ATLASGAL-selected massive clumps in the inner Galaxy
Résumé
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 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 ~86 GHz and NH 2 D 1 11 −1 01 para line at ~110 GHz. 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 ~74 GHz. Any variation of NH 3 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 93 GHz with the Mopra telescope and from 84 to 115 GHz using the IRAM 30m telescope. A subsample was followed up in the NH 2 D transition at 74 GHz 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 110 GHz 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 110 GHz transitions as an estimate of the NH 2 D ortho-to-para ratio. Results. 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 110 GHz simultaneously to determine the NH 2 D temperature directly. We determine a median ortho-to-para column density ratio of 3.7 ± 1.2. Conclusions. 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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