Deuterium Fractionation and Ionization Degree in Massive Protostellar/cluster Cores

Abstract

We have conducted a survey of deuterium fractionation of N₂H⁺, R_D(N₂H⁺) ≡ N(N₂D⁺)/N(N₂H⁺), with the Arizona Radio Observatory (ARO) Submillimeter Telescope (SMT) to assess the use of R_D(N₂H⁺) as an evolutionary tracer among massive protostellar/cluster cores in early stages. Our sample includes 32 dense cores in various evolutionary stages, from high-mass starless cores (HMSCs), high-mass protostellar objects (HMPOs), to ultra-compact (UC) HII regions, in infrared dark clouds (IRDCs) and high infrared extinction clouds. The results show a decreasing trend in deuterium fractionation with evolutionary stage traced by gas temperature and line width (Fig. 1). A moderate increasing trend of deuterium fractionation with the CO depletion factor is also found among cores in IRDCs and HMSCs. These suggest a general chemical behavior of deuterated species in low- and high-mass protostellar candidates. Upper limits to the ionization degree are also estimated to be in the range of 4 × 10⁻⁸ − 5 × 10⁻⁶.