3. Intrinsic scatter in the L/M ratios
As recalled above, the constancy of the L/M ratios in the framework of SB models relies on the mixing of the primary SN ejecta with the secondary light elements produced by the SBEPs, before the formation of a new generation of stars. In practice, however, such a mixing cannot be perfect and the value of the local L/M ratio is expected to vary from one place to another. In addition, the formation of new stars can occur before all the massive stars explode and/or the induced LiBeB production occurs. As a result, stars with somewhat different L/M ratios should form from a given superbubble, and this should be observed as a scatter in the Be and B data. This is a common prediction of any SB model. However, quantitatively, the amplitude of the scatter depends on the mixing of the gas inside the SB, and of the SB gas with the ambient supershell. Therefore, a model which assumes that the SN ejecta are well mixed with the ISM evaporated inside the SB (Parizot & Drury) predicts a smaller dispersion than a model in which the SBEPs are almost pure ejecta (Ramaty et al.), not diluted with ambient gas. The exact distribution of the L/M ratios expected in the framework ot these two models is not calculated in this paper, because it depends on the details of the gas dynamics inside the SB and the surrounding shell, as well as on the star formation dynamics. Instead, we argue that the accumulation of Be, B and O data could optimistically provide an interesting way to constrain the models, through the statistical description of the scatter in the elemental ratios.
In fact, one might already conclude from the very existence of a well defined correlation between Be and O (or B and O) that the secondary light elements must be quite well mixed with the primary ejecta (CNO) before new stars form. This is an argument in favour of our model, because a good mixing between the CNO inside the SB and the LiBeB produced in the SB shell first requires a good mixing of the gas inside the SB itself. However, stronger conclusions cannot be drawn until more data are available and the error bars become small enough to allow for a direct measure of the scatter in the L/M ratios. It is worth noting also that a larger dispersion should be found for Be than for B, since part of the boron is expected to be produced along with C and O in the course of SN explosions (by neutrino-spallation; Woosley, et al., 1990), and thus be `ready-mixed' inside the SBs.
The above source of scatter in the L/M ratios is inherent in the SB models. It results from the fact that the light elements are produced in a different place from CNO, namely in the shell rather than inside the SB, where the gas in well mixed. In the following section, we discuss another source of dispersion in the L/M ratios of low-metallicity stars, resulting from the fact that SBs do not cover the whole volume of the Galaxy and therefore an other Be and B production mechanism dominates in the regions distant from SBs.
© European Southern Observatory (ESO) 2000
Online publication: April 17, 2000