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arXiv · 2311.00547

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

Y. P. Zou·J. S. Zhang·C. Henkel·D. Romano·W. Liu·Y. H. Zheng·Y. T. Yan·J. L. Chen·Y. X. Wang·J. Y. Zhao
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Abstract

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 30m and the SMT 10m telescopes a systematic study of Galactic interstellar 18O/17O 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 C18O and C17O detections. The previously suggested 18O/17O gradient is confirmed. For the 41 sources detected with both facilities, good agreement is obtained. A correlation of 18O/17O 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 18O/17O = (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 C18O 2-1 lines, w.r.t the corresponding 1-0 transitions, which is supported by RADEX calculations and the fact that C18O/C17O is positively correlated with 13CO/C18O. After considering optical depth effects with C18O J=2-1 reaching typically an optical depth of 0.5, corrected 18O/17O 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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Y. P. Zou, J. S. Zhang, C. Henkel, D. Romano, W. Liu, Y. H. Zheng, Y. T. Yan, J. L. Chen, Y. X. Wang, J. Y. Zhao. 2023-11-01. A Systematic Observational Study on Galactic Interstellar Ratio 18O/17O. II. C18O and C17O J=2-1 Data Analysis. https://doi.org/10.3847/1538-4365/acee6b

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