Two-component scattering model and the electron density spectrum

In this paper, we discuss a rigorous treatment of the refractive scintillation caused by a two-component interstellar scattering medium and a Kolmogorov form of density spectrum. It is assumed that the interstellar scattering medium is composed of a thin-screen interstellar medium (ISM) and an extended interstellar medium. We consider the case that the scattering of the thin screen concentrates in a thin layer represented by a δ function distribution and that the scattering density of the extended irregular medium satisfies the Gaussian distribution. We investigate and develop equations for the flux density structure function corresponding to this two-component ISM geometry in the scattering density distribution and compare our result with the observations. We conclude that the refractive scintillation caused by this two-component ISM scattering gives a more satisfactory explanation for the observed flux density variation than does the single extended medium model. The level of refractive scintillation is strongly sensitive to the distribution of scattering material along the line of sight (LOS). The theoretical modulation indices are comparatively less sensitive to the scattering strength of the thin-screen medium, but they critically depend on the distance from the observer to the thin screen. The logarithmic slope of the structure function is sensitive to the scattering strength of the thin-screen medium, but is relatively insensitive to the thin-screen location. Therefore, the proposed model can be applied to interpret the structure functions of flux density observed in pulsar PSR B2111 + 46 and PSR B0136 + 57. The result suggests that the medium consists of a discontinuous distribution of plasma turbulence embedded in the interstellar medium. Thus our work provides some insight into the distribution of the scattering along the LOS to the pulsar PSR B2111 + 46 and PSR B0136 + 57.     

Electromagnetic instability of a homogeneous interstellar medium

The electromagnetic instability of an interstellar medium with an arbitrary velocity distribution is examined over the large scale lengths typical of gas-dust clouds without a significant magnetic field. It is shown that over a moderate time scale (months and years) these instabilities can develop and that the requirement of stability is satisfied by a narrow class of distributions that are close to spherical. __________ Translated from Astrofizika, Vol. 49, No. 2, pp. 289–297 (May 2006).     

Equilibrium and stability of interstellar medium in its own gravitational field

The statistical equilibrium of a heterogeneous self-gravitating layer of the interstellar medium is examined. Electrostatic fields and the different reactions to them by electrons and the ions of different elements are taken into account. In general, different, uncoupled temperatures are specified for the individual components of the interstellar medium. It is emphasized that some details of the structure of an equilibrium layer cannot be interpreted in terms of gravitation alone, despite the weakness of the electrostatic fields. A mixture of H II, He II, and electrons is examined as an important example. The asymptotic behavior of the total density and of the ratio of partial densities over large distances is found. A method for finding (in quadrature) these characteristics at any point of the medium is described. __________ Translated from Astrofizika, Vol. 50, No. 3, pp. 453–462 (August 2007).     

SKA studies of atomic gas in the interstellar medium of the Milky Way

The λ21-cm line is an excellent tracer of the neutral interstellar medium (ISM). Atomic hydrogen (HI) is found in a variety of environments, from dense clouds to the diffuse galactic halo, and its filling factor is often high, so structures with sizes over a wide range of scales can be mapped with this line. Galactic HI surveys show small scale structure that is consistent with a spectrum of interstellar turbulence similar to what is measured in the ionized component of the ISM. But our sampling of the spectrum of this turbulence is limited to a few size ranges, based on the sensitivities of existing telescopes for emission and absorption studies. The Square Kilometer Array (SKA) will provide the sensitivity and resolution to give continuous coverage of the turbulence spectrum from hundreds of parsecs to a few tens of Astronomical Units. By showing us the full spectrum of interstellar turbulence in the neutral medium, the physical processes driving hydrodynamic and magneto-hydrodynamic instabilities will be illuminated. Ultimately the turbulence governs the passage of the gas from the warm phases of the medium to the cold phases where gravitational collapse can initiate star formation. The SKA is needed to fill in this missing link in the cycle of star formation and chemical enrichment that drives the evolution of galaxies. In the Milky Way halo, SKA mapping of HI high velocity clouds will trace the structure and motion of both the warm phase gas and the hot medium. The interaction between these two phases of halo gas is a great unsolved problem in Galactic astrophysics.     

