Oxygen isotopic ratios toward molecular clouds in the Galactic disk

We present our observations of the J = 1 − 0 rotation transitions in molecular isotopes C 18 O and C 17 O toward a sample of molecular clouds with different galactocentric distances, using the Delingha 13.7 m (DLH 13.7 m) telescope, administered by Purple Mountain Observatory, and its 9-beam SIS receiver. Complementary observations toward several sources with large galactocentric distance are obtained with the IRAM 30m and Mopra 22m telescopes. C 18 O/C 17 O abundance ratios reflecting the 18 O/ 17 O isotope ratios are obtained from integrated intensity ratios of C 18 O and C 17 O. We derived the ratio value for 13 sources covering a galactocentric distance range of 3 kpc to 16 kpc. In combination with our mapping results that provide a ratio value of 3.01±0.14 in the Galactic center region, it shows that the abundance ratio tends to increase with galactocentric distance, i.e., it supports a radial gradient along the Galactic disk for the abundance ratio. This is consistent with the inside-out formation scenario of our Galaxy. However, our results may suffer from small samples with large galactocentric distance. Combining our data with multitransition lines of C 18 O and C 17 O will be helpful for constraining opacities and abundances and further confirming the Galactic radial gradient shown by the isotope ratio 18 O/ 17 O.