The Chemical Structure of Young High-mass Star-forming Clumps. I. Deuteration

The chemical structure of high-mass star nurseries is important for a general understanding of star formation. Deuteration is a key chemical process in the earliest stages of star formation because its efficiency is sensitive to the environment. Using the IRAM-30 m telescope at 1.3--4.3 mm wavelengths, we have imaged two parsec-scale high-mass protostellar clumps (P1 and S) that show different evolutionary stages but are located in the same giant filamentary {infrared dark cloud} G28.34+0.06. Deep spectral images at subparsec resolution reveal the dust and gas physical structures of both clumps. We find that (1) the low-$J$ lines of $\rm N_2H^+$, HCN, HNC, and $\rm HCO^+$ isotopologues are subthermally excited; and (2) the deuteration of $\rm N_2H^+$ is more efficient than that of $\rm HCO^+$, HCN, and HNC by an order of magnitude. The deuterations of these species are enriched toward the chemically younger clump S compared with P1, indicating that this process favors the colder and denser environment ($\rm T_{kin} \sim14 K$, $\rm N(NH_3) \sim 9\times 10^{15}\,cm^{-2}$). In contrast, single deuteration of $\rm NH_3$ is insensitive to the environmental difference between P1 and S; and (3) single deuteration of $\rm CH_3OH$ ($\rm > 10\%$) is detected toward the location where CO shows a depletion of $\sim10$. This comparative chemical study between P1 and S links the chemical variations to the environmental differences and shows chemical similarities between the early phases of high- and low-mass star-forming regions.