DEUTERIUM FRACTIONATION AS AN EVOLUTIONARY PROBE IN MASSIVE PROTOSTELLAR/CLUSTER CORES

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 (CO)-O-18 (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 Dfrac 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 2 x 10(-7) and 5 x 10(-6). The four quiescent cores have marginal field-neutral coupling and perhaps favor turbulent cooling flows.