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We present ALMA maps of the starless molecular cloud core Ophiuchus/H-MM1 inthe lines of deuterated ammonia (ortho-NH2D), methanol (CH3OH), and sulphurmonoxide (SO). The dense core is seen in NH2D emission, whereas the CH3OH andSO distributions form a halo surrounding the core. Because methanol is formedon grain surfaces, its emission highlights regions where desorption from grainsis particularly efficient. Methanol and sulphur monoxide are most abundant in anarrow zone that follows the eastern side of the core. This side is shelteredfrom the stronger external radiation field coming from the west. We show thatphotodissociation on the illuminated side can give rise to an asymmetricmethanol distribution, but that the stark contrast observed in H-MM1 is hard toexplain without assuming enhanced desorption on the shaded side. The region ofthe brightest emission has a wavy structure that rolls up at one end. This isthe signature of Kelvin-Helmholtz instability occurring in sheared flows. Wesuggest that in this zone, methanol and sulphur are released as a result ofgrain-grain collisions induced by shear vorticity.

Efficient methanol production on the dark side of a prestellar core