The identification of diffuse bands

Our present knowledge of the diffuse interstellar bands (two of which were first noted by Wilson in 1958) is briefly summarized. Other broad and very broad interstellar features (the 220nm extinction bump, the very broad structure in the extinction curve, the extended red emission in reflection and other nebulae, and the unidentified-sometimes called the “overidentified”-infrared bands) are also briefly described. The origins of all these features remain unknown, in spite of intensive study. Possible relations between these various features and families is briefly discussed. Recent observations are shown to support the hypothesis that the carriers of the diffuse bands are free-flying molecules, whereas the alternative hypothesis of dust grains now seems to be untenable. Candidate types of molecular carriers are mentioned, and the possible source of such molecules in the diffuse interstellar medium is briefly discussed.     

Effect of the heliospheric interface on the distribution of interstellar hydrogen atom inside the heliosphere

This paper deals with the modeling of the interstellar hydrogen atoms (H atoms) distribution in the heliosphere. We study influence of the heliospheric interface, that is the region of the interaction between solar wind and local interstellar medium, on the distribution of the hydrogen atoms in vicinity of the Sun. The distribution of Hatoms obtained in the frame of the self-consistent kinetic-gasdynamicmodel of the heliospheric interface is compared with a simplified model which assumes Maxwellian distribution of H atoms at the termination shock and is called often as “hot” model. This comparison shows that the distribution of H atoms is significantly affected by the heliospheric interface not only at large heliocentric distances, but also in vicinity of the Sun at ∼1–5 AU. Hence, for analysis of experimental data connected with direct or undirect measurements of the interstellar atoms one necessarily needs to take into account effects of the heliospheric interface. In this paper we propose a new model that is relatively simple but takes into account all major effects of the heliospheric interface. This model can be applied for analysis of backscattered La-alpha radiation data obtained on board of different spacecraft.     

On the model of the solar wind-interstellar medium interaction with two shock waves

The model of the solar wind interaction with interstellar medium suggested by Baranovet al. (1970) is developed. In this model (TSM) the presence of two shock waves is assumed, through which the solar wind and interstellar gas pass, the latter moving relative to the Sun at supersonic speed (20 km s^–1).The distance between shocks was considered earlier (Baranovet al., 1970; Baranov and Krasnobaev, 1971) to be small compared with their distance from the Sun, due to the hypersonic character of the flow. The structure of the subsonic flow portion may not be taken into account.In the present paper the distribution of the gas parameters in the region between shocks is calculated which, in particular, allows us to estimate the possibility of its experimental detection, observing radio-scintillation on interstellar irregularities (Baranovet al., 1975).The possible influence on the model of galactic hydrogen neutral atoms penetrating into interplanetary medium is estimated.     

Possible mechanism of formation of interstellar clouds

A possible mechanism of formation of interstellar clouds in the Galaxy by aggregation from gas “cloudlets” ejected by red giants is considered. A numerical model of the motion of a “cloudlet” in the interstellar medium of the Galaxy is constructed. The resulting clouds are fractal with a fractal dimensionality that coincides with the observed one. The characteristic time of formation of a cloud agrees with observations. Translated from Astrofizika, Vol. 43, No. 2, pp. 229–237, April–June, 2000.     

