Abstract

This paper describes how we have used numerical simulations and laboratory combustion experiments to learn about Type Ia thermonuclear supernova explosions. We discuss detonations, deflagrations, and the transition from deflagrations to detonations, and how these relate to exploding white dwarf stars.

Introduction

This paper is for Craig Wheeler (aka Professor J. Craig Wheeler, Captain, ISS Bunbry, often stationed in the Virgo Cluster), who has been a good friend and fellow traveler for many years. Craig is wonderfully enthusiastic, persistently curious, and always asking those painfully “simple” questions for which we have no answers. He has motivated and driven research programs that have brought combustion science to astrophysics.

A cursory study of the multivolume Proceedings of the Combustion Institute shows that combustion can now be loosely defined as the result of fluid dynamics combined with exothermic reactions, and everything this implies. The definition has expanded with the understanding of the controlling phenomena and the range of applications. In the early 1900's, there was combustion and detonation, and the concepts seemed separated. Combustion was defined as oxidation with energy release, with an emphasis on specific chemical reactions. Detonation studies emphasized the fluid dynamics with shocks and explosions. Now these fields have merged and expanded. We now consider exothermic reactions, including the physics, chemistry, structure and dynamics of flames and detonations, including the production products such as pollutants, soot, diamonds, fullerenes, microparticles, and nanoparticles.

The purpose of this paper is to introduce some aspects of combustion and the combustion community to astrophysicists.

Abstract

I use photometry and spectroscopy data for 24 Type II plateau supernovae to examine their observed and physical properties. This dataset shows that these objects encompass a wide range in their observed properties (plateau luminosities, tail luminosities, and expansion velocities) and their physical parameters (explosion energies, ejected masses, initial radii, and 56Ni yields). Several regularities emerge within this diversity, which reveal (1) a continuum in the properties of Type II plateau supernovae, (2) a one parameter family (at least to first order), (3) evidence that stellar mass plays a central role in the physics of core collapse and the fate of massive stars.

Introduction

Type II supernovae (SNe II, hereafter) are exploding stars characterized by strong hydrogen spectral lines and their proximity to star forming regions, presumably resulting from the gravitational collapse of the cores of massive stars (MZAMS > 8 M⊙). SNe II display great variations in their spectra and lightcurves depending on the properties of their progenitors at the time of core collapse and the density of the medium in which they explode. Nearly 50% of all SNe II belong to the plateau subclass (SNe IIP) which constitutes a well-defined family distinguished by 1) a characteristic “plateau” lightcurve (Barbon et al. 1979), 2) Balmer lines exhibiting broad P-Cygni profiles, and 3) low radio emission (Weiler et al. 2002).

Abstract

The six galactic supernovae within the last millennium are critical to all work on the relationships between supernovae and their remnants. Yet this field has been dogged by controversy and discarded arguments. Even during the Wheeler Symposium, we had successive speakers give different type assignments to individual events. In an effort to at least define the confusion, I have polled a group of leading experts as to their current thinking on the types for each of the historical events. This complements a similar poll made a decade ago. We must realize that these results are not voting-on-the-truth, but is rather an expression of community opinion. The recent poll has the following results. SN1006 is universally agreed to be a Type Ia event. SN1054 (the Crab) is puzzling in many ways, but it must be from some sort of a core collapse event. SN1181 is thought to be a core collapse event primarily on the basis of its remnant being a plerion like the Crab. SN1572 (Tycho's) is agreed to be a Type Ia event. SN1604 (Kepler's) has no consensus, with all types being claimed and denied. Cas A is unanimously agreed to not be a Type Ia event, but after that all possibilities find their champions.

The polls

The six galactic supernovae within the last millennium (SN1006, SN1054, SN1181, SN1572, SN1604, and Cas A) all have very well observed remnants. A key question for understanding these remnants is the type of the original explosion.

Abstract

Combining sub-arcsec imaging with moderate spectral resolution and high throughput, the Chandra X-ray Observatory enables spectacular views of Galactic supernova remnants as well as X-ray studies of compact remnants, young extragalactic supernovae, and gamma-ray burst afterglows. In this contribution, I briefly review the capabilities of Chandra and then describe some recent observations of supernovae and supernova remnants made with Chandra.

The Chandra X-ray observatory — an overview

Chandra (see, e.g., Weisskopf et al. 2002) was launched from space shuttle Columbia 23 July 1999 and is now late into the fourth year of its ten year mission. The heart of the facility is the High-Resolution Mirror Assembly consisting of 4 nested mirror pairs with a 120 cm outer shell diameter. The mirrors provide about 800 cm2 of collecting area at 1 keV and about 400 cm2 at 5 keV. Most importantly, the mirror design results in less than 0.″5 on-axis spatial resolution; an order-of-magnitude higher resolution than any other X-ray facility yet flown. This corresponds to a resolution of ∼100 AU at the distance of the Crab Nebula; making Chandra ideal for probing the fine structure of supernova remnants on spatial scales comparable to that achievable by some of the best ground-based optical telescopes. Equally important, high spatial resolution improves the sensitivity of X-ray measurements by concentrating source photons into a small area thereby minimizing the contribution from the underlying background. There are two types of focal plane instrument onboard Chandra.

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.