THE BROWN DWARF DESERT AT 75–1200 AU

We present results of a comprehensive infrared coronagraphic search for substellar companions to nearby stars. The research consisted of (1) a 178-star survey at Steward and Lick observatories, with optical follow-up from Keck Observatory, capable of detecting companions with masses greater than 30 MJ, and semimajor axes between about 140 to 1200 AU; (2) a 102-star survey using the Keck Telescope, capable of detecting extrasolar brown dwarfs and planets typically more massive than 10 MJ, with semimajor axes between about 75 and 300 AU. Only one brown dwarf companion was detected, and no planets. The frequency of brown dwarf companions to G, K, and M stars orbiting between 75 and 300 AU is measured to be 1 % 1%, the most precise measurement of this quantity to date. The frequency of massive (greater than 30 MJ) brown dwarf companions at 120–1200 AU is found to be f 0:7 % 0:7%. The frequency of giant planet companions with masses between 5 and 10 MJ orbiting between 75 and 300 AU is measured here for the first time to be no more than 3%. Together with other surveys that encompass a wide range of orbital separations, these results imply that substellar objects with masses between 12 and 75 MJ form only rarely as companions to stars. Theories of star formation that could explain these data are only now beginning to emerge