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

Abstract

Context. The discovery of radionuclides like 60 Fe with half-lives of million years in deep-sea crusts and sediments offers the unique possibility to date and locate nearby supernovae. Aims. We want to quantitatively establish that the 60 Fe enhancement is the result of several supernovae which are also responsible for the formation of the Local Bubble, our Galactic habitat. Methods. We performed three-dimensional hydrodynamic adaptive mesh refinement simulations (with resolutions down to subparsec scale) of the Local Bubble and the neighbouring Loop I superbubble in different homogeneous, self-gravitating environments. For setting up the Local and Loop I superbubble, we took into account the time sequence and locations of the generating core-collapse supernova explosions, which were derived from the mass spectrum of the perished members of certain stellar moving groups. The release of 60 Fe and its subsequent turbulent mixing process inside the superbubble cavities was followed via passive scalars, where the yields of the decaying radioisotope were adjusted according to recent stellar evolution calculations. Results. The models are able to reproduce both the timing and the intensity of the 60 Fe excess observed with rather high precision, provided that the external density does not exceed 0.3 cm −3 on average. Thus the two best-fit models presented here were obtained with background media mimicking the classical warm ionised and warm neutral medium. We also found that 60 Fe (which is condensed onto dust grains) can be delivered to Earth via two physical mechanisms: either through individual fast-paced supernova blast waves, which cross the Earth's orbit sometimes even twice as a result of reflection from the Local Bubble's outer shell, or, alternatively, through the supershell of the Local Bubble itself, injecting the 60 Fe content of all previous supernovae at once, but over a longer time range.

A nearby recent supernova that ejected the runaway star ζ Oph, the pulsar PSR B1706−16, and 60Fe found on Earth

“…However, this fit is inconsistent with the Schaller et al (1992) isochrones at the lowest SN progenitor masses: e.g. at 9 M , one would obtain 21 Myr from the above fit (as used in Fuchs et al 2006, B16, andSchulreich et al 2017), but 29-32 Myr from the original publication in Schaller et al (1992);Wallner et al (2016) give ∼ 27 Myr as life-time for such a progenitor mass in their table S8. The above fit is valid only above ∼ 12 M (Schaller et al 1992).…”

Undiscovered Pulsar in the Local Bubble as an Explanation of the Local High Energy Cosmic Ray All-Electron Spectrum

“…These population simulations predict a pulsar within 80 pc in around 5-10% of the realizations. The existence of such a pulsar implies the occurrence of a recent local supernova, the case for which is strengthened by studies of 60 Fe transport [27,28]. By modeling the formation of the Local Bubble by several supernovae they find that the most recent supernovae must have exploded at a distance of 90-100 pc from the Earth, 2-3 Myr ago with a suitable initial mass to produce a pulsar [29].…”

Cosmic ray models

“…The Local Bubble extends roughly 200 pc in the Galactic plane, and 600 pc perpendicular to it, with an inclination of about 20 • [78]. Observations [79,80] and simulations [81,82] show that the bubble walls are fragmented and twisted. Moreover, outflows away from the Galactic plane may open up the bubble [81].…”