Simulations of $^{60}$Fe entrained in ejecta from a near-Earth supernova: Effects of observer motion

Abstract

Recent studies have shown that live (not decayed) radioactive 60 Fe is present in deep-ocean samples, Antarctic snow, lunar regolith, and cosmic rays. 60 Fe represents supernova (SN) ejecta deposited in the Solar System around 3 Myr ago, and recently an earlier pulse ≈ 7 Myr ago has been found. These data point to one or multiple near-Earth SN explosions that presumably participated in the formation of the Local Bubble. We explore this theory using 3D high-resolution smooth-particle hydrodynamical simulations of isolated supernovae with ejecta tracers in a uniform interstellar medium (ISM). The simulation allows us to trace the supernova ejecta in gas form and those eject in dust grains that are entrained with the gas. We consider two cases of diffused ejecta: when the ejecta are well-mixed in the shock and when they are not. In the latter case, we find that these ejecta remain far behind the forward shock, limiting the distance to which entrained ejecta can be delivered to ≈ 100 pc in an ISM with 𝑛 H = 0.1 cm −3 mean hydrogen density. We show that the intensity and the duration of 60 Fe accretion depend on the ISM density and the trajectory of the Solar System. Furthermore, we show the possibility of reproducing the two observed peaks in 60 Fe concentration with this model by assuming two linear trajectories for the Solar System with 30-km s −1 velocity. The fact that we can reproduce the two observed peaks further supports the theory that the 60 Fe signal was originated from near-Earth SNe.

Heliospheric Compression due to Recent Nearby Supernova Explosions

“…At late times, the blast speed slows and Earth's speed could become important; these effects are discussed in (Chaikin et al 2021) in the context of 60 Fe deposition and the local environment encountered by the solar system. It remains for future work to model such effects on the heliosphere, including the possibility that the Earth's velocity is misaligned with that of the supernova blast.…”