We have shown that the SB models predict a scatter in the L/M ratios observed in halo stars. In the next few years, the statistical analysis of this scatter (measured thanks to smaller error bars) should provide information about the SB dynamics and the star formation mechanism around SBs. The accumulation of data should allow us to distinguish between the two current SB models (good or poor mixing of the gas inside and around SBs).
The models also predict a `bi-modal' production of Be and B in the early Galaxy, with collective SNe giving rise to a high-efficiency mechanism providing the observed L/M ratios (SB model), and isolated SNe giving rise to a low-efficiency mechanism and L/M ratios 10 to 30 times lower (SNR model). Both processes are local, respectively inside superbubbles (or their shells) and inside SNRs, and independent of the ambient ISM metallicity, as required by the observed or inferred constancy of the L/M ratios at very-low metallicity. In addition to these processes, the standard GCRN is expected to occur on the Galactic scale, but at a lower rate until the ISM metallicity reaches about 3 to 10% of the solar metallicity. In any case, if two populations of stars can be identified with respectively high and low L/M ratios, the determination of their relative weight will give information about the statistics of SN explosions in OB associations.
We also predict that Be will be found more deficient than B in the so-called B-depleted stars, of which HD 160617 could only be a first example. On the other hand, if low-metallicity stars can be observed with both strongly deficient Be and B, with approximately the same apparent "depletion", this would imply that the primary component, i.e. the -process, is not dominant for B, and that we have to find an other process to account for the observed 11B/10B ratio. This would put a strong constraint on light element production, probably requiring the existence of very abundant low-energy `cosmic-rays' ( MeV/n), powered by an energy source still to be determined.
© European Southern Observatory (ESO) 2000
Online publication: April 17, 2000