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 ser ies from a homogeneous reanalysis of high quality data for a large sample o f stars are combined with stellar kinematics in order to discuss the histor y of star formation in the solar neighborhood. We found that the Fe/O and F e/Mg abundance ratios are roughly constant in the (inner) halo and the thic k 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 (simila r to 1 Gyr), this conclusion being somewhat relaxed (referring to star form ation in the individual fragments) in an accretion model for the Galaxy for mation. 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 th in 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 di ssipational collapse and accretions were active on a quite similar timescal e.