Theoretical light curves and spectra of X-rays and γ-rays from SN 1987A are calculated by the Monte Carlo method, based on a model built up from the early observations of neutrinos and optical light. Comparison of the predicted radiation with observational results obtained later confirms the radiation mechanism of supernovae: γ-rays are emitted in the decays of radioactive 56Co and X-rays are generated by the Compton degradation of these γ-rays. It also suggests that large scale mixing occurred and clumpy structure was formed inside the ejecta. These findings lead us to construct the model with a new distribution of elements, which is determined through comparisons of observations of X-rays and γ-rays with numerical simulations based on the assumed distribution. Using this model, the subsequent X-ray and γ-ray emission is predicted: the light curves of X-rays and γ-rays as well as their spectral evolution are in very good agreement with that expected from the radioactive decays of 56Co and 57Co. The mass of newly synthesized 44Ti and the emission from the neutron star will be determined by future satellite and balloon-borne observations.

Introduction

SN 1987A has given us an invaluable chance to examine supernova theory, which has predicted the emergence of X-ray and γ-ray radiation from supernovae. Several possible mechanisms for the X-ray and γ-ray emission have been discussed, such as collision of the ejecta with circumstellar matter, nonthermal radiation from a pulsar, and Compton degradation of the line γ-rays emitted by radioactive nuclei.

X-ray spectroscopy can provide vital information about the progenitors and environments of supernova remnants. Plasma diagnostics and spectral modelling can be used to infer the energy of the remnant, the density and composition of the surrounding medium, and the degree of equilibrium in the shock heated gas. A new generation of X-ray spectrometers, the first of which was the Broad-Band X-Ray Telescope (BBXRT), has improved our ability to make precise measurements of X-ray line fluxes and energies. We summarize the results obtained from the BBXRT mission. These include a definitive measurement of the Fe K line centroid in the Tycho remnant, production of the first narrow-band X-ray maps (of Puppis A) and the first measurement of an electron-ion equipartition timescales in evolved remnants.

Introduction

Supernova remnants may be grouped into three broad categories, based on their X-ray and radio morphologies. The first of these shows shell-like structure in both bands. The X-rays from these are thermal, arising from the shock heating of ejecta and interstellar material. Prominent examples of this class of remnant are Tycho and the Cygnus Loop. The second category shows centrally peaked emission in both bands; these are the plerions, or Crab-like remnants, after the class archetype. The X-ray emission is a non-thermal power law, dominated by synchrotron processes from the energetic electrons produced by the pulsar. A third category combines elements of the previous two.

Introduction

Several multidimensional computations of hydrodynamics related to supernovae have been completed, and are summarized here. More detail may be found in Arnett 1994a,b, Arnett & Livne 1994a,b, and Livne & Arnett 1993. The hydro code PROMETHEUS is based upon an implementation of the piecewise-parabolic method (PPM) of Colella & Woodward 1984, as described in Fryxell et al. 1991. A detailed comparison of PPM with other schemes is given in Woodward & Colella 1984. The method constructs the physics of the flow between grid points by a nonlinear solution of the equations of continuity of mass, momentum and energy (the Riemann problem) rather than the usual mathematical approach of a Taylor expansion about the grid points. This gives it better resolution per grid point, which is highly desirable for multidimensional problems. Although the effort required per grid point is greater, the number of such points is less (often much less) for a given level of accuracy. Because the computational load per grid point is greater, more realistic physics (reactions, radiation, gravity, etc.) may be added before affecting the runtime significantly. Thus PPM is well suited for multidimensional problems with significant physics beyond the bare hydrodynamics.

The Prometheus project was an effort by the author, Bruce Fryxell and Ewald Müller, to implement a “state of the art” hydrodynamic method with realistic microphysics for stellar problems. After an extensive study of several methods, and direct comparison of the resulting codes (Fryxell, Müller & Arnett 1989), PPM was chosen as the preferred method.

We review critical physics affecting the observational characteristics of those supernovae that occur in massive stars. Particular emphasis is given to 1) how mass loss, either to a binary companion or by a radiatively driven wind, affects the type and light curve of the supernova, and 2) the interaction of the outgoing supernova shock with regions of increasing ρr3 in the stellar mantle. One conclusion is that Type II-L supernovae may occur in mass exchanging binaries very similar to the one that produced SN 1993J, but with slightly larger initial separations and residual hydrogen envelopes (∼1 M⊙ and radius ∼ several AU). The shock interaction, on the other hand, has important implications for the formation of black holes in explosions that are, near peak light, observationally indistinguishable from ordinary Type II-p and Ib supernovae.

