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Observations of Mass Loss from the Transiting Exoplanet HD 209458b | | Jeffrey L. Linsky
; Hao Yang
; Kevin France
; Cynthia S. Froning
; James C. Green
; John T. Stocke
; Steven N. Osterman
; | | Date: |
10 May 2010 | | Abstract: | 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 $7.8pm 1.3$\% for the C~II 1334.5323~AA and 1335.6854~AA lines and by
$8.2pm 1.4$\% 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 $8pm 1$\% 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)$ imes 10^{10}$
g~s$^{-1}$. Our mass-loss rate estimate is consistent with theoretical
hydrodynamic models that include metals in the outflowing gas. | | Source: | arXiv, 1005.1633 | | Services: | Forum | Review | PDF | Favorites | |
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