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\nusing the 10.4 m Gran Telescopio Canarias (GTC) and the OSIRIS instrument.\nUsing tuneable filters we observed a total of five transits, three of which\nwere observed at two wavelengths nearly simultaneously, producing a total of\neight individual light curves, six of these probed the possible existence of a\nmethane absorption feature in the 8770 - 8850 {\\AA} region at high resolution.\nWe detect no increase in the planet-to-star radius ratio across the methane\nfeature with a change in radius ratio of $\\Delta$R = -0.0007 $\\pm$ 0.0017\ncorresponding to a scale height (H) change of -0.5 $\\pm$ 1.2 H across the\nmethane feature, assuming a hydrogen dominated atmosphere. We find a variety of\nwater and cloudy atmospheric models fit the data well, but find that cloud-free\nmodels provide poor fits. These observations support a flat transmission\nspectrum resulting from the presence of a high-altitude haze or a water-rich\natmosphere, in agreement with previous studies. In this study the observations\nare predominantly limited by the photometric quality and the limited number of\ndata points (resulting from a long observing cadence), which make the\ndetermination of the systematic noise challenging. With tuneable filters\ncapable of high resolution measurements (R ~ 600 - 750) of narrow absorption\nfeatures, the interpretation of our results are also limited by the absence of\nhigh resolution methane models below 1 $\\mu$m.\n