FORS2 observes a multi-epoch transmission spectrum of the hot Saturn-mass exoplanet WASP-49b

Abstract

Context. Transmission spectroscopy has proven to be a useful tool for the study of exoplanet atmospheres, because the absorption and scattering signatures of the atmosphere manifest themselves as variations in the planetary transit depth. Several planets have been studied with this technique, leading to the detection of a small number of elements and molecules (Na, K, H2O), but also revealing that many planets show flat transmission spectra consistent with the presence of opaque high-altitude clouds.

Aims. We apply this technique to the MP = 0.40MJ, Rp = 1.20RJ, P = 2.78 d planet WASP-49b, aiming to characterize its transmission spectrum between 0.73 and 1 ¯m and search for the features of K and H2O. Owing to its density and temperature, the planet is predicted to possess an extended atmosphere and is thus a good target for transmission spectroscopy.

Methods. Three transits of WASP-49b have been observed with the FORS2 instrument installed at the VLT/UT1 telescope at the ESO Paranal site. We used FORS2 in MXU mode with grism GRIS_600z, producing simultaneous multiwavelength transit light curves throughout the i' and z' bands. We combined these data with independent broadband photometry from the Euler and TRAPPIST telescopes to obtain a good measurement of the transit shape. Strong correlated noise structures are present in the FORS2 light curves, which are due to rotating flat-field structures that are introduced by inhomogeneities of the linear atmospheric dispersion corrector’s transparency. We accounted for these structures by constructing common noise models from the residuals of light curves bearing the same noise structures and used them together with simple parametric models to infer the transmission spectrum.

Results. We present three independent transmission spectra of WASP-49b between 0.73 and 1.02 ¯m, as well as a transmission spectrum between 0.65 and 1.02 ¯m from the combined analysis of FORS2 and broadband data. The results obtained from the three individual epochs agree well. The transmission spectrum of WASP-49b is best fit by atmospheric models containing a cloud deck at pressure levels of 1 mbar or lower.