The crucial role of higher order multiplicity in wide binary formation: a case study using the β-Pictoris moving group

Abstract

The "in-situ" formation of very wide binaries is hard to explain as their physical separations are beyond the typical size of a collapsing cloud core ($\approx$5000-10,000 au). Here we investigate the formation process of such systems. We compute population statistics such as the multiplicity fraction (MF), companion-star fraction (CSF) and physical separation distribution of companions in the $\beta$-Pictoris moving group (BPMG). We compare previous multiplicity studies in younger and older regions and show that the dynamic evolution of a young population with a high degree of primordial multiplicity can lead to a processed separation distribution, similar to the field population. The evolution of outer components is attributed to the dynamical unfolding of higher-order (triple) systems; a natural consequence of which is the formation of wide binaries. We find a strong preference for wide systems to contain three or more components ($>$1000 au: 11 / 14, 10,000 au: 6 / 7). We argue that the majority of wide binaries identified in young moving groups are primordial. Under the assumption that stellar populations, within our galaxy, have statistically similar primordial multiplicity, we can infer that the paucity of wide binaries in the field is the result of dynamical evolution.