origin of the early absorption, we use the Parker model for the stellar wind and a particle-in-cell code for the magnetosphere to show that its orientation should be deflected similar to 10-30 degrees from the planet-star line, while its nose's position should be at least similar to 16.7 R-p upstream of the exoplanet in order to fit the C II transit light curve. The derived stand-off distance is consistent with a surface magnetic field strength of similar to 5.3 Gauss for the exoplanet, and a supersonic stellar wind impinging at similar to 250 km s(-1), with a temperature of 1.2 x 10(5) K and a density similar to 6.3 x 10(6) cm(-3) at the planetary orbit, yet the fit is not unique.