Low-Mass Stellar and Brown Dwarf Binary Systems

Binary systems have long been recognised as providing one of the most powerful methods of probing the physical properties of stars drawn from the full range of the HR diagram. Most notably, the overwhelming majority of measurements of mass, the fundamental stellar parameter, rest on observations of astrometric or spectroscopic binaries. For main sequence stars, those individual measurements can be combined to define a mass-luminosity relation which, applied to a suitably-constructed sample, permits derivation of Ψ(M), the mass function. The latter, in its turn, provides a valuable constraint on star formation theory by defining the end product of an assortment of processes, including collapse, fragmentation, coagulation and accretion, which combine to transform gas into stars. In similar fashion, the frequency of binaries as a function of mass and the distribution of mass ratios and separations offer important clues to the underlying mechanism(s) which conspire reduce individual molecular cloud cores to to gaseous spheres in hydrostatic equilibrium