HST COS Observations of Mass Loss from HD 209458 b

HST COS Observations of Mass Loss from HD 209458 b

Observations of Mass Loss from the Transiting Exoplanet HD 209458b
http://fr.arxiv.org/abs/1005.1633

Abstract wrote:Using the new Cosmic Origins Spectrograph (COS) on the {\it Hubble Space Telescope (HST)}, 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 $7.8\pm 1.3$\% for the C~II 1334.5323~\AA\ and 1335.6854~\AA\ lines and by $8.2\pm 1.4$\% for the Si~III 1206.500~\AA\ 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~\AA\ absorption are in agreement with \citet{Vidal-Madjar2004}, but we find absorption during transit in the Si~III line contrary to the earlier result. The $8\pm 1$\% 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, but models that assume that the absorbers are high-temperature thermal ions are inconsistent with the COS spectra. 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)$\times 10^{10}$ g~s$^{-1}$. Our mass-loss rate estimate is consistent with theoretical hydrodynamic models that include metals in the outflowing gas.

_________________
Caps Lock: Cruise control for 'Cool'!

Sirius_Alpha

Number of posts : 3493
Location : Earth
Registration date : 2008-04-06

Re: HST COS Observations of Mass Loss from HD 209458 b

Velocity structure has been confirmed, as has the extended tail.

http://hubblesite.org/newscenter/archive/releases/2010/21

_________________
Caps Lock: Cruise control for 'Cool'!

Sirius_Alpha

Number of posts : 3493
Location : Earth
Registration date : 2008-04-06

Permissions in this forum:
You cannot reply to topics in this forum