Primordial mass segregation of star clusters with primordial binaries

¹ Astronomical Institute of the Czech Academy of Sciences, Boční II 1401, 141 31 Prague 4, Czech Republic
e-mail: pavlik@asu.cas.cz
² Astronomical Institute of Charles University, V Holešovičkách 2, 180 00 Prague 8, Czech Republic
³ Observatory and Planetarium of Prague, Královská obora 233, 170 21 Prague 7, Czech Republic

Received: 13 January 2020
Accepted: 29 April 2020

Abstract

Context. Observations of young star-forming regions suggest that star clusters are born completely mass segregated. These initial conditions are, however, gradually lost as the star cluster evolves dynamically. For star clusters with single stars only and a canonical initial mass function, it has been suggested that traces of these initial conditions vanish at a time τ_v between 3 and 3.5 t_rh (initial half-mass relaxation times).

Aims. 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 aim to investigate what role a primordial binary population (even up to 100% binaries) plays in the loss of primordial mass segregation of young star clusters.

Methods. We used 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 compared our models to the observed ONC.

Results. We found that τ_v depends on the binary star fraction and the distribution of initial binary parameters that include a semi-major axis, eccentricity, and mass ratio. For instance, in the models with 50% binaries, we find τ_v = (2.7 ± 0.8) t_rh, while for 100% binary fraction, we find a lower value τ_v = (2.1 ± 0.6) t_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.