Binary mass ratios: system mass not primary mass

TL;DR

This paper argues that using system mass rather than primary mass provides a more fundamental understanding of binary star properties, revealing a universal flat mass ratio distribution across various stellar types, except for brown dwarfs.

Contribution

It demonstrates that the observed variations in mass ratio distributions are consistent with a universal flat distribution when considering system mass, challenging previous primary-mass-based analyses.

Findings

System masses follow a log-normal distribution.

Mass ratio distributions are consistent with a universal flat distribution for stars.

Brown dwarf mass ratios differ from stellar distributions.

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.