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4 Discussion

We have reviewed the literature in order to find out if our stars have already been detected as variable stars by other surveys. In the original sample of 5168 objects, we found 8 stars classified as Be stars by Keller et al. (1999). We classified four of them as type-4 (ngc 330/219, ngc 330/235, ngc 346/259 and ngc 346/377), and the others were missed by our visual inspection search method since they show I-band rms lower than 0.02 mag (ngc 330/238, ngc 330/1054, ngc 330/1064 and ngc 330/1239). We also found one type-4 star listed as extreme Be-star by Garmany & Humphreys (1985, their star AV433). Another two type-4 stars are listed as Be/X-ray binaries and one as possible Be/X-ray binary in the compilation by Haberl & Sasaki (2000, their stars 20, 23 and 6, respectively). The fact that eight type-4 stars have been previously classified as Be stars or Be/X-ray binaries suggests that all type-4 stars might be related with Be stars in some way. Their light curve morphology, quite similar to Galactic Be stars, supports this conjecture.

Type-1 light curves are reminiscent of those observed in FU Orionis stars, thought to be caused by accretion events in protostellar disks around young solar-type stars. There is a notable similarity between the shape of the sharp type-1 outbursts with those observed in the extreme classical T Tauri star EX Lupi (Herbig et al. 2001) and the FU Ori star V 1057 Cyg (Bell et al. 1995). However, the amplitude and duration of the outbursts are respectively factors 20 and 1000 smaller in type-1 stars. On the other hand, the general appearance of type-2 light curves, i.e., a rapid rise of luminosity followed by a "plateau'' of rather constant brightness resemble (except for a scale factor) the initial stages of the outbursts observed in V1515 Cyg and FU Ori (Bell et al. 1995). Type-1 light curves are also quite similar in shape to those observed in cataclysmic variables (CVs) of the dwarf novae type. However, these objects experience in general much larger amplitude outbursts ($\sim$2-7 mags), have a rather constant luminosity at maximum and usually return to the same quiescence level after outburst.

The outbursts observed in CVs and FU Ori stars have been modeled by mass transfer instabilities in accretion disks (Meyer & Meyer-Hofmeister 1981; Bell et al. 1995). A heating front propagates across the disk after a certain critical density is reached, pushing the disk into a hot state with high mass transfer rate and consequently high accretion luminosity. Rapid outburst rising times can be produced by outbursts starting in the outer disk and propagating inwards, and slower rising times can be produced by outbursts triggered in the inner disk and propagating outwards. We can speculate that the same phenomenon responsible for FU Ori outbursts is also present in more massive young stars, producing light curve patterns like the ones observed in type-1 and type-2 stars. In this view, the smaller outburst amplitude and shorter duration of type-1 outburst when compared with those observed in FU Ori stars should correspond to accretion of matter with lower mass transfer rates. Sharp and hump-like outbursts should correspond to outside-in and inside-out outbursts, respectively. Type-2 stars could be explained by outside-in outbursts followed by a sustained period of a high mass-transfer rate. A characteristic of the FU Ori outbursts is their larger amplitude at shorter wavelengths, contrary to what is observed in type-1 stars. However, dwarf novae are redder at maximum.

In connection with the above ideas, it is interesting that one of our objects classified as type-1 was identified by Beaulieu et al. (2001) as a possible pre-main sequence star (ESHC2). In principle, this is in contradiction with the Cook et al. (1995) classification of some of these objects as Be stars. It is possible that the presence of Balmer emission not necessarily implies a Be star classification for the "bumpers" detected in the LMC by Cook and collaborators. In fact, the low ionization metal absorption lines usually found in optical spectra of Herbig Ae/Be stars could be hard to detect in low metallicity stellar photospheres like those found in the SMC and LMC. Therefore, it is reasonable to ask if perhaps all of type-1 and type-2 objects are actually pre-main sequence stars with FU Ori-like accretion disk activity. In this case, the mass accretion rates need to be much higher than those observed in normal Be star discs, or even in Be/X-ray binaries, to yield luminosities comparable or larger than the luminosity of the accreting B-type star. Since the nebulosities defining Herbig Ae/Be objects cannot be resolved at the distance of the Magellanic Clouds, the answer to the above question needs to await future spectroscopic studies, especially in the infrared, and possibly very high-resolution imaging studies which could reveal the surrounding nebulosities. We must keep in mind, however, that Hipparcos photometry of Galactic Herbig Ae/Be stars apparently does not reveal type-1 like activity (van den Ancker et al. 1998). Also, the optical light of some Galactic Be stars showing outbursts, like $\lambda$ Eri and $\mu$ Cen, is apparently dominated by the stellar continuum during quiescence, and the spectrum does not reveal the strong emission lines typical of massive Herbig Ae/Be envelopes. Although it is not clear if these Galactic Be stars are the same kind of object that type-1/type-2 stars, we recognize that these two facts are the weak points in the pre-main sequence scenario. We therefore offer an alternative explanation in the next paragraph.

