This dissertation presents a comprehensive infrared coronagraphic search for substellar companions to stars. The research consists of: (1) a 102 star infrared survey using the Keck telescope, capable of detecting extrasolar brown dwarfs and planets typically more massive than 10 MJupiter , orbiting between about 75 and 300 AU, (2) a 178 star infrared survey at Steward and Lick Observatories, with optical followup from Keck Observatory, capable of detecting companions with masses greater than 30 MJupiter, orbiting between about 120 to 1200 AU and (3) a 24 star survey of two very young stellar associations TW Hydrae and MBM 12, capable of detecting companions down to 2 MJupiter, between about 25 and 1000 AU. These searches are the first to place limits on the frequency of massive planets orbiting beyond 75 AU, and the most sensitive to encompass such a large number of stars.; This research resulted in the discovery of one brown dwarf companion, zero planets and 23 double stars. The frequency of brown dwarf companions to G, K & 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 (>30MJupiter) brown dwarf companions is found to be 0.6% +/- 0.6%. The frequency of giant planet companions with masses between 5 and 10 MJupiter orbiting between 75 and 300 AU is measured here for the first time to be no more than 3%.; Taken together with results of other searches encompassing a wide range of orbital separations, this research implies that objects with masses between 12 and 75 MJupiter form very rarely as companions to stars. Theories of star formation which could explain these data are only now beginning to emerge.
展开▼
