# Efficient methanol production on the dark side of a prestellar core

## TL;DR

This study uses ALMA observations to analyze the asymmetric distribution of methanol and other molecules in a starless core, revealing that grain collisions from shear flows likely enhance desorption on the shaded side.

## Contribution

It provides new evidence linking shear-induced grain collisions to methanol desorption in prestellar cores, highlighting the role of dynamical instabilities.

## Key findings

- Methanol and SO form a halo around the core, concentrated on the shaded side.
- Kelvin-Helmholtz instability signatures are observed in the emission structure.
- Enhanced desorption due to grain collisions explains the asymmetric methanol distribution.

## Abstract

We present ALMA maps of the starless molecular cloud core Ophiuchus/H-MM1 in the lines of deuterated ammonia (ortho-NH2D), methanol (CH3OH), and sulphur 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 sulphur 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 sulphur are released as a result of grain-grain collisions induced by shear vorticity.