O abundance ratios toward the GC region relative to molecular clouds in the Galactic disk. Furthermore, even inside the GC region, ratios appear not to be uniform. The low GC values are consistent with an inside-out formation scenario for our Galaxy.

To investigate the relative amount of ejecta from high-mass versus intermediate-mass stars and to trace the chemical evolution of the Galaxy, we have performed with the IRAM 30 m and the SMT 10 m telescopes a systematic study of Galactic interstellar 18 O/ 17 O ratios toward a sample of 421 molecular clouds, covering a galactocentric distance range of ∼1 -22 kpc. The results presented in this paper are based on the J=2-1 transition and encompass 364 sources showing both C 18 O and C 17 O detections. The previously suggested 18 O/ 17 O gradient is confirmed. For the 41 sources detected with both facilities, good agreement is obtained. A correlation of 18 O/ 17 O ratios with heliocentric distance is not found, indicating that beam dilution and linear beam sizes are not relevant. For the subsample of IRAM 30 m high-mass star-forming regions with accurate parallax distances, an unweighted fit gives 18 O/ 17 O = (0.12 ± 0.02)R GC + (2.38 ± 0.13) with a correlation coefficient of R = 0.67. While the slope is consistent with our J=1-0 measurement, ratios are systematically lower. This should be caused by larger optical depths of C 18 O 2-1 lines, w.r.t the corresponding 1-0 transitions, which is supported by RADEX calculations and the fact that C 18 O/C 17 O is positively correlated with 13 CO/C 18 O. After considering optical depth effects with C 18 O J=2-1 reaching typically an optical depth of ∼0.5, corrected 18 O/ 17 O ratios from the J=1-0 and J=2-1 lines become consistent. A good numerical fit to the data is provided by the MWG-12 model, including both rotating stars and novae.

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