How Dry is the Brown Dwarf Desert? Quantifying the Relative Number of Planets, Brown Dwarfs, and Stellar Companions around Nearby Sun‐like Stars

Abstract

Sun-like stars have stellar, brown dwarf, and planetary companions. To help constrain their formation and migration scenarios, we analyze the close companions (orbital period <5 yr) of nearby Sun-like stars. By using the same sample to extract the relative numbers of stellar, brown dwarf, and planetary companions, we verify the existence of a very dry brown dwarf desert and describe it quantitatively. With decreasing mass, the companion mass function drops by almost 2 orders of magnitude from 1 M☉ stellar companions to the brown dwarf desert and then rises by more than an order of magnitude from brown dwarfs to Jupiter-mass planets. The slopes of the planetary and stellar companion mass functions are of opposite sign and are incompatible at the 3 σ level, thus yielding a brown dwarf desert. The minimum number of companions per unit interval in log mass (the driest part of the desert) is at M = 31MJ. Approximately 16% of Sun-like stars have close (P < 5 yr) companions more massive than Jupiter: 11% ± 3% are stellar, <1% are brown dwarf, and 5% ± 2% are giant planets. The steep decline in the number of companions in the brown dwarf regime, compared to the initial mass function of individual stars and free-floating brown dwarfs, suggests either a different spectrum of gravitational fragmentation in the formation environment or post-formation migratory processes disinclined to leave brown dwarfs in close orbits.