2020ApJS..249....6Z

The interstellar oxygen isotopic ratio of ¹⁸O/¹⁷O can reflect the relative amount of the secular enrichment by ejecta from high-mass versus intermediate-mass stars. Previous observations found a Galactic gradient of ¹⁸O/¹⁷O, i.e., low ratios in the Galactic center and large values in the Galactic disk, which supports the inside-out formation scenario of our Galaxy. However, there are not many observed objects and, in particular, there are not many at large galactocentric distances. For this reason, we started a systematic study on Galactic interstellar ¹⁸O/¹⁷O, through observations of C¹⁸O and C¹⁷O multi-transition lines toward a large sample of 286 sources (at least one order of magnitude larger than previous ones), from the Galactic center region to the far outer Galaxy (∼22 kpc). In this article, we present our observations of J = 1-0 lines of C¹⁸O and C¹⁷O, with the 12 m antenna of the Arizona Radio Observatory (ARO 12 m) and the Institut de Radio Astronomie Millimetrique (IRAM) 30 m telescopes. Among our IRAM 30 m sample of 50 targets, we detected successfully both C¹⁸O and C¹⁷O 1-0 lines for 34 sources. Similarly, our sample of 260 targets for ARO 12 m observations resulted in the detection of both lines for 166 sources. The C¹⁸O optical depth effect on our ratio results, evaluated by fitting results of C¹⁷O spectra with hyperfine components (assuming τ_C18O=4τ_C17O) and our radiative transfer and excitation model nonlocal thermodynamic equilibrium (non-LTE) model calculation for the strongest source, was found to be insignificant. Beam dilution does not seem to be a problem either, which was supported by the fact that there is no systematic variation between the isotopic ratio and the heliocentric distance, and ratios are consistently measured from two telescopes for most of those detected sources. With this study we obtained ¹⁸O/¹⁷O isotopic ratios for a large sample of molecular clouds with different galactocentric distances. Our results, though there are still very few detections made for sources in the outer Galaxy, confirm the apparent ¹⁸O/¹⁷O gradient of ¹⁸O/¹⁷O=(0.10 ± 0.03)R_GC+(2.95 ± 0.30), with a Pearson's rank correlation coefficient of R = 0.69. This is supported by the newest Galactic chemical evolution model including the impact of massive stellar rotators and novae. Our future J = 2-1 and J = 3-2 observations of C¹⁸O and C¹⁷O toward the same sample would be important to determine their physical parameters (opacities, abundances, etc.) and further accurately determine the Galactic radial gradient of the isotopic ratio ¹⁸O/¹⁷O.