Abundances of light elements in metal-poor stars IV. [Fe/O] and [Fe/Mg] ratios and the history of star formation in the solar neighborhood

The accurate O, Mg and Fe abundances derived in previous papers of this series from a homogeneous reanalysis of high quality data for a large sample of stars are combined with stellar kinematics in order to discuss the history of star formation in the solar neighborhood. We found that the Fe/O and Fe/Mg abundance ratios are roughly constant in the (inner) halo and the thick disk; this means that the timescale of halo collapse was shorter than or of the same order of typical lifetime of progenitors of type Ia SNe (similar to 1 Gyr), this conclusion being somewhat relaxed (referring to star formation in the individual fragments) in an accretion model for the Galaxy formation. Both Fe/O and Fe/Mg ratios raised by similar to 0.2 dex while the O/H and Mg/H ratios hold constant during the transition from the thick to thin disk phases, indicating a sudden decrease in star formation in the solar neighbourhood at that epo;ch. These results are discussed in the framework of current views of Galaxy formation; they fit in a scenario where both dissipational collapse and accretions were active on a quite similar timescale.