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

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

Jun 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003iaus..211..249c&link_type=abstract

Brown Dwarfs, Proceedings of IAU Symposium #211, held 20-24 May 2002 at University of Hawaii, Honolulu, Hawaii. Edited by Eduard

Physics

Optics

1

Scientific paper

Use of the highly sensitive Hokupa`a/Gemini curvature wavefront 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 (Δ Ks<0.8 mag; 0.852.38 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<0.18 Msun) 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.

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