Formation rates of star clusters in the hierarchical merging scenario

TL;DR

This study uses N-body simulations to show that hierarchical star cluster formation mainly results from stellar scattering out of clumps, with two-body relaxation and initial virial ratio significantly influencing the erasure of substructure and cluster formation rate.

Contribution

It reveals that star cluster formation from substructured stars is driven primarily by stellar scattering and relaxation, not clump merging, highlighting the importance of relaxation time and initial virial ratio.

Findings

Hierarchical formation occurs mainly through stellar scattering, not merging.

Two-body relaxation accelerates substructure erasure beyond merging effects.

Lower initial virial ratios lead to faster substructure loss.

Abstract

Stars form with a complex and highly structured distribution. For a smooth star cluster to form from these initial conditions, the star cluster must erase this substructure. We study how substructure is removed using N-body simulations that realistically handle two-body relaxation. In contrast to previous studies, we find that hierarchical cluster formation occurs chiefly as a result of scattering of stars out of clumps, and not through clump merging. Two-body relaxation, in particular within the body of a clump, can significantly increase the rate at which substructure is erased beyond that of clump-merging alone. Hence the relaxation time of individual clumps is a key parameter controlling the rate at which smooth, spherical star clusters can form. The initial virial ratio of the clumps is an additional key parameter controlling the formation rate of a cluster. Reducing the initial virial ratio causes a star cluster to lose its substructure more rapidly.