SOPHIE velocimetry of Kepler transit candidates

XII. KOI-1257 b: a highly eccentric three-month period transiting exoplanet^{⋆,⋆⋆,⋆⋆⋆}

¹ Centro de Astrofísica, Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
e-mail: alexandre.santerne@astro.up.pt
² Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
³ Aix-Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
⁴ Institut d’Astrophysique de Paris, UMR 7095 CNRS, Université Pierre & Marie Curie, 98bis boulevard Arago, 75014 Paris, France
⁵ Observatoire de Haute-Provence, Université d’Aix-Marseille & CNRS, 04870 Saint Michel l’Observatoire, France
⁶ Laboratoire Lagrange, UMR 7239, Université de Nice-Sophia Antipolis, CNRS, Observatoire de la Côte d’Azur, 06300 Nice, France
⁷ Departamento de Física, I3N, Universidade de Aveiro, Campus de Santiago, 3810-193 Aveiro, Portugal
⁸ Astronomie et Systèmes Dynamiques, IMCCE-CNRS UMR 8028, 77 Av. Denfert-Rochereau, 75014 Paris, France
⁹ Instituto de Astrofísica de Canarias (IAC), 38200, La Laguna, Tenerife, Spain
¹⁰ Universidad de La Laguna, Dept. Astrofísica, 38206, La Laguna, Tenerife, Spain
¹¹ Observatoire Astronomique de l’Université de Genève, 51 chemin des Maillettes, 1290 Versoix, Switzerland
¹² CdR & CdL Group: Lightcurves of Minor Planets and Variable Stars, 1290 Sauverny, Switzerland
¹³ Observatoire des Engarouines, 1606 chemin de Rigoy, 84570 Mallemort-du-Comtat, France
¹⁴ INAF – Osservatorio Astrofisico di Torino, via Osservatorio 20, 10025 Pino Torinese, Italy
¹⁵ Centre d’Astronomie, Plateau du Moulin à Vent, 04870 St.-Michel-l’Observatoire, France
¹⁶ CNRS, Canada-France-Hawaii Telescope Corporation, 65-1238 Mamalahoa Hwy., Kamuela, HI 96743, USA
¹⁷ Observatoire Oukaimeden, 40273 Oukaimeden, Morocco
¹⁸ Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, 4169-007 Porto, Portugal
¹⁹ Observatoire de Chinon B92, Mairie de Chinon, 37500 Chinon, France
²⁰ Club d’Astronomie Lyon Ampère, Bât. Planétarium, Place de la Nation, 69120 Vaulx-en-Velin, France
²¹ Atalaia Group – Crow Observatory, 4169-007 Porto, Portalegre, Portugal
²² Observatoire ADAGIO, 31540 Bélesta, France
²³ 2 rue des Écoles, 34920 Le Cres, France
²⁴ Observatoire de Blauvac, 293 Ch. de St. Guillaume 84570 Blauvac, France
²⁵ OverSky, 47 allée des Palanques, BP 12, 33127, Saint-Jean d’Illac, France
²⁶ Montreurs de Grande Ourse, les Mairies, 26190 St. Laurent en Royans, France

Received: 8 May 2014
Accepted: 23 June 2014

Abstract

In this paper we report a new transiting warm giant planet: KOI-1257 b. It was first detected in photometry as a planet-candidate by the Kepler space telescope and then validated thanks to a radial velocity follow-up with the SOPHIE spectrograph. It orbits its host star with a period of 86.647661 d ± 3 s and a high eccentricity of 0.772 ± 0.045. The planet transits the main star of a metal-rich, relatively old binary system with stars of mass of 0.99 ± 0.05 M_☉ and 0.70 ± 0.07 M_☉ for the primary and secondary, respectively. This binary system is constrained thanks to a self-consistent modelling of the Kepler transit light curve, the SOPHIE radial velocities, line bisector and full-width half maximum (FWHM) variations, and the spectral energy distribution. However, future observations are needed to confirm it. The PASTIS fully-Bayesian software was used to validate the nature of the planet and to determine which star of the binary system is the transit host. By accounting for the dilution from the binary both in photometry and in radial velocity, we find that the planet has a mass of 1.45 ± 0.35 M_♃ , and a radius of 0.94 ± 0.12 R_♃ , and thus a bulk density of 2.1 ± 1.2 g cm^-3. The planet has an equilibrium temperature of 511 ± 50 K, making it one of the few known members of the warm-Jupiter population. The HARPS-N spectrograph was also used to observe a transit of KOI-1257 b, simultaneously with a joint amateur and professional photometric follow-up, with the aim of constraining the orbital obliquity of the planet. However, the Rossiter-McLaughlin effect was not clearly detected, resulting in poor constraints on the orbital obliquity of the planet.