The HNC/HCN Ratio in Star-Forming Regions

TL;DR

This study investigates how environmental factors and chemical reactions influence the HNC/HCN ratio in star-forming regions, revealing the importance of gas-phase reactions and star formation dynamics in explaining observed temperature dependence.

Contribution

It provides a detailed parameter study using updated models to identify key reactions affecting the HNC/HCN ratio and its temperature dependence in star-forming environments.

Findings

H + HNC reaction controls the ratio in static models.

Gas-grain chemistry significantly influences the ratio in warm-up models.

Understanding star formation dynamics is crucial for accurate modeling.

Abstract

HNC and HCN, typically used as dense gas tracers in molecular clouds, are a pair of isomers that have great potential as a temperature probe because of temperature dependent, isomer-specific formation and destruction pathways. Previous observations of the HNC/HCN abundance ratio show that the ratio decreases with increasing temperature, something that standard astrochemical models cannot reproduce. We have undertaken a detailed parameter study on which environmental characteristics and chemical reactions affect the HNC/HCN ratio and can thus contribute to the observed dependence. Using existing gas and gas-grain models updated with new reactions and reaction barriers, we find that in static models the H + HNC gas-phase reaction regulates the HNC/HCN ratio under all conditions, except for very early times. We quantitively constrain the combinations of H abundance and H + HNC reaction barrier that can explain the observed HNC/HCN temperature dependence and discuss the implications in light of new quantum chemical calculations. In warm-up models, gas-grain chemistry contributes significantly to the predicted HNC/HCN ratio and understanding the dynamics of star formation is therefore key to model the HNC/HCN system.