Evolution of the binary population in young dense star clusters

¹ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
e-mail: kaczmarek@ph1.uni-koeln.de
² Astronomisches Rechen-Institut (ARI), Zentrum für Astronomie der Universitaet Heidelberg (ZAH) Mönchhofstr. 12-14, 69120 Heidelberg, Germany
³ Max-Planck-Institut für Astronomie (MPIA), Königstuhl 17, 69117 Heidelberg, Germany
⁴ National Astronomical Observatories of China (NAOC), Chinese Academy of Sciences (CAS) 20A Datun Lu, Chaoyang District, 100012 Beijing, PR China
⁵ The Kavli Institute for Astronomy and Astrophysics (KIAA), Peking University (PKU) Yi He Yuan Lu 5, Hai Dian Qu, 100871 Beijing, PR China
⁶ Max-Planck-Institut für Radioastronomie (MPIfR), Auf dem Hügel 69, 53121 Bonn, Germany

Received: 18 June 2010
Accepted: 2 February 2011

Abstract

Context. Field stars are not always single stars, but can often be found in bound double systems. Since binary frequencies in the birth places of stars, young embedded clusters, are sometimes even higher than on average the question arises of how binary stars form in young dense star clusters and how their properties evolve to those observed in the field population.

Aims. We assess, the influence of stellar dynamical interactions on the primordial binary population in young dense cluster environments.

Methods. We perform numerical N-body simulations of the Orion nebula cluster like star cluster models including primordial binary populations using the simulation code nbody6++.

Results. We find two remarkable results that have yet not been reported: The first is that the evolution of the binary frequency in young dense star clusters is independent predictably of its initial value. The time evolution of the normalized number of binary systems has a fundamental shape. The second main result is that the mass of the primary star is of vital importance to the evolution of the binary. The more massive a primary star, the lower the probability that the binary is destroyed by gravitational interactions. This results in a higher binary frequency for stars more massive than 2 M_⊙ compared to the binary frequency of lower mass stars. The observed increase in the binary frequency with primary mass is therefore most likely not due to differences in the formation process but can be entirely explained as a dynamical effect.

Conclusions. Our results allow us to draw conclusions about the past and the future number of binary systems in young dense star clusters and demonstrate that the present field stellar population has been influenced significantly by its natal environments.