Metallicities of old open clusters located in the third galactic quadrant

Abstract

One hundred and thirty six stars associated with nine old open clusters with galactic longitudes between 215 and 295 degrees have been investigated with the Washington system. The data yield a luminosity class, temperature and metal abundance for each star. The luminosity classification, supplemented with additional membership criteria, is used to separate field stars from cluster giants; definitive membership status is available for virtually the entire sample. Mean metal abundances for an average of twelve member giants per cluster are determined to an accuracy of 0.2 dex (standard deviation), including observational and reddening errors. For the five clusters in common with the recent spectroscopic study by Friel & Janes (1992, A&A, in press), agreement is generally good, although our metallicities are 0.1 dex lower, on average. Two of the other four clusters, NGC 2324 and NGC 2660, are found to be surprisingly metal-poor, with [Fe/H] ∼ 1. The existence of such metal-poor clusters, without other exceptional properties (Galactocentric distance, distance from the plane, or age), is anomalous. A third cluster, NGC 3960, has a Galactocentric distance of only 8 kpc but a metallicity of ∼ -0.7. Such clusters indicate substantial scatter actually exists in the tight relation found by Friel and Janes between the metallicity of an open cluster and its current Galactocentric distance. We concur with their findings that Galactocentric distance is the major factor in determining open cluster metallicities and that age and metallicity are not correlated for open clusters, unlike the case for Large Magellanic Cloud clusters. We emphasize, however, that outer disk clusters have a metallicity at a given age that is much more like that of their LMC counterparts than that of solar neighborhood disk field stars or clusters. Indeed, these latter two populations appear to have distinct age-metallicity distributions as well, with the solar neighborhood open clusters more metal-poor by ∼ 0.15 dex than local disk field stars of the same age, except for the oldest clusters. An offset in metallicity scales is the most likely explanation for this effect. This paper will appear in the november issue of The Astronomical Journal.