1999A&A...348..831R

New high-resolution echelle spectra are presented for all 6 B-type stars currently known to exhibit so-called central quasi-emission peaks (CQE's). Empirical requirements are derived on the circumstances which favour the appearance of such features. The presence of a circumstellar disk seen edge-on is the most important. Furthermore, the disk should be optically thin in the continuum, have a small spatial extent, and show little line broadening. Both this set of conditions and the observed line profiles are compared to a model by Hanuschik (1995A&A...295..423H) for the formation of shell lines in circumstellar disks with Keplerian rotation. This model predicts not only the existence of CQE's, but also the same actual circumstances of their occurrence. They result from the local minimum at zero radial velocity in the fraction of the stellar disk that is occulted by circumstellar gas in Keplerian orbital motion, i.e. by gas moving perpendicular to the line of sight. In particular, CQE's are in spite of their name not related to any emission process but are a pure absorption phenomenon. All currently available observations of CQE's are consistent with this model, which appears to have considerable diagnostic potential for the understanding of the structure and dynamics of Be star disks. Only one of the 6 stars (ν Pup) was not previously known to ever have displayed shell or only emission lines. But the new Hα and Hβ profiles clearly show the presence of variable amounts of circumstellar matter. Previously suggested photospheric explanations for CQE's are nevertheless briefly examined. In a given star, CQE's seem to appear with the highest probability at times when the innermost regions of the disk are being re-supplied with matter. With the success of Hanuschik's model, CQE's furthermore become one of the most important indicators of rotational support of disks of Be stars. Together with other evidence for rotation compiled from the literature, this leads to the conclusion that models for the formation of disks need to include a mechanism for sufficient angular momentum transfer. In the context of Hanuschik's model for CQE's, the considerable acceleration inherent to wind-compressed disks (WCD's) presents an additional difficulty for the WCD model in its basic form.