2002ApJ...575..845F

Recent halo-star observations of heavy elements produced by rapid neutron capture (r-process) show a striking behavior: within a single star, the relative abundances of r-process elements heavier than Ba are the same as those of solar system matter, while across stars with similar metallicity Fe/H, the r/Fe ratio varies by more than 2 orders of magnitude. In this paper we present a simple analytic model that describes a star's abundances in terms of its ``ancestry'', i.e., the number of nucleosynthesis events (e.g., supernova explosions) that contributed to the star's composition. This model leads to a very simple analytic expression for the abundance scatter versus Fe/H, which is in good agreement with the data and with more sophisticated numerical models. We investigate two classes of scenarios for r-process nucleosynthesis, one in which r-process synthesis events occur in only ∼4% of supernovae but iron synthesis is ubiquitous, and one in which iron nucleosynthesis occurs in only about 9% of supernovae (the Wasserburg-Qian model). We find that the predictions in these scenarios are similar for [Fe/H]≳-2.5, but that these models can be readily distinguished observationally by measuring the dispersion in r/Fe at [Fe/H]≲-3.