Kepler-423b: a half-Jupiter mass planet transiting a very old solar-like star^⋆,⋆⋆

¹ Landessternwarte Königstuhl, Zentrum für Astronomie der Universität Heidelberg, Königstuhl 12, 69117 Heidelberg, Germany
e-mail: davide.gandolfi@lsw.uni-heidelberg.de
² INAF–Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy
³ Department of Physics, University of Oxford, Oxford, OX1 3RH, UK
⁴ Instituto de Astrofísica de Canarias, C/ Vía Láctea s/n, 38205 La Laguna, Spain
⁵ Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Spain
⁶ Institute of Planetary Research, German Aerospace Center, Rutherfordstrasse 2, 12489 Berlin, Germany
⁷ Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, 439 92, Onsala, Sweden
⁸ Leiden Observatory, University of Leiden, PO Box 9513, 2300 RA, Leiden, The Netherlands
⁹ Physics Department “E. Fermi”, University of Pisa, Largo B. Pontecorvo 3, 56127 Pisa, Italy
¹⁰ Istituto Nazionale di Fisica Nucleare, Largo B. Pontecorvo 3, 56127 Pisa, Italy
¹¹ Nordic Optical Telescope, Apartado 474, 38700 Santa Cruz de La Palma, Spain
¹² Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
¹³ School of Physics and Astronomy, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv, Israel
¹⁴ Tuorla Observatory, Department of Physics and Astronomy, University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland
¹⁵ Stellar Astrophysics Centre, Department of Physics and Astronomy, Århus Uni., Ny Munkegade 120, 8000 Århus C, Denmark

Received: 26 September 2014
Accepted: 17 November 2014

Abstract

We report the spectroscopic confirmation of the Kepler object of interest KOI-183.01 (Kepler-423b), a half-Jupiter mass planet transiting an old solar-like star every 2.7 days. Our analysis is the first to combine the full Kepler photometry (quarters 1−17) with high-precision radial velocity measurements taken with the FIES spectrograph at the Nordic Optical Telescope. We simultaneously modelled the photometric and spectroscopic data-sets using Bayesian approach coupled with Markov chain Monte Carlo sampling. We found that the Kepler pre-search data conditioned light curve of Kepler-423 exhibits quarter-to-quarter systematic variations of the transit depth, with a peak-to-peak amplitude of ~4.3% and seasonal trends reoccurring every four quarters. We attributed these systematics to an incorrect assessment of the quarterly variation of the crowding metric. The host star Kepler-423 is a G4 dwarf with M_⋆ = 0.85 ± 0.04 M_⊙, R_⋆ = 0.95 ± 0.04 R_⊙, T_eff= 5560 ± 80 K, [M/H] = − 0.10 ± 0.05 dex, and with an age of 11 ± 2 Gyr. The planet Kepler-423b has a mass of M_p= 0.595 ± 0.081M_Jup and a radius of R_p= 1.192 ± 0.052R_Jup, yielding a planetary bulk density of ρ_p = 0.459 ± 0.083  g cm^-3. The radius of Kepler-423b is consistent with both theoretical models for irradiated coreless giant planets and expectations based on empirical laws. The inclination of the stellar spin axis suggests that the system is aligned along the line of sight. We detected a tentative secondary eclipse of the planet at a 2σ confidence level (ΔF_ec = 14.2 ± 6.6 ppm) and found that the orbit might have asmall non-zero eccentricity of 0.019^+0.028_-0.014. With a Bond albedo of A_B = 0.037 ± 0.019, Kepler-423b is one of the gas-giant planets with the lowest albedo known so far.