Monte-Carlo Modeling of Extinct Short-lived r-Process Radioactivities

The recently discovered evidence for live Hf-182 in the early solar system opens up important new possibilities for constraining time scales for r-process nucleosynthesis in the Galaxy. The fascinating puzzle that emerges is that Hf-182 does not fit with the other short-lived r-process radioactivities. From one point of view, the inferred Hf-182/Hf-180 value in the early solar system, about 3 x 10 exp -4 agrees well with the value estimated to be present in molecular cloud cores. The problem is that in this case the other r-process radioactivities have meteoritic values much larger than their steady synthesis values in molecular clouds. On the other hand, allowing a long free-decay interval for steady-state I-129 to reach its meteoritic value would give too little Hf-182. An obvious conclusion would be that the r-process yields of the short-lived radioactivities can vary from supernova to supernova. For varying yields to be the explanation of the nonsteady-state abundances of the short-lived r-process radioactivities, contributions from individual supernovae must play a crucial role. In this case, we must find some way of following the individual supernovae. To do this, we have chosen to make Monte Carlo calculations of the galactic evolution of the bundances of the r-process short-lived radioactivities and the stable reference isotopes.