The inflow signature toward different evolutionary phases of massive star formation

TL;DR

This study investigates inflow motions in massive star formation regions across different evolutionary stages using line profile analysis, revealing early-phase inflow activity and the importance of chemistry in interpreting dynamics.

Contribution

It demonstrates that HCN is a reliable tracer of inflow across all stages, while HNC is less effective in evolved stages, highlighting the role of chemistry in star formation dynamics.

Findings

Inflow signatures are prevalent in all stages, especially in IRDCs.

HNC is less suitable for tracing inflow in later stages due to chemical changes.

Red-shifted absorption dips suggest potential global collapse in some sources.

Abstract

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.