Orbital Phase Variations of the Eccentric Giant Planet Hat-P-2b

We present the first secondary eclipse and phase curve observations for the highly eccentric hot Jupiter HAT-P-2b in the 3.6, 4.5, 5.8, and 8.0 μm bands of the Spitzer Space Telescope. The 3.6 and 4.5 μm data sets span an entire orbital period of HAT-P-2b (P = 5.6334729 d), making them the longest continuous phase curve observations obtained to date and the first full-orbit observations of a planet with an eccentricity exceeding 0.2. We present an improved non-parametric method for removing the intrapixel sensitivity variations in Spitzer data at 3.6 and 4.5 μm that robustly maps position-dependent flux variations. We find that the peak in planetary flux occurs at 4.39±0.28, 5.84±0.39, and 4.68±0.37 hours after periapse passage with corresponding maxima in the planet/star flux ratio of 0.1138%±0.0089%, 0.1162%±0.0080%, and 0.1888%±0.0072% in the 3.6, 4.5, and 8.0 μm bands respectively. Our measured secondary eclipse depths of 0.0996%±0.0072%, 0.1031%±0.0061%, 0.071% −0.013%, and 0.1392% ± 0.0095% in the 3.6, 4.5, 5.8, and 8.0 μm bands respectively indicate that the planet cools significantly from its peak temperature before we measure the dayside flux during secondary eclipse. We compare our measured secondary eclipse depths to the predictions from a one-dimensional radiative transfer model, which suggests the possible presence of a transient day side inversion in HAT-P-2b’s atmosphere near periapse. We also derive improved estimates for the system parameters, including its mass, radius, and orbital ephemeris. Our simultaneous fit to the transit, secondary eclipse, and radial velocity data allows us to determine the eccentricity (e = 0.50910 ± 0.00048) and argument of periapse (ω = 188.09◦ ± 0.39◦) of HAT-P-2b’s orbit with a greater precision than has been achieved for any other eccentric extrasolar planet. We also find evidence for a long-term linear trend in the radial velocity data. This trend suggests the presence of another substellar companion in the HAT-P-2 system, which could have caused HAT-P-2b to migrate inward to its present-day orbit via the Kozai mechanism. Subject headings: planets and satellites: general, planets and satellites: individual: HAT-P-2b, techniques: photometric, methods: numerical, atmospheric effects 1 Department of Planetary Sciences and Lunar and Planetary Laboratory, The University of Arizona, Tucson, AZ 85721, USA 2 Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA; [email protected] 3 Sagan Postdoctoral Fellow 4 Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA 5 Center for Interdisciplinary Exploration and Research in Astrophysics and Department of Physics and Astronomy, Northwestern University, 2131 Tech Drive, Evanston, IL 60208, USA 6 Department of Astronomy & Astrophysics, University of California, Santa Cruz, CA 95064, USA 7 Dept. of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA 8 Dept. of Astronomy, University of Maryland, College Park, MD, 20742, USA 9 Department of Physics, University of Notre Dame, Notre Dame, IN 46556, USA 10 National Optical Astronomy Observatories, Tucson, AZ 85726, USA 11 Dept. of Astronomy, University of Washington, Seattle, WA 98195, USA 12 Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, USA 13 Department of Astronomy, Yale University, New Haven, CT 06511, USA 14 Department of Astrophysics, California Institute of Technology, MC 249-17, Pasadena, CA 91125, USA 15 Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA 16 Department of Physics, Principia College, 1 Maybeck Place, Elsah, IL 62028, USA 17 Department of Astronomy, University of California, Berkeley, CA 94720, USA 18 Department of Physics, and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139 ar X iv :1 30 2. 50 84 v1 [ as tr oph .E P] 2 0 Fe b 20 13