2018A&A...615A..88B

Context. The abundance of deuterated molecules in a star-forming region is sensitive to the environment in which they are formed. Deuteration fractions, in other words the ratio of a species containing D to its hydrogenated counterpart, therefore provide a powerful tool for studying the physical and chemical evolution of a star-forming system. While local low-mass star-forming regions show very high deuteration ratios, much lower fractions are observed towards Orion and the Galactic centre. Astration of deuterium has been suggested as a possible cause for low deuteration in the Galactic centre.
Aims. We derive methanol deuteration fractions at a number of locations towards the high-mass star-forming region NGC 6334I, located at a mean distance of 1.3kpc, and discuss how these can shed light on the conditions prevailing during its formation.
Methods. We use high sensitivity, high spatial and spectral resolution observations obtained with the Atacama Large Millimeter/submillimeter Array to study transitions of the less abundant, optically thin, methanol-isotopologues: ¹³CH₃OH, CH₃¹⁸OH, CH₂DOH and CH₃OD, detected towards NGC 6334I. Assuming local thermodynamic equilibrium (LTE) and excitation temperatures of ∼120-330K, we derive column densities for each of the species and use these to infer CH₂DOH/CH₃OH and CH₃OD/CH₃OH fractions.
Results. We derive column densities in a range of (0.8-8.3)x10¹⁷cm^–2 for ¹³CH₃OH, (0.13-3.4)x10¹⁷cm^–2 for CH₃¹⁸OH, (0.03-1.63)x10¹⁷cm^–2 for CH₂DOH and (0.15-5.5)x10¹⁷cm^–2 for CH₃OD in a ∼1'' beam. Interestingly, the column densities of CH₃OD are consistently higher than those of CH₂DOH throughout the region by factors of 2-15. We calculate the CH₂DOH to CH₃OH and CH₃OD to CH₃OH ratios for each of the sampled locations in NGC 6334I. These values range from 0.03% to 0.34% for CH₂DOH and from 0.27% to 1.07% for CH₃OD if we use the ¹³C isotope of methanol as a standard; using the ¹⁸ O-methanol as a standard, decreases the ratios by factors of between two and three.
Conclusions. All regions studied in this work show CH₂DOH/CH₃OH as well as CH₂DOH/CH₃OD values that are considerably lower than those derived towards low-mass star-forming regions and slightly lower than those derived for the high-mass star-forming regions in Orion and the Galactic centre. The low ratios indicate a grain surface temperature during formation ∼30K, for which the efficiency of the formation of deuterated species is significantly reduced. Therefore, astration of deuterium in the Galactic centre cannot be the explanation for its low deuteration ratio but rather the high temperatures characterising the region.