Multi-Wavelength Constraints on the Day-Night Circulation Patterns of HD 189733b

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  • 12 February 2008
Abstract
We present new Spitzer observations of the phase variation of the hot Jupiter HD 189733b in the MIPS 24 micron bandpass, spanning the same part of the planet's orbit as our previous observations in the IRAC 8 micron bandpass (Knutson et al. 2007). We find that the minimum hemisphere-averaged flux from the planet in this bandpass is 73+/-5% of the maximum flux; this corresponds to minimum and maximum hemisphere-averaged brightness temperatures of 982+/-53 K and 1248+/-53 K, respectively. The planet reaches its maximum flux 4.9+/-1.3 hours before the center of the secondary eclipse, corresponding to a hot region shifted 30-40 degrees east of the substellar point. The small amplitude of the observed phase variation indicates that the planet's atmosphere is effectively homogenized at the 24 micron photosphere, in contrast to previous results for the more highly-irradiated planet v Andromedae b. The similarities between the 8 and 24 micron phase curves for HD 189733b lead us to conclude that the circulation on this planet behaves in a fundamentally similar fashion across the range of pressures sensed by these two wavelengths. This similarity could be explained by a higher methane abundance on the planet's night side, which would increase the opacity in the 8 micron bandpass to a level comparable to that at 24 micron. If these two bandpasses do probe different pressures, it would indicate that the temperature varies only weakly with depth, and hence that the atmosphere is not convective at these altitudes. In this paper we also present a detailed analysis of the possible contribution of star spots to the time series at both 8 and 24 micron based on near-simultaneous ground-based observations. (abridged)

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