The ESA “Ultraviolet Bright Star Spectrophotometric Catalogue” (Jamar et al. 1976) and the “Catalogue of 0.2 A Resolution Far-ultraviolet Stellar Spectra Measured with Copernicus” (Snow and Jenkins 1976) have been used to try to separate, by means of simple diagrams, O-type stars belonging to different luminosity classes and having different temperatures. The ESA catalogue gives absolute fluxes for 43 O-type stars, mostly reddened, with a spectral resolution of 35–40 A, depending on the channel, in the wavelength range 1350–2550 A. The Copernicus catalogue gives spectra between 1000 and 1450 A for 17 O-type stars, with low reddening, at 0.2 A resolution.

Far ultraviolet photometric observations (in 5 bands between λ = 1550 Å and 3300 Å) are presented for ten O stars which are also the exciting stars of diffuse nebulae. Since the number of photons shortwards of the Lyman limit is known for these stars, sufficient information on the total flux is available to determine the effective temperature without making any assumptions concerning a model atmosphere. Conversely, the distribution of flux with wavelength can be used to determine the applicability of a given model. A detailed discussion of this is presented.

A comparison of these ‘normal’ O stars and stars with O type spectra which excite planetary nebulae is given. It is concluded that the atmospheres of these two types of O stars are very similar; probably they have the same effective gravity. Further, a discussion of the Zanstra He II temperature is presented.

In addition to O-type stars, this review will discuss the radio and infrared properties of B supergiants and Wolf-Rayet stars, since it is generally accepted that these objects represent later stages in the evolution of O stars. The radio properties of X-ray binaries which exhibit non-thermal emission will not be discussed. Hjellming (1977) gives an excellent review of the radio properties of close binaries, including X-ray binaries.

The λ 10124 He II line has been found to be a measurable emission in three stars. Our new data are compared to theoretical predictions. The λ 10830 He I line is observed in emission in 74% of the 05–08 supergiants but only seen in 29 % of the dwarfs, all of the latter exhibiting some “pecularities” i.e. classified as Oef, Oe, ON or On. An envelope with a sufficient amount of material seems to be a favorable condition to get the λ 10830 line in emission. However the mechanism leading to the observed emission is temperature dependent as well.

We know from Copernicus ultraviolet observations that all O-type stars are losing mass by stellar wind. The ionized expanding circumstellar envelope formed by the stellar wind is emitting through free-free and bound-free radiation processes. This radiation is detectable at the infrared wavelengths where the stellar continuum is negligible. The measurement of the IR excess (defined as the difference between the total flux and the stellar continuum at a given wavelength) and the knowledge of the terminal velocity of the envelope, allow us to derive for OB stars the mass loss rate. From the analysis of our IR observations of two O stars, HDE 226868 and HDE 245770, identified as optical counterpart of X-ray sources, we give an estimate of their mass loss rate. The IR observations were carried out with the Jungfraujoch 76 cm telescope using a GE bolometer with a focal plane chopping system and with the Merate 132 cm telescope using an InSb detector.

A method of deriving mass loss rates using near IR excesses is established and applied to OB stars in Carina and the LMC.

Many emission line stars are radio sources including early type stars such as ζ Pup and P Cyg, peculiar objects such as MWC 349 (a possible proto-planetary system) and V1016 Cyg, nebular variables such as T Tau, and at least one late M giant. These “radio stars” are characterized by (1) a rising spectrum with spectral index α = 0.6 to 1.1, (2) positional coincidence between star and radio source, and (3) no rapid (less than 107 sec) time variability. These characteristics are in contrast with the non-thermal spectrum and rapid variability of Algol type “radio stars” and the radio emission associated with x-ray sources. The radio component of the spectrum of emission line stars originates in the substantial mass loss flows which are a feature of their outer atmospheres and which, when ionized by stellar radiation, radiate by free-free electron thermal bremsstrahlung. These circumstellar H II regions are, in general, detectable with present radiotelescopes if the characteristic size of the optically thick region of the flow is greater than about 10−7 of the distance to the star.

A program to search for steady-state thermal emissions from stars has been in progress for several years in Canada (Purton 1976). In this program we have specifically excluded flaring objects (such as β Lyr or HR1099) where non-thermal emission is probably responsible. Out of the 30 or so detections, about 1/4 of them show a ~+1 spectral index, suggesting a 1/r2 density profile in the emitting region. This implies the existence of a non-static circumstellar envelope which is mostly likely the result of continuous mass outflow.

Radio fluxes of 7.2 ± 1.1 and 69 ± 5 mJy at 14.7 GHz have been detected from the stars ζ Pup (O4If) and γ Vel (WC8+O9I) respectively with the 64-m telescope at Parkes, and upper limits have been determined for 9 more hot stars. The interpretation of these fluxes as free-free emission from a shell of ionized gas resulting from a stellar wind gives mass-loss rates = (6.5 ± 1.4) x 10−6 M⊙ yr−1 for ζ Pup and (3.9 ± 0.7) x 10−5 M⊙ yr−1 for γ Vel if H/He = 10 by number and the He is fully ionized. If the gas around γ Vel originates mainly from the WC8 star, the helium predominates and = (5.2 ± 0.9) x 10−5 or (17 ± 3) x 10−5 M⊙ yr−1 depending on whether the helium is doubly or singly ionized.

