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

Abstract

We have observed the 70 μ m dark infrared dark cloud (IRDC) G14.492-00.139 in the N 2 D + J = 3–2, DCO + J = 3–2, DCN J = 3–2, and C 18 O J = 2–1 lines, using the Atacama Large Millimeter/submillimeter Array (ALMA) as part of the ALMA Survey of 70 μ 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 N 2 D + 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 + /N 2 D + 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 + /N 2 D + ratio in this IRDC clump. The low DCO + /N 2 D + ratio can be explained if the temperature of the dense cores is in the range between the sublimation temperatures of N 2 (∼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.