David Charbonneau et al. 2002 ApJ 568 377 doi:10.1086/338770
David Charbonneau1,2, Timothy M. Brown3, Robert W. Noyes2 and Ronald L. Gilliland4
Show affiliationsWe report high-precision spectrophotometric observations of four planetary transits of HD 209458, in the region of the sodium resonance doublet at 589.3 nm. We find that the photometric dimming during transit in a bandpass centered on the sodium feature is deeper by (2.32 ± 0.57) × 10-4 relative to simultaneous observations of the transit in adjacent bands. We interpret this additional dimming as absorption from sodium in the planetary atmosphere, as recently predicted from several theoretical modeling efforts. Our model for a cloudless planetary atmosphere with a solar abundance of sodium in atomic form predicts more sodium absorption than we observe. There are several possibilities that may account for this reduced amplitude, including reaction of atomic sodium into molecular gases and/or condensates, photoionization of sodium by the stellar flux, a low primordial abundance of sodium, and the presence of clouds high in the atmosphere.
binaries: eclipsing; planetary systems; stars: atmospheres; stars: individual (HD 209458); techniques: photometric
Issue 1 (2002 March 20)
Received 2001 October 25, accepted for publication 2001 November 19
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