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We analyzed both HCN J=1-0 and HNC J=1-0 line profiles to study the inflowmotions in different evolutionary stages of massive star formation: 54 infrareddark clouds (IRDCs), 69 high-mass protostellar object (HMPOs), and 54ultra-compact HII regions (UCHIIs). The inflow asymmetry in HCN spectra seemsto be prevalent throughout all the three evolutionary phases, with IRDCsshowing the largest excess in blue profile. In the case of HNC spectra, theprevalence of blue sources does not appear, excepting for IRDCs. We suggestthat this line is not appropriate to trace inflow motion in evolved stages ofmassive star formation because the abundance of HNC decreases at hightemperatures. This result spotlights the importance of considering chemistry inthe dynamics study of massive star-forming regions. The fact that the IRDCsshow the highest blue excess in both transitions indicates that the most activeinflow occurs in the early phase of star formation, i.e., the IRDC phase ratherthan in the later phases. However, mass is still inflowing onto some UCHIIs. Wealso found that the absorption dips of the HNC spectra in 6 out of 7 bluesources are red-shifted relative to their systemic velocities. Thesered-shifted absorption dips may indicate global collapse candidates, althoughmapping observations with better resolution are needed to examine this featurein more detail.

The inflow signature toward different evolutionary phases of massive star formation