Simulations of 60Fe entrained in ejecta from a near-Earth supernova: effects of observer motion

Abstract

Recent studies have shown that live (not decayed) radioactive 60Fe is present in deep-ocean samples, Antarctic snow, lunar regolith, and cosmic rays. 60Fe represents supernova (SN) ejecta deposited in the Solar System around $3 \, \rm Myr$ ago, and recently an earlier pulse $\approx 7 \ \rm Myr$ 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 $n_\mathrm{H}=0.1\; \rm cm^{-3}$ mean hydrogen density. We show that the intensity and the duration of 60Fe 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 60Fe 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 60Fe signal was originated from near-Earth SNe.

Supernova Dust Evolution Probed by Deep-sea 60Fe Time History

“…In combination with the observer motion effect from the solar system's motion relative to the supernova (see, e.g., Chaikin et al 2022), these processes can account for extended delivery time.…”

“…Under this assumption, Fry et al (2015) showed that if the 60 Fe is well mixed in a Sedov blast, the signal appears discontinuously with the arrival of the forward shock, and decreases thereafter from this maximum. Chaikin et al (2022) included the effects of incomplete mixing of 60 Fe in the remnant gas, and also allowed for effects of the Earthʼs motion. They too found that the 60 Fe pulse begins abruptly and is concentrated in a pulse, but found that the signal can linger thereafter.…”

“…(2) The 60 Fe flux could reflect the motion of the Sun through the supernova material (Chaikin et al 2022), with a complex time history needed to accommodate the two pulses.…”

Deep-Sea and Lunar Radioisotopes from Nearby Astrophysical Explosions

“…Abbreviations: An, AGB star of mass n M ࣻ ; ECSN, electron capture SN; KN, kilonova; SN, supernova; Sn, core-collapse SN of mass n M ࣻ ; SN Plio, Pliocene supernova; SNIa, Type Ia SNe. Another possibility is that the SN enriches a cloud, through which the Sun later moves (23,148,149). This raises issues of cloud survival and 60 Fe transport to Earth (147), but in this picture, D rad roughly corresponds to the distance from the SN to the cloud.…”

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 Earth's velocity is misaligned with that of the supernova blast.…”