Recent observations of the galactic supernova remnants the Crab Nebula, SN 1006, Cas A, and the Cygnus Loop are reviewed. New studies of the Crab Nebula suggest its progenitor may have had appreciable mass loss in the form of a circumstellar disk resulting in both a bipolar expansion and formation of the synchrotron ‘bays’. Unusually high proper motion knots near to and possibly directed away from the pulsar also have been reported. In the Cas A remnant, a NE jet of ejecta appears to be a plume of mantle material with expansion velocities up to 12000 km s−1 or nearly twice that seen in the main ejecta shell. HST observations of the sdOB star located behind SN 1006 indicate symmetrically expanding Fe II ejecta out to 8100 km s−1. Lastly, deep images of the Cygnus Loop reveal emission structures similar to those seen in 2D & 3D shocked cloud simulations.

Optical research on the properties of galactic supernova remnants (SNRs) continues to yield important new results. Though only a small fraction of the radio catalogued 170+ galactic SNRs are optically detectable, optical measurements permit one to investigate such SNR properties as chemical abundances relative to hydrogen, expansion velocities, gas densities and temperatures, and ejecta filament morphologies and distribution. With the advent of the International Ultraviolet Explorer (IUE) in 1978 and now the Hubble Space Telescope (HST), UV observations on the brighter and less reddened optical SNRs are possible, substantially adding to our knowledge.

Radio observations have shown that some supernovae are powerful radio emitters which increase rapidly in brightness to radio luminosities which are hundreds to thousands of times greater than even the brightest known supernova remnant, Cas A. They then fade over a period of weeks, months, or years. This radio emission has been found to provide important information about the nature of the progenitor stars, their mass loss rates, and the circumstellar material surrounding them. RSN observations may also offer the possibility of extragalactic distance measurements and the presence of radio emission appears to be indicator of strong x-ray emission and late time optical emission.

Introduction

Detailed studies of radio emission from supernovae have now been carried out for over a decade with SN1979C providing the first example of a radio supernova (RSN) which could be detected and monitored in detail over a lengthy time span. The monitoring of the radio emission from SN1979C is still continuing. Additionally, in the intervening 13 years a number of other SNe have been detected at radio wavelengths and these are listed in Table 1. This list is complete at the present time. However, it is limited to objects which show most or all of the RSN properties which are listed in Section 5, and in practice includes only “young” SNe occurring since the first radio detection of an SN, SN1970G, by Gottesman et al. (1972).

Hydrodynamical simulations of type-II supernovae in one and two dimensions are performed for the revival phase of the delayed shock by neutrino energy deposition. Starting with a postcollapse model of the 1.31 M⊙ iron core of a 15 M⊙ star immediately after the stagnation of the prompt shock about 10 ms after core bounce, the models are followed for several hundred milliseconds with varied neutrino fluxes from the neutrino sphere. The variation of the neutrino luminosities is motivated by the considerable increase of the neutrino emission due to convective processes inside and close to the neutrino sphere (see Janka 1993), which are driven by negative gradients of entropy and electron concentration left behind by the prompt shock (Burrows & Fryxell 1992, Janka & Müller 1992). The size of this luminosity increase remains to be quantitatively analyzed yet and may require multi-dimensional neutrino transport. However, in the presented simulations the region below the neutrino sphere is cut out and replaced by an inner boundary condition, so that the convective zone is only partially included and the neutrino flows are treated as a freely changeable energy source.

For small neutrino luminosities the energy transfer to the matter is insufficient to revive the stalled shock. However, there is a sharp transition to successful explosions, when the neutrino luminosities lie above some ‘threshold value’. Once the shock is driven out and the density and temperature of the matter between neutrino sphere and shock start to decrease during the expansion, suitable conditions for further neutrino energy deposition are maintained, and an explosion results.

Extragalactic supernova rates are reviewed. The main uncertainties in calculated rates are due to (1) the influence of the (still poorly known) luminosity function of supernova of a given type on “control times”, to (2) uncertain corrections for possible inclination – dependent bias in supernova discovery probabilities, and (3) interstellar absorption. The total supernova rate in late-type galaxies is found to be ∼ 2(Ho/75)2 supernovae (SNe) per century per 1010 LB(⊙). This is consistent with the rate of 3 SNe per century that is derived from the historical data on Galactic supernovae. It is, however, a source of some concern that none of the three Galactic SNe expected to have occurred during the last century was actually observed!

