Probing the initial conditions of high-mass star formation

IV. Gas dynamics and NH₂D chemistry in high-mass precluster and protocluster clumps^★

¹ National Astronomical Observatories, Chinese Academy of Sciences, 100101 Beijing, PR China
e-mail: cpzhang@nao.cas.cn
² CAS Key Laboratory of FAST, National Astronomical Observatories, Chinese Academy of Sciences, 100101 Beijing, PR China
³ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁴ South-Western Institute for Astronomy Research, Yunnan University, Kunming, 650500 Yunnan, PR China
⁵ Institute for Astrophysical Research, 725 Commonwealth Ave, Boston University, Boston, MA 02215, USA
⁶ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁷ Istituto di Fisica dello Spazio Interplanetario – INAF, Via Fosso del Cavaliere 100, 00133 Roma, Italy

Received: 17 June 2019
Accepted: 29 April 2020

Abstract

Context. The initial stage of star formation is a complex area of study because of the high densities (n_H2 > 10⁶ cm⁻³) and low temperatures (T_dust < 18 K) involved. 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, which would indicate that they may serve as ideal tracers.

Aims. We investigate the gas dynamics and NH₂D chemistry in eight massive precluster and protocluster clumps (G18.17, G18.21, G23.97N, G23.98, G23.44, G23.97S, G25.38, and G25.71).

Methods. We present NH₂D 1₁₁–1₀₁ (at 85.926 GHz), NH₃ (1, 1), and (2, 2) observations in the eight clumps using the PdBI and the VLA, respectively. We used 3D GAUSSCLUMPS to extract NH₂D cores and provide a statistical view of their deuterium chemistry. We used NH₃ (1, 1) and (2, 2) data to investigate the temperature and dynamics of dense and cold objects.

Results. We find that the distribution between deuterium fractionation and kinetic temperature shows a number density peak at around T_kin = 16.1 K and the NH₂D cores are mainly located at a temperature range of 13.0 to 22.0 K. The 3.5 mm continuum cores have a kinetic temperature with a median width of 22.1 ± 4.3 K, which is obviously higher than the temperature in NH₂D cores. We detected seven instances of extremely high deuterium fractionation of 1.0 ≤ D_frac ≤ 1.41. We find that the NH₂D emission does not appear to coincide exactly with either dust continuum or NH₃ peak positions, but it often surrounds the star-formation active regions. This suggests that the NH₂D has been destroyed by the central young stellar object (YSO) due to heating. The detected NH₂D lines are very narrow with a median width of 0.98 ± 0.02 km s⁻¹, which is dominated by non-thermal broadening. The extracted NH₂D cores are gravitationally bound (α_vir < 1), they are likely to be 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 evidence of 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.

Conclusions. High deuterium fractionation strongly depends on the temperature condition. Tracing NH₂D is a poor evolutionary indicator of high-mass star formation in evolved stages, but it is a useful tracer in starless and prestellar cores.