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

Abstract

While the abundances of Be and B observed in metal-poor halo stars are well explained as resulting from spallation of CNO-enriched cosmic rays (CRs) accelerated by supernova shocks, accounting for the observed 6Li in such stars with supernova CRs is more problematic. Here we propose that gravitational shocks induced by infalling and merging subgalactic clumps during hierarchical structure formation of the Galaxy should dissipate enough energy at early epochs so that the CRs accelerated by such shocks can provide a natural explanation of the observed 6Li. In clear contrast to supernovae, structure formation shocks do not eject freshly synthesized CNO nor Fe, so that ?-? fusion, which is the only effective production channel at low metallicity, is capable of generating sufficient 6Li with no accompanying Be or B and no direct correspondence with Fe. Correlations between the 6Li abundance and the kinematic properties of the halo stars may also be expected in this scenario. Further, more extensive observations of 6Li in metal-poor halo stars, e.g., by the Subaru High Dispersion Spectrograph or the Very Large Telescope UV-Visual Echelle Spectrograph, may offer us an invaluable fossil record of dissipative dynamical processes that occurred during the formation of our Galaxy.