The emergence of super-canonical stars in R136-type star-burst clusters
  [abridged] Among the most remarkable features of the stellar population of
R136, the central, young, massive star cluster in the 30 Doradus complex of the
Large Magellanic Cloud, are the single stars whose masses substantially exceed
the canonical stellar upper mass limit of 150 M_sun. A recent study by us,
viz., that of Banerjee, Kroupa & Oh (2012; Paper I), which involves realistic
N-body computations of star clusters mimicking R136, indicates that such
"super-canonical" (SC) stars can be formed out of a dense stellar population
with a canonical initial mass function (IMF) through dynamically induced
mergers of the most massive binaries. Here we study the formation of SC stars
in the R136 models of Paper I in detail. To avoid forming extraneous SC stars
from initially highly eccentric primordial binaries as in Paper I, we compute
additional models with only initially circular primordial binaries. We also
take into account the mass-evolution of the SC stars using detailed stellar
evolutionary models that incorporate updated treatments of stellar winds. We
find that SC stars begin to form via dynamical mergers of massive binaries from
approx. 1 Myr cluster age. We obtain SC stars with initial masses up to approx.
250 M_sun from these computations. Multiple SC stars are found to remain bound
to the cluster simultaneously within a SC-lifetime. These properties of the
dynamically formed SC stars are consistent with those observed in R136. In
fact, the stellar evolutionary models of SC stars imply that had they formed
primordially along with the rest of the R136 cluster, i.e., violating the
canonical upper limit, they would have evolved below the canonical 150 M_sun
limit by approx. 3 Myr, the likely age of R136, and would not have been
observable as SC stars at the present time in R136. This strongly supports the
dynamical formation scenario of the observed SC stars in R136.
