Planets Around Low-mass Stars (PALMS). V. Age-dating Low-mass Companions to Members and Interlopers of Young Moving Groups
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; 8–120 Myr) 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 (~10–100 MJup) 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 (40–60 M_Jup) shows clear spectroscopic signs of low gravity and, hence, youth. The primary, possibly a member of the ~40 Myr 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 (≲200 Myr) 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.
Additional Information
© 2015 The American Astronomical Society. Received 2015 February 24; accepted 2015 April 4; published 2015 June 9. We thank the referee for their helpful suggestions and Niall Deacon for obtaining some of the IRTF observations presented here. M.C.L. has been supported by NASA Grant NNX11AC31G and NSF Grant AST09-09222. This paper is based on observations at Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatory (NOAO Prop. ID: 2014B-0083; PI: Bowler), which is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. This research is also based in part on observations obtained at the Gemini Observatory, which is operated by AURA under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the Science and Technology Facilities Council (United Kingdom), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), CNPq (Brazil) and CONICET (Argentina). The Robo-AO system was developed by collaborating partner institutions, the California Institute of Technology and the Inter-University Centre for Astronomy and Astrophysics, and with the support of the National Science Foundation under Grants AST-0906060, AST-0960343, and AST-1207891, the Mt. Cuba Astronomical Foundation, and by a gift from Samuel Oschin. Ongoing science operation support of Robo-AO is provided by the California Institute of Technology and the University of Hawai'i. C.B. acknowledges support from the Alfred P. Sloan Foundation. B.T.M. is supported by a National Science Foundation Graduate Research Fellowship under Grant DGE1144469. We utilized data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. NASA's Astrophysics Data System Bibliographic Services together with the VizieR catalog access tool and SIMBAD database operated at CDS, Strasbourg, France, were invaluable resources for this work. This research has made use of the Washington Double Star Catalog maintained at the U.S. Naval Observatory. Finally, mahalo nui loa to the kama'āina of Hawai'i for their support of Keck and the Maunakea observatories. We are grateful to conduct observations from this mountain. Facilities: Keck:II (NIRC2, ESI), Subaru (IRCS), Gemini: South (NICI), PO:1.5 m (Robo-AO), Keck:I (OSIRIS, HIRES), IRTF (SpeX), Mayall (RC-Spec), SOAR (Goodman), UH:2.2 m (SNIFS), CFHT (ESPaDOnS), Du Pont (Echelle), Max Planck:2.2 m (FEROS), CTIO:1.5 m (CHIRON). Note Added in Proof. Recently we were notified that the All Sky Automated Survey light curve of 2MASS J06475229- 2523304 shows particularly large amplitude variations of 0.33 mag and has structure resembling an eclipsing binary with a period of 7.561 days (or alias thereof). The K7 spectral type we found is cooler than the standard subgiant branch, so this may be an example of a poorly understood but relatively nearby "sub-subgiant" or "red straggler" system, a class of objects that occupy an unusual position in the color-magnitude diagram and have primarily been found in open and globular clusters. We thank E. Mamajek and A. Kraus for bringing this to our attention. 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)).Attached Files
Published - 0004-637X_806_1_62.pdf
Submitted - 1505.01494v1.pdf
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Additional details
- Eprint ID
- 58940
- Resolver ID
- CaltechAUTHORS:20150720-100844839
- NNX11AC31G
- NASA
- AST09-09222
- NSF
- AST-0906060
- NSF
- AST-0960343
- NSF
- AST-1207891
- NSF
- Alfred P. Sloan Foundation
- DGE-1144469
- NSF Graduate Research Fellowship
- Mt. Cuba Astronomical Foundation
- Created
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2015-07-21Created from EPrint's datestamp field
- Updated
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2021-11-10Created from EPrint's last_modified field