Characteristics of Massive Star-forming Molecular Cores: The Spectral Observations of 12CO, 13CO and C18O and the Statistical Comparison

With the 13.7 m millimeter wave telescope of Purple Mountain Observatory at Qinghai Station, the simultaneous mapping observations at the 12CO(J=1-0), 13CO(J=1-0) and C18O(J=1-0) lines were performed towards the 24 Galactic high-mass star-forming cores, which are associated with water masers and have available Spitzer's infrared data. The average mapping range was 8′ × 8′. The C18O line emission was detected in all the cores, in which 11 cores were observed to the half maximum of their C18O integrated intensities and the rather extended (5′ − 8′) C18O maps were obtained, while the others were failed to make such a large scale mapping because of the low SNR or the intrinsically extended morphology of the cores. On the 11 completely mapped dense cores, we analyzed their characteristics and made the statistics and comparisons on the integrated intensity ratios between 12CO and 13CO (R12/13), 13CO and C18O(R13/18), as well as 12CO and C18O(R12/18). We concluded that as a tracer of dense gas, C18O is absolutely optically thin and can be used to detect the detailed structures of the cores, and that in general the 3 ratios increase gradually from the core center to the periphery. We found that the integrated intensity ratio R12/13 ranges from 2 to 6; R13/18 fluctuates between 4 and 20, but in central regions it is concentrated in the range 6–12 with a small fluctuation; and R12/18 occupies a wider range 13–90, but it is concentrated between 13 and 50 in the denser regions of the cores.