The TMRT K Band Observations towards 26 Infrared Dark Clouds: NH\(_{3}\), CCS, and HC\(_{3}\)N

Abstract

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 NH\(_{3}\) together with CCS (2\(_{1}\)-1\(_{0}\)) and HC\(_{3}\)N \(J\,\)=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 NH\(_{3}\) inversion lines, we found the gas temperature to be between 10 and 18 K. The column density ratios of CCS to NH\(_{3}\) 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 NH\(_{3}\) are consistent with chemical evolutionary ages of \(\lesssim\)10\(^{5}\) yr in the models. Comparisons of the data and chemical models suggest that CCS, HC\(_{3}\)N, and NH\(_{3}\) are sensitive to the chemical evolutionary stages of the sources.