Possible Habitats for NH₃, NH₂D, H¹³CN, HC¹⁵N, SO, and C¹⁸O in the Initial Conditions of High-mass Star Formation

The initial condition of high-mass 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 N-bearing and deuterated species could remain gaseous under these extreme conditions, suggesting that they may serve as ideal tracers. In this paper, using the Plateau de Bure Interferometer and Very Large Array observations at 1.3 mm, 3.5 mm, and 1.3 cm, we investigate the possible habitats for NH 3 , NH 2 D, H 13 CN, HC 15 N, SO, and C 18 O in eight massive precluster and protocluster clumps G18.17, G18.21, G23.97N, G23.98, G23.44, G23.97S, G25.38, and G25.71. We found that the NH 3 cores are in good agreement with the 3.5 mm peak emission, but the NH 3 is much more extended than the 3.5 mm emission structure. The SO distributions agree well with the 3.5 mm peaks for the evolved star formation stage, but we did not detect any SO emission in the four earliest star formation sources. C 18 O is a poor tracer in conditions of the cold (≲18 K) and dense (≳10 4 cm −3 ) cores, e.g., the prestellar cores. We also found that the NH 2 D cores are mainly located in the temperature range of 13.0–20.0 K, and the NH 2 D lines may be strongly depleted above 20 K.