Abstract

We present optical and near-infrared adaptive optics (AO) imaging and spectroscopy of 13 ultracool (>M6) companions to late-type stars (K7–M4.5), most of which have recently been identified as candidate members of nearby young moving groups (YMGs; 8–120Myr) in the literature. Three of these are new companions identified in our AO imaging survey, and two others are confirmed to be comoving with their host stars for the first time. The inferred masses of the companions (∼10–100MJup) are highly sensitive to the ages of the primary stars; therefore we critically examine the kinematic and spectroscopic properties of each system to distinguish bona fide YMG members from old field interlopers. The new M7 substellar companion 2MASS J02155892–0929121 C (40–60MJup) showsclear spectroscopic signs of low gravity and, hence, youth. The primary, possibly a member of the ∼40Myr Tuc-Hor moving group, is visually resolved into three components, making it a young low-mass quadruple system in a compact (100 AU) configuration. In addition, Li I λ6708 absorption in the intermediate-gravity M7.5 companion 2MASS J15594729+4403595 B provides unambiguous evidence that it is young (200Myr) and resides below the hydrogen-burning limit. Three new close-separation (<1′′) companions (2MASS J06475229–2523304 B, PYC J11519+0731 B, and GJ 4378 Ab) orbit stars previously reported as candidate YMG members, but instead are likely old (1 Gyr) tidally locked spectroscopic binaries without convincing kinematic associations with any known moving group. The high rate of false positives in the form of old active stars with YMG-like kinematics underscores the importance of radial velocity and parallax measurements to validate candidate young stars identified via proper motion and activity selection alone. Finally, we spectroscopically confirm the cool temperature and substellar nature of HD 23514 B, a recently discovered M8 benchmark brown dwarf orbiting the dustiest-known member of the Pleiades. The Astrophysical Journal, 806:62 (36pp), 2015 June 10 doi:10.1088/0004-637X/806/1/62 © 2015. The American Astronomical Society. All rights reserved. * Some of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. † Based on observations obtained at the Southern Astrophysical Research (SOAR) telescope, which is a joint project of the Ministério da Ciência, Tecnologia, e Inovação (MCTI) da República Federativa do Brasil, the U.S. National Optical Astronomy Observatory (NOAO), the University of North Carolina at Chapel Hill (UNC), and Michigan State University (MSU). ‡ Based in part on data collected at Subaru Telescope, which is operated by the National Astronomical Observatory of Japan. § Based on observations collected at the European Organization for Astronomical Research in the Southern Hemisphere, Chile (ESO Program 090.A-9010(A)). 19 Caltech Joint Center for Planetary Astronomy Fellow. 20 NSF Graduate Research Fellow. 21 Visiting Astronomer, Cerro Tololo Inter-American Observatory,National Optical Astronomy Observatory, which isoperated by the Association of Universities for Research in Astronomyunder contract with the National Science Foundation. 22 Visiting Astronomer, Kitt Peak National Observatory, National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA) under cooperative agreement with the National Science Foundation. 23 Visiting Astronomer at the Infrared Telescope Facility, which is operated by the University of Hawaii under Cooperative AgreementNNX-08AE38A with the National Aeronautics and Space Administration, Science Mission Directorate, Planetary Astronomy Program. 24 Based on observations obtained at the Canada–France–Hawaii Telescope (CFHT),which is operated by the National Research Council of Canada, the Institut National des Sciences de l’Univers of the Centre National de la Recherche Scientifique of France, and the University of Hawaii.

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