Low levels of methanol deuteration in the high-mass star-forming region
  NGC 6334I
  The abundance of deuterated molecules in a star-forming region is sensitive
to the environment in which they are formed. Deuteration fractions 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. 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. We use high sensitivity, high
spatial and spectral resolution observations obtained with ALMA to study
transitions of the less abundant, optically thin, methanol-isotopologues:
(13)CH3OH, CH3(18)OH, CH2DOH and CH3OD, detected towards NGC 6334I. Assuming
LTE and excitation temperatures of 120-330 K, we derive column densities for
each of the species and use these to infer CH2DOH/CH3OH and CH3OD/CH3OH
fractions. Interestingly, the column densities of CH3OD are consistently higher
than those of CH2DOH throughout the region. All regions studied in this work
show CH2DOH/CH3OH as well as CH2DOH/CH3OD ratios 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.
