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

Abstract

Abstract. We have obtained high resolution, high signalto-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 galactic globular cluster system. The analysis of this newly acquired material reveals no significant evidence of starto-star variation of the [Fe/H] ratio in these three clusters. Moreover, a large set of high quality literature data (equivalent widths from high dispersion CCD spectra) was reanalyzed in an homogeneous and self-consistent way to integrate our observations and derive new metal abundances 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 clusters which is the result of high quality, direct spectroscopic data, of new and updated model atmospheres from the grid of Kurucz (1992), and of a careful fine abundance analysis; this last, in turn, is based on a common set of both atomic 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 (24 clusters) on average, and can be interpreted also as the mean precision of the cluster ranking. Compared to our system, metallicities on the widely used Zinn and West's scale are about 0.10 dex higher for [Fe/H]> −1, 0.23 dex lower for −1 <[Fe/H]< −1.9 and 0.11 dex too high for [Fe/H]< −1.9. The non-linearity of the Zinn and West's scale is significant even at 3σ level. A quadratic transformation is given to correct older values to the new scale in the range of our calibrating clusters (−2.24 ≤[Fe/H] ZW ≤ −0.51). A minor disagreement is found at low metallicities between the metallicity scale based on field and cluster RR Lyrae variables (via a new Send offprint requests to: E. Carretta Table 9 also available in electronic form at the CDS via anonymous ftp 130.79.128.5 or via http://cdsweb.ustrabg.fr/Abstract.html calibration of the ∆S index) and our new cluster metallicities. It could be tentatively ascribed to non-linearity in the [Fe/H]−∆S relationship. The impact of new metallicities on major astrophysical problems is exemplified through a simple exercise on the Oosterhoff effect in the classical pair M 3 and M 15.

The metallicity gradient as a tracer of history and structure: the Magellanic Clouds and M33 galaxies

“…The study by Pompéia et al (2008) (Carretta & Gratton 1997). Their RGB stars are >8 Gyr old (Sculptor) and 3−6 Gyr old (Fornax) while LMC RGB stars are mostly young (1−3 Gyr old).…”

Metal Abundances in Extremely Distant Galactic Old Open Clusters. II. Berkeley 22 and Berkeley 66

“…taken from Carretta & Gratton (1997). In this equation, the mass M/M was derived from the comparison between the position of the star in the Hertzsprung-Russell diagram and the Padova isochrones (Girardi et al 2000).…”

The Relative Age of the Thin and Thick Galactic Disks

“…This correction was determined using the theoretical HB models of Demarque et al (2000). (Carretta & Gratton 1997), which gives a Z value intermediate between the above two isochrones sets. For this reason, we average the two ages and adopt an age for 47 Tuc of 12.9^0.7 Gyr.…”