Although emerging from a range of progenitor stars and the product of different explosion mechanisms the light curves of the various supernova types are shaped mainly by radioactive power. Core-collapse supernovae have in addition early peaks from shock breakout with a subsequent cooling phase and massive extended stars a recombination (plateau) phase. Variations occur mostly due to differences of the progenitor stars. While there appears to be a fair understanding of the light curves of SNe II, new wrinkles are emerging for SNe Ia. The photometry of SNe Ib and SNe Ic remains unsatisfactory.

Introduction

The temporal brightness variation of supernovae (SNe) as measured by photometry contains valuable and unique information on the evolution of the progenitor star and the explosion event. Combined with optical spectroscopy broad-band light curves have been the main tools for supernova investigations in the past (e.g. Minkowski 1964, Woosley & Weaver 1986, Wheeler & Harkness 1990, Kirshner 1990). The light curves are shaped by the size and mass of the progenitor star, various processes within the explosion itself, the radioactive ashes, and, in certain cases, the local environment.

Accurate photometry is mandatory to disentangle the physics driving the emission and the colors provide information on the temperature evolution. Telltale deviations from blackbody emission arise from the effects of the rapidly expanding atmosphere. The decline rates at different epochs and for supernovae of different types are indicative of the power sources, the explosion energy, and the envelope mass.

Infrared spectra of SN 1987A have been obtained at the Anglo-Australian Telescope since the explosion of this supernova. I present highlights from this program which include the analysis of the molecular emission, the determination of the mass of 57Co in the ejecta and the analysis of the emission due to dust in the ejecta. I also show the spectrum of the supernova in the infrared 5 years after explosion.

Introduction

Prior to the explosion of supernova 1987A only a few supernovae had been studied spectroscopically in the infrared (see Frogel et al. 1987; Graham et al. 1986). Although recently a number of very powerful common-user infrared spectrographs have become available to the community most of the observations of supernova 1987A were made using the previous generation of instrumentation. The near-infrared data discussed here were obtained at the Anglo-Australian Telescope using the FIGS and IRIS spectrographs through a collaboration of the author with Peter Meikle (Imperial College London) and David Allen (Anglo-Australian Observatory). Mid-infrared data were also obtained at the AAT using the UCLIR spectrograph by David Aitken (Australian Defence Forces Academy), Pat Roche (University of Oxford) and Craig Smith (ADFA).

Other groups have also been involved in infrared studies of supernova 1987A. The other major southern observatories (CTIO & ESO) have also presented infrared spectra of SN 1987A although these will not be discussed here.

A catalog of all supernovae discovered between 1885 and 1988 December 31 has been published by Barbon, Cappellaro & Turatto (1989). In Table 1 a similar listing is given for all 203 supernovae found between 1989 January 1 and 1993 April 1. A statistical discussion of these new data, and references to original sources, will be given in van den Bergh (1994). The main results of that paper are the following: (1) The most recently discovered supernovae, almost all of which were found during systematic search programs, show no evidence for the inclination effect. If no inclination corrections need to be applied then supernova rates in spirals are only half as large as previously believed (van den Bergh & Tammann 1989, Cappellaro et al. 1993). (2) The data for supernovae of type II (SNe II) show clear evidence for the Shaw (1979) effect. However, no evidence for such an effect (which is due to the fact that some supernovae are not discovered when they appear projected on the bright nuclear bulges of distant galaxies) is seen for the more luminous supernovae of Type Ia (SNe Ia). (3) Due to more intensive surveillance of bright galaxies supernovae with m (discovery) < 16 are frequently found before maximum light, whereas fainter supernovae are more often discovered at a later phase.

We review the fundamental classification scheme and statistical analysis of supernovae, emphasizing recently introduced subtypes, SN 1987K, and SN 1993J. Type Ib/Ic and Type II supernovae are of interest for starburst galaxies. We discuss possible progenitors of SNIb and SN Ic, and the possibility that they may be Wolf-Rayet stars.

