r-Process Radioisotopes from Near-Earth Supernovae and Kilonovae

The astrophysical sites where r-process elements are synthesized remain mysterious: it is clear that neutron star mergers (kilonovae (KNe)) contribute, and some classes of core-collapse supernovae (SNe) are also possible sources of at least the lighter r-process species. The discovery of Fe-60 on the Earth and Moon implies that one or more astrophysical explosions have occurred near the Earth within the last few million years, probably SNe. Intriguingly, Pu-244 has now been detected, mostly overlapping with Fe-60 pulses. However, the Pu-244 flux may extend to before 12 Myr ago, pointing to a different origin. Motivated by these observations and difficulties for r-process nucleosynthesis in SN models, we propose that ejecta from a KN enriched the giant molecular cloud that gave rise to the Local Bubble, where the Sun resides. Accelerator mass spectrometry (AMS) measurements of Pu-244 and searches for other live isotopes could probe the origins of the r-process and the history of the solar neighborhood, including triggers for mass extinctions, e.g., that at the end of the Devonian epoch, motivating the calculations of the abundances of live r-process radioisotopes produced in SNe and KNe that we present here. Given the presence of Pu-244, other r-process species such as Zr-93, Pd-107, I-129, Cs-135, Hf-182, U-236, Np-237, and Cm-247 should be present. Their abundances and well-resolved time histories could distinguish between the SN and KN scenarios, and we discuss prospects for their detection in deep-ocean deposits and the lunar regolith. We show that AMS I-129 measurements in Fe-Mn crusts already constrain a possible nearby KN scenario.

Automated summary

The astrophysical sites where r-process elements are synthesized are still uncertain, but neutron star mergers and certain core-collapse supernovae are possible sources. Recent discoveries of Fe-60 and Pu-244 on Earth and the Moon suggest that astronomical explosions, possibly supernovae, have occurred near Earth within the last few million years. This has led to discussions on the origins of r-process elements and the potential history of the solar neighborhood, with the focus on distinguishing between supernova and kilonova scenarios for the synthesis of these elements.