The formation of multiples in small-$N$ subclusters

TL;DR

This study models small stellar subclusters to determine how their initial conditions influence the formation of binary and multiple star systems, finding that typical core sizes of around 4-5 stars best match observations.

Contribution

It introduces a dynamical model of small stellar subclusters with specific initial conditions that accurately reproduces observed multiplicity statistics, suggesting larger initial core sizes than previously assumed.

Findings

Optimal core size for matching observed multiplicity is around 4.3 to 5.2 stars.

Approximately 50% of the internal kinetic energy is in ordered rotation.

Results are robust against variations in mass segregation and rotation law.

Abstract

We explore the relative percentages of binary systems and higher-order multiples that are formed by pure stellar dynamics, within a small subcluster of $N$ stars. The subcluster is intended to represent the fragmentation products of a single isolated core, after most of the residual gas of the natal core has dispersed. Initially the stars have random positions, and masses drawn from a log-normal distribution. For low-mass cores spawning multiple systems with Sun-like primaries, the best fit to the observed percentages of singles, binaries, triples and higher-order systems is obtained if a typical core spawns on average between $N=$ 4.3 and 5.2 stars, specifically a distribution of $N$ with mean $\mu_{_{N}}\sim4.8$ and standard deviation $\sigma_{_N}\sim2.4$. This fit is obtained when $\sim 50\%$ of the subcluster's internal kinetic energy is invested in ordered rotation and $\sim 50\%$ in isotropic Maxwellian velocities. There is little dependence on other factors, for example mass segregation or the rotation law. Whilst such high values of $N$ are at variance with the lower values often quoted (i.e. $N=$ 1 or 2), very similar values ($N=4.3\pm0.4$ and $N=4.5\pm1.9$) have been derived previously by completely independent routes, and seem inescapable when the observed distribution of multiplicities is taken into account.