Brown Dwarfs and Planets Around Solar-Type Stars: Searches by Precise Velocities

Abstract

We review eight major searches for both brown dwarfs and extra-solar planets which use precise velocity measurements to detect the reflex motion of the host primary star. Two searches have reported 11 brown dwarf candidates in total, while the searches having highest velocity precision (~15 ras-1) have revealed none. We examine this apparent conflict by modelling the occurrence and detectability of brown-dwarf companions, including their distribution of mass, orbital period and inclination. The velocities constrain the frequency of brown dwarf companions to be no more than 1% within 5 AU of G, K stars. However, the observed distribution of M2 sini is consistent with two forms of the true companion mass distribution: 1) It is approximately constant at all masses, implying that 1. 0% of solar-type stars have a brown dwarf companion within 5 AU, or 2) It rises gently toward low masses, i. e., dN/dM ∝ M -0.8, but drops for masses below 0.1 M⊙, requiring few or no brown dwarfs. A significant fraction of the current brown dwarf candidates are actually H-burning stars that have somewhat face-on orbital planes. Extra-solar “planets” having masses of ≳3 MJUP are detectable by four different velocity surveys. No such planets have been reported, from 0 to 5 AU, placing the occurrence of such super-Jupiters at less than 5%.