Abstract We investigate the infall properties in a sample of 11 infrared dark clouds (IRDCs) showing blue-asymmetry signatures in HCO+J=1–0 line profiles. We used JCMT to conduct mapping observations in HCO+J =4–3 as well as single-point observations in HCO+J =3–2, towards 23 clumps in these IRDCs. We applied the HILL model to fit these observations and derived infall velocities in the range of 0.5−2.7 km s−1, with a median value of 1.0 km s−1, and obtained mass accretion rates of 0.5– 14 ×10−3 \(M_{\odot}\) yr−1. These values are comparable to those found in massive star forming clumps in later evolutionary stages. These IRDC clumps are more likely to form star clusters. HCO+J =3–2 and HCO+J =1–0 were shown to trace infall signatures well in these IRDCs with comparable inferred properties. HCO+J =4–3, on the other hand, exhibits infall signatures only in a few very massive clumps, due to smaller opacities. No obvious correlation for these clumps was found between infall velocity and the NH3/CCS ratio.
Keywords stars: formation — ISM: clouds — star formation: kinematics and dynamics
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