The VLT-FLAMES Tarantula Survey IV: Candidates for isolated high-mass
  star formation in 30 Doradus
  Whether massive stars can occasionally form in relative isolation or if they
require a large cluster of lower-mass stars around them is a key test in the
differentiation of star formation theories as well as how the initial mass
function of stars is sampled. Previous attempts to find O-type stars that
formed in isolation were hindered by the possibility that such stars are merely
runaways from clusters, i.e., their current isolation does not reflect their
birth conditions. We introduce a new method to find O-type stars that are not
affected by such a degeneracy. Using the VLT-FLAMES Tarantula Survey and
additional high resolution imaging we have identified stars that satisfy the
following constraints: 1) they are O-type stars that are not detected to be
part of a binary system based on RV time series analysis; 2) they are
designated spectral type O7 or earlier ; 3) their velocities are within 1\sigma
of the mean of OB-type stars in the 30 Doradus region, i.e. they are not
runaways along our line-of-sight; 4) the projected surface density of stars
does not increase within 3 pc towards the O-star (no evidence for clusters); 5)
their sight lines are associated with gaseous and/or dusty filaments in the
ISM, and 6) if a second candidate is found in the direction of the same
filament with which the target is associated, both are required to have similar
velocities. With these criteria, we have identified 15 stars in the 30 Doradus
region, which are strong candidates for being high-mass stars that have formed
in isolation. Additionally, we employed extensive MC stellar cluster
simulations to confirm that our results rule out the presence of clusters
around the candidates. Eleven of these are classified as Vz stars, possibly
associated with the zero-age main sequence. We include a newly discovered W-R
star as a candidate, although it does not meet all of the above criteria.
