Dynamical ejections of massive stars from young star clusters under diverse initial conditions

¹ Helmholtz-Institut für Strahlen- und Kernphysik (HISKP), University of Bonn, Nussallee 14-16, 53115 Bonn, Germany
e-mail: skoh@astro.uni-bonn.de
² Argelander Institut für Astronomie, University of Bonn, Auf dem Hügel 71, 53121 Bonn, Germany

Received: 1 February 2016
Accepted: 23 March 2016

Abstract

We study the effects that initial conditions of star clusters and their massive star population have on dynamical ejections of massive stars from star clusters up to an age of 3 Myr. We use a large set of direct N-body calculations for moderately massive star clusters (M_ecl ≈ 10^3.5 M_⊙). We vary the initial conditions of the calculations, such as the initial half-mass radius of the clusters, initial binary populations for massive stars and initial mass segregation. We find that the initial density is the most influential parameter for the ejection fraction of the massive systems. The clusters with an initial half-mass radius r_h(0) of 0.1 (0.3) pc can eject up to 50% (30)% of their O-star systems on average, while initially larger (r_h(0) = 0.8 pc) clusters, that is, lower density clusters, eject hardly any OB stars (at most ≈ 4.5%). When the binaries are composed of two stars of similar mass, the ejections are most effective. Most of the models show that the average ejection fraction decreases with decreasing stellar mass. For clusters that are efficient at ejecting O stars, the mass function of the ejected stars is top-heavy compared to the given initial mass function (IMF), while the mass function of stars that remain in the cluster becomes slightly steeper (top-light) than the IMF. The top-light mass functions of stars in 3 Myr old clusters in our N-body models agree well with the mean mass function of young intermediate-mass clusters in M 31, as reported previously. This implies that the IMF of the observed young clusters is the canonical IMF. We show that the multiplicity fraction of the ejected massive stars can be as high as ≈ 60%, that massive high-order multiple systems can be dynamically ejected, and that high-order multiples become common especially in the cluster. We also discuss binary populations of the ejected massive systems. Clusters that are initially not mass-segregated begin ejecting massive stars after a time delay that is caused by mass segregation. When a large kinematic survey of massive field stars becomes available, for instance through Gaia, our results may be used to constrain the birth configuration of massive stars in star clusters. The results presented here, however, already show that the birth mass-ratio distribution for O-star primaries must be near uniform for mass ratios q ≳ 0.1.