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

TL;DR

This study tests how the interstellar $^{18}$O/$^{17}$O ratio traces Galactic chemical evolution by using C$^{18}$O and C$^{17}$O J=2-1 transitions across a large, diverse sample of 421 molecular clouds. Combining IRAM 30 m and SMT 10 m data yields 364 sources with detections in both isotopologues, enabling a robust assessment of beam dilution, optical depth, and radiative-transfer effects beyond previous J=1-0 work. A pronounced radial gradient is observed, especially among high-mass star-forming regions with precise parallax distances, with $^{18}$O/$^{17}$O = $(0.12 \,\pm\ 0.02)R_{GC} + (2.38 \,\pm\ 0.13)$ and $R=0.67$, in agreement with recent Galactic chemical evolution models that include rotating massive stars and novae. After correcting for optical depth in C$^{18}$O J=2-1 (which has $\tau$ around 0.5 on average), the J=2-1 results align with J=1-0 measurements, validating the multi-transition approach as a reliable probe of isotopic enrichment across the Galaxy.

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.

Figures (11)

• Figure 1: The IRAM 30 m spectra of C$^{18}$O (upper panels) and C$^{17}$O (lower panels) with green fit lines of the 96 sources detected in both isotopologues. (An extended version of this figure is available).
• Figure 2: The SMT 10 m spectra of C$^{18}$O (upper panels) and C$^{17}$O (lower panels) with green fit lines of the 325 sources detected in both isotopologues. (An extended version of this figure is available).
• Figure 3: Abundance ratios of the entire sample plotted against heliocentric distance. Red circles and black triangles represent our IRAM 30 m and SMT 10 m measurements, respectively.
• Figure 4: The spectra of C$^{18}$O (upper panels) and C$^{17}$O (lower panels) with green fit lines of those 41 sources detected by both the SMT 10 m (left columns) and the IRAM 30 m (right columns) telescopes. (An extended version of this figure is available).
• Figure 5: The comparison of estimated source sizes from C$^{18}$O and C$^{17}$O J=2-1 lines (left panel) and the isotope ratio measured by IRAM 30 m and SMT 10 m (right panel) for those 41 sources detected by both telescopes toward identical positions. The green dashed lines indicate $\theta_{s}^{18}$ = $\theta_{s}^{17}$ and Ratio${\rm _{IRAM}}$ = Ratio${\rm _{SMT}}$, while the dark and light green shaded areas indicate the $\pm$ 10% and $\pm$ 20% error ranges, respectively.