Kinematics of young open clusters and the rotation curve of the galaxy

Recent radio and optical observations show that the rotation-curves of nearly all spiral galaxies are flat out to large galactocentric radii. Since this suggests the existence of massive galactic halos, much effort is being expended on the study of the rotation-curve of our Galaxy. A group of objects well suited for this purpose are young open clusters. However, the rather small number of clusters with available radial-velocities did not yet allow a more detailed investigation of their galactic kinematics. Thus, new radial-velocities for 28 southern young open clusters were obtained for this work. These data and already published radial-velocities and distances were used to compile a catalog of 105 O-B3 clusters which served as basis for the kinematical study. The main results are: The rotation curve shows no decline of rotational velocities up to (R-R0)approximately5kpc, but it has a 'wavelike' structure with maximum deviations from a flat curve of about +/-10km/s at (R-R0)approximately-2/+2. This feature is confirmed by a comparison with the published galactic rotation curves from HII-regions and molecular clouds. The solar motion obtained from this sample of open clusters does not confirm any noncircular motions of the LSR. The effect of a galactic abundance gradient of -0.1dex/kpc on the rotation curve is shown to be nearly negligible. The radial velocities of open clusters in the Perseus-Arm are about 10km/s larger than those HII-regions and their associated molecular clouds. This difference is found to be significant for the older (BO-B3) clusters not surrounded by HII-regions and probably significant for those clusters embedded in HII-regions. (Author)