Cosmic‐Ray Production of6Li 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 wellexplained as resulting from spallation of CNO-enriched cosmic rays (CRs)accelerated by supernova shocks, accounting for the observed $^6$Li in suchstars with supernova CRs is more problematic. Here we propose thatgravitational shocks induced by infalling and merging sub-Galactic clumpsduring hierarchical structure formation of the Galaxy should dissipate enoughenergy at early epochs, and CRs accelerated by such shocks can provide anatural explanation of the observed $^6$Li. In clear constrast to supernovae,structure formation shocks do not eject freshly synthesized CNO nor Fe, so thatthe only effective production channel at low metallicity is $\alpha-\alpha$fusion, capable of generating sufficient $^6$Li with no accompanying Be or Band no direct correspondence with Fe. Correlations between the $^6$Li abundanceand the kinematic properties of the halo stars may also be expected in thisscenario. Further, more extensive observations of $^6$Li in metal-poor halostars, e.g. by the Subaru HDS or VLT/UVES, may offer us an invaluable fossilrecord of dissipative dynamical processes which occurred during the formationof our Galaxy.Comment: Ap.J. in press; 6 pages, 1 figur