Volume 572, December 2014
|Number of page(s)||11|
|Section||Planets and planetary systems|
|Published online||04 December 2014|
XIII. KOI-189 b and KOI-686 b: two very low-mass stars in long-period orbits
Observatoire Astronomique de l’Université de Genève,
Chemin des Maillettes 51,
2 Aix-Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
3 Institut d’Astrophysique de Paris, UMR 7095 CNRS, Université Pierre & Marie Curie, 98bis boulevard Arago, 75014 Paris, France
4 Observatoire de Haute-Provence, CNRS/OAMP, 04870, Saint-Michel-l’Observatoire, France
5 Canadian Institute for Theoretical Astrophysics, 60 St. George St., University of Toronto, Toronto, ON M5S 3H8, Canada
6 INAF – Osservatorio Astrofisico di Torino, via Osservatorio 20, 10025 Pino Torinese, Italy
7 Canada France Hawaii Telescope Corporation, Kamuela, USA
8 Centro de Astrofísica, Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
9 Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
Received: 15 June 2014
Accepted: 6 October 2014
We present the radial-velocity follow-up of two Kepler planetary transiting candidates (KOI-189 and KOI-686) carried out with the SOPHIE spectrograph at the Observatoire de Haute Provence. These data promptly discard these objects as viable planet candidates and show that the transiting objects are in the regime of very low-mass stars, where a strong discrepancy between observations and models persists for the mass and radius parameters. By combining the SOPHIE spectra with the Kepler light curve and photometric measurements found in the literature, we obtain a full characterization of the transiting companions, their orbits, and their host stars. The two companions are in significantly eccentric orbits with relatively long periods (30 days and 52.5 days), which makes them suitable objects for a comparison with theoretical models, since the effects invoked to understand the discrepancy with observations are weaker for these orbital distances. KOI-189 b has a mass M = 0.0745 ± 0.0033 M⊙ and a radius R = 0.1025 ± 0.0024 R⊙. The density of KOI-189 b is significantly lower than expected from theoretical models for a system of its age. We explore possible explanations for this difference. KOI-189 b is the smallest hydrogen-burning star with such a precise determination of its fundamental parameters. KOI-686 b is larger and more massive (M = 0.0915 ± 0.0043 M⊙; R = 0.1201 ± 0.0033 R⊙), and its position in the mass-radius diagram agrees well with theoretical expectations.
Key words: techniques: photometric / techniques: radial velocities / stars: low-mass / stars: fundamental parameters / stars: individual: KIC11391018 / stars: individual: KIC7906882
© ESO, 2014
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