Two step ejection of massive stars and the issue of their formation in isolation

TL;DR

This study introduces the two-step-ejection scenario where massive stars are expelled from clusters through binary ejection and supernova acceleration, explaining isolated massive stars and their velocities.

Contribution

It presents a novel two-step-ejection model that accounts for the formation and high velocities of isolated massive stars, aligning with observed fractions.

Findings

Most ejected massive stars cannot be traced back to clusters.

The fraction of isolated O stars from this process matches observations (1-4%).

Ejected stars can attain velocities 1.5-2 times higher than their pre-supernova velocities.

Abstract

In this paper we investigate the combined effect of massive binary ejection from star clusters and a second acceleration of a massive star during a subsequent supernova explosion. We call this the "two-step-ejection" scenario. The main results are: i) Massive field stars produced via the two-step-ejection process can not in the vast majority of cases be traced back to their parent star clusters. These stars can be mistakenly considered as massive stars formed in isolation. ii) The expected O star fraction produced via the two-step-ejection process is of the order of 1-4 per cent, in quantitative agreement with the observed fraction of candidates for isolated-O-star formation. iii) Stars ejected via the two-step-ejection process can get a higher final velocity (up to 1.5-2 times higher) than the pre-supernova velocity of the massive-star binary.