Deuterium Fractionation as an Evolutionary Probe in the Infrared Dark Cloud G28.34+0.06

TL;DR

This study investigates deuterium fractionation in an infrared dark cloud, revealing significant enhancement and potential as an evolutionary indicator for high-mass star-forming regions.

Contribution

It demonstrates the use of N(N2D+)/N(N2H+) ratio as an evolutionary tracer in high-mass star formation, extending previous low-mass core studies.

Findings

Deuterium fractionation is enhanced by about 3 orders of magnitude.

The N(N2D+)/N(N2H+) ratio decreases moderately with evolutionary stage.

N2H+ emission avoids warm regions in the most evolved core.

Abstract

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 (SMT) and the Submillimeter Array (SMA). 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 what 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).