The late B-supergiant HR 4049 is peculiar in different respects: (1) It is located far from the galactic plane (b = 23°); (2) It is a variable with a large amplitude and on a long time scale (Waelkens and Rufener, 1983); (3) It has a spectacular infrared excess (Lamers et al., 1986). Two models were proposed: (i) HR 4049 is a runaway hypergiant embedded in a dust cloud, or (ii) HR 4049 is a low-mass star in a post-AGB stage of evolution. In this paper we present evidence that favours the second hypothesis. This evidence consists of new observational data on HR 4049 itself and of the discovery of a second very similar object, that is located still farther from the galactic plane.

We report various recent observations of molecules in circumstellar envelopes of late type stars, mainly possible proto-planetary nebulae at various stages of evolution, and supergiants, including.

First observation of SO2 (Lucas et al. 1986) in five stars and SO in one. S O 2 is particularly strong in OH 231.8+4.2 (OH 0739-14) and OH 26.5+0.6, where one of its lines is the strongest non-maser line observed in these envelopes (Guilloteau et al. 1986).

The existence of circumstellar dust in late-type stars is connected with stellar pulsations since periodic shocks accompanying pulsations seem to be the most probable mechanism of the gas density increase needed for condensation of dust particles in outer layers of the stellar atmosphere. Most abundant solid materials formed in O-rich stars are forsterite (or enstatite), silicon monoxide and iron, whereas in C-rich stars these are carbon, silicon carbide, magnesium sulphur and iron. Application of the homogeneous nucleation theory shows that condensation proceeds at extremely high departures from thermal equilibrium due to a very long time between collisions of condensable monomers. The most efficient mechanism of destruction of dust grains is thermal evaporation caused by variations of the stellar effective temperature.

Radiative instabilities due to simple molecules may convert chromospheric material into masering molecules and dust capable of being removed from the star by radiation pressure.

A mechanism is proposed for the initiation of dust formation in stellar winds of M-type giants and supergiants. If Mg and Fe are ionized (M0…M4) dust formation is initiated by homogeneous nucleation of SiO, otherwise (later ≍ M2) by homogeneous nucleation of MgS. The condensation temperatures for these mechanisms agree well with observations.

Infrared speckle measurements and photometry are used to assess the shape and thickness of the dust shell of the carbon rich Mira variable IRC + 10216. A spherically symmetric radiative transfer model has been developed to approximate the physical parameters.

Optical/infrared observations of RV Tauri stars obtained at SAAO have allowed the natures of the dust shells around stars with infrared excess to be investigated. The data suggest that dust formation occurs sporadically and that some stars have multiple shells. There is no photometrically discernible difference between carbon- and oxygen-rich stars or their dust shells. There is some evidence that stars with higher metallicity have more dust.

Infrared photometry shows that while all RV Tauri stars have circumstellar dust shells, the RVb stars with slow cyclic variations in mean light as well as the 30–100 day variations common to all RV stars have more hot dust close to the star (Lloyd Evans 1985). Many M giant stars which are variables of semiregular type also show long-period variations in the mean light (O'Connell 1933; Payne-Gaposchkin 1954), with a roughly constant ratio between the two periods. Payne-Gaposchkin (1954) found P2/P1 ~9.4 for red variables of type M and P2/P1 ~19.4 for stars of type F-K, most of which are FV Tauri stars. Re-analysis using the more extensive data available now indicates P2/P1 ~10 for the M giants and P2/P1 ~15 for the FV Tauri stars. The nature of the long-period variability is unknown (Wood 1975).

The formation of carbon grains is described by a chemical pathway from acetylene via polyaromatic hydrogens (PAHs). The proposed mechanism is in excellent agreement with the observations and provides in particular the observed low condensation temperature which cannot be explained by classical nucleation theory.

