Astrophysical Shrapnel: Discriminating Among Near-Earth Stellar Explosion Sources of Live Radioactive Isotopes

Abstract: We consider the production and deposition on Earth of isotopes with half-lives in the range 10$<sup>{5}$ to 10$<sup>{8}$ years that might provide signatures of nearby stellar explosions, extending previous analyses of Core-Collapse Supernovae (CCSNe) to include Electron-Capture Supernovae (ECSNe), Super-Asymptotic Giant Branch (SAGBs) stars, Thermonuclear/Type Ia Supernovae (TNSNe), and Kilonovae/Neutron Star Mergers (KNe). We revisit previous estimates of the $<sup>{60}$Fe and $<sup>{26}$Al signatures, and extend these estimates to include $<sup>{244}$Pu and $<sup>{53}$Mn. We discuss interpretations of the $<sup>{60}$Fe signals in terrestrial and lunar reservoirs in terms of a nearby stellar ejection ~2.2 Myr ago, showing that (i) the $<sup>{60}$Fe yield rules out the TNSN and KN interpretations, (ii) the $<sup>{60}$Fe signals highly constrain a SAGB interpretation but do not completely them rule out, (iii) are consistent with a CCSN origin, and (iv) are highly compatible with an ECSN interpretation. Future measurements could resolve the radioisotope deposition over time, and we use the Sedov blast wave solution to illustrate possible time-resolved profiles. Measuring such profiles would independently probe the blast properties including distance, and would provide additional constraints the nature of the explosion.