Light Element Production in the Galactic Disc by Delayed Low Energy Cosmic Ray Flux from Intermediate Mass Stars

Abstract

The mean abundance of Li in stars formed within the past 5 Gyr is log N(Li) = 3.2(±0.2) while the corresponding value for the oldest stars in the galaxy is log N(Li) = 2.2(±0.2). As long as the potential source of Li in the galactic disc cannot be identified there remains reasonable doubt about whether the latter represents the primordial Li abundance. Spallation of interstellar CNO nuclei by the ⁴ He and proton components of galactic cosmic rays (GCR) was proposed, over 20 years ago, as a source of Li but production falls short by an order of magnitude. Recently α + α fusion was invoked to explain some Li production in the early Galaxy, but this reaction yielded, apparently, to much Li in the old disc, or too-little Li in young stars.

Failures of this sort led to the proposal of other Li-producing mechanisms: in the helium flash in AGB stars, in the nova phase, and even at supernova in specific shock mechanisms. Here we show that by assuming that the low energy GCR flux depends on the gas expulsion rate from low and intermediate mass stars rather than the supernova rate we obtain a model which explains the log N(Li) v. log N(Fe) evolution in the solar neighborhood. Incorporating interstellar depletion in the hot plasma around OB associations we can account for the observed ⁷ Li/⁶ Li ratios, as well as the ¹¹ B/¹⁰ B ratio in the solar system, and can give a fair description of the secular decline in the D/H ratio during the disc lifetime.