Introduction

Eta Carinae is a hot, massive, very luminous star which has erupted with episodes of greatly enhanced mass loss several times during the past few centuries. It is surrounded by an expanding shell of material, commonly known as the Homunculus, which was ejected during an outburst between 1830 – 1860. Fainter condensations are visible at greater distances. Their composition and radial proper motions suggest clumps of ejecta, some of which are contemporaries of the Homunculus, while others may have been expelled during earlier outbursts. Eta Carinae represents a dramatic although possibly brief phase of the interaction between a massive star in an advanced stage of evolution and its environment.

The spatial and temporal characteristics of the nebulosity make it an attractive target for observations with the Hubble Space Telescope. The smallest structures currently known have sizes between 1/10 to 1/4 arc second, beyond the reach of ground based instruments, but well matched to the resolution of HST. Proper motions are typically 0.05 to 0.1 arc seconds per year. Displacements of several pixels per year will be seen with HST. Over the planned 15 year HST mission the kinematics of the expanding debris should be clearly revealed.

Introduction

Of the three sub–types of interacting binary considered in this review, there is no doubt that classical novae (CN) are the best defined at present in terms of our knowledge of the composition of the central binary (white dwarf plus late-type main sequence star) and cause of outburst (thermonuclear runaway - see e.g. Bode and Evans 1989 and references therein). Recurrent novae (RN) on the other hand form a small, and surprisingly heterogeneous group of nine known members, with either red giant or main sequence mass–donating stars and either white dwarf or main–sequence accretors (see e.g. Bode 1987, Webbink et al. 1987). The much larger class of symbiotic stars (SS) is equally heterogeneous, and as with RN, the cause of outburst is less clear than for classical novae, though it seems that the evidence in favour of most of these systems containing a white dwarf is increasing (e.g. Mürset et al. 1991). What has been clear since the class was first defined is that their cool components are evolved.

Introduction

Abell 30 and Abell 78 are the best-known members of a small but important class of planetary nebulae (PNe) which are characterized by H-poor, dusty ejecta. Other members of this group include Abell 58 (V605 Aql), IRAS 18333-2357 (in the globular cluster M22) and IRAS 15154-5258. In these objects the H-poor material is surrounded by an outer envelope of normal composition (except for IRAS 18333-2357, where the ram pressure of the ISM would have stripped off the outer envelope: Borkowski et al. 1993a). Clearly, a secondary ejection of highly processed material has occurred after the loss of the hydrogen envelope of the AGB progenitor. A detailed interpretation was put forward by Iben et al. (1983), who proposed a final helium shell flash after nearly all of the H-rich envelope had been expelled.

The H-poor PNe are important because the composition of the ejecta opens a window upon the final phase of AGB nucleosynthesis and dredge-up, and also because the high dust to gas ratio lets us study the physics of dusty plasmas (e.g., gas heating by photoelectrons from grains: Borkowski & Harrington 1991). Here, however, we wish to point out that at least two of these objects also provide an exceptional opportunity to study mass-loaded flows. Mass-loading occurs when a tenuous, fast wind, as it streams around dense, slow-moving knots, entrains and mixes with bits of the dense material.

In order to understand the evolution of Wolf-Rayet (WR) stars and their interaction with the surrounding circumstellar and interstellar gas we have undertaken an emission-line imaging survey of the (almost complete) WR star population in the Magellanic Clouds (Dopita. et al. 1994).

Interference filter CCD images have been obtained in Hα and [O iii] λ5007 for all WR stars in the LMC and the SMC. The survey was conducted using the 2.3 m telescope at the Siding Spring Observatory ANU. The field of view was 6′.7, and the pixel size was 0″.65/pix.

A total of 115 WR. stars in the LMC (Breysacher 1981; Lortet, 1991) and 9 WR stars in the SMC (Azzopardi & Breysacher, 1979; Morgan et al. 1991) were observed in this survey.

