The TMRT K band observations towards 26 infrared dark clouds: NH3, CCS, and HC3N

We present one of the first Shanghai Tian Ma Radio Telescope (TMRT) K Band observations towards a sample of 26 infrared dark clouds (IRDCs). We observed the (1,1), (2,2), (3,3), and (4,4) transitions of NH3 together with CCS (21–>10) and HC3N J = 2–1, simultaneously. The survey dramatically increases the existing CCS-detected IRDC sample from 8 to 23, enabling a better statistical study of the ratios of carbon-chain molecules (CCM) to N-bearing molecules in IRDCs. With the newly developed hyperfine group ratio (HFGR) method of fitting NH3 inversion lines, we found the gas temperature to be between 10 and 18 K. The column density ratios of CCS to NH3 for most of the IRDCs are less than 10−2, distinguishing IRDCs from low-mass star-forming regions. We carried out chemical evolution simulations based on a three-phase chemical model NAUTILUS. Our measurements of the column density ratios between CCM and NH3 are consistent with chemical evolutionary ages of ≲105 yr in the models. Comparisons of the data and chemical models suggest that CCS, HC3N, and NH3 are sensitive to the chemical evolutionary stages of the sources.