The GTC exoplanet transit spectroscopy survey

II. An overly large Rayleigh-like feature for exoplanet TrES-3b^⋆

¹ Sub-department of Astrophysics, Department of Physics, University of Oxford, Oxford, OX1 3RH, UK
e-maiil: hannu.parviainen@astro.ox.ac.uk
² Instituto de Astrofísica de Canarias (IAC), 38200, La Laguna, Tenerife, Spain
³ Dept. Astrofísica, Universidad de La Laguna (ULL), 38206, La Laguna, Tenerife, Spain
⁴ Institut für Astrophysik, Georg-August-Universität, Friederich-Hund-Platz 1, 37077 Göttingen, Germany
⁵ Theoretical Meteorology group, Klimacampus, University of Hamburg, Grindelberg 5, 20144 Hamburg, Germany
⁶ Univ. Grenoble Alpes, IPAG, 38000 Grenoble, France
⁷ CNRS, IPAG, 38000 Grenoble, France

Received: 14 April 2015
Accepted: 8 October 2015

Abstract

Aims. We search for Rayleigh scattering and K and Na absorption signatures from the atmosphere of TrES-3b using ground-based transmission spectroscopy covering the wavelength range from 530 to 950 nm as observed with the OSIRIS instrument at the Gran Telescopio CANARIAS.

Methods. Our analysis is based on a Bayesian approach where the light curves covering a set of given passbands are fitted jointly with PHOENIX-calculated stellar limb darkening profiles. The analysis is carried out assuming both white and red noise that is temporally correlated, with two approaches (Gaussian processes and divide-by-white) to account for the red noise.

Results. An initial analysis reveals a transmission spectrum that shows a strong Rayleigh-like increase in extinction towards the blue end of the spectrum, and enhanced extinction around the K I resonance doublet near 767 nm. However, the signal amplitudes are significantly larger than expected from theoretical considerations. A detailed analysis reveals that the K I-like feature is entirely due to variability in the telluric O₂ absorption, but the Rayleigh-like feature remains unexplained.