Low levels of methanol deuteration in the high-mass star-forming region NGC 6334I

Abstract

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 1.3 kpc, 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: 13 CH 3 OH, CH 183 OH, CH 2 DOH and CH 3 OD, detected towards NGC 6334I. Assuming local thermodynamic equilibrium (LTE) and excitation temperatures of ∼120-330 K, we derive column densities for each of the species and use these to infer CH 2 DOH/CH 3 OH and CH 3 OD/CH 3 OH fractions. Results: We derive column densities in a range of (0.8-8.3) × 10 17 cm −2 for 13 CH 3 OH, (0.13-3.4) × 10 17 cm −2 for CH 183 OH, (0.03-1.63) × 10 17 cm −2 for CH 2 DOH and (0.15-5.5) × 10 17 cm −2 for CH 3 OD in a ∼1 00 beam. Interestingly, the column densities of CH 3 OD are consistently higher than those of CH 2 DOH throughout the region by factors of 2-15. We calculate the CH 2 DOH to CH 3 OH and CH 3 OD to CH 3 OH ratios for each of the sampled locations in NGC 6334I. These values range from 0.03% to 0.34% for CH 2 DOH and from 0.27% to 1.07% for CH 3 OD if we use the 13 C 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 CH 2 DOH/CH 3 OH as well as CH 2 DOH/CH 3 OD 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 ∼30 K, 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.