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Abundances for globular cluster giants. I. Homogeneous metallicities for 24 clusters

¹ Osservatorio Astronomico di Bologna, Via Zamboni 33, I-40126 Bologna, Italy
² Dipartimento d'Astronomia, Università di Padova, Vicolo dell'Osservatorio 5, I-35122 Padova, Italy
³ Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 5, I-35122 Padova, Italy

Send offprint request to: E. Carretta. Table 9 also available in electronic form at the CDS via anonymous ftp 130.79.128.5 or viahttp://cdsweb.u-strabg.fr/Abstract.html

Received: 28 February 1996
Accepted: 21 May 1996

Abstract

We have obtained high resolution, high signal-to-noise ratio CCD echelle spectra of 10 bright red giants in 3 globular clusters (47 Tuc, NGC 6752 and NGC 6397) roughly spanning the whole range of metallicities of the galacticglobular cluster system. The analysis of this newly acquired material reveals no significant evidence of star-to-star variation of the [Fe/H] ratio in thesethree clusters.Moreover, a large set of high quality literature data (equivalent widths from highdispersion CCD spectra) was re-analyzed in an homogeneous andself-consistent way to integrate our observations and derive new metalabundances for more than 160 bright red giants in 24 globular clusters (i.e.about 16% of the known population of galactic globulars).This set was then used to define a new metallicity scale for globular clusterswhich is the result of high quality, direct spectroscopic data, of new andupdated model atmospheres from the grid of Kurucz(1992), and of a carefulfine abundance analysis; this last, in turn, is based on a common set of bothatomic and atmospheric parameters for all the stars examined.Given the very high degree of internal homogeneity, our new scale supersedes the offsets and discrepancies existing in previous attempts to obtain a metallicity scale.The internal uncertainty in [Fe/H] is very small: 0.06 dex (24clusters) on average, and can be interpreted also as the mean precision of the clusterranking.Compared to our system, metallicities on the widely used Zinn and West's scaleare about 0.10 dex higher for [Fe/H], 0.23 dex lower for[Fe/H]and 0.11 dex too high for [Fe/H]. The non-linearity ofthe Zinn and West'sscale is significant even at 3σ level. A quadratic transformation isgiven to correct older values to the new scale in the range of our calibratingclusters ([Fe/H]).A minor disagreement is found at low metallicities between the metallicityscale based on field and cluster RR Lyrae variables (via a new calibration ofthe index) and our new cluster metallicities. It could betentatively ascribed to non-linearity in the [Fe/H] relationship.The impact of new metallicities on major astrophysical problems is exemplifiedthrough a simple exercise on the Oosterhoff effect in the classical pair M 3and M 15.