Abstract

The death of massive stars and the processes which govern the formation of compact remnants are not fully understood. Observationally, this problem may be addressed by studying different classes of cosmic explosions and their energy sources. Here we discuss recent results on the energetics of γ-ray bursts (GRBs) and Type Ib/c Supernovae (SNe Ib/c). In particular, radio observations of GRB 030329, which allow us to undertake calorimetry of the explosion, reveal that some GRBs are dominated by mildly relativistic ejecta such that the total explosive yield of GRBs is nearly constant, while the ultra-relativistic output varies considerably. On the other hand, SNe Ib/c exhibit a wide diversity in the energy contained in fast ejecta, but none of those observed to date (with the exception of SN 1998bw) produced relativistic ejecta. We therefore place a firm limit of 3% on the fraction of SNe Ib/c that could have given rise to a GRB. Thus, there appears to be clear dichotomy between hydrodynamic (SNe) and engine-driven (GRBs) explosions.

The death of massive stars

The death of massive stars (M ≳ 8M⊙) is a chapter of astronomy that is still being written. Recent advances in modeling suggests that a great diversity can be expected. Indeed, such diversity has been observed in the neutron star remnants: radio pulsars, AXPs, and SGRs. We know relatively little about the formation of black holes.

The compact objects form following the collapse of the progenitor core.

Abstract

It is the conventional wisdom that overluminous Type Ia supernovae have an overproduction of their elemental powerhouse, 56Ni, leading to broader light curves, higher temperatures, higher ionization states, and peculiar spectra similar to that of SN1991T. However, this simple picture is incomplete: we show that a broad lightcurve width does not necessarily predict spectroscopic peculiarity, nor does a spectrum resembling SN1991T guarantee a broad lightcurve. There is circumstantial evidence that asymmetry may play a role in the explanation of the diverse properties of broad lightcurve and SN1991T-like SNe Ia.

As an illustrative example, we present optical and NIR light curves, and Lick 3m and HST STIS spectra of the SN Ia with the broadest light curve observed to date, SN 2001ay. SN 2001ay has Δm15(B) = 0.6 and stretch s = 1.6, yet at maximum light is fairly spectroscopically normal. The exception is an extremely high Si velocity, v = 15,000 km s–1. The secondary peak in the I-band lightcurve is higher than the primary peak, and the Js and H lightcurves remain flat over the entire 55 days of observation. SN 2001ay also does not appear to obey lightcurve shape-luminosity relationships, at least as they are currently formulated. Despite its broad lightcurve, the SN has normal absolute magnitudes after correction for Milky Way and host galaxy extinction. Thus, if a stretch or Δm15(B) correction is applied, the resulting magnitude would be overcorrected by ∼1 mag.

Abstract

We present a brief summary of asphericity effects in thermonuclear and core collapse supernovae (SN), and how to distinguish the underlying physics by their observable signatures. Electron scattering is the dominant process to produce polarization which is one of the main diagnostical tools. Asphericities result in a directional dependence of the luminosity which has direct implications for the use of SNe in cosmology. For core collapse SNe, the current observations and their interpretations suggest that the explosion mechanism itself is highly aspherical with a well defined axis and, typically, axis ratios of 2 to 3. Asymmetric density/chemical distributions and off-center energy depositions have been identified as crucial for the interpretation of the polarization P. For thermonuclear SNe, polarization turned out to be an order of magnitude smaller strongly supporting rather spherical, radially stratified envelopes. Nevertheless, asymmetries have been recognized as important signatures to probe A) for the signatures of the progenitor system, B) the global asymmetry with well defined axis, likely to be caused by rotation of an accreting white dwarf or merging WDs, and C) possible remains of the deflagration pattern.

Introduction

During the last decade, advances in observational, theoretical and computational astronomy have provided new insights into the nature and physics of SNe and gamma-ray bursts. Due to the extreme brightness of these events, they are expected to continue to play important role in cosmology. SNe Ia allowed good measurements of the Hubble constant both by statistical methods and theoretical models.

