On the true shape of the upper end of the stellar initial mass function: The case of R136

TL;DR

This study estimates the initial high-mass stellar mass function of the young cluster R136, revealing it was likely more top-heavy at birth than current observations suggest due to dynamical ejections of massive stars.

Contribution

It provides a preliminary correction for the high-mass IMF of R136 accounting for dynamical ejections, highlighting the potential top-heaviness of the initial IMF.

Findings

The true high-mass IMF of R136 was likely more top-heavy at birth.

Dynamical ejections significantly alter the observed high-mass stellar content.

Massive, young clusters tend to have a top-heavy initial IMF after correction.

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, Kroupa & Oh (2012) 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.