2021MNRAS.505..726R

We present results of a multiline study of the filamentary infrared dark cloud G351.78-0.54 in the 1.3 and 0.8 mm wavelength bands. The lines of the three isotopologues of carbon monoxide CO, N₂H⁺, CH₃CCH, and HNCO were observed. The aim was to study the general structure of the filamentary cloud, its fragmentation, and physical parameters with the emphasis on properties of dense clumps in this cloud. Several dense clumps are identified from the N₂H⁺ (3-2) data, their masses and virial parameters are determined using the C¹⁸O (2-1) line. Temperatures of some clumps are estimated from the CH₃CCH and HNCO data. Almost all clumps appear to be gravitationally unstable. The density estimates obtained from the C¹⁸O (3-2)/(2-1) and N₂H⁺ (3-2)/(1-0) intensity ratios are in the range n ∼ (0.3-3) x 10⁵ cm^–3. The HNCO emission is detected exclusively towards the first clump which contains the luminous IR source IRAS 17233-3606, and indicates an even higher density. It is observed in the outflow, too. The velocity shift of the higher excitation HNCO lines may indicate a movement of the hot core relative to the surrounding medium. In some clumps there is a velocity shift ∼1 km s^–1 between N₂H⁺ (3-2) and CO isotopologues. The large widths of the N₂H⁺ (3-2) line in the clumps indicate an increase of the velocity dispersion in their dense interiors, which may be related to the star formation process. The N₂H⁺ abundance drops towards the luminous IR source.