ALMA Observations of the IRDC Clump G34.43+00.24 MM3: DNC/HNC Ratio

TL;DR

This study uses ALMA to observe the IRDC clump G34.43+00.24 MM3, revealing the distribution and ratio of DNC/HNC, which provides insights into the chemical environment and density structure of the star-forming region.

Contribution

First high-resolution ALMA observations of DNC and HN$^{13}$C in G34.43+00.24 MM3, showing different spatial distributions and a higher DNC/HNC ratio than previous single-dish measurements.

Findings

DNC emission is more extended and stronger than HN$^{13}$C near the hot core.

DNC/HNC ratio exceeds 0.06, much higher than single-dish results.

High-density regions have higher DNC/HNC ratios, indicating density-dependent chemistry.

Abstract

We have observed the clump G34.43+00.24 MM3 associated with an infrared dark cloud in DNC $J$=3--2, HN$^{13}$C $J$=3--2, and N$_2$H$^+$ $J$=3--2 with the Atacama Large Millimeter/submillimeter Array (ALMA). The N$_2$H$^+$ emission is found to be relatively weak near the hot core and the outflows, and its distribution is clearly anti-correlated with the CS emission. This result indicates that a young outflow is interacting with cold ambient gas. The HN$^{13}$C emission is compact and mostly emanates from the hot core, whereas the DNC emission is extended around the hot core. Thus, the DNC and HN$^{13}$C emission traces warm regions near the protostar differently. The DNC emission is stronger than the HN$^{13}$C emission toward most parts of this clump. The DNC/HNC abundance ratio averaged within a $15^{\prime\prime} \times 15^{\prime\prime}$ area around the phase center is higher than 0.06. This ratio is much higher than the value obtained by the previous single-dish observations of DNC and HN$^{13}$C $J$=1--0 ($\sim$0.003). It seems likely that the DNC and HNC emission observed with the single-dish telescope traces lower density envelopes, while that observed with ALMA traces higher density and highly deuterated regions. We have compared the observational results with chemical-model results in order to investigate the behavior of DNC and HNC in the dense cores. Taking these results into account, we suggest that the low DNC/HNC ratio in the high-mass sources obtained by the single-dish observations are at least partly due to the low filling factor of the high density regions.