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Metallicities and Velocities of Old Open Clusters

E. N. Thogersen, E. D. Friel and B. V. Fallon
Publications of the Astronomical Society of the Pacific
Vol. 105, No. 693 (1993 November), pp. 1253-1259
Stable URL: http://www.jstor.org/stable/40680185
Page Count: 7
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Metallicities and Velocities of Old Open Clusters
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Abstract

We have obtained moderate-resolution CCD spectra for giants in six old open clusters in the direction of the galactic anticenter [NGC 188, 6705 (M11), 6819, 6939, 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 four and six 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 are centered around the solar radius, to supplement published results by Friel and Janes (1993, A&A, 267, 75), and used the combined data set to better define the Galactic metallicity gradient. We found that Δ[Fe/H]/ΔRgc=-0.097±0.017 dex kpc⁻¹ 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 dependences with age and height above the Galactic plane.

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