Recent observations imply magnetic activity in atmospheres of early-type stars. We explore the possibility that stressed surface magnetic fields can be driven by inertial oscillations, such as r-modes which are vorticity waves. We show that vorticIAL MOTIOns are able to supply helicity to drive magnetic activity in stellar atmospheres.

We explore the possibility that the magnetic field generated by a dynamo at the interface between the convective core and radiative envelope of a massive star can be transported to the surface by buoyancy.

New results of high-resolution spectroscopy of four pre-main sequence Ae/Be stars are presented. An analysis of parameters of lines originating in different regions of the circumstellar (CS) envelope (Hα, Hβ, He I 5876, DNal) allows to reconstruct a picture of the interaction between the star and the CS environment which can be displayed in different forms. At least two separate processes seem to impact the structural and kinematical properties of the envelope: the stellar wind from the stellar surface and the matter infall onto the star from the CS media. A possible relation between these two phenomena is discussed in the framework of different models. Some similarity between observational phenomena in Herbig Ae/Be and classical Be stars is noted in spite of their difference in evolutionary status.

The number of Be stars which showed short- and long-lived outbursts during the Hipparcos photometric survey, previously reported in Hubert and Floquet (1998), have been increased by examination of light curves of fainter objects. Long-lived outbursts have been modelled as being due to an ejected layer/slab which becomes gradually diluted.

The origin of the hot X-ray emission from γ Cas has been a subject of long debate, with some authors advocating that it arises from a supposed undetected degenerate companion. Parmar et al. (1996) concluded that X-ray properties alone limited one’s ability to understand this source. Thus, Smith et al. (1998, “SRC”) obtained ≈1 day’s simultaneous X-ray and UV spectroscopic and photometric coverage with the RXTE/PCA and HST/GHRS space-borne instruments on 14-15 March 1996. The observed 2–10 keV X-ray light curve was found to have two distinct components; short duration bursts (‘shots)’ superimposed on a slowly varying background, termed the “basal” flux, which modulates on a timescales of hours. SRC found two maxima in the RXTE light curve separated by 10 hours. These features coincided with the appearance of two “dips” in the UV continuum light curve sampled near λ1400. From these light curves and other arguments, SRC concluded that γ Cas’s X-ray and UV continuum flux originated near the surface of the star and were modulated in intensity as the star rotated with a period of 1.123 days. This period was deduced by phasing of the X-ray minima obtained from our RXTE data with ASCA observations taken 10 days earlier and from comparisons of UV minima observed with the GHRS with similar minima seen in IUE observations taken two months earlier.

Systematic Be star observations with the HEROS spectrograph have been carried out in several long observing runs and seasons, mainly at the ESO La Silla Observatory. The HEROS data represents one of the most extended spectroscopic data sets for the Be star research. More complete and efficient diagnostics and a higher frequency resolution in analyses of periodic spectral variations were achieved, thanks to a combination of relatively high spectral resolution, a large sample of monitored spectral lines of different formation properties, long observing runs, and a relatively high frequency of observations of main targets.

Important results concerning different parts of the extended atmospheres of Be stars and having strong impact on our understanding of these objects have been achieved.

In the early 1980’s, astronomers at the Beijing Astronomical Observatory (BAO) entered a collaboration in a long term photometric observing project on Be stars which included about 30 stars. For example, in 1983 the group at BAO organized a multi-site campaign to measure the short term variability of EW Lac, KX And, KY And, o And, and LQ And. In 1989, 48 Per was selected to be one of the targets of MuSiCoS campaign in which many stellar astronomers at the BAO participated. In 1991, the Be group at the BAO joined a campaign on the prototypical nonradially pulsating B star, 53 Per. In 1992, French & Chinese astronomers proposed a joint campaign on o And and EW Lac. Astronomers in Japan are also active in Be star campaigns and in 1993-5 they organized several campaigns on few well known Be stars (ζ Oph, EW Lac, ϒ Cas, λ Eri and 28 Cyg).

