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Detection of Nine M8.0–L0.5 Binaries: The Very Low Mass Binary Population and Its Implications for Brown Dwarf Formation Theories

Published online by Cambridge University Press:  26 May 2016

Laird M. Close ,
Nick Siegler  and
Melanie Freed
Laird M. Close
Affiliation:
Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson AZ, USA 85721–0065; [email protected]
Nick Siegler
Affiliation:
Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson AZ, USA 85721–0065; [email protected]
Melanie Freed
Affiliation:
Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson AZ, USA 85721–0065; [email protected]

Abstract

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Use of the highly sensitive Hōkūpa'a/Gemini curvature wave-front sensor has allowed for the first time direct adaptive optics (AO) guiding on very low mass (VLM) stars with SpT=M8.0–L0.5. A survey of 39 such objects detected 9 VLM binaries (7 of which were discovered for the first time to be binaries). Most of these systems (55%) are tight (separation < 5 AU) and have similar masses (ΔKs < 0.8 mag; 0.85 < q < 1.0). However, 2 systems (LHS 2397a, and 2M2331016-040618) have large ΔKs > 2.38 mag and consist of a VLM star orbited by a much cooler L6.5–L8.5 brown dwarf companion. Based on our initial flux limited (Ks < 12 mag) survey of 39 M8.0–L0.5 stars (mainly from the sample of Gizis et al. 2000) we find a binary fraction in the range 19±7% for M8.0–L0.5 binaries with separations > 2.6 AU. This is slightly less than the 32 ± 9% measured for more massive (M0–M4) stars over the same separation range (Fischer & Marcy 1992). It appears M8.0–L0.5 binaries (as well as L and T dwarf binaries) have a much smaller semi-major axis distribution peak (~ 4 AU) compared to more massive M and G stars which have a broad peak at larger ~ 30 AU separations. We also find no VLM binaries (Mtot < 0.18M⊙) with separations > 20 AU. We find that a velocity “kick” of ~ 3 km/s can reproduce the observed cut-off in the semi-major axis distribution at ˜ 20 AU. This kick may have been from the VLM system being ejected from its formation mini-cluster.

Type
Part 6. Searches for Substellar Companions
Information
Symposium - International Astronomical Union , Volume 211: Brown Dwarfs , 2003 , pp. 249 - 256
Copyright
Copyright © Astronomical Society of the Pacific 2001 

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