On the oxygen isotope composition of the solar system
The O-18/O-17 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 O-18-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 Al-26 and Ca-41 in the early Solar System. Proposals for the introduction of O-18-rich material into the Solar System must also be consistent with any explanation for the origin of the observed slope-one relationship between O-17/O-16 and O-18/O-16. Instead, the difference in O-18/O-17 ratios could be the result of the enrichment of the ISM with O-17 from asymptotic giant branch (AGB) stars, the sequestration of comparatively O-18-rich gas from star-forming regions into long-lived, low-mass stars, and a monotonic decrease in the O-18/O-17 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 oxygen isotope plot, but instead moves along a steeper trajectory towards a O-17-rich state. Evolution of the ISM and star-forming gas by AGB ejecta also explains the difference in the carbon isotope ratios of the Solar System and ISM.