OBSERVATIONS OF MASS LOSS FROM THE TRANSITING EXOPLANET HD 209458b

Abstract

Using the new Cosmic Origins Spectrograph (COS) on the {\\it Hubble Space\nTelescope (HST)}, we obtained moderate-resolution, high signal/noise\nultraviolet spectra of HD 209458 and its exoplanet HD 209458b during transit,\nboth orbital quadratures, and secondary eclipse. We compare transit spectra\nwith spectra obtained at non-transit phases to identify spectral features due\nto the exoplanet's expanding atmosphere. We find that the mean flux decreased\nby $7.8\\pm 1.3$% for the C II 1334.5323\\AA\\ and 1335.6854\\AA\\ lines and by\n$8.2\\pm 1.4$% for the Si III 1206.500\\AA\\ line during transit compared to\nnon-transit times in the velocity interval --50 to +50 km s$^{-1}$. Comparison\nof the C II and Si III line depths and transit/non-transit line ratios shows\ndeeper absorption features near --10 and +15 km s$^{-1}$ and less certain\nfeatures near --40 and +30--70 km s$^{-1}$, but future observations are needed\nto verify this first detection of velocity structure in the expanding\natmosphere of an exoplanet. Our results for the C II lines and the\nnon-detection of Si IV 1394.76\\AA\\ absorption are in agreement with\n\\citet{Vidal-Madjar2004}, but we find absorption during transit in the Si III\nline contrary to the earlier result. The $8\\pm 1$% obscuration of the star\nduring transit is far larger than the 1.5% obscuration by the exoplanet's disk.\nAbsorption during transit at velocities between --50 and +50 km s$^{-1}$ in the\nC II and Si III lines requires high-velocity ion absorbers, but models that\nassume that the absorbers are high-temperature thermal ions are inconsistent\nwith the COS spectra. Assuming hydrodynamic model values for the gas\ntemperature and outflow velocity at the limb of the outflow as seen in the C II\nlines, we find mass-loss rates in the range (8--40)$\\times 10^{10}$ g s$^{-1}$.\n