Atomic processes in astrophysics

We review the atomic processes responsible for astrophysical emission line spectra, paying particular attention to the reliability of the rates used to determine the physical conditions in the emiting gas. We discuss particular cases where often-neglected processes play important roles.     

Multi-ion plasmas in astrophysics

The steady-state motion of a quasi-neutraln-ion plasma is investigated using a fluiddynamical model. The main results obtained are that there are only two distinct ways in which such a plasma can make a transition from a subsonic state to a supersonic one. There is one unique possibility for which there exists one critical point where all the ion gases have their Mach numbers exactly to unity and where the individual ion forcing functios (the inhomogeneous terms) are non-zero but linearly related to each other. The other possibility which we find is that at a critical point all inhomogeneous terms are identically equal to zero and there exist as many critical points as there exist simultianeous zeros of the ion forcing functions. These results are necessary and sufficient forn3. For one- and two-ion plasmas special results can be obtained, some of which fit into the above two classes and some of which do not.Operated by the Association of Universities for Research in Astronomy, under contract with the National Science Foundation.     

Mulit-ion plasmas in astrophysics

An isothermal hydrodynamic model of the motions of a multi-ion plasma in a gravitational field is developed and the properties of the flow are discussed for the case of major astrophysical interest in which the gas undergoes a subsonic-supersonic transition. It is shown that the existence of critical points thorough which the plasma has to pass will determine a large number of the plasma parameters, especially the temperature of the minor ions. The equation of motion of a two ion gas (hydrogen-helium) are solved numerically and yield the interesting result that the bulk velocity of the plasma constituents are not equal at 1 AU.Operated by the Association of Universities for Research in Astronomy, under contract with the National Science Foundation.     

Key problems in cool-star astrophysics

Selected key problems in cool-star astrophysics are reviewed, with emphasis on the importance of new ultraviolet missions to tackle the unresolved issues.UV spectral signatures are an essential probe of critical physical processes related to the production and transport of magnetic energy in astrophysical plasmas ranging, for example, from stellar coronae, to the magnetospheres of magnetars, and the accretion disks of protostars and Active Galactic Nuclei. From an historical point of view, our comprehension of such processes has been closely tied to our understanding of solar/stellar magnetic activity, which has its origins in a poorly understood convection-powered internal magnetic dynamo. The evolution of the Sun's dynamo, and associated magnetic activity, affected the development of planetary atmospheres in the early solar system, and the conditions in which life arose on the primitive Earth. The gradual fading of magnetic activity as the Sun grows old likewise will have profound consequences for the future heliospheric environment. Beyond the Sun, the magnetic activity of stars can influence their close-in companions, and vice versa.Cool star outer atmospheres thus represent an important laboratory in which magnetic activity phenomena can be studied under a wide variety of conditions, allowing us to gain insight into the fundamental processes involved. The UV range is especially useful for such studies because it contains powerful diagnostics extending from warm (∼ 10⁴ K) chromospheres out to hot (1–10 MK) coronae, and very high-resolution spectroscopy in the UV has been demonstrated by the GHRS and STIS instruments on HST but has not yet been demonstrated in the higher energy EUV and X-ray bands. A recent example is the use of the hydrogen Lyα resonance line—at 110 000 resolution with HST STIS—study, for the first time, coronal winds from cool stars through their interaction with the interstellar gas. These winds cannot be detected from the ground, for lack of suitable diagnostics; or in the X-rays, because the outflowing gas is too thin.A 2m class UV space telescope with high resolution spectroscopy and monitoring capabilities would enable important new discoveries in cool-star astronomy among the stars of the solar neighborhood out to about 150 pc. A larger aperture facility (4–6 m) would reach beyond the 150 pc horizon to fainter objects including young brown dwarfs and pre-main sequence stars in star-forming regions like Orion, and magnetic active stars in distant clusters beyond the Pleiades and α Persei. This would be essential, as well, to characterize the outer atmospheres of stars with planets, that will be discovered by future space missions like COROT, Kepler, and Darwin.Deceased October 23, 2005     

Spectral theory and stability in astrophysics

This paper is part II of a limited review of the applications of the spectral theory of linear operators in an astrophysical context. A major part of the paper is devoted to describing the results obtained by Dyson and Schutz (1979) for differentially rotating perfect fluid stars. The functional-analytic techniques used and the results so obtained are compared with those in Paper I. As in the case of ideal magnetohydrodynamics, the mathematical structure of the rotating star problem is very rich indeed. Many questions remain unanswered in both areas.     

White and grey holes in astrophysics

Astrophysical applications of white and grey holes are considered. Four types of anticollapsars in extended manifolds of general relativity are distinguished: canonical white and grey holes, light- and dark-grey holes. White and grey holes can be revealed in the form of bursts of gravitational and electromagnetic radiation, neutrino, and cosmic rays. Quasars, active galactic nuclei, jets, and cosmic voids can be associated with relicts of white and light-grey holes, and black holes do with relicts of canonical grey and dark-grey holes.     

Spectral theory and stability in astrophysics

This paper is part I of a limited review of the applications of the spectral theory of linear operators in an astrophysical context. The ideal magnetohydrodynamic equations arise in the study of magnetic flux tubes in the solar corona, and in the plasma physics of nuclear containment devices. The system described by these equations is very rich both mathematically and physically, and there are many open problems associated with these models. The underlying mathematical principles are discussed in a qualitative manner in Paper I, and in a more technical manner in subsequent Paper II.     

The development of astrophysics in South Africa

A brief overview is given of the history of astrophysics in South Africa up to the beginning of the modern era.     

Applying machine learning to catalogue matching in astrophysics

We present the results of applying automated machine learning techniques to the problem of matching different object catalogues in astrophysics. In this study, we take two partially matched catalogues where one of the two catalogues has a large positional uncertainty. The two catalogues we used here were taken from the H  i Parkes All Sky Survey (HIPASS) and SuperCOSMOS optical survey. Previous work had matched 44 per cent (1887 objects) of HIPASS to the SuperCOSMOS catalogue.
A supervised learning algorithm was then applied to construct a model of the matched portion of our catalogue. Validation of the model shows that we achieved a good classification performance (99.12 per cent correct).
Applying this model to the unmatched portion of the catalogue found 1209 new matches. This increases the catalogue size from 1887 matched objects to 3096. The combination of these procedures yields a catalogue that is 72 per cent matched.     

‘Time-integrated” synchrotron radiation in high-energy astrophysics

We survey the observational data on infrared, optical and X-radiation sources associated with energetic cosmic events, and note the occurrence of an apparently preferred value of the spectral index,n=1, for the radiation continua. We review the essentials of standard synchrotron radiation theory; the conventional interpretation of the observational data in terms of an energy distribution of electrons injected into a constant, low valued magnetic field; and the somewhat unsatisfactory attempts that can be made to explain this electron energy distribution in terms of the Fermi acceleration mechanism. We examine the evidence for the presence in the radiation sources of high magnetic fields, which cause evolution of the synchrotron radiation power spectrum to occur. We work out the consequences of this evolution, and obtain a new form of synchrotron radiation theory, which we describe astime-integrated synchrotron radiation theory, the particular advantage of which is that it is able to give a unique value (n=1/2 of the spectral index for radiation produced by a single high energy electron, independently of the initial electron energy. We consider the consequences of there being a distribution of magnetic field values in a radiation source; and in particular we consider a uniform distribution (in which all values are equally probable), which is capable of producing the required spectral indexn=1. We show that this uniform distribution can be explained in terms of a model in which there exist condensations of material containing high magnetic fields and within which electrons can be generatedin situ, through the familiar pion production and decay processes. We also consider systems in which electrons in a radiation source have injection patterns that enable the radiation continua to be interpreted in terms of time-integrated synchrotron radiation theory, originally devised for a single electron. We apply these considerations to sources of optical and higher frequency radiation; we also show that they have limited application to certain types of radio source. We suggest in conclusion that the condensations that feature in our model could act as basic units of structure for complex radiation sources associated with different types of energetic cosmic event, and that therein could lie the clue to the evident similarity of their radiation continua.     

Gamma-ray astrophysics: parsecs to megaparsecs

A rapidly spinning, slowly accreting magnetic white dwarf (or X-ray pulsar) in hibernation is expected to result in rapid spindown as a result of the stretching and reconnection of magnetic field lines, leading to particle acceleration at the magnetospheric radiusoutside the corotation radius, and the propeller type ejection of magnetized synchrotron-emitting clouds. This may explain the non-thermal (radio and-rays) emission seen from the unique nearby AE Aquarii. Moving to Galactic distances we show how TeV-ray observations of pulsar-driven supernova remnants (with well-measured synchrotron X-ray spectra) allow us to obtain a direct measurement of the average magnetic field strength in the nebula. Finally, GeV to TeV observations of-ray blazars out to redshifts of 2 allow us to probe the intergalactic infrared radiation field, the Hubble constant and possibly the parameter of the Universe.     

Introduction to plasma astrophysics and cosmology

The year 1996 marks the Centennial Celebration of the founding of Plasma Astrophysics and Cosmology; its origins may be traced to the seminal research first published by Kristian Birkeland in 1896. This special workshop issue reports on advances in issues of importance to the plasma universe; topics as timely as when first raised a century ago.     

Sinc-Collocation method for solving astrophysics equations

In this paper we propose Sinc-Collocation method for solving Lane–Emden equation which is a nonlinear ordinary differential equation on a semi-infinite interval. It is found that Sinc procedure converges with the solution at an exponential rate. This method is utilized to reduce the computation of this problem to some algebraic equations. We also compare this solution with some well-known results and show that it is accurate.     

A DUAL mission for nuclear astrophysics

DUAL will study the origin and evolution of the elements and explores new frontiers of physics: extreme energies that drive powerful stellar explosions and accelerate particles to macroscopic energies; extreme densities that modify the laws of physics around the most compact objects known; and extreme fields that influence matter in a way that is unknown on Earth. The variability of these extreme objects requires continuous all-sky coverage, while detailed study demands an improvement in sensitivity over previous technologies by at least an order of magnitude. The DUAL payload is composed of an All-Sky Compton Imager (ASCI), and two optical modules, the Laue-Lens Optic (LLO) and the Coded-Mask Optic (CMO). The ASCI serves dual roles simultaneously, both as an optimal focal-plane sensor for deep observations with the optical modules and as a sensitive true all-sky telescope in its own right for all-sky surveys and monitoring. While the optical modules are located on the main satellite, the All-Sky Compton Imager is situated on a deployable structure at a distance of 30?m from the satellite. This configuration not only permits to maintain the less massive payload at the focal distance, it also greatly reduces the spacecraft-induced detector background, and, above all it provides ASCI with a continuous all-sky exposure.     

White holes and high energy astrophysics

The role of exploding objects in high-energy astrophysics is discussed. Quantitative results are obtained for the simple case of the canonical white hole which explodes from a singularity and consists of pressure-free matter in the comoving frame of reference. General relativity is used for calculating the dynamical results. Applications to X-ray background, transient X-ray sources, γ-ray bursts and high energy cosmic rays are considered. White holes of more general types are discussed in a qualitative manner.     

Computable extensions of generalized fractional kinetic equations in astrophysics

Fractional calculus and special functions have contributed a lot to mathematical physics and its various branches.The great use of mathematical physics in distinguished astrophysical problems has attracted astronomers and physicists to pay more attention to available mathematical tools that can be widely used in solving several problems of astrophysics/physics.In view of the great importance and usefulness of kinetic equations in certain astrophysical problems,the authors derive a generalized fractional kin...     

Field approach to gravitation and its significance in astrophysics

A field modification of classical gravitational theory which is analogous to the classical electrodynamics is proposed. Within its framework it is possible to account for some types of behaviour of matter occurring under certain extreme physical conditions. Especially, the energy release in quasars and pulsars may be calculated, under some plausible physical assumptions, to obtain values comparable with the observable ones. Several astrophysical effects (e.g. the occurrence of non-thermal radiation in pulsars and quasars, etc.) find reasonable explanations within this field approach to gravitation.     

Electrodynamic and gravitational effects of proca stresses in astrophysics

If the photon were to have a rest mass near the best present upper limit,m3×10^–53 g, Maxwell's equations would be inapplicable over distances exceeding about 10¹⁵ cm, with profound implications for cosmic electrodynamics. This paper deals with electrodynamic and gravitational effects of the non-Maxwellian stresses which would be associated with large-scale magnetic fields in quasi-static plasmas in these circumstances. The existence of moderately dense interstellar gas clouds with relatively strong magnetization shows thatm10^–58 g. General relativity must be used to calculate even the Newtonian gravitational effects of electromagnetic fields, and it is shown here that the Newtonian potential caused by the non-Maxwellian stresses just cancels that caused by the non-Maxwellian energy density. Previous arguments based on the latter alone are therefore invalid.