The ALMA Survey of 70 mu m Dark High-mass Clumps in Early Stages (ASHES). V. Deuterated Molecules in the 70 mu m Dark IRDC G14.492-00.139

We have observed the 70 mu m dark infrared dark cloud (IRDC) G14.492-00.139 in the N2D+ J = 3-2, DCO+ J = 3-2, DCN J = 3-2, and (CO)-O-18 J = 2-1 lines, using the Atacama Large Millimeter/submillimeter Array (ALMA) as part of the ALMA Survey of 70 mu m 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 N-2 (similar to 20 K) and CO (similar to 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.