Frequency Of Brown Dwarf Companions Research Articles

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.

Brown Dwarf Companions to Solar-Type Stars and other Brown Dwarfs

We report on our ongoing imaging efforts to detect brown dwarfs orbiting solar-type stars and other brown dwarfs. We study the properties of brown dwarf companions as a function of primary mass. Our results indicate that the frequency of brown dwarf companions around solar-type stars for separations larger than about 40 AU is non-negligible. The frequency of brown dwarf binaries is about 20%. There is a dearth of brown dwarf binaries with separations larger than ∼20 AU. We propose that brown dwarf systems are a scaled down version of stellar systems, which probably form via triggered collapse of small molecular cores.

There is a Brown Dwarf Desert of Companions Orbiting Stars between 75 and 1000 AU

We present results of the first large (> 100 stars) infrared coronographic search for substellar companions to nearby stars. The search consisted of two surveys of stars chosen for their youth and proximity to Earth: 1.) a 178 star infrared survey at Steward and Lick Observatories, with optical followup from Keck Observatory, capable of detecting companions with masses greater than 30MJ, orbiting between about 75 and 300 AU, 2.) a 102 star survey using the Keck telescope, capable of detecting extrasolar brown dwarfs and planets typically more massive than 10 MJ, orbiting between about 75 and 300 AU.This research resulted in the discovery of one brown dwarf companion, zero planets and 23 double stars. The frequency of brown dwarf companions (of any mass) to G, K and M stars orbiting between 75 and 300 AU is measured to be 1 ± 1 %. The frequency of massive (> 30 MJ) brown dwarf companions is found to be 0.6 ± 0.6 %. The frequency of giant planets with masses larger than 10 MJ, between 75 and 300 AU, is measured here for the first time to be no more than about 3 %.

Predicting the properties of binary stellar systems: the evolution of accreting protobinary systems

We investigate the formation of binary stellar systems. We consider a model where a ‘seed’ protobinary system forms, via fragmentation, within a collapsing molecular cloud core and evolves to its final mass by accreting material from an infalling gaseous envelope. This accretion alters the mass ratio and orbit of the binary, and is largely responsible for forming the circumstellar and/or circumbinary discs. Given this model for binary formation, we predict the properties of binary systems and how they depend on the initial conditions within the molecular cloud core. We predict that there should be a continuous trend such that closer binaries are more likely to have equal-mass components and are more likely to have circumbinary discs than wider systems. Comparing our results with observations, we find that the observed mass-ratio distributions of binaries and the frequency of circumbinary discs as a function of separation are most easily reproduced if the progenitor molecular cloud cores have radial density profiles between uniform and 1/r (e.g., Gaussian) with near-uniform rotation. This is in good agreement with the observed properties of pre-stellar cores. Conversely, we find that the observed properties of binaries cannot be reproduced if the cloud cores are in solid-body rotation and have initial density profiles which are strongly centrally condensed. Finally, in agreement with the radial-velocity searches for extrasolar planets, we find that it is very difficult to form a brown dwarf companion to a solar-type star with a separation ≲10 au, but that the frequency of brown dwarf companions should increase with larger separations or lower mass primaries.