The Theory of the Formation of Brown Dwarfs

Abstract

We investigate the dynamical decay of non-hierarchical accreting triple systems and its implications on the ejection model as Brown Dwarf formation scenario. A modified chain-regularization scheme is used to integrate the equations of motion, that also allows for mass changes over time as well as for momentum transfer from the accreted gas mass onto the bodies. We integrate an ensemble of triple systems within a certain volume with different accretion rates, assuming several prescriptions of how momentum is transferred onto the bodies. We follow their evolution until the systems have decayed. We find that the formation probability of Brown Dwarfs depends strongly on the assumed momentum transfer which is related to the motion of the gas. Due to ongoing accretion and consequent shrinkage of the systems, the median escape velocity is increased by a factor of 2 and the binary separations are decreased by a factor of 5 compared with non-accreting systems. Furthermore, the obtained semi-major axis distribution drops off sharply to either side of the median, which is also supported by observations. However, the disks around the ejected Brown Dwarfs seem to have too low accretion rates and masses to account for many of the observed disks in typical low-mass star-forming regions. We conclude that accretion of gas and the kinematic properties of the accreted gas during dynamical interactions strongly influence the abundance as well as the dynamical properties of Brown Dwarfs and, that the ejection scenario seems to be a promising scenario to produce both, very close Brown Dwarf binaries as well as single Brown Dwarfs, whereas it seems less likely to find very long-lived accretion disks around them.