LRG-BEASTS: sodium absorption and Rayleigh scattering in the atmosphere of WASP-94A b using NTT/EFOSC2

Abstract

We present an optical transmission spectrum for WASP-94A b, the first atmospheric characterisation of this highly-inflated hot Jupiter. The planet has a reported radius of 1.72 +0.06 −0.05 R Jup , a mass of only 0.456 +0.032 −0.036 M Jup , and an equilibrium temperature of 1508 ± 75 K. We observed the planet transit spectroscopically with the EFOSC2 instrument on the ESO New Technology Telescope (NTT) at La Silla, Chile: the first use of NTT/EFOSC2 for transmission spectroscopy. We achieved an average transitdepth precision of 128 ppm for bin widths of ∼ 200 Å. This high precision was achieved in part by linking Gaussian Process hyperparameters across all wavelength bins. The resulting transmission spectrum, spanning a wavelength range of 3800−7140 Å, exhibits a sodium absorption with a significance of 4.9𝜎, suggesting a relatively cloud-free atmosphere. The sodium signal may be broadened, with a best fitting width of 78 +67 −32 Å in contrast to the instrumental resolution of 27.2 ± 0.2 Å. We also detect a steep slope in the blue end of the transmission spectrum, indicating the presence of Rayleigh scattering in the atmosphere of WASP-94A b. Retrieval models show evidence for the observed slope to be super-Rayleigh and potential causes are discussed. Finally, we find narrow absorption cores in the CaII H&K lines of WASP-94A, suggesting the star is enshrouded in gas escaping the hot Jupiter.