Abstract
We report Gemini-North GMOS observations of the inflated hot Jupiter HAT-P-32b during two primary transits. We simultaneously observed two comparison stars and used differential spectro-photometry to produce multi-wavelength light curves. 'White' light curves and 29 'spectral' light curves were extracted for each transit and analysed to refine the system parameters and produce transmission spectra from 520-930nm in ~14nm bins. The light curves contain time-varying white noise as well as time-correlated noise, and we used a Gaussian process model to fit this complex noise model. Common mode corrections derived from the white light curve fits were applied to the spectral light curves which significantly improved our precision, reaching typical uncertainties in the transit depth of ~2x10^-4, corresponding to about half a pressure scale height. The low resolution transmission spectra are consistent with a featureless model, and we can confidently rule out broad features larger than about one scale height. The absence of Na/K wings or prominent TiO/VO features is most easily explained by grey absorption from clouds in the upper atmosphere, masking the spectral features. However, we cannot confidently rule out clear atmosphere models with low abundances (~10^-3 solar) of TiO, VO or even metal hydrides masking the Na and K wings. A smaller scale height or ionisation could also contribute to muted spectral features, but alone are unable to to account for the absence of features reported here.
Highlights
Remarkable progress has been achieved in understanding the diversity of extrasolar systems in our Galaxy, owing to the success of radial velocity and transit surveys
Results for the simultaneous Gaussian process (GP) fit to the white light curves are shown in Tab. 1
The transit parameters were inferred from i, z and g bands, and we caution that the HAT-P-32b’s radius could be marginally larger than that reported in Hartman et al (2011), depending on the apertures used for the photometry and the relative contribution to the final parameters of each of the light curves
Summary
Remarkable progress has been achieved in understanding the diversity of extrasolar systems in our Galaxy, owing to the success of radial velocity and transit surveys. Wavelength calibrated spectra for the 3 stars were extracted by summing in the cross-dispersion direction after sky subtraction using the gsextract routine, with an aperture (diameter) of 4 (≈ 28 binned pixels). Combined with the estimated few percent of light that falls within the aperture we calculate that the contaminant could dilute the transit depth at the reddest wavelengths by less than 10−5, far below the expected precision of our observations. For this crude calculation we assumed a Gaussian PSF, and median seeing.
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