The Deuterium Fractionation of NH3 in Massive Star-forming Regions

Abstract

Deuteration is sensitive to environmental conditions in star-forming regions. To investigate NH _2 D chemistry, we compared the spatial distribution of ortho-NH _2 D ${1}_{11}^{s}-{1}_{01}^{a}$ , NH _3 (1,1), and NH _3 (2,2) in 12 late-stage massive star-forming regions. By averaging several pixels along the spatial slices of ortho-NH _2 D ${1}_{11}^{s}-{1}_{01}^{a}$ , we obtained the deuterium fractionation of NH _3 . In seven targets, the deuterium fractionation of NH _3 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 NH _3 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 NH _3 is offset from the NH _3 peak in each source, likely because the temperature is higher near the NH _3 peaks and NH _2 D 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.

Keywords

• Massive stars
• Star formation
• Interstellar molecules
• Interstellar abundances