A Systematic Observational Study on Galactic Interstellar Ratio 18O/17O. II. C18O and C17O J = 2–1 Data Analysis

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 a systematic study of Galactic interstellar ^18 O/ ^17 O ratios toward a sample of 421 molecular clouds with IRAM 30 m and the 10 m Submillimeter Telescope, 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 the ^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, the ratios are systematically lower. This should be caused by larger optical depths of C ^18 O 2–1 lines with respect to 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. When we consider that optical depth effects with C ^18 O J = 2–1 typically reach an optical depth of ∼0.5, the corrected ^18 O/ ^17 O ratios from the J = 1–0 and J = 2–1 lines are consistent. A good numerical fit to the data is provided by the MWG-12 model, which includes both rotating stars and novae.