The revival of galactic cosmic-ray nucleosynthesis?

Because of the roughly linear correlation between Be/H and Fe/H in low-meta llicity halo stars, it has been argued that a "primary" component in the nu cleosynthesis of Be must be present in addition to the "secondary" componen t from standard Galactic cosmic-ray nucleosynthesis. In this paper we criti cally reevaluate the evidence for the primary versus secondary character of Li, Be, and B (LiBeB) evolution, analyzing both the observations and Galac tic chemical evolution models. Although it appears that [Be/H] versus [Fe/H ] has a logarithmic slope near 1, it is rather the Be-O trend that directly arises from the physics of spallation production. Using new abundances for oxygen in halo stars based on UV OH lines, we find that in Population II s tars for which O has been measured, the Be-O slope has a large uncertainty due to systematic effects. Namely, the Be-O logarithmic slope lies in the r ange 1.3-1.8, rendering it difficult to distinguish from the data between t he secondary slope of 2 and the primary slope of 1. The possible difference between the Be-Fe and Be-O slopes is a consequence of the variation in O/F e versus Fe: recent data suggest that the best-fit O/Fe-Fe slope for Popula tion II is in the range -0.5 to -0.2, rather than zero (i.e., Fe proportion al to O) as is often assumed. In addition to this phenomenological analysis of Be and B evolution, we have also examined the predicted LiBeB, O, and F e trends in Galactic chemical evolution models that include outflow. Based on our results, it is possible that a good fit to the LiBeB evolution requi res only the traditional Galactic cosmic-ray spallation and the (primary) n eutrino-process contribution to B-11. We thus suggest that these two proces ses might be sufficient to explain Li-6, Be, and B evolution in the Galaxy, without the need for an additional primary source of Be and B. However, th e uncertainties in the data at this time prevent one from reaching a defini tive conclusion. Fortunately, several observational tests of this "neoclass ical" scenario are available; we note in particular the importance of furth er observations to secure the O/Fe Population II trend, as well as accurate measurements of B/Be, Li-6/Be, and B-11/B-10 in halo stars.