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.

中文翻译：

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丝状红外暗云 G351.78-0.54 的多线研究

我们展示了在 1.3 和 0.8 毫米波段中对丝状红外暗云 G351.78-0.54 进行多线研究的结果。观察到一氧化碳 CO、N2H+、CH3CCH 和 HNCO 三种同位素体的谱线。目的是研究丝状云的一般结构、碎裂和物理参数，重点是该云中致密团块的性质。从 N2H+ (3-2) 数据中确定了几个致密团块，它们的质量和维里参数使用 C18O (2-1) 线确定。从 CH3CCH 和 HNCO 数据估计一些团块的温度。几乎所有的团块似乎都是重力不稳定的。从 C18O (3-2)/(2-1) 和 N2H+ (3-2)/(1-0) 强度比获得的密度估计值在 n ∼ (0.3-3) × 105 cm-3 范围内。HNCO 发射仅在包含发光 IR 源 IRAS 17233-3606 的第一个团块中检测到，并表明密度更高。它也在流出中观察到。较高激发 HNCO 线的速度变化可能表明热芯相对于周围介质的运动。在一些团块中，在 N2H+ (3-2) 和 CO 同位素体之间存在 ∼1 km s-1 的速度偏移。团块中 N2H+ (3-2) 线的大宽度表明其致密内部的速度分散增加，这可能与恒星形成过程有关。N2H+ 丰度向发光的 IR 源下降。较高激发 HNCO 线的速度变化可能表明热芯相对于周围介质的运动。在一些团块中，在 N2H+ (3-2) 和 CO 同位素体之间存在 ∼1 km s-1 的速度偏移。团块中 N2H+ (3-2) 线的大宽度表明其致密内部的速度分散增加，这可能与恒星形成过程有关。N2H+ 丰度向发光的 IR 源下降。较高激发 HNCO 线的速度变化可能表明热芯相对于周围介质的运动。在一些团块中，在 N2H+ (3-2) 和 CO 同位素体之间存在 ∼1 km s-1 的速度偏移。团块中 N2H+ (3-2) 线的大宽度表明其致密内部的速度分散增加，这可能与恒星形成过程有关。N2H+ 丰度向发光的 IR 源下降。