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

• Published in: The Astrophysical journal. Supplement series 2023-10, Vol.268 (2), p.56
• Main Authors: Zou 邹, Y. P. 益鹏, Zhang 张, J. S. 江水, Henkel, C., Romano, D., Liu 刘, W. 玮, Zheng 郑, Y. H. 映慧, Yan 闫, Y. T. 耀庭, Chen 陈, J. L. 家梁, Wang 汪, Y. X. 友鑫, Zhao 赵, J. Y. 洁瑜
• Format: Article
• Language: English
• ISSN: 0067-0049; 1538-4365
• DOI: 10.3847/1538-4365/acee6b

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.