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.

We calculate a nonlinear growth of the Rayleigh-Taylor instability in the exploding red supergiant stars with a two-dimensional hydrodynamical code, and examine how the extent of mixing depends on the progenitor's core mass and the envelope mass. The results are compared with the observations of type II-P supernovae and the recent type II-b supernova 1993J.

Introduction

Large scale mixing in supernova ejecta has been indicated in spectroscopic and photometric observations of various types of supernovae. This has stimulated 2D and 3D hydrodynamical calculations of the Rayleigh-Taylor (R-T) instabilities during supernova explosions for SN 1987A (Arnett et al. 1989; Hachisu et al. 1990, 1992; Fryxell et al. 1991; Müller et al. 1991; Den et al. 1990; Yamada et al. 1990; Yamada & Sato 1991; Herant & Benz 1991, 1992), type Ib/Ic supernovae (Hachisu et al. 1991, 1994a), type II-P supernovae (Herant & Woosley 1994; Hachisu et al. 1994b), and the type II-b supernova 1993J (Iwamoto et al. 1994). In particular, Hachisu et al. (1991, 1994a) found that development of the R-T instabilities depend sensitively on the presupernova structure, so that the comparison between hydrodynamical simulations and observations can provide a new clue to the understanding of supernova progenitors, their structure, and the explosion mechanism.

In the present paper, we follow a nonlinear growth of the R-T instabilities in the exploding red supergiant stars, i.e., type II-P and II-b supernovae. We find that the extent of mixing depends on the core mass and the envelope mass of red supergiants.

We summarize various explosion models of Type Ia supernovae and their nucleosynthesis features for both Chandrasekhar and sub-Chandrasekhar mass white dwarf models. These models provide different predictions of the photometric and spectroscopic variations among Type Ia supernovae, which are compared with observations. Some attempts to model the peculiar SNe 1991T and 1991bg are shown.

Introduction

Type I supernovae (SNe I) are spectroscopically defined by the absence of hydrogen in their optical spectra and further subclassified into Ia, Ib, and Ic (see, e.g., Branch et al. 1991). The early–time optical spectra of SNe Ia are characterized by the presence of a deep absorption Si II line near 6150 Å, and their late–time spectra are dominated by strong blends of Fe emission lines (Harkness & Wheeler 1990). Relatively uniform light curves and spectral evolution of SNe Ia have led to the use of SNe Ia as a standard candle to determine cosmological parameters, H0 and q0 (Branch & Tammann 1992).

Recent attention has been paid to variations of light curves and spectra among SNe Ia. SNe 1991T and 1991bg have clearly revealed the presence of both spectroscopically and photometrically peculiar SNe Ia. Photometrically, maximum brightness and the decline rate of the light curve show some systematic variations, where SNe 1991T and 1991bg are situated at the two extreme ends of the brighter–slower tendency (Phillips 1993; Branch et al. 1993). Spectroscopically, the pre-maximum spectra of SNe Ia reveal a significant variation of the composition and expansion velocities of the outermost layers, whereas the post-maximum spectra are relatively uniform except for SN 1991bg.

We give an update of current research on the use of nebular spectra of SNe Ia as distance indicators. Results of the application of the method to a group of SNe Ia are reported. We describe the status of the research including theoretical and observational requirements of the method. Our results point toward a shorter distance scale than methods based on the “standard candle” hypothesis for Type Ia SNe.

Introduction

The use of SNe Ia as “standard candles” to determine the extragalactic distance scale has been recurrently debated. The correlation found by Pskovskii (1977, 1984) and by Branch (1981) between the postmaximum decline rate of the light curve and the magnitude at maximum cast doubts concerning this method. The validity of the correlation was questioned by Boisseau & Wheeler (1991), who found that such an effect might reflect contamination from the light of the underlying galaxy. But new evidence on differences in the light curve decline rate (Phillips 1993; Suntzeff, this volume) opens again the question of the correlation of magnitude at maximum and slope of the light curve soon after maximum. The value of the absolute magnitude of SNe Ia as a class loses much of its meaning if the considerable spread in magnitudes found in recent work is confirmed.

Uncertainties in the absolute magnitudes of SNe Ia are amplified by extinction. The discrepant “observationally-inferred” values obtained for SN 1986G (Phillips et al. 1992a; Delia Valle & Panagia 1992; Phillips 1993) show that when reddening is high the usual prescriptions to obtain this quantity from the color curves and from the equivalent width of the Na I D interstellar line towards the supernova give different results.

