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.

arXiv comment14 pages, 4 figures, accepted by SCPMA

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