Abstract
We present a new primary transit observation of the hot-jupiter HD189733b, obtained at 3.6 microns 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. 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 microns. 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. We obtained the following system parameters: R_p/R_\star=0.15566+0.00011-0.00024, b=0.661+0.0053-0.0050, and a/R_\star=8.925+0.0490-0.0523 at 3.6 microns. 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 is in agreement with the previous measurement obtained in the same bandpass (Desert et al. 2009). We also discuss that water vapour could not be evidenced by comparison of the planetary radius measured at 3.6 and 5.8 microns, because the radius measured at 3.6 microns is affected by absorption by other species, possibly Rayleigh scattering by haze.
Highlights
The ransiting hot jupiter HD 189733b orbits a small, bright, main-sequence K2V star (K = 5.5) and produces deep transits of ∼2.5% (Bouchy et al 2005)
The raw aperture photometry of the transit light curve obtained in subarray mode binned by four is plotted in Fig. 2 together with the lightcurve decorrelated from the dependence on instrumental parameters
We find that the center of our transit is Tc = 2 454 430.310594 (HJD), which corresponds to an observed minus calculated (O−C) transit time of 47 s ± 4(±16)
Summary
The star have made possible the study of the atmosphere’s emission spectrum (Charbonneau et al 2005; Deming et al 2005, 2006, 2007) This planet has been extensively observed during secondary transits and throughout the phases of its orbit (Knutson et al 2007a,b, 2009). We derived a slightly larger radius at 4.5 μm that cannot be explained by H2O or Rayleigh scattering We interpreted this small absorption excess as caused by to the presence of CO molecules (Désert et al 2009). This is consistent with the low level of emission measured from the planetary eclipse at the same wavelength (Charbonneau et al 2008).
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