The bound fraction of young star clusters

Abstract

Context. The residual gas within newly formed star clusters is expelled through stellar feedback on timescales 1 Myr. The subsequent expansion of the cluster results in an unbinding of a fraction of stars, before the remaining cluster members can re-virialize and form a surviving cluster. Aims. We investigate the bound fraction after gas expulsion as a function of initial cluster mass in stars M ecl and gauge the influence of primordial mass segregation, stellar evolution and the tidal field at solar distance. We also assess the impact of the star-formation efficiency ε SFE and gas expulsion velocity v g . Methods. We perform N-body simulations using Sverre Aarseth's NBODY7 code, starting with compact clusters in their embedded phase and approximate the gas expulsion by means of an exponentially depleting external gravitational field. We follow the process of re-virialization through detailed monitoring of different Lagrange radii over several Myr, examining initial half-mass radii of 0.1 pc, 0.3 pc and 0.5 pc and M ecl usually ranging from 5 × 10 3 M ⊙ to 5 × 10 4 M ⊙ . Results. The strong impact of the relation between the gas expulsion timescale and the crossing time means that clusters with the same initial core density can have very different bound fractions. The adopted ε SFE = 0.33 in the cluster volume results in a distinct sensitivity to v g over a wide mass range, while a variation of ε SFE can make the cluster robust to the rapidly decreasing external potential. We confirm that primordial mass segregation leads to a smaller bound fraction, its influence possibly decreasing with mass. Stellar evolution has a higher impact on lower mass clusters, but heating through dynamical friction could expand the cluster to a similar extent. The examined clusters expand well within their tidal radii and would survive gas expulsion even in a strong tidal field.

Very massive stars in not so massive clusters

“…We note that the true initial mass of the cluster could have been higher because of the possible loss of stars as a result of residual-gas expulsion (e.g. Kroupa, Aarseth & Hurley 2001;Brinkmann, Banerjee, Motwani & Kroupa 2017). If so then this work presents a conservative estimate because more massive and thus denser clusters lead to more stellar mergers.…”

Realistic modelling of wind and supernovae shocks in star clusters: addressing 22Ne/20Ne and other problems in Galactic cosmic rays

“…Although diffuse or 'leaky' clusters (Pfalzner 2009) have sizes of about an order of magnitude larger than compact clusters (e.g., Cyg OB2), they can be thought of as assemblies of compact clusters, given that they have substantial substructures (Wright et al 2014). Finally, if the initial cluster is small enough, it can be dissolved (or nearly dissolved) due to gas dispersion and mass loss during SNe (Brinkmann et al 2017, Shukirgaliyev et al 2017.…”

Disruption of giant molecular clouds and formation of bound star clusters under the influence of momentum stellar feedback

“…The second method is to use the "Lagrangian radius", R lag , defined as a series of radii within which the star cluster contains a sequence of fractions of stellar mass. Brinkmann et al (2017) suggest to use the evolution of the Lagrangian radii to determine Figure 4. The solid line is the best-fit model with f sat = 0.95 and α * = 0.48 using Equation 17.…”