Complete ejection of OB stars from very young star clusters and the formation of multiple populations

TL;DR

This study uses advanced N-body simulations to demonstrate that dynamical ejection of OB stars from young star clusters can explain the observed multiple stellar populations in the Orion Nebula Cluster within about 1 million years.

Contribution

It provides the first detailed simulation-based evidence supporting OB star ejection as a mechanism for multiple population formation in young clusters.

Findings

High probability (50-70%) of OB star ejection within 1 Myr

Ejection occurs if OB stars are initially mass-segregated in the cluster center

Supports dynamical ejection as a viable explanation for observed multiple populations

Abstract

Recently, three stellar sequences separated in age by about 1 Myr were discovered in the Orion Nebula Cluster (ONC; Beccari et al. 2017). Kroupa et al. (2018) suggest that such small dense subpopulations eject all their OB stars via the decay of unstable few-body systems such that the gas can recombine and form new stars. This explains the multisequence phenomenon without introducing an extra mechanism into star formation theory. In this work, we apply the recently updated primordial binary distribution model (Belloni et al. 2017; implemented here in a new version of MCLUSTER) and perform a large set of direct N-body simulations to investigate the feasibility of this dynamical scenario. Our results suggest that if 3-4 OB stars in the ONC formed primordially mass-segregated in the cluster centre with a maximum separation of about 0.003 pc, all OB stars have a high chance (50-70 per cent) to escape from the centre and do not come back within 1 Myr and the dynamical ejection scenario is a viable channel to form short-age-interval multipopulation sequences as observed in the ONC. This is also consistent with self-regulated star formation.