Mass segregation and elongation of the starburst cluster Westerlund 1

TL;DR

This study analyzes Westerlund 1, a massive young stellar cluster, revealing rapid mass segregation and elongation likely caused by subcluster merging during its formation, based on near-infrared photometry and stellar models.

Contribution

It provides the first detailed spatially dependent IMF and density profile analysis of Westerlund 1, demonstrating early mass segregation and elongation.

Findings

Strong evidence of mass segregation in Westerlund 1.

Elongation with an axis ratio of 3:2 confirmed.

Mass segregation likely caused by subcluster merging.

Abstract

Massive stellar clusters are the best available laboratories to study the mass function of stars. Based on NTT/SofI near-infrared photometry, we have investigated the properties of the massive young cluster Westerlund 1. From comparison with stellar models, we derived an extinction A_{Ks} = 0.91 +/- 0.05 mag, an age \tau = 4 +/- 0.5 Myr and a distance d = 4.0 +/- 0.2 kpc for Westerlund 1, as well as a total mass of M_{Wd1} = 4.91_{-0.49}^{+1.79} x 10^4 M_{sun}. Using spatially dependent completeness corrections we performed a 2D study of the cluster's IMF and, in addition, of the stellar density profiles of the cluster as a function of mass. From both IMF slope variations and stellar density, we find strong evidence of mass segregation. For a cluster with some 10^5 stars, this is not expected at such a young age as the result of two-body relaxation alone. We also confirm previous findings on the elongation of Westerlund 1; assuming an elliptical density profile, we found an axis ratio of a:b = 3:2. Rapid mass segregation and elongation could be well explained as the results of subclusters merging during the formation of Westerlund 1.