A Systematic Observational Study on Galactic Interstellar Ratio ¹⁸O/¹⁷O. I. C¹⁸O and C¹⁷O J = 1–0 Data Analysis

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 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.

A 12.2 GHz Methanol Maser Survey toward the 6.7 GHz Counterparts Associated with/without UC H ii Regions

“…Alternatively, from a statistical perspective, this finding may trace the overall evolution of our Galaxy and supports the inside-out formation scenario of our Galaxy (see Zhang et al 2020, and references therein), since the 12.2 GHz methanol masers usually appear later than the 6.7 GHz methanol masers from the evolutionary stages of interstellar masers during the massive star formation outlined by Breen et al (2010b). It can be further explored by combining the measurements of the Galactic gradient of isotopic ratios using particular molecules (e.g., C, N, O, and S; see Yan et al 2019;Yu et al 2020;Zhang et al 2020) for sources a certain arm.…”

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

“…Briefly, 18 O is primarily synthesized in massive stars, while 17 O is predominantly ejected by intermediate-mass stars through a longer production timescale. This can lead to a mildly positive gradient of 18 O/ 17 O throughout the disk, assuming an inside-out formation scenario for our Galaxy (see the detailed description in Zhang et al 2020). Recent theoretical results mainly based on numerical calculations suggest a complex interdependence of the yields on mass, metallicity, and rotation of the stars.…”

The evolution of CNO elements in galaxies

“…Nowadays, the increased sensitivity and high spectral resolution of millimeter telescopes allow to reach far outer Galactic disc regions and probe their low-metallicity ISM. This provides unique information on the synthesis of the minor CNO isotopes in metalpoor environments (see Romano et al 2017Romano et al , 2019bZhang et al 2020;Fontani et al 2022). Spectroscopy of unevolved stars, in fact, provides only fragments of the local enrichment histories of 12 C= 13 C and 16 O= 18 O (see Fig.…”