The Spatial Structure of Young Stellar Clusters. III. Physical Properties and Evolutionary States

TL;DR

This study examines the physical properties and evolutionary stages of young stellar clusters in massive star-forming regions, revealing correlations indicative of dynamical evolution and supporting a hierarchical cluster formation model.

Contribution

It provides a detailed analysis of subcluster properties, evidence for gravitational binding, and supports a hierarchical merger model for cluster formation.

Findings

Significant correlations between subcluster size, density, and age.

Many subclusters are gravitationally bound.

Evidence for hierarchical merging in cluster formation.

Abstract

We analyze the physical properties of stellar clusters that are detected in massive star-forming regions in the MYStIX project--a comparative, multiwavelength study of young stellar clusters within 3.6 kpc that contain at least one O-type star. Tabulated properties of subclusters in these regions include physical sizes and shapes, intrinsic numbers of stars, absorptions by the molecular clouds, and median subcluster ages. Physical signs of dynamical evolution are present in the relations of these properties, including statistically significant correlations between subcluster size, central density, and age, which are likely the result of cluster expansion after gas removal. We argue that many of the subclusters identified in Paper I are gravitationally bound because their radii are significantly less than what would be expected from freely expanding clumps of stars with a typical initial stellar velocity dispersion of ~3 km/s for star-forming regions. We explore a model for cluster formation in which structurally simpler clusters are built up hierarchically through the mergers of subclusters--subcluster mergers are indicated by an inverse relation between the numbers of stars in a subcluster and their central densities (also seen as a density vs. radius relation that is less steep than would be expected from pure expansion). We discuss implications of these effects for the dynamical relaxation of young stellar clusters.