Galactic interstellar 18O/17O ratios - a radial gradient?

Abstract

(Abridged) Our aim is to determine 18O/17O abundance ratios across the entire Galaxy. These provide a measure of the amount of enrichment by high-mass versus intermediate-mass stars. Such ratios, derived from the C18O and C17O J=1-0 lines alone, may be affected by systematic errors. Therefore, the C18O and C17O (1-0), (2-1), and (3-2), as well as the 13CO (1-0) and (2-1) lines, were observed towards 18 prominent galactic targets (a total of 25 positions). The combined dataset was analysed with an LVG model, accounting for optical depth effects. The data cover galactocentric radii R between 0.1 and 16.9 kpc (solar circle at 8.5 kpc). Near the centre of the Galaxy, 18O/17O = 2.88 +/- 0.11. For the galactic disc out to an R of ca. 10 kpc, 18O/17O = 4.16 +/- 0.09. At ca. R = 16.5 kpc, 18O/17O = 5.03 +/- 0.46. Assuming that 18O is synthesised predominantly in high-mass stars (M > 8 Msun), while C17O is mainly a product of lower-mass stars, the ratio from the inner Galaxy indicates a dominance of CNO-hydrogen burning products that is also apparent in the C- and N-isotope ratios. The high 18O/17O value of the solar system (5.5) relative to that of the ambient ISM suggests contamination by nearby high-mass stars during its formation. High values in the metal-poor environment of the outer Galaxy are not matched by the low values observed towards the even more metal-poor LMC. Apparently, the outer Galaxy cannot be considered as an intermediate environment between the solar neighbourhood and the ISM of small metal-poor galaxies. The apparent 18O/17O gradient along the galactic disc and the discrepancy between outer disc and LMC isotope ratios may be explained by different ages of the respective stellar populations.