The GTC exoplanet transit spectroscopy survey II: An overly-large Rayleigh-like feature for exoplanet TrES-3b

TL;DR

This study uses ground-based transmission spectroscopy to analyze the atmosphere of exoplanet TrES-3b, finding an unexpectedly large Rayleigh-like feature that remains unexplained after accounting for telluric contamination.

Contribution

It introduces a Bayesian analysis method incorporating red noise modeling to study exoplanet atmospheres using ground-based data, revealing an anomalously strong Rayleigh-like feature.

Findings

Detected a strong Rayleigh-like increase in extinction towards blue wavelengths.

Identified a K I resonance feature caused by telluric O₂ variability.

Observed the Rayleigh-like feature remains unexplained after correction.

Abstract

We set to 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 OSIRIS@GTC. 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 -- temporally correlated -- noise, with two approaches (Gaussian processes and divide-by-white) to account for the red noise. 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 O$_2$ absorption, but the Rayleigh-like feature remains unexplained.