Possible Implications of Relatively High Levels of Initial $^{60}$Fe in Iron Meteorites for the Non-Carbonaceous -- Carbonaceous Meteorite Dichotomy and Solar Nebula Formation

Abstract

Cook et al. (2021) found that iron meteorites have an initial abundance ratio of the short-lived isotope $^{60}$Fe to the stable isotope $^{56}$Fe of $^{60}$Fe/$^{56}$Fe $\sim$ $(6.4 \pm 2.0) \times 10^{-7}$. This appears to require the injection of live $^{60}$Fe from a Type II supernova (SN II) into the presolar molecular cloud core, as the observed ratio is over a factor of ten times higher than would be expected to be found in the ambient interstellar medium (ISM) as a result of galactic chemical evolution. The supernova triggering and injection scenario offers a ready explanation for an elevated initial $^{60}$Fe level, and in addition provides a physical mechanism for explaining the non-carbonaceous -- carbonaceous (NC-CC) dichotomy of meteorites. The NC-CC scenario hypothesizes the solar nebula first accreted material that was enriched in supernova-derived nuclides, and then later accreted material depleted in supernova-derived nuclides. While the NC-CC dichotomy refers to stable nuclides, not short-lived isotopes like $^{60}$Fe, the SN II triggering hypothesis provides an explanation for the otherwise unexplained change in nuclides being accreted by the solar nebula. Three dimensional hydrodynamical models of SN II shock-triggered collapse show that after triggering collapse of the presolar cloud core, the shock front sweeps away the local ISM while accelerating the resulting protostar/disk to a speed of several km/s, sufficient for the protostar/disk system to encounter within $\sim$ 1 Myr the more distant regions of a giant molecular cloud complex that might be expected to have a depleted inventory of supernova-derived nuclides.