DNC/HNC Ratio of Massive Clumps in Early Evolutionary Stages of High-Mass Star Formation

TL;DR

This study investigates the DNC/HNC ratio in massive star-forming regions, revealing it is lower in high-mass sources than in low-mass cores, and explores how this ratio relates to temperature and evolutionary history.

Contribution

It provides new observational data on DNC/HNC ratios in massive star-forming regions and discusses their dependence on physical conditions and evolutionary history using chemical models.

Findings

DNC/HNC ratio is lower in high-mass sources than in low-mass cores.

The ratio does not depend solely on current kinetic temperature.

Chemical models suggest the ratio decreases after protostar formation.

Abstract

We have observed the HN13C J=1-0 and DNC J=1-0 lines toward 18 massive clumps, including infrared dark clouds (IRDCs) and high-mass protostellar objects (HMPOs), by using the Nobeyama Radio Observatory 45 m telescope. We have found that the HN13C emission is stronger than the DNC emission toward all the observed sources. The averaged DNC/HNC ratio is indeed lower toward the observed high-mass sources (0.009\pm0.005) than toward the low-mass starless and star-forming cores (0.06). The kinetic temperature derived from the NH3 (J, K) = (1, 1) and (2, 2) line intensities is higher toward the observed high-mass sources than toward the low-mass cores. However the DNC/HNC ratio of some IRDCs involving the Spitzer 24 {\mu}m sources is found to be lower than that of HMPOs, although the kinetic temperature of the IRDCs is lower than that of the HMPOs. This implies that the DNC/HNC ratio does not depend only on the current kinetic temperature. With the aid of chemical model simulations, we discuss how the DNC/HNC ratio decreases after the birth of protostars. We suggest that the DNC/HNC ratio in star-forming cores depends on the physical conditions and history in their starless-core phase, such as its duration time and the gas kinetic temperature.