Deuterated methanol in the pre-stellar core L1544
  Using the IRAM 30m telescope, we mapped the methanol emission in the
pre-stellar core L1544 and observed singly deuterated methanol (CH$_2$DOH and
CH$_3$OD) towards the dust peak of L1544. Non-LTE radiative transfer modelling
was performed on three CH$_3$OH emissions lines at 96.7 GHz, using a
Bonnor-Ebert sphere as a model for the source. We have also assumed a centrally
decreasing abundance profile to take the molecule freeze-out in the inner core
into account. The column density of CH$_2$DOH was derived assuming LTE
excitation and optically thin emission. The CH$_3$OH emission has a highly
asymmetric morphology, resembling a non-uniform ring surrounding the dust peak,
where CO is mainly frozen onto dust grains. The observations provide an
accurate measure of methanol deuteration in the cold pre-stellar gas. The
derived abundance ratio is [CH$_2$DOH]/[CH$_3$OH] $= 0.10\pm 0.03$, which is
significantly smaller than the ones found in low-mass Class 0 protostars and
smaller than the deuterium Fraction measured in other molecules towards L1544.
The low deuterium fractionation observed in L1544 and the morphology of the
CH$_3$OH emission suggest that we are mainly tracing the outer parts of the
core, where CO just started to freeze-out onto dust grains.
