Planetary and Brown Dwarf Companion Mass Ratio Distribution versus Stellar Mass and Orbital Separation

Understanding the demographic properties of multiple stars as well as planet populations can provide profound constraints on theories of star and planet formation. Surveys for very low mass companions to stars from M to A types as a function of orbital separation naturally detect very low mass brown dwarfs which are a logical extension of binary star formation as well as exoplanets formed through diverse processes in circumstellar disks in Keplerian rotation. We fit the orbital distribution of planetary mass companions around M, FGK, and A stars with a log-normal distribution that peaks between 3-6 AU (Meyer et al. 2018; Meyer et al. submitted). Combining these new results with extrapolations of binary star data, we propose a new parametric model for the companion mass ratio distribution (CMRD) as a function of central host star mass from 0.3-3 MSUN sun that includes very low mass brown dwarf companions formed through “binary-like processes” as well as a “planet-like” mass function. The model predicts binary companions well into the brown dwarf regime as well as exoplanet populations as a function of planet mass and orbital radius. These predictions are found to be consistent with many point estimates of companion fraction as a function of mass ratio and orbital separation ranges for diverse host star samples. We find: a) the brown dwarf “desert” is expected as the natural extension of binary and gas giant planet properties; b) local minima in the CMRD occur between 10-40 Jupiter masses depending on host star mass and orbital separation; and c) in binary companions as well as gas giant planet formation, a key parameter is the ratio of the companion to the host star mass from M to A stellar types.