Extinct radioactivities and protosolar cosmic rays: Self-shielding and light elements

We study the effects of self-shielding in the X-wind model of protosolar cosmic-ray irradiation of early solar-system rocks. We adopt a two-component picture of protoCAIs consisting of cores with the elemental abundances of type B1 CAIs (calcium-aluminum-rich inclusions) and mantles of less refractory material. The cores have a power-law distribution of sizes between R-min and R-max. The mantles have a uniform thickness, whose value is chosen to bring the total inventory of elements at least as refractory as sulfur to cosmic abundances for the entire population of protoCAIs. Each object is irradiated with a fluence consistent with the product of their residence time in the reconnection ring and the flux of solar cosmic rays obtained by a scaling of impulsive flares from the hard X-rays observed from low-mass protostars. For R-min in the 50 mum regime and R-max in the few centimeter regime, which corresponds to the range of sizes of observed CAIs in micrometeorites and chondrites, we recover approximately the canonical values quoted for the ratios Al-26/Al-27, Mn-53/Mn-55, and Ca-41/Ca-40 in CV3 meteorites. Moreover, the excess La-138 (denoted as La-138*) produced by proton bombardment of Ba-138 lies within the CAI range obtained in the experiments of Shen et al. When we include fragmentation reactions that produce Be-10 from the impact of protons, alphas, and He-3 on the O-16 that is bound up in rocks, we further obtain a level of Be-10/Be-9 that agrees approximately with the report of McKeegan et al. for a CAI from the Allende meteorite. Similar calculations for the expected anomalies in the stable isotopes of lithium show rough consistency with the measured values and further support our interpretation. The value for Be-10/Be-9 is particularly difficult to produce by any other astrophysical mechanism. Thus, the Be-10 discovery greatly strengthens the case for an origin in early solar-system irradiation, rather than external stellar seeding, for the shortest-lived radionuclides inferred from CAIs in chondritic meteorites.