Observations of Mass Loss from the Transiting Exoplanet HD 209458b

• Published in: The Astrophysical journal Vol. 717; no. 2; pp. 1291 - 1299
• Main Authors: Linsky, Jeffrey L, Yang, Hao, France, Kevin, Froning, Cynthia S, Green, James C, Stocke, John T, Osterman, Steven N
• Format: Journal Article
• Language: English
• Published: Bristol IOP Publishing 10-07-2010; IOP
• Subjects: ABSORPTION; ABUNDANCE; Astronomy; ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ATMOSPHERES; Earth, ocean, space; ECLIPSE; ELEMENTS; EVOLUTION; Exact sciences and technology; HYDRODYNAMIC MODEL; MATHEMATICAL MODELS; METALS; PARTICLE MODELS; PLANETS; SATELLITE ATMOSPHERES; SATELLITES; SOLAR SYSTEM; SOLAR SYSTEM EVOLUTION; SORPTION; SPECTRA; STARS; STATISTICAL MODELS; TELESCOPES; THERMODYNAMIC MODEL; ULTRAVIOLET SPECTRA; Satellite atmospheres; planets and satellites: atmospheres; Extrasolar planets; Mass loss; planets and satellites: physical evolution; stars: individual (HD 209458); ultraviolet: stars; planets and satellites: individual (HD 209458b)
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1088/0004-637X/717/2/1291

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 A and 1335.6854 A lines and by 8.2% {+-} 1.4% for the Si III 1206.500 A line during transit compared to non-transit times in the velocity interval -50 to +50 km s{sup -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{sup -1} and less certain features near -40 and +30-70 km s{sup -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 A 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{sup -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)x10{sup 10} g s{sup -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.