r-Process Radioisotopes from Near-Earth Supernovae and Kilonovae

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 the live isotope 60 Fe on the Earth and Moon implies that one or more astrophysical explosions occurred near the Earth within the last few Myr, probably a SN. Intriguingly, several groups have reported evidence for deposits of 244 Pu, some overlapping with the 60 Fe pulse. However, the putative 244 Pu flux appears to extend to at least 12 Myr ago, pointing to a different origin. Motivated by this observation, we propose that ejecta from a KN enriched the giant molecular cloud that gave rise to the Local Bubble in which the Sun resides. Accelerator Mass Spectrometry (AMS) measurements of 244 Pu 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., 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 244 Pu, other r-process species such as 93 Zr, 107 Pd, 129 I, 135 Cs, 182 Hf, 236 U, 237 Np and 247 Cm should be present. Their abundances could distinguish between SN and KN scenarios, and we discuss prospects for their detection in deep-ocean deposits and the lunar regolith. We show that AMS 129 I measurements in Fe-Mn crusts already constrain a possible nearby KN scenario.

Future radioisotope measurements to clarify the origin of deep-ocean 244Pu

“…The yellow band indicates the observed 60 Fe/ 244 Pu ratio [18] for 3 Mya. We also show the 60 Fe/ 244 Pu ratios calculated [17] in forced ν wind and MHD SN models (SA and SB), and in KN models (KA and KB). We present results for each model both without and including an additional nonr-process SN source of 60 Fe at 100 pc; calculations are for events 3 Mya, but with a 10 Myr kilonova in the two-step KA/B+SNnonr models.…”

“…These are interesting because the 244 Pu would have been produced by the astrophysical r-process, whose likely sites include neutron-star mergers (kilonovae, KNe), and possibly some atypical SNe. Motivated by these hints, in a recent paper we studied possible signatures of SN and KN r-process events, analyzing the potential implications of 244 Pu detection, estimating the strengths of other rprocess radioisotope signatures, and discussing how they could help distinguish between potential sites [17].…”

“…SNe may produce the r-process via a ν-driven wind or via magnetohydrodynamic (MHD) effects, but both struggle to make actinides, see [17] and references therein. If SNe are confirmed as robust sources of actinides such as 244 Pu, the available models must have major omissions.…”

“…If SNe are confirmed as robust sources of actinides such as 244 Pu, the available models must have major omissions. We thus discussed in Wang et al [17] a modified FIG. 1.…”

“…1 results from these models, both without and including ordinary (non-r-process) SN 60 Fe production. Our calculations are made using the nuclear reaction network code Portable Routines for Integrated nucleoSynthesis Modeling (PRISM) [21,22], as implemented in Wang et al [17], with baseline nuclear data from [23] and [24] (FRDM+QRPA), and variations in the masses [25] (HFB), β-decay rates [26] (MKT), and fission yields [27]. The non-r-process SN 60 Fe yields are for an explosion at 100 pc with M ej,60 ∼ 10 −4.5 M with an uncertainty discussed in the Supplemental Materials.…”