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Probing the initial conditions of high-mass star formation -- IV. Gas dynamics and NH2_2D chemistry in high-mass precluster and protocluster clumps

Abstract

The initial stage of star formation is a complex area study because of its high density and low temperature. Under such conditions, many molecules become depleted from the gas phase by freezing out onto dust grains. However, the deuterated species could remain gaseous and are thus ideal tracers. We investigate the gas dynamics and NH2_2D chemistry in eight massive pre/protocluster clumps. We present NH2_2D 111_{11}-101_{01} (at 85.926 GHz), NH3_3 (1, 1) and (2, 2) observations in the eight clumps using the PdBI and the VLA, respectively. We find that the distribution between deuterium fractionation and kinetic temperature shows a number density peak at around Tkin=16.1T_{\rm kin}=16.1 K, and the NH2_2D cores are mainly located at a temperature range of 13.0 to 22.0 K. We detect seven instances of extremely high deuterium fractionation of 1.0β©½Dfracβ©½1.411.0 \leqslant D_{\rm frac} \leqslant 1.41. We find that the NH2_2D emission does not appear to coincide exactly with either dust continuum or NH3_3 peak positions, but often surrounds the star-formation active regions. This suggests that the NH2_{2}D has been destroyed by the central young stellar object (YSO) due to its heating. The detected NH2_2D lines are very narrow with a median width of 0.98Β±0.02km/s\rm 0.98\pm0.02 km/s. The extracted NH2_2D cores are gravitationally bound (Ξ±vir<1\alpha_{\rm vir} < 1), are likely prestellar or starless, and can potentially form intermediate-mass or high-mass stars. Using NH3_3 (1, 1) as a dynamical tracer, we find very complicated dynamical movement, which can be explained by a combined process with outflow, rotation, convergent flow, collision, large velocity gradient, and rotating toroids. High deuterium fractionation strongly depends on the temperature condition. NH2_2D is a poor evolutionary indicator of high-mass star formation in evolved stages, but a useful tracer in the starless and prestellar cores.Comment: 27 pages, 25 figures, 6 tables, accepted for publication in A&

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