The emergence of super-canonical stars in R136-type starburst clusters

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?. A recent study by Banerjee, Kroupa & Oh (hereafter Paper I), 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. The above study consists of realistic N-body computations of fully mass-segregated star clusters mimicking R136 in which all the massive stars are in primordial binaries. In the present work, we study the formation of SC stars in the computed R136 models of Paper I in detail. Taking into consideration that extraneous SC stars form in the computed models of Paper I due to the primordial binaries initial eccentricities, we compute additional models where all primordial binaries are initially circular. We also take into account the evolution of the mass of the SC stars and the resulting lifetime in their SC phase using detailed stellar evolutionary models over the SC mass range that incorporate updated treatments of the stellar winds. In all these computations, we find that SC stars begin to form via dynamical mergers of massive binaries from similar to 1?Myr cluster age. We obtain SC stars with initial masses up to similar to 250?M? from these computations. Multiple SC stars are found to remain bound to the cluster simultaneously within an SC lifetime. However, we also note that SC stars can be formed at runaway velocities which escape the cluster at birth. These properties of the dynamically formed SC stars, as obtained from our computations, are consistent with the observed SC stellar population in R136. In fact, the 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? limit by similar to 3?Myr, the likely age of R136. Thus, according to the new stellar evolutionary models, primordially formed SC stars should not be observable at the present time in R136. This strongly supports the dynamical formation scenario of the observed SC stars in R136.