Comparison with other populations

In order to gather clues on the origin of metal-rich clusters, in Figure 9.5 we compare their [X/Fe] (for the most abundant a-elements apart from oxygen) versus [Fe/H] distribution with those of other Galactic populations, namely bulge stars, thin- and thick-disk stars, bulge-like dwarf stars (stars with metallicities and kinematics characteristics of a probable inner-disk or bulge origin - e.g. Castro et al. (1997)). Note that a-element measurements are not available for all of the ten

-0.4 -0.2 0 0.2 [Fe/H]

Figure 9.5. Plots of [X/Fe] versus [Fe/H] for metal-rich OCs (filled circles) and other Galactic populations, namely thin- and thick-disk stars, open circles (Bensby et al. 2003); bulge stars, open stars (McWilliam & Rich 1994; Fulbright et al. 2007); and bulge-like stars, open triangles and squares (Pompeia et al. 2003; Castro et al. 1997).

clusters listed in Table 9.1. As demonstrated by several other authors, Figure 9.5 shows a large spread in [a/Fe] ratios for any given population/metallicity; it is therefore difficult to discern whether OCs fit well into one or other of the Galactic components. With the exception of a few outliers, however, metal-rich clusters are characterized by a rather homogeneous abundance pattern. They do not exhibit significant a-enhancement (most [a/Fe] ratios are indeed close to zero), suggesting that there is a better agreement with the trend of disk stars rather than with that of the bulge or bulge-like populations. The latter, bulge stars in particular, may be characterized by a-enhancement at supersolar metallicities; see also Zoccali et al. (2006). Therefore, we tentatively conclude that the majority of metal-rich clusters

Figure 9.6. The distribution of [Fe/H] versus Galactocentric distance (Rgc). Filled symbols denote the 10 metal-rich clusters listed in Table 9.1, while open circles indicate the 58 non-metal-rich clusters with a spectroscopic determination of metallicity. The [Fe/H] values come from various sources and are not on the same scale. The figure is not meant to provide a quantitative estimate of the slope of the gradient, but merely to illustrate qualitatively the influence of metal-rich clusters on the gradient.

Figure 9.6. The distribution of [Fe/H] versus Galactocentric distance (Rgc). Filled symbols denote the 10 metal-rich clusters listed in Table 9.1, while open circles indicate the 58 non-metal-rich clusters with a spectroscopic determination of metallicity. The [Fe/H] values come from various sources and are not on the same scale. The figure is not meant to provide a quantitative estimate of the slope of the gradient, but merely to illustrate qualitatively the influence of metal-rich clusters on the gradient.

most likely originated in the local disk; a different origin might hold for NGC 6791 (Carraro et al. 2006; Bedin et al. 2006).

Regarding the discrepant clusters in Figure 9.5, namely NGC 5822 and IC 4725, their abundances come from Luck (1994) and are based on only one and two stars, respectively. Additional observations are needed before conclusions concerning their anomalous abundance trends can be drawn.

Finally, under the hypothesis that most metal-rich clusters were born in the disk, we can use them to trace the radial metallicity gradient. Since, as mentioned, all of these clusters are located at small Galactocentric radii, their inclusion in the determination of the gradient would make it steeper (see Figure 9.6). Note, however, that at any given Rgc there is a dispersion in metallicity; the reason for this dispersion and the possible dependence of metallicity on other parameters must be ascertained before the exact slope of the gradient can be derived.

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