Introduction

Up to May 1, 1993, 890 supernovae have been discovered, of which 480 have been classified. Today, with data of increasing quantity and precision, we have categorized supernovae into several groups, not just Types I and II, but Types Ia, Ib/Ic, II-L, IIp, 87A, and probably more to come. But, except for SNIIp and SN1987A, we are still unclear as to the evolution of the progenitor star (or stars). As the rich diversity of supernovae becomes more evident, we are increasingly challenged to ask: what are the stellar progenitors and the explosion mechanisms of the various types and subtypes of supernovae? What makes a SN Ia and what would a progenitor system look like? Are the progenitors of SN Ib/Ic all Wolf-Rayet stars? Here we attempt a brief overview of these questions. For more extensive discussions of issues bearing on supernova progenitors, see recent reviews by Wheeler(1990, 1991) and Branch et al. (1990).

We have reached a basic understanding of how the observable properties of SNe depends on the characteristics of the immediate presupernova stars and their explosion parameters.

In this paper, I summarize two new developments in the theory of core-collapse supernovae. The first is the recent establishment of an analytic context for understanding neutrino-driven explosions. Converting the supernova problem into an eigenvalue problem, Burrows & Goshy (1993) have derived a critical condition on neutrino luminosity and mass accretion rate through a stalled bounce shock for instability and explosion. The second development is the recent calculation of Burrows & Fryxell (1993) of the boost in the neutrino luminosities by the Rayleigh-Taylor-like overturn of the shocked mantle of a protoneutron star. This boost may turn duds into explosions and may be the missing ingredient of supernova theory.

Introduction

Core-collapse supernova predominate in the supernova bestiary (van den Bergh & Tammann 1991), but have challenged theorists during the entire post-war era of astrophysics. The sparseness of data that directly probe the dynamics of collapse and shock generation has hobbled advances in supernova theory, as has the wider than normal range of physical inputs required from the gravitational, neutrino, hydrodynamic, transport, thermodynamic, and nuclear realms. Extracting the essential elements of the explosion mechanism has not been easy. As a result, supernova theory has been perceived at various times to be confusing, arcane, hopeless, muddled, or vulnerable to the quick fix by a well-meaning Cincinnatus.

The environment of the SN1987A is quite complex but also very regularly structured. Detailed analyses of direct images taken under good seeing conditions (0.3–0.8 arcsec) from the European Southern Observatory (ESO)'s New Technology Telescope (NTT) show that there are two nebular loops within the 3 arcsec environment of the SN. The inner loop is elliptical in shape. The kinematics of this loop as revealed by spectroscopic data with a spectral resolving power λ/Δλ ≈ 30000 provide further clues for the three dimensional structure of these two loops. The data show that the overall structure of the nebulosity can be understood by an hourglass-shaped shell with significant mass enhancement on its equatorial plane. A diffuse nebulosity called Napoleon's Hat is observed at a distance of about 5 arcsec to the north of the SN. It showed little size evolution since the first observation on Aug. 1989, until it disappeared on Jan, 1992. The Napoleon's Hat nebula appears to be a bow-shock coming from an interaction between the supernova progenitor's stellar wind and the interstellar medium, as the supernova progenitor moved through the interstellar medium with a velocity of around 5 km s−1. On an even larger scale, there is a huge dark bay of size around 100 arcsec in diameter, we suggested that this bay was also formed by interactions between the supernova progenitor and the interstellar medium.

New observations of supernova remnants in the far ultraviolet, especially in the sub-Ly α region, are changing the way we look at the interaction between blast waves and the interstellar medium. I briefly review some of the recent FUV observations of supernova remnants from the Hopkins Ultraviolet Telescope, the Voyager Ultraviolet Spectrometers, as well as IUE and HST.