The expansion velocities of SNe Ia are found to correlate strongly with parent galaxy Hubble type. This relation is in the sense that low expansion velocities are only observed for those SNe Ia that occur in early-type galaxies. This suggests that V(exp) correlates with the ages of SNe Ia progenitors. It is speculated that the progenitors of a few SNe Ia with high V(exp) values in E and S0 galaxies were formed during recent starbursts.

SNe Ia rates appear to be enhanced in post-starburst galaxies. It is suggested that supernova rates might be quite high in the recently discovered population of faint blue galaxies at intermediate redshifts.

Extragalactic Supernova Rates

The first estimate of extragalactic supernova rates was made by Zwicky (1938), who introduced the idea that “control time” was a critical factor needed to determine the supernova frequency.

The quality of observational data on Type Ia supernovae has improved remarkably in the last few years, due mainly to monitoring programs with CCD-equipped detectors on small aperture telescopes at observatories across the world, and at the space observatories. I will review the recent observational characteristics of Type Ia supernovae, focusing the discussion on our observations of SN1992A in the S0 galaxy NGC 1380 in the Fornax cluster as a reference to other Type Ia events. We now have strong evidence that Type Ia events are not a homogeneous class, but vary in both color and brightness at maximum light, vary in rise time and decline from maximum, and have spectral characteristics at maximum light that are correlated with these photometric parameters. Insofar as the SBF, PNLF, and infrared Tully-Fisher distance scales are correct, the observed (uvoir) bolometric light curves also indicate that these supernovae are less luminous than expected from the models of the explosion of a C-O white dwarf at the Chandrasekhar mass.

Introduction

A stellar explosion is an unlikely physical environment to produce a homogeneous energy flux, given the fantastic brightness of a supernova at maximum light which can reach 10% of the luminosity of the whole galaxy for a period of a few weeks. Yet it is the brightness of the event that makes the use of supernovae as “standard candles” so attractive, since they can be readily observed to cosmologically interesting distances.

Core collapse in very massive stars can lead to a cerntral black hole that swallows the rest of the star and in less massive stars to a central neutron star and explosion. There is probably an intermediate mass range that gives an explosion and a central black hole; supernova remnants with no observable central object are candidates. The association of pulsars with Type II supernovae gives an estimate of the pulsar power to be expected in a supernova, but the uncertainty in the initial pulsar periods gives a wide range in possible powers. The relativistic wind bubble model for the Crab Nebula has steadily developed and there are now predictions regarding particle acceleration in the optical wisps. The bubble model with expansion into supernova gas can also be applied to other young pulsar nebulae.

Introduction

The study of compact objects in supernova remnants has long been troubled by the lack of evidence for such objects. For many years, the Crab and Vela pulsars were the only compact objects observed in remnants. More recently, the number of pulsar/remnant associations has increased to 9 or 10 (Kaspi et al. 1992; Kulkarni et al. 1993). In other cases, the presence of a pulsar is inferred from a centrally condensed, flat radio spectrum nebula thought to be created by a pulsar. The study of these objects, as well as more detailed study of the Crab Nebula, has led to a general theoretical picture, although many basic uncertainties remain.

Existing evidence of photometric and spectroscopic diversity among Type Ia supernovae is compared with the predictions from physical modeling of the explosions. Concerning light curves, changes in the central ignition density of massive (M ≃ Mch) C+O white dwarfs alone do not give appreciable variation. Spectroscopic diversity has been found in the nebular phase, the underluminous SN 1991bg providing an extreme case. A range of 0.4–0.8 M⊙ of 56Ni synthesized in the explosions is derived from the nebular spectra of a sample of SNe Ia. For SN 1991bg, however, a 56Ni mass of ∼ 0.1 M⊙ only is obtained. That leads us to explore models based on the detonation of low–mass WDs for this SN. Additionally, a nebular spectrum of SN 1991bg shows narrow Hα emission at the position of the SN. If this emission is confirmed against background contamination from the galaxy, it would be first evidence of a nondegenerate, H–rich companion in a SNIa.

