The GTC exoplanet transit spectroscopy survey: V. A spectrally-resolved Rayleigh scattering slope in GJ 3470b

Aims. As a sub-Uranus-mass low-density planet, GJ 3470b has been found to show a flat featureless transmission spectrum in the infrared and a tentative Rayleigh scattering slope in the optical. We conducted an optical transmission spectroscopy project to assess the impacts of stellar activity and to...

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Bibliographic Details
Published in:Astronomy and astrophysics (Berlin) 2017-04, Vol.600, p.A138
Main Authors: Chen, G, Guenther, E W, Palle, E, Nortmann, L, Nowak, G, Kunz, S, Parviainen, H, Murgas, F
Format: Article
Language:English
Subjects:
Clouds
Extrasolar planets
High resolution
Rayleigh scattering
Slopes
Spectroscopy
Stellar activity
Transit
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Summary:Aims. As a sub-Uranus-mass low-density planet, GJ 3470b has been found to show a flat featureless transmission spectrum in the infrared and a tentative Rayleigh scattering slope in the optical. We conducted an optical transmission spectroscopy project to assess the impacts of stellar activity and to determine whether or not GJ 3470b hosts a hydrogen-rich gas envelop. Methods. We observed three transits with the low-resolution Optical System for Imaging and low-Intermediate-Resolution Integrated Spectroscopy (OSIRIS) at the 10.4 m Gran Telescopio Canarias, and one transit with the high-resolution Ultraviolet and Visual Echelle Spectrograph (UVES) at the 8.2 m Very Large Telescope. Results. From the high-resolution data, we find that the difference of the CaiiH+K lines in- and out-of-transit is only 0.67 + or - 0.22%, and determine a magnetic filling factor of about 10-15%. From the low-resolution data, we present the first optical transmission spectrum in the 435-755 nm band, which shows a slope consistent with Rayleigh scattering. Conclusions. After exploring the potential impacts of stellar activity in our observations, we confirm that Rayleigh scattering in an extended hydrogen-helium atmosphere is currently the best explanation. Further high-precision observations that simultaneously cover optical and infrared bands are required to answer whether or not clouds and hazes exist at high-altitude.
ISSN:0004-6361
1432-0746
DOI:10.1051/0004-6361/201630228