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.
@article{arxiv.0711.1779,
title = {NIR spectroscopy of the most massive open cluster in the Galaxy: Westerlund 1},
author = {S. Mengel and L. E. Tacconi-Garman},
journal= {arXiv preprint arXiv:0711.1779},
year = {2009}
}
Comments
To appear in the proceedings of IAU 246: "Dynamical Evolution of Dense Stellar Systems" (E. Vesperini, M. Giersz, A. Sills, eds.)