LRG-BEASTS: Detection of sodium and evidence for water absorption in the hot Saturn HAT-P-44b

We present the low-resolution optical transmission spectrum of the inflated hot Saturn HAT-P-44b. The planet is a close sibling in radius (1.24 RJup), temperature (1100 K), and mass (0.35 MJup) to the exceedingly well-characterized WASP-39b. Using the ACAM instrument on the William Herschel Telescope (WHT), we obtain a transmission spectrum with sub-scale height precision of 246 ppm, with a wavelength range of 495 – 874 nm and a 20 nm resolution, despite a relatively faint host star (Vmag = 13.2). We detect absorption due to sodium with 3.9σ confidence. Atmospheric retrieval of the transmission spectrum also reveals evidence for H2O absorption and Rayleigh scattering from H2 gas consistent with a cool 800 K atmosphere and a super-solar metallicity of 7⁺¹⁶₋₅×solar. Comparison of retrieval models disfavour the inclusion of a super-Rayleigh scattering slope or high-altitude clouds (at <1 mbar) while being agnostic towards the presence of mid-altitude clouds. Our transmission spectrum of HAT-P-44b shows strong similarity to that of its sibling WASP-39b. This is the tenth planet in the LRG-BEASTS (Low-Resolution Ground-Based Exoplanet Atmosphere Survey using Transmission Spectroscopy) survey.