2021A&A...649A..21W

Context. Deuteration has been used as a tracer of the evolutionary phases of low- and high-mass star formation. The APEX Telescope Large Area Survey (ATLASGAL) provides an important repository for a detailed statistical study of massive star-forming clumps in the inner Galactic disc at different evolutionary phases.
Aims. We study the amount of deuteration using NH₂D in a representative sample of high-mass clumps discovered by the ATLASGAL survey covering various evolutionary phases of massive star formation. The deuterium fraction of NH₃ is derived from the NH₂D 1₁₁-1₀₁ ortho transition at ∼86GHz and NH₂D 1₁₁-1₀₁ para line at ∼110GHz. This is refined for the first time by measuring the NH₂D excitation temperature directly with the NH₂D 2₁₂-2₀₂ para transition at ∼74GHz. Any variation of NH₃ deuteration and ortho-to-para ratio with the evolutionary sequence is analysed.
Methods. Unbiased spectral line surveys at 3 mm were conducted towards ATLASGAL clumps between 85 and 93GHz with the Mopra telescope and from 84 to 115GHz using the IRAM 30m telescope. A subsample was followed up in the NH₂D transition at 74GHz with the IRAM 30m telescope. We determined the deuterium fractionation from the column density ratio of NH₂D and NH₃ and measured the NH₂D excitation temperature for the first time from the simultaneous modelling of the 74 and 110GHz line using MCWeeds. We searched for trends in NH₃ deuteration with the evolutionary sequence of massive star formation. We derived the column density ratio from the 86 and 110GHz transitions as an estimate of the NH₂D ortho-to-para ratio.
Results. We find a large range of the NH₂D to NH₃ column density ratio up to 1.6±0.7 indicating a high degree of NH₃ deuteration in a subsample of the clumps. Our analysis yields a clear difference between NH₃ and NH₂D rotational temperatures for a fraction. We therefore advocate observation of the NH₂D transitions at 74 and 110GHz simultaneously to determine the NH₂D temperature directly. We determine a median ortho-to-para column density ratio of 3.7±1.2.
Conclusions. The high detection rate of NH₂D confirms a high deuteration previously found in massive star-forming clumps. Using the excitation temperature of NH₂D instead of NH₃ is needed to avoid an overestimation of deuteration. We measure a higher detection rate of NH₂D in sources at early evolutionary stages. The deuterium fractionation shows no correlation with evolutionary tracers such as the NH₃ (1,1) line width, or rotational temperature.