Friday, January 30, 2015

Forming Brown Dwarf Binaries From Disintegrating Trinary Systems

Brown Dwarf Binaries from Disintegrating Triple Systems


Reipurth et al


We have carried out 200,000 N-body simulations of three identical stellar embryos with masses from a Chabrier IMF and embedded in a molecular core. The bodies are initially non-hierarchical and undergo chaotic motions, while accreting using Bondi-Hoyle accretion. The coupling of dynamics and accretion often leads to one or two dominant bodies controlling the center of the cloud core, while banishing the other(s) to the lower-density outskirts, leading to stunted growth. Eventually each system transforms either to a bound hierarchical configuration or breaks apart into separate single and binary components. The orbital motion is followed for 100 Myr. To illustrate the simulations we introduce the 'triple diagnostic diagram', which plots two dimensionless numbers against each other, representing the binary mass ratio and the mass ratio of the third body to the total system mass. Numerous freefloating BD binaries are formed in these simulations. The separation distribution function is in good correspondence with observations, showing a steep rise at close separations, peaking around 13 AU and then declining more gently. Unresolved BD triple systems may appear as wider BD binaries. Mass ratios are strongly peaked towards unity, as observed, but this is partially due to the initial assumptions. Eccentricities gradually increase towards higher values, due to the lack of viscous interactions in the simulations, which would both shrink the orbits and decrease their eccentricities. The main threat to newly born triple systems is internal instabilities, not external perturbations. Dynamical interactions in newborn triple systems of stellar embryos embedded in and accreting from a cloud core naturally form a population of freefloating BD binaries, and this mechanism may constitute a significant pathway for the formation of BD binaries.

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