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Sing D.K.,University of Exeter | Huitson C.M.,University of Exeter | Lopez-Morales M.,Institute Of Ciencies Of Lespai Csic Ice | Pont F.,University of Exeter | And 7 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2012

We present two transits of the hot-Jupiter exoplanet XO-2b using the Gran Telescopio Canarias (GTC). The time series observations were performed using long-slit spectroscopy of XO-2 and a nearby reference star with the Optical System for Imaging and low Resolution Integrated Spectroscopy (OSIRIS) instrument, enabling differential spectrophotometric transit light curves capable of measuring the exoplanet's transmission spectrum. Two optical low-resolution grisms were used to cover the optical wavelength range from 3800 to 9300Å. We find that sub-mmag-level slit losses between the target and reference star prevent full optical transmission spectra from being constructed, limiting our analysis to differential absorption depths over ∼1000Å regions. Wider long slits or multi-object grism spectroscopy with wide masks will likely prove effective in minimizing the observed slit-loss trends. During both transits, we detect significant absorption in the planetary atmosphere of XO-2b using a 50-Å bandpass centred on the Nai doublet, with absorption depths of Δ(R pl/R {black star}) 2 = 0.049 ± 0.017 per cent using the R500R grism and 0.047 ± 0.011 per cent using the R500B grism (combined 5.2σ significance from both transits). The sodium feature is unresolved in our low-resolution spectra, with detailed modelling also likely ruling out significant line-wing absorption over an ∼800Å region surrounding the doublet. Combined with narrow-band photometric measurements, XO-2b is the first hot Jupiter with evidence for both sodium and potassium present in the planet's atmosphere. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.

Sing D.K.,University of Exeter | Desert J.-M.,Harvard - Smithsonian Center for Astrophysics | Fortney J.J.,University of California at Santa Cruz | Des Etangs A.L.,University Pierre and Marie Curie | And 8 more authors.
Astronomy and Astrophysics | Year: 2011

We present Gran Telescopio Canarias (GTC) optical transit narrowband photometry of the hot-Jupiter exoplanet XO-2b using the OSIRIS instrument. This unique instrument has the capabilities to deliver high-cadence narrowband photometric lightcurves, allowing us to probe the atmospheric composition of hot Jupiters from the ground. The observations were taken during three transit events that cover four wavelengths at spectral resolutions near 500, necessary for observing atmospheric features, and have near-photon limited sub-mmag precisions. Precision narrowband photometry on a large aperture telescope allows for atmospheric transmission spectral features to be observed for exoplanets around much fainter stars than those of the well studied targets HD 209458b and HD 189733b, providing access to the majority of known transiting planets. For XO-2b, we measure planet-to-star radius contrasts of Rpl/R * = 0.10508 ± 0.00052 at 6792 Å, 0.10640 ± 0.00058 at 7582 Å, and 0.10686 ± 0.00060 at 7664.9 Å, and 0.10362 ± 0.00051 at 8839 Å. These measurements reveal significant spectral features at two wavelengths, with an absorption level of 0.067 ± 0.016% at 7664.9 Å caused by atmospheric potassium in the line core (a 4.1-σ significance level), and an absorption level of 0.058 ± 0.016% at 7582 Å, (a 3.6-σ significance level). When comparing our measurements to hot-Jupiter atmospheric models, we find good agreement with models that are dominated in the optical by alkali metals. This is the first evidence for potassium in an extrasolar planet, an element that has along with sodium long been supposed to be a dominant source of opacity at optical wavelengths for hot Jupiters. © 2011 ESO.

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