Cosmic Ray Production of Lithium-6 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 $^6$Li in such stars with supernova CRs is more problematic. Here we propose that gravitational shocks induced by infalling and merging sub-Galactic clumps during hierarchical structure formation of the Galaxy should dissipate enough energy at early epochs, and CRs accelerated by such shocks can provide a natural explanation of the observed $^6$Li. In clear constrast to supernovae, structure formation shocks do not eject freshly synthesized CNO nor Fe, so that the only effective production channel at low metallicity is $\alpha-\alpha$ fusion, capable of generating sufficient $^6$Li with no accompanying Be or B and no direct correspondence with Fe. Correlations between the $^6$Li abundance and the kinematic properties of the halo stars may also be expected in this scenario. Further, more extensive observations of $^6$Li in metal-poor halo stars, e.g. by the Subaru HDS or VLT/UVES, may offer us an invaluable fossil record of dissipative dynamical processes which occurred during the formation of our Galaxy.