2021RAA....21..208X

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 x 10-3 M☉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.