Abstract

The linear stability of stalled accretion shocks is investigated in the context of core collapse of type II supernovae. We focus on a particular instability mechanism based on the coupling of acoustic perturbations with advected ones (vorticity, entropy). This advective-acoustic cycle takes place between the shock and the nascent neutron star. Both adiabatic and non-adiabatic processes may contribute to this coupling, but only adiabatic ones are considered in this first approach. The growth time of the adiabatic instability scales like the advection time, and is dominated by low degree modes l = 0,1,2. Non radial modes (l = 1,2) found unstable by Blondin et al. (2003) can be related to this mechanism.

Introduction

Shocked accretion onto the surface of a compact star is known to be unstable in the context of magnetized white dwarfs, leading to shock oscillations (from Langer, Chanmugam & Shaviv 1981, hereafter LCS81, to Saxton & Wu 2001). Houck & Chevalier (1992, hereafter HC92) made a linear stability analysis of shocked accretion onto a neutron star, and found an instability reminiscent of the instability found by LCS81. HC92 showed specific cases where the cooling occurs mostly in a thin layer at the surface of the neutron star, while the flow is essentially adiabatic above it. The mechanism of the instability was described by LCS81 and subsequent authors as a kind of thermal instability: if the shock surface is moving outwards, the higher incident velocity in the frame of the shock produces a higher temperature blob, which pushes the shock further out if the increased cooling time exceeds the increased advection time.

Abstract

We explore whether the observed variations in the peak luminosities of Type Ia supernovae originate in part from a scatter in metallicity of the main-sequence stars that become white dwarfs. Previous, numerical, studies have not self-consistently explored metallicities greater than solar. One-dimensional, Chandrasekhar mass models of SNe Ia produce most of their 56Ni in a burn to nuclear statistical equilibrium between the mass shells 0.2 M⊙ and 0.8 M⊙, for which the electron to nucleon ratio Ye is constant during the burn. We show analytically that, under these conditions, charge and mass conservation constrain the mass of 56Ni produced to depend linearly on the original metallicity of the white dwarf progenitor. This effect is most evident at metallicities greater than solar. Detailed post-processing of W7-like models confirms this linear dependence, and our calculations are in agreement with previous self-consistent calculations over the metallicity range common to both calculations. The observed scatter in the metallicity (1/3 Z⊙-3 Z⊙) of the solar neighborhood is enough to induce a 25% variation in the mass of 56Ni ejected by Type Ia supernova and is sufficient to vary the peak V-band brightness by |ΔMV| ≈ 0.2. This scatter in metallicity is present out to the limiting redshifts of current observations (z ≲ 1). Sedimentation of 22Ne can possibly amplify the variation in 56Ni mass to ≲50%. Further numerical studies can determine if other metallicity-induced effects, such as a change in the mass of the 56Ni-producing region, offset or enhance the variation we identify.

Abstract

We study the evolution of supernova remnants in the circumstellar medium formed by mass loss from the progenitor star. The properties of this interaction are investigated, and the specific case of a 35 M⊙ star is studied in detail. The evolution of the SN shock wave in this case may have a bearing on other SNRs evolving in wind-blown bubbles, especially SN 1987A.

Introduction

Type II Supernovae are the remnants of massive stars (M > 8 M⊙). As these stars evolve along the main sequence, they lose a considerable amount of mass, mainly in the form of stellar winds. The properties of this mass loss may vary considerably among different evolutionary stages. The net result of the expelled mass is the formation of circumstellar wind-blown cavities, or bubbles, around the star, bordered by a dense shell. When the star ends its life as a supernova, the resulting shock wave will interact with this circumstellar bubble rather than with the interstellar medium. The evolution of the shock wave, and that of the resulting supernova remnant (SNR), will be different from that in a constant density ambient medium.

In this work we study the evolution of supernova remnants in circumstellar wind-blown bubbles. The evolution depends primarily on a single parameter, the ratio of the mass of the shell to that of the ejected material. Various values of this parameter are explored.

Abstract

It is a weird and unlikely circumstance that a collapse supernova (Type II) should explode. The peculiar mechanism that facilitates this explosion is the formation and preservation of large scale structures in a high entropy atmosphere residing on the surface of a nearly formed neutron star. The high entropy atmosphere is maintained by two sources: the gravitational energy of initial formation of the neutron star, released by diffusion and transport of neutrinos and secondly and possibly dominantly by the gravitational energy released at the suface by additional low entropy matter falling through to the neutron star surface. The preservation of this entropy contrast between up and down flows requires thermal isolation between the low entropy down flows and the high entropy up flows. This entropy contrast allows an efficient Carnot cycle to operate and thus allows the efficient conversion of thermal energy to mechanical, which in turn drives the explosion. The P-V diagram of various up and down going mass elements in the calculations demonstrates the existence of the cycle and its efficiency. Greater thermal isolation should occur in 3-D as opposed to 2-D calculations because of the difference in relative thickness or surface to mass ratio for the same mass flow in 2 and 3-D. This may explain the observed stronger explosion in 3-D calculations.

