Abstract

HETE-2 has confirmed the connection between GRBs and Type Ic supernovae. Thus we now know that the progenitors of long GRBs are massive stars. HETE-2 has also provided strong evidence that the properties of X-Ray Flashes (XRFs) and GRBs form a continuum, and therefore that these two types of bursts are the same phenomenon. We show that both the structured jet and the uniform jet models can explain the observed properties of GRBs reasonably well. However, if one tries to account for the properties of both XRFs and GRBs in a unified picture, the uniform jet model works reasonably well while the structured jet model fails utterly. The uniform jet model of XRFs and GRBs implies that most GRBs have very small jet opening angles (∼ half a degree). This suggests that magnetic fields play a crucial role in GRB jets. The model also implies that the energy radiated in gamma rays is ∼100 times smaller than has been thought. Most importantly, the model implies that there are ∼ 104–105 more bursts with very small jet opening angles for every such burst we see. Thus the rate of GRBs could be comparable to the rate of Type Ic core collapse supernovae. Accurate, rapid localizations of many XRFs, leading to identification of their X-ray and optical afterglows and the determination of their redshifts, will be required in order to confirm or rule out these profound implications.

Abstract

We present the V light curve and optical/infrared spectra of the Type Ic SN 1997B. We show that (1) this SN displayed lines of the He I series; (2)the expansion velocities were higher than those of SNe with traces of H or large He masses in their envelopes (like SN 1993J); the light curve of SN 1997B decayed slower than that of SN 1993J. The smaller mass to kinetic energy ratio and shallower light curve of SN 1997B are inconsistent with it being a He stripped version of some of the best studied Type Ib or II-transition SNe. We infer that Type Ib/c and II-transition SN progenitors come, at least, with two different types of inner structure.

Introduction

A few years ago the presence of He in the atmospheres of Type Ic SNe, the nature of their progenitors, and the relation between Type Ib and Type Ic SNe was subject of debate. On the one hand, empirical evidence and theoretical interpretation supported the view that SNe of Type Ib and Ic are different enough to insure that their progenitors result from different paths of stellar evolution. If so, Type Ic SNe originated in bare C+O cores and were expected not to display He I lines in their spectra. On the other hand, it was stressed that Type Ib and Ic SNe could originate in similar stars evolving as interacting binaries.

Abstract

There are many interesting topics at the intersection of physics and astrophysics we call Supernova Theory. A small subset of them include the origin of pulsar kicks, gravitational radiation signatures of core bounce, and the possible roles of neutrinos and rotation in the mechanism of explosion. In this brief communication we summarize various recent ideas and calculations that bear on these themes.

What is the mechanism of pulsar kicks?

Radio pulsars are observed to have large proper motions that average ∼400–500 km s-1 (Lyne & Lorimer 1994) and whose velocity distribution might be bimodal (Fryer, Burrows, and Benz 1998; Arzomanian, Chernoff, & Cordes 2002). If bimodal, the slow peak would have a mean speed near ∼100 km s-1 and the fast peak would have a mean speed near 500–600 km s-1. A bimodal distribution implies different populations and different mechanisms, but what these populations could be remains highly speculative.

Many arguments suggest that pulsars are given “kicks” at birth (Lai 2000; Lai, Chernoff, and Cordes 2001), and are not accelerated over periods of years or centuries. The best explanation is that these kicks are imparted during the supernova explosion itself. We think that this view is compelling. The two suggested modes of acceleration and impulse are via net neutrino anisotropy during the neutrino emission phase (which lasts seconds) and anisotropic mass motions and aspherical explosion which impart momentum to the residual core.

Abstract

In the popular progenitor scenario, Type Ia supernova are the result of a white dwarf exploding in a binary system. The presence of a nearby companion star could cause a substantial asymmetry in the supernova ejecta — according to the models of Marietta et. al. (2000), the companion carves out an hole in the ejecta. The opening angle of the hole is as large as 40°. Such an asymmetry would leave signatures in the supernova flux and polarization spectra. We explore this possibility using a three-dimensional Monte-Carlo LTE radiative transfer code which includes gamma ray transport and a temperature correction procedure. We calculate synthetic spectra and polarization levels from multiple lines of sight to see how an ejecta hole model compares to observations.

