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A search for methane in the atmosphere of GJ 1214b via GTC narrow-band transmission spectrophotometry

Abstract : We present narrow-band photometric measurements of the exoplanet GJ 1214b using the 10.4 m Gran Telescopio Canarias and the Optical System for Imaging and low Resolution Integrated Spectroscopy instrument. Using tuneable filters, we observed a total of five transits, three of which were observed at two wavelengths nearly simultaneously, producing a total of eight individual light curves, six of these probed the possible existence of a methane absorption feature in the 8770-8850 Å region at high resolution. We detect no increase in the planet-to-star radius ratio across the methane feature with a change in radius ratio of Δ overline{R} = -0.0007 ± 0.0017 corresponding to a scaleheight (H) change of -0.5 ± 1.2H across the methane feature, assuming a hydrogen-dominated atmosphere. We find that a variety of water and cloudy atmospheric models fit the data well, but find that cloud-free models provide poor fits. These observations support a flat transmission spectrum resulting from the presence of a high-altitude haze or a water-rich atmosphere, in agreement with previous studies. In this study, the observations are pre-dominantly limited by the photometric quality and the limited number of data points (resulting from a long observing cadence), which make the determination of the systematic noise challenging. With tuneable filters capable of high-resolution measurements (R ≈ 600-750) of narrow absorption features, the interpretation of our results are also limited by the absence of high-resolution methane models below 1 μm.
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P. A. Wilson, K. D. Colón, D. K. Sing, G. E. Ballester, J. -M. Désert, et al.. A search for methane in the atmosphere of GJ 1214b via GTC narrow-band transmission spectrophotometry. Monthly Notices of the Royal Astronomical Society, Oxford University Press (OUP): Policy P - Oxford Open Option A, 2014, 438, pp.2395-2405. ⟨10.1093/mnras/stt2356⟩. ⟨insu-03645691⟩

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