Clusters metal rich

We present CCD photometry in the Johnson V, Kron-Cousins I and Washington system CTi passbands for NGC2324, a rich open cluster located near the Galactic anticentre direction. We believe that the high discrepancy in the basic cluster parameters derived in previous studies, particularly in the cluster metal content, warrants their redetermination on the basis of more reliable data. 

The metallicity distribution of globular clusters in the Galaxy has a metal-rich peak at [Fe/H] -0.5 and a metal-poor peak at [Fe/H] -1.6 (e.g. Cote 1999), where most of the metal-rich ones are bulge clusters. Metallicities for samples of field stars were derived by McWilliam Rich (1994, hereafter MR94), Sadler et al. (1996), Ramirez et al. (2000). Zoccali et al. (2003) presented the... 

In addition to results from this study, Table 1 includes two of the relatively more metal-rich globular clusters associated with the Sgr dSph. There appears to be little in common between the two metallicity groups in their < a>-abundances relative to iron. Abundances reported so far for in situ Sgr dSph field stars of comparable metallicities [4] are in accord with those of its metal-rich clusters. 

Abstract. I present preliminary results for a sample of 700 red giants in u> Cen, observed during the Ital-FLAMES Consortium GTO time in May 2003, for the Bologna Project on u Cen. Preliminary Fe and Ca abundances confirm previous results while the metal-poor and intermediate populations show a normal halo o-enhancement of [a/Fe] +0.3, the most metal-rich stars show a significantly lower [a/Fe] +0.1. If the metal-rich stars have evolved within the cluster in a process of self-enrichment, the only way to lower their a-enhancement would be SNe type la intervention. 

Spectroscopic observations of globular clusters (GCs) have revealed star-to-star inhomogeneities in the light metals that are not observed in field stars. These light metal anomalies could be interpreted with a self-pollution scenario. But what about heavier (Z > 30) elements Do they also show abundance anomalies Up to now, no model has been developed for the synthesis of n-capture elements in GCs, and the self-pollution models do not explain the origin of their metallicity. In 1988, Truran suggested a test for the self-enrichment scenario [4], which could possibly explain the metallicity and the heavy metal abundances in GCs if self-enrichment occurred in GCs, even the most metal-rich clusters would show both high [a/Fe] ratios and r-process dominated heavy elements patterns, which characterize massive star ejecta as it is seen in the most metal-poor stars. 

Abstract. From VLT/UT2 GIRAFFE GTO, we performed a lithium abundance survey along the red giant branch of the metal-rich globular cluster 47 TUC (NGC 104), in order to investigate the efficiency of extra mixing occurring at the RGB bump. 

The distribution function for globular clusters is somewhat more complicated, as there appear to be two (probably overlapping) distributions corresponding to the halo and the thick disk, respectively. These have been tentatively fitted in Fig. 8.20 with a Simple model truncated at [Fe/H] = —1.1 for the halo and a model for the thick disk clusters with an initial abundance [Fe/H] = —1.6 (the mean metallicity of the halo) and truncated at [Fe/H] = —0.35. The disk-like character of the more metal-rich clusters is supported by their spatial distribution (Zinn 1985). Furthermore, there is a marginally significant shortage of globular clusters in the lowest... 

The clusters described so far have in common, that the number of metal atoms is less or equal than the number of substituents. However, there is a still growing number of both neutral and anionic clusters in which the number of metal atoms is larger than the number of substituents. As a consequence, these metal-rich clusters contain naked metal centers which are only bonded to other metal centers. Schnockel referred these ones to as metalloid clusters. Several metalloid A1 and Ga clusters were prepared by standard salt elimination reactions using metastable solutions of metal subhalides MX (M = Al, Ga X = Cl, Br) as well as solutions of Gal. Since the metal subhalides were found to play the key role for the successful synthesis of this particular class of compounds, they will be discussed first. (For excellent review articles see Refs 273 and 274.)... 

Compounds with metal-metal bonding occur frequently throughout the Periodic Table. The trivial but necessary condition for covalent M-M bonding is a low oxidation state which leaves valence electrons with the metal atom. This condition, however, is not sufficient. Compounds need to be metal-rich to allow for sufficiently close contacts between metal atoms, and the extension of the valence electron orbitals in space must be large in order to provide good overlap. Hence, it is no surprise that M-M bonding and cluster formation dominates with the heavier elements in the Periodic Table, involving s, p, d, and even f electrons. 

Of course, valence electron concentration is not only related to the metal atoms but also to the number and valence of the ligands. Ligand deficiency creates vacant coordination sites at metal atoms and results in cluster condensation, which is the fusion of clusters via short M-M contacts into larger units ranging from zero- to three-dimensional. The chemistry of metal-rich halides of rare earth metals comprises both principles, incorporation of interstitial atoms and cluster condensation, with a vast number of examples [22, 23]. 

When it comes to metal-rich compounds of the alkaline earth and alkali metals with their pronounced valence electron deficiencies it is no surprise that both principles play a dominant role. In addition, there is no capability for bonding of a ligand shell around the cluster cores. The discrete and condensed clusters of group 1 and 2 metals therefore are bare, a fact which leads to extended inter-cluster bonding and results in electronic delocalization and metallic properties for all known compounds. 

Homonuclear clusters of the heavier elements of the third main-group aluminum, gallium, indium and thallium having direct element-element interactions form a fascinating new class of compounds. As discussed in the previous Chapter 2.3, in some cases their structures resemble those known with the lightest element of that group, boron, while in other cases novel, metal-rich compounds were obtained which do not have any analogue in boron chemistry. 

Some general comments on the solid-state chemistry ( From a molecular view on solids to molecules in solids ) have been reported by Simon (1995) emphasis was especially placed on the structural chemistry of metal-rich compounds formed by the metals in groups 1 to 6 and it was underlined that it is largely based on discrete and condensed clusters. In the chemistry of metals in low oxidation states, the residual valence electrons can be used for metal—metal bonding. Metal-rich compounds lie between normal valence compounds and the elemental metals themselves, with respect to their compositions, and often also with respect to their structures fragments of usual metal structures (close-packed, b.c.c., etc.) are often component units in the structures of metal-rich compounds. 

The rich variety of active sites that can be present in zeolites (i) protonic acidic sites, which catalyze acid reactions (ii) Lewis-acid sites, which often act in association with basic sites (acid-base catalysis) (iii) basic sites (iv) redox sites, incorporated either in the zeolite framework (e.g., Ti of titanosHicates) or in the channels or cages (e.g., Pt clusters, metal complexes). Moreover, redox and acidic or basic sites can act in a concerted way for catalyzing bifunctional processes. 

A brief historical note on the structure of the iron-sulfur clusters in ferredoxins is relevant. After the first analytical results revealed the presence of (nearly) equimolar iron and acid-labile sulfur, it was clear that the metal center in ferredoxins did not resemble any previously characterized cofactor type. The early proposals for the Fe S center structure were based on a linear chain of iron atoms coordinated by bridging cysteines and inorganic sulfur (Blomstrom et al., 1964 Rabino-witz, 1971). While the later crystallographic analyses of HiPIP, PaFd, and model compounds (Herskovitz et al., 1972) demonstrated the cubane-type structure of the 4Fe 4S cluster, the original proposals have turned out to be somewhat prophetic. Linear chains of sulfide-linked irons are observed in 2Fe 2S ferredoxins and in the high-pH form of aconitase. Cysteines linked to several metal atoms are present in metallothionein. The chemistry of iron-sulfur clusters is rich and varied, and undoubtedly many other surprises await in the future. 

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