Some Generalities

There is broad agreement regarding the qualitative evolution of single stars sufficiently massive to ignite carbon burning non-degenerately (e.g., Woosley & Weaver 1986; Weaver & Woosley 1993; Nomoto & Hashimoto 1986, 1988). Given the usual, relevant caveats about the treatment of convective mixing, convective overshoot, and semiconvection, it is agreed that stars of approximately 8 to 12 M⊙(±1 M⊙ depending upon initial helium abundance and convective parameters) will not proceed to silicon burning in hydrostatic equilibrium, but will stop prior to central neon ignition and experience a complicated subsequent evolution in which degenerate flashes play an important role.

Cosmology is a general relativistic topic. General relativity must replace Newtonian mechanics for systems in which the mass and spatial dimensions have similar orders of magnitude, M ~ r. If the density of the Universe were a constant, M/r ≈ ρr3 /r ≈ ρr2, and for large enough length scales, the condition M ≈ r would eventually be met. Let us take the current density of 10-29g cm-3 (= 7.4 x 10-58cm-2 in geometrized units). Then, for length scales of r ≈ 3.7 x 1028cm (≈ 104 Mpc), calculations must be carried out using general relativity. This length is of the order of magnitude of the observable Universe, and corresponds to objects about 1010, years old, near the commonly accepted age of the Universe.

The cosmological principle

The Newtonian interpretation of the cosmological principle must now be reconsidered. It stated that at a given time all thermodynamic parameters are homogeneously and isotropically distributed. But what kind of time? Now every observer has his proper time and thus the time that the cosmological principle takes as a reference must be specified without invoking privileged observers or systems with peculiar characteristics. General relativity assures us that at any point there is a free-falling observer, for whom nature is explained in terms of the laws of special relativity. Observers in free-fall are called fundamental observers in cosmology and the particle of cosmic fluid that they ride is called a fundamental particle, or it could be called a fundamental galaxy, taking galaxies as the pieces from which the Universe is built.

Main properties

The fluid of stars in a galaxy closely resembles the fluid of molecules. The fluid of galaxies does not. Let us examine three main differences.

(a) The distribution of galaxies is not chaotic. There are groups, clusters, superclusters, and there is a large-scale structure, which we are now beginning to realize. The distinction between clusters and superclusters is not sharp, and as superclusters are elements of larger structures their limits and sizes are difficult to establish. The largest observed structures are as large as the limits of the deepest surveys. A continuous spectrum of inhomogeneities describes structures larger than a galaxy better than it describes discrete objects such as stars and galaxies.

The relative increase in the density with respect to the mean density δ is about 102 –103 for clusters. Typical intercluster distances of about 5 Mpc and inter-supercluster distances of about 25 Mpc are revealed by cross-correlation studies. The value of δ for a galaxy is about 105.

The large-scale structure of the Universe is complex. Many clusters are aligned in huge filaments. Others seem to form sheets. There are also voids which are apparently deplete of galaxies. A simplified picture of the large-scale structure might consist of an ensemble of large polyhedral voids. In the limiting sheets separating two adjacent voids there are clusters. Along the limiting line intersections there are more clusters. At the limiting vertices there are still more clusters. The linear dimensions of the voids are typically 20 – 50 Mpc.

This small book is intended as a general introduction to astrophysical fluid dynamics. The reader is presumed to possess a knowledge of basic physics, namely, classical physics, elements of relativity, and introductory ideas about quantum mechanics. No previous knowledge of fluid dynamics or of astrophysics is required, these topics being introduced in the book. Although fluid dynamics may constitute a complementary, original, natural, fecund, unexplored, simple, and enjoyable way to introduce astrophysics, the topic of astrophysical fluid dynamics is a promising, distinct, and particularly wide branch of astrophysics at the present time.

The first part of the book (Chapters 1–4) deals with basic fluid dynamics. Although it could also be used for non-astrophysical purposes, it was written with the former in mind. It often includes cosmic examples that are mainly related to a stationary, static, and stratified atmosphere. These conditions provide the greatest simplification while maintaining a high degree of astrophysical interest.

