The Climate of HD 189733b from Fourteen Transits and Eclipses Measured by Spitzer

Abstract

We present observations of six transits and six eclipses of the transiting planet system HD 189733 taken with the Spitzer Space Telescope's Infrared Array Camera (IRAC) at 8 μm, as well as a re-analysis of previously published data. We use several novel techniques in our data analysis, the most important of which is a new correction for the detector "ramp" variation with a double-exponential function, which performs better and is a better physical model for this detector variation. Our main scientific findings are (1) an upper limit on the variability of the dayside planet flux of 2.7% (68% confidence); (2) the most precise set of transit times measured for a transiting planet, with an average accuracy of 3 s; (3) a lack of transit-timing variations, excluding the presence of second planets in this system above 20% of the mass of Mars in low-order mean-motion resonance at 95% confidence; (4) a confirmation of the planet's phase variation, finding the night side is 64% as bright as the day side, as well as an upper limit on the nightside variability of 17% (68% confidence); (5) a better correction for stellar variability at 8 μm causing the phase function to peak 3.5 hr before secondary eclipse, confirming that the advection and radiation timescales are comparable at the 8 μm photosphere; (6) variation in the depth of transit, which possibly implies variations in the surface brightness of the portion of the star occulted by the planet, posing a fundamental limit on non-simultaneous multi-wavelength transit absorption measurements of planet atmospheres; (7) a measurement of the infrared limb darkening of the star, which is in good agreement with stellar atmosphere models; (8) an offset in the times of secondary eclipse of 69 s, which is mostly accounted for by a 31 s light-travel time delay and 33 s delay due to the shift of ingress and egress by the planet hot spot; this confirms that the phase variation is due to an offset hot spot on the planet; (9) a retraction of the claimed eccentricity of this system due to the offset of secondary eclipse, which is now just an upper limit; and (10) high-precision measurements of the parameters of this system. These results were enabled by the exquisite photometric precision of Spitzer IRAC; for repeat observations the scatter is less than 0.35 mmag over the 590 day timescale of our observations after decorrelating with detector parameters.

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