The contribution of the MuSiCoS project (a multisite network of 2m class telescopes) to Be-star research is summarized. Some preliminary results of the 1998 campaign on ω Ori are shown.

From 1985–96 twelve multiwavelength campaigns on 15 Be stars were carried through using data from the IUE spacecraft to study the phenomenon of short-term spectroscopic and photometric variability. Highlights from recent work on this database are presented here. Three classes of variability have been identified: 1) The FUV flux and wind strength are correlated, 2) The FUV flux varies cyclically but wind variability if present does not correlate with the flux, and 3) The wind strength cyclically varies but does not correlate with light variations. For Class 1 the period is usually less than the star’s expected rotational period, but for Classes 2 & 3, the period is sometimes close to Prot. We have employed a cross-correlation technique to extract information on the nature of the line profile variability (lpv). Evidence for nonradial pulsations (NRP) in low-order modes is found for objects in Classes 1 & 2, the light variations appear to be caused by a modulation of the star's photospheric temperature, and the hot crest of the NRP wave is in front when the star is bright. For Class 1 stars the wind is enhanced over the hot crest. The mass loss in Class 3 may originate from a localized active region on the star and the importance of NRP in these stars remains unknown.

Polarimetric observations of stars can provide useful diagnostics of the geometry and physical conditions of the circumstellar envelope. This paper reviews the application of polarimetric techniques to hot stars (with particular emphasis on Be stars) and the status of polarization observations, and discusses some recent developments that improve our ability to make and to interpret such observations. Polarization measurements, bolstered by the increasing availability of new instruments on larger telescopes, continue to improve toward both higher signal-to-noise ratio and higher spectral resolution. Because of this we can begin to address specific questions of the physics of the circumstellar envelopes, since polarization provides a means to separate the effects of the photosphere from those of the envelope. New theoretical and analytic approaches are improving our ability to interpret spectropolarimetric data. We now are able to determine characteristics such as disk opening angle, density, and temperature. Furthermore, polarimetric observations in new wavelength regimes (e.g. ultraviolet and infrared) are opening new avenues to investigate circumstellar disks. The combination of spectropolarimetry with other kinds of data, such as spectroscopy, photometry, and interferometry, shows promise for providing stringent tests for theoretical models of the circumstellar envelopes of Be stars.

In this talk I give a short overview of models for spectral lines in Be stars disks which I think are quite instructive. I then examine the arguments for my own model assumptions and discuss selected topics concerning radiative line transfer calculations in three dimensions. We show that symmetric Hα emission lines of Be stars can be understood in terms of Keplerian disks. It is also demonstrated that one-armed global disk oscillations provide the best available solution to understand the long-term V/R variations in Be star emission lines.

We discuss the characteristics of global oscillation modes in Be disks and review recent studies related to the disk oscillation model. Since the m = 1 modes are present only in near Keplerian disks and the mode confinement occurs only in the region in which the radial flow is subsonic, the model of global disk oscillation strongly prefers the viscous decretion disk scenario proposed by Lee et al. (1991), whereas it is incompatible with the wind-compressed disk scenario of Bjorkman & Cassinelli (1993), which predicts angular-momentum conserving disks with supersonic radial flow. Based on the viscous decretion disk scenario, we discuss transonic solutions of decretion and examine the effect of viscosity on the global one-armed modes.

Be stars can switch between non-disk, gradual disk growth, disk-loss events, and shell-line phases. Many aspects of this Be phenomenon are still not understood. In this paper I review recent work on variability in Be-star disks, divided in four different topics: disk growth (Section 1), long-term V/R variations and global disk oscillations (Section 2), spectacular variations (Section 3), and, concisely, the disk variability in Be/X-ray binaries (Section 4).