This survey is the first complete survey of the ionized material around WR stars in the Magellanic Clouds, and indeed is the first complete survey in any galaxy. We have almost doubled the number of ring nebula known in the MCs, and have revealed a number of cases in which stellar ejecta has almost certainly been identified. As a consequence, we find that the incidence of ring nebulae around WR stars in the LMC is very similar to that in the solar neighborhood. (According to Lozinskaya, 1982; 1983; 1992 only 30-40% of WR. and Of stars in the distance-complete sample in the Galaxy are associated with ring nebulae; the nebula types of stellar ejecta and wind-blown bubble are even more scarce, about. 10–15%.

Introduction

The detection of SN 1993J in NGC3031 (≡M81) by F. Garcia on March 28.86 was reported in an IAU Circular by Ripero (1993) on March 30. This was the nearest supernova (SN) detected in the northern hemisphere since SN 1937C, and it has been, and will be, studied at many wavelengths in more detail than any other SN except SN 1987A in the LMC. SN 1993J showed hydrogen lines in its early spectra, and was initially thought to be a type II SN, but subsequently its spectra developed to resemble those of type Ib SN. Thus it has been classed as a peculiar type IIb SN (Filippenko & Matheson 1993), and it is thought to be the result of the explosion of a massive star which has almost lost its outer hydrogen-rich envelope.

Introduction

The morphology of Planetary Nebulae (PNe) has been studied for many years. Early catalogues of images of PNe have been presented by Keeler (1908) and Curtis (1918). These were the first comprehensive photographic samples of PNe images, and a lot of work on the classification and morphology has been based on these papers.

The importance of a nebula's morphology, of which the diameter of the object is the simplest form, lies in the physical parameters that can be derived from it.

The diameter of a nebula is related to its evolutionary stage, to its distance and is needed for the modeling of its spectrum. One method of distance determination uses the diameter and the Hβ flux of a nebula, under the assumption of a constant given mass for all PNe (Shklovskii 1956).

Introduction

A large fraction of all stars are members of binary systems. It is therefore reasonable to consider the possibility that the properties of many massive supernovae (i.e., supernovae whose progenitors had initial main-sequence masses, Mms, greater than ∼ 8 M⊙) are influenced by prior interactions of the progenitor with a binary companion star.

Introduction

Most long-period variables (LPVs) are surrounded by extended dusty circumstellar shells accompanied by considerable mass loss, which is often large enough to influence their evolution and to provide substantial amounts of chemically processed material to the ISM (Lafon & Berruyer 1991). Due to the improved instrumental capabilities at infrared and longer wavelengths a large amount of observational data is available for these objects (for a review see Habing 1990). On the other hand based on the progress in theoretical dynamical modelling of LPV atmospheres achieved during the last years (e.g. Bowen 1988, Fleischer et al. 1992) a detailed comparison of theory and observations seems to become accessible now.

The modelling method

Dynamical model calculations

Our approach for the dynamical modelling of circumstellar dust shells of LPVs, which is described in detail in Fleischer et al. 1992, comprises the explicit solution of the hydro-equations in spherical symmetry adopting Lagrangian coordinates, the treatment of radiative transfer by the Eddington approximation for a spherical grey atmosphere according to Lucy (1976), the description of postshock cooling of the gas either by limiting cases (e.g. isothermal shocks) or by cooling functions, and the consistent detailed treatment of the formation, growth and destruction of carbon grains by means of a moment method (cf. Gauger et al. 1990).

