An overview of the Be phenomenon is given with particular emphasis on the problem of the mass loss mechanism. We assume that the principal mechanism of mass loss in these stars is magneto-centrifugal acceleration (MCA). Within this framework, we discuss how particular features of Be stars which distinguish them from normal B stars can be understood. The frequency distribution of Be stars relative to normal B stars in the Galaxy and Magellanic Clouds is discussed. The distribution of equatorial velocities, activity in the visual and X-ray regions and photospheric activity all point towards a magnetic model and seem to be qualitatively consistent with the MCA hypothesis. Periodic variations in light and line profiles offer particularly important clues. In this respect, the recent discovery of co-rotating photospheric clouds in several Be stars strongly suggests gas trapped in closed field lines. We investigate the dynamics of an outburst and suggest that the localized nature of these events imply magnetic flaring and acceleration of mass due to connection and disconnection of field lines. Finally, suggestions for further progress are presented.

The determination of accurate and reliable basic physical properties of Be stars is a very complicated task which, among other things, requires a good knowledge of the variability pattern of each particular object. Possible confusion between a rotationally distorted photosphere and a pseudophotosphere – an inner optically thick part of a gradually formed envelope which temporarily mimics a photosphere – is discussed in some detail in relation to the stellar effective temperature, brightness and rate of rotation. It is shown that the currently available data allow us to set relatively stringent limits on possible radii of at least some brighter Be stars, but very few direct determinations of mass have been made. The only exceptions are the Be stars in eclipsing binaries. The trouble is, however, that all of them are interacting binaries for which reliable orbital solutions are seriously hampered by the presence of gas streams and by strong interactions between the binary components. Moreover, their masses need not be representative for the whole population of Be stars.

An answer to the question about the evolutionary stage of Be stars is even more uncertain since it is closely related to the principal, as yet unanswered question about the origin of the Be phenomenon itself.

The characteristics of the various types of B[e] stars are discussed and compared with those of classical Be stars. Both groups of stars are characterized by the presence of emission lines in their spectra, in particular of hydrogen. However, there are also significant differences between these classes. Classical Be stars lack hot circumstellar dust and strong forbidden low-excitation emission lines, which are typical characteristics produced by B[e]-type stars. While classical Be stars are a rather uniform group of early-type stars, B[e]-type stars form a quite heterogeneous group, very often of poorly known evolutionary status, comprising such diverse types of objects as near main-sequence objects, evolved lowmass proto-planetray nebulae and massive evolved hot supergiants. Even pre-main sequence Herbig Ae/Be stars sometimes find their way into the group of B[e] stars. However, despite these dissimilarities classical Be stars and B[e]-type stars, share a common property, namely the nonsphericity of their circumstellar envelopes.

In this contribution the spectral characteristics, physical properties, and the evolutionary status of B-type supergiants is reviewed with respect to the Be phenomenon. Long-term monitoring campaigns in the UV and the optical have recently revealed the distinct variability patterns of the photospheric and the emission-line spectra of this class. The various proposed scenarios for the spatially structured circumstellar envelopes and their rotational modulation are presented. The possibility of generating and maintaining large-scale wind structures by photospheric processes which structure the underlying stellar surfaces is discussed. Some open questions in connection with these hypotheses will be identified.

We are developing an atlas of spectropolarimetric observations of 61 bright northern Be stars obtained from 1989-94 using the halfwave polarimeter (HPOL) at the 0.9m telescope of the University of Wisconsin Pine Bluff Observatory (PBO). The data cover the wavelength range from about 3400-7600Å, with a spectral resolution of about 25Å. This atlas will contain all data (297 observations total) obtained as part of a survey program with HPOL during the time when the detector in use was a dual Reticon array; the survey observations with HPOL continue, using a new CCD detector which extends the spectral coverage out to 1.05μm and improves the spectral resolution to about 12Å. The CCD observations will be presented later in a second volume of the atlas.

Only a brief summary of the findings of the survey from the first 5 years of the project is presented here. A full analysis of the data will be included in a paper to be published elsewhere. The general wavelength dependence of polarization for classical Be stars can be considered on the basis of these observations, and results on polarimetric variability are available. In particular, we find that 56% (20 of 36) of the Be stars observed 3 or more times from 1989-94 show significantly variable polarization at the level of 0.1% changes (inclusion of preliminary results from the continuing CCD survey indicates that the percentage is even higher). The timescales for these changes range from as short as night-to-night to as long as several months. Several of the stars showed evidence for polarimetric “outbursts” during the time period covered by the observations.

This is an attempt at determining the IMF (Initial Mass Function) of Be stars relative to that of B stars in the vicinity of the Sun. We have represented the IMF as a power function of the stellar mass Mβ, so that the relative IMF is then proportional to Mβ(Be)–β(B). Allowing for systematic differences in the counting of Be stars due to their apparent overluminosity, a difference β(Be) – β(B) ~ 0 is found, which may indicate that there are no huge intrinsic differences between the two types of objects. In these calculations changes of spectral types due to high rotation were not taken into account. This effect may still strongly affect the results obtained. By extrapolating the MβBe) – β(B) curve to spectral types later than B7, we reckon that in a volume limited to the apparent magnitude V = 7 there may be about 150 still undetected Be stars of late spectral types.

