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Session 72 - Open Clusters.
Display session, Wednesday, January 15
At 6--8 Gyrs, Collinder 261 is one of the oldest known open clusters (Phelps et al.\ 1994). Estimates of the metallicity range from [Fe/H] = -0.14\pm0.19 (Friel et al.) from low resolution spectroscopy to higher than or comparable to solar by analogy to the red giant branch of NGC 6791. Stellar evolution model isochrone fitting yields best fits for metallicities of at most solar. As part of a project to explore the chemical enrichment of old open clusters compared to both globular clusters and to more typical young open clusters, we have obtained high resolution echelle spectra on the CTIO 4m of four red giant stars in the cluster. Radial velocities were determined by cross- correlating the spectra against stellar radial velocity standards taken during the run. The average cluster radial velocity is -25.12\pm0.56 km/s. Based on this determination, the 4 stars appear to be members of the cluster. Measured equivalent widths of 36 selected FeI and 6 FeII lines, atomic transition probabilities (gf values) determined by comparison to the solar spectrum, and Kurucz stellar atmosphere models were used as input to the spectral analysis program MOOG (Sneden 1973), giving abundances as functions of excitation potential (EP) and equivalent width (EW). Initial stellar models were chosen based on photometrically calculated values of temperature and gravity. Final stellar abundances are calculated by iterating the T_eff, log g, input metallicity, and microturbulent velocity until the FeI and FeII abundances agree and no trends with EP or EW exist. We will present an analysis of the Fe abundances and resulting spectroscopically determined stellar parameters.
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