Lévy model for interstellar scintillations

Observations of radio signals from distant pulsars provide a valuable tool for investigation of interstellar turbulence. The time shapes of the signals are the result of pulse broadening by the fluctuating electron density in the interstellar medium. While the scaling of the shapes with the signal frequency is well understood, the observed anomalous scaling with respect to the pulsar distance has remained a puzzle for more than 30 years. We propose a new model for interstellar electron density fluctuations, which explains the observed scaling relations. We suggest that these fluctuations obey Lévy statistics rather than Gaussian statistics, as assumed in previous treatments of interstellar scintillations.     

Magnetohydrodynamics turbulence: An astronomical perspective

Early work on magnetohydrodynamic (MHD) turbulence in the 1960s due, independently, to Iroshnikov and Kraichnan (IK) considered isotropic inertial-range spectra. Whereas laboratory experiments were not in a position to measure the spectral index, they showed that the turbulence was strongly anisotropic. Theoretical horizons correspondingly expanded in the 1980s, to accommodate both the isotropy of the IK theory and the anisotropy suggested by the experiments. Since the discovery of pulsars in 1967, many years of work on interstellar scintillation suggested that small-scale interstellar turbulence must have a hydromagnetic origin; but the IK spectrum was too flat and the ideas on anisotropic spectra too qualitative to explain the observations. In response, new theories of balanced MHD turbulence were proposed in the 1990s, which argued that the IK theory was incorrect, and made quantitative predictions of anisotropic inertial-range spectra; these theories have since found applications in many areas of astrophysics. Spacecraft measurements of solar-wind turbulence show that there is more power in Alfvén waves that travel away from the Sun than towards it. Theories of imbalanced MHD turbulence have now been proposed to address interplanetary turbulence. This very active area of research continues to be driven by astronomy.     

Scintillation as a tool for studying the interstellar plasma

The interstellar plasma is characterized by variations of electron density having a wide range of physical scales. Radiation propagating through this inhomogeneous plasma is scattered, causing the received intensity to scintillate on a variety of time scales. Observations of the radio frequency spectrum and temporal variation of scintillation give information on the turbulence spectrum of the plasma and the distribution of density irregularities throughout the galaxy. Some uses of scintillation as a means of probing the interstellar plasma will be described.Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE, United Kingdom. Published in Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 37, No. 6, pp. 685–692, June, 1994.     

Cosmic ray spectrum produced by galactic supernova remnants

Cosmic ray acceleration by supernova shocks is considered. A new numerical code is used to describe the cosmic ray acceleration and shock wave evolution. The magnetohydrodynamic turbulence generation in the shock precursor by streaming instability of accelerated particles is taken into account. The cosmic ray spectrum produced by supernova explosion in uniform interstellar medium is simulated.     

Nonlinear propagation of cosmic rays in the galaxy

Propagation of cosmic rays in the interstellar medium after their emergence from sources—supernova remnants—may be accompanied by the development of flow instability which forms high magnetohydrodynamic turbulence and leads to nonlinear cosmic ray diffusion. A self-similar solution to the nonlinear diffusion equations is found and it is shown that the noted mechanism leads to an effective diffusion coefficient of cosmic rays, which coincides with the empirical value.     

Non-Gaussian Phase Screen Based on log-Poisson Distribution and Effect on Imaging

Gaussian models without intermittency are extensively used in estimating the effect of turbulence, but it also brings some puzzles, for example the observed pulse shape that disagrees with the result of the standard theory of interstellar scintillations. Indeed the property of intermittency is inherent in turbulence, i.e., all the quantities that characterize it suffer from strong fluctuations. So it is necessary to consider turbulent intermittency in manyapplications. In this paper we propose a non-Gaussian phase screen, which obeys log-Poisson statistics, and also offers the corresponding point spread function (PSF). These results describe that intermittency leads to the more extent and different directional distribution of PSF. Theoretical analysis is made under the hypothesis of the phase difference satisfying log-Poissonstatistics, and the average point spread function, which accord qualitatively with the result of the above generated phase screen, is derived.     

Structure function scaling in compressible super-Alfvénic MHD turbulence

Supersonic turbulent flows of magnetized gas are believed to play an important role in the dynamics of star-forming clouds in galaxies. Understanding statistical properties of such flows is crucial for developing a theory of star formation. In this Letter we propose a unified approach for obtaining the velocity scaling in compressible and super-Alfvénic turbulence, valid for the arbitrary sonic Mach number, M(S). We demonstrate with numerical simulations that the scaling can be described with the She-Lévêque formalism, where only one parameter, interpreted as the Hausdorff dimension of the most intense dissipative structures, needs to be varied as a function of M(S). Our results thus provide a method for obtaining the velocity scaling in interstellar clouds once their Mach numbers have been inferred from observations.     

