The True Ultracool Binary Fraction of M7-L5 Dwarfs

Multiplicity is a key statistic for understanding star and brown dwarf formation. Here, we present a novel calculation of the true binary fraction of M7−L5 dwarfs, which straddle the hydrogen-burning mass limit. We have compiled the most complete near-infrared spectroscopic sample of M7-L5 ultracool dwarfs, volume-limited to 25pc, and used the method of Bardalez Gagliuffi et al. (2014) to identify all spectral binaries within the local volume. To infer the true binary fraction, we simulated 72 distinct populations of binary systems taking into consideration different assumptions of the mass, age, and mass ratio distributions, and evolutionary models. We have generated synthetic spectral-blend binary systems from these populations and applied our spectral binary identification technique to calculate the fraction of binaries that are spectral binaries. Combining these results with the observed spectral binary fraction, we recover the underlying ultracool binary fraction of M7-L5 dwarfs, as well as the most likely combination of population parameters that formed these objects in the solar neighborhood.