Near-Earth Supernova Explosions: Evidence, Implications, and Opportunities

There is now solid experimental evidence of at least one supernova explosion within 100 pc of Earth within the last few million years, from measurements of the short-lived isotope ⁶⁰Fe in widespread deep-ocean samples, as well as in the lunar regolith and cosmic rays. This is the first established example of a specific dated astrophysical event outside the Solar System having a measurable impact on the Earth, offering new probes of stellar evolution, nuclear astrophysics, the astrophysics of the solar neighborhood, cosmic-ray sources and acceleration, multi-messenger astronomy, and astrobiology. Interdisciplinary connections reach broadly to include heliophysics, geology, and evolutionary biology. Objectives for the future include pinning down the nature and location of the established near-Earth supernova explosions, seeking evidence for others, and searching for other short-lived isotopes such as ²⁶Al and ²⁴⁴Pu. The unique information provided by geological and lunar detections of radioactive ⁶⁰Fe to assess nearby supernova explosions make now a compelling time for the astronomy community to advocate for supporting multi-disciplinary, cross-cutting research programs

Radioactive Iron Rain: Transporting 60^{60}Fe in Supernova Dust to the Ocean Floor

Several searches have found evidence of $^{60}$Fe deposition, presumably from a near-Earth supernova (SN), with concentrations that vary in different locations on Earth. This paper examines various influences on the path of interstellar dust carrying $^{60}$Fe from a SN through the heliosphere, with the aim of estimating the final global distribution on the ocean floor. We study the influences of magnetic fields, angle of arrival, wind and ocean cycling of SN material on the concentrations at different locations. We find that the passage of SN material through the mesosphere/lower thermosphere (MLT) is the greatest influence on the final global distribution, with ocean cycling causing lesser alteration as the SN material sinks to the ocean floor. SN distance estimates in previous works that assumed a uniform distribution are a good approximation. Including the effects on surface distributions, we estimate a distance of $46^{+10}_{-6}$ pc for a $8-10 \ M_{\odot}$ SN progenitor. This is consistent with a SN occurring within the Tuc-Hor stellar group $\sim$2.8 Myr ago with SN material arriving on Earth $\sim$2.2 Myr ago. We note that the SN dust retains directional information to within $1^{\circ}$ through its arrival in the inner Solar System, so that SN debris deposition on inert bodies such as the Moon will be anisotropic, and thus could in principle be used to infer directional information. In particular, we predict that existing lunar samples should show measurable $^{60}$Fe differences.Comment: 18 pages, 8 figures. Comments welcom

The Evolution of Li6 in Standard Cosmic-Ray Nucleosynthesis

We review the Galactic chemical evolution of Li6 and compare these results with recent observational determinations of the lithium isotopic ratio. In particular, we concentrate on so-called standard Galactic cosmic-ray nucleosynthesis in which Li, Be, and B are produced (predominantly) by the inelastic scattering of accelerated protons and \alpha's off of CNO nuclei in the ambient interstellar medium. If O/Fe is constant at low metallicities, then the Li6 vs Fe/H evolution-as well as Be and B vs Fe/H-has difficulty in matching the observations. However, recent determinations of Population II oxygen abundances, as measured via OH lines, indicate that O/Fe increases at lower metallicity; if this trend is confirmed, then the Li6 evolution in a standard model of cosmic-ray nucleosynthesis is consistent with the data. We also show that another key indicator of Li6BeB origin is the Li6/Be ratio which also fits the available data if O/Fe is not constant at low metallicity. Finally we note that Li6 evolution in this scenario can strongly constrain the degree to which Li6 and Li7 are depleted in halo stars.Comment: 15 pages, latex, 2 ps figure