Although the first ultraviolet (UV) observation of an astronomical source was obtained in 1946, the first UV observations of Be stars were not obtained until 1964. In this review of UV data covering the period since 1964, the term Be star will be assumed to include Oe stars as well (Frost and Conti, 1976). An earlier review of this subject is by Heap (1976).

Resonance-line profiles of SiIII and SiIV lines in 22 B and Be stars have been analyzed in the derivation of mass-loss rates. Of the 19 known Be or shell stars in the sample group, all but one show evidence of winds. It is argued that for stars of spectral type B1.5 and later, SiIII and SiIV are the dominant stages of ionization, and this conclusion, together with theoretical fits to the line profiles, leads to mass-loss rates between 10-11 and 3 × 10-9 for the stars. The rate of mass loss does not correlate simply with stellar parameters, and probably is variable with time. The narrow FeIII shell lines often seen in the ultraviolet spectra of Be stars may arise at low levels in the wind, below the strong acceleration zone. The mass-loss rates from Be stars are apparently insufficient to affect stellar evolution.

One of the interesting results of ultraviolet (UV) astronomy was the discovery of ions from unexpected stages of ionization in spectra of O,B stars. The most common ions concerned are O VI and N V, but also C IV and Si IV in B stars. The presence of these ions is anomalous because generally their abundance is expected to be negligible if they are produced by photoionization by stellar radiation, either in the photosphere or in a cool circumstellar envelope (CE). The same ions are observed in the UV spectra of Be stars. Previous investigations, largely with Copernicus spectra, have reported O VI and N V in late Oe and early Be stars and Si IV in stars as cool as B5 (Marlborough, 1981 and references therein). In this paper we present the results of a preliminary survey of IUE spectra of Be stars covering a wide range of spectral type.

Fe III lines from metastable states have been detected in the UV spectrum of π Aqr (HD 212571). The line profiles are asymmetric and they are probably formed in a expanding envelope. From the observed fine widths and using the Sobolev approximation we have derived a mass loss rate of about 2.5 × 10−9M⊙ y−1.

Visible and far ultraviolet high resolution spectra of the B2 binary star HD 218393 have been obtained at one day interval, shortly before the phase of maximum outwards velocity, adopting the period of 38.908 days. Both spectra show the existence of an extended atmosphere, accelerated outwards. The SiIV resonance lines show some indication of mass loss. With only one set of observations, the origin of the strong and wide (about 130 km/s FWHM) ultraviolet absorption lines of once ionized elements is difficult to determine.

IUE observations as well as optical spectral of the peculiar star HD 87643 were obtained. The observed optical Fe II lines are probably formed in a expanding envelope and the rate of mass-loss derived from the analysis of these lines is about 7 × 10−7 M⊙ y−1.

IUE observations of the “pole-on” Be stars ω Ori and 66 Oph have revealed the unexpected presence of high velocity (v ≃ −750 km s−1), relatively narrow (Δλ ≃ 1A) absorption components in the resonance lines of C IV, Si III, and Si IV. The C IV features show structure indicative of multiple shells or clouds. Similar high velocity lines were not observed in other pole-on Be stars considered in the program. The nature of these unusual features and the column densities and mass loss rates implied by them are discussed in this paper.

Initial results from the analysis of a series of timed, high resolution IUE observations of HR 2142, ϕ Per, CX Dra, KX And, AU Mon, and TT Hya are presented. The data base for HR 2142 also includes Copernicus U1 and U2 observations. Variable absorption lines, indicative of mass flow in the system, are observed in all objects except ϕ Per. We also, in general, find evidence of mass outflow in the form of winds and/or discrete components. We observe variable N V absorption in CX Dra and AU Mon and emission features in KX And and ϕ Per (C IV only). U1 data reveals the presence of complex structure in the gas stream in HR 2142. These observations are compared with those of Be stars which are not thought to be interacting binaries.

High resolution IUE observations of the Be star HR 2855 (HD 58978) spaced five months apart have revealed striking variations in the resonance lines of C IV and N V. The nature of these variations can be seen in Figures 1 and 2.

Coordinated UV and visual observations of 59 Cyg in 1978–81 show strong mass ejection activity and strong variability in displacements and profiles of superionized lines during the new Be phase, starting from a “quasi normal B” phase in 1977, and increasing irregularly through 1981 to a low and then moderate Hα emission. These data show that visual data alone cannot describe the activity of the star.

A method free of interstellar reddening of comparing the energy distributions in the far-UV of Be/Shell stars to those of normal B stars is presented. The deviations of Be/Shell stars from the sequences of normal stars are correlated with other physical parameters observed in these stars. The largest UV color differences to the sequences of normal stars are found for those Be/Shell stars having the largest IR color excesses.

