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.

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.