Do O-stars form in isolation?

TL;DR

This paper investigates the origin of apparently isolated O-stars, showing that their observed frequency can be explained by random sampling from standard initial mass and cluster mass functions, challenging the idea of a strict cluster-mass relationship.

Contribution

It demonstrates that the fraction of isolated O-stars aligns with predictions from random sampling models, questioning the necessity of a fundamental cluster-mass to star-mass relationship.

Findings

The observed isolated O-star fraction matches random sampling predictions.

Isolated O-stars can be explained as low-mass clusters with massive stars.

Challenges the idea of a strict cluster-mass to star-mass relationship.

Abstract

Around 4% of O-stars are observed in apparent isolation, with no associated cluster, and no indication of having been ejected from a nearby cluster. We define an isolated O-star as a star > 17.5 M_\odot in a cluster with total mass <100 M_\odot which contains no other massive (>10 M_\odot) stars. We show that the fraction of apparently isolated O-stars is reproduced when stars are sampled (randomly) from a standard initial mass function and a standard cluster mass function of the form N(M) \propto M^-2. This result is difficult to reconcile with the idea that there is a fundamental relationship between the mass of a cluster and the mass of the most massive star in that cluster. We suggest that such a relationship is a typical result of star formation in clusters, and that `isolated O-stars' are low-mass clusters in which massive stars have been able to form.