Characterization of the gaseous companion κ Andromedae b^⋆

New Keck and LBTI high-contrast observations^⋆⋆

¹ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
e-mail: bonnefoy@mpia-hd.mpg.de
² Department of Astronomy & Astrophysics, University of Toronto. 50 St. George Street, Room 101, Toronto M5S 3H4, Canada
³ HL Dodge Department of Physics & Astronomy, University of Oklahoma, 440 W Brooks St, Norman OK 73019, USA
⁴ Department of Physics & Astronomy, College of Charleston, 58 Coming St., Charleston SC 29424, USA
⁵ Lowell Observatory, 1400 W. Mars Hill Road, Flagstaff AZ 86001, USA
⁶ Steward Observatory, Department of Astronomy, University of Arizona, 933 N. Cherry Ave, Tucson AZ 85721, USA
⁷ Lehigh University, College of Art and Science, Department of Physics, 27 Memorial Dr. W, Bethlehem PA 18015, USA
⁸ Department of Astrophysical Sciences, Princeton University, NJ 08544, USA
⁹ Exoplanets and Stellar Astrophysics Laboratory, Code 667, Goddard Space Flight Center, Greenbelt MD 20771, USA
¹⁰ Astronomical Institute Anton Pannekoek, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
¹¹ Department of Astronomy, University of Virginia, Charlottesville, VA 22904, USA
¹² CRAL, UMR 5574, CNRS, Université de Lyon, École Normale Supérieure de Lyon, 46 allée d’Italie, 69364 Lyon Cedex 07, France
¹³ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, 181-8588 Tokyo, Japan
¹⁴ Department of Earth and Planetary Science, Graduate School of Science, 7-3-1 Hongo, Bunkyo-ku, 113-0033 Tokyo, Japan
¹⁵ Department of Physics, McGill University, 3600 rue University, Montréal, Québec H3A 2T8, Canada
¹⁶ SUPA, School of Physics & Astronomy, University of St. Andrews, St. Andrews KY16 9SS, UK
¹⁷ Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
¹⁸ Institute for Astronomy, University of Hawaii, 640 N. Aohoku Place, Hilo HI 96720, USA
¹⁹ Department of Earth and Space Science, Graduate School of Science, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan
²⁰ Subaru Telescope, 650 North Aohoku Place, Hilo HI 96720, USA
²¹ Laboratoire Lagrange (UMR 7293), Universite de Nice-Sophia Antipolis, CNRS, Observatoire de la Cote d’Azur, 28 avenue Valrose, 06108 Nice Cedex 2, France
²² Department of Astronomy, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan
²³ Hiroshima University, 1-3-2, Kagamiyama, Higashihiroshima, 739-8511 Hiroshima, Japan
²⁴ Departamento de Astrofísica, CAB (INTA-CSIC), Instituto Nacional Tecnica Aeroespacial, Torrejón de Ardoz, 28850 Madrid, Spain
²⁵ Jet Propulsion Laboratory, California Institute of Technology, Pasadena CA 171-113, USA
²⁶ Department of Astronomical Sciences, Graduate University for Advanced Studies (Sokendai), Mitaka, 181-8858 Tokyo, Japan
²⁷ Institute of Astronomy and Astrophysics, Academia Sinica, PO Box 23-141, 10617 Taipei, Taiwan
²⁸ Kavli Institute for Physics and Mathematics of the Universe, The University of Tokyo, 5-1-5, Kashiwanoha, Kashiwa, 277-8568 Chiba, Japan
²⁹ Department of Cosmosciences, Hokkaido University, Kita-ku, Sapporo 060-0810, Japan
³⁰ Astronomical Institute, Tohoku University, Aoba-ku, Sendai 980-8578, Japan

Received: 21 June 2013
Accepted: 5 August 2013

Abstract

Context. We previously reported the direct detection of a low-mass companion at a projected separation of 55 ± 2 AU around the B9-type star κ Andromedae. The properties of the system (mass ratio, separation) make it a benchmark for understanding the formation and evolution of gas giant planets and brown dwarfs on wide orbits.

Aims. We present new angular differential imaging (ADI) images of the system at 2.146 (K_s), 3.776 (L′), 4.052 (NB_4.05), and 4.78 μm (M′) obtained with Keck/NIRC2 and LBTI/LMIRCam, as well as more accurate near-infrared photometry of the star with the MIMIR instrument. We aim to determine the near-infrared spectral energy distribution of the companion and use it to characterize the object.

Methods. We used analysis methods adapted to ADI to extract the companion flux. We compared the photometry of the object to reference young, and old objects and to a set of seven PHOENIX-based atmospheric models of cool objects accounting for the formation of dust. We used evolutionary models to derive mass estimates considering a wide range of plausible initial conditions. Finally, we used dedicated formation models to discuss the possible origin of the companion.

Results. We derive a more accurate J = 15.86 ± 0.21, H = 14.95 ± 0.13, K_s = 14.32 ± 0.09 mag for κ And b. We detect the companion in all our high-contrast observations. We confirm previous contrasts obtained at K_s and L′ band. We derive NB_4.05 = 13.0 ± 0.2, and M′ = 13.3 ± 0.3 mag and estimate log ₁₀(L/L_⊙) = −3.76 ± 0.06. Atmospheric models yield T_eff = 1900⁺¹⁰⁰_-200 K. They do not set any constraint on the surface gravity. “Hot-start” evolutionary models predict masses of 14⁺²⁵_-2 M_Jup based on the luminosity and temperature estimates, and when considering a conservative age range for the system (30⁺¹²⁰_-10 Myr), “warm-start” evolutionary tracks constrain the mass to M ≥ 10M_Jup.

Conclusions. The mass of κ Andromedae b mostly falls in the brown-dwarf regime, owing to remaining uncertainties in age and in mass-luminosity models. According to the formation models, disk instability in a primordial disk may account for the position and a wide range of plausible masses of κ And b.