VLT FORS2 comparative transmission spectroscopy: Detection of Na in the atmosphere of WASP-39b from the ground
Sing, David K.
Gibson, Neale P.
Fortney, J. J.
Evans, Thomas M.
Barstow, Joanna K.
Wilson, Paul A.
American Astronomical Society
We present transmission spectroscopy of the warm Saturn-mass exoplanet WASP-39b made with the Very Large Telescope FOcal Reducer and Spectrograph (FORS2) across the wavelength range 411–810 nm. The transit depth is measured with a typical precision of 240 parts per million (ppm) in wavelength bins of 10 nm on a V = 12.1 mag star. We detect the sodium absorption feature (3.2σ) and find evidence of potassium. The ground-based transmission spectrum is consistent with Hubble Space Telescope (HST) optical spectroscopy, supporting the interpretation that WASP-39b has a largely clear atmosphere. Our results demonstrate the great potential of the recently upgraded FORS2 spectrograph for optical transmission spectroscopy, with which we obtained HST-quality light curves from the ground.
The research leading to these results received funding from the European Research Council under the European Union Seventh Framework Program (FP7/2007-2013) ERC grant agreement no. 336792. N.P.G. gratefully acknowledges support from the Royal Society in the form of a University Research Fellowship. J.K.B. and E.R.C. acknowledge support from project 617119 (ExoLights). P.A.W. acknowledges the support of the French Agence Nationale de la Recherche (ANR), under program ANR-12-BS05-0012 "Exo-Atmos."
This is the author accepted manuscript. The final version is available from American Astronomical Society via the DOI in this record.
Vol. 832, No. 2