The Origin of Massive O-type Field Stars. Part II: Field O stars as runaways

TL;DR

This study investigates whether Galactic field O-type stars are ejected from clusters by analyzing their velocities, positions, and surroundings, finding that about 4% are likely runaways, supporting the idea most form in clusters.

Contribution

The paper provides statistical evidence that a small percentage of field O stars are runaways, reinforcing the cluster origin hypothesis for massive stars.

Findings

Approximately 4% of O-type stars are runaways.

Most field O stars are single or in binaries, not in clusters.

The observed runaway fraction aligns with models assuming a universal cluster richness distribution.

Abstract

In two papers we try to confirm that all Galactic high-mass stars are formed in a cluster environment, by excluding that O-type stars found in the Galactic field actually formed there. In de Wit et al. (2004) we presented deep K-band imaging of 5 arcmin fields centred on 43 massive O-type field stars that revealed that the large majority of these objects are single objects. In this contribution we explore the possibility that the field O stars are dynamically ejected from young clusters, by investigating their peculiar space velocity distribution, their distance from the Galactic plane, and their spatial vicinity to known young stellar clusters. We (re-)identify 22 field O-type stars as candidate runaway OB-stars. The statistics show that ~4% of all O-type stars with V < 8 can be considered as formed outside a cluster environment. Most are spectroscopically single objects, some are visual binaries. The derived percentage for O-type stars that form isolated in the field based on our statistical analyses is in agreement with what is expected from calculations adopting a universal cluster richness distribution with power index of beta = 1.7, assuming that the cluster richness distribution is continuous down to the smallest clusters containing one single star.