Prior to Stothers and Chin (1993, 1994, 1995), LBV eruptions were thought to be irregular (Humphreys and Davidson 1994). Hence proposed explanations were constructed around this aspect of the behaviour of LBVs. Since the shell phase of Cas seems to be a rare event, perhaps the process responsible either is rare itself, or is a rare occurrence in an otherwise ongoing process. As an example of the latter in the context of LBVs, Hummer (1989) suggested that fluctations at the base of the photosphere due to non-radial pulsations may account for eruptions in the LBV P Cyg. Although many non-radial modes may be excited regularly at some location at or near the surface, these modes may normally interfere destructively so that nothing significant happens. However, if they interfere constructively at irregular intervals so that some critical amplitude is exceeded, then a major change in the structure of the atmosphere could occur, perhaps leading to ejection of some matter by the radiation force. Small scale fluctuations of order do occur in LBVs. For example Percy et al. (1988) detected irregular variations of in P Cyg on time scales from a few days to a few months. Perhaps these are manifestations of non-radial pulsations. Non-radial pulsations are strongly suspected to occur in main sequence stars having the same range of spectral type as those of the LBVs at minimum (Baade, 1988; Gies, 1991). Even more appropriate may be the photospheric activity detected over a wide range of wavelengths in the Be star Cen (Peters 1986) and the variations observed in the Be stars Cas and Eri (Smith 1991, 1994).
Lastly, the B[e] stars have properties which seem to suggest some connection between the LBVs and Be stars. The B[e] stars occupy the same area of the HR diagram as do the LBVs and are surrounded by circumstellar matter which has a more disc-like than a spherically symmetric structure (Zickgraf 1989, 1992).
Stothers and Chin (1994) initially suggested that the luminous B[e] stars might be the evolutionary descendents of the LBVs. However, Stothers and Chin (1996) now suspect that the majority of B[e] stars are more likely to be in a late main sequence phase. More interestingly Gummersbach et al. (1995) have discovered 4 B[e] stars in the LMC, whose luminosities suggest they evolved from main sequence stars with masses between 10 and 15 M , the same mass range of many Be stars. Perhaps these B[e] stars are Be stars in a shell phase more extensive than that of Cas during 1933-1942?
© European Southern Observatory (ESO) 1997