GTC OSIRIS transiting exoplanet atmospheric survey: detection of potassium in HAT-P-1b from narrow-band spectrophotometry

We present the detection of potassium in the atmosphere of HAT-P-1b using optical transit narrow-band photometry. The results are obtained using the 10.4-m Gran Telescopio Canarias together with the OSIRIS instrument in tunable filter imaging mode. We observed four transits, two at continuum wavelengths outside the potassium feature, at 6792 and 8844 angstrom, and two probing the potassium feature in the line wing at 7582.0 angstrom and the line core at 7664.9 angstrom using a 12 angstrom filter width (R similar to 650). The planet-to-star radius ratios in the continuum are found to be R-pl/R-star = 0.1176 +/- 0.0013 at 6792 angstrom and R-pl/R-star = 0.1168 +/- 0.0022 at 8844 angstrom, significantly lower than the two observations in the potassium line: R-pl/R-star = 0.1248 +/- 0.0014 in the line wing at 7582.0 angstrom and R-pl/R-star = 0.1268 +/- 0.0012 in the line core at 7664.9 angstrom With a weighted mean of the observations outside the potassium feature R-pl/R-star = 0.1174 +/- 0.0010, the potassium is detected as an increase in the radius ratio of Delta R-pl/R-star = 0.0073 +/- 0.0017 at 7582.0 angstrom and Delta R-pl/R-star = 0.0094 +/- 0.0016 at 7664.9 angstrom (a significance of 4.3 sigma and 6.1 sigma, respectively). We hypothesize that the strong detection of potassium is caused by a large scaleheight, which can be explained by a high temperature at the base of the upper atmosphere. A lower mean molecular mass caused by the dissociation of molecular hydrogen into atomic hydrogen by the extreme ultraviolet flux from the host star may also partly explain the amplitude of our detection.