Research in Astronomy and Astrophysics Oxygen isotopic ratios toward molecular clouds in the Galactic disk

Abstract We present our observations on the J=1-0 rotation transitions of isotope molec-ular C18O and C17O toward a sample of molecular clouds with different galactocen-tric distance, through the Purple Mountain Observatory the Delingha 13.7m telescope (DLH13.7m) and its 9-beam SIS receiver. Complementary observations toward several sources with large galactocentric distance are obtained with IRAM 30m and Mopra 22m telescopes. C18O/C17O abundance ratios reflecting the 18O/17O isotope ratios are ob-tained from integrated intensity ratios of C18O and C17O. We got the ratio value for 13 sources covering a galactocentric distance range of 3 kpc to 16 kpc. Combination with our mapping results of 3.01±0.14 in the Galactic center region, it shows that the abundance ratio tend to be rising with the galactocentric distance, i.e., it supports a radial gradient along the Galactic disk for the abundance ratio. This is consistent with the inside-out for-mation scenario of our Galaxy. However, our results may suffer from small samples with large galactocentric distance. And combination with the multi-transition lines of C18O and C17O will be helpful to constrain opacities and abundances and further confirm the Galactic radial gradient on isotope ratio 18O/17O