Narrow band Hαphotometry of the super-Earth GJ 1214b with GTC/OSIRIS tunable filters

Abstract

The super-earth planet GJ 1214b has recently been the focus of several studies, using the transit spectroscopy technique, trying to determine the nature of its atmosphere. Here we focus on the Halpha line as a tool to further restrict the nature of GJ1214's atmosphere. We used the Gran Telescopio Canarias (GTC) OSIRIS instrument to acquire narrow band photometry with tunable filters. With our observations, we were able to observe the primary transit of the super-Earth GJ 1214b in three bandpasses: two centered in the continuum around Halpha (653.5 nm and 662.0 nm) and one centered at the line core (656.3 nm). We measure the depth of the planetary transit at each wavelength interval.By fitting analytic models to the measured light curves we were able to compute the depth of the transit at the three bandpasses. Taking the difference in the computed planet to star radius ratio between the line and the comparison continuum filters, we find Delta (Rp/Rstar)_{Halpha-653.5} = (6.60 +/- 3.54) 10^-3 and Delta (Rp/Rstar)_{Halpha-662.0} = (3.30 +/- 3.61) 10^-3. Although the planet radius is found to be larger in the Halpha line than in the surrounding continuum, the quality of our observations and the sigma level of the differences (1.8 and 1.0, respectively) does not allow us to claim an Halpha excess in GJ1214's atmosphere. Further observations will be needed to resolve this issue.