EVOLUTION OF SPIRAL GALAXIES .4. THE THICK DISK IN THE SOLAR REGION AS AN INTERMEDIATE COLLAPSE PHASE

We extend the multiphase nonlinear approach for galactic evolution to a three-zone description of the Milky Way, where halo, thick disk, and thin disk populations have been considered as coupled but distinguishable components of the same unitary system which is our Galaxy. The attention is focused on the possible delays between the main star formation episodes in each zone, as a natural consequence of the formation of more flattened structures via the accumulation of diffuse gas. A main consequence of this is a better fit of the G-dwarf metallicity distribution in the thin-disk component. Another consequence is the simultaneous presence of three populations, kinematically and chemically distinguished, which prevents us from drawing a simple unique evolutionary curve for the evolution of element abundances. The presence of these distinct stellar populations can provide a natural explanation for part of the observed abundance spread in the solar neighborhood.