Introduction: a brief time for history

There has been a great deal of progress in the thirty-five years or so that I have been working on supernovae and related topics. Two of the classical problems have been with us the whole time: what makes core collapse explode, and what are the progenitors of Type Ia supernovae? This workshop, indeed, the perspectives of three-dimensional astrophysics applied to these problems, gave encouraging evidence that breakthroughs may be made in both of these venerable areas.

On the other hand, what a marvelous array of progress has rolled forth with ever increasing speed. We have an expanded botany of supernovae classification: Type Ia, Ib, Ic, Type IIP, IIL IIb, IIn; but, of course, more than mere classification, a growing understanding of the physical implications of these categories. Neutron stars were discovered as rotating, magnetized pulsars when I was a graduate student, and the extreme form, magnetars, has now been revealed (Duncan & Thompson 1992). The evidence that we are seeing black holes in binary systems and the centers of galaxies has grown from suspicion to virtual certainty, awaiting only the final nail of detecting the black spot in a swirl of high-gravity effects. Supernova 1987A erupted upon us over 16 years ago and is still teaching us important lessons as it reveals its distorted ejecta and converts to a young supernova remnant before our eyes.

There have also been immense theoretical developments.

Abstract

There are currently a few cases where a supernova was associated with a Gamma-Ray Burst, proving that GRBs arise from the death of massive stars. Other lines of evidence supporting this conclusion are the spatial location of bursts in the host galaxy, the detection of multiple high velocity absorption lines in GRB 021004, and of X-ray emission lines and edges for a few afterglows. Massive stars drive powerful winds, shaping the circumstellar medium up to tens of parsecs. Modeling of the broadband afterglow emission with a relativistic fireball interacting with the circumburst medium, yields estimations of its particle density. The resulting values, ranging from 0.1 cm-3 to 50 cm-3, are consistent with the density of the wind from a Wolf-Rayet star at the typical distance (0.1 ÷ 1 pc) where the afterglow is expected to occur. The r˗2 density profile expected around a massive star is consistent with the results of afterglow modeling in a majority of cases; nevertheless there are a few afterglows for which a homogeneous medium accommodates much better the sharpness of the optical light-curve break. Afterglow modeling also shows that the kinetic energy of GRB jets spans the range 1050 and 3 × 1051 ergs, i.e. slightly less than that of the supernova ejecta. The burst γ-ray energy output, corrected for collimation, has a similar range.

Abstract

Delayed detonations in exploding carbon-oxygen (C-O) white dwarfs, are bound to ignite and propagate in an expanding Rayleigh-Taylor (R-T) unstable region. Therefore, non-spherical detonations are expected to evolve due to a possible off-center ignition and due to the inhomogeneous composition ahead of the detonation front. We examine some of the possible consequences of such non-spherical explosions, using two-dimensional axisymmetric simulations.

We find that the explosion products, namely the amount of energy released and the composition of the burnt material, are rather sensitive to the asphericity. This sensitivity follows from the fact that the expansion speed is not negligible with respect to the detonation speed. With lower transition density we get less Fe group elements, smaller explosion energy and higher asphericity in the distribution of elements. We also show that the delayed detonation cannot directly induce a second detonation in a nearby isolated bubbles or channels of cold fuel. Therefore, pockets of unburnt C-O mixture may survive deep inside the ejecta.

Introduction

The delayed detonation model for Type Ia supernovae assumes that transition from deflagration to detonation occurs during the combustion of a carbon oxygen (C-O) Chandrasekhar mass white dwarf. In order to fit observations, the transition should occur after a significant expansion that reduces the density of the fuel ahead of the front. Traditionally, the transition point is parametrized by a transition density ρtr, which is the density ahead of the deflagration front at the transition moment.

