The Morphologies and Kinematics of Supernova Remnants

We review the major advances in understanding the morphologies and kinematics of supernova remnants (SNRs). Simulations of SN explosions have improved dramatically over the last few years, and SNRs can be used to test models through comparison of predictions with SNRs’ observed large-scale compositional and morphological properties as well as the three-dimensional kinematics of ejecta material. In particular, Cassiopeia A—the youngest known core-collapse SNR in the Milky Way—offers an up-close view of the complexity of these explosive events that cannot be resolved in distant, extragalactic sources. We summarize the progress in tying SNRs to their progenitors’ explosions through imaging and spectroscopic observations, and we discuss exciting future prospects for SNR studies, such as X-ray microcalorimeters.     

Magnetic Fields in Massive Stars, Their Winds, and Their Nebulae

Massive stars are crucial building blocks of galaxies and the universe, as production sites of heavy elements and as stirring agents and energy providers through stellar winds and supernovae. The field of magnetic massive stars has seen tremendous progress in recent years. Different perspectives—ranging from direct field measurements over dynamo theory and stellar evolution to colliding winds and the stellar environment—fruitfully combine into a most interesting and still evolving overall picture, which we attempt to review here. Zeeman signatures leave no doubt that at least some O- and early B-type stars have a surface magnetic field. Indirect evidence, especially non-thermal radio emission from colliding winds, suggests many more. The emerging picture for massive stars shows similarities with results from intermediate mass stars, for which much more data are available. Observations are often compatible with a dipole or low order multi-pole field of about 1 kG (O-stars) or 300 G to 30?kG (Ap/Bp stars). Weak and unordered fields have been detected in the O-star ζ Ori A and in Vega, the first normal A-type star with a magnetic field. Theory offers essentially two explanations for the origin of the observed surface fields: fossil fields, particularly for strong and ordered fields, or different dynamo mechanisms, preferentially for less ordered fields. Numerical simulations yield the first concrete stable (fossil) field configuration, but give contradictory results as to whether dynamo action in the radiative envelope of massive main sequence stars is possible. Internal magnetic fields, which may not even show up at the stellar surface, affect stellar evolution as they lead to a more uniform rotation, with more slowly rotating cores and faster surface rotation. Surface metallicities may become enhanced, thus affecting the mass-loss rates.     

Observational Signatures of Particle Acceleration in Supernova Remnants

We evaluate the current status of supernova remnants as the sources of Galactic cosmic rays. We summarize observations of supernova remnants, covering the whole electromagnetic spectrum and describe what these observations tell us about the acceleration processes by high Mach number shock fronts. We discuss the shock modification by cosmic rays, the shape and maximum energy of the cosmic-ray spectrum and the total energy budget of cosmic rays in and surrounding supernova remnants. Additionally, we discuss problems with supernova remnants as main sources of Galactic cosmic rays, as well as alternative sources.     

Dust in Supernovae and Supernova Remnants I: Formation Scenarios

Supernovae are considered as prime sources of dust in space. Observations of local supernovae over the past couple of decades have detected the presence of dust in supernova ejecta. The reddening of the high redshift quasars also indicate the presence of large masses of dust in early galaxies. Considering the top heavy IMF in the early galaxies, supernovae are assumed to be the major contributor to these large amounts of dust. However, the composition and morphology of dust grains formed in a supernova ejecta is yet to be understood with clarity. Moreover, the dust masses inferred from observations in mid-infrared and submillimeter wavelength regimes differ by two orders of magnitude or more. Therefore, the mechanism responsible for the synthesis of molecules and dust in such environments plays a crucial role in studying the evolution of cosmic dust in galaxies. This review summarises our current knowledge of dust formation in supernova ejecta and tries to quantify the role of supernovae as dust producers in a galaxy.     

Nonlinear Kinetic Theory of Cosmic-ray Acceleration in Supernova Remnants

A review of kinetic nonlinear theory for cosmic-ray (CR) acceleration and subsequent -ray production due to CR nuclear component in supernova remnants (SNRs) is presented. The correspondence of the expected spectrum and composition of CRs produced inside SNRs in the Galaxy with the experimental data is discussed. Possible explanations of negative results in searching high energy -ray emission from nearby SNRs are analyzed.     

Nonlinear Shock Acceleration and Cosmic-Ray Production in Young Supernova Remnants

A number of young supernova remnants (SNRs) are now known to have nonthermal X-ray spectra. The steepness of the X-ray emission suggests that it is synchrotron from TeV electrons, and if this is the case, efficient shock acceleration is likely occurring in these objects. Here we use a model of nonlinear diffusive shock acceleration to fit the broad-band emission from SN1006, Tychos, and Keplers SNRs. Our fits confirm that all of these SNRs are producing TeV particles, but also show that the electron and ion spectra do not extend as a power law above a few TeV, well below the cosmic ray `knee at 10¹⁵ eV.     

Dust in Supernovae and Supernova Remnants II: Processing and Survival

Observations have recently shown that supernovae are efficient dust factories, as predicted for a long time by theoretical models. The rapid evolution of their stellar progenitors combined with their efficiency in precipitating refractory elements from the gas phase into dust grains make supernovae the major potential suppliers of dust in the early Universe, where more conventional sources like Asymptotic Giant Branch (AGB) stars did not have time to evolve. However, dust yields inferred from observations of young supernovae or derived from models do not reflect the net amount of supernova-condensed dust able to be expelled from the remnants and reach the interstellar medium. The cavity where the dust is formed and initially resides is crossed by the high velocity reverse shock which is generated by the pressure of the circumstellar material shocked by the expanding supernova blast wave. Depending on grain composition and initial size, processing by the reverse shock may lead to substantial dust erosion and even complete destruction. The goal of this review is to present the state of the art about processing and survival of dust inside supernova remnants, in terms of theoretical modelling and comparison to observations.     

Kinematics of Supernova Remnants: Status of X-Ray Observations

A supernova (SN) explosion drives stellar debris into the circumstellar material (CSM) filling a region on a scale of parsecs with X-ray emitting plasma. The velocities involved in supernova remnants (SNRs), thousands of km?s^?1, can be directly measured with medium and high-resolution X-ray spectrometers and add an important dimension to our understanding of the last stages of the progenitor, the explosion mechanism, and the physics of strong shocks. After touching on the ingredients of SNR kinematics, I present a summary of the still-growing measurement results from SNR X-ray observations. Given the advances in 2D/3D hydrodynamics, data analysis techniques, and especially X-ray instrumentation, it is clear that our view of SNRs will continue to deepen in the decades ahead.     

Magnetic Fields in Galaxies

Radio synchrotron emission, its polarization and its Faraday rotation are powerful tools to study the strength and structure of magnetic fields in galaxies. Unpolarized emission traces turbulent fields which are strongest in spiral arms and bars (20–30?μG) and in central starburst regions (50–100?μG). Such fields are dynamically important, e.g. they can drive gas inflows in central regions. Polarized emission traces ordered fields which can be regular or anisotropic random, generated from isotropic random fields by compression or shear. The strongest ordered fields of 10–15?μG strength are generally found in interarm regions and follow the orientation of adjacent gas spiral arms. Ordered fields with spiral patterns exist in grand-design, barred and flocculent galaxies, and in central regions of starburst galaxies. Faraday rotation measures (RM) of the diffuse polarized radio emission from the disks of several spiral galaxies reveal large-scale patterns, which are signatures of regular fields generated by a mean-field dynamo. However, in most spiral galaxies observed so far the field structure is more complicated. Ordered fields in interacting galaxies have asymmetric distributions and are an excellent tracer of past interactions between galaxies or with the intergalactic medium. Ordered magnetic fields are also observed in radio halos around edge-on galaxies, out to large distances from the plane, with X-shaped patterns. Future observations of polarized emission at high frequencies, with the EVLA, the SKA and its precursors, will trace galactic magnetic fields in unprecedented detail. Low-frequency telescopes (e.g. LOFAR and MWA) are ideal to search for diffuse emission and small RMs from weak interstellar and intergalactic fields.     

Cosmic Rays From Supernova Remnants: a Brief Description of the Shock Acceleration of gas and Dust

Space Science Reviews - We summarize our model of galactic cosmic-ray (GCR) origin and acceleration, wherein a mixture of interstellar and/or circumstellar gas and dust is accelerated by a...     

Magnetic Fields in Galactic Haloes

Magnetic fields on a range of scales play a large role in the ecosystems of galaxies, both in the galactic disk and in the extended layers of gas away from the plane. Observing magnetic field strength, structure and orientation is complex, and necessarily indirect. Observational data of magnetic fields in the halo of the Milky Way are scarce, and non-conclusive about the large-scale structure of the field. In external galaxies, various large-scale configurations of magnetic fields are measured, but many uncertainties about exact configurations and their origin remain. There is a strong interaction between magnetic fields and other components in the interstellar medium such as ionized and neutral gas and cosmic rays. The energy densities of these components are comparable on large scales, indicating that magnetic fields are not passive tracers but that magnetic field feedback on the other interstellar medium components needs to be taken into account.     

Galactic and Extragalactic Magnetic Fields

The current state of research of the Galactic magnetic field is reviewed critically. The average strength of the total field derived from radio synchrotron data, under the energy equipartition assumption, is 6±2 G locally and about 10±3 G at 3 kpc Galactic radius. These values agree well with the estimates using the locally measured cosmic-ray energy spectrum and the radial variation of protons derived from -rays. Optical and synchrotron polarization data yield a strength of the local regular field of 4±1 G, but this value is an upper limit if the field strength fluctuates within the beam or if anisotropic fields are present. Pulsar rotation measures, on the other hand, give only 1.4±0.2 G, a lower limit if fluctuations in regular field strength and thermal electron density are anticorrelated along the pathlength. The local regular field may be part of a `magnetic arm between the optical arms. However, the global structure of the regular Galactic field is not yet known. Several large-scale field reversals in the Galaxy were detected from rotation measure data, but a similar phenomenon was not observed in external galaxies. The Galactic field may be young in terms of dynamo action so that reversals from the chaotic seed field are preserved, or a mixture of dynamo modes causes the reversals, or the reversals are signatures of large-scale anisotropic field loops. The Galaxy is surrounded by a thick disk of radio continuum emission of similar extent as in edge-on spiral galaxies. While the local field in the thin disk is of even symmetry with respect to the plane (quadrupole), the global thick-disk field may be of dipole type. The Galactic center region hosts highly regular fields of up to milligauss strength which are oriented perpendicular to the plane. A major extension of the data base of pulsar rotation measures and Zeeman splitting measurements is required to determine the structure of the Galactic field. Further polarization surveys of the Galactic plane at wavelengths of 6 cm or shorter may directly reveal the fine structure of the local magnetic field.     

Small-Scale Solar Magnetic Fields

As we resolve ever smaller structures in the solar atmosphere, it has become clear that magnetism is an important component of those small structures. Small-scale magnetism holds the key to many poorly understood facets of solar magnetism on all scales, such as the existence of a local dynamo, chromospheric heating, and flux emergence, to name a few. Here, we review our knowledge of small-scale photospheric fields, with particular emphasis on quiet-sun field, and discuss the implications of several results obtained recently using new instruments, as well as future prospects in this field of research.     

The First Magnetic Fields

We review current ideas on the origin of galactic and extragalactic magnetic fields. We begin by summarizing observations of magnetic fields at cosmological redshifts and on cosmological scales. These observations translate into constraints on the strength and scale magnetic fields must have during the early stages of galaxy formation in order to seed the galactic dynamo. We examine mechanisms for the generation of magnetic fields that operate prior during inflation and during subsequent phase transitions such as electroweak symmetry breaking and the quark–hadron phase transition. The implications of strong primordial magnetic fields for the reionization epoch as well as the first generation of stars are discussed in detail. The exotic, early-Universe mechanisms are contrasted with astrophysical processes that generate fields after recombination. For example, a?Biermann-type battery can operate in a proto-galaxy during the early stages of structure formation. Moreover, magnetic fields in either an early generation of stars or active galactic nuclei can be dispersed into the intergalactic medium.     

Magnetic Fields in the Inner Heliosphere

The structure of Heliospheric Magnetic Field (HMF) is a function of both the coronal conditions from which it originates and dynamic processes which take place in the solar wind. The division between the inner and outer regions of the heliosphere is the result of dynamic processes which form large scale structures with increasing heliocentric distance. The structure of the HMF is normally described in the reference frame based on Parker's geometric model, but is better understood as an extension of potential field models of the corona. The Heliospheric Current Sheet (HCS) separates the two dominant polarities in the heliosphere; its large scale geometry near solar minimum is well understood but its topology near solar maximum remains to be investigated by Ulysses. At solar minimum, Corotating Interaction Regions (CIRs) dominate the near-equatorial heliosphere and extend their influence to mid-latitudes; the polar regions of the heliosphere are dominated by uniform fast solar wind streams and large amplitude, long wavelength, mostly transverse magnetic fluctuations. Coronal Mass Ejections (CMEs) introduce transient variability into the large scale heliospheric structure and may dominate the inner heliosphere near solar maximum at all latitudes.     

Cosmic Rays in Galactic and Extragalactic Magnetic Fields

We briefly review sources of cosmic rays, their composition and spectra as well as their propagation in the galactic and extragalactic magnetic fields, both regular and fluctuating. A special attention is paid to the recent results of the X-ray and gamma-ray observations that shed light on the origin of the galactic cosmic rays and the challenging results of Pierre Auger Observatory on the ultra high energy cosmic rays. The perspectives of both high energy astrophysics and cosmic-ray astronomy to identify the sources of ultra high energy cosmic rays, the mechanisms of particle acceleration, to measure the intergalactic radiation fields and to reveal the structure of magnetic fields of very different scales are outlined.     

Particle Scattering by Magnetic Fields

The need for a correct quantitative treatment of the interactions between cosmic rays and turbulent magnetic fields continues to be one of the fundamental problems of modern astrophysics. It is the aim of this paper to review new developments in the understanding of mechanisms involved in the scattering of charged particles by magnetic field fluctuations. Special emphasis is given to a comparison of transport parameters determined from the modeling of spacecraft and neutron monitor observation of solar particle events, with theoretical predictions derived from a spectral analysis of simultaneously measured fluctuation spectra. It appears that the traditional quasi-linear theory of particle scattering requires only a slight modification, and the major problem still is our lack of knowledge of the three-dimensional structure of the magnetic turbulence. Possibilities to better reconcile the theory with observations by properly taking into account the microphysics of wave and turbulence aspects of the fluctuations, and to use energetic particles as probes to study certain properties of the magnetic turbulence, are discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Planetary Magnetic Fields: Achievements and Prospects

The past decade has seen a wealth of new data, mainly from the Galilean satellites and Mars, but also new information on Mercury, the Moon and asteroids (meteorites). In parallel, there have been advances in our understanding of dynamo theory, new ideas on the scaling laws for field amplitudes, and a deeper appreciation on the diversity and complexity of planetary interior properties and evolutions. Most planetary magnetic fields arise from dynamos, past or present, and planetary dynamos generally arise from thermal or compositional convection in fluid regions of large radial extent. The relevant electrical conductivities range from metallic values to values that may be only about one percent or less that of a typical metal, appropriate to ionic fluids and semiconductors. In all planetary liquid cores, the Coriolis force is dynamically important. The maintenance and persistence of convection appears to be easy in gas giants and ice-rich giants, but is not assured in terrestrial planets because the quite high electrical conductivity of an iron-rich core guarantees a high thermal conductivity (through the Wiedemann-Franz law), which allows for a large core heat flow by conduction alone. This has led to an emphasis on the possible role of ongoing differentiation (growth of an inner core or “snow”). Although planetary dynamos mostly appear to operate with an internal field that is not very different from (2ρΩ/σ)^1/2 in SI units where ρ is the fluid density, Ω is the planetary rotation rate and σ is the conductivity, theoretical arguments and stellar observations suggest that there may be better justification for a scaling law that emphasizes the buoyancy flux. Earth, Ganymede, Jupiter, Saturn, Uranus, Neptune, and probably Mercury have dynamos, Mars has large remanent magnetism from an ancient dynamo, and the Moon might also require an ancient dynamo. Venus is devoid of a detectable global field but may have had a dynamo in the past. Even small, differentiated planetesimals (asteroids) may have been capable of dynamo action early in the solar system history. Induced fields observed in Europa and Callisto indicate the strong likelihood of water oceans in these bodies. The presence or absence of a dynamo in a terrestrial body (including Ganymede) appears to depend mainly on the thermal histories and energy sources of these bodies, especially the convective state of the silicate mantle and the existence and history of a growing inner solid core. As a consequence, the understanding of planetary magnetic fields depends as much on our understanding of the history and material properties of planets as it does on our understanding of the dynamo process. Future developments can be expected in our understanding of the criterion for a dynamo and on planetary properties, through a combination of theoretical work, numerical simulations, planetary missions (MESSENGER, Juno, etc.) and laboratory experiments.     

Magnetic Fields in the Multiphase Interstellar Medium

We discuss the interstellar magnetic field and its measurement in the different phases of the interstellar medium. The measurement techniques include Faraday rotation, Zeeman splitting, linear polarization from aligned dust grains, and the Chandrasekhar-Fermi method. The phases include the classical ones: molecular clouds, the Cold and Warm Neutral Media, the Warm and Hot Ionized Media. We also include a less well-known phase, the Warm Partially Ionized Medium, which can be surprisingly prominent in Faraday rotation.     

Induced Magnetic Fields in Solar System Bodies

Electromagnetic induction is a powerful technique to study the electrical conductivity of the interior of the Earth and other solar system bodies. Information about the electrical conductivity structure can provide strong constraints on the associated internal composition of planetary bodies. Here we give a review of the basic principles of the electromagnetic induction technique and discuss its application to various bodies of our solar system. We also show that the plasma environment, in which the bodies are embedded, generates in addition to the induced magnetic fields competing plasma magnetic fields. These fields need to be treated appropriately to reliably interpret magnetic field measurements in the vicinity of solar system bodies. Induction measurements are particularly important in the search for liquid water outside of Earth. Magnetic field measurements by the Galileo spacecraft provide strong evidence for a subsurface ocean on Europa and Callisto. The induction technique will provide additional important constraints on the possible subsurface water, when used on future Europa and Ganymede orbiters. It can also be applied to probe Enceladus and Titan with Cassini and future spacecraft.