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 possible sources of at least the lighter r -process species. The discovery of 60 Fe 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, 244 Pu has now been detected, mostly overlapping with 60 Fe pulses. However, the 244 Pu 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 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., 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 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 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 129 I measurements in Fe–Mn crusts already constrain a possible nearby KN scenario.
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Inhomogeneous Enrichment of Radioactive Nuclei in the Galaxy: Deposition of Live 53 Mn, 60 Fe, 182 Hf, and 244 Pu into Deep-sea Archives. Surfing the Wave?
While modeling the galactic chemical evolution (GCE) of stable elements provides insights to the formation history of the Galaxy and the relative contributions of nucleosynthesis sites, modeling the evolution of short-lived radioisotopes (SLRs) can provide supplementary timing information on recent nucleosynthesis. To study the evolution of SLRs, we need to understand their spatial distribution. Using a three-dimensional GCE model, we investigated the evolution of four SLRs:53Mn,60Fe,182Hf, and244Pu with the aim of explaining detections of recent (within the last ≈1–20 Myr) deposition of live53Mn,60Fe, and244Pu of extrasolar origin into deep-sea reservoirs. We find that core-collapse supernovae are the dominant propagation mechanism of SLRs in the Galaxy. This results in the simultaneous arrival of these four SLRs on Earth, although they could have been produced in different astrophysical sites, which can explain why live extrasolar53Mn,60Fe, and244Pu are found within the same, or similar, layers of deep-sea sediments. We predict that182Hf should also be found in such sediments at similar depths.
more » « less- Award ID(s):
- 1927130
- NSF-PAR ID:
- 10397482
- Publisher / Repository:
- DOI PREFIX: 10.3847
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 944
- Issue:
- 2
- ISSN:
- 0004-637X
- Format(s):
- Medium: X Size: Article No. 121
- Size(s):
- ["Article No. 121"]
- Sponsoring Org:
- National Science Foundation
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Abstract 244Pu has recently been discovered in deep-sea deposits spanning the past 10 Myr, a period that includes two60Fe pulses from nearby supernovae.244Pu is among the heaviest
r -process products, and we consider whether it was created in supernovae, which is disfavored by nucleosynthesis simulations, or in an earlier kilonova event that seeded the nearby interstellar medium with244Pu that was subsequently swept up by the supernova debris. We discuss how these possibilities can be probed by measuring244Pu and otherr -process radioisotopes such as129I and182Hf, both in lunar regolith samples returned to Earth by missions such as Chang’e and Artemis, and in deep-sea deposits. -
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Abstract Analysis of inclusions in primitive meteorites reveals that several short-lived radionuclides (SLRs) with half-lives of 0.1–100 Myr existed in the early solar system (ESS). We investigate the ESS origin of107Pd,135Cs, and182Hf, which are produced by
slow neutron captures (thes -process) in asymptotic giant branch (AGB) stars. We modeled the Galactic abundances of these SLRs using theOMEGA+ galactic chemical evolution (GCE) code and two sets of mass- and metallicity-dependent AGB nucleosynthesis yields (Monash and FRUITY). Depending on the ratio of the mean-lifeτ of the SLR to the average length of time between the formations of AGB progenitorsγ , we calculate timescales relevant for the birth of the Sun. Ifτ /γ ≳ 2, we predict self-consistent isolation times between 9 and 26 Myr by decaying the GCE predicted107Pd/108Pd,135Cs/133Cs, and182Hf/180Hf ratios to their respective ESS ratios. The predicted107Pd/182Hf ratio indicates that our GCE models are missing 9%–73% of107Pd and108Pd in the ESS. This missing component may have come from AGB stars of higher metallicity than those that contributed to the ESS in our GCE code. Ifτ /γ ≲ 0.3, we calculate instead the time (T LE) from the last nucleosynthesis event that added the SLRs into the presolar matter to the formation of the oldest solids in the ESS. For the 2M ⊙,Z = 0.01 Monash model we find a self-consistent solution ofT LE= 25.5 Myr. -
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Live (not decayed) radioisotopes on the Earth and Moon are messengers from recent nearby astrophysical explosions. Measurements of60Fe in deep-sea samples, Antarctic snow, and lunar regolith reveal two pulses about 3 Myr and 7 Myr ago. Detection of244Pu in a deep-sea crust indicates a recent r-process event. We review the ultrasensitive accelerator mass spectrometry techniques that enable these findings. We then explore the implications for astrophysics, including supernova nucleosynthesis, particularly the r-process, as well as supernova dust production and the formation of the Local Bubble that envelops the Solar System. The implications go beyond nuclear physics and astrophysics to include studies of heliophysics, astrobiology, geology, and evolutionary biology.
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Abstract There is a wealth of data on live, undecayed60Fe (
t 1/2= 2.6 Myr) in deep-sea deposits, the lunar regolith, cosmic rays, and Antarctic snow, which is interpreted as originating from the recent explosions of at least two near-Earth supernovae. We use the60Fe profiles in deep-sea sediments to estimate the timescale of supernova debris deposition beginning ∼3 Myr ago. The available data admits a variety of different profile functions, but in all cases the best-fit60Fe pulse durations are >1.6 Myr when all the data is combined. This timescale far exceeds the ≲0.1 Myr pulse that would be expected if60Fe was entrained in the supernova blast wave plasma. We interpret the long signal duration as evidence that60Fe arrives in the form of supernova dust, whose dynamics are separate from but coupled to the evolution of the blast plasma. In this framework, the >1.6 Myr is that for dust stopping due to drag forces. This scenario is consistent with the simulations in Fry et al. (2020), where the dust is magnetically trapped in supernova remnants and thereby confined around regions of the remnant dominated by supernova ejects, where magnetic fields are low. This picture fits naturally with models of cosmic-ray injection of refractory elements as sputtered supernova dust grains and implies that the recent60Fe detections in cosmic rays complement the fragments of grains that survived to arrive on the Earth and Moon. Finally, we present possible tests for this scenario.
