The chemical structure of young high-mass star-forming clumps: (I) Deuteration.

Abstract

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.