Metal Enrichment in the Early Galactic Halo

The early history of metal enrichment in the galactic halo was studied. We investigated the chemical inhomogeneity by using a stochastic chemical evolution model. The model confronts the metallicity distribution function of long-lived halo stars, which has been found to be a clue to obtain the best model prescriptions. We have found that star formation in the halo virtually terminated by ∼ 1 Gyr and that the halo has never been chemically homogeneous in its star-formation history. This conclusion does not depend whether the mass loss from the halo is taken into account or not. The observed ratios of a-elements with respect to iron do not show scatters on a [a/Fe]-[Fe/H] plane, but this does not imply that interstellar matter in the halo was homogeneous because a chemical evolution path on this diagram is degenerate in the star-formation rate. On the other hand, an apparent spread of the [Sr/Fe] ratio among metal-poor halo stars does not reflect an inhomogeneous metal enrichment; instead, it is due to a sharp increase in the production rate of strontium, which is probably synthesised in slightly less-massive stars than the progenitor of iron-producing type II supernovae.