Measuring the Substellar IMF using a 25 pc Volume-Limited Sample of L0–T8 Brown Dwarfs

Brown dwarfs cool through their lifetimes and progress through spectral types M, L, T, and Y, implying that a younger, less massive brown dwarf can have the same spectral type and luminosity as an older, more massive brown dwarf. Thus, it becomes exceedingly complex to utilize photometry and spectroscopy to determine physical properties such as mass, age, metallicity etc., hindering our understanding of a brown dwarf’s evolutionary path. A statistical approach that can overcome this challenge is to model populations of brown dwarfs characterized by an initial mass function (IMF), a birth history, and an evolutionary model. We report our efforts to constrain the substellar IMF using a 25 pc volume-limited sample of 369 L0-T8 brown dwarfs defined entirely by parallaxes. We constrain the index alpha of the IMF (dN/dM vs M) using the exponential age distribution, single-age distributions, and the Besançon Galaxy Model (BGM) of the solar neighborhood through a chi-square goodness-of-fit test utilizing the ‘hybrid’ evolutionary models of Saumon & Marley (SM08) and the BT-Settl evolutionary models. We present the strongest substellar IMF constraints to date, finding a strong preference for a negative value of alpha, i.e. an average higher mass (>40 MJup) population of brown dwarfs in the solar neighborhood. We obtain consistent values of alpha for SM08 and BT-Settl, indicating that the uncertainties on our constraints have not been underestimated. Our sample also agrees with previous results on a near uniform brown dwarf formation rate in the solar neighborhood but stands in contrast to the empirical Dupuy & Liu (2017) age distribution and the age distribution of the solar neighborhood presented by BGM.