The last observations (until April 1993) of SN 1987A made at ESO, La Silla, are presented. Our data show that: (i) the criterion of line shifts proves that dust is still present and is absorbing more strongly than ever; (ii) the I magnitude decreases faster than the other ones after day ∼1700; (iii) the 1.3mm flux is constant at about 9mJy, and comes most probably from free-free emission produced by the cooling of the former star envelope still weakly ionized. Previous analyses of the bolometric light curve until day 1444 are briefly reviewed. In spite of the large uncertainties, the flattening of the light curve, observed after day ∼900, extends until our latest data points (day 2172). This can be explained by theoretical models including time-dependent effects due to long recombination and cooling times (Fransson and Kozma 1993). However, one cannot rule out the presence of a compact object such as a neutron star, radiating as a pulsar or accreting matter from a disk either continuously or intermittently.

The Dust

In order to understand many aspects of the observed behaviour of SN 1987A at later phases, one must appreciate the role of dust in the expanding ejecta of the supernova. Molecules such as CO and SiO were formed at a very early phase (<100 days after outburst) (Bouchet and Danziger 1993). Probably as a result of the presence of molecules, dust formed at approximately day 530 and has since continued to play a dominant role in absorbing much of the harder radiation and thermalizing it.

At its peak, SN 1993J was one of the brightest supernovae in this century, and it is being studied more thoroughly than any supernova except SN 1987A. It is proving to be similar to the transition object SN 1987K, which metamorphosed from being a hydrogen-rich Type II near peak to having a hydrogen-deficient nebular phase. SN 1993J has been observed throughout the electromagnetic spectrum and with optical spectropolarimetry. It is interacting with a dense circumstellar nebula and is generating radio and X-ray flux, but it has probably not been detected in gamma rays. The photometric and spectral evolution are consistent with a star of original mass ∼ 15 M⊙ that lost appreciable mass to a binary companion leaving an extended, helium-rich hydrogen envelope of ≲ 0.5 M⊙ and a helium core of ∼ 4 M⊙. The spectral evolution will put strong constraints on the mixing of 56Ni and other species.

Introduction

SN 1993J was discovered on March 28.9 by F. Garcia (Ripero 1993) in the Sab galaxy NGC 3031 = M81. It was the brightest supernova observable from mid-northern latitudes since SN 1972E and has been the subject of intense observation by a large number of major and minor optical observatories, the VLA and other radio telescopes, IUE, the Compton Gamma Ray Observatory, ROSAT, and the newly launched ASCA satellite, as well as by a host of amateur astronomers. In addition, SN 1993J has proven exceptional on a number of grounds and has prompted considerable theoretical modeling.

Presupernova evolution and explosive nucleosynthesis in massive stars for main-sequence masses from 13 M⊙ to 70 M⊙ are calculated. We examine the dependence of the supernova yields on the stellar mass, 12C(α,γ)16O rate, and explosion energy. The supernova yields integrated over the initial mass function are compared with the solar abundances.

Presupernova models and the 12C(α,γ)16O rate

Presupernova models are obtained for helium stars with masses of Mα = 3.3, 4, 5, 6, 8, 16, and 32 M⊙ as an extension of the studies by Nomoto & Hashimoto (1988), Thielemann et al. (1993), and Hashimoto et al. (1993). These helium star masses correspond approximately to main-sequence masses of Mms = 13, 15, 18, 20, 25, 40, and 70 M⊙, respectively (Sugimoto & Nomoto 1980). The systematic study for such a dense grid of stellar masses enables us to understand how explosive nucleosynthesis depends on the presupernova stellar structure and to apply the results to the chemical evolution of galaxies. We use the Schwarzschild criterion for convection and neglect overshooting. The initial composition is given by X(4He) = 0.9879 and X(14N) = 0.0121. These helium stars are evolved from helium burning through the onset of the Fe core collapse.

Nuclear reaction rates are mostly taken from Caughlan & Fowler (1988). For the uncertain rate of 12C(α,γ)16O, we use the rate by Caughlan et al. (1985; CFHZ85), which is larger than the rate by Caughlan & Fowler (1988; CF88) by a factor of ∼ 2.4.