Abstract

The effects of rotation in progenitor models for Type Ia supernovae are addressed. After discussing processes of angular momentum transport in carbon+oxygen white dwarfs, we investigate pre-explosion conditions of accreting white dwarfs. It is shown that differential rotation will persist throughout the mass accretion phase, with a shear strength near the threshold value for the dynamical shear instability. It is also found that rotational effects stabilise the helium shell source and reduce the carbon abundance in the accreted envelope.

Introduction

Unlike core collapse supernovae, Type Ia supernovae (SNe Ia) occur exclusively in binary systems (e.g. Livio 2000). Although it is still unclear which kinds of binary systems lead to SNe Ia, non-degenerate stars such as main sequence stars, red giants or helium stars are often assumed as the white dwarf companion (e.g. Hachisu et al. 1999, Langer et al. 2000, Han & Podsiadlowski 2003, Yoon & Langer 2003). This leads us to consider the spin-up of the white dwarf, since the transfered matter from those companions should form a Keplerian disk that carries a large amount of angular momentum. The observation that white dwarfs in cataclysmic variables rotate much faster than isolated ones (Sion 1999) provides evidence that accreting white dwarfs are indeed spun up. A rapidly rotating progenitor may also explain the asphericity implied by the polarizations observed in SNe Ia explosions (Wang, this volume). Here we discuss implications of the spin-up of accreting white dwarfs for the progenitors of SNe Ia.

Abstract

Bright outbursts from Soft Gamma Repeaters (SGRs) and Anomalous X-ray Pulsars (AXPs) are believed to be caused by instabilities in ultramagnetized neutron stars, powered by a decaying magnetic field. It was originally thought that these outbursts were due to reconnection instabilities in the magnetosphere, reached via slow evolution of magnetic footpoints anchored in the crust. Later models considered sudden shifts in the crust's structure. Recent observations of magnetars give evidence that at least some outburst episodes involve rearrangements and/or energy releases within the star. We suggest that bursting episodes in magnetars are episodes of rapid plastic yielding in the crust, which trigger “swarms” of reconnection instabilities in the magnetosphere. Magnetic energy always dominates; elastic energy released within the crust does not generate strong enough Alfvén waves to power outbursts. We discuss the physics of SGR giant flares, and describe recent observations that give useful constraints and clues.

Introduction: a neutron star's crust

The crust of a neutron star has several components: (1) a Fermi sea of relativistic electrons, which provides most of the pressure in the outer layers; (2) another Fermi sea of neutrons in a pairing-superfluid state, present only at depths below the “neutron drip” level where the mass-density exceeds ρdrip ≈ 4.6 × 1011 gm cm-3; and (3) an array of positively-charged nuclei, arranged in a solid (but probably not regular crystalline) lattice-like structure throughout much of the crust.

Abstract

Emission morphologies of young, Galactic supernova remnants can be used for investigating SN expansion dynamics, elemental distributions, and progenitor mass loss history and properties at the time of outburst. The remnants of two suspected Galactic Type Ia SNe, Tycho and SN 1006, show spherical morphologies, with Si-rich ejecta near the forward shock front suggestive of significant mixing. Searches for possible surviving binary companions near the centers of these remnants may help clarify the progenitor binary system(s) involved in SNe Ia. On the other hand, high mass, core collapse remnants, such as SNR 1987A and Cas A, exhibit strongly asymmetrical morphologies, with Cas A showing some evidence for bipolar ejecta jets. However, it is currently unclear if such ejecta jets are consistent with any of the recently proposed jet induced SN explosion models.

Introduction

For a workshop on the 3-D signatures of stellar explosions, it seems worth-while to first explain why one might be interested in the properties of supernova remnants (SNRs). Even the youngest Galactic SN remnants are hundreds and even thousands of years removed from the actual SN events, so SNRs may seem at first to be relatively poor tools for any meaningful testing of SN models or explosion theories. However, young supernova remnants, and especially the nearby Galactic ones, offer chemical and kinematic data on SN ejecta on much finer spatial scales than possible from extragalactic SN/SNR investigations.