Following the first chapter on classical fluids, Chapter 2 is devoted to relativistic fluids. The early introduction of relativistic fluids is necessary, as many cosmic fluids, and the cosmic fluid itself, are relativistic. One important advantage is that radiative transfer can be developed as transport in a relativistic fluid, thereby avoiding the usual classical mis-interpretation of the radiative Boltzmann equation. Plasmas and magnetohydrodynamics are also included because of their growing interest in the field of astrophysics. The important role played by magnetic fields in a large sample of cosmic systems is only now beginning to be appreciated.

The fluid of stars

In contrast with the fluid in a star, defined as the fluid of which stars are constituted, we will now deal with a fluid of stars, defined as a fluid whose microscopic particles are stars. Our first example will be a galaxy. It is true that a galaxy is not composed solely of stars. It also possesses gas and dust in different amounts, and gas influences the dynamics of the star system decisively. However, as an introduction to galactic dynamics we will first study the dynamics of the stellar system, then that of the interstellar gas, and will finally try to combine the two. In this chapter, those terms in the hydrodynamical equations that are not influenced by the presence of gas will be considered.

Galaxies are not the only examples of fluids of stars. Globular clusters and even open clusters can be systems with these characteristics. Clusters of galaxies can also be analysed using the same theoretical approach, with the stars being replaced by galaxies, that is, assuming a fluid of galaxies. The galactic scenario is assumed in general here, but other types of stellar system will be considered. The number of stars in our Galaxy is of the order of 1011, a sufficiently large number to justify the use of hydrodynamics.

A great simplification in the hydrodynamical equations consists of azimuthal or axial symmetry, which is appropriate for the majority of galaxies.

Some naive thoughts provide a first insight into Newtonian cosmology. Olbers' paradox, introduced in Chapter 6 (6.7), suggests that the Universe is finite, either in space or in time, or both. If the Universe were finite in space and infinite in time, the whole Universe would have collapsed gravitationally. Therefore, the Universe is finite in time. Are there other simple arguments with which to assess the fmiteness of space, that is, of matter content of the Universe? The cosmological principle may provide such an argument.

The cosmological principle is widely accepted as a reasonable, basically compatible with observations, philosophically attractive principle. From the Newtonian point of view its statement and interpretation does not present any difficulty: the point in space at which we are in the Universe is not a special point; all points in the Universe are similar, or, more precisely, all thermodynamic parameters have the same value at any point in the Universe: the Universe is homogeneous. The cosmological principle also ensures the isotropy of the Universe, that is, all directions are equivalent. Isotropy implies homogeneity, but homogeneity does not imply isotropy. For example, if the Universe were embedded in a constant magnetic field, it could be homogeneous but anisotropic. However, the Newtonian interpretation of the cosmological principle usually states that the Universe is both homogeneous and isotropic. This would only be true on a very large scale in the Universe, that is, larger than characteristic sizes of superclusters.

Introduction

Most cosmic fluids are plasmas. As was shown in Chapter 4, the internal motions of the fluid produce magnetic fields, which in turn affect this motion. Magnetic forces are, then, fundamental to the birth, structure, and evolution of most cosmic systems. However, considerable effort was made in the past to explain everything in astrophysics by taking only the gravitational force into account. This approach often yielded inadequate results, and the importance of magnetism as a basic cosmic interaction has now begun to be fully appreciated. In attemping to explain any astrophysical system, systematically ignoring magnetic fields is simply naive.

In this chapter particular attention will be paid to two topics: the Sun and the interstellar medium. A basic introduction to the physics of active galactic nuclei is also included. Observations clearly show that we are dealing with complicated systems. Because of the proximity of the Sun, we are able to appreciate how complex a star can be. There is a large variety of transient phenomena at all observable outer layers in the Sun which are now interpreted as structures in which magnetism plays a leading role. Also, the morphology of galaxies and the complex structures of interstellar clouds may reveal the presence of magnetism. Steady-state phenomena, both in the Sun and in the galactic gas, will also be treated here, taking magnetism into account.

The Sun

In order to determine the general properties of the solar atmosphere plasma, we must determine the characteristic quantities defined in Chapter 4.