Aims. Excitation of far-infrared and submillimetric molecular lines may originate from
nonreactive collisions, chemical formation, or far infrared, near-infrared, and optical
fluorescences. As a template, we investigate the impact of each of these processes on the
excitation of the methylidyne cation CH+ and on the intensities of its rotational transitions
recently detected in emission in dense photodissociation regions (PDRs) and in planetary
nebulae. Methods. We have developed a nonlocal thermodynamic equilibrium excitation�…

Context. Particle trapping in local or global pressure maxima in protoplanetary disks is one
of the new paradigms in the theory of the first stages of planet formation. However, finding
observational evidence for this effect is not easy. Recent work suggests that the large ring-
shaped outer disks observed in transition disk sources may in fact be lopsided and
constitute large banana-shaped vortices. Aims. We wish to investigate how effectively dust
can accumulate along the azimuthal direction. We also want to find out if the size-sorting�…