Early star formation in the Galaxy from beryllium and oxygen abundances

We investigate the evolution of the star formation rate in the early Galaxy using beryllium and oxygen abundances in metal poor stars. Specifically, we show that stars belonging to two previously identified kinematical classes (the so-called ``accretion'' and ``dissipative'' populations) are neatly separated in the [O/Fe] vs. Log(Be/H) diagram. The dissipative population follows the predictions of our model of Galactic evolution for the thick disk component, suggesting that the formation of this stellar population occurred on a timescale significantly longer (by a factor ~5-10) than the accretion component. The latter shows a large scatter in the [O/Fe] vs. Log(Be/H) diagram, probably resulting from the inhomogeneous enrichment in oxygen and iron of the protogalactic gas. Despite the limitation of the sample, the data suggest that the combined use of products of spallation reactions (like beryllium) and elemental ratios of stellar nucleosynthesis products (like [O/Fe]) can constrain theoretical models for the formation and early evolution of our Galaxy.