Cosmic-ray nucleosynthesis and the infall rate of extragalactic matter in the solar neighborhood

Abstract

We present an up-to-date discussion of the galactic abundances of lithium, beryllium, and boron, and show that new insights into a number of astrophysical problems arise from recent improvements in the quantity and quality of the data.The observed galactic abundances of these elements remain surprisingly constant in time, at values significantly lower than predictions made by conventional models of closed galactic evolution. This fact suggests that intergalactic hydrogen may enter our neighborhood. We favor a local infall rate of about 2 M/sub sun/ per 10/sup 9/ years per pc/sup 2/, and discuss the rather large uncertainties on this rate.The measured ratios of element and isotope abundances among the light elements suggest that the average energy spectrum of fast particles in our galactic neighborhood turns up below 50--100 MeV per nucleon. Such a turnup implies that two source families for cosmic rays must exist, perhaps supernovae and ordinary stars.In carbonaceous chondrites, not only boron, but apparently also beryllium and perhaps even lithium, seem to be enriched beyond their stellar abundance values.And even after many years of study, the origin of /sup 7/Li remains uncertain.