Introduction

Type Ia supernovae (SNIa) are attributed to the thermonuclear explosion of C+O white dwarfs. Explosive ignition would be the outcome of accretion of matter from a close companion in a binary system and it would completely burn the star, leaving no bound remnant. In most models, explosive C burning starts at the center of the WD as a result of the increase in density and temperature induced by quasistatic mass growth.

The oldest historical supernova (SN), recorded by ancient Chinese in 14th Century B.C. on pieces of tortoise shells or bones, is identified with the aid of modern space γ-ray observations. Hard X-rays with energy up to 20 keV were observed from IC 443 by the X-ray satellite Ginga. We infer from these observations the age of IC 443 is ∼ 1000 – 1400 yrs. The result supports the hypothesis that IC 443 is the remnant of the historical SN 837 that occurred during the Tang Dynasty.

The association between the supernova remnant (SNR) CTB 80 and SN 1408 has been hotly debated for about ten years and is briefly reviewed and discussed here. A new picture is presented to explain this association.

High energy emission from historical SNRs can persist in a multiphase interstellar medium (ISM). As a result, the study of the relationship between SNRs and ancient guest stars has gained new vitality.

The First Supernova Observed by Mankind

SN 1987A, the first supernova observed by the naked eye in nearly 400 years, stimulates a high tide in supernova research. It also tempts us to ask: what is the earliest supernova recorded by mankind? Recently, we have discussed this topic in a few articles (Wang 1987 a,b; Xu, Wang & Qu 1992). The earliest supernova recorded by mankind is the great new star that occurred in 14th century B.C. recorded by the ancient Chinese on a piece of Tortoise shell or bone in Yin-Shang Dynasty (Fig. 1).

Knowledge of the size and age of the Universe depends on understanding supernovae. The direct geometric measurement of the circumstellar ring of SN1987A using IUE spectra and HST images provides an independent test of the Cepheid distance scale to the Large Magellanic Cloud. Understanding the details of the mass distribution in the circumstellar matter is important to improving the precision of this distance. Type Ia supernovae have a narrow distribution in absolute magnitude, and new Cepheid distances to IC 4182 (the site of SN 1937C) and to NGC 5253 (the site of SN 1972E) obtained with HST by Sandage and his collaborators allow that absolute magnitude to be calibrated. Comparison with more distant SNIa gives H0 = 56 ± 8 km s−1 Mpc−1. Recent work in supernova spectroscopy and photometry shows that the apparent homogeneity of SNIa is not quite what it seems, and a deeper understanding of these variations is needed to use the SNIa to best advantage. The Expanding Photosphere Method (EPM) allows direct measurement to each Type II supernova that has adequate photometry and spectroscopy. There are now 18 such objects. The sample of EPM distances from 4.5 Mpc to 180 Mpc indicates H0 = 73±6(statistical)±7(systematic) km s−1 Mpc−1. Better understanding of supernova atmospheres can reduce the systematic error in this approach, which is completely independent of all other astronomical distances.

The Catalogue

This catalogue of Galactic supernova remnants (SNRs) is an updated version of those presented in detail in Green (1984, 1988) and in summary form in Green (1991). The basic parameters of the 182 SNRs included in this (1993 May) version of the catalogue are presented below. Notes on how these parameters are derived from observational data are given in Green (1988). It should be noted that there are serious selection effects which apply to the identification of Galactic SNRs (see Green 1991), so that great care should be taken if these data are used in statistical studies. There are many objects that have been identified as SNRs and are listed in the catalogue, although they have been barely resolved in the available observations, or are faint, and have not been well separated from confusing background or nearby thermal emission. The identification of these objects as SNRs, or at least their parameters remain uncertain (see Green 1988).

Revisions

Since Green (1991) the following eight SNRs have been added to the catalogue:

• Three new remnants (G59.5+0.1, G67.7+1.8 and G84+0.5) of the the eleven possible SNRs reported by Taylor, Wallace & Goss (1992).

• G156.2+5.7, which was first identified from X-ray observations by ROSAT (Pfeffermann, Aschenbach & Predehl 1991).

• G318.9+0.4, a complex of radio arcs reported by Whiteoak (1990).

• G322.5−0.1, reported by Whiteoak (1992).

