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Astron. Astrophys. 360, 509-519 (2000)

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4. Metallicity

It is difficult to derive an error estimate on the abundances. If we assume that each star has a solar distribution of heavy elements, then an error estimate is given by the dispersion of the heavy element abundances around the mean abundance (relative to solar). The abundances of all measured elements are shown in Fig. 3 for each of our stars, with the mean iron-peak element abundance indicated by a dashed line in each panel. From visual inspection (and excluding Ni which seems underabundant in some of our stars, probably only because of some systematic error in the oscillator strengths), we estimate the error on the abundance of a given element to be [FORMULA] dex. Table 4 summarizes the abundances derived in all our stars together with the number of lines used for their determination.

[FIGURE] Fig. 3. Abundances relative to solar for the 7 red giants. The dotted line represents the solar abundances, the dashed line is [M/H], defined as the average abundance of the iron-peak elements weighted by the number of lines.

The metallicity of each star is defined as a weighted average of the abundances of the iron-group elements, the weight being the number of lines. Since these weights are roughly the same for all stars, each cluster's metallicity is defined as the arithmetic mean of the metallicities of its stars. The values found are [M/H]=0.10 for NGC 2360 and [M/H]=0.05 for NGC 2447. The iron abundances are slightly smaller: [Fe/H]=0.07 for NGC 2360 and [M/H]=0.03 for NGC 2447. Those are reported in Table 4.

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© European Southern Observatory (ESO) 2000

Online publication: August 17, 2000