The 4.5 μm Full-orbit Phase Curve of the Hot Jupiter HD 209458b
The hot Jupiter HD 209458b is particularly amenable to detailed study as it is among the brightest transiting exoplanet systems currently known (V-mag = 7.65; K-mag = 6.308) and has a large planet-to-star contrast ratio. HD 209458b is predicted to be in synchronous rotation about its host star with a hot spot that is shifted eastward of the substellar point by superrotating equatorial winds. Here we present the first full-orbit observations of HD 209458b, in which its 4.5 μm emission was recorded with Spitzer/IRAC. Our study revises the previous 4.5 μm measurement of HD 209458b's secondary eclipse emission downward by ~35% to 0.1391%^(+0.0072%)_(-0.0069%), changing our interpretation of the properties of its dayside atmosphere. We find that the hot spot on the planet's dayside is shifted eastward of the substellar point by 40°.9 ± 6°.0, in agreement with circulation models predicting equatorial superrotation. HD 209458b's dayside (T_(bright) = 1499 ± 15 K) and nightside (T_(bright) = 972 ± 44 K) emission indicate a day-to-night brightness temperature contrast smaller than that observed for more highly irradiated exoplanets, suggesting that the day-to-night temperature contrast may be partially a function of the incident stellar radiation. The observed phase curve shape deviates modestly from global circulation model predictions potentially due to disequilibrium chemistry or deficiencies in the current hot CH4 line lists used in these models. Observations of the phase curve at additional wavelengths are needed in order to determine the possible presence and spatial extent of a dayside temperature inversion, as well as to improve our overall understanding of this planet's atmospheric circulation.
Additional Information© 2014 The American Astronomical Society. Received 2014 March 20; accepted 2014 June 2; published 2014 July 2. R.Z. and C.A.G. are supported by the NASA Planetary Atmospheres Program. N.K.L. performed this work in part under contract with the California Institute of Technology (Caltech) funded by NASA through the Sagan Fellowship Program executed by the NASA Exoplanet Science Institute. A.P.S. is supported by the NASA Origins program. R.Z. would like to thank Travis Barman, Ian J. M. Crossfield, Julien de Wit, Davin Flateau, Joe Giacalone, Tiffany Kataria, Michael R. Line, Julianne I. Moses, Kyle A. Pearson, Emily Rauscher, Tamara M. Rogers, David K. Sing, and Mark R. Swain for their helpful discussions. We would like to thank the two referees for their helpful comments and suggestions.
Published - 0004-637X_790_1_53.pdf
Submitted - 1405.5923v1.pdf