[Paper Review] The TMRT K Band Observations towards 26 Infrared Dark Clouds: NH$_{3}$, CCS, and HC$_{3}$N

Jinjin Xie, G. A. Fuller+10 more|arXiv (Cornell University)|Mar 24, 2021

Astrophysics and Star Formation StudiesPhysics and Astronomy83 references23 citations

TL;DR

This study presents the first TMRT K-band survey of 26 infrared dark clouds (IRDCs), simultaneously detecting NH₃, CCS, and HC₃N to probe early massive star formation. Using the hyperfine group ratio (HFGR) method, it finds gas temperatures of 10–18 K and shows CCS/NH₃ column density ratios <10⁻², indicating IRDCs are chemically young (≲10⁵ yr), consistent with NAUTILUS chemical models.

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.

Motivation & Objective

• To investigate the chemical evolution of infrared dark clouds (IRDCs) at the onset of massive star formation.
• To measure gas temperatures, column densities, and velocity dispersions in a representative sample of 26 IRDCs.
• To determine the column density ratios of carbon-chain molecules (CCMs) like CCS and N-bearing molecules like NH₃ to assess chemical evolutionary stages.
• To test the consistency of observed molecular abundances with the NAUTILUS three-phase chemical evolution model.
• To evaluate the reliability of CCS and NH₃ as chemical age indicators in high-mass star-forming regions.

Proposed method

• Conducted single-pointing observations with the 65 m Shanghai Tian Ma Radio Telescope (TMRT) at K-band (18–26.5 GHz) toward 26 IRDCs.
• Simultaneously observed NH₃ (1,1), (2,2), (3,3), (4,4) inversion transitions and CCS (21→10) and HC₃N (J=2–1) rotational lines.
• Applied the hyperfine group ratio (HFGR) method to fit NH₃ inversion lines for accurate excitation temperature and optical depth determination.
• Used CLASS/GILDAS software for data reduction, baseline subtraction, and spectral fitting with 1.431 kHz spectral resolution.
• Calculated column densities using partition functions and main beam brightness temperatures, with beam efficiency and calibration corrections applied.
• Performed chemical evolution simulations using the NAUTILUS model to compare observed column density ratios with evolutionary tracks.

Experimental results

Research questions

• RQ1 What are the physical conditions (temperature, velocity dispersion, column density) in a representative sample of 26 IRDCs?
• RQ2 How do the column density ratios of CCS to NH₃ in IRDCs compare to those in low-mass star-forming regions, and what does this imply for chemical evolution?
• RQ3 What is the inferred chemical evolutionary age of these IRDCs based on observed CCM and N-bearing molecule abundances?
• RQ4 Are the observed molecular abundances consistent with the predictions of the NAUTILUS three-phase chemical model?
• RQ5 How do the distributions and abundances of CCS, HC₃N, and NH₃ trace the early stages of massive star formation?

Key findings

• Gas temperatures in the 26 IRDCs were determined to be between 10 and 18 K using the HFGR method on NH₃ inversion lines.
• The CCS/NH₃ column density ratio is less than 10⁻² for most IRDCs, indicating a low abundance of carbon-chain molecules relative to ammonia.
• The survey increased the number of CCS-detected IRDCs from 8 to 23, enabling robust statistical analysis of CCM/N-bearing molecule ratios in massive star-forming regions.
• Chemical evolution modeling with NAUTILUS indicates that the observed column density ratios are consistent with evolutionary ages ≤10⁵ yr.
• The observed molecular abundances of CCS, HC₃N, and NH₃ are sensitive to the chemical evolutionary stage, supporting their use as chemical clocks in IRDCs.
• The study confirms that CCS abundance is suppressed in IRDCs, suggesting this is an intrinsic feature of high-mass star-forming regions.