* Astronomy

Title: WASP-8b: Characterisation of a Cool and Eccentric Exoplanet with Spitzer
Authors: Patricio Cubillos (1,2), Joseph Harrington (1,2), Nikku Madhusudhan (3), Kevin B. Stevenson (4), Ryan A. Hardy (1), Jasmina Blecic (1), David R. Anderson (5), Matthew Hardin (1), Christopher J. Campo (1) ((1) Department of Physics, University of Central Florida, (2) Max-Plank-Institut für Astronomie, Heidelberg, (3) Department of Physics and Department of Astronomy, Yale University, (4) Department of Astronomy and Astrophysics, University of Chicago, (5) Astrophysics Group, Keele University)

WASP-8b has 2.18 times Jupiter's mass and is on an eccentric (e=0.31) 8.16-day orbit. With a time-averaged equilibrium temperature of 948 K, it is one of the least-irradiated hot Jupiters observed with the Spitzer Space Telescope. We have analysed six photometric light curves of WASP-8b during secondary eclipse observed in the 3.6, 4.5, and 8.0 m Infrared Array Camera bands. The eclipse depths are 0.113 ± 0.018%, 0.069 ± 0.007%, and 0.093 ± 0.023%, respectively, giving respective brightness temperatures of 1552, 1131, and 938 K. We characterised the atmospheric thermal profile and composition of the planet using a line-by-line radiative transfer code and a Markov-chain Monte Carlo sampler. The data indicated no thermal inversion, independently of any assumption about chemical composition. We noted an anomalously high 3.6-m brightness temperature (1552 K); by modelling the eccentricity-caused thermal variation, we found that this temperature is plausible for radiative time scales less than ~ 10² hours. However, as no model spectra fit all three data points well, the temperature discrepancy remains as an open question.

Read more (1215kb, PDF)