Galactic Chemical Evolution and the Oxygen Isotopic Composition of the
  Solar System
  We review current observational and theoretical constraints on the Galactic
chemical evolution (GCE) of oxygen isotopes in order to explore whether GCE
plays a role in explaining the lower 17O/18O ratio of the Sun, relative to the
present-day interstellar medium, or the existence of distinct 16O-rich and
16O-poor reservoirs in the Solar System. Although the production of both 17O
and 18O are related to the metallicity of progenitor stars, 17O is most likely
produced in stars that evolve on longer timescales than those that produce 18O.
Therefore the 17O/18O ratio need not have remained constant over time, contrary
to preconceptions and the simplest models of GCE. An apparent linear, slope-one
correlation between delta17O and delta18O in the ISM need not necessarily
reflect an O isotopic gradient, and any slope-one galactocentric gradient need
not correspond to evolution in time. Instead, increasing 17O/18O is consistent
both with observational data from molecular clouds and with modeling of the
compositions of presolar grains. Models in which the rate of star formation has
decelerated over the past few Gyr or in which an enhanced period of star
formation occurred shortly before solar birth ("starburst") can explain the
solar-ISM O-isotopic difference without requiring a local input of supernova
ejecta into the protosolar cloud. "Cosmic chemical memory" models in which
interstellar dust is on average older than interstellar gas predict that
primordial Solar System solids should be 16O-rich, relative to the Sun, in
conflict with observations. However, scenarios in which the 16O-rich
contribution of very massive stars could lead to 16O-poor solids and a 16O-rich
bulk Sun, if the Solar System formed shortly after a starburst, independent of
the popular scenario of photochemical self-shielding of CO.
