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J. Harrington, S. Luszcz (Cornell), D. Deming (NASA's GSFC), L. J. Richardson (NRC/NASA's GSFC), S. Seager (DTM/CIW)
We used the Multiband Imaging Photometer on the Spitzer Space Telescope to obtain 24-\micron lightcurves for one secondary eclipse and half a transit of extrasolar planet HD 209458b behind/in front of its star. Optimal photometric extraction of the total system brightness gives S/N of ~100 per 10-sec exposure. We measure the deficit of planetary flux in secondary eclipse at 55 ± 10 \muJy, or 0.260 ± 0.046% of the stellar flux, giving a brightness temperature of 1130 ± 150 K. The secondary eclipse occurs within 7 min (1\sigma) of the mid-time between transits, essentially eliminating the hypothesis that HD 209458b's radius is inflated by tidal dissipation due to an elliptical orbit. Such an orbit would have required a second planet in a resonant orbit. This measurement and a similar, simultaneously-submitted result for TrES-1 by Charbonneau et al. (2005) are the first direct measurements of radiation from confirmed extrasolar planets.
We measure a preliminary transit depth of 1.559 ± 0.065% of the stellar flux. The stellar limb-darkening curve is flat to within the errors, as expected for this wavelength. Adopting a stellar radius of 1.18 ± 0.10 R\sun (Cody and Sasselov 2002) gives a planetary radius of 1.43 ± 0.13 RJ, consistent with that derived from optical measurements. The large error bar prevents us from ruling out a significant wavelength dependence of the eclipsing radius, as might be be expected if a particulate coma were responsible for the inflated optical radius. Spitzer has acquired an additional complete transit; updated results will be presented at the conference.
This work was supported by the NASA Origins of Solar Systems program and the Spitzer Space Telescope. LJR is an NRC research associate.
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Bulletin of the American Astronomical Society, 37 #3
© 2004. The American Astronomical Soceity.