The distinction between star clusters and associations

TL;DR

This paper proposes an objective criterion to distinguish star clusters from associations based on the ratio of stellar age to crossing time, aiding classification especially for barely resolved distant objects.

Contribution

It introduces a new, quantifiable definition for separating star clusters and associations using the ratio of stellar age to crossing time.

Findings

Bound objects have stellar age exceeding crossing time.

Unbound objects have crossing time exceeding stellar age.

The ratio equals unity at the boundary between clusters and associations.

Abstract

In Galactic studies a distinction is made between (open) star clusters and associations. For barely resolved objects at a distance of several Mpc this distinction is not trivial to make. Here we provide an objective definition by comparing the age of the stars to the crossing time of nearby stellar agglomerates. We find that a satisfactory separation can be made where this ratio equals unity. Stellar agglomerates for which the age of the stars exceeds the crossing time are bound, and are referred to as star clusters. Alternatively, those for which the crossing time exceeds the stellar age are unbound and are referred to as associations. This definition is useful whenever reliable measurements for the mass, radius and age are available.