Prolog

This paper is written in honor of a long and lasting friendship between Craig Wheeler and the first author for more than half his current life.

Abstract

Current observations stimulate the production of fully three-dimensional explosion models, which in turn motivates three-dimensional spectrum synthesis for supernova atmospheres. We briefly discuss techniques adapted to address the latter problem, and consider some fundamentals of line formation in supernovae without recourse to spherical symmetry. Direct and detailed extensions of the technique are discussed, and future work is outlined.

Introduction

Spectrum synthesis is the acid test of supernova modelling. Unless synthetic spectra calculated from a hydrodynamical stellar explosion model agree with observations, the model is not descriptive. Some explosion modellers contend that only three-dimensional (3-D) models faithfully describe the physics of the real events. If this is so, then the evaluation of those models requires solutions to the 3-D model supernova atmosphere problem. These solutions require full detail, the inclusion of as much radiation transfer physics as possible. Otherwise, a bad fit of a synthetic spectrum to an observed one might have less to do with the accuracy of the hydrodynamical model, and more to do with the shortcomings of the radiation transfer procedure.

On the other hand, solutions (of a sort) to the ill-posed inverse problem constrain parameter space available to hydrodynamical models. Fast, iterative, parameterized fits to observed spectra characterize the ejection velocities and identities of species found in the line forming region. Most importantly, the procedure reveals species that cannot be identified by simply Doppler-shifting line lists on top of observed spectra in search of feature coincidences.

Abstract

Polarization and other observations indicate that supernova explosions are aspherical and often axisymmetric, implying a necessary departure from spherical models. Akiyama et al. investigated the effects of the magneto-rotational instability (MRI) on collapsing and rotating cores. Their results indicate that the MRI dynamo generates magnetic fields of greater than the Q.E.D. limit (4.4 × 1013 G). We present preliminary results of the effects of the super-strong magnetic field on degenerate electron pressure in core collapse.

Introduction

Although core collapse cannot be observed directly, except with neutrinos, observations of explosion ejecta can provide us with information about the explosion mechanism itself. Such observations indicate that explosions of core collapse supernovae are aspherical and often bipolar. HST observations clearly show that 1987A has aspherical ejecta for which the axis aligns roughly with the small axis of the rings (Pun et al. 2001; Wang et al. 2002). Spectropolarimetry is a powerful tool for probing ejecta asphericity, and it reveals that most, if not all, core collapse supernovae possesses asphericity and often times bipolar structure (Wang et al. 1996, 2001). Explosions of Type Ib and Ic are more strongly aspherical, while the asphericity of Type II supernovae increases with time as the ejecta expand and the photosphere recedes (Wang et al. 2001; Leonard et al. 2000, 2001). The indication is that it is the core collapse mechanism itself that is responsible for the asphericity.

The observational evidence of asphericity motivates the inclusion of rotation in core collapse physics.

Introduction

The assumption commonly made is that Supernovae of Type Ia (SN Ia) are the result of thermonuclear runaways (TNR) in the cores of carbon-oxygen white dwarfs (WD) which are members of binary systems and have accreted material from a companion until their masses exceed the Chandrasekhar Limit (Leibundgut 2000, 2001). However, the binary star systems that end in this explosion are not yet known although there have been numerous proposals. Nevertheless, the importance of SN Ia, both to our understanding of the evolution of the Universe and to the formation of iron in the Galaxy, demands that we determine the progenitors of these explosions.

Originally proposed by Whelan and Iben (1973), virtually every type of close binary which contains a WD has been suggested at one time or another. However, based purely on observational concerns, most of the systems that have been proposed cannot be the progenitors (Starrfield 2003). For example, one of the first suggestions was a classical nova system (CN), but the amount of core material ejected during the outburst implies strongly that the WD is losing mass as a result of the outburst (Gehrz et al. 1998). Other suggestions such as Symbiotic Novae (T CrB or RS Oph, for example) can probably be ruled out.because there is too much hydrogen present in the system (the explosion takes place inside the outer layers of a red giant) and the defining characteristic of a SN Ia outburst is the absence of hydrogen in the spectrum (Filippenko 1997).

The diversity of chondrules

Chondrules are highly diverse in their properties. They all appear to have been melted to some degree at some point; some are clearly melt droplets that have crystallized; some appear to be igneous systems that have been subsequently rounded. Most are deficient in metal and sulfide in comparison with the host rock, some have additional compositional differences. Some have thick rims of material, some do not, and in some cases those rims are rich in metal and sulfide, and in some cases they are not. The nature of the chondrules, and their diversity, tells us something about conditions in the early Solar System. Chondrules are the major component of chondrites and must account for many of their bulk properties, as does their relative abundance in the various classes of meteorites (Huang et al., 1996b). It also seems clear that accounting for the diversity of properties will provide insights into their formation process.

