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.