Galactic chemical evolution with the short-lived isotopes Mn-53, Fe-60, Hf-182, and Pu-244

Abstract: We run a three-dimensional Galactic chemical evolution (GCE) model to follow the propagation of Mn-53 from supernovae of type Ia (SNIa), Fe-60 from core-collapse supernovae (CCSNe), Hf-182 from intermediate mass stars (IMSs), and Pu-244 from neutron star mergers (NSMs) in the Galaxy. We compare the GCE of these short-lived radioactive isotopes (SLRs) to recent detections on the deep-sea floor. We find that although these SLRs originate from different sites, they often arrive conjointly on Earth.

• The paper presents a 3D Galactic chemical evolution model that simulates the production, transport, and deposition of four short-lived radioisotopes.
• It incorporates detailed ISM processes including gas infall, star formation, and shock-driven transport to match deep-sea sediment measurements.
• The study reveals correlated increases in SLR abundances and predicts detectable levels of Hf-182 alongside Fe-60 and Pu-244.

Galactic Chemical Evolution of Short-Lived Radioisotopes: Modeling the Propagation of $^{53}$Mn, $^{60}$Fe, $^{182}$Hf, and $^{244}$Pu

Introduction

This study presents a comprehensive three-dimensional Galactic chemical evolution (GCE) model to investigate the production, propagation, and deposition of four key short-lived radioactive isotopes (SLRs): $^{53}$Mn, $^{60}$Fe, $^{182}$Hf, and $^{244}$Pu. These isotopes are synthesized in distinct astrophysical sites—Type Ia supernovae (SNIa), core-collapse supernovae (CCSNe), intermediate-mass stars (IMSs), and neutron star mergers (NSMs), respectively. The work leverages recent detections of these SLRs in deep-sea sediments to constrain the model and elucidate the mechanisms by which SLRs are transported through the interstellar medium (ISM) and ultimately deposited on Earth.

Model Framework and Methodology

The GCE model is implemented in a cubic, periodic simulation volume of 2 kpc per side, with a sub-grid resolution of $(50\,\mathrm{pc})^2$ and a temporal resolution of 1 Myr per timestep. The model incorporates the following key physical processes:

• Gas Infall: The simulation volume is replenished with gas following a prescription that mimics the early rise and late exponential decline of infall rates.
• Star Formation: The Schmidt law ($$\Sigma_{\mathrm{SFR}} \propto \Sigma_{\mathrm{gas}}^{1.5}$$) governs star formation, with a Salpeter IMF ($^{60}$0, $^{60}$1).
• Stellar Evolution: Stellar lifetimes are computed as a function of mass and metallicity using Geneva group formulae.
• Binary Evolution and Explosive Events: Probabilistic treatment of binary systems yields SNIa and NSM rates, with $^{60}$2 for IMSs and $^{60}$3 for HMSs.
• Nucleosynthetic Yields: Isotope yields are adopted from recent nucleosynthesis calculations for each event type, with explicit treatment of radioactive decay.

The model tracks the ejection and subsequent mixing of SLRs into the ISM following each explosive event. The spatial and temporal evolution of SLR abundances is then compared to measured concentrations in deep-sea sediment layers, which serve as a proxy for the local ISM composition at the time of deposition.

Results

Temporal and Spatial Evolution of SLRs

The simulation reveals that the abundances of $^{60}$4Mn, $^{60}$5Fe, $^{60}$6Hf, and $^{60}$7Pu in the ISM are subject to both gradual decay and abrupt, order-of-magnitude fluctuations. The latter are attributed to the stochastic nature of nearby explosive events, which can both clear local ISM cells of SLRs and inject fresh nucleosynthetic products.

A key finding is that SLRs originating from distinct nucleosynthetic sites can be deposited simultaneously in the same ISM region. This occurs when a shock front from a subsequent explosive event (e.g., a CCSN) sweeps up and transports pre-existing SLRs (e.g., $^{60}$8Pu from a prior NSM) along with its own ejecta (e.g., $^{60}$9Fe). This mechanism leads to correlated increases in multiple SLRs within a given ISM cell, consistent with the co-detection of these isotopes in deep-sea samples.

Comparison with Deep-Sea Measurements

The model is calibrated to reproduce the temporal profiles of SLR deposition inferred from deep-sea sediment data. While the absolute timing is adjusted for optimal fit, the model successfully captures the shape and magnitude of observed SLR abundance variations. Notably, the simulation predicts that $^{182}$0Hf, which has not yet been detected in deep-sea samples, should be present at levels comparable to $^{182}$1Fe and $^{182}$2Pu, providing a testable prediction for future measurements.

Transport Mechanisms

The results demonstrate that SLRs are not confined to their production sites but are efficiently redistributed by subsequent energetic events. The dominant transport mechanism is the propagation of SLR-enriched shells driven by supernova and NSM shock waves. This "surfing" of SLRs on shock fronts leads to their spatial and temporal co-localization, even when their nucleosynthetic origins are distinct.

Implications and Future Directions

Theoretical Implications

The study provides a quantitative framework for interpreting the co-occurrence of SLRs with disparate nucleosynthetic origins in terrestrial and meteoritic samples. The results challenge the assumption that SLRs detected in the early Solar System or on Earth must originate from a single, recent nucleosynthetic event. Instead, the ISM mixing and transport processes can lead to the simultaneous delivery of multiple SLRs from independent sources.

Practical Implications

The model offers a predictive tool for interpreting SLR measurements in terrestrial archives and meteorites, enabling constraints on the frequency and proximity of recent nucleosynthetic events in the Solar neighborhood. The prediction of $^{182}$3Hf in deep-sea sediments provides a concrete target for future geochemical analyses.

Prospects for Future Research

Further refinement of the model could incorporate more detailed treatments of ISM turbulence, anisotropic mixing, and the role of Galactic structure. Improved nucleosynthetic yield calculations, particularly for NSMs and rare SNIa channels, will enhance the fidelity of SLR abundance predictions. The integration of additional observational constraints, such as gamma-ray line measurements and meteoritic isotope ratios, will further test and calibrate the model.

Conclusion

This work advances the modeling of Galactic chemical evolution by explicitly simulating the production, propagation, and deposition of multiple short-lived radioisotopes with distinct astrophysical origins. The results underscore the importance of ISM mixing and shock-driven transport in shaping the spatial and temporal distribution of SLRs. The model's ability to reproduce observed SLR abundances in deep-sea sediments and its prediction of yet-undetected isotopes highlight its utility for interpreting both astrophysical and geochemical data. Future developments in both modeling and measurement will further elucidate the complex interplay between nucleosynthesis, ISM dynamics, and the chemical evolution of the Galaxy.