An Upper Limit on the Albedo of HD 209458b: Direct Imaging Photometry with theMOSTSatellite

We present space-based photometry of the transiting exoplanetary system HD209458 obtained with the MOST (Microvariablity and Oscillations of STars)satellite, spanning 14 days and covering 4 transits and 4 secondary eclipses.The HD 209458 photometry was obtained in MOST's lower-precision Direct Imagingmode, which is used for targets in the brightness range $6.5 < V < 13$. Wedescribe the photometric reduction techniques for this mode of observing, inparticular the corrections for stray Earthshine. We do not detect the secondaryeclipse in the MOST data, to a limit in depth of 0.053 mmag (1 \sigma). We seta 1 \sigma upper limit on the planet-star flux ratio of 4.88 x 10^-5corresponding to a geometric albedo upper limit in the MOST bandpass (400 to700 nm) of 0.25. The corresponding numbers at the 3 \sigma level are 1.34 x10^-4 and 0.68 respectively. HD 209458b is half as bright as Jupiter in theMOST bandpass. This low geometric albedo value is an important constraint fortheoretical models of the HD209458b atmosphere, in particular ruling out thepresence of reflective clouds. A second MOST campaign on HD 209458 is expectedto be sensitive to an exoplanet albedo as low as 0.13 (1 sigma), if the stardoes not become more intrinsically variable in the meantime.Comment: 29 pages, 9 figures. Accepted for publication in the Astrophysical Journal (July 2006, v645n1