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Published October 1, 2012 | Published
Journal Article Open

Multiple Star Formation to the Bottom of the Initial Mass Function


The frequency and properties of multiple star systems offer powerful tests of star formation models. Multiplicity surveys over the past decade have shown that binary properties vary strongly with mass, but the functional forms and the interplay between frequency and semimajor axis remain largely unconstrained. We present the results of a large-scale survey of multiplicity at the bottom of the initial mass function in several nearby young associations, encompassing 78 very low mass members observed with Keck laser guide star adaptive optics. Our survey confirms the overall trend observed in the field for lower-mass binary systems to be less frequent and more compact, including a null detection for any substellar binary systems with separations wider than ~7 AU. Combined with a Bayesian re-analysis of existing surveys, our results demonstrate that the binary frequency and binary separations decline smoothly between masses of 0.5 M_☉ and 0.02 M_☉, though we cannot distinguish the functional form of this decline due to a degeneracy between the total binary frequency and the mean binary separation. We also show that the mass ratio distribution becomes progressively more concentrated at q ~ 1 for declining masses, though a small number of systems appear to have unusually wide separations and low-mass ratios for their mass. Finally, we compare our results to synthetic binary populations generated by smoothed particle hydrodynamic simulations, noting the similarities and discussing possible explanations for the differences.

Additional Information

© 2012 American Astronomical Society. Received 2011 September 20; accepted 2012 June 12; published 2012 September 12. We thank Russel White for many insightful discussions of multiple star formation that have helped to shape our conclusions, as well as the anonymous referee for providing a detailed and very helpful critique of this work.We also 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., by a SIM Science Study, and by NASA through Hubble Fellowship grant 51257.01 awarded by STScI, which is operated by AURA, Inc. for NASA, under contract NAS 5-26555. 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. The observations 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.

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