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

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 µm. 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.

Ground-based optical transmission spectrum of the hot Jupiter HAT-P-1b

“…This level of uncertainty in the wavelength solution has been shown to be sufficient to avoid systematic effects caused by wavelength-dependent differences in the limb-darkening models we used to fit the transits in Sect. 4 (Huitson et al 2017), and in addition, it is also smaller than our resolution element (∼ 4 nm in R150 and ∼2 nm in B600).…”

“…We based our data reduction on the custom GMOS transit spectroscopy pipeline discussed in detail in Huitson et al (2017). We used this code to correct the raw images for gain and bias levels, to perform cosmic ray identification, to remove bad columns, to flat-field the data, and to correct for slit tilt (as needed), and finally, to extract the 1D spectra.…”

“…The combined background and diluted-light fluxes correspond to ∼ 1% of the target stellar flux. To ensure that using the PSF shape on the opposite side of the target is reasonable, we tested the symmetry of the GMOS spectral PSF for a single star using the observations of WASP-4 (Huitson et al 2017). We found that at 11.2", the spectroscopic PSF is symmetric in the cross-dispersion direction within 0.2-0.4% of the overall flux level at this location.…”

“…Because GMOS has no atmospheric dispersion compensator, we expect the wavelength solution to shift with time. Huitson et al (2017) reported that throughout the night, individual spectra are shifted and stretched in the dispersion direction. The shift and stretch are both time and wavelength dependent.…”

“…This work, along with the study by Huitson et al (2017), is part of a transit spectroscopy survey using the Gemini/GMOS instrument that observed nine transiting exoplanets during a total of about 40 transits with high spectrophotometric precision in visible wavelengths. The results from Huitson et al (2017) have been used by Bouma et al (2019) along with other observations, including TESS transits, to detect transit timing variations of the hot Jupiter WASP-4b. The goal of the GMOS survey is to robustly estimate systematic effects and extract highfidelity transit spectra for the target planets, ultimately constraining Rayleigh scattering and cloud deck prevalence in irradiated atmospheres.…”