Broad-band transmission spectrum and K-band thermal emission of WASP-43b as observed from the ground ^⋆,⋆⋆

¹ Purple Mountain Observatory & Key Laboratory for Radio Astronomy, Chinese Academy of Sciences, 2 West Beijing Road, 210008, Nanjing, PR China
e-mail: guochen@pmo.ac.cn
² University of Chinese Academy of Sciences, 19A Yuquan Road, 100049 Beijing, PR China
³ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁴ Astrophysics Group, University of Exeter, Stocker Road, EX4 4QL, Exeter, UK
⁵ Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064, USA
⁶ Institut für Astrophysik, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
⁷ Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, 100012 Beijing, PR China

Received: 24 September 2013
Accepted: 8 January 2014

Abstract

Aims. WASP-43b is the closest-orbiting hot Jupiter, and it has high bulk density. It causes deep eclipse depths in the system’s light curve in both transit and occultation that is attributed to the cool temperature and small radius of its host star. We aim to secure a broad-band transmission spectrum and to detect its near-infrared thermal emission in order to characterize its atmosphere.

Methods. We observed one transit and one occultation event simultaneously in the g′, r′, i′, z′, J, H, K bands using the GROND instrument on the MPG/ESO 2.2-m telescope, where the telescope was heavily defocused in staring mode. After modeling the light curves, we derived wavelength-dependent transit depths and flux ratios and compared them to atmospheric models.

Results. From the transit event, we have independently derived WASP-43’s system parameters with high precision and improved the period to be 0.81347437(13) days based on all the available timings. No significant variation in transit depths is detected, with the largest deviations coming from the i′-, H-, and K-bands. Given the observational uncertainties, the broad-band transmission spectrum can be explained by either (i) a flat featureless straight line that indicates thick clouds; (ii) synthetic spectra with absorption signatures of atomic Na/K, or molecular TiO/VO that in turn indicate cloud-free atmosphere; or (iii) a Rayleigh scattering profile that indicates high-altitude hazes. From the occultation event, we detected planetary dayside thermal emission in the K-band with a flux ratio of 0.197 ± 0.042%, which confirms previous detections obtained in the 2.09 μm narrow band and K_S-band. The K-band brightness temperature 1878⁺¹⁰⁸_-116 K favors an atmosphere with poor day- to nightside heat redistribution. We also have a marginal detection in the i′-band (0.037^+0.023_-0.021%), corresponding to T_B = 2225⁺¹³⁹_-225 K, which is either a false positive, a signature of non-blackbody radiation at this wavelength, or an indication of reflective hazes at high altitude.