Deuterated methanol in the pre-stellar core L1544

Abstract

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.

8 pages, 9 figures. Accepted in A&A

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