Type-1 star outbursts resemble also in shape the optical outbursts observed in some Be/X-ray binaries (e.g. A 0538-66, Densham et al.  1983), except for their much smaller amplitudes and redder colours. It is currently assumed that in these objects a neutron star companion undergoes episodes of mass accretion when passing through the envelope of a Be star primary (e.g., Okazaki & Negueruela 2001). These high-energy phenomena are no doubt relevant in the X-ray domain, but should not show up at optical and near infrared wavelengths. In addition, the optical counterparts of Galactic and Magellanic Cloud Be/X-ray binaries have spectral types earlier than B2 (Negueruela 1998). This is in principle contrary to the distribution of B-V colours in Fig. 7. However, the similarity between the X-ray type-I and type-II outbursts observed in Be/X-ray binaries and the narrow and hump-like outburst observed in type-1 stars suggest that they could be due to accretion by a white dwarf, not a neutron star. The Be+WD system should produce less energetic outburst, eventually much prominent in the optical and infrared. Favouring this hypothesis is the fact that many Be+WD binaries, even with late-type B stars, are expected from evolutionary calculations (Raguzova 2000; Waters et al. 1989). Only few candidates for these systems have been found up to now, likely due that the white dwarfs, if present, should be hard to detect (Apparao 1991). The hypothesis of accreting white dwarf companions would naturally explain some of the type-1 light curves. The same model for periodic X-ray outbursts in Be/X-ray binaries by Okazaki & Negueruela (2001) could eventually be applied to Be+WD systems. In this model, periodic narrow outbursts occur in systems with high orbital eccentricity, every time that the compact object captures gas from the disc at every periastron passage. Hump-like outbursts occurs in systems with low eccentricity, when the outer disc eventually passes beyond the orbit of the companion. Accordingly to Apparao (1991), observations of time delays between enhancements of optical line and continuum emission could identify Be stars with white dwarf companions. We must keep in mind, that only a small fraction of type-1 objects shows periodic or quasiperiodic outbursts, however.

Concerning type-3 stars, they can be ruled out to be Cepheids, since their large periods (sometimes hundreds of days) are incompatible with their inferred luminosities. On the other hand, the periods are one or two orders of magnitude longer than those observed in Galactic pulsating B stars. This hints at the possibility that type-3 variability is linked to the envelope rather than to the star itself. There is evidence for global one-armed oscillations in the envelopes of Galactic Be stars (Okazaki 1997), but the associated photometric variations are not so strictly periodic as those observed in type-3 stars, the time-scales are longer, about 7 years in average, and, from a theoretical point of view, the oscillations are expected to disappear in the low metallicity environment of the SMC or, alternatively, to slow down, yielding periods in excess of several years (Hummel et al. 2001), in disagreement with the observed type-3 oscillations. The distribution of type-3 stars in the colour-colour diagram of Fig. 8 suggests that these stars are perhaps not be linked to the Be star phenomenon at all. Preliminary spectroscopy of one type-3 object seems to corroborate this view. It shows double H$\alpha$ emission with a deep central absorption and total equivalent width $\sim$100 Å, too large for an isolated Be star (Mennickent et al.  in preparation). We speculate that type-3 stars could be young objects with massive envelopes ongoing some kind of oscillation.


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