The HCN/HNC abundance ratio toward different evolutionary phases of massive star formation

TL;DR

This study measures the HCN/HNC abundance ratio across different stages of massive star formation, revealing an increase with evolutionary progression and temperature, supported by observational data and chemical modeling.

Contribution

It provides the first comprehensive observational analysis of HCN/HNC ratios across multiple star formation stages and tests chemical models explaining their variation.

Findings

HCN/HNC ratio increases from quiescent IRDCs to UCHIIs

Ratios are 0.97, 2.65, 4.17, and 8.96 in respective stages

Chemical modeling suggests a lower energy barrier for HNC to HCN conversion

Abstract

Using the HCN and HNC J=1--0 line observations, the abundance ratio of HCN/HNC has been estimated for different evolutionary stages of massive star formation: Infrared dark clouds (IRDCs), High-mass protostellar object (HMPOs), and Ultra-compact HII regions (UCHIIs). IRDCs were divided into `quiescent IRDC cores' and `active IRDC cores', depending on star formation activity. The HCN/HNC ratio is known to be higher at active and high temperature regions related to ongoing star formation, compared to cold and quiescent regions. Our observations toward 8 quiescent IRDC cores, 16 active IRDC cores, 23 HMPOs, and 31 UCHIIs show consistent results; the ratio is 0.97~($\pm~0.10$), 2.65~($\pm~0.88$), 4.17~($\pm~1.03$) and 8.96~($\pm~3.32$) in these respective evolutionary stages, increasing from quiescent IRDC cores to UCHIIs. The change of the HCN/HNC abundance ratio, therefore, seems directly associated with the evolutionary stages of star formation, which have different temperatures. One suggested explanation for this trend is the conversion of HNC to HCN, which occurs effectively at higher temperatures. To test the explanation, we performed a simple chemical model calculation. In order to fit the observed results, the energy barrier of the conversion must be much lower than the value provided by theoretical calculations.