The two-step ejection of massive stars and the issue of their formation in isolation

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 as follows. (i) Massive field stars produced via the two-step-ejection process cannot 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.