Characteristics of Massive Star-forming Molecular Cores: The Spectral Observations of 12CO, 13CO and C 18O and the Statistical Comparison

Abstract

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 C 18O(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 C 18O line emission was detected in all the cores, in which 11 cores were observed to the half maximum of their C 18O integrated intensities and the rather extended (5′ − 8′) C 18O 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 (R 12/13), 13CO and C 18O(R 13/18), as well as 12CO and C 18O(R 12/18). We concluded that as a tracer of dense gas, C 18O 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 R 12/13 ranges from 2 to 6; R 13/18 fluctuates between 4 and 20, but in central regions it is concentrated in the range 6–12 with a small fluctuation; and R 12/18 occupies a wider range 13–90, but it is concentrated between 13 and 50 in the denser regions of the cores.