LRG-BEASTS: evidence for clouds in the transmission spectrum of HATS-46 b

We have performed low-resolution ground-based spectroscopy of HATS-46 b in transmission, using the EFOSC2 instrument on the ESO New Technology Telescope (NTT). HATS-46 b is a highly inflated exoplanet that is a prime target for transmission spectroscopy, having a Jupiter-like radius (0.95 R_Jup) but a much lower mass (0.16 M_Jup). It orbits a G-type star with a 4.7 d period, giving an equilibrium temperature of 1100 K. We observed one transit of HATS-46 b with the NTT, with the time-series spectra covering a wavelength range of 3900-9000 Å at a resolution (R) of ~380. We achieved a remarkably precise transmission spectrum of 1.03 × photon noise, with a median uncertainty of 357 ppm for ~200 Å-wide bins, despite the relative faintness of the host star with V_mag = 13.6. The transmission spectrum does not show strong absorption features and retrievals favour a cloudy model, ruling out a clear atmosphere with 3.0σ confidence. We also place a conservative upper limit on the sodium abundance under the alternative scenario of a clear atmosphere. This is the eighth planet in the LRG-BEASTS (Low-Resolution Ground-Based Exoplanet Atmosphere Survey using Transmission Spectroscopy) survey, which uses 4 m-class telescopes such as the NTT to obtain low-resolution transmission spectra of hot Jupiters with precisions of around one atmospheric scale height.