1996A&A...308..170H

We investigate the geometrical structure of the emitting part of circumstellar envelopes around Be stars from an empirical point of view. We use new high-resolution, high-S/N spectroscopic data of the FeII λ5317 and some other faint FeII emission lines in 27 Be stars showing symmetrical emission lines (class 1). We find a clear correlation between its total width (measuring the maximum velocities of circumstellar matter) and the stellar rotational velocity. This correlation means that a typical Be envelope (or, more precisely, that part of it which is visible in optical emission lines) is an axisymmetric, rotationally supported disk. For empirical investigation of the vertical structure, we use the occurrence of shell lines. We define, as shell criterion based on FeII lines, a Be shell star as one with FeII central intensity F_cd/F_*(FeII)<1. Using this for calibrating an appropriate parameter for the much more frequently observed Hα line, we find that shell stars are those with F_p/F_cd(Hα)≥1.5 where F_p is the mean peak intensity at Hα. In a sample of 114 programme stars, we find a shell star fraction of 22.8%. This number is readily transformed into a half opening angle of Be star disks, φ=13deg. We furthermore show that Be disks must be thin at the inner edge, and may become fairly thick at the outer rim. This, together with the small value of φ, is evidence for a conical or concave shape, the latter typical of a hydrostatically balanced disk. Finally we provide evidence that the famous "shell-Be" phase transitions can naturally occur in such disks as a geometrical effect if they are seen under inclination i≃70deg and if their outer radius is variable with time.