Long-term visual spectrophotometric behaviour of Be stars II. Correlationswith fundamental stellar parameters and interpretation

The long-term visual spectrophotometric (SPh) behaviour of Be stars as a fu nction of fundamental stellar parameters is studied. Some previous SPh resu lts obtained by other authors are confirmed. Moreover, a tendency for tempe rature and aspect angle dependency of SPh variations is found. From the cha racteristics of visual SPh behaviour in Be stars we derive constraints for models of regions in circumstellar envelopes where the Visual continuum spe ctrum is formed: (i) The SPh emission and absorption phases should not impl y preferential aspect angles, as they can both appear whatever the stellar inclination. This phenomenon cannot be always accounted for by strongly fla ttened circumstellar envelopes; (ii) Radii of the visible continuum forming regions cannot be larger than a few R-*; (iii) Electron densities of these regions should not exceed N-e similar to 10(13) cm(-3); (iv) Electron temp erature of circumstellar layers producing the SPh emission phases compare w ith the stellar Balmer continuum radiation temperature and it is much lower in those producing the SPh absorption phases. Three scenarios were studied to produce the observed characteristics of emissions in the V magnitude an d in the second component of Balmer discontinuity (Delta D) during the SPh emission phases: (a) expansion of a massive circumstellar shell that preser ves circumstellar envelope flattening; (b) expansion of a circumstellar she ll which increases the global flattening, so that a disc-like structure is formed; (c) continuous mass ejection that increases the storage of mass in a constant volume with a given flattening. Mechanisms (a) and (b) produce a double valued (Delta V, Delta D) SPh relation, while (c) produces a single valued relation. Only mechanisms (a) and (c) can easily produce the observ ed amounts of emission Delta V and Delta D without violating the modeling c onstraints from (i) to (iv) imposed by observations. The model SPh slopes o f (Delta V, Delta D) show the global sin i and T-eff observed dependencies. The scenarios used to describe the double valued (Delta V, Delta D) sugges t another possible way how to build up circumstellar envelopes around Be st ars.