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Astron. Astrophys. 362, 1020-1040 (2000)

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8. Conclusions

The most important results presented in this study are:

  • the finding that the underlying star is in fact a B1.5-B2 rapidly rotating main sequence object and the estimation of corresponding mass and radius

  • the confirmation of multiperiodicity in line profile variations and the presence of the same periods (within the error bars) in two sets obtained in very different conditions and at a 4-year interval. Many frequencies are commonly found in EW, FWHM, centroid RV, V and R emissions and lpv of He I 6678, and some of them are also present in H[FORMULA]

  • the beat phenomenon between the 1.39 and 1.55 c/d frequencies which is rather conspicuous in the RV of the He I centroid

  • the linking of the four main detected frequencies to nrp. Possible solutions for frequencies 1.39, 2.76, 1.55 and 3.20 c/d indicate modes with [FORMULA] and [FORMULA]

  • the detection of pronounced minima in He I 6678 V emission with a period coherent with the stellar rotation period suggesting rotational modulation of CS material close to the star

  • the evidence of signs of episodic matter outflows during the 1993 campaign such as a variable broad additional "pseudo-photospheric" component, discrete narrow blue-shifted absorptions and polarized flux enhancements

  • the detection of blobs temporarily orbiting the star and giving rise to an expelled ring-like envelope

  • the close correlation between continuum polarization and He I 6678 strength variations

  • we also present an interpretation which reproduces the behaviour of the 1993 additional pseudo-photospheric contribution.

The pseudo-photosphere in EW Lac was found to be highly variable during the 8-day run in August-September 1993. It was probably the result of successive mass loss events by which matter is more or less continuously transferred to the envelope, and it seems that we observed at least one of these discrete events. We can imagine a rapid rotational acceleration of the upper photospheric layers and the expulsion of patchy blobs in the form of a ring-like structure with decreasing rotational speed. The different examples mentioned in Sect. 7.2 do support the formation of a rather cold, dense, rapidly rotating region between the "classical" photosphere and the envelope responsible for emission and shell lines. An important point is the close correlation between continuum polarization and He I 6678, which is expected from models, as the polarized flux comes from regions close to the photosphere (Poeckert & Marlborough 1979). The gradual mid-term increase of H[FORMULA] can be explained both by the vanishing of the pseudo-photosphere and by some of the gas being added to the overall density of the line-forming region farther out. But the relation between H[FORMULA] and polarization is not clear. Note that Sonneborn et al. (1988) found for [FORMULA] Ori an increase in polarization rate correlated with brightness, but unrelated to H[FORMULA] behaviour.

The description of He I 6678 line behaviour, considered in the context of circumstellar rings, may actually reveal the presence of phenomena related to circumstellar envelope building mechanisms. In fact, the expansion velocities needed to account for the shapes of line profiles are supersonic. The rings may then represent shock fronts due to wind-circumstellar envelope interaction phenomena. These shock fronts are in particular expected in the ablation process of parcels of matter ejected by the stellar wind. The mechanism leads to a mass-loaded flow which can be considered as embodying the circumstellar envelope of the Be star (Zorec et al. 2000).

As EW Lac is not among the brighter Be stars, spectroscopic monitoring was restricted in 1989 and 1993 to one of the strongest He I lines, due to the limited available wavelength range with current instrumentation. He I 6678 has been revealed as very sensitive to the formation and development of a pseudo-photosphere related to discrete mass loss events; consequently it is not suitable for the search for non-radial pulsations, though multiperiodicity in the line profile was nicely confirmed. Newly operating, more sensitive echelle spectrographs will enable us to extend the investigation of nrp and stellar activity by observing many lines formed at different depths of the photosphere and to obtain a better image of atmospheric phenomena in this star.

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© European Southern Observatory (ESO) 2000

Online publication: October 30, 2000