ar X iv : a st ro - p h / 04 11 56 9 v 1 1 9 N ov 2 00 4 Li , Be , B and Cosmic Rays in the Galaxy

A short overview is presented of current issues concerning the production and evolution of Li, Be and B in the Milky Way. It is argued that the currently popular idea that Galactic Cosmic rays are accelerated inside metal-rich superbubbles (which leads “naturally” to the production of primary Be and B, as observed) encounters the same problems as the previously popular idea of supernovae accelerating their own ejecta. A major challenge to theories of light element production is presented by the recent (and still preliminary) data suggesting a surprisingly high and ∼constant abundance of Li in halo stars; attempts to explain such a “plateau” are critically examined. 1 The 1970s: problems with late Li, B The idea that the light and fragile elements Li, Be and B are produced by the interaction of the energetic nuclei of galactic cosmic rays (CGR) with the nuclei of the interstellar medium (ISM) was introduced 35 years ago (Reeves et al. 1970, Meneguzzi et al. 1971). In those early works it was shown that, taking into account the relevant cross-sections and with plausible assumptions about the GCR properties (injected and propagated spectra, intensities etc.; see Fig. 1, right column) one may reproduce reasonably well the abundances of those light elements observed in meteorites and in GCR. Two problems were identified with the GCR production, compared to meteoritic composition: the Li/Li ratio (∼2 in GCR but ∼12 in meteorites) and the B/B ratio (∼2.5 in GCR but ∼4 in meteorites). Modern solutions to those problems involve stellar production of ∼70% of Li (in the hot envelopes of AGB stars and/or novae) and of ∼40% of B (through ν-induced spallation of C in SNII). In both cases, however, uncertainties in the yields are such that observations are used to constrain the yields of the candidate sources rather than to confirm the validity of the scenario. 2 The 1980s: the Li plateau; primordial, but low or high? One of the major cosmological developments of the 1980s was the discovery of the Li plateau in low metallicity halo stars (Spite and Spite 1982, see Fig. 1 top left panel). The unique behaviour of that element, i.e. the constancy of the Li/H ratio with metallicity, strongly suggests a primordial origin. The observed value has been extensively used (along with those of D and He) to constrain the