ORBITAL PHASE VARIATIONS OF THE ECCENTRIC GIANT PLANET HAT-P-2b

Abstract

We present the first secondary eclipse and phase curve observations for the\nhighly eccentric hot Jupiter HAT-P-2b in the 3.6, 4.5, 5.8, and 8.0 \\mu m bands\nof the Spitzer Space Telescope. The 3.6 and 4.5 \\mu m data sets span an entire\norbital period of HAT-P-2b, making them the longest continuous phase curve\nobservations obtained to date and the first full-orbit observations of a planet\nwith an eccentricity exceeding 0.2. We present an improved non-parametric\nmethod for removing the intrapixel sensitivity variations in Spitzer data at\n3.6 and 4.5 \\mu m that robustly maps position-dependent flux variations. We\nfind that the peak in planetary flux occurs at 4.39+/-0.28, 5.84+/-0.39, and\n4.68+/-0.37 hours after periapse passage with corresponding maxima in the\nplanet/star flux ratio of 0.1138%+/-0.0089%, 0.1162%+/-0.0080%, and\n0.1888%+/-0.0072% in the 3.6, 4.5, and 8.0 \\mu m bands respectively. We compare\nour measured secondary eclipse depths to the predictions from a one-dimensional\nradiative transfer model, which suggests the possible presence of a transient\nday side inversion in HAT-P-2b's atmosphere near periapse. We also derive\nimproved estimates for the system parameters, including its mass, radius, and\norbital ephemeris. Our simultaneous fit to the transit, secondary eclipse, and\nradial velocity data allows us to determine the eccentricity and argument of\nperiapse of HAT-P-2b's orbit with a greater precision than has been achieved\nfor any other eccentric extrasolar planet. We also find evidence for a\nlong-term linear trend in the radial velocity data. This trend suggests the\npresence of another substellar companion in the HAT-P-2 system, which could\nhave caused HAT-P-2b to migrate inward to its present-day orbit via the Kozai\nmechanism.\n