Warm Spitzer Observations of Three Hot Exoplanets: XO-4b, HAT-P-6b, and HAT-P-8b
We analyze Warm Spitzer/Infrared Array Camera observations of the secondary eclipses of three planets, XO-4b, HAT-P-6b, and HAT-P-8b. We measure secondary eclipse amplitudes at 3.6 μm and 4.5 μm for each target. XO-4b exhibits a stronger eclipse depth at 4.5 μm than at 3.6 μm, which is consistent with the presence of a temperature inversion. HAT-P-8b shows a stronger eclipse amplitude at 3.6 μm and is best described by models without a temperature inversion. The eclipse depths of HAT-P-6b can be fitted with models with a small or no temperature inversion. We consider our results in the context of a postulated relationship between stellar activity and temperature inversion and a relationship between irradiation level and planet dayside temperature, as discussed by Knutson et al. and Cowan & Agol, respectively. Our results are consistent with these hypotheses, but do not significantly strengthen them. To measure accurate secondary eclipse central phases, we require accurate ephemerides. We obtain primary transit observations and supplement them with publicly available observations to update the orbital ephemerides of the three planets. Based on the secondary eclipse timing, we set upper boundaries for e cos(ω) for HAT-P-6b, HAT-P-8b, and XO-4b and find that the values are consistent with circular orbits.
Additional Information© 2012 American Astronomical Society. Received 2011 July 7; accepted 2011 November 23; published 2012 January 27. This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. Support for this work was provided by NASA through an award issued by JPL/Caltech. The Center for Exoplanets and Habitable Worlds is supported by the Pennsylvania State University, the Eberly College of Science, and the Pennsylvania Space Grant Consortium. We thank the anonymous referee for a careful review of this paper.
Published - Todorov2012p18199Astrophys_J.pdf