Probing the initial conditions of high-mass star formation -- IV. Gas dynamics and NH$_2$D chemistry in high-mass precluster and protocluster clumps

• Main Authors: Zhang, Chuan-Peng, Li, Guang-Xing, Pillai, Thushara, Csengeri, Timea, Wyrowski, Friedrich, Menten, Karl M, Pestalozzi, Michele R
• Format: Journal Article
• Language: English
• Published: 28-04-2020
• Subjects: Physics - Astrophysics of Galaxies; Physics - Solar and Stellar Astrophysics
• DOI: 10.48550/arxiv.2004.13946

A&A 638, A105 (2020) 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 NH$_2$D chemistry in eight massive
pre/protocluster clumps. We present NH$_2$D 1$_{11}$-1$_{01}$ (at 85.926 GHz),
NH$_3$ (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
$T_{\rm kin}=16.1$ K, and the NH$_2$D 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 $1.0 \leqslant D_{\rm frac} \leqslant 1.41$. We find that the
NH$_2$D emission does not appear to coincide exactly with either dust continuum
or NH$_3$ 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 $\rm 0.98\pm0.02 km/s$. The extracted
NH$_2$D cores are gravitationally bound ($\alpha_{\rm vir} < 1$), are likely
prestellar or starless, and can potentially form intermediate-mass or high-mass
stars. Using NH$_3$ (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. 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.