X Per is a variable emission line star which shows among other peculiarities a weak X-ray emission (4U 0352+30) and a strongly variable IR excess (Ferrari-Toniolo et al. 1978, Viotti et al. 1980). In the past decade the star has undergone three phases of enhanched “activity” (1972–73, 1978 and 1980) characterized by brighter visual luminosity, excess in the Balmer continuum and in the IR, stronger X-ray emission, with intermediate periods of minimum activity (1974–77, 1979) when the optical-infrared energy distribution was closer to that of a normally reddened early type star (figure 2). But during most of its history the energy distribution largely deviated from that of a non-emission line early type star, and the first problem is to determine the interstellar extinction, disregarding any “local” effect. The strength of the 2200 A band in the UV spectrum of X Per is consistent with E(B-V)=0.35, a value close to the extinction towards other Per II stars: Per (0.34), o Per (0.31), Per (0.32, Viotti & Lamers 1975). The i.s. Ly∝ line observed in the high resolution IUE spectrum of X Per obtained on 1979, December 23, has a FWHM of 11.0 A corresponding to N(HI)=4.9 1020 cm−2. The Copernicus observation of H2 lines (of not good quality) gives N(H2)≅1.1 1021 (Mason et al. 1976). A much lower value of 3–5 1020was derived by Snow (1976, 1977) for o and Per. Taking for X Per N(H2)≅5 1020we have N(H total)≅1.5 1021, yielding to N(H)/E(B-V)=4.3 1021 cm−2/mag in agreement with Bohlin law (1975).

New visible and ultraviolet spectra of the peculiar emission line star HD 51585 are described. Interstellar lines and the λ 2200 feature are rather weak. A colour excess E(B-V) = 0.33 is derived. The extinction curve resembles the one obtained from LMC stars.

Twenty one high resolution IUE observations made over a two year interval revealed remarkable changes in the profiles of the resonance lines of C IV, N V and Si IV in the B 0.5 IVe star λ Cas. Narrow (FWHM < 100 km s−1), blue shifted (v < 1500 km s−1), absorption components of varying strenghts are observed in addition to the broad, asymmetric, low-velocity absorption lines which seem to be steady.

The spectrum of 17 Lep is dominated by numerous and strong shell lines of once-ionized metals. The shell shows a composite radial velocity structure which indicates the presence of multiple components with a velocity range between −50 and −250 km s−1. Unlike in the visible, where these multiple components are present only during outburst, in the ultraviolet such components seem to be a permanent feature.

Using the IUE satellite we obtained high resolution UV spectrograms (1200 < λ < 3200 Å) of the S Dor type variable R 71 in the LMC. The IUE observations were supplemented by coordinated groundbased high dispersion spectroscopy and by photometric observations. From these observations we derive for the minimum state of R 71 the following stellar parameters: L = 2.0 × 105 L⊙, R ≈ 81 R⊙, Teff ≈ 13 600 K. For the expanding envelope we find a surprisingly low temperature of only about 6000 K and an apparently decelerated velocity field with a maximum outflow velocity of ~ 127 km s−1. The minimum state mass loss rate is in the order of 3 × 10−7 M⊙ y−1. Our results support the suggestion that the visual light variations of the S Dor type variables are produced by strong density variations of the expanding envelopes of these objects. A detailed study is forthcoming in Astronomy & Astrophysics.

I.U.E. low-dispersion spectra and spectral scans made with the Lick Observatory IDS scanners have been combined for 16 shell stars. Eleven objects can be represented by Kurucz model atmospheres, although some of them display strong shell-type line spectra. Five among them are known binaries. The six remaining objects (all interacting binaries) display complex spectra. A model involving continuum and line radiation from a hydrogen cloud surrounding the accreting component is proposed. A generalization of this model with optically thick segments of the cloud promises to explain even more exotic objects such as β Lyrae, W Serpentis and possibly ∊ Aurigae.

The basic concept of what a Be star is we owe to Struve (1931). His proposal that these stars are rapidly rotating and have an extended, disc-like, circumstellar envelope is still the basis for all models of Be stars. Unfortunately, progress in understanding the dynamics of Be stars has been painfully slow, a consequence of the very complex nature of the problem. As yet there does not exist a self-consistent unique model of any Be star.

Numerical investigations of rotating stellar winds driven by a combination of magnetic and line radiation forces show a clear dichotomy: shallow winds have their azimuthal velocity enhanced by magnetic corotation, whereas steep winds may have a diminished circular velocity.

Abstracted comparison of Struve's 1942 quasi-empirical model of the shell-atmosphere of Be and similar stars, based on visual data and quasi-radiative equilibrium, to that demanded by current visual+nonvisual data, requiring nonradiative+mass fluxes characterized by variability and individuality.