Light element production in the circumstellar matter of Type Ic supernovae at low metallicity

We investigate energetic Type Ic supernovae as production sites for ⁶Li and Be in the early stages of the Milky Way. Recent observations have revealed that some very metal-poor stars with [Fe/H]< -2.5 possess unexpectedly high abundances of ⁶Li. Some also exhibit enhanced abundances of Be as well as N. From a theoretical point of view, recent studies of the evolution of metal-poor massive stars show that rotation-induced mixing can enrich the outer H and He layers with C, N, and O (CNO) elements, particularly N, and at the same time cause the intense mass loss of these layers. Here we consider energetic supernova explosions occurring after the progenitor star has lost all but a small fraction of the He layer. The fastest portion of the supernova ejecta and the circumstellar matter (CSM), both of which are composed of He and CNO, can interact directly and induce light-element production through spallation and He-He fusion reactions. The CSM should be sufficiently thick to energetic particles so that the interactions terminate within its innermost regions. We calculate the resulting ⁶Li/O and ⁹Be/O ratios in the ejecta+CSMmaterial out of which the very metal-poor stars may form. We find that they are consistent with the observed values if the mass of the He layer remaining on the pre-explosion core is ∼0.01-0.1 Ṁand if the mass fraction of N mixed in the He layer is ∼0.01. Further observations of ⁶Li, Be, and N at low metallicity should provide us with critical tests of this production scenario.