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Description
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. We study the amount of deuteration using NH_2_D 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 NH_3_ is derived from the NH_2_D 1_11_-1_01_ortho transition at ~86GHz and NH_2_D 1_11_-1_01_para line at ~110GHz. This is refined for the first time by measuring the NH_2_D excitation temperature directly with the NH_2_D 2_12_-2_02_para transition at ~74GHz. Any variation of NH_3_ deuteration and ortho-to-para ratio with the evolutionary sequence is analysed. Unbiased spectral line surveys at 3mm 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 NH_2_D transition at 74GHz with the IRAM 30m telescope. We determined the deuterium fractionation from the column density ratio of NH_2_D and NH_3_ and measured the NH_2_D excitation temperature for the first time from the simultaneous modelling of the 74 and 110GHz line using MCWeeds. We searched for trends in NH_3_ 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 NH_2_D ortho-to-para ratio. We find a large range of the NH_2_D to NH_3_ column density ratio up to 1.6+/-0.7 indicating a high degree of NH_3_ deuteration in a subsample of the clumps. Our analysis yields a clear difference between NH_3_ and NH_2_D rotational temperatures for a fraction. We therefore advocate observation of the NH_2_D transitions at 74 and 110GHz simultaneously to determine the NH_2_D temperature directly. We determine a median ortho-to-para column density ratio of 3.7+/-1.2. The high detection rate of NH_2_D confirms a high deuteration previously found in massive star-forming clumps. Using the excitation temperature of NH_2_D instead of NH_3_ is needed to avoid an overestimation of deuteration. We measure a higher detection rate of NH_2_D in sources at early evolutionary stages. The deuterium fractionation shows no correlation with evolutionary tracers such as the NH_3_ (1,1) line width, or rotational temperature.
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