Stellar and circumstellar activity of the Be star (omega) CMa - I. Line and continuum emission in 1996-2002

Echelle spectroscopy and mostly unaided-eye photometry of the southern Be star CMa were obtained in the period 19962003. The monitoring is bracketed by two brightenings by ... . The results of a literature search suggest that such phases occur about once a decade and have various commonalities. Along with these photometric events goes enhanced line emission. This is due to an increased total mass of the disk as well as to a change in its density profile. The models by Poeckert & Marlborough (1978, 1979) imply that the enhanced continuum flux originates from the inner disk. Higher-order Balmer line emission is correlated with brightness. The increase in H(alpha) is retarded by some months, possibly indicating a time delay in filling up and ionizing the outer disk. In the (U-B) vs. (B-V) colour diagram and the D54 vs. D34 Balmer decrement diagram the path from the ground to the bright state is distinct from the return path. This could result from the bulk of the disk matter being in the outer (inner) disk during the photometric ground (high) state, while the two transitions between the two states are both due to changes progressing radially outward. Some (mu) Cen-like outbursts (Rivinius et al. 1998c) seem to occur in all phases. It is conceivable that the build-up of the inner disk is caused by more frequent or more effective outbursts. During the photometric bright state various other phenomena gain in prominence and suggest this to be a phase of increased activity. Of particular interest, but possibly only apparently related to this phase, are absorption components at redshifts well beyond the range covered by the combination of rotation and nonradial pulsation.