Introduction

While some Type Ia supernovae (SNe Ia) are known to be aspherical, the exact nature of the asymmetry is unknown. The direct evidence of the asphericity is the detection of non-zero intrinsic polarization in, for example, SN 1999by [4] and SN 2001el [17]. In both cases, the polarization level was rather low (∼0.7% for SN 1999by, ∼0.4% for SN 2001el), which indicates a mild asymmetry along the line of sight. In addition, the polarization angle was fairly constant across the majority of line features, indicating that the bulk of the ejecta obeyed a near axial symmetry. The exact shape of the supernova ejecta is an important question, as it must be closely tied to the explosion processes and progenitor systems of SNe Ia.

Abstract

Current massive single star evolution models with rotation, especially when magnetic fields are included, appear to get close in reproducing the spin rates of young neutron stars. This, however, excludes them as progenitors of gamma-ray bursts within the collapsar model. Close binary evolution models with rotation, on the other hand, suggest that the mass receiving star is spun-up appreciably and may retain enough angular momentum in its core until collapse, while the mass donor is spun-down to produce core rotation rates below those of single stars.

Introduction

The evolution of a single star can be strongly influenced by its rotation (e.g., Heger & Langer 2000; Meynet & Maeder 2000), and evolutionary models of rotating stars are now available for many masses and metallicities. While the treatment of the rotational processes in these models is not yet in a final stage (e.g., magnetic dynamo processes are just about to be included; Heger et al. 2003), they provide first ideas of what rotation can really do to a star.

Effects of rotation, as important as they are in single stars, can be much stronger in the components of close binary systems: Estimates of the angular momentum gain of the accreting star in mass transferring binaries show that critical rotation may be reached quickly (Packet 1981; Langer et al. 2000). Therefore, we need binary evolution models which include a detailed treatment of rotation in the stellar interior, as in recent single star models.

Abstract

We study a thermonuclear explosion of a carbon-oxygen white dwarf (WD) using a three-dimensional hydrodynamic model with a simplified mechanism for nuclear reactions and energy release. The explosion begins as a deflagration with the flame front highly distorted by the Rayleigh-Taylor instability. Turbulent combustion and convective flows produce an inhomogeneous mixture of burned and unburned materials that extends from the center to about 0.8 of the radius of the expanding WD. At this stage, a detonation is ignited and propagates through the layers of unburned material with the velocity about 12,000 km/s, which is comparable to the expansion velocities induced in outer layers of the WD by the subsonic burning. During the period of detonation propagation, the density of the expanding unreacted material ahead of the shock can decrease by an order of magnitude compared to its value before the detonation started. Because the detonation burns material to different products at different densities, it can create a large-scale asymmetry in composition if it starts far from the WD center. In contrast to the 3-D deflagration model, the 3-D delayed-detonation model of SN Ia explosions does not leave carbon, oxygen, and intermediate-mass elements in central parts of a WD. This removes the key disagreement between simulations and observations, and confirms that the delayed detonation is currently the most promising mechanism for SN Ia explosions.

Introduction

Type Ia supernovae (SNe Ia) [1–10] result from the most powerful thermonuclear explosions in the Universe.

Abstract

Stellar abundance observations are providing important clues about the relationship between supernovae (SNe) and the rapid neutron capture process (i.e., the r-process). Although the site for the r-process is still not identified, events in and around SNe have long been suspected. Abundances of heavy neutron-capture elements in a number of stars suggest a robust r-process operating over billions of years, constraining astrophysical and nuclear conditions in supernova models. Variations in lighter n-capture element abundances — observed only very recently in any stars — could be explained as a signature of certain supernova models, or might require multiple r-process sites with different mass ranges or frequencies of SNe. Recent observations of elemental abundance scatter in the early Galaxy are consistent with earlier suggestions of a restricted range of SNe responsible for the r-process.

Introduction

The elements heavier than iron are synthesized in neutron processes, either in the (s)-low or (r)-apid process. In the s-process the timescale for neutron capture (τn) is much longer than the electron (beta)-decay (τβ) timescale. For the r-process, however, τn << τβ with many neutrons captured in a very short time period. As a result, neutron captures proceed into very neutron-rich regions far from the stable nuclei, where very little experimental nuclear data is available. This element synthesis is intimately connected to the late stages of stellar evolution, with the s-process occurring in the thermally pulsing helium shells of asymptotic giant branch (AGB) stars of low- and intermediate-mass (M ∼ 0.8–8 M⊙) (see, e.g., the review by Busso, Gallino, & Wasserburg 1999).

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.