[Paper Review] The GTC exoplanet transit spectroscopy survey XI. Possible detection of Rayleigh scattering in the atmosphere of the Saturn-mass planet WASP-69b

F. Murgas, G. Chen+3 more|arXiv (Cornell University)|Jul 6, 2020

Stellar, planetary, and galactic studies67 references4 citations

TL;DR

This study presents ground-based optical spectrophotometry of WASP-69b's transit using the GTC/OSIRIS instrument, detecting a wavelength-dependent increase in transit depth toward bluer wavelengths. The observed slope is consistent with Rayleigh scattering in a hydrogen-dominated atmosphere, though stellar activity cannot be fully ruled out; spectral retrieval favors hazes over stellar spots as the dominant source of scattering.

ABSTRACT

One of the main atmospheric features in exoplanet atmospheres, detectable both from ground- and space-based facilities, is Rayleigh scattering. In hydrogen-dominated planetary atmospheres, Rayleigh scattering causes the measured planetary radius to increase toward blue wavelengths in the optical range. We obtained a spectrophotometric time series of one transit of the Saturn-mass planet WASP-69b using the OSIRIS instrument at the Gran Telescopio Canarias. From the data we constructed 19 spectroscopic transit light curves representing 20 nm wide wavelength bins spanning from 515 nm to 905 nm. We derived the transit depth for each curve individually by fitting an analytical model together with a Gaussian process to account for systematic noise in the light curves. We find that the transit depth increases toward bluer wavelengths, indicative of a larger effective planet radius. Our results are consistent with space-based measurements obtained in the near infrared using the Hubble Space Telescope, which show a compatible slope of the transmission spectrum. We discuss the origin of the detected slope and argue between two possible scenarios: a Rayleigh scattering detection originating in the planet's atmosphere or a stellar activity induced signal from the host star.

Motivation & Objective

• To detect and characterize atmospheric features in the optical transmission spectrum of WASP-69b, a Saturn-mass exoplanet.
• To determine whether the observed wavelength-dependent transit depth variation is due to atmospheric Rayleigh scattering or stellar activity.
• To compare ground-based optical data with prior Hubble Space Telescope near-infrared measurements to constrain atmospheric composition and structure.
• To assess the role of atmospheric hazes versus stellar spots in producing the observed spectral slope.
• To improve understanding of atmospheric scattering mechanisms in highly irradiated, low-density exoplanets.

Proposed method

• Obtained a spectrophotometric time series of one transit of WASP-69b using the OSIRIS instrument on the 10.4 m Gran Telescopio Canarias.
• Constructed 19 spectroscopic transit light curves across 20 nm-wide wavelength bins from 515 nm to 905 nm.
• Fitted each light curve with an analytical transit model combined with a Gaussian process to correct for systematic noise.
• Calculated the planet-to-star radius ratio as a function of wavelength to derive the transmission spectrum.
• Modeled stellar activity effects using spot and faculae parameters to test if they could reproduce the observed slope.
• Combined GTC optical data with HST/WFC3 near-infrared data for joint spectral retrieval analysis using atmospheric models with varying cloud and haze properties.

Experimental results

Research questions

• RQ1 Does the optical transmission spectrum of WASP-69b exhibit a blueward-increasing transit depth consistent with Rayleigh scattering?
• RQ2 Could the observed spectral slope be explained by unocculted stellar spots or faculae rather than atmospheric scattering?
• RQ3 How do the optical and near-infrared transmission spectra of WASP-69b compare in terms of slope and absorption features?
• RQ4 What atmospheric models best explain the combined optical and near-infrared data, including haze and metallicity effects?
• RQ5 Is the detected signal more consistent with a planetary atmospheric origin or stellar activity, given the star's measured activity level?

Key findings

• The optical transmission spectrum shows a significant increase in transit depth toward shorter wavelengths, with a slope of α = -3.35 ± 0.75, consistent with Rayleigh scattering in a hydrogen-dominated atmosphere.
• Stellar activity modeling suggests that spots and faculae with filling factors δ_spot = 0.55+0.30-0.27 and δ_facu = 0.15+0.46-0.13, and temperatures T_spot = 4594+48-77 K and T_facu = 4788+308-68 K, could reproduce the observed slope.
• Joint analysis of GTC and HST/WFC3 data rules out both cloud-free and completely cloudy atmospheric models as explanations for the observed spectrum.
• Spectral retrieval favors models with enhanced scattering (e.g., hazes) and super-solar metallicity to explain the optical and near-infrared features.
• Despite the star's moderate activity (log R′_HK ≈ -4.54), the lack of spot-crossing features in the white-light light curve and consistency with HST data support a planetary origin for the slope.
• The study cannot definitively rule out stellar activity as the source, and independent observations at different epochs are recommended to confirm the atmospheric interpretation.