Probing the initial conditions of high-mass star formation

Abstract

Context. The initial stage of star formation is a complex area of study because of the high densities ( n H 2 > 10 6 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 NH 2 D 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 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 used 3D GAUSSCLUMPS to extract NH 2 D cores and provide a statistical view of their deuterium chemistry. We used NH 3 (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 NH 2 D 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 NH 2 D cores. We detected seven instances of 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 NH 3 peak positions, but it 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 heating. The detected NH 2 D 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 NH 2 D 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 NH 3 (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 NH 2 D is a poor evolutionary indicator of high-mass star formation in evolved stages, but it is a useful tracer in starless and prestellar cores.