SPITZER OBSERVATIONS OF THE THERMAL EMISSION FROM WASP-43b

WASP-43b is one of the closest-orbiting hot Jupiters, with a semimajor axis of a = 0.01526 +/- 0.00018 AU and a period of only 0.81 days. However, it orbits one of the coolest stars with a hot Jupiter (T-* = 4520 +/- 120 K), giving the planet a modest equilibrium temperature of T-eq = 1440 +/- 40 K, assuming zero Bond albedo and uniform planetary energy redistribution. The eclipse depths and brightness temperatures from our jointly fit model are 0.347% +/- 0.013% and 1670 +/- 23 K at 3.6 mu m and 0.382% +/- 0.015% and 1514 +/- 25 K at 4.5 mu m. The eclipse timings improved the estimate of the orbital period, P, by a factor of three (P = 0.81347436 +/- 1.4 x 10(-7) days) and put an upper limit on the eccentricity (e = 0.010(-0.007)(+0.010)). We use our Spitzer eclipse depths along with four previously reported ground-based photometric observations in the near- infrared to constrain the atmospheric properties of WASP- 43b. The data rule out a strong thermal inversion in the dayside atmosphere of WASP- 43b. Model atmospheres with no thermal inversions and fiducial oxygen-rich compositions are able to explain all the available data. However, a wide range of metallicities and C/ O ratios can explain the data. The data suggest low day-night energy redistribution in the planet, consistent with previous studies, with a nominal upper limit of about 35% for the fraction of energy incident on the dayside that is redistributed to the nightside.