2010ApJ...713L..50C

We have observed the J = 3 - 2 transition of N₂H⁺ and N₂D⁺ to investigate the trend of deuterium fractionation with evolutionary stage in three selected regions in the infrared dark cloud (IRDC) G28.34+0.06 with the Submillimeter Telescope and the Submillimeter Array. A comprehensible enhancement of roughly 3 orders of magnitude in deuterium fractionation over the local interstellar D/H ratio is observed in all sources. In particular, our sample of massive star-forming cores in G28.34+0.06 shows a moderate decreasing trend over a factor of 3 in the N(N₂D⁺)/N(N₂H⁺) ratio with evolutionary stage, a behavior resembling that previously found in low-mass protostellar cores. This suggests a possible extension for the use of the N(N₂D⁺)/N(N₂H⁺) ratio as an evolutionary tracer to high-mass protostellar candidates. In the most evolved core, MM1, the N₂H⁺(3-2) emission appears to avoid the warm region traced by dust continuum emission and emission of ¹³CO sublimated from grain mantles, indicating an instant release of gas-phase CO. The majority of the N₂H⁺ and N₂D⁺ emission is associated with extended structures larger than 8'' (∼0.2 pc).