DuneXpress

The DuneXpress observatory will characterize interstellar and interplanetary dust in-situ, in order to provide crucial information not achievable with remote sensing astronomical methods. Galactic interstellar dust constitutes the solid phase of matter from which stars and planetary systems form. Interplanetary dust, from comets and asteroids, represents remnant material from bodies at different stages of early solar system evolution. Thus, studies of interstellar and interplanetary dust with DuneXpress in Earth orbit will provide a comparison between the composition of the interstellar medium and primitive planetary objects. Hence DuneXpress will provide insights into the physical conditions during planetary system formation. This comparison of interstellar and interplanetary dust addresses directly themes of highest priority in astrophysics and solar system science, which are described in ESA’s Cosmic Vision. The discoveries of interstellar dust in the outer and inner solar system during the last decade suggest an innovative approach to the characterization of cosmic dust. DuneXpress establishes the next logical step beyond NASA’s Stardust mission, with four major advancements in cosmic dust research: (1) analysis of the elemental and isotopic composition of individual interstellar grains passing through the solar system, (2) determination of the size distribution of interstellar dust at 1 AU from 10^− 14 to 10^− 9 g, (3) characterization of the interstellar dust flow through the planetary system, (4) establish the interrelation of interplanetary dust with comets and asteroids. Additionally, in supporting the dust science objectives, DuneXpress will characterize dust charging in the solar wind and in the Earth’s magnetotail. The science payload consists of two dust telescopes of a total of 0.1 m² sensitive area, three dust cameras totaling 0.4 m² sensitive area, and a nano-dust detector. The dust telescopes measure high-resolution mass spectra of both positive and negative ions released upon impact of dust particles. The dust cameras employ different detection methods and are optimized for (1) large area impact detection and trajectory analysis of submicron sized and larger dust grains, (2) the determination of physical properties, such as flux, mass, speed, and electrical charge. A nano-dust detector searches for nanometer-sized dust particles in interplanetary space. A plasma monitor supports the dust charge measurements, thereby, providing additional information on the dust particles. About 1,000 grains are expected to be recorded by this payload every year, with 20% of these grains providing elemental composition. During the mission submicron to micron-sized interstellar grains are expected to be recorded in statistically significant numbers. DuneXpress will open a new window to dusty universe that will provide unprecedented information on cosmic dust and on the objects from which it is derived.     

On the role of stochastic Fermi acceleration in setting the dissipation scale of turbulence in the interstellar medium

We consider the dissipation by Fermi acceleration of magnetosonic turbulence in the Reynolds layer of the interstellar medium. The scale in the cascade at which electron acceleration via stochastic Fermi acceleration (STFA) becomes comparable to further cascade of the turbulence defines the inner scale. For any magnetic turbulent spectra equal to or shallower than Goldreich–Sridhar this turns out to be ≥10¹² cm, which is much larger than the shortest length-scales observed in radio scintillation measurements. While STFA for such spectra then contradict models of scintillation which appeal directly to an extended, continuous turbulent cascade, such a separation of scales is consistent with the recent work of Boldyrev & Gwinn and Boldyrev & Konigl suggesting that interstellar scintillation may result from the passage of radio waves through the Galactic distribution of thin ionized boundary surfaces of H  ii regions, rather than density variations from cascading turbulence. The presence of STFA dissipation also provides a mechanism for the non-ionizing heat source observed in the Reynolds layer of the interstellar medium. STFA accommodates the proper heating power, and the input energy is rapidly thermalized within the low-density Reynolds layer plasma.     

Absorption bands and extinction in small interstellar particles

The production of discrete line and broad-band extinction by small interstellar oxide and silicate particles is discussed quantitatively. Restrictions on particle size and refractive index that are required to produce ‘pure’ absorption features are reviewed. The relationship between optical depth in interstellar extinction and absorption coefficients for bulk materials is used to reach some general conclusions concerning the diffuse interstellar features, VUV extinction and the composition of interstellar dust. It is noted that charge transfer bands of ions such as Fe³⁺ may be detectable in the VUV spectrum of dust. Several effects that lead to the enhancement of oscillator strength by 10³–10⁴ in small particles are discussed.     

