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: 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.
