Chemical evolution of the milky way galaxy .3. Is the disk formed through mergers of fragments?

A model for the chemical evolution of the solar neighbourhood cylinder has been constructed assuming that i) the protogalaxy is a system of fragments which evolve independently before merge, ii) stars which have been formed in these fragments before merger produce the stellar halo, iii) the disk is formed from the gas of these fragments through the successive mergers of fragments. The evolution of oxygen and iron abundances has been considered. The model predictions agree with available observational data (age-metallicity relation, iron-to-oxygen abundances ratio as a function of the iron abundance, and abundance distribution function) both for the halo stars and for the solar neighbourhood disk stars. It has been found that the halo-star-producing fragments contain only a part of the gas of the protogalaxy, and that the rest of the protogalactic gas does not take part in the halo star formation.