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Description
The initial stage of star formation is very difficult to study because of its high density (n_H_2> 10^6^cm^-3^) and low temperature (T_dust_<18K). Under such conditions, many molecules become depleted from the gas phase by freezing out onto dust grains. However, the deuterated species could remain gaseous under these extreme conditions and are thus ideal tracers. We investigate the gas dynamics and NH_2_D chemistry in eight massive pre/protocluster clumps (G18.17, G18.21, G23.97N, G23.98, G23.44, G23.97S, G25.38, and G25.71). We present NH_2_D 1_11-1_01 (at 85.926GHz), NH_3_ (1,1) and (2,2) observations in the eight clumps using the PdBI and the VLA, respectively. We use 3D GAUSSCLUMPS to extract NH_2_D cores and provide a statistical view of their deuterium chemistry. We use NH_3_ (1,1) and (2,2) data to investigate the temperature and dynamics of dense and cold objects. We find that the distribution between deuterium fractionation and kinetic temperature shows a number density peak at around T_kin_=16.1K, and the NH_2_D cores are mainly located at a temperature range of 13.0 to 22.0K. The 3.5mm continuum cores have a kinetic temperature with the median width of 22.1+/-4.3K, which is obviously higher than the temperature in NH_2_D cores. We detect seven extremely high deuterium fractionation of 1.0<=D_frac_<=1.41. We find that the NH_2_D emission does not appear to coincide exactly with either dust continuum or NH3 peak positions, but often surrounds the star-formation active regions. This suggests that the NH_2_D has been destroyed by the central young stellar object (YSO) due to its heating. The detected NH_2_D lines are very narrow with a median width of 0.98+/-0.02km/s, which is dominated by non-thermal broadening. The extracted NH_2_D cores are gravitationally bound ({alpha}_vir_<1), are likely prestellar or starless, and can potentially form intermediate-mass or high-mass stars in future. Using NH3 (1,1) as a dynamical tracer, we find very complicated dynamical movement in all the eight clumps, 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. NH_2_D is a poor evolutionary indicator of high-mass star formation in evolved stages, but a useful tracer in the starless and prestellar cores.
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