Binaries and the L Dwarf/T Dwarf Transition

High-resolution imaging has revealed an unusually high binary fractionamongst objects spanning the transition between the L dwarf and T dwarfspectral classes. In an attempt to reproduce and unravel the origins of thisapparent binary excess, I present a series of Monte Carlo mass function andmultiplicity simulations of field brown dwarfs in the vicinity of the Sun.These simulations are based on the solar metallicity brown dwarf evolutionarymodels and incorporate empirical luminosity and absolute magnitude scales,measured multiplicity statistics and observed spectral templates in theconstruction and classification of composite binary spectra. In addition toproviding predictions on the number and surface density distributions of L andT dwarfs for volume-limited and magnitude-limited samples, these simulationssuccessfully reproduce the observed binary fraction distribution assuming anintrinsic (resolved) binary fraction of 11(+6)(-3)% (95% confidence interval),consistent with prior determinations. However, the true binary fraction may beas high as 40% if, as suggested by Liu et al., a significant fraction of L/Ttransition objects (~66%) are tightly-bound, unresolved multiples. Thesimulations presented here demonstrate that the binary excess amongst L/Ttransition objects arises primarily from the flattening of the luminosity scaleover these spectral types and is not inherently the result of selection effectsincurred in current magnitude-limited imaging samples. Indeed, the existence ofa binary excess can be seen as further evidence that brown dwarfs traverse theL/T transition rapidly, possibly driven by a nonequilibrium submergence ofphotospheric condensates.Comment: 51 pages, 14 figures; accepted for publication in Ap