Binary mass ratios: system mass not primary mass

Abstract Binary properties are usually expressed (for good observational reasons) as a function of primary mass. It has been found that the distribution of companion masses – the mass ratio distribution – is different for different primary masses. We argue that system mass is the more fundamental physical parameter to use. We show that if system masses are drawn from a log-normal mass function, then the different observed mass ratio distributions as a function of primary mass, from M-dwarfs to A-stars, are all consistent with a universal, flat, system mass ratio distribution. We also show that the brown dwarf mass ratio distribution is not drawn from the same flat distribution, suggesting that the process which decides upon mass ratios is very different in brown dwarfs and stars.