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Abstract:
We present Atacama Large Millimeter/submillimeter Array maps of the starless molecular cloud core Ophiuchus/H-MM1 in the lines of deuterated ammonia (ortho-NH2D), methanol (CH3OH), and sulfur monoxide (SO). The dense core is seen in NH2D emission, whereas the CH3OH and SO distributions form a halo surrounding the core.
Because methanol is formed on grain surfaces, its emission highlights regions where desorption from grains is
particularly efficient. Methanol and sulfur monoxide are most abundant in a narrow zone that follows the eastern
side of the core. This side is sheltered from the stronger external radiation field coming from the west. We show
that photodissociation on the illuminated side can give rise to an asymmetric methanol distribution but that the
stark contrast observed in H-MM1 is hard to explain without assuming enhanced desorption on the shaded side.
The region of the brightest emission has a wavy structure that rolls up at one end. This is the signature of Kelvin–Helmholtz instability occurring in sheared flows. We suggest that in this zone, methanol and sulfur are released as aresult of grain–grain collisions induced by shear vorticity.
