摘要
We present our observations of the J = 1-0 rotation transitions in molecular isotopes C^18O and C^17O 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 30 m and Mopra 22 m telescopes. C^18O/C^17O abundance ratios reflecting the ^（18）O/^（17）O isotope ratios are obtained from integrated intensity ratios of C^18O and C^17O. 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^18O and C^17O will be helpful for constraining opacities and abundances and further confirming the Galactic radial gradient shown by the isotope ratio ^（18）O/^（17）O.
We present our observations of the J = 1-0 rotation transitions in molecular isotopes C^18O and C^17O 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 30 m and Mopra 22 m telescopes. C^18O/C^17O abundance ratios reflecting the ^（18）O/^（17）O isotope ratios are obtained from integrated intensity ratios of C^18O and C^17O. 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^18O and C^17O will be helpful for constraining opacities and abundances and further confirming the Galactic radial gradient shown by the isotope ratio ^（18）O/^（17）O.
基金
supported by the National Basic Research Program of China (973 program, 2012CB821800)
the National Natural Science Foundation of China (Nos. 11178009 and 11473007)
