Standard cosmic ray energetics and light element production

Abstract

The recent observations of an approximately linear relationship between both Be and B and iron in metal-poor stars has led to a reassessment of the origin of the light elements in the early Galaxy. In addition to standard secondary production of BeB, it is necessary to introduce a production mechanism which is independent of the interstellar metallicity (primary), and in which freshly synthesized C, O and He are accelerated by supernova shock waves. Primary mechanisms are expected to be dominant at low metallicity. At metallicities higher than [{\rm O/H}] \ga -1.75, some existing data indicate that secondary production is dominant. In this paper, we focus on the secondary process, related to the standard Galactic cosmic rays, and we examine the cosmic ray energy requirements for both present and past epochs. We find the power input to maintain the present-day Galactic cosmic ray flux is about 1.5 1041 erg/s = 5 1050 erg/century; this estimate includes energy losses from both the escape of high-energy particle and ionization losses from low-energy particles. This implies that, if supernovae are the sites of cosmic ray acceleration, the fraction of explosion energy going to accelerated particles is about ~30% , a value which we obtain consistently both from considering the present cosmic ray flux and confinement and from the present 9Be and 6Li abundances. Using the abundances of 9Be (and 6Li) in metal-poor halo stars, we extend the analysis to show the effect of the interstellar gas mass on the standard Galactic cosmic ray energetic constraints on models of Li, Be, and B evolution. The efficiency of the beryllium production per erg may be enhanced in the past by a factor of about 10; thus the energetic requirement by itself cannot be used to rule out a secondary origin of light elements. Only a clear and indisputable observational determination of the O-Fe relation in the halo will discriminate between the two processes