A multiline study of the filamentary infrared dark cloud G351.78–0.54

Abstract

ABSTRACT 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) × 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.