THE EVOLUTION OF BE-9

In the light of recent observations of the Be/H ratio in stars with very low iron content, we reassess the evolution of beryllium. A dedicated Galactic evolutionary model allows us to follow the history of 9Be from the birth of the Galaxy up to now. The star formation rate determines both the production of this isotope though the cosmic-ray intensity, itself taken as proportional to the number of potential accelerators (essentially supernovae and Wolf-Rayet stars) and its destruction by astration. Various star formation rates and initial mass functions are tried. We conclude the following. 1. The sharp rise of the Be/H ratio is essentially due to the rapid growth of the abundance of its progenitors C, N, and O in the interstellar medium, thanks to intense stellar nucleosynthesis and ejection in the early Galaxy Due to the lack of data this effect has been previously ignored, though acknowledged. 2. Infall of extragalactic matter has only a marginal importance in the beryllium behavior compared to other factors, as for instance the variation of the star formation rate. It has, however, as emphasized by previous authors, the interesting effect of keeping the Be/H ratio constant in the last billion years, as observed. So infall is not mandatory, but we cannot exclude it. 3. 9Be sets interesting constraints on the formation rate of massive stars in the first epochs of galactic evolution. Certain kinds of models with enhanced massive star formation at early times produce iron and oxygen too quickly, and therefore beryllium.