METALLICITIES AND VELOCITIES OF OLD OPEN CLUSTERS

Abstract

ABSTRACT We have obtinaed moderate resoluiton CCD spectra for giants in six old open clusters in the direction of the galactic anti-center (NGC 188, 6705 (M11), 6819, 6940, and IC 4756). Using cross correlation techniques, we determined radial velocities for stars in each cluster and used these values to compute a cluster mean accurate to ~10 km s-1. These means agree quite well with published values. We then used between 4 and 6 spectroscopic indices to measure the metallicity of each star and produce a mean cluster metallicity. Following the Galactic metallicity gradient, more distant clusters are metal poor compared with clusters closer to the solar neighborhood. We used our data, which is centered around the solar radius, to supplement published results by Friel and Janes (1993), and used the combined data set to better define the Galactic metallicity gradient. We found that delta[Fe/H]/deltaRgc = -0.097 ± 0.017 dex/kpc for Rgc > 7 kpc. This result agrees quite well with that of Friel and Janes and with that of other authors. We discuss a possible correlation between age and Mg-enhancement which is indicated by the differences between Fe and Mg line strength among cluster giants. We also briefly consider metallicity dependencies with age and height above the Galactic plane.