Reaction of a uniform astrophysical plasma with a magnetic field to an electric disturbance

A uniform collisionless interstellar or intergalactic medium with a constant magnetic field is considered. The reaction of this medium to a slowly varying electromagnetic field from external sources or generated in the medium itself is determined using an improved method which is more compact than the existing methods. Convenient formulas are obtained for determining the reaction of an interstellar or intergalactic medium in various wave number and frequency ranges and the characteristics of magnetoacoustic waves are discussed. __________ Translated from Astrofizika, Vol. 50, No. 4, pp. 633–640 (November 2007).     

Electromagnetic instability of partially overlapping flows in the interstellar medium

The stability of the charged particle velocity distributions resulting from intermixing of neighboring flows of interstellar medium are examined, in particular, within the confines of spiral galaxies. It is shown for a specific example that the customary electrostatic instability shows up when the difference in the systematic velocities of both flows is sufficiently large compared to the thermal speed. On the other hand, the electromagnetic instability occurs, in principle, for an arbitrarily small difference in the flow velocities. The characteristic time for this instability to develop is much longer than for the electrostatic instability, but is still short compared to the ordinary evolution time for the interstellar medium. __________ Translated from Astrofizika, Vol. 49, No. 4, pp.637–649 (November 2006).     

A search for interstellar meteoroids using the Canadian Meteor Orbit Radar (CMOR)

Using the CMOR system, a search was conducted through 2.5 years (more than 1.5 million orbits) of archived data for meteoroids having unbound hyperbolic orbits around the Sun. Making use of the fact that each echo has an individually measured error, we were able to apply a cut-off for heliocentric speeds both more than two, and three standard deviations above the parabolic limit as our main selection criterion. CMOR has a minimum detectable particle radius near 100 μm for interstellar meteoroids. While these sizes are much larger than reported by the radar detections of extrasolar meteoroids by AMOR or Arecibo, the interstellar meteoroid population at these sizes would be of great astrophysical interest as such particles are more likely to remain unperturbed by external forces found in the interstellar medium, and thus, more likely to be traceable to their original source regions. It was found that a lower limit of approximately 0.0008% of the echoes (for the 3σ case) were of possible interstellar origin. For our effective limiting mass of 1×10⁻⁸ kg, this represents a flux of meteoroids arriving at the Earth of 6×10⁻⁶ meteoroids/km²/h. For our 2σ results, the lower limit was 0.003%, with a flux of 2×10⁻⁵ meteoroids/km²/h. The total number of events was too low to be statistically meaningful in determining any temporal or directional variations.     

Low density ionized gas in the inner galaxy — Interpretation of recombination line observations at 325 MHz

The recent survey of H 272α recombination line (324.99 MHz) in the direction of 34 Hn regions, 12 SNRs and 6 regions of continuum minimum (‘blank’ regions) in the galactic plane is used to derive the properties of diffuse ionized gas in the inner Galaxy. The intensity of radio recombination lines at high frequencies is dominated by spontaneous emission in high-density gas and that at low frequencies (325 MHz) by stimulated emission in low-density gas. We have used this property to obtain the electron density in the gas in the direction of blank regions and SNRs, by combining the H 272 α measurements (preceeding paper) with the published data at higher frequencies. Further, we have imposed constraints on the electron temperature and pathlength through this gas using the observed high-frequency continuum emission, average interstellar electron density and geometry of the line-emitting regions. The derived properties of the gas are (i) electron density 0.5–6 cm^-3, (ii) electron temperature 3000–8000 K and (iii) emission measures 500–3000 pc cm^-6 The corresponding pathlengths are 50–200 pc. As the derived sizes of the low-density regions are small compared to the pathlength through the Galaxy, the low-frequency recombination lines cannot be considered as coming from a widely distributed component of the interstellar medium. The Hn regions studied in the above survey cannot themselves produce the H 272α lines detected towards them because of pressure broadening, optical depth, and beam dilution. However, the agreement in velocity of these lines with those seen at higher frequencies suggests that the low-frequency recombination lines arise in low-density envelopes of the Hn regions. Assuming that the temperature of the envelopes are similar to those of the cores and invoking geometrical considerations we find that these envelopes should have electron densities in the range 1–10 cm^-3 and linear sizes of 30–300 pc in order to produce the observed H 272α lines.     