Abstract

We present the first unrestricted, three-dimensional relativistic hydrodynamical calculations of the blob of gas associated with the jet producing a gamma-ray burst as applied to the time when afterglow radiation is produced. Our main findings are that (ⅰ) gas ahead of the advancing blob does not accrete onto and merge with the blob material but rather flows around the blob, (ⅱ) the decay light curve steepens at a time corresponding roughly to γ˗1 ≈ θ (in accord with earlier studies), and (ⅲ) the rate of decrease of the forward component of momentum in the blob is well-fit by a simple model in which the gas in front of the blob exerts a drag force on the blob, and the cross sectional area of the blob increases quadratically with laboratory time.

Introduction

Gamma-ray bursts are the most powerful explosions in the Universe. If GRBs were isotropic, then the measured redshifts would imply total explosion energies of ∼1052–1054 ergs (Frail et al. 2001). Theoretical work on relativistic jet expansion, however, shows that one expects a steepening in the decay light curve if one is looking down the axis of a jet as the flow decelerates from a bulk Lorentz factor γ˗1 < θ to γ˗1 > θ, where θ is the jet beaming angle (Rhoads 1999). The concept of a “break” corresponding to γ˗1 ≃ θ has been used to infer the presence of strong beaming in GRBs (Frail et al.

Abstract

This article will cover two topics at the intersection of Gamma-Ray Bursts and supernovae that have been much studied by the Texas group with relevance for the 3-D structure of core collapse explosions. The first topic is the high-velocity and high-excitation absorption lines seen in GRB 021004 (and other more recent events). These lines must come from (likely clumpy) shells around the progenitor star, and hence can provide a unique means of knowing the nature of the exploding star. In particular, the lines imply that normal GRBs form from the core collapse of a massive star, and thus that GRBs are closely related to supernovae. The second topic is the four luminosity indicators for Gamma-Ray Bursts and their implications for cosmology. The validity of the luminosity (and hence distance) indicators is already well demonstrated, although the current accuracy of the distances is roughly a factor of three times worse than for Type Ia supernovae. With GRBs serving as standard candles visible out to redshifts of >12 or farther, they can be used for many of the same purposes in cosmology now reserved for supernovae at low redshifts. With the launch of Swift in 2004, hundreds of bursts can then be used to construct Hubble diagrams to z ≥ 5, to measure the star formation rate to z ∼ 12 or farther, and to serve as beacons for discovering the Gunn-Peterson effect.

GRB/SN connections

Gamma-Ray Bursts (GRBs) and supernovae (SNe) have long been connected. Before the discovery of GRBs, S.

Abstract

Observations of gamma-ray burst (GRB) afterglows have yielded tantalizing hints that supernovae (SNe) and GRBs are related. The case had been circumstantial, though, relying on irregularities in the light curve or the colors of the afterglow. I will present observations of the optical afterglow of GRB 030329. The early spectra show a power-law continuum, consistent with other GRB afterglows. After approximately one week, broad peaks in the spectrum developed that were remarkably similar to those seen in the spectra of the peculiar Type Ic SN 1998bw. This is the first direct, spectroscopic confirmation that at least some GRBs arise from SNe.

Introduction

The mechanism that produces gamma-ray bursts (GRBs) has been the subject of considerable speculation during the four decades since their discovery (see Mészáros 2002 for a recent review of the theories of GRBs). Optical afterglows (e.g., GRB 970228: Groot et al. 1997; van Paradijs et al. 1997) opened a new window on the field (see, e.g., van Paradijs, Kouveliotou, & Wijers 2000). Subsequent studies of other bursts yielded the redshifts of several GRBs (e.g., GRB 970508: Metzger et al. 1997), providing definitive evidence for their cosmological origin.

Models that invoked supernovae (SNe) to explain GRBs were proposed from the very beginning (e.g., Colgate 1968; Woosley 1993; Woosley & MacFadyen 1999). A strong hint was provided by GRB 980425. In this case, no traditional GRB optical afterglow was seen, but a supernova, SN 1998bw, was found in the error box of the GRB (Galama et al. 1998a).

