On the true shape of the upper end of the stellar initial mass function

The case of R136

Argelander-Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany
e-mail: sambaran@astro.uni-bonn.de; pavel@astro.uni-bonn.de

Received: 6 February 2012
Accepted: 30 August 2012

Abstract

Context. The shape of the stellar initial mass function (IMF) of a star cluster near its upper mass limit is a focal topic of investigation as it determines the high mass stellar content and hence the dynamics of the cluster at its embedded phase as well as during its young gas-free phase. The massive stellar content of a young cluster, however, can be substantially modified due to the dynamical ejections of the massive stars so that the present-day high-mass stellar mass function (hereafter MF) can be different than that with which the cluster is born.

Aims. In the present study, we provide a preliminary estimate of this evolution of the high-mass IMF of a young cluster due to early ejections of massive stars, using the Large Magellanic Cloud massive, young cluster R136 as an example.

Methods. To that end, we utilize the results of the state-of-the-art calculations by Banerjee et al. (2012, ApJ, 746, 15) comprising direct N-body computations of realistic, binary-rich, mass-segregated models of R136. In particular, these calculations provide the ejection fraction of stars as a function of stellar mass.

Results. We find that if the measured IMF of R136 is granted to be canonical, as observations indicate, then the “true” high-mass IMF of R136 at its birth must be at least moderately top-heavy when corrected for the dynamical escape of massive stars.

Conclusions. The top-heaviness of the true high-mass IMF over the observationally determined one is a general feature of massive, young clusters where the dynamical ejection of massive stars is efficient. We discuss its implications and possible improvements over our current estimate.