Superbubbles as the source of 6 Li , Be and B in the early Galaxy

We investigate the spallative production of the light elements, Li, Be and B (LiBeB), associated with the evolution of a superbubble (SB) blown by repeated supernovae (SNe) in an OB association. It is shown that if about ten percent of the SN energy can power the acceleration of particles from the material inside the SB, the observed abundances of LiBeB in halo stars, as a function of O, can be explained in a fully consistent way over several decades of metallicity. In this model, the energetic particles (EPs) reflect the SB material, which is a mixing of the ejecta of previous SNe and of the swept-up ISM gas evaporated off the shell. We investigated two different energy spectra for the EPs: the standard cosmic ray source spectrum, or ‘SNR spectrum’, and a specific ‘SB spectrum’, E−α exp(−E/E0), where α = 1.–1.5 and E0 is of order a few hundreds of MeV/n, as results from the SB acceleration mechanism of Bykov & Fleishman (1992). While the latter spectrum is more efficient in producing LiBeB, the SNR spectrum can be reconciled with the observational data if an imperfect mixing of the SN ejecta with the rest of the SB material and/or a selective acceleration is invoked (enhancing the C and O abundance amongst the EPs by a factor of ∼ 6). One of the main consequences of our model is that the observed linear growth of Be and B abundances as a function of Fe/H expresses a dilution line rather than a continuous, monotonic increase of the metallicity. We propose an observational test of this feature. We also show that the recent 6Li observations in halo stars fit equally well in the framework of the SB model. Finally, we conjecture the existence of two sets of low-metallicity stars, differing in their Be/Fe or B/O abundance ratios, resulting from a ‘bimodal’ LiBeB production in the Galaxy, namely from correlated (in SBs) or isolated SN explosions.