Zhilyaev & Zubko described white dwarf model atmospheres in which carbon might condense. Whittaker2 presented a version of the phase diagram for carbon on which there is a region where carbyne is thermodynamically favoured over graphite. Because the ratio of cross-section to mass is much higher for a thin cylinder, a carbyne cylinder has a much better chance of being blown out of a white dwarf atmosphere by radiation pressure. However even for small grains, gravity overcomes radiation pressure for cylinders, as it does for spherical grains. In the case of a non-polar cataclysmic variable (CV) system, the white dwarf is surrounded by an accretion disk and the luminosity of the disk may provide sufficient additional radiation pressure to drive a grain out of the system. If a grain can initially be transported out of the white dwarf atmosphere there may be several CV systems in which the combined radiation pressure of the disk and white dwarf can blow grains out of the system. For small grains and white dwarf masses ≳ 0.5 Mo radiation pressure can overcome gravity. These remarks also apply to grains that originate in the cooler regions of the disk, where the density may be high enough to sustain grain formation. Thus some non-polar CV systems may possess circumstellar dust shells, whose composition may reflect the nature of the white dwarf or secondary. For cases in which grains originate in the disk any circumstellar dust may be transient, as the mass transfer varies. Furthermore, in classical nova systems, this process may provide grain precursors, on which larger grains might grow during a noya outburst. A more complete discussion may be found in Albinson & Evans.

The observation of a deep minimum in the light curves of some novae, accompanied by a simultaneous rise in the infrared some weeks after outburst, is attributed to the rapid formation and growth of dust grains in the ejecta (Clayton & Wickramasinghe 1976). The observed nucleation rate J(obs) ~ 10−10 cm−3 s−1 for typical dusty novae, whereas the expected homogeneous nucleation rate J(hom) ~ 6 × 10−26 cm−3 s−1. We suggest that heterogeneous nucleation on ions could be a possible grain forming mechanism. In this case J(het) ~ 3.3 × 10−12 cm−3 s−1, which is within an order of magnitude of the observed nucleation rate.

A status report is given of the continuing development of the photochemical model for circumstellar envelopes around cool evolved stars, with emphasis on molecular ions in both O-rich and C-rich envelopes.

We have developed a time-dependent chemical kinetic model to describe the chemistry in the circumstellar envelopes of cool stars, with particular reference to IRC + 10216. Our detailed calculations show that ion-molecule reactions are important in the formation of many of the species observed in IRC + 10216.

Heterodyne spectroscopy at 11 μm combines high spectral resolution (λ/Δ λ ~106), high spatial resolution (< 1 arcsec at 3 m telescopes) and high penetration depth. Therefore, it seems promising to use it also for the investigation of bright circumstellar atmospheres.

Polarimetric observations of the red semi-regular variable L2 Puppis, obtained over a period of several years, confirm significant variations across the CaI 4226 line and several TiO bands. Together with published infra-red data, the observations point to a fundamental symmetry plane to which all variations with wavelength and time are related. The data are also consistent with a combination of photospheric effects, including a non-uniform distribution of calcium across the stellar disk, and scattering from grains in a cloud in which there is a systematic variation in grain size with distance to the star. Grain growth and dissipation, as evidenced by the observations, occur on a time scale of several years, in contrast to the optical variability time scale of months.

The knowledge of the temperature distribution in circumstellar dust shells (CDS) is needed for both the calculation of the emergent IR spectrum and the modelling of physical processes, e.g. dust formation.

Over the last few years a better understanding of the chemistry inherent in the refractory nucleation process has emerged from experimental studies performed in several laboratories. These studies shed light not only upon the factors which might control the onset of grain formation, but also on the spectral characteristics of freshly condensed grains.

The Astrophysics program of the National Aeronautics and Space Administration of the United States emphasizes use of vehicles to obtain above-the-atmosphere observational advantages, including expanded electromagnetic frequency access, enhanced sensitivity resulting from reduced or eliminated atmospheric absorption of light and image smearing. Space technology provides a superior means for astrophysical inquiry, particularly in the case of circumstellar material. Much of the flight program is undergoing intensive review following the Space Shuttle disaster of January 1986.

Circumstellar matter exists in many forms, such as winds from early and from late type stars, in infra-red sources and OH and H2O masers, in X-ray sources and jets, and molecular flows and Herbig-Haro objects. There is considerable diversity in the nature of the starlike bodies that underlie these phenomena. Here an attempt is made to trace the connection between some of the observations related to pre-main sequence stars and their surroundings, with emphasis on jets and molecular flows.