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

TL;DR

This study presents the optical transmission spectrum of hot Saturn HAT-P-44b, detecting sodium and water absorption features, and suggests a cool, metal-rich atmosphere with minimal high-altitude clouds, using ground-based observations.

Contribution

First low-resolution optical transmission spectrum of HAT-P-44b with high precision, revealing atmospheric composition and properties using ground-based telescopes.

Findings

Detection of sodium absorption at 3.9σ confidence

Evidence for water vapor and Rayleigh scattering in the atmosphere

Atmospheric retrieval indicates a cool, metal-rich atmosphere with minimal high-altitude clouds

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 (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$\sigma$ 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$\substack{+16 \\ -5}$$\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.