The formation of broad molecular line wings in turbulent boundary layers

The broad (∼ 10 km/s) component seen in the CH + absorption feature towards ζ Oph and the narrower (∼ 3 km/s) component are compatible with the CH + existing at the interface between a diffuse molecular cloud and a more tenuous medium flowing at about 300 km/s. The broad component arises in the turbulent boundary layer at the interface and the narrow component forms in underlying molecular gas which has approached the interface but has not entered the more turbulent parts of the boundary layer. The narrow-component gas is heated by the dissipation of waves generated in the turbulent boundary layer and propagating towards the cloud