We present recent observations from the HST-Cosmic Origins Spectrograph aimedat characterizing the auroral emission from the extrasolar planet HD209458b. Weobtained medium-resolution (R~18-20,000) far-ultraviolet (1150-1700A) spectraat both the Phase 0.25 and Phase 0.75 quadrature positions as well as a stellarbaseline measurement at secondary eclipse. This analysis includes a catalog ofstellar emission lines and a star-subtracted spectrum of the planet. We presentan emission model for planetary H2 emission, and compare this model to theplanetary spectrum. No unambiguously identifiable atomic or molecular featuresare detected, and upper limits are presented for auroral/dayglow linestrengths. An orbital velocity cross-correlation analysis finds a statisticallysignificant (3.8 sigma) feature at +15 (+/- 20) km/s in the rest frame of theplanet, at 1582 A. This feature is consistent with emission from H2 B-X (2-9)P(4) (lambda_{rest} = 1581.11 A), however the physical mechanism required toexcite this transition is unclear. We compare limits on relative line strengthsseen in the exoplanet spectrum with models of ultraviolet fluorescence toconstrain the atmospheric column density of neutral hydrogen between the starand the planetary surface. These results support models of short periodextrasolar giant planets with weak magnetic fields and extended atomicatmospheres.
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