Numerical studies on the link between radioisotopic signatures on Earth and the formation of the Local Bubble

Abstract

Context. Measurements of long-lived radioisotopes, which have grown rapidly in quantity and sensitivity over the last few years, provide a means, completely independent of other observational channels, to draw conclusions about near-Earth supernovae (SNe) and thus the origin of the Local Bubble (LB), our Galactic habitat. First and foremost in this context is 60Fe, which has already been detected across the Earth and on the Moon. Aims. The present study constitutes a significant step in further refining the coherent picture of the formation of the LB, constrained by radioisotopic anomalies, that we have drawn earlier and is based on the most sophisticated initial conditions determined to date. Methods. Using Gaìa EDR3, we identified 14 SN explosions, with 13 occurring in Upper Centaurus-Lupus and Lower Centaurus-Crux, and one in V1062 Sco, all being subgroups of the Scorpius-Centaurus OB association. The timing of these explosions was obtained by us through interpolation of modern rotating stellar evolution tracks via the initial masses of the already exploded massive stars. We further developed a new Monte Carlo-type approach for deriving the trajectories of the SN progenitors, utilising a plethora of test-particle simulations in a realistic Milky Way potential and selecting explosion sites based on maximum values in six-dimensional phase-space probability distributions constructed from the simulations. We then performed high-resolution three-dimensional hydrodynamic simulations based on these initial conditions to explore the evolution of the LB in an inhomogeneous local interstellar medium and the transport of radioisotopes to Earth. The simulations include the effects of age- and initial mass-dependent stellar winds from the SN progenitors and additional radioisotopes (26Al, 53Mn, and 244Pu) besides 60Fe using wind-derived and explosive yields from rotating models. Results. From our modelling of the LB, we find for our main results that (i) our simulations are consistent with measurements of 60Fe, in particular, a peak 2–3 Myr before present, as well as 26Al, 53Mn, and 244Pu data; (ii) stellar winds contribute to the distribution of radioisotopes and also to the dynamics of the LB; (iii) the Solar System (SS) entered the LB about 4.6 Myr ago; and (iv) the recent influx of 60Fe, discovered in Antarctic snow and deep-sea sediments, can be naturally explained by turbulent radioisotopic transport (in dust grains) mainly originating from the SN explosions and from the shock waves reflected at the LB shell. Conclusions. Our simulations not only support the recent hypothesis that the LB triggered star formation in the solar vicinity through its expansion, but they also suggest that the second, separate 60Fe peak measured at 6–9 Myr ago was generated by the passage of the SS through a neighbouring superbubble (SB), possibly the Orion-Eridanus SB, prior to its current residence in the LB.