Several theories have been proposed to explain the origin of Be star disks. Among them are Wind-Compressed Disks, accretion disks, decretion disks, and “explosive” ejections. In reviewing these mechanisms, I first concentrate on the current status of the Wind-Compressed Disk model. In particular, I discuss how non-radial forces may prevent disk formation and then discuss various physical effects that may restore the disk. Second, I examine the observational evidence and what it tells us about the structure of the disk. Of particular interest is evidence in favor of Keplerian disks. Finally, I discuss theories for Keplerian disk formation and some of the constraints such theories must satisfy.

A mechanism to create circumstellar envelopes in Be stars is proposed. The mechanism assumes a mass-loaded wind produced by ablated discrete stellar ejecta. The discrete mass ejections are suggested by the long-term spectrophotometric behaviours of Be stars and by their short and long-lived light outbursts discovered recently.

The polarization of Be disks that are not axisymmetric is discussed, with results given for one-armed density waves or global disk oscillations (GDO). Polarimetric diagnostics of electron scattering Be disks are presented, including the expected behavior at (a) optical wavelengths where the star is the dominant source of luminosity and (b) infrared wavelengths where the disk is more luminous than the star. Results for detached disks are given. A qualitative discussion of polarization at Hα is also presented.

Okazaki (1991) and Papaloizou, Savonije, & Henrichs (1992) suggested that the quasi-cyclic V/R variability observed in the emission line profiles of many Be stars is caused by a precessing one-armed density wave in the circumstellar disk. It seems likely that the changing aspect of such a non-axisymmetric density pattern might also lead to a related variation of the continuum polarization. We have searched for such an effect in two well-studied Be shell stars, ζ Tau and 48 Lib, based on data compiled from several groups of observers from 1984 to 1998. Using the Monte Carlo radiation transfer code of Wood, Bjorkman, Whitney, & Code (1996), we have calculated the polarization due to electron scattering in Be disks in the presence of one-armed density perturbations. Although the notorious long and short term deviations from strict periodicity present in Be stars make it difficult to rigorously demonstrate the connection between the V/R variability and the polarization variations, we have been able to find specific modes that are consistent with the observed V/R line profile variations together with the suspected polarization cycles.

Polarimetry has proven to be a useful observational tool in the study of Be stars. In recent years, Monte Carlo radiative transfer methods have often been used in modeling polarimetric measurements, since they allow accurate predictions of flux and polarization via numerical simulation of the interaction of light with matter, even in systems with complex configurations. We have extended these two complementary techniques to the investigation of binary Be stars by developing a Monte Carlo code that models the flux and polarization phase curves of a binary-disk system. We here present the latest results matching geometrical model configurations with our own spectropolarimetric observations of the Algol binary β Lyrae.

The medium resolution (R=1000) near infrared spectra of 30 early type Be stars obtained during the period October 1998-April 1999 are presented. The Paschen β and Brackett γ lines are seen in emission in majority of these stars. The higher order Brackett series lines from m=10 to 19 are detected in emission with significant strength compared to the Brackett γ emission line flux indicating the effect of optical depth. The Fe II line at 1.6873μm is detected in a small number of stars indicating existence of higher density and lower temperature emission zone.

The activities detected in Be stars indicate that the formation of the circumstellar envelope and its structure cannot be studied independently of the phenomena taking place in the outermost layers of the central stars. Assuming that related to the stellar activity there is an expanding atmospheric region with temperatures Te > Teff followed by an envelope with a decreasing temperature, we calculated hydrogen line profiles for different velocity fields and different positions of temperature maxima relative to the underlying photosphere. Results show that the Hα line is not very sensitive to changes introduced to the stellar atmosphere and to the nearby circumstellar layers. Moreover, the Hα emission line profiles look like those produced by disc-like circumstellar envelopes seen pole-on, although the model for the circumstellar envelope is spherical. However, the first members of the Paschen and Brackett series are strongly sensitive to any changes introduced in the photospheric and exophotospheric layers. We conclude that the study of these lines may then be valuable to obtain new insight on the activity of central stars and on the phenomena involved in circumstellar envelope formation in Be stars.