摘要
We have observed the J = 3 - 2 transition of N 2 H + and N 2 D + 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(N 2 D + )/N(N 2 H + ) 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(N 2 D + )/N(N 2 H + ) ratio as an evolutionary tracer to high-mass protostellar candidates. In the most evolved core, MM1, the N 2 H + (3-2) emission appears to avoid the warm region traced by dust continuum emission and emission of 13 CO sublimated from grain mantles, indicating an instant release of gas-phase CO. The majority of the N 2 H + and N 2 D + emission is associated with extended structures larger than 8″ (∼0.2 pc). © 2010. The American Astronomical Society. All rights reserved.