Production of lithium in the galactic disk

The 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 Galax y is log N(Li)= 2.2(+/- 0.2). The global evidence suggests that the latter represents the full, or the major, part of the primordial abundance, so tha t the difference of an order of magnitude is due to Li produced in the Gala xy. It is well known that spallation of interstellar CNO by He-4 and proton s in Galactic cosmic rays (GCRs) can produce Li, but models yield a shortfa ll of almost an order of magnitude compared with the current observed abund ance range. Another GCR reaction, alpha + alpha fusion, has been invoked to explain some Li production in the early Galaxy, but application of this to the disk yielded too much early Li or too little current Li. These failure s led to a search for alternative mechanisms, essentially stellar, at parti cular phases of evolution: the helium flash phase in asymptotic giant branc h stars, in novae, and in supernovae (SNe). Here we stress the importance of the observed upper envelope in the plot of Li versus Fe in stars as a constraint on any mechanism in any model aiming to account for disk Li. We show that a good match can be found assuming th at low-energy GCRs produce the Li, with the alpha + alpha reaction as the k ey mechanism, although production in supernovae cannot at this stage be exc luded. There is an apparent time delay in the Li production, relative to O and Fe, which if con firmed could be explained by the origin of a low-energ y alpha-particle component in processes associated with stars of intermedia te and low mass. The alpha-flux at a given epoch would then be proportional to the amount of gas expelled by low- and intermediate-mass stars in the G alaxy, though the acceleration of these alpha-particles could still be link ed to more energetic events as supernova explosions. The present scenario a ppears to account coherently for the closely related observations of the te mporal evolution in the Galaxy (halo + disk) of abundances of C-12, C-13, N -14, O-16, Fe-26, the two main peaks (one in the halo and one in the disk) in the G-dwarf stellar frequency distribution, and the evolution of Be-9 an d B-10 + B-11 via GCR spallation reactions without requiring the very high local cosmicray fluxes implied by the spallation close to SN. Adding a natu ral mechanism of differential depletion in red supergiant envelopes, we can explain the observed time evolution of the abundance of D and that of the isotopic ratios Li-7/Li-6 and B-11/B-10 Starting from a standard big bang n ucleosynthesis model with baryon density similar to 0.05. Our model also pr edicts the second Li "plateau" found for [Fe/H] between -0.2 and + 0.2, due to the "loop back" implied for Li (also for Be-9 and B) because of the req uired infall of low-metallicity gas to the disk. Without ruling out other m echanisms for the main production of Li in the Galactic disk, the low-energ y alpha + alpha fusion reaction in the interstellar medium offers a promisi ng contribution.