Early chemical evolution of the Milky Way

Abstract

The earliest phases of the chemical evolution of our Galaxy are analysed in the light of the recent VLT results (concerning abundance patterns in the most metal-poor stars of the Galactic halo) and of stellar nucleosynthesis calculations. It is argued that, among the various suggestions made in order to explain the observed abundance patterns, nucleosynthesis in asymmetric supernova explosions appears most promising. The data suggest a correlation between asymmetry and metallicity, which is hard to justify theoretically. The data also confirm the absence of dispersion in abundance ratios, at least up to the Fe peak, in the early Galaxy; this may be related to the (presently poorly known) timescales of homogeneisation of the interstellar medium, but also on small yield variations among massive stars. Finally, the metallicity distribution of halo stars may provide important constraints on the formation of the Milky Way; at present, it is not clear whether observations support the hierarchical formation scenario.