Although some evidence had been available earlier, it was the work of Morton (1967) that proved that the OB Supergiants are losing mass. He obtained spectra with a rocket borne spectrograph of the three stars in Orion's belt, δ, ε and ζ Orionis. These stars are all supergiants of spectral type 09.5 or BO. The spectra showed resonance lines of ions such as C III, C IV, Si IV, N V with P-Cygni type profiles. The deepest part of the absorption profile corresponded to a velocity away from the star of 1400 km s−1, and the absorption profile extended out to wavelengths corresponding to velocities of 2000 km s−1 away from the star. Since these velocities are much greater than the escape velocities from the surface of these stars, it is clear that matter is escaping from them. With a number of simplifying assumptions Morton deduced that the rate of mass loss is about 10−6 M⊙ yr−1. This order of magnitude has remained unchanged by the many high quality observations made since that time.

Recent improvements to theoretical stellar wind models and the results of empirical modelling of the ionization balance and the infrared continuum are discussed. The model of a wind driven by radiation pressure in spectral lines is improved by accounting for overlap of the driving lines, dependence of ionization balance on density, and stellar rotation. These effects produce a softer velocity law than that given by Castor, Abbott and Klein (1975). The ionization balance in ζ Puppis is shown to agree with that estimated for an optically thick wind at a gas temperature of 60,000 K. The ionization model is not unique. The infrared continuum of ζ Pup measured by Barlow and Cohen is fitted to a cool model with a linear rise of velocity with radius; this fit is also not unique. It is concluded we should try to find a model that fits several kinds of evidence simultaneously.

The analysis of the UV spectra of τ Sco (BO V) and ζ Pup (O4 f) resulted in the empirical warm wind model. In this model the presence of O5+ and N4+ ions in the envelopes of early type stars is explained by collisional ionization in a ‘warm’ envelope of T ≃ 2 105 K.

The anomalously strong OVI and NV lines in O stars and the CIV and Si IV lines in B supergiants may be due to Auger ionization by x-rays from a thin coronal zone at the base of their cool stellar winds. This paper summarizes the results of several studies to determine constraints on the size and temperature of coronal zones from calculations of the effects of coronae on continuum and line spectra. The model that has resulted can be tested by observations from HEAO-B of the predicted 2 kev emergent fluxes. The model explains very well the persistence of the OVI and NV in the ultraviolet spectra of supergiants to class B0.5 and B2 respectively, and it predicts that CIV should be observable in the ultraviolet spectra at and beyond B5I.

If I understand correctly, this session of the Symposium on Mass-Loss and Evolution of O Stars aims at clarifying the merits and demerits of four “theories” for the observed, nonthermal mass-loss from these stars. Hearn has summarized what he considers to be the essential characteristics of each of the four, especially relative to a set of questions which, he considers, put the observational requirements on the “theories” in focus. Representatives of each of the “radiation-pressure initiated, radiative-equilibrium controlled”, “hot corona”, and “warm corona” alternatives have elaborated on Hearn's summary, to stress what they consider essential. So, now, I would do the same for what we caricature as the “imperfect wind-tunnel” model — not theory, which I assert does not yet exist — both for nonthermal mass-flux, and for other observed nonthermal phenomena, in stellar atmospheres generally, not just in O stars particularly. I assert, that in studying nonthermal mass-flux from O stars, if you limit your attention only to nonthermal mass-flux, and only to O stars, you handicap, a priori, your chance to understand what is required for the general model of a stellar atmosphere, in order to produce this variety of nonthermal phenomena observed, alike in all varieties of stars.

Previous work by Castor, Abbott, and Klein (1975) presented a self-consistent model of a steady-state stellar wind. They also showed qualitatively that for O stars at least a static atmosphere could not exist. This paper extends that result by calculating in detail the minimum luminosity as a function of effective temperature required for the line radiation force to exceed gravity. Within the observational and theoretical uncertainty there is a one-to-one correspondence between a star's calculated ability to self-initiate a stellar wind by radiation pressure alone and the observed presence of outflowing material in the UV resonance lines.

A new method of calculating models for stellar coronae is being developed with the aim of providing models of coronae around hot stars capable of explaining the observed mass loss as a stellar wind from a hot corona. The method gives a stationary solution to the coupled equations of motion, continuity and energy balance. The equation of motion contains only the gradient of the gas pressure and the acceleration due to gravity. Radiation pressure is not yet included. The continuity equation assumes spherical symmetry. The energy equation includes heating by weak shock waves, heating by the absorption of photospheric radiation through continuous opacity, radiation losses, heating or cooling by conductivity and the effect on the energy balance of the velocity of the expanding atmosphere. The model is assumed to be optically thin. The velocity distribution is calculated from the solution going through the critical point.