• […]

Models for SNIa

In order to study the question whether the appearance of SNIa should be uniform from theoretical point of view, we present light curves (LC) for a broad variety of models using our elaborated LC scheme, including implicit LTE-radiation transport, expansion opacities, MC-γ transport, etc. For more details see Khokhlov (1991), Höflich et al. (1992), Höflich et al. (1993), Khokhlov et al. (1993), and Müller et al. (1993).

We consider a set of 19 SNIa explosion models, which encompass all currently discussed explosion scenarios. The set consists of three deflagration models (DF1, DF1MIX, W7 o), two detonation models (DET1, DET2 *), two delayed detonation models (N21, N32 •), detonations in low density white dwarfs (CO095, CO10, CO11 ⋆), six pulsating delayed detonation models (PDD3, PDD5-9 Δ) and three tamped detonation models (DET2ENV2, DET2ENV4, DET2ENV6 Δ). We also included the widely-used deflagration model W7 of Nomoto et al. (1984)

Different explosion models can be discriminated well by the slopes of the LCs and changes of spectral features (e.g. line shifts ⇒ expansion velocities). The differences can be understood in terms of the expansion rate of the ejecta, the total energy release, the distribution of the radioactive matter, and the total mass and density structure of the envelope.

The structure and the size of the core of massive presupernova stars are determined by the electron fraction and entropy of the core during its late stages of evolution; these in turn affect the subsequent evolution during gravitational collapse and supernova explosion phases. Beta decay and electron capture on a number of neutron rich nuclei can contribute substantially towards the reduction of the entropy and possibly the electron fraction in the core. Methods for calculating the weak transition rates for a number of nuclei for which no reliable rates exist (particularly for A > 60) are outlined. The calculations are particularly suited for presupernova matter density (ρ = 107 − 109 g/cc) and temperature (T = 2 − 6 × 109 °K). We include besides the contributions from the ground state and the known excited states, the Gamow-Teller (GT) resonance states (e.g. for beta decay rates, the GT+ states) in the mother nucleus which are populated thermally. For the GT strength function for transitions from the ground state (as well as excited states) we use a sum rule calculated by the spectral distribution method where the centroid of the distribution is obtained from experimental data on (p,n) reactions. The contribution of the excited levels and GT+ resonances turn out to be important at high temperatures which may prevail in presupernova stellar cores.

Presupernova Evolution of Massive Stars

Beta decay (β−) and electron (e−) capture of neutron rich nuclei play important roles in determining presupernova core structure (Nomoto et al, 1991).

The status for the identification of specific astronomical objects as SNIa progenitors is reviewed. Single or double degenerate progenitors? Chandrasekhar or sub-Chandrasekhar mass exploders? These are the two main questions still to be answered concerning the progenitors of Type Ia supernovae. Although all four combinations may be represented in nature, searches for double degenerates seem to indicate that such systems provide a minor channel for the production of SNIa's. The more promising candidates appear to be symbiotic stars, consisting of a single degenerate star and a sub-Chandrasekhar mass star.

Introduction

The nature of the progenitors of Type Ia SNe remains highly conjectural. The fact that SNIa's occur in elliptical galaxies – where star formation ceased a very long time ago – indicates that at least in some cases there is a long delay between the formation of the progenitor and the explosion. Attention has generally concentrated on white dwarfs (WD) in binary systems, in which the explosion of the WD is triggered by accretion from the companion. Various WD explosion mechanisms are discussed by Ken'ichi Nomoto and Eli Livne at this meeting, and I will here deal with the identification of specific astronomical objects as suitable precursor candidates.

We discuss a new scenario for the production of SNII explosion and present the results of numerical modelling studies of SNe II light curves which are being done in our group.

Exploding Neutron Star

The outburst of SN1987A has given a powerful impetus for theoretical work on the physical mechanism of supernova explosions. The one-dimensional theory of the SN mechanism has met certain difficulties in explaining the SN II explosion (see, e.g. Imshennik 1992a). Multidimensional effects might be required to resurrect the delayed explosion mechanism (Bethe & Wilson 1985), owing to neutrino heating (see contributions by Burrows 1993 and Janka 1993). Hillebrandt et al. (1990) have remarked that we may have to invent complicated scenarios in order to account for the explosions of massive stars, M = 20M⊙. We discuss here a bizarre scenario proposed by Imshennik (1992b), where the interested reader can find further details. Here we give only a brief sketch of the main idea and report on the present status of the project.

