How dry is the brown dwarf desert? Quantifying the relative number of planets, brown dwarfs, and stellar companions around nearby Sun-like stars

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-circle dot 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 sigma 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 = 31(-18)(+25) M-J. 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.