NIR spectroscopy of the most massive open cluster in the Galaxy: Westerlund 1

TL;DR

This study uses near-infrared spectroscopy to measure the velocities of stars in Westerlund 1, determining its mass and dynamical state, and finding it to be a massive, stable cluster without signs of expansion.

Contribution

First detailed spectroscopic analysis of all NIR-bright stars in Westerlund 1, providing precise velocity measurements and dynamical mass estimation.

Findings

Cluster mass estimated at 1.25x10^5 solar masses

Velocity dispersion of 8.4 km/s consistent with virial equilibrium

No evidence of cluster expansion or dissolution

Abstract

Using ISAAC/VLT, we have obtained individual spectra of all NIR-bright stars in the central 2'x2' of the cluster Westerlund 1 (Wd 1) with a resolution of R~9000 at a central wavelength of 2.30 micron. This allowed us to determine radial velocities of ten post-main-sequence stars, and from these values a velocity dispersion. Assuming virial equilibrium, the dispersion of sigma=8.4 km/s leads to a total dynamical cluster mass of 1.25x10^5 solar masses, comparable to the photometric mass of the cluster. There is no extra-virial motion which would have to be interpreted as a signature of cluster expansion or dissolution.