Starbursts in four galaxy locations are discussed: on the periphery and in tidally ejected debris, in the main disk, in inner Lindblad resonance rings, and in the nucleus. Starbursts in dwarfs are also briefly mentioned. Possible reasons for the starbursts are summarized, mostly in the context of two theoretical models, one where star formation is initiated spontaneously by gravitational instabilities in disks, spiral arms or rings, and another where star formation is stimulated by high-pressure star clusters. The observed rates, efficiencies, and durations of star formation in all five regions follow from the models. We emphasize the importance of a critical density for star formation, which is approximately κ2/G for epicyclic frequency κ, and the importance of large-scale radial gas flows. Star formation tends to occur wherever the density exceeds the critical value. The rate of star formation is very large in inner rings and nuclear regions because the critical density is very high there. Normal galaxy disks have lower rates because of their lower κ. This difference in rates implies that inner rings and nuclear regions of galaxies can maintain their star formation for much shorter times than the main disks following an episode of gas accretion that makes the density exceed the critical value. Thus only the inner regions will have major fluctuations in the star formation rate. Normal galaxy disks probably have fluctuations too, but with lower amplitudes and longer durations.

I review recent results on the cosmological evolution of QSOs identified at optical, X-ray and radio frequencies. In all these regimes, it is now clear that the redshift range 2 ≲ z ≲ 3 corresponds to the epoch of maximum QSO activity. I demonstrate that QSO models invoking supermassive black holes or the starburst cores of young elliptical galaxies are equally successful at reproducing the observed space densities of even the most luminous QSOs in the Universe at these redshifts. In addition, both models can also account for the strong decline in QSO luminosity, L(z) ∝ (1 + z)3.0±0.4, observed in all regimes at lower redshifts (z ≲ 2). In the infrared and X-ray passbands, recent results suggest that starburst galaxies may also exhibit a remarkably similar rate of evolution to QSOs, L(z) ∝ (1 + z)3±1.

Introduction

In the last few years there has been a dramatic increase in the numbers of QSOs identified in complete spectroscopic surveys. In particular, a significant number of high redshift QSOs have now been identified, including more than 40 QSOs at z > 4. The mere existence of these high redshift QSOs, coupled with their inferred high metal abundances (Hamann & Ferland 1993; Ferland, this conference), indicates the presence of a significant number of massive, evolved systems only ∼ 1 Gyr after the Big Bang.

SN 1979C (a type II-L), known for its powerful radio emission, was the first SN II where the late-time Hα luminosity was attributed to the ejecta-wind interaction (Chevalier & Fransson 1985). Yet the success of the radioactive model for the late-time luminosity of SN 1987A raised the problem of choosing between radioactive and shock-wave mechanisms in SN II. One possible solution was prompted by the observed excess in the Hα luminosity of SN 1980K (also type II-L and a strong radio emitter) at t = 670 days, relative to the predictions of the radioactive model (Chugai 1988). The interpretation of the excess in terms of the ejecta-wind interaction was supported by the strong radio luminosity and wide flat-top profile of Hα.

Introduction

Red giant stars on the asymptotic giant branch (AGB) lose considerable amounts of matter in a slow wind, and a circumstellar envelope (CSE) of gas and dust is formed. The CSE gradually becomes thicker as the star evolves towards the end of the AGB. The mass loss decreases substantially as the star leaves the AGB and the CSE detaches itself from the star. Eventually, the central post-AGB object becomes hot enough to ionize the inner regions of the remnant AGB–CSE and a planetary nebula (PN) forms. Thus, the AGB–CSE provides a common link through this evolutionary sequence, and hopefully much can be learnt about the late stages of stellar evolution as well as the metal enrichment of the interstellar medium through the study of its properties. Unfortunately, space does not permit a discussion of CSEs around supergiants (see e.g. Knapp & Woodhams 1993).

Introduction

Circumstellar interaction has turned out to be of crucial importance for the interpretation of observations of supernovae at both early and late stages. Much of the progress in this field is a result, of the combination of radio, optical, UV and X-ray observations. Here I review the basic evidence for circumstellar interaction, some of the most important physical processes, and specific examples at both early and late, stages in the supernova evolution. For a complementary review see especially the excellent review by Chevalier (1990).

Observational evidence for circumstellar interaction

The first evidence that circumstellar interaction is important for supernovae came from observations of SN 1979C. UV observations during the first few weeks showed a number of emission lines, interpreted as a result of circumstellar interaction (§4.3). Unambiguous evidence for circumstellar interaction came from radio observations more than a year later, showing a wavelength-dependent turn-on of the radio emission (Sramek & Weiler 1990; Van Dyk, this volume). Emission was first seen at short wavelengths, and later at longer. This behavior is interpreted as a result of decreasing free-free absorption by the ionized gas in a circumstellar medium around the supernova (Chevalier 1982b).

Introduction

The ring nebulae that are observed around Luminous Blue Variables (LBVs) and Wolf-Rayet (WR) stars are examples of circumstellar media in the late stages of stellar evolution of hot, massive stars. These nebulae are excellent laboratories for studying the interaction of winds and ejecta with the interstellar medium (ISM). They also provide unique insights into the central stars, particularly from an evolutionary point, of view. In Sect. 2, LBVs and WR stars are introduced, and Sect. 3 discusses the formation and composition of their nebulae. Sect.

Introduction

Over the last decade, we have come to realize that mass loss on the asymptotic giant branch (AGB) has major effects on the formation of planetary nebulae (PN), and many observable characteristics (e.g. haloes, molecular envelopes) of PN can be traced back to the circumstellar envelopes of their AGB progenitors (Kwok 1982). The large infrared excesses observed in PN are certainly due to the remnant of the AGB envelopes which have cooled as the result of expansion (Kwok 1990, Zhang & Kwok 1991). The detections of the 9.7 µm silicate and the 11.3 µm SiC features, both commonly observed in AGB stars, provide confirmations to this link between AGB and PN (Aitken & Roche 1982, Zhang & Kwok 1990).

However, the infrared spectra of PN also show features not found in AGB stars. The most prominent are the family of features at 3.3, 6.2, 7.7, 8.6, and 11.3 µm, which are attributed to the PAH molecule (Allamandola et al. 1989). It is clear that these molecules must either be synthesized during the transition from the AGB to the PN phase, or they are produced in the AGB atmosphere but only excited in the PN environment. In either case, it would be useful to study the infrared spectra of young PN and transition objects between AGB and PN (or proto-PN) in order to understand the origin of the PAH features.

Analytical beginnings

Balick (1987, 1988) suggested that the interacting-winds model for planetary nebulae, if generalized to two dimensions, might explain the morphologies of nearly all PNs. He supposed that the fossil red giant envelope (RGE) is cylindrically symmetric and that the density is higher at the equator than at the poles. The PN morphology and its evolution is then purely a consequence of the mass distribution in the RGE and the properties of the fast stellar wind.

Analytical models for aspherical PNs have been quite successful in describing the propagation of the outer shock of a two-wind configuration. One uses either a snowplow-type approximation (Kahn & West 1985, Soker & Livio 1989) or a generalization of the work of Kompaneets (1960; Balick et al. 1987, Icke 1988, Icke et al. 1989), in which the wind is supposed to generate a uniform pressure inside the expanding bubble. The method is easy to apply and shows the generic features of the outer shocks clearly. It works as follows.

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.