Gemini/GMOS Transmission Spectral Survey: Complete Optical Transmission Spectrum of the hot Jupiter WASP-4b

TL;DR

This study presents a high-precision optical transmission spectrum of the hot Jupiter WASP-4b, revealing cloud-dominated atmosphere with condensate grains around 1 micron, based on multiple transits observed with GMOS.

Contribution

First complete optical transmission spectrum of WASP-4b from 440-940 nm using GMOS, demonstrating repeatability and identifying cloud-dominated atmosphere with custom data reduction pipeline.

Findings

Transmission spectrum consistent with cloud-dominated atmosphere

No Rayleigh scattering slope detected, indicating cloud opacity

Achieved precision roughly twice the photon noise level

Abstract

We present the complete optical transmission spectrum of the hot Jupiter WASP-4b from 440-940 nm at R ~ 400-1500 obtained with the Gemini Multi-Object Spectrometers (GMOS); this is the first result from a comparative exoplanetology survey program of close-in gas giants conducted with GMOS. WASP-4b has an equilibrium temperature of 1700 K and is favorable to study in transmission due to a large scale height (370 km). We derive the transmission spectrum of WASP-4b using 4 transits observed with the MOS technique. We demonstrate repeatable results across multiple epochs with GMOS, and derive a combined transmission spectrum at a precision about twice above photon noise, which is roughly equal to to one atmospheric scale height. The transmission spectrum is well fitted with a uniform opacity as a function of wavelength. The uniform opacity and absence of a Rayleigh slope from molecular hydrogen suggest that the atmosphere is dominated by clouds with condensate grain size of ~1 um. This result is consistent with previous observations of hot Jupiters since clouds have been seen in planets with similar equilibrium temperatures to WASP-4b. We describe a custom pipeline that we have written to reduce GMOS time-series data of exoplanet transits, and present a thorough analysis of the dominant noise sources in GMOS, which primarily consist of wavelength- and time- dependent displacements of the spectra on the detector, mainly due to a lack of atmospheric dispersion correction.