¹⁸²Hf chronometer for the early Solar System

Abstract

It is now generally believed that ²⁶Al (t_1/2=7.2×10⁵ yr) and ¹⁰⁷Pd (t_1/2=6.5×10⁶ yr) were present in the early Solar System¹. Thus, the nucleosynthetic event (supernova?) responsible for the production of these nuclei must have occurred no more than a few million years before the formation of solid bodies^2,3. (However, an alternative model based on grain carriers of extinct radioactivities has also been discussed^4,5.) It is possible that this event also produced the ¹²⁹I believed to be present in the early Solar System^6–8. However, the last event to contribute ²⁴⁴Pu to the Solar System occurred ∼10⁸ yr before the time of solidification^9,10. This latter time scale is also consistent with the lack of evidence for a ²⁴⁷Cm chronometer^11,12. We propose here that ¹⁸²Hf (t_1/2=9×10⁶ yr) can resolve the question of whether heavy-element non-actinide nucleosynthesis occurred during the ²⁶Al-producing event. The answer to this question will help to clarify the chronology of the formation of the Solar System and will help to determine the astrophysical sites of heavy-element nucleosynthesis.