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 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.