The Decay of Accreting Triple Systems as Brown Dwarf Formation Scenario

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 analyze the end states and decay times of these systems and determine the fraction of Brown Dwarfs formed, their escape speeds as well as the semi-major axis distribution of the formed Brown Dwarf binaries. 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. We conclude that accretion and momentum transfer of accreted gas during the dynamical decay of triple systems is able to produce the observed distribution of close binary Brown Dwarfs, making the ejection model a viable option as Brown Dwarf formation scenario.Comment: 31 pages, 8 figures, accepted for publication in Ap

Close binary systems among very low‐mass stars and brown dwarfs

“…These distributions were truncated at a = 10 au in order to better match what is known about wider binary systems. We also trialled the prediction from a triple-body decay calculation for BD binaries from Umbreit et al (2005) (see Fig. 1).…”

“…Also shown are the various model log a distributions used to simulate the VLM/BD binary population (see text). They are from Umbreit et al (2005) or are Gaussian distributions with peaks at 4 au and with various indicated widths (truncated for a > 10 au). The model distributions are normalised such that the binary frequency is 15 per cent for 2.6 < a < 10 au.…”

Binarity at the L/T brown dwarf transition

“…Finally, the binary fraction is an important prediction of brown dwarf formation scenarios, and, with other parameters, will eventually allow to discriminate between the various scenarios (Billères et al 2005;Umbreit et al 2005;Caballero 2007). In particular, Allen (2007) has analysed several high-resoution imaging surveys using a Bayesian algorithm.…”

First Millimeter Detection of the Disk around a Young, Isolated, Planetary-mass Object

“…Several scenarios have been proposed to prevent a substellar core in a dense environment from accreting to stellar mass (e.g., dynamical interactions, Reipurth & Clarke 2001;Umbreit et al 2005; disk fragmentation, Goodwin & Whitworth 2007;Stamatellos et al 2007; or photoevaporation, Whitworth & Zinnecker 2004). Alternatively, brown dwarfs and free-floating planets (with masses below the deuterium-burning mass limit, 13M Jup ) could form in an isolated mode by direct collapse.…”