In the suggested scenario (Imshennik 1992b), the decisive role is played by the rotation of a presupernova core. The idea to connect an SN explosion with the fission instability in a rapidly rotating collapsing star was first put forward by von Weizsäcker (1947). Shklovsky (1970) had also pointed out the possible importance of the rotational instability for type II SNe. Those ideas were expressed in quite general form.

We present here preliminary results of the ASCA satellite. ASCA is equipped with X-ray telescopes that can observe the energy range up to 12 keV. There are two types of detector systems: GIS and SIS. The energy resolution of the SIS is 130 eV (FWHM at 7 keV) and can resolve emission lines clearly. For the PV phase, we planned to observe about 150 sources. Among them, there are 23 SNR's, some of which are presented here. We will be able to study the evolution of thin hot plasma in the SNRs.

Introduction

The fourth Japanese X-ray Astronomy satellite was successfully launched on February 20, 1993, from Kagoshima Space Center. The satellite's pre-launch name, Astro-D, was changed to it ASCA once it achieved orbit. ASCA is equipped with four thin foil X-ray mirror telescopes (XRT) that can collect X-rays up to 12 keV. Fig. 1 shows the effective area of the XRT. The XRT has a point spread function (PSF) with a half power diameter (HPD) about 2.7 arcmin. There is a sharp core of about 20 arcsec diameter in the PSF that enables us to separate point sources separated by less than one arcmin.

ASCA has two types of detectors: one is the imaging gas scintillation proportional counter, (IGSPC, Ohashi et al, 1991) and the other is the X-ray CCD camera (Burke, et al., 1993). They are called the gas imaging spectrometers (GIS) and the solid-state imaging spectrometers (SIS), respectively.

A series of early-time optical spectra of the peculiar SNIa 1991T, obtained from 2 weeks before to 4 weeks after maximum, have been computed with our Monte Carlo code.

The earlier spectra can be successfully modelled if 56Ni and its decay products, 56Co and 56Fe, dominate the composition of the outer part of the ejecta. This atypical distribution confirms that the explosion mechanism in SN 1991T was different from a simple deflagration wave, the model usually adopted for SNe Ia.

As the photosphere moves further into the ejecta the Ni Co Fe fraction drops, while intermediate mass elements become more abundant. The spectra obtained 3–4 weeks after maximum look very much like those of the standard SN Ia 1990N. A mixed W7 composition produces good fits to these spectra, although Ca and Si are underabundant. Thus, in the inner parts of the progenitor white dwarf the explosion mechanism must have been similar to the standard deflagration model.

The fits were obtained adopting a reddening E(B − V) = 0.13. A Tully-Fisher distance modulus µ = 30.65 to NGC 4527 implies that SN 1991T was about 0.5 mag brighter than SN 1990N. At comparable epochs, the photosphere of SN 1991T was thus hotter than that of SN 1990N. The high temperature, together with the anomalous composition stratification, explains the unusual aspect of the earliest spectra of SN 1991T.

Large samples of supernova remnants are needed in order to study the global distribution of supernovae in galaxies, for determining how the environment in which a SN explodes affects the appearance of a SNR, for studying abundances and abundance gradients in galaxies, for estimating SN rates, and in order to determine the energetics of SNRs and their expansion. Here we describe techniques which are currently being used to expand SNR samples in nearby spirals.

Introduction

The Cygnus Loop is thought to be about 18,000 years old (Ku et al. 1984). Assuming a SN rate of 5 per century (van den Bergh & Tammann 1991), there should be about 900 SNR in the galaxy younger than the Cygnus Loop. However, Green's revised catalog of Galactic SNRs contains 182 SNRs, some of which are clearly more evolved than the Cygnus Loop (Green, these proceedings). As a result, it is clear that the Galactic sample is very incomplete. In the Galaxy, nearly all SNRs have been first recognized as SNRs from radio observations. Since SNRs are found primarily in the Galactic plane and since X-rays and optical light are strongly absorbed by material in the Galactic plane, they are hard to detect in these wavelength bands. In fact, only about 40 Galactic SNRs have been detected at optical wavelengths and only about 50 have been detected at X-ray wavelengths.