Abstract
We present an analysis of Spitzer/IRAC primary transit and secondary eclipse lightcurves measured for HD209458b, using Gaussian process models to marginalise over the intrapixel sensitivity variations in the 3.6 micron and 4.5 micron channels and the ramp effect in the 5.8 micron and 8.0 micron channels. The main advantage of this approach is that we can account for a broad range of degeneracies between the planet signal and systematics without actually having to specify a deterministic functional form for the latter. Our results do not confirm a previous claim of water absorption in transmission. Instead, our results are more consistent with a featureless transmission spectrum, possibly due to a cloud deck obscuring molecular absorption bands. For the emission data, our values are not consistent with the thermal inversion in the dayside atmosphere that was originally inferred from these data. Instead, we agree with another re-analysis of these same data, which concluded a non-inverted atmosphere provides a better fit. We find that a solar-abundance clear-atmosphere model without a thermal inversion underpredicts the measured emission in the 4.5 micron channel, which may suggest the atmosphere is depleted in carbon monoxide. An acceptable fit to the emission data can be achieved by assuming that the planet radiates as an isothermal blackbody with a temperature of $1484\pm 18$ K.
Highlights
Over the past decade, the Spitzer Space Telescope has proven to be a productive facility for characterizing the atmospheres of transiting exoplanets (e.g. Charbonneau et al 2005; Deming et al 2006; Knutson et al 2007a; Desert et al 2009; Crossfield et al 2012; Lewis et al 2013; Todorov et al 2014)
This paper focuses on observations made with the Infrared Array Camera (IRAC), which has been the most widely used Spitzer instrument for observing exoplanets
We present an alternative method for treating intrapixel sensitivity variations and the ramp effect in IRAC light curves based on Gaussian processes (GPs)
Summary
The Spitzer Space Telescope has proven to be a productive facility for characterizing the atmospheres of transiting exoplanets (e.g. Charbonneau et al 2005; Deming et al 2006; Knutson et al 2007a; Desert et al 2009; Crossfield et al 2012; Lewis et al 2013; Todorov et al 2014). The ability of its instruments to probe the ∼3–25 μm wavelength range has provided constraints on the thermal emission of numerous exoplanets, as well as atmospheric transmission in a region dominated by absorption from molecular species such as water, methane, carbon monoxide, and carbon dioxide. Hot Jupiters have offered especially favourable targets for such observations, given their large atmospheric scaleheights and relatively strong emission at these wavelengths. This paper focuses on observations made with the Infrared Array Camera (IRAC), which has been the most widely used Spitzer instrument for observing exoplanets. HD 209458b IRAC light curves with GP models 681 Programme P.I. Type
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