Cosmic Ray Production of Lithium-6 by Structure Formation Shocks in the Early Milky Way: A Fossil Record of Dissipative Processes during Galaxy Formation

While the abundances of Be and B observed in metal-poor halo stars are well explained as arising from spallation of CNO-enriched cosmic rays (CRs) accelerated by supernova (SN) shocks, accounting for the observed $^6$Li in such stars with SN CRs is more problematic. Here we propose that gravitational shocks induced by infalling and merging sub-Galactic clumps during hierarchical structure formation in the early Galaxy should dissipate enough energy at early epochs, so that CRs accelerated by such shocks can provide a natural explanation of the observed $^6$Li. In clear constrast to SNe, structure formation shocks do not eject freshly synthesized CNO nor Fe, and the only effective production channel at low metallicity is $\alpha-\alpha$ fusion, capable of generating sufficient $^6$Li with little Be/B production 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 High Dispersion Spectrograph, may offer us an invaluable fossil record of dissipative dynamical processes which occurred during the formation of our Galaxy.