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 $3\mathrm{Myr}$ ago, and recently an earlier pulse $\approx 7\mathrm{Myr}$ 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 $\approx 100$ pc in an ISM with $n_{\mathrm{H}}=0.1\mathrm{c}{\mathrm{m}}^{-3}$ 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.

17 pages, 11 figures (including appendix); Accepted for publication in MNRAS

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