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

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1. Introduction

The comparison of chemical abundances observed at the surface of stars with those predicted by stellar model calculations offers a valuable tool to test our knowledge of stellar evolution. Among the most studied objects are red giants. Those objects are formed after the main sequence (MS) phase when hydrogen has been exhausted in the core. The core contracts, the outer layers expand, and the convective envelope deepens into the core. The consequent mixing of the ashes of hydrogen burning from the deep layers to the surface is called the first dredge-up process (1DUP).

In most cases, the surface abundances after 1DUP are predicted to be sensitive to stellar mass. The observation of red giants belonging to given clusters presents in this respect a unique advantage since the age of the cluster, and thus of its individual stars, is known by comparing the cluster's distribution in the Hertzsprung-Russell diagram with predicted isochrones. The masses of the individual stars are then determined, enabling a thorough comparison of their surface abundances with predictions.

In this paper, we determine the abundances of oxygen, sodium and heavier elements in four red giants of the open clusters NGC 2360 and in three red giants of NGC 2447. All these giants belong to the clumps of their respective clusters. The initial masses of our stars are estimated to 2 and [FORMULA] for those in NGC 2360 and NGC 2447, respectively. These masses turn out to be in a critical range for surface sodium enhancement after the 1DUP. Oxygen, on the other hand, is an interesting element to study since it has already been observed to be slightly deficient relative to iron not only in supergiants but also in giants of globular clusters (Brown & Wallerstein 1991) and open clusters (Luck 1994). Finally, the abundances of the iron-group elements are used to directly derive the metallicity of the two clusters. This direct technique, which contrasts with the classical, but less reliable, technique based on color indices and photometric calibrations, has so far been used only in a small number of open clusters (about 20, see Strobel 1991; Twarog et al. 1997). Our observations enable to enlarge that list.

The observational material is presented in Sect. 2 and the abundance determination procedure in Sect. 3. The metallicity is derived in Sect. 4, and the sodium and oxygen abundances analyzed in Sects. 5 and 6, respectively. Conclusions are drawn in Sect. 7.

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© European Southern Observatory (ESO) 2000

Online publication: August 17, 2000