Using the circumstellar envelope parameters (mean temperature, extent and opacity) derived by fitting theoretical energy distributions to those observed in a sample of 20 Be stars, we estimate the respective magnitude difference ΔV = V – V* of the continuum flux excess produced by their circumstellar envelopes. Using then the HIPPARCOS parallaxes of these stars and the ISM E(B-V) colour excesses derived with the surrounding normal B stars, we estimate the visual absolute magnitude of the central objects in the program Be stars. In most cases the results obtained are in good agreement with the absolute magnitudes obtained from the photospheric (λ1, D) components derived in the BCD spectrophotometric system. There are, however, strong discrepancies in 4 program stars which deserve further studies. The magnitude excess seems to be ΔV ≲ 0 for the higher effective temperatures only. There is no correlation between ΔV and Vsin i, as would be expected if the circumstellar envelopes were strongly flattened.

We present here standard uvbyβ CCD photometry for five open clusters of the Galaxy in order to determine abundances of Be stars, as a function of age.

We present CCD Hα and Hβ photometry of young open clusters. We show that the comparison of the α and β photometric indices provides an efficient tool for identifying emission line stars. We report on the discovery of several new Be stars.

The preliminary results of our survey are the following: i. the younger clusters (age < 10 Myr) are almost lacking of Be stars, ii. clusters in the age interval 10–30 Myr are rich in Be stars. Almost all of them are of spectral types earlier than B5, while late-type Be stars are scarce. These results point towards an evolutionary interpretation of the Be phenomenon, in the sense that Be stars are close to the end of their main sequence lifetime.

We present medium resolution spectroscopy (λ/Δλ ≈ 7,000) covering 6500-6750 Å of three emission stars in the highly reddened young open cluster NGC7128. Two of them were previously unknown. The known Be star (V1814 Cyg) is an irregular variable; one of the newly discovered stars is an eclipsing binary (V1481 Cyg) (Jerzykiewicz et al., 1996.). Physical parameters of the cluster have been determined from new UBV and uvbyHβ photometric measurements.

A spectroscopic investigation of four main-sequence stars in the association Cas OB2 (HD 17327A and B, HD 16893, HD 17443) and of two stars in the open cluster Platais 1 (No. 1 (1921) and No. 111 (1600)) was carried out using high-resolution CCD spectra, obtained with echelle-spectrometer ”LYNX” (6-m telescope, Special Astrophysical Observatory, Russian Academy of Sciences). We have determined the atmospheric parameters for the program stars, their projected rotational velocities and elemental abundances in their atmospheres. Two new HgMn stars - HD 17327A and Platais No. 1 - were detected. HD 17443 is not a Be star.

We present the results of our studies of Be stars within the Magellanic Clouds. We have studied the Be population within four young populous clusters, NGC 330 in the SMC, NGC 1818, 2004 and 2100 in the LMC as well as the field population around NGC 2004. The Be fraction within each cluster is seen to peak towards the luminosity of the main sequence turnoff. The field, which exhibits a large spread of ages, has a Be fraction more uniformly distributed in luminosity. We interpret this difference as evidence for an evolutionary enhancement of the Be phenomenon that occurs towards the end of the main sequence lifetime.

We present low resolution spectroscopy of about 20 Be stars in the SMC cluster NGC 330 to confirm the high contents of Be stars in this cluster.

The star MWC 84 (CI Cam) was first discovered to be an emission line object classified Bep (Merrill & Burwell, 1933), then classified as a symbiotic star and finally identified as the possible optical counterpart of the X-ray transient XTE J0421+560 (Wagner & Starrfield, 1998)

We have observed MWC 84 at the Haute Provence Observatory (CNRS) with a resolution of about 1.3 Å in 1992 (wavelength regions Hα and near infrared), 1998 and 1999 (from 4080 to 8880 Å).

Stars in different evolutionary stages and/or environments can show the B[e] phenomenon. In order to establish possible relations between the stellar evolutionary stage and the presence of this phenomenon, the fundamental parameters of these stars need to be determined, as in most cases they are unknown. With this aim we undertook systematic low resolution spectroscopic observations of stars with the B[e] phenomenon. Preliminary determinations of fundamental parameters based on the BCD (Barbier-Chalonge-Divan Paris spectrophotometric system) analysis are reported in this paper.

Multiwavelength observations of a large sample of A-type supergiants reveal that these stars display several features associated with the classical Be phenomenon.

The morphology and variability of spectral lines in the P Cygni high-resolution and high signal-to-noise near-UV spectra have been studied in detail over a period of few days. It is found that several Balmer lines have profiles with a complicated structure. All those lines which are well developed, but still not saturated, show two absorption components. A striking profile variability is discovered on a time-scale of from 1 to 3 days. The most dominant part of rapid variations is the drift of absorption components. It appears that there are two different kinds of components: long lived (with a characteristic time-scale of 200 days) and short-lived (time-scale of few days). The DAC phenomenon in the P Cygni stellar wind is a problems that still remains unsolved.