Transit spectrophotometry of the exoplanet HD 189733b

Context. We present a new primary transit observation of the hot-jupiter HD 189733b, obtained at 3.6 μm with the Infrared Array Camera (IRAC) onboard the Spitzer Space Telescope. Previous measurements at 3.6 microns suffered from strong systematics, and conclusions could hardly be obtained with confidence on the water detection by comparison of the 3.6 and 5.8 microns observations. Aims. We aim at constraining the atmospheric structure and composition of the planet and improving previously derived parameters. Methods. We use a high-S/N Spitzer photometric transit light curve to improve the precision of the near infrared radius of the planet at 3.6 μm. The observation has been performed using high-cadence time series integrated in the subarray mode. We are able to derive accurate system parameters, including planet-to-star radius ratio, impact parameter, scale of the system, and central time of the transit from the fits of the transit light curve. We compare the results with transmission spectroscopic models and with results from previous observations at the same wavelength. Results. We obtained the following system parameters of Rp/R = 0.15566+0.00011 −0.00024, b = 0.661 +0.0053 −0.0050, and a/R = 8.925 +0.0490 −0.0523 at 3.6 μm. These measurements are three times more accurate than previous studies at this wavelength because they benefit from greater observational efficiency and less statistic and systematic errors. Nonetheless, we find that the radius ratio has to be corrected for stellar activity and present a method to do so using ground-based long-duration photometric follow-up in the V-band. The resulting planet-to-star radius ratio corrected for the stellar variability agrees with our previous measurement obtained in the same bandpass. We also discuss that water vapour could not be detected by a comparison of the planetary radius measured at 3.6 and 5.8 μm, because the radius measured at 3.6 μm is affected by absorption by other species, possibly Rayleigh scattering by haze.