OBSERVATIONS OF MASS LOSS FROM THE TRANSITING EXOPLANET HD 209458b

Abstract

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 Å and 1335.6854 Å lines and by 8.2% ± 1.4% for the Si iii 1206.500 Å 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 Å 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 Si 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) × 10 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.