2. The superbubble models
In the very early Galaxy, the interaction of energetic particles (EPs) having the ISM composition would produce very little Be and B, because the C and O nuclei are so rare. Simple energetics considerations thus indicate that Be and B can be significantly produced only if the EPs have a composition richer in C and O than the global ISM. This is the reason why the only viable models proposed so far involve the acceleration of C- and O-rich material inside a superbubble (Higdon, et al., 1998; Parizot & Drury, 1999c,2000, Bykov, 1999; Ramaty et al., 1999; Bykov., et al., 2000). Indeed, repeated SNe occurring in an OB association are known to generate a superbubble (SB) with a typical radius of order 300 pc, filled with hot, tenuous gas, and composed of the metal-rich ejecta of the SNe having already exploded, possibly diluted by the swept-up ambient material (of essentially zero metallicity in the early Galaxy).
As discussed in detail in Parizot & Drury (1999c), the SB models for Be and B Galactic evolution are based on the following sequence of events: 1) CNO nuclei are ejected by SNe inside the superbubble; 2) the CNO nuclei are mixed with some ambient, metal-poor material; 3) the resulting material (including CNO) is accelerated; 4) LiBeB is produced by spallation through the interaction of these `superbubble energetic particles' (SBEPs) with the metal-poor material in the supershell and at the surface of the adjacent molecular cloud; 5) the LiBeB produced is mixed with the CNO ejected by the SNe, which leads to a unique value of the L/M ratios throughout the superbubble (and part of the supershell); 6) new stars form by condensation of this gas, after possible dilution by ambient, metal-poor gas (from the supershell or the adjacent molecular cloud. All these new stars then have the same L/M ratios, but possibly different overall metallicity.
Apart from this common `astrophysical background', the models proposed differ in some of their assumptions, notably relating to the composition and spectrum of the metal-rich EPs. Ramaty et al. note that the current composition and spectrum of the cosmic rays (CRs) provide a Be production efficiency sufficient to explain the high L/M ratios observed in halo stars. This is reminiscent of the original result of Meneguzzi et al. (1971), which is the heart of the GCRN scenario for light element production (Vangioni-Flam, et al., 1990; Fields & Olive, 1999): multiplying the light element production rates from GCRs by the age of the Galaxy, one obtains approximately the total amount of Be and B present today in the Galaxy. However, while the GCRN scenario assumes that the CR composition follows that of the ISM (i.e. is richer and richer in C and O) and therefore does not reproduce the primary behavior of Be and B in the early Galaxy, Ramaty et al. assume that the CR composition does not change during the whole Galactic evolution. This is indeed expected if the CRs are made of SN ejecta accelerated inside a superbubble, by the shock of subsequent SNe. Their composition is then almost independent of the ISM metallicity, provided that the SN ejecta are not well mixed with the ambient matter before the acceleration occurs.
In our model (Parizot & Drury, 1999c), we argue that an acceleration mechanism different from the diffusive shock acceleration could occur inside SBs, because of the specific physical conditions prevailing there (hot, tenuous gas, strong magnetic turbulence, multiple weak shocks...). Such a mechanism has been described by Bykov (1995,1999) and leads to a different energy spectrum, which we refer to as the `SB spectrum', and which is flatter than the cosmic-ray source spectrum (CRS) at low energy, say below a few hundreds of MeV/n (for a discussion, see Parizot & Drury, 2000). The actual shape of the spectrum above this `break' (whether a steep power-law or the standard CRS shape in ) is irrelevant here, since it does not affect the Be and B production efficiency. We adopt the following shape for the SB spectrum: up to MeV/n, and above.
As can be seen from Fig. 1, this spectrum makes the light element production more efficient than the standard CRS spectrum, so that the same amount of Be and B can be produced by less metal-rich EPs. In particular, the observed L/M ratios in halo stars can still be accounted for if one allows for a perfect mixing of the SN ejecta with the metal-free material swept-up and evaporated off the supershell, before the acceleration occurs (Parizot & Drury, 1999c,2000). This is suggested by the comparison between the mixing time inside a superbubble ( yr; see Parizot & Drury 1999c) and the typical age of a superbubble ( yr). In the following, we analyse the common and distinctive implications of the above models for the distribution of the L/M ratios in halo stars.
© European Southern Observatory (ESO) 2000
Online publication: April 17, 2000