138La anomaly in the early solar system

For every 1100 lanthanum atoms in the solar system, only one is La-138. Relative to this low abundance, even a tiny additional La-138 made by irradiating its more abundant neighboring nuclides with energetic protosolar flare particles would cause a large, hence detectable, percentage increase in La-138. Such early solar irradiation can produce many now-extinct short-lived radio nuclides (e.g., Ca-41, Mn-53, and Al-26) and is the only way to make the newly discovered Be-10 and the possibly detected Be-7, because stars destroy rather than produce Be. The alternative hypothesis to produce extinct nuclides is the injection of freshly synthesized radioactivity from a nearby asymptotic giant branch star or supernovae during solar system formation. Hoping to clarify the origin of extinct nuclides, we have been searching for La-138 excess and its possible correlation with extinct nuclides. Here we report the detection of up to 0.6% (similar to 7.5 sigma) La-138 excesses in five Allende calcium-aluminum-rich inclusions. Surprisingly, they do not correlate with 26Al, thus offering no support for making 26Al by early irradiation. Instead, La-138 excess correlates with Ti-50 excess. Current nucleosynthesis models produce Ti-50 in a rare subset of Type I supernovae whose core underwent significant gravitational collapse before carbon deflagration. Our observed correlation thus suggests that 138La also came from these rare sources, perhaps in the mantle of the white dwarf, by reactions induced by the neutrino burst emitted during core neutronization. After the explosion, La-138 was incorporated into Ti-rich dusts that later became the building material of our solar system.