Standard cosmic ray energetics and light element production

Abstract

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 [O/H] > ∼ −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 10 41 erg/s = 5 10 50 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 9 Be and 6 Li abundances. Using the abundances of 9 Be (and 6 Li) 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.

Cosmic‐Ray Production of6Li by Structure Formation Shocks in the Early Milky Way: A Fossil Record of Dissipative Processes during Galaxy Formation

“…We set à esc ¼ 100 g cm À2 in our calculations below, although the exact value is irrelevant so long as this is larger than à ion or à n;i . It is emphasized that under such conditions, the LiBeB production rate does not depend on the gas density n g (as opposed to the situation considered in, e.g., Fields et al 2001): since both ionization and nuclear destruction loss rates are proportional to n g , any increase (decrease) in the density of gas containing target particles is always compensated by a decrease (increase) in the propagated CR flux due to increased (decreased) losses.…”

Production and evolution of Li, Be, and B isotopes in the Galaxy

“…); the true scatter would be less than suggested by the figure. a similar reasoning was adopted in Fields et al (2001). Fields & Olive (1999) explored the possibility of a high O/Fe at low metallicities, which increases the contribution of the term Y ISM CNO .…”

Strong magnetic fields and cosmic rays in very young galaxies

“…The net Galactic cosmic ray injection power due to supernovae dW SN dt is proportional to the supernova rate and average mechanical energy output of a supernova times some efficiency which is transformed into cosmic rays. Since the supernova rate is proportional to the star formation rate dW SF dt one finally ends up with a scaling that Fields et al (2001) derived this intuitive scaling from quantitative arguments by a careful and consistent model for the energetics of cosmic ray acceleration and the abundances of LiBeB, which are due to cosmic ray nucleosynthesis in the interstellar medium. Doing so, they could go a step further and they derive an even more direct scaling of the star formation rate and the cosmic ray acceleration rate, as has been implicitly assumed by most previous work on early Galactic cosmic ray nucleosynthesis (see next section).…”