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-pointing 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$\times$10$^{-3}$ Msun
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 opacties. No obvious correlation for these
clumps was found between infall velocity and the NH3/CCS ratio.
