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C.H. Lineweaver (Planetary Sciences Institute, Research School of Astronomy and Astrophysics &), D. Grether (University of New South Wales)
Sun-like stars have stellar, brown dwarf and planetary companions. To help constrain their formation and migration scenarios, we analyse the close companions (orbital period < 5 years) 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 two orders of magnitude from 1 MSun 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 +25-18 MJup. Linear fits to the companion mass function marginally suggest that the driest part of the desert scales with host mass and we predict a Jupiter-mass desert and a stellar companion desert for hosts of mass < 0.5 MSun and >2 MSun respectively. However, we find no evidence that companion mass scales with host mass in general. Approximately 16% of Sun-like stars have close (P < 5 years) 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. We acknowledge funding from ANU's Planetary Science Institute, the Australian Research Council and UNSW.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.