2022ApJ...925..144S

We have observed the 70 µm dark infrared dark cloud (IRDC) G14.492-00.139 in the N₂D⁺J = 3-2, DCO⁺J = 3-2, DCN J = 3-2, and C¹⁸O J = 2-1 lines, using the Atacama Large Millimeter/submillimeter Array (ALMA) as part of the ALMA Survey of 70 µm Dark High-mass Clumps in Early Stages. We find that the spatial distribution is different among the observed emission from the deuterated molecular lines. The N₂D⁺ emission traces relatively quiescent regions, while both the DCO⁺ and DCN emission emanate mainly from regions with signs of active star formation. In addition, the DCO⁺/N₂D⁺ ratio is found to be lower in several dense cores than in starless cores embedded in low-mass star-forming regions. By comparing the observational results with chemical-model calculations, we discuss the origin of the low DCO⁺/N₂D⁺ ratio in this IRDC clump. The low DCO⁺/N₂D⁺ ratio can be explained if the temperature of the dense cores is in the range between the sublimation temperatures of N₂ (∼20 K) and CO (∼25 K). The results suggest that the dense cores in G14.492-00.139 are warmer and denser than the dense cores in low-mass star-forming regions.