VLT/FORS2 comparative transmission spectroscopy II: confirmation of a cloud-deck and Rayleigh scattering in WASP-31b, but no potassium?
Gibson, NP; Nokolov, N; Sing, DK; et al.Barstow, J; Evans, TE; Kataria, T; Wilson, PA
Date: 1 January 2017
Journal
Monthly Notices of the Royal Astronomical Society
Publisher
Oxford University Press (OUP) / Royal Astronomical Society
Publisher DOI
Abstract
We present transmission spectroscopy of the hot-Jupiter WASP-31b using FORS2
on the VLT during two primary transits. The observations cover a wavelength range
of ≈400–840 nm. The light curves are corrupted by significant systematics, but these
were to first order invariant with wavelength and could be removed using a commonmode
correction ...
We present transmission spectroscopy of the hot-Jupiter WASP-31b using FORS2
on the VLT during two primary transits. The observations cover a wavelength range
of ≈400–840 nm. The light curves are corrupted by significant systematics, but these
were to first order invariant with wavelength and could be removed using a commonmode
correction derived from the white light curves. We reach a precision in the
transit depth of ≈140 ppm in 15 nm bins, although the precision varies significantly
over the wavelength range. Our FORS2 observations confirm the cloud-deck previously
inferred using HST/STIS. We also re-analyse the HST/STIS data using a Gaussian
process model, finding excellent agreement with earlier measurements. We reproduce
the Rayleigh scattering signature at short wavelengths (. 5300 ˚A) and the
cloud-deck at longer wavelengths. However, our FORS2 observations appear to rule
out the large potassium feature previously detected using STIS, yet it is recovered from
the HST/STIS data, although with reduced amplitude and significance (≈ 2.5σ). The
discrepancy between our results and the earlier STIS detection of potassium (≈ 4.3σ) is
either a result of telluric contamination of the ground-based observations, or an underestimate
of the uncertainties for narrow-band features in HST/STIS when using linear
basis models to account for the systematics. Our results further demonstrate the use
of ground-based multi-object spectrographs for the study of exoplanet atmospheres,
and highlight the need for caution in our interpretation of narrow-band features in
low-resolution spectra of hot-Jupiters.
Physics and Astronomy
Faculty of Environment, Science and Economy
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