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 co smic-ray irradiation of early solar-system rocks. We adopt a two-component picture of protoCAIs consisting of cores with the elemental abundances of t ype B1 CAIs (calcium-aluminum-rich inclusions) and mantles of less refracto ry 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 c osmic abundances for the entire population of protoCAIs. Each object is irr adiated 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 sc aling of impulsive flares from the hard X-rays observed from low-mass proto stars. For R-min in the 50 mum regime and R-max in the few centimeter regim e, which corresponds to the range of sizes of observed CAIs in micrometeori tes and chondrites, we recover approximately the canonical values quoted fo r the ratios Al-26/Al-27, Mn-53/Mn-55, and Ca-41/Ca-40 in CV3 meteorites. M oreover, the excess La-138 (denoted as La-138*) produced by proton bombardm ent 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 ca lculations for the expected anomalies in the stable isotopes of lithium sho w rough consistency with the measured values and further support our interp retation. The value for Be-10/Be-9 is particularly difficult to produce by any other astrophysical mechanism. Thus, the Be-10 discovery greatly streng thens the case for an origin in early solar-system irradiation, rather than external stellar seeding, for the shortest-lived radionuclides inferred fr om CAIs in chondritic meteorites.