Ground-Based Transmission Spectroscopy with VLT FORS2: Evidence for faculae and clouds in the optical spectrum of the warm Saturn WASP-110b

TL;DR

This study presents the optical transmission spectrum of exoplanet WASP-110b, revealing evidence of faculae and clouds, and highlights the importance of stellar activity and clouds in interpreting exoplanet atmospheres.

Contribution

First ground-based optical transmission spectrum of WASP-110b showing the influence of faculae and clouds, advancing understanding of atmospheric composition in warm Saturn-mass exoplanets.

Findings

Transmission spectrum shows increasing radius with wavelength.

Lack of pressure-broadened sodium and potassium features.

Evidence for faculae and clouds affecting the spectrum.

Abstract

We present a ground-based optical transmission spectrum for the warm Saturn-mass exoplanet WASP-110b from two transit observations made with the FOcal Reducer and Spectrograph (FORS2) on the Very Large Telescope (VLT). The spectrum covers the wavelength range from 4000 to 8333\AA, which is binned in 46 transit depths measured to an averaged precision of 220 parts per million (ppm) over an averaged 80\AA~bin for a Vmag=12.8 star. The measured transit depths are unaffected by a dilution from a close A-type field dwarf, which was fully resolved. The overall main characteristic of the transmission spectrum is an increasing radius with wavelength and a lack of the theoretically predicted pressure-broadened sodium and potassium absorption features for a cloud-free atmosphere. We analyze archival high-resolution optical spectroscopy and find evidence for low to moderate activity of the host star, which we take into account in the atmospheric retrieval analysis. Using the AURA retrieval code, we find that the observed transmission spectrum can be best explained by a combination of unocculted stellar faculae and a cloud deck. Transmission spectra of cloud-free and hazy atmospheres are rejected at a high confidence. With a possible cloud deck at its terminator, WASP-110b joins the increasing population of irradiated hot-Jupiter exoplanets with cloudy atmospheres observed in transmission.