Primordial mass segregation of star clusters with primordial binaries

Václav Pavlík

Published 2020-04-29Version 1

Observational results of young star-forming regions suggest that star clusters are completely mass segregated at birth. As a star cluster evolves dynamically, these initial conditions are gradually lost. For star clusters with single stars only and a canonical IMF, it has been suggested that traces of these initial conditions vanish at a time $\tau_\mathrm{v}$ between $3$ and $3.5\,t_\mathrm{rh}$ (initial half-mass relaxation times). Since a significant fraction of stars are observed in binary systems and it is widely accepted that most stars are born in binary systems we investigate what role a primordial binary population (even up to $100\,\%$ binaries) plays in the loss of primordial mass segregation of young star clusters. We use numerical $N$-body models similar in size to the Orion Nebula Cluster (ONC) -- a representative of young open clusters -- integrated over several relaxation times to draw conclusions on the evolution of its mass segregation. We also compare our models to the observed ONC. We found that $\tau_\mathrm{v}$ depends on the binary star fraction and the distribution of initial binary parameters that include semi-major axis, eccentricity and mass ratio. For instance, in the models with $50\,\%$ binaries, we find $\tau_\mathrm{v} = (2.7 \pm 0.8)\,t_\mathrm{rh}$, while for $100\,\%$ binary fraction, we find a lower value, i.e. $\tau_\mathrm{v} = (2.1 \pm 0.6)\,t_\mathrm{rh}$. We also conclude that the initially completely mass segregated clusters, even with binaries, are more compatible with the present-day ONC than the non-segregated ones.