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Astron. Astrophys. 356, 873-887 (2000)

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1. Introduction

The low metal abundances and high peculiar velocities of halo stars indicate that the halo is an old, if not the oldest component of the Milky Way. Age determinations of globular clusters and halo field stars point to an age of 14-15 billion years with no detectable age gradient with galactocentric distance (Harris et al. 1997). The halo has therefore special significance for the formation of the Milky Way. There are two quantities which play important rôles in the investigation of the formation of the halo: the orbits of halo stars and their chemical composition. Since halo stars form a collisionless system, their orbits contain information about the dynamics at the time of star formation and thus the formation of the halo (e.g. Carney et al. 1996; Chiba & Yoshii 1998). Information about the chemical composition of the interstellar medium (ISM) of the halo is more direct. The element abundances observed in low mass halo stars directly reflect the chemical abundances and the chemical inhomogeneity of the ISM during halo formation (McWilliam 1997).

Examinations of element abundance ratios as function of metallicity [Fe/H] show that star-to-star differences rise with decreasing metallicity (Ryan et al. 1996). Most of the chemical elements are ejected during supernovae Type II (SNe II) explosions. The enrichment of the halo depends on how many SNe II explode and how effectively the ejected gas is mixed with the surrounding ISM. If the ejected metals are distributed over a large volume, a spatially homogeneous enrichment takes place. If the mixing volume is small, the ISM in the vicinity of a core-collapse supernova (SN II) is highly enriched, while large parts of the halo gas remain metal-poor. In this case the ISM is chemically highly inhomogeneous and newly formed stars are of different chemical composition, depending on where they form. In this scenario one should moreover expect that the metal-poorest stars have chemical compositions corresponding to the stellar yields of single SNe II (Ryan et al. 1996).

In this paper we present a stochastic chemical evolution model and investigate the inhomogeneous enrichment of the halo ISM. The description of the model is given in Sect. 2, followed by an overview of theoretical SN II yields and their uncertainties in Sect. 3. The employed observational data is presented in Sect. 4. The results of our model and the conclusions are given in Sect. 5 and 6, respectively.

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© European Southern Observatory (ESO) 2000

Online publication: April 17, 2000