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

ABSTRACT 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 $\mathrm{R_{Jup}}$), temperature (1100 K), and mass (0.35 $\mathrm{M_{Jup}}$) to the exceedingly well-characterized WASP-39b. Using the ACAM instrument on the William Herschel Telescope, 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 ($V\mathrm{_{mag} = 13.2}$). We detect absorption due to sodium with 3.9$\sigma$ confidence. Atmospheric retrieval of the transmission spectrum also reveals evidence for H$_2$O absorption and Rayleigh scattering from H$_2$ gas consistent with a cool 800 K atmosphere and a supersolar metallicity of 7$^{+16}_{-5}$ $\times$ solar. Comparison of retrieval models disfavour the inclusion of a super-Rayleigh scattering slope or high-altitude clouds (at ${\lt} 1$ 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 Low-Resolution Ground-Based Exoplanet Atmosphere Survey using Transmission Spectroscopy survey.