Introduction

Strong winds from massive stars can sweep up the ambient gas forming stellar wind bubbles, also called ring nebulae. Classically, ring nebulae around Wolf-Rayet (WR) stars have been modeled assuming a homogeneous interstellar medium (ISM), following Weaver et al. (1977). However, theory and observations have progressed to the point that, this simplification can no longer be justified. The evolution of massive stars has been studied by Maeder (1990). He shows tracks for 15–120 M⊙ in his plots (Figures 1-4). Main sequence (MS) stars between 25–40 M⊙ evolve to WR stars after passing through a red supergiant (RSG) phase. Observations of MS stars (Herrero et al. 1992) and WR stars (Willis 1991) reveal fast winds, as opposed to RSG stars (Humphreys 1991), where the winds are dense and slow (see also Chevalier & Liang 1989, Stencel et al. 1989). The above studies, suggest to us that the ISM initially encountered by a WR wind is far from homogeneous. This is the base of our three-wind model. In order to explain WR ring nebulae, we must take into account the history of the central stars, not just their interstellar environment. We have already presented a brief description of an analytic calculation of the dynamical behavior of the swept-up shell of RSG wind (Garcia-Segura. and Mac Low 1993). In this paper, we present numerical computations of the shell that follow it after instability sets in and it can no longer be modeled analytically.

Introduction

Wolf-Rayet Shell Nebulae (WRSN) provide a “quick look” at an intermediate stage of evolution of massive stars between the main sequence O stage and their ultimate demise as SNII. During this evolutionarily brief epoch, the O star develops a strong wind which affects the surrounding ISM, and can even have significant mass loss which enriches the ISM with H-burning products –specifically He and N (Maeder 1990). Therefore, studies of these objects are both interesting and important regarding the physics of wind-shock effects on the ISM and in the role they have in galactic chemical evolution.

In this short contribution I will present some of the results of two recent students of mine who completed Ph.D. theses studying the morphology and spectra of the WRSN NGC 6888 (Mitra 1990) and NGC 2359 (Jernigan 1988). A more comprehensive review of the literature on WRSN is given by the fine paper by L. Smith in this volume. The theses studies incorporated CCD imagery mapping of the ionization structure of the nebulae in the emission lines of Hβ, [OIII]λ5007, Hα, [NII]λ6583, & [SII]λ6717+30; followed by spectroscopy of parts of the two nebulae that were of special interest from the imagery. Herein I will note some of the spectroscopic results regarding the hot wind-driven gas; the imagery mapping is available in their theses and moreso in a forthcoming Atlas of CCD Imagery of Galactic HII Regions (Hester et al. 1994).

Introduction

A variety of supernova interactions with circumstellar material (CSM) has been observed to date. The best, and most direct, example is the ring of emission around SN 1987A (Jakobsen et al. 1991). This material has been ionized by the UV and soft X-ray flash of the shock breakout at the surface of the supernova (Fransson et al. 1989, Lundqvist & Fransson 1989). The density enhancement in the ring is caused by the interaction of the fast blue supergiant wind colliding with the slow red supergiant wind of a previous epoch (Blondin & Lundqvist 1993). In the case of SN 1993J, the early detection of radio and X-ray emission, in combination with narrow emission lines in the UV and optical, are indicative of interaction with the CSM. Blue optical continua, X-ray detection at early phases, as well as the UV emission have been proposed as characteristics of a shock in the CSM around SN 1979C (Fransson 1984).

Introduction

Mass loss from late type stars is a ubiquitous phenomenon which has important ramifications for the further evolution of the star. Typically, the nuclear burning timescale of a giant is 10−7 M⊙/yr. For comparison, the mass loss rate on the Asymptotic Giant Branch (AGB) varies from 10−6 to a few times 10−4M⊙/yr.

Supernova 1993J in the spiral galaxy M81 is the brightest supernova since SN 1987A and, like the latter, appears to be another peculiar type II supernova. Its early light curve is characterized by a very sharp initial peak (lasting for less than ten days) followed by a less rapid secondary brightening, which was qualitatively similar to the secondary brightening observed in SN 1987A.

