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 ${\mathrm{R}}_{\mathrm{Jup}}$), temperature (1100 K), and mass (0.35 ${\mathrm{M}}_{\mathrm{Jup}}$) 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 ($V{}_{\mathrm{mag}}=13.2$). We detect absorption due to sodium with 3.9$\ddot{I}ƒ$ confidence. Atmospheric retrieval of the transmission spectrum also reveals evidence for \ch{H2O} absorption and Rayleigh scattering from \ch{H2} gas consistent with a cool 800 K atmosphere and a super-solar metallicity of 7$\begin{array}{c}+16\\ -5\end{array}$$\times$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.

21 pages, 22 figures, accepted for publication in Monthly Notices of the Royal Astronomical Society