  The 18O/17O ratio of the Solar System is 5.2 while that of the interstellar
medium (ISM) and young stellar objects is ~4. This difference cannot be
explained by pollution of the Sun's natal molecular cloud by 18O-rich supernova
ejecta because (1) the necessary B-star progenitors live longer than the
duration of star formation in molecular clouds; (2) the delivery of ejecta gas
is too inefficient and the amount of dust in supernova ejecta is too small
compared to the required pollution (2% of total mass or ~20% of oxygen); and
(3) the predicted amounts of concomitant short-lived radionuclides (SLRs)
conflicts with the abundances of 26Al and 41Ca in the early Solar System.
Proposals for the introduction of 18O-rich material must also be consistent
with any explanation for the origin of the observed slope-one relationship
between 17O/16O and 18O/16O in the high-temperature components of primitive
meteorites. The difference in 18O/17O ratios can be explained by enrichment of
the ISM by the 17O-rich winds of asymptotic giant branch (AGB) stars, the
sequestration of comparatively 18O-rich gas from star-forming regions into
long-lived, low-mass stars, and a monotonic decrease in the 18O/17O ratio of
interstellar gas. At plausible rates of star formation and gas infall, Galactic
chemical evolution does not follow a slope-one line in an three-isotope plot,
but instead moves along a steeper trajectory towards an 17O-rich state.
Evolution of the ISM and star-forming gas by AGB winds also explains the
difference in the carbon isotope ratios of the Solar System and ISM.
