The interstellar oxygen isotopic ratio of ^ { 18 } O/ ^ { 17 } O 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 ^ { 18 } O/ ^ { 17 } O , 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 , the observed objects are not many and , in particular , not so many at large galactocentric distances .
For this reason , we started a systematic study on Galactic interstellar ^ { 18 } O/ ^ { 17 } O , through observations of C ^ { 18 } O and C ^ { 17 } O 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 ( \sim 22 kpc ) .
In this article , we present our observations of J=1-0 lines of C ^ { 18 } O and C ^ { 17 } O , with the 12 m antenna of the Arizona Radio Observatory ( ARO 12 m ) and the IRAM 30 m telescope .
Among our IRAM30 m sample of 50 targets , we detected successfully both C ^ { 18 } O and C ^ { 17 } O 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 C ^ { 18 } O optical depth effect on our ratio results , evaluated by fitting results of C ^ { 17 } O spectra with hyperfine components ( assuming \tau _ { C 18 O } = 4 \tau _ { C 17 O } ) and our RADEX 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 of no systematic variation between the isotopic ratio and the heliocentric distance , and consistent measured ratios from two telescopes for most of those detected sources .
With this study we obtained ^ { 18 } O/ ^ { 17 } O 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 ^ { 18 } O/ ^ { 17 } O gradient of ^ { 18 } O/ ^ { 17 } O = ( 0.10 \pm 0.03 ) R _ { GC } + ( 2.95 \pm 0.30 ) , with a Pearson ’ s rank correlation coefficient 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 C ^ { 18 } O and C ^ { 17 } O toward the same sample would be important to determine their physical parameters ( opacities , abundances , etc . )
and further determine accurately the Galactic radial gradient of the isotopic ratio ^ { 18 } O/ ^ { 17 } O .