Humphreys et al. (1993) have identified a candidate progenitor consistent with the position of the supernova. Combining their UBVR. photometry with the I magnitude obtained by Blakeslee & Tonry (1993), they concluded that the colors of the apparent progenitor require the presence of at least two bright stars. One star is an early-type supergiant (most likely a late-B to early-A supergiant), the other a late-type supergiant (most likely a G to early-K supergiant). The bolometric magnitudes of both stars are in the range of –6 to –8, with best-fit values of –7 to –7.5 (for an assumed distance of 3.3 Mpc). We have performed our own fits to the photometric data and obtained similar results. These best-fit magnitudes imply mainsequence masses of ∼ 15 M⊙, but the masses could be as low as 8 M⊙ or as large as 20 M⊙. The image of the candidate progenitor appears extended on some plates (Blakeslee & Tonry 1993). This suggests that, at the distance of M81, the two stars do not form a close binary (although either star could have an undetected binary companion).

The historic series of Herstmonceux Conferences was started by the new Astronomer Royal, Sir Richard Woolley, in the late 1950's. The Royal Greenwich Observatory had by then recently finished moving its scientific operations from Greenwich to Herstmonceux Castle in East Sussex. Evidently the first few such conferences were relatively small private affairs, and there are few written records of them, but in later years they grew in size. After the moving of the RGO to Cambridge in 1989, they have been organised jointly with the University's Institute of Astronomy.

Our idea for the 34th Conference was to bring together different astronomical communities who study stellar evolution, stellar winds and the physics of circumstellar media, and to bring out the common physics affecting matter around both high and low-mass stars. This volume presents the proceedings. We have included all the invited reviews and the contributed oral talks, and there is a summary listing the titles and authors of all the poster papers.

Thanks are due to many people for helping to put together what was the largest-ever Herstmonceux Conference. The Organising Committee were Robin Catchpole, Robin Clegg, Peter Meikle, Jim Pringle, Anne Reynolds and Ian Stevens; Anne did a huge job as Conference Secretary and deserves special mention. The Advisory Committee were John Dyson, Alex Filippenko, Claes Fransson, Harm Habing, Alain Omont and Guillermo Tenorio-Tagle. The RGO and the IoA gave financial support, and the International Science Foundation funded speakers from Russia.

Introduction

The detection of narrow emission lines from supernova 1993J gives us a rare opportunity to study the circumstellar medium produced in the preexplosion phase. This can give insights into the evolution and mass loss history of the progenitor star, and test theories of the interaction of a supernova with its surroundings.

Observations

Optical spectra of SN 1993J were obtained with the IDS and ISIS spectrographs on, respectively, the INT and WHT on La Palma. For the first two weeks, spectroscopy was carried out nightly, and less frequently thereafter. The resolution and wavelength coverage varied according to the scheduled observing programme being carried out at the time.

Circumstellar matter in symbiotic stars

By definition, symbiotic stars exhibit simultaneously the absorption features of a cool giant and high excitation emission lines, e.g. HeII, [OIII], of an ionised nebula. Thus the presence of circumstellar material is a necessary classification criterion of these objects. It is now generally accepted that symbiotic stars are binary systems consisting of a red giant and a hot radiation source, in most cases a hot white dwarf. The strong emission lines originate in a dense nebula which is thought to be wind material lost by the cool giant and ionised by the hot companion.

Besides the compact nebula other components of circumstellar material are observed. Enhanced IR emission due to circumstellar dust is found in D-type (D for dust) symbiotic systems. Very extended ionised regions have been mapped with radio interferometers or optical imaging techniques (e.g. Taylor 1988, Solf 1988, Corradi & Schwarz 1993, Schwarz 1993). Some features of the extended structures can be associated with bipolar outflow (velocities ∼ 100 km/s) from the central, unresolved binary system.

In this paper we discuss how the geometric structure of the circumstellar environment of symbiotic systems can be clarified from an analysis of light scattering processes.

Scattering processes in the circumstellar environment of symbiotic stars

Polarisation measurements are a well known tool for studying scattering processes. Polarimetric observations of symbiotic stars in broad and narrow band filters have shown that these objects are often intrinsically polarised (e.g. Piirola 1983, Schulte-Ladbeck 1985, Schulte-Ladbeck et al. 1990).