The inflow signature toward different evolutionary phases of massive
  star formation
  We analyzed both HCN J=1-0 and HNC J=1-0 line profiles to study the inflow
motions in different evolutionary stages of massive star formation: 54 infrared
dark clouds (IRDCs), 69 high-mass protostellar object (HMPOs), and 54
ultra-compact HII regions (UCHIIs). The inflow asymmetry in HCN spectra seems
to be prevalent throughout all the three evolutionary phases, with IRDCs
showing the largest excess in blue profile. In the case of HNC spectra, the
prevalence of blue sources does not appear, excepting for IRDCs. We suggest
that this line is not appropriate to trace inflow motion in evolved stages of
massive star formation because the abundance of HNC decreases at high
temperatures. This result spotlights the importance of considering chemistry in
the dynamics study of massive star-forming regions. The fact that the IRDCs
show the highest blue excess in both transitions indicates that the most active
inflow occurs in the early phase of star formation, i.e., the IRDC phase rather
than in the later phases. However, mass is still inflowing onto some UCHIIs. We
also found that the absorption dips of the HNC spectra in 6 out of 7 blue
sources are red-shifted relative to their systemic velocities. These
red-shifted absorption dips may indicate global collapse candidates, although
mapping observations with better resolution are needed to examine this feature
in more detail.
