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The interstellar oxygen isotopic ratio of 18O/17O can reflect the relativeamount of the secular enrichment by ejecta from high-mass versusintermediate-mass stars. Previous observations found a Galactic gradient of18O/17O, i.e., low ratios in the Galactic center and large values in theGalactic disk, which supports the insideout formation scenario of our Galaxy.However, the observed objects are not many and, in particular, not so many atlarge galactocentric distances. Thus we started a systematic study on Galacticinterstellar 18O/17O, through observations of C18O and C17O multi-transitionlines toward a large sample of 286 sources (at least one order of magnitudelarger than previous ones), from the Galactic center region to the far outerGalaxy (~22 kpc). Here we present our observations of J=1-0 lines of C18O andC17O, with the ARO12m and the IRAM 30m telescope. We detected successfully bothC18O and C17O 1-0 lines for 34 sources among our IRAM30m sample of 50 targetsand for 166 sources among our ARO12m sample of 260 targets. The C18O opticaldepth effect on our ratio results, evaluated by fitting results of C17O spectrawith hyperfine components and our RADEX non-LTE model calculation for thestrongest source, was found to be insignificant. Beam dilution does not seem tobe a problem either, which was supported by the fact of no systematic variationbetween the isotopic ratio and the heliocentric distance, and consistentmeasured ratios from two telescopes for most of those detected sources. Ourresults, though there are still very few detections made for sources in theouter Galaxy, confirm the apparent 18O/17O gradient of 18O/17O =(0.10+-0.03)R_GC+(2.95+-0.30), with a Pearson's rank correlation coefficient R= 0.69. This is supported by the newest Galactic chemical evolution modelincluding the impact of massive stellar rotators and novae.

A systematic observational study on Galactic interstellar ratio 18O/17O: I. C18O and C17O J=1-0 data analysis