Studying infall in infrared dark clouds with multiple HCO+ transitions

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 ⊙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.