The Starburst Cluster Westerlund 1: The Initial Mass Function and Mass Segregation

TL;DR

This study investigates Westerlund 1, a key starburst cluster, by analyzing its initial mass function, age, distance, and mass segregation, revealing a massive, young, and dynamically evolving stellar environment.

Contribution

It provides the first detailed IMF and mass segregation analysis of Westerlund 1 using multi-band photometry and recent stellar evolution models with rotation.

Findings

IMF slope of -0.8 indicating a shallow distribution

Total cluster mass exceeds 5 x 10^4 solar masses

Clear evidence of mass segregation within the cluster

Abstract

Westerlund 1 is the most important starburst cluster in the Galaxy due to its massive star content. We have performed BVIc and JKs photometry to investigate the initial mass function (IMF). By comparing the observed color with the spectral type - intrinsic color relation, we obtain the mean interstellar reddening of <E(B-V)>=4.19+/-0.23 and <E(J-Ks)>=1.70+/-0.21. Due to the heavy extinction toward the cluster, the zero-age main sequence fitting method based on optical photometry proved to be inappropriate for the distance determination, while the near-infrared photometry gave a reliable distance to the cluster -- 3.8 kpc from the empirical relation. Using the recent theoretical stellar evolution models with rotation, the age of the cluster is estimated to be 5.0+/-1.0 Myr. We derived the IMF in the massive part and obtained a fairly shallow slope of {\Gamma} = -0.8 +/- 0.1. The integration of the IMF gave a total mass for the cluster in excess of 5.0 x 10^4 solar mass. The IMF shows a clear radial variation indicating the presence of mass segregation. We also discuss the possible star formation history of Westerlund 1 from the presence of red supergiants and relatively low-luminosity yellow hypergiants.