Distances to Recent Near-Earth Supernovae from Geological and Lunar ⁶⁰Fe

Near-Earth supernova blasts which engulf the solar system have left traces of their ejecta in the geological and lunar records. There is now a wealth of data on live radioactive ⁶⁰Fe pointing to a supernova at 3 Myr ago, as well as the recent discovery of an event at 7 Myr ago. We use the available measurements to evaluate the distances to these events. For the better analyzed supernova at 3 Myr, samples include deep-sea sediments, ferromanganese crusts, and lunar regolith; we explore the consistency among and across these measurements, which depends sensitively on the uptake of iron in the samples as well as possible anisotropies in the ⁶⁰Fe fallout. There is also significant uncertainty in the astronomical parameters needed for these calculations. We take the opportunity to perform a parameter study on the effects that the ejected ⁶⁰Fe mass from a core-collapse supernova and the fraction of dust that survives the remnant have on the resulting distance. We find that with an ejected ⁶⁰Fe mass of 3 × 10⁻⁵ M _⊙ and a dust fraction of 10%, the distance range for the supernova 3 Myr ago is D ∼ 20-140 pc, with the most likely range between 50 and 65 pc. Using the same astrophysical parameters, the distance for the supernova at 7 Myr ago is D ∼ 110 pc. We close with a brief discussion of geological and astronomical measurements that can improve these results.