  Meteoritic evidence shows that the Solar system at birth contained
significant quantities of short-lived radioisotopes (SLRs) such as 60Fe and
26Al (with half-lives of 2.6 and 0.7 Myr respectively) produced in supernova
explosions and in the Wolf-Rayet winds that precede them. Proposed explanations
for the high SLR abundance include formation of the Sun in a
supernova-triggered collapse or in a giant molecular cloud (GMC) that was
massive enough to survive multiple supernovae (SNe) and confine their ejecta.
However, the former scenario is possible only if the Sun is a rare outlier
among massive stars, while the latter appears to be inconsistent with the
observation that 26Al is distributed with a scale height significantly larger
than GMCs. In this paper, we present a high-resolution chemo-hydrodynamical
simulation of the entire Milky-Way Galaxy, including stochastic star formation,
HII regions, SNe, and element injection, that allows us to measure for the
distribution of 60Fe/56Fe and 26Al/27Al ratios over all stars in the Galaxy. We
show that the Solar System's abundance ratios are well within the normal range,
but that SLRs originate neither from triggering nor from confinement in
long-lived clouds as previously conjectured. Instead, we find that SLRs are
abundant in newborn stars because star formation is correlated on galactic
scales, so that ejecta preferentially enrich atomic gas that will subsequently
be accreted onto existing GMCs or will form new ones. Thus new generations of
stars preferentially form in patches of the Galaxy contaminated by previous
generations of stellar winds and supernovae.
