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.

中文翻译：

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在前恒星核心的阴暗面高效生产甲醇

我们在氘化氨 (ortho-NH2D)、甲醇 (CH3OH) 和一氧化硫 (SO) 的谱线上展示了无星分子云核心蛇夫座/H-MM1 的 ALMA 地图。在 NH2D 发射中可以看到致密的核心，而 CH3OH 和 SO 分布在核心周围形成一个晕圈。因为甲醇是在颗粒表面形成的，它的排放突出了从颗粒中解吸特别有效的区域。甲醇和一氧化硫在岩心东侧的狭窄区域中含量最高。这一侧可以避开来自西方的更强的外部辐射场。我们表明，光照侧的光解会引起不对称的甲醇分布，但如果不假设阴影侧的解吸增强，则很难解释在 H-MM1 中观察到的鲜明对比。最亮的发射区域具有一端卷起的波浪状结构。这是发生在剪切流中的开尔文-亥姆霍兹不稳定性的特征。我们认为，在该区域中，由于剪切涡流引起的颗粒-颗粒碰撞而释放出甲醇和硫。