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

Abstract 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 60 Fe 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 60 Fe 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 60 Fe 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 60 Fe 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.