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Published October 1, 2009 | Published
Journal Article Open

Unusually Wide Binaries: Are They Wide or Unusual?


We describe an astrometric and spectroscopic campaign to confirm the youth and association of a complete sample of candidate wide companions in Taurus and Upper Sco. Our survey found 15 new binary systems (three in Taurus and 12 in Upper Sco) with separations of 3"-30" (500-5000 AU) among all of the known members with masses of 2.5-0.012 M_⊙. The total sample of 49 wide systems in these two regions conforms to only some expectations from field multiplicity surveys. Higher mass stars have a higher frequency of wide binary companions, and there is a marked paucity of wide binary systems near the substellar regime. However, the separation distribution appears to be log-flat, rather than declining as in the field, and the mass ratio distribution is more biased toward similar-mass companions than the initial mass function or the field G-dwarf distribution. The maximum separation also shows no evidence of a limit at ≾ 5000 AU until the abrupt cessation of any wide binary formation at system masses of ~0.3 M_⊙. We attribute this result to the post-natal dynamical sculpting that occurs for most field systems; our binary systems will escape to the field intact, but most field stars are formed in denser clusters and undergo significant dynamical evolution. In summary, only wide binary systems with total masses ≾ 0.3 M_⊙ appear to be "unusually wide."

Additional Information

© 2009 The American Astronomical Society. Received 2008 April 13; accepted 2009 August 12; published 2009 September 10. We thank Russel White and Michael Ireland for several helpful discussions regarding wide binary formation, Cathy Slesnick and Greg Herczeg for discussions regarding the interpretation of young star spectra, and Brian Cameron and Stan Metchev for discussions of astrometric precision and accuracy in AO imaging. We also thank the referee for a prompt and insightful report. Finally, we thank the Keck LGSAO team for their efforts in developing and supporting a valuable addition to the observatory. A.L.K. was supported by a NASA Origins grant to L.A.H. This work makes use of data products from 2MASS, which is a joint project of the University of Massachusetts and the IPAC/Caltech, funded by NASA and the NSF. This work also makes use of data products from the DENIS project, which has been partly funded by the SCIENCE and the HCM plans of the European Commission under grants CT920791 and CT940627. Finally, our research has made use of the USNOFS Image and Catalogue Archive operated by the USNO, Flagstaff Station (http://www.nofs.navy.mil/data/fchpix/). Some of the data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership between Caltech, the University of California, and NASA. The observatory was made possible by the generous financial support of the W.M. Keck Foundation. The authors also wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. Facilities: Hale, Keck II, CTIO:2MASS, FLWO:2MASS

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