Radial-velocity measurements in 20 young open clusters

The further results of a program to determine the radial velocities of young open clusters are presented. Using the KPNO coude spectrograph coupled with the 1-m feed and 2.1-m telescopes, radial velocities have been measured for nearly one hundred stars, most of which are of spectral type B and A, in 20 young clusters. The combination of instruments and the use of cross-correlation techniques show that radial velocities of B and A type stars as faint as 10th magnitude can be determined with an internal precision of less than about 2 km/s. As expected, the uncertainties in the velocity determination for the young clusters are dominated by spectroscopic binary stars in these clusters. A third of the stars in the sample are found to be spectroscopic binaries, but with a large variation in the frequency of binaries from cluster to cluster. Because the time coverage is still limited, this should be considered a lower limit to the binary frequency. Cluster velocities are determined after eliminating binaries and known nonmember stars. The new velocities are compared with a model galactic rotation curve, as well as with previous velocity determinations.