Transit and Eclipse Analyses of the Exoplanet HD 149026b Using BLISS Mapping

The dayside of HD 149026b is near the edge of detectability by the Spitzer Space Telescope. We report on 11 secondary-eclipse events at 3.6, 4.5, 3 × 5.8, 4 × 8.0, and 2 × 16 μm plus three primary-transit events at 8.0 μm. The eclipse depths from jointly fit models at each wavelength are 0.040% ± 0.003% at 3.6 μm, 0.034% ± 0.006% at 4.5 μm, 0.044% ± 0.010% at 5.8 μm, 0.052% ± 0.006% at 8.0 μm, and 0.085% ± 0.032% at 16 μm. Multiple observations at the longer wavelengths improved eclipse-depth signal-to-noise ratios by up to a factor of two and improved estimates of the planet-to-star radius ratio (R_p /R _sstarf = 0.0518 ± 0.0006). We also identify no significant deviations from a circular orbit and, using this model, report an improved period of 2.8758916 ± 0.0000014 days. Chemical-equilibrium models find no indication of a temperature inversion in the dayside atmosphere of HD 149026b. Our best-fit model favors large amounts of CO and CO₂, moderate heat redistribution (f = 0.5), and a strongly enhanced metallicity. These analyses use BiLinearly-Interpolated Subpixel Sensitivity (BLISS) mapping, a new technique to model two position-dependent systematics (intrapixel variability and pixelation) by mapping the pixel surface at high resolution. BLISS mapping outperforms previous methods in both speed and goodness of fit. We also present an orthogonalization technique for linearly correlated parameters that accelerates the convergence of Markov chains that employ the Metropolis random walk sampler. The electronic supplement contains light-curve files.