We present an optical to near-infrared transmission spectrum of the hot Jupiter HAT-P-1b, based on HST observations, covering the spectral regime from 0.29 to 1.027{\mu}m with STIS, which is coupled ...with a recent WFC3 transit (1.087 to 1.687{\mu}m). We derive refined physical parameters of the HAT-P-1 system, including an improved orbital ephemeris. The transmission spectrum shows a strong absorption signature shortward of 0.55{\mu}m, with a strong blueward slope into the near-ultraviolet. We detect atmospheric sodium absorption at a 3.3{\sigma} significance level, but find no evidence for the potassium feature. The red data implies a marginally flat spectrum with a tentative absorption enhancement at wavelength longer than ~0.85{\mu}m. The STIS and WFC3 spectra differ significantly in absolute radius level (4.3 +/- 1.6 pressure scale heights), implying strong optical absorption in the atmosphere of HAT-P-1b. The optical to near-infrared difference cannot be explained by stellar activity, as simulta- neous stellar activity monitoring of the G0V HAT-P-1b host star and its identical companion show no significant activity that could explain the result. We compare the complete STIS and WFC3 transmission spectrum with theoretical atmospheric mod- els which include haze, sodium and an extra optical absorber. We find that both an optical absorber and a super-solar sodium to water abundance ratio might be a sce- nario explaining the HAT-P-1b observations. Our results suggest that strong optical absorbers may be a dominant atmospheric feature in some hot Jupiter exoplanets.
In June 2011 we attempted to measure the first atmospheric transmission spectrum of an exoplanet with the GTC. We observed the hot Jupiter TrEs-2b during two transit epochs using the long-slit ...spectroscopic mode of OSIRIS. The technique consists of doing differential spectrophotometry by placing the planet host and a nearby star on a wide slit during a planetary transit. We then monitor both objects for the duration of the event to obtain simultaneous transit coverage at wavelengths between ~5000 and 9000 Å. We present here the results of our first attempt with this new technique, which holds the potential of providing a 5-10 fold improvement on the efficiency of exoplanet primary and secondary eclipse observations with the GTC. Finally, we also present a summary of the science done so far with observations from the ESO/GTC program 182.C.2018. Fernandez, J. M., et al. 2009, ApJ, 137, 4911 Sing, D. K., et al. 2011, A&A, 527, 73
