Deuterium Fractionation as an Evolutionary Probe in Massive Proto-stellar/cluster Cores

Abstract: Clouds of high infrared extinction are promising sites of massive star/cluster formation. A large number of cloud cores discovered in recent years allows investigation of 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 Hii 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 (SMT). Spectra of N2H+ (3 - 2), N2D+ (3 - 2), and C18O (2 - 1) were observed to derive the deuterium fractionation of N2H+, Dfrac \equiv N(N2D+)/N(N2H+), as well as the CO depletion factor for every selected core. Our results show a decreasing trend in Dfrac with both gas temperature and linewidth. Since colder and quiescent gas is likely to be associated with less evolved cores, larger Dfrac appears to correlate with early phases of core evolution. Such decreasing trend resembles the behavior of Dfrac 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 Dfrac with the CO depletion factor, suggesting that sublimation of ice mantles alters the competition in the chemical reactions and reduces Dfrac. Our findings suggest a general chemical behavior of deuterated species in both low- and high-mass proto-stellar candidates at early stages. In addition, upper limits to the ionization degree are estimated to be within 2 \times 10^-7 and 5 \times 10^-6. The four quiescent cores have marginal field-neutral coupling and perhaps favor turbulent cooling flows.