Two paths of cluster evolution: global expansion versus core collapse

TL;DR

The paper investigates two distinct evolutionary paths of gravitationally bound clusters—either undergoing core collapse or experiencing continuous expansion—using N-body simulations to identify the critical initial conditions that determine their fate.

Contribution

It introduces the concept of a 'collapse line' in initial parameter space, delineating clusters that will undergo core collapse from those that will not, based on initial size and population.

Findings

Clusters with initial population above the collapse line tend to undergo core collapse.

A minimum population for core collapse exists, which scales with Galactocentric radius as R_G^{-9/8}.

Most observed open clusters are below the collapse line and will not undergo core collapse.

Abstract

All gravitationally bound clusters expand, due to both gas loss from their most massive members and binary heating. All are eventually disrupted tidally, either by passing molecular clouds or the gravitational potential of their host galaxies. However, their interior evolution can follow two very different paths. Only clusters of sufficiently large initial population and size undergo the combined interior contraction and exterior expansion that leads eventually to core collapse. In all other systems, core collapse is frustrated by binary heating. These clusters globally expand for their entire lives, up to the point of tidal disruption. Using a suite of direct N-body calculations, we trace the "collapse line" in r_v-N space that separates these two paths. Here, r_v and N are the cluster's initial virial radius and population, respectively. For realistic starting radii, the dividing N-value is from 10^4 to over 10^5. We also show that there exists a minimum population, N_min, for core collapse. Clusters with N < N_min tidally disrupt before core collapse occurs. At the Sun's Galactocentric radius, R_G = 8.5 kpc, we find N_min >~ 300. The minimum population scales with Galactocentric radius as R_G^{-9/8}. The position of an observed cluster relative to the collapse line can be used to predict its future evolution. Using a small sample of open clusters, we find that most lie below the collapse line, and thus will never undergo core collapse. Most globular clusters, on the other hand, lie well above the line. In such a case, the cluster may or may not go through core collapse, depending on its initial size. We show how an accurate age determination can help settle this issue.