A multi-line study of the filamentary infrared dark cloud G351.78-0.54

We present results of a multi-line 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, N$_2$H$^+$, CH$_3$CCH 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 N$_2$H$^+$ (3-2) data, their masses and virial parameters are determined using the C$^{18}$O (2-1) line. Temperatures of some clumps are estimated from the CH$_3$CCH and HNCO data. Almost all clumps appear to be gravitationally unstable. The density estimates obtained from the C$^{18}$O (3-2)/(2-1) and N$_2$H$^+$ (3-2)/(1-0) intensity ratios are in the range $n \sim (0.3-3)\times 10^5$ cm$^{-2}$. The HNCO emission is detected exclusively toward 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 the surrounding medium. In some clumps there is a velocity shift $\sim 1$ km s$^{-1}$ between N$_2$H$^+$ (3-2) and CO isotopologues. The large widths of the N$_2$H$^+$ (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 N$_2$H$^+$ abundance drops toward the luminous IR source.