Is multiplicity universal? A study of multiplicity in the young moving groups

Abstract

The young moving groups are collections of nearby (<200 pc), young (5-150 Myr) pre-main sequence stars; these stars offer us one of the best opportunities to characterise stellar multiplicity, sub-stellar phenomena, disc evolution and planet formation.

Here we present results from a series of multiplicity studies aimed at producing comprehensive multiplicity statistics of the young moving groups. The aim was to compare the derived statistics of the young moving groups to other populations in order to investigate whether the abundance and properties of multiple systems are environment independent.

We have combined high-resolution spectroscopy, AO-imaging and direct imaging to identify and characterise multiple systems across a huge range of orbital periods (1- 10e10 day). The observational techniques also allow us to constrain the abundance of multiple systems in these populations by calculating detection limits.

We found many similarities (frequency of spectroscopic binaries; frequency, mass-ratio and physical separation of visual binaries) between the young moving groups and both younger and older regions, for multiple systems with physical separations smaller than 1000 au. We did, however, identify a significant number of new wide (>1000 au) companions. We reconciled the apparent excess of wide binary systems, when compared to the field population, by arguing that the wide systems are weakly bound and most likely decaying. By comparing the multiplicity statistics in one particular moving group we showed that the dynamical evolution of non-hierarchical protostars could lead to the population of wide binaries we can observe today.

Our results indicate that the majority of low-mass stars form in small groups with 3 or 4 components that undergo significant dynamical evolution. The multiplicity properties of the young nearby moving groups are statistically similar to many other populations, supporting the environment-independent formation of multiple systems.