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 \approx 9000 at a central wavelength of 2.30 \mu m. 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 1.25 \times 10 ^ { 5 } M _ { \odot } , 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 .