The Deuterium Fractionation of NH₃ in Massive Star-forming Regions

Deuteration is sensitive to environmental conditions in star-forming regions. To investigate NH₂D chemistry, we compared the spatial distribution of ortho-NH₂D 1 11 s − 1 01 a , NH₃(1,1), and NH₃(2,2) in 12 late-stage massive star-forming regions. By averaging several pixels along the spatial slices of ortho-NH₂D 1 11 s − 1 01 a , we obtained the deuterium fractionation of NH₃. In seven targets, the deuterium fractionation of NH₃ shows a decreasing trend with increasing rotational temperature. This trend is less clear in the remaining five sources, likely due to limited spatial resolution. However, when considering all 12 sources together, the anticorrelation between NH₃ deuterium fractionation and rotational temperature becomes less significant, suggesting that other physical parameters may influence the fractionation. Additionally, we found that the region of highest deuterium fractionation of NH₃ is offset from the NH₃ peak in each source, likely because the temperature is higher near the NH₃ peaks and NH₂D may be depleted from the gas phase as the molecular cloud core evolves, as well as the increased release of CO from grains into the gas phase.