Abstract

Multi-dimensional models of supernovae show radial and non-radial variation in the density and composition not seen in one-dimensional models. Many of the questions about the flow of radiation through the expanding, multi-dimensional atmosphere will require multi-dimensional radiation transport calculations, but some may be tested with existing one-dimensional transport codes. So far, tests with models of Type Ia supernovae have shown that the unburned fuel (C+O) mixed down into deeper layers in multi-dimensional models have only a simple (C II lines) and modest signature in the spectra. This places only light constraints on the mixing of C+O into the lower layers of Type Ia supernovae.

3-D effects on supernova spectra

The proliferation of multi-dimensional explosion models for supernovae has made it clear that using one dimensional models for spectrum synthesis is not fully adequate. How adequate are the old 1-D spectral models? What are the multidimensional signals present in the light received from distant supernovae? Can we calculate spectra with 1-D codes that can explore the multi-dimensional effects?

The clearest multi-dimensional signal in spectrum of supernova is polarization. The number of polarization observations of supernovae have increased dramatically in the last decade, and polarization has now been detected in all types and sub-types of supernovae. Since it is impossible to calculate polarization in the 1-D models we calculate, we will not discuss polarization further, but Kasen discusses polarization elsewhere in this volume.

Abstract

As we investigate the manifestly multi-dimensional nature of core collapse supernovae, the connection between microscopic physics and macroscopic fluid motion must not be forgotten. As an example, we discuss nuclear electron capture and its impact on the supernova shock. Though electron capture on nuclei with masses larger than 60 is the most important nuclear interaction to the dynamics of stellar core collapse, in prior simulations of core collapse it has been treated in a highly parameterized fashion, if not ignored. With a realistic treatment of electron capture on heavy nuclei come significant changes in the hydrodynamics of core collapse and bounce. We discuss these as well as their ramifications for the post-bounce evolution in core collapse supernovae.

Introduction

The many observations of asymmetries in core collapse supernovae, coupled with the failure of spherically symmetric simulations of the neutrino reheating paradigm to produce explosions, has persuaded the community that multidimensional effects like convection and other fluid instabilities must be vital elements of the supernova mechanism (Wilson & Mayle 1993, Herant et al. 1994, Burrows et al. 1995, Fryer & Warren 2002) though, even with these convective enhancements, explosions are not guaranteed (Janka & Müller 1996, Mezzacappa et al. 1998, Buras et al. 2003). This view has been reinforced in recent years by the failure of more accurate spherically symmetric multigroup Boltzmann simulations to produce explosions (Rampp & Janka 2000, Mezzacappa et al. 2001, Liebendörfer et al. 2001, Thompson et al. 2003).

Abstract

We review recent progress in the theory of jet production, with particular emphasis on the possibility of 1) powerful jets being produced in the first few seconds after collapse of a supernova core and 2) those jets being responsible for the asymmetric explosion itself. The presently favored jet-production mechanism is an electrodynamic one, in which charged plasma is accelerated by electric fields that are generated by a rotating magnetic field anchored in the protopulsar. Recent observations of Galactic jet sources provide important clues to how all such sources may be related, both in the physical mechanism that drives the jet and in the astrophysical mechanisms that create conditions conducive to jet formation. We propose a grand evolutionary scheme that attempts to unify these sources on this basis, with MHD supernovae providing the missing link. We also discuss several important issues that must be resolved before this (or another scheme) can be adopted.

Introduction: a cosmic zoo of galactic jet sources

The last few decades have seen the discovery of a large number of different types of Galactic sources that produce jets. The purpose of this talk is to show that all of these jets sources are related, in both a physical sense and an astrophysical sense. Furthermore, Craig Wheeler's idea that most core collapse supernovae (SNe) are driven by MHD jets from a protopulsar provides the missing link in an attractive unified scheme of all stellar jet sources.