NIR Spectroscopy of the Most Massive Open Cluster in the Galaxy: Westerlund 1

Using ISAAC/VLT, we have obtained individual spectra of all NIR-bright stars in the central 2' × 2' of the cluster Westerlund 1 (Wd 1) with a resolution of R ≈ 9000 at a central wavelength of 2.30 μm. 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 σ = 8.4 km/s leads to a total dynamical cluster mass of 1.25 × 10⁵M_⊙, 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.