Attempting to understand objects as diverse as chondrules starts with their classification and there have been many proposed schemes. However, all the schemes have common features and it is possible to trace a development of ideas. In this way I hope to focus on the important observations without getting bogged down in the plethora of detail, much of which may be extraneous. There has been a long-term trend of textural classification schemes giving way to composition-based schemes as analytical instruments improved and trends became clearer.

Chondrules and chondrite classes as impact pyroclastics

If chondrules formed by impact into a regolith, and chondrules behaved as open systems during their formation, then the diversity of chondrule compositions presumably reflects the diversity in the intensity of impact. It is then a small step to assume that the redox state of the resultant chondrite similarly depends on the violence of impacts locally. The remaining factor in forming chondrites concerns the matter of assembling the components, and producing small variations in the amount of matrix and metal in relation to the chondrules. The size and distribution of the chondrules and metal, which are characteristic of many classes of chondrites, suggests sorting before or during accumulation. Again, a great many mechanisms have been proposed for how this might have been achieved in the nebula, but I think it unlikely that this process occurred in the nebula because the meteorites managed to preserve compositions so close to cosmic and because aerodynamic sorting alone fails quantitatively. Density sorting is also required and this in turn needs the presence of at least a weak gravity field. Some meteorite parent bodies must have experienced degassing in their early stage to turn CI compositions into ordinary chondrite compositions and may have had thick dusty surfaces that were easily mobilized by gases evolving from the interior. Density and size sorting may have occurred in the surface layers as the upward drag forces of gases (mainly water) acted against the downward force of gravity.

We suggested in Section 3.3 that the two features that need to be explained in understanding chondrites are the formation of chondrules and the metal–silicate fractionation. Having understood, at least partially, these two processes, much of the remaining properties of chondrites will fall into place. We discussed chondrule formation in the previous chapter, now we discuss metal–silicate fractionation and how the chondrules and metal were assembled together to produce chondrites. This, we argue, amounts to discussing the origin of chondrites. Finally, we can examine the extent to which other chondrite properties relate to the ideas we have developed for these processes.

Chondrule sorting

We will consider several processes for sorting chondrules. First, we will consider the idea that size sorting is a primary property and a result of the formation mechanism. Second, we will discuss the idea that it is the passage of the chondrule through the local gas environment that sorts the chondrules. We will call this aerodynamic sorting. Next, we will discuss the idea that the sorting is a result of the process by which chondrules were made to move from their formation location to the location at which the present meteorite formed. We call this ballistic sorting. Finally, we suggest that abrasion could result in size sorting in some cases.

Primary processes

Most theories for sorting chondrules and metal grains are “secondary,” in the sense that they assume that a range of sizes previously existed and were later sorted.

Some general comments

The data gathered on chondrules and chondrites over the last two hundred years are rich and diverse. There have been evolutions in techniques and instruments and large amounts of time and energy have been spent on the study of these data. Despite this, there is uncertainty as to the origin of chondrules and chondrites and this is reflected in the great many theories for their formation. This wealth of theories and lack of consensus on any particular theory is an indication of the complexity of the processes, the rudimentary state of our knowledge about conditions in the early Solar System, and, perhaps, the absence of some key element of information about the objects themselves.

Grossman (1988) reviewed chondrule formation theories and summarized them in table form, listing 19 theories. He concluded that chondrules formed in the nebula by an unknown flash heating event. Hewins (1989) presented a very different review to that of Grossman (1988), but came to essentially the same conclusion. Boss (1996b) presented a discussion of nine chondrule formation theories (that they are impact melts, meteor ablation products, the products of a hot inner nebula, FU Orionis, bipolar outflows, nebula lightning, magnetic flares, or that they were produced in accretion shocks, or nebular shocks). He gave an uncritical listing of the pros and cons of each. On the other hand, chondrule researcher J. N. Grossman once observed in a private communication that he had counted over 60 theories for chondrule formation.

Rocks falling from the sky have a long and colorful history. I mean this both in a socio-economic sense and, perhaps more obviously, in a scientific sense. Stories of stones from the heavens have been with us for as long as humans have left traces of themselves. In ancient tombs and burial sites, in their earliest writings and during the faltering steps of the industrial revolution and the creation of modern science, people wrote about rocks from the sky now known as meteorites. In many respects the history of modern science instrumention is inextricably linked with the history meteorite studies.

Meteorites are major witnesses of the history of our Solar System. Everyone agrees that meteorites are ancient materials from the earliest stages in the history of the Solar System. Their age, composition, and texture clearly point to this conclusion. Everyone also agrees that meteorites are fragments from near-Earth asteroids, which occasionally threaten us with impact, and it seems that such asteroids largely come out of the Main Asteroid Belt between Mars and Jupiter although a small fraction of them are probably related to comets. These rocks are fascinating to study. They are sufficiently like terrestrial rocks that similar techniques and approaches can be used, yet they present a whole new range of physical and chemical processes to consider, processes that take the researcher from petrologist, mineralogist, and geochemist to the astronomer and the astrophysicist.