An Observation of Diffuse Soft X-Ray Background and Distribution of Hot Interstellar Medium

The diffuse soft X-ray background in 0.07∼ 2.0 keV has been observed along a small circle with the angular width of 10– covering b=—57– ∼ 77– and crossing the galactic plane at l=125– and 283–. Observed spectra are well fitted with models emitted from a thin hot gas with two or multi-temperature components and attenuated by a slab or interspersed neutral gas clouds. However the multi-temperature component model has some difficulties to explain the diffuse component expected from observations of OVI absorption lines. The two temperature component model with interspersed clouds consistently interprets a physical state of the hot interstellar medium. It turned out that the hot interstellar medium interspersed with neutral clouds of a representative hydrogen column density of 1× 10²⁰ cm^-2has two temperature components (10^5.8 K,(1.2-4.5)× 10⁶ K) and a disk-like distribution with the thickness of 580(± 100). {(p/k)/10⁴cm^-3 K} ^-2 pc at the pressure of p/k cm^-3 K along the galactic plane. Its pressure (p/k) is obtained to be 1.4(+0.7,-0.5)× 10⁴cm^-3 K, assuming that hot gases responsible for the diffuse soft X-rays and OVI ions are in pressure equilibrium. This interpretation qualitatively agrees with the theoretical prediction proposed by McKee and Ostriker. This revised version was published online in July 2006 with corrections to the Cover Date.     

Diffuse Interstellar Band at 862 nm as a Reddening Tracer for GAIA

We investigate the plausibility of using diffuse interstellar band at862 nm for tracing interstellar extinction with the ESA's astrometric space mission GAIA. For this purpose we perform numerical tests to simulate the conditions of real observations, covering a wide range of stellar parameters and different amounts of interstellar extinction. Our simulations indicate that with the present Radial Velocity Spectrometer setup the uncertainty in color excess of σ_E(B-V)≤ 0.05 can be achieved only for the interstellar reddening tracers brighter than V ∼ 13. None of the plausible tracers can provide accurate color excesses (σ _E(B-V) ≤ 0.05) at the distances beyond 2 kpc. We therefore conclude that with the currently planned instrumentation onboard GAIA this method can not be used as a stand-alone approach for probing interstellar extinction on the Galactic distance scales within the framework of the GAIA mission. This revised version was published online in July 2006 with corrections to the Cover Date.     

A Model for Interstellar Extinction

From an analysis of the interstellar extinction we conclude that interstellar grains are of three main kinds: graphite spheres of radii ∼ 0.02 μmmaking up ∼ ;10% of the total grain mass, small dielectric spheres of radius about 0.04 μm making up ∼ 25% of the mass, and hollow dielectric cylinders containing metallic iron with diameters of ∼ 2/3 μmmaking up ∼ 45% of the mass. The remaining ∼ 20%consists of other metals and metal oxides. The main dielectric component of the grains appears to be comprised of organic material. This revised version was published online in July 2006 with corrections to the Cover Date.     

Diffusion of cosmic rays in a multiphase interstellar medium swept-up by a supernova remnant blast wave

Supernova remnants (SNRs) are one of the most energetic astrophysical events and are thought to be the dominant source of Galactic cosmic rays (CRs). A recent report on observations from the Fermi satellite has shown a signature of pion decay in the gamma-ray spectra of SNRs. This provides strong evidence that high-energy protons are accelerated in SNRs. The actual gamma-ray emission from pion decay should depend on the diffusion of CRs in the interstellar medium. In order to quantitatively analyse the diffusion of high-energy CRs from acceleration sites, we have performed test particle numerical simulations of CR protons using a three-dimensional magnetohydrