r-Process Radioisotopes from Near-Earth Supernovae and Kilonovae

TL;DR

This paper explores the origins of r-process radioisotopes like 244Pu in the solar neighborhood, proposing that kilonova ejecta, rather than supernovae, enriched the local environment, with implications for understanding nearby astrophysical events.

Contribution

It introduces a model where kilonova ejecta enriched the Local Bubble, and discusses how measurements of various r-process isotopes can distinguish between supernova and kilonova origins.

Findings

AMS measurements of 244Pu and 129I constrain nearby kilonova scenarios.

Detection of multiple r-process isotopes can differentiate between supernova and kilonova sources.

The presence of 244Pu suggests a recent nearby astrophysical event, likely a kilonova.

Abstract

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 likely sources of at least the lighter r-process species. The discovery of 60Fe 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, 244Pu has now been detected, mostly overlapping with 60Fe pulse. However, the 244Pu 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 244Pu 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 244Pu, other r-process species such as 93Zr, 107Pd, 129I, 135Cs, 182Hf, 236U, 237Np and 247Cm 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 129I measurements in Fe-Mn crusts already constrain a possible nearby KN scenario.