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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.