Hierarchical formation of Westerlund 1: a collapsing cluster with no primordial mass segregation?

TL;DR

This study investigates the structure and mass distribution of Westerlund 1, suggesting it formed without primordial mass segregation and has evolved into a relatively smooth, dense cluster.

Contribution

It provides evidence that Westerlund 1 likely formed without primordial mass segregation, challenging previous assumptions about its initial conditions.

Findings

Westerlund 1 shows little to no primordial mass segregation.

Massive stars are located in regions of higher surface density.

The cluster likely formed with no primordial mass segregation.

Abstract

We examine the level of substructure and mass segregation in the massive, young cluster Westerlund 1. We find that it is relatively smooth, with little or no mass segregation, but with the massive stars in regions of significantly higher than average surface density. While an expanding or bouncing-back scenario for the evolution of Westerlund 1 cannot be ruled out, we argue that the most natural model to explain these observations is one in which Westerlund 1 formed with no primordial mass segregation and at a similar or larger size than we now observe.