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 (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$σ$ 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.

Figures (20)

• Figure 1: Example ACAM frames demonstrating the used extraction (solid blue lines) and background (dashed blue lines) regions from the first night (first panel) and second night (second panel). We also observed a strong 2D enhancement on the detector in the second night for some of the transit, depicted in the third panel, and highlighted with a difference frame between without-scattering and with-scattering frames with similar FWHM for the second night in the final panel. The approximate wavelength of each Y pixel is indicated. Each X pixel spans 0.25 arcseconds.
• Figure 2: Ancillary data for the first night of observation of HAT-P-44b. The blue crosses correspond to HAT-P-44b and the red crosses correspond to the comparison star. Each panel represents the variation of various quantities throughout the night, from top to bottom: target airmass, the shift in trace x and y centroid across the slit, the full width half maximum (FWHM) of the stellar lines, the normalized flux, the total sky background counts, and the white-light light-curve differential flux. The impact of clouds is evident as dips in the normalized flux in the fifth and final twelve exposures. The poor seeing at the beginning of the night is apparent in the FWHM plot, and the kink in the sky background curve near the end of the night is due to moonset.
• Figure 3: Ancillary data for the second night of observation of HAT-P-44b. The blue crosses correspond to HAT-P-44b and the red crosses correspond to the comparison star. From top to bottom: target airmass, the shift in trace centroid across the slit, the full width half maximum (FWHM) of the stellar lines, the total sky background counts, and the white-light light-curve differential flux. The minimum in normalized flux immediately before transit is due to clouds, while we believe differential enhancement in background to be indirect scattered moonlight due to its abrupt appearance and disappearance.
• Figure 4: Flux ratio between the target and comparison (top panel), and normalized flux of the target (bottom panel, blue) and comparison (bottom panel, red) as a function of wavelength, for an example exposure. The wavelength bins used in the creation of our transmission spectra as displayed as black dotted lines, and the wavelengths of Na I and K I absorption are highlighted as green lines at 589 nm and 770 nm respectively. Additional bins discluded from our final analysis are indicated with grey dotted lines.
• Figure 5: Top panel: White-light light-curve of our first night of observation of HAT-P-44b (black data points), with an analytic transit model (grey dotted), polynomial systematics model (green), and the combined fit to the data (red). Middle panel: Detrending components contributing to the systematics model: quadratic in sky background (poly 1, blue) and quadratic in FWHM (poly 2, orange). Bottom panel: Residuals from the combined systematics and transit model.