ON THE STRUCTURE OF BE STAR DISKS

We investigate the geometrical structure of the emitting part of circu mstellar envelopes around Be stars from an empirical point of view. We use new high-resolution, high-S/N spectroscopic data of the Fe II lam bda 5317 and some other faint Fe II emission lines in 27 Be stars show ing symmetrical emission lines (class 1). We find a clear correlation between its total width (measuring the maximum velocities of circumste llar matter) and the stellar rotational velocity. This correlation mea ns that a typical Be envelope (or, more precisely, that part of it whi ch is visible in optical emission lines) is an axisymmetric, rotationa lly supported disk. For empirical investigation of the vertical struct ure, we use the occurrence of shell lines. We define, as shell criteri on based on Fe II lines, a Be shell star as one with Fe II central int ensity F-cd/F(Fe II) < 1. Using this for calibrating an appropriate p arameter for the much more frequently observed H alpha line, we find t hat shell stars are those with F-p/F-cd(H alpha) greater than or equal to 1.5 where F-p is the mean peak intensity at H alpha. In a sample o f 114 programme stars, we find a shell star fraction of 22.8%. This nu mber is readily transformed into a half opening angle of Be star disks , phi = 13 degrees. We furthermore show that Be disks must be thin at the inner edge, and may become fairly thick at the outer rim. This, to gether with the small value of phi, is evidence for a conical or conca ve shape, the latter typical of a hydrostatically balanced disk. Final ly 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 approximate to 70 degrees and if their outer radius is variable with time.