2011ApJ...743..196C

Clouds of high infrared extinction are promising sites of massive star/cluster formation. A large number of cloud cores discovered in recent years allow for the investigation of a possible evolutionary sequence among cores in early phases. We have conducted a survey of deuterium fractionation toward 15 dense cores in various evolutionary stages, from high-mass starless cores to ultracompact H II regions, in the massive star-forming clouds of high extinction, G34.43+0.24, IRAS 18151-1208, and IRAS 18223-1243, with the Submillimeter Telescope. Spectra of N₂H⁺ (3-2), N₂D⁺ (3-2), and C¹⁸O (2-1) were observed to derive the deuterium fractionation of N₂H⁺, D_frac≡ N(N₂D⁺)/N(N₂H⁺), as well as the CO depletion factor for every selected core. Our results show a decreasing trend in D_frac with both gas temperature and line width. Since colder and quiescent gas is likely to be associated with less evolved cores, larger D_frac appears to correlate with early phases of core evolution. Such decreasing trend resembles the behavior of D_frac in the low-mass protostellar cores and is consistent with several earlier studies in high-mass protostellar cores. We also find a moderate increasing trend of D_frac with the CO depletion factor, suggesting that sublimation of ice mantles alters the competition in the chemical reactions and reduces D_frac. Our findings suggest a general chemical behavior of deuterated species in both low- and high-mass protostellar candidates at early stages. In addition, upper limits to the ionization degree are estimated to be within 2x10^–7 and 5x10^–6. The four quiescent cores have marginal field-neutral coupling and perhaps favor turbulent cooling flows.