Introduction

Observations with the IUE satellite over the last 16 years have permitted great strides to be made in better understanding supernova remnants (SNRs) and their interaction with the interstellar medium (ISM). In particular, many filaments have been observed in the galactic SNRs Vela and the Cygnus Loop (Raymond et al. 1988; Raymond, Wallerstein, &, Balick 1991; Hester, Raymond, & Blair 1993; and references therein), and a few studies have been made of bright remnants in the Magellanic Clouds (Vancura et al. 1992a; Blair et al. 1989). However, IUE (and even HST) is limited to wavelengths longer than about 1200 Å, and it is only in the last few years that significant inroads have been made at FUV wavelengths down to the Lyman limit at 912 Å. These observations have been made with the Ultraviolet Spectrometers (UVSs) onboard the Voyager interplanetary spacecraft, and the Hopkins Ultraviolet Telescope (HUT) onboard the Astro-1 space shuttle mission in December 1990. In separate sections below I will discuss some of the recent advances from each of these instruments.

Most of the supernova remnants known in the Galaxy have only been detected at radio frequencies. The reason for this is absorption in the Galactic plane at both optical and X-ray wavelengths. All available evidence suggests that the shock fronts which accompany supernova remnants accelerate enough cosmic rays to GeV energies to produce readily detectable radio emission. This is fortunate, for it enables us to study remnants throughout the Galactic disk, although existing catalogues may be anywhere from 50 to 90 % incomplete. Cosmic rays and the magnetic fields in which they gyrate are the essential ingredients for producing the synchrotron radiation which is observed at radio frequencies. Various methods for estimating magnetic field strengths can be applied to a small number of remnants, and produce values not far from those based upon equipartition between the energy contents of particles and fields. From this, the particle energy content is derived for a number of objects.

Introduction

If we could view the heavens with radio eyes, then the majestic sweep of the Galactic disk would dominate the large scale structure we see. This radio version of the Milky Way has greater symmetry about a more dominant Sagittarius than its optical counterpart, as can be seen in the fine 408 MHz map of Haslam et al (1982). The main reason for this difference is simply dust, for although both stars and the cosmic rays responsible for decimeter radio emission are similarly distributed in the Galaxy, great clouds of the stuff obscure our optical vision, especially toward the Galactic center.

We review the first six years of radio observations of Supernova 1987A. The evolution can be divided into two phases: the initial radio outburst which lasted a few weeks, and the period from mid-1990 to the present, during which the radio emission has steadily increased. Both phases can be explained by a small fraction (0.1–0.5%) of the post-shock thermal energy being converted to energy in relativistic particles and magnetic fields, which give rise to synchrotron radiation. The optical depths, densities and density profiles for the pre-shocked circumstellar material are somewhat different for the two phases, but consistent with models of the density structure of the material within the circumstellar ring. New high-resolution radio observations show that the SN shock front is already at about three-quarters of the radius of the circumstellar ring, and that there exists a bright equatorial component of emission aligned with this ring which is probably due to a polar density gradient in the ‘hourglass’ structure.

Introduction

Radio studies of supernovae began with the detection of SN 1970G in M101 (Gottesman et al. 1972; Allen et al. 1976), though it wasn't for another decade that detailed radio light curves were available for a statistically useful sample of supernovae. Mainly through the work of Weiler, Sramek and collaborators (this volume) at the Very Large Array, there are now over a dozen well-studied examples of radio supernovae (RSN).

The bright O I λ11287 line observed in SN1987A is produced by the Bowen fluorescence with Lyβ and comes from regions that lie within a Sobolev length (δR ∼ 10−3RSN, the maximum distance over which fluorescence can work) from hydrogen rich gas ionized by the 56Co decay. Its strength relative to hydrogen lines (e.g. Brγ) depends on the O/H relative abundance in the ‘fluorescent region’ and on the density (i.e. the filling factor) of the gas. The observed evolution of λ11287 can be successfully understood using a relatively simple theory which takes into account the effects of transfer in the O I lines and is the generalization of the classical theory of Bowen fluorescence.

The most important result is that the time evolution of the relative intensities and profiles of O Iλ11287 and Brγ is a powerful diagnostic to determine:

1. – The filling factor of the hydrogen rich gas;

2. – The pre-SN O/H relative abundance;

3. – The amount of small scale mixing between hydrogen and oxygen rich regions and its radial stratification.

In SN1987A the results are the following:

1. –Inside 2000 km/s the hydrogen rich material is clumped with f ≃ 0.1

2. – Outside 2000 km/s the gas has f ≃ 1 and the oxygen relative abundance is quite low: O/H≃ 5 × 10−5, indicating that only the pre-SN oxygen is fluorescently coupled with hydrogen.

3. […]

Freeze out effects and the IR-catastrophe are discussed for SN 1987A and for Type Ia SNe. We show that the light curves of the optical lines in SN 1987A provide strong evidence for the IR-catastrophe. We also argue that most optical lines are dominated by non-thermal excitation after ∼ 800 days. The level of this emission is set mainly by the total mass of the elements. Models of the [OI]λλ6300 – 64 light curve show that an oxygen mass of ∼ 1.5M⊙ is needed. Light curve models for Type Ia SNe display a sharp decrease in the optical flux as a result of the IR-catastrophe at ∼ 500 days, producing UBV-photometry inconsistent with observations of SN 1972E by Kirshner & Oke (1975).

Introduction

Observations of SN 1987A, but also a number of other Type II and Type Ia SNe, at late stages have made it possible to study a number of new features in the evolution of the SN ejecta from explosion to the remnant stage. Here we discuss some recent results in this evolution. A more complete review of the background physics can be found in Fransson (1993).

SN 1987A

It is now well established from the bolometric light curve that ∼ 0.07 M⊙ of 56Ni was created in SN 1987A, and that this is responsible for most of the observed emission from the SN during the first ∼ 800 days. Being based on the bolometric light curve, this is a fairly model independent conclusion.

The nebular spectra of supernovae differ from those of better-known emission nebulae in that many of the emission lines are optically thick. Here we sketch the theory for interpreting such spectra, and show how it can be used to interpret prominent emission line systems in the spectrum of SN 1987A. As examples, we describe: (1) a simple method to infer the density of O I from observations of the evolution of the doublet ratio in [OI]λλ6300; (2) new kind of hydrogen recombination line spectrum; (3) an analysis showing that the Ca II infrared emission lines must come from primordial, not newly-synthesized, calcium; (4) a theory for the Fe/Co/Ni emission lines that shows that the inner envelope of SN 1987A must have a foamy texture, in which low density radioactive bubbles of Fe/Co/Ni reside in a massive substrate of hydrogen, helium, and other elements.

Introduction

Conventional wisdom holds that supernova explosions produce most of the heavy elements in the universe, and a major goal of astronomy is to test this hypothesis through observations of supernova spectra. For this purpose, SN 1987A should be a Rosetta Stone. We have observed its spectrum in far greater detail than that of any other supernova: at wavelength bands, such as gamma rays and far infrared, where no other supernova has been observed; with almost daily (nightly!) observations continuing for more than seven years after outburst; and with unprecedented spectral resolution (McCray 1993).

We perform hydrodynamical calculations of the collision between the supernova ejecta and circumstellar matter for SN 1987A and SN 1993J. For SN 1987A we predict light curves of X-ray emissions from the shocked ring. For SN 1993J, thermal X-rays from the shocked circumstellar matter can consistently account for the observations with ROSAT, ASCA, and OSSE.

Introduction

The supernova ejecta collides with the circumstellar matter (CSM) if its progenitor was undergoing significant mass loss. Shock waves arising from this collision compress and heat the ejecta and the CSM. The emission from the shocked material strongly depends on the density distributions of the ejecta and the CSM, thereby providing important information about the nature of the CSM.

SN 1987A

The images from the European Southern Observatory (ESO) (Wampler et al. 1990) and the Hubble Space Telescope (HST) (Jakobsen et al. 1991) revealed the presence of a ring-like structure at ∼ 6 × 1017 cm from SN 1987A. The outermost part of the supernova ejecta is expanding at ∼ 104 km s−1 (Shigeyama & Nomoto 1990), thus being expected to collide with the ring at ∼ 10 years after the explosion.

Hydrodynamical model

The progenitor of SN 1987A had once become a red supergiant (RSG) and then contracted to a blue supergiant (BSG) before the explosion (for reviews, see Arnett et al. 1989, Hillebrandt & Höflich 1989, Podsiadlowski 1992, and Nomoto et al. 1993a).