OBSERVATIONS OF MASS LOSS FROM THE TRANSITING EXOPLANET HD 209458b*

Using the new Cosmic Origins Spectrograph on the Hubble Space Telescope, we obtained moderate-resolution, high signal/noise ultraviolet spectra of HD 209458 and its exoplanet HD 209458b during transit, both orbital quadratures, and secondary eclipse. We compare transit spectra with spectra obtained at non-transit phases to identify spectral features due to the exoplanet's expanding atmosphere. We find that the mean flux decreased by 7.8% +/- 1.3% for the C II 1334.5323 angstrom and 1335.6854 angstrom lines and by 8.2% +/- 1.4% for the Si III 1206.500 angstrom line during transit compared to non-transit times in the velocity interval -50 to + 50 km s(-1). Comparison of the C II and Si III line depths and transit/non-transit line ratios shows deeper absorption features near -10 and +15 km s(-1) and less certain features near -40 and +30-70 km s(-1), but future observations are needed to verify this first detection of velocity structure in the expanding atmosphere of an exoplanet. Our results for the C II lines and the non-detection of Si IV 1394.76 angstrom absorption are in agreement with Vidal-Madjar et al., but we find absorption during transit in the Si III line contrary to the earlier result. The 8% +/- 1% obscuration of the star during transit is far larger than the 1.5% obscuration by the exoplanet's disk. Absorption during transit at velocities between -50 and +50 km s(-1) in the C II and S III lines requires high-velocity ion absorbers. Assuming hydrodynamic model values for the gas temperature and outflow velocity at the limb of the outflow as seen in the C II lines, we find mass-loss rates in the range (8-40)x10(10) g s(-1). These rates assume that the carbon abundance is solar, which is not the case for the giant planets in the solar system. Our mass-loss rate estimate is consistent with theoretical hydrodynamic models that include metals in the outflowing gas.