Abundances of Actinides and Short-lived Nonactinides in the Interstellar Medium: Diverse Supernova Sources for the r-Processes

Abstract

Abundances of ^(244)Pu, ^(235)U, ^(238)U, and ^(232)Th in the early solar system are about those expected for uniform production over most of galactic history. The inferred abundance of 182Hf is also compatible with this model. We here associate production of ^(182)Hf with the same r-process SN sources that produce actinides (SNACS). This requires that r-process nucleosynthesis in SNACS took place rather uniformly over the age of the galaxy until ~10^7 yr prior to solar system formation. The low abundance of ^(107)Pd and ^(129)I in the early solar system indicates that SNACS cannot produce these nuclei at the high yields expected from standard r-process models. We propose that there are distinctive SN sources for different r-process nuclei with a sharp distinction in different SN contributions below and above A ~ 140. Abundances in stars with very low metallicities will vary depending on the type of SN contributing to the local region of star formation. A time scale of ~10^7 yr is much shorter than the 108 yr time usually associated with processes in the galaxy and with the last time of r-process injection accounting for ^(129)I, but may be compatible with the rate of SN occurrence. The hypothesis of a nearby SN polluting the protosolar nebula is critically discussed.

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