Rayleigh scattering in the transmission spectrum of HAT-P-18b

TL;DR

This study presents ground-based optical transmission spectroscopy of HAT-P-18b, revealing Rayleigh scattering and a haze that masks sodium absorption, demonstrating ground-based methods can achieve Hubble-quality precision.

Contribution

First ground-based detection of Rayleigh scattering slope in a hot Jupiter's atmosphere, showing the effectiveness of ground observations for exoplanet atmospheric characterization.

Findings

Detected a Rayleigh scattering slope from 4750 to 9250 Å.

No sodium absorption feature detected, indicating haze presence.

Ground-based transmission spectra can match Hubble's precision.

Abstract

We have performed ground-based transmission spectroscopy of the hot Jupiter HAT-P-18b using the ACAM instrument on the William Herschel Telescope (WHT). Differential spectroscopy over an entire night was carried out at a resolution of $R \approx 400$ using a nearby comparison star. We detect a bluewards slope extending across our optical transmission spectrum which runs from 4750 to 9250\AA. The slope is consistent with Rayleigh scattering at the equilibrium temperature of the planet (852K). We do not detect enhanced sodium absorption, which indicates that a high-altitude haze is masking the feature and giving rise to the Rayleigh slope. This is only the second discovery of a Rayleigh scattering slope in a hot Jupiter atmosphere from the ground, and our study illustrates how ground-based observations can provide transmission spectra with precision comparable to the Hubble Space Telescope.