Title: The GTC exoplanet transit spectroscopy survey. VII. Detection of sodium in WASP-52b's cloudy atmosphere,
Abstract: We report the first detection of sodium absorption in the atmosphere of the
hot Jupiter WASP-52b. We observed one transit of WASP-52b with the
low-resolution Optical System for Imaging and low-Intermediate-Resolution
Integrated Spectroscopy (OSIRIS) at the 10.4 m Gran Telescopio Canarias (GTC).
The resulting transmission spectrum, covering the wavelength range from 522 nm
to 903 nm, is flat and featureless, except for the significant narrow
absorption signature at the sodium doublet, which can be explained by an
atmosphere in solar composition with clouds at 1 mbar. A cloud-free atmosphere
is stringently ruled out. By assessing the absorption depths of sodium in
various bin widths, we find that temperature increases towards lower
atmospheric pressure levels, with a positive temperature gradient of 0.88 +/-
0.65 K/km, possibly indicative of upper atmospheric heating and a temperature
inversion.