It''s a dusty Universe: surface science in space

We live in a dusty Universe! Dust is not only found in our solar system among the planets but is found in a wide variety of objects throughout the Universe, mainly in those regions between the stars known as interstellar clouds. Interstellar dust particles, which consist of cores of silicates and carbonaceous material often surrounded by icy mantles, are most probably highly irregular in shape with a size distribution from micro- to nanometers. Interstellar dust is important for many reasons, including the template it provides for surface chemical reactions that form, among other species, the most important interstellar molecule––H₂. In this article, we discuss the evidence for interstellar dust, its physical and chemical properties, its role in interstellar surface chemistry, and what remains to be learned.     

星际26Al核合成的研究进展

介绍了星际 26Al衰变所发射1 809 keV γ射线的最新空间探测结果， 综述了目前关于星际 26Al起源的各种天体模型的研究进展。最后， 简要阐述了14N(p,γ)15O反应截面的最新测量结果及其对 26Al起源研究的影响， 探讨了一种星际 26Al合成的可能新途径。The recently detection outcome about interstellar 26Al is introduced, as well as the investigation progresses of interstellar 26Al nucleosynthesis in all models are reviewed. Finally, the new nuclear physics experimental results for the14N(p,γ)15O reaction effect on sources of interstellar 26Al and an alternative new way for nucleosynthesis 26Al in SNIa are discussed.     

Chemistry in space

Summary Molecules have been observed in widely disparate astronomical objects, from comets to supernova remnants. Interstellar space appear to be a real chemical laboratory, able to produce a large number of molecules, some of them relatively complex. The abundances show a very high sensitivity to local physical properties and dynamical history. This sensitivity renders molecular observations and astrochemical modelling very flexible tools for investigating the properties and evolution of the interstellar medium. A brief outline of morfology of interstellar medium is given, together with a discussion of the basic chemical processes leading to the molecular formation. Since interstellar-dust particles play a crucial role in the chemical evolution of interstellar medium, physical and chemical properties of dust are reviewed. Finally, by way of an example of modelling exercise, a toy model of the chemical evolution of interstellar gas is presented. Paper presented at the V Cosmic Physics National Conference, S. Miniato, November 27–30, 1990.     

Spiral magneto-electron waves in interstellar gas dynamics

We discuss possible observational consequences resulting from the propagation of transverse magneto-electron waves in the interstellar medium. We briefly describe a magnetohydrodynamic model for the cyclotron waves with emphasis on their analogy with hydrodynamic inertial waves. It is shown that the cyclotron waves are heavily damped in the interstellar medium and, therefore, cannot affect the gas dynamics of star-forming molecular clouds. We developed an analytical model of the helicoidal magneto-electron waves based on the electromagnetic induction equation for the magnetic flux density driven by the Hall and Ohmic components of the electric field generated by flows of thermal electrons. It is established that the helicons can propagate in the interstellar medium without any noticeable attenuation. The presented numerical estimates for the group velocity of the intercloud helicons suggest that spiral circularly polarized magneto-electron waves of this type can be responsible for the broadening of molecular lines detected from dark interstellar clouds.     

Carbon onions: carriers of the 217.5 nm interstellar absorption feature

Ultraviolet-visible absorption measurements of high purity and well separated carbon onion samples are reported. The results show that, after purification, absorption features from carbon onions match well with the interstellar UV spectrum. The measurements show that the absorption peak position remains constant at 4.55+/-0.1 microm(-1), and the width varies from 1.2-1.6 microm(-1), a key feature of the interstellar spectrum. The similarities between the experimental and observed absorption spectra indicate that carbon onions are very strong candidates for the origin of the UV interstellar absorption peak at 4.6 microm(-1).     

The quantum nonthermal effect of a nonstationary Kerr－Newman black hole and the average range of the effective particles

We have found that the nonthermal radiation of a nonstationary Kerr－Newman black hole is affected by interstellar materials. In particular, the interstellar gas deeply influences the average range of nonthermal radiation particles, while the average range depends on the maximum energy of the radiation and the energy extent of the radiation.     

Radiation transfer in dense interstellar clouds

Summary The scattering properties of interstellar dust in the far ultraviolet as derived from the observations are reviewed. The transfer of ultraviolet radiation within a spherically symmetric inhomogeneous cloud is analysed using the method of successive scattering. The effects of different assumptions, both on the albedo and asymmetry factor of interstellar dust, and also of the extinction law on UV radiation penetration into clouds are studied. The results are used to determine the lifetime against photodestruction of interstellar formaldehyde in dense clouds. Paper presented at the 2° Convegno Nazionale di Fisica Cosmica, held at L'Aquila, 29 May–2 June 1984.     

Diffuse interstellar absorption bands

The diffuse interstellar bands (DIBs) are a large number of absorption bands that are superposed on the interstellar extinction curve and are of interstellar origin. Since the discovery of the first two DIBs in the 1920s, the exact nature of DIBs still remains unclear. This article reviews the history of the detections of DIBs in the Milky Way and external galaxies, the major observational characteristics of DIBs, the correlations or anti-correlations among DIBs or between DIBs and other interstellar features (e.g. the prominent 2175 Angstrom extinction bump and the far-ultraviolet extinction rise), and the proposed candidate carriers. Whether they are also present in circumstellar environments is also discussed. Supported by the NSF of USA (Grant No. AST-0707866), the National Natural Science Foundation of China (Grant No. 10878012), the Outstanding Overseas Young Chinese Scholarship, and the Laboratory of Particle Astrophysics, Institute of High Energy Physics, CASS     

Ultimate energy of cosmic rays generated in supernova shells

It is shown that the ultimate energy of cosmic rays accelerated in a supernova shell due to the surfing acceleration mechanism is determined by the shell radius and the interstellar magnetic field. The ultimate energy of cosmic rays accelerated in the supernova shock does not exceed 10¹⁷ eV for typical values of the interstellar magnetic field in the vicinity of a supernova and the radii of observed supernova shells.     

The ulysses space mission

Summary We describe briefly the collaborative ESA-NASA Ulysses mission which will provide, for the first time in the history of the Solar-System exploration,in situ observations of the heliosphere over a broad range of heliographic latitudes. Launched on October 6, 1990, Ulysses has been injected in a high-inclination orbit by means of a gravity-assisted manoeuvre at the time of his encounter with Jupiter, in February 1992. On-board instrumentation will obtain data on the solar wind, the heliospheric magnetic field, radio, X-and γ-bursts plasma waves and interplanetary and interstellar gas and dust. Italy participates to the mission with an experiment, led by Professor Bruno Bertotti, of the University of Pavia, aimed at detecting gravitational waves, and with an interdisciplinary investigation, led by Professor Giancarlo Noci, of the University of Florence, where scientists from the University of Florence, the Arcetri Astrophysical Observatory, the Institute for the Physics of the Interplanetary Space of Frascati and the Observatory of Trieste will use data from different experiments to study the dependence of mass loss, ion composition and turbulence of the solar wind on heliographic latitude. Paper presented at the V Cosmic Physics National Conference, S. Miniato, November 27–30, 1990.     

Cosmic dust and our origins

The small solid particles in the space between the stars provide the surfaces for the production of many simple and complex molecules. Processes involving the effects of ultraviolet irradiation of the thin (hundredth micron) mantles are shown to produce a wide range of molecules and ions also seen in comets. Some of the more complex ones inferred from laboratory experiments are expected to play an important role in the origin of life. An outline of the chemical evolution of interstellar dust as observed and as studied in the laboratory is presented. Observations of comets are shown to provide substantial evidence for their being fluffy aggregates of interstellar dust as it was in the protosolar nebula, i.e. the interstellar cloud which collapsed to form the solar system. The theory that comets may have brought the progenitors of life to the earth is summarized.     

Theoretical support for buckyonions as carriers of the UV interstellar extinction feature

Theoretically simulated UV-visible photoabsorption spectra of the buckyonions are compared with the most recent and complete experimental data. The very good agreement found provides support for our theoretical model of the buckyonions and allows for a detailed analysis of the correspondence between these spectra and the UV interstellar extinction feature. The excellent agreement found between the theoretical and observational features and the consistency of the former with the observational data constraints gives very strong support for the buckyonion origin of the UV interstellar spectrum.     

Spallation processes and nuclear interaction products of cosmic rays

Most cosmic-ray nuclei heavier than helium have suffered nuclear collisions in the interstellar gas, with transformation of nuclear composition. The isotopic and elemental composition at the sources has to be inferred from the observed composition near the Earth. The source composition permits tests of current ideas on sites of origin, nucleosynthesis in stars, evolution of stars, the mixing and composition of the interstellar medium and injection processes prior to acceleration. The effects of nuclear spallation, production of radioactive nuclides and the time dependence of their decay provide valuable information on the acceleration and propagation of cosmic rays, their nuclear transformations, and their confinement time in the Galaxy. The formation of spallation products that only decay by electron capture and are relatively long-lived permits an investigation of the nature and density fluctuations (like clouds) of the interstellar medium. Since nuclear collisions yield positrons, antiprotons, gamma rays and neutrinos, we shall discuss these topics briefly.