Very metal poor stars in the Milky Way: constraints on stellar nucleosynthesis

Recently Cayrel et al. [Cayrel, R., Depagne, E., Spite, M. et al. 2004, AfeA 416, 1117] have derived the abundances of several a and Fe-peak elements for a sample of very metal poor giants ([Fe/H] from -4.0 to -3.0 dex) thus allowing us to test chemical evolution models in a metallicity range never reached before. Moreover, the small spread in the [el/Fe] ratios in the metallicity range from [Fe/H]=-4.0 up to -3.0 dex [Cayrel, R., Depagne, E., Spite, M. et al. 2004, AfeA 416, 1117] is a clear sign that the halo of the Milky Way was well mixed even in the earliest phases of its evolution. We computed the evolution of the abundances of O, Mg, Si, Ca, K, Ti, Sc, Ni, Mn, Co, Fe and Zn in the Milky Way. We made use of the most widely adopted nucleosynthesis calculations and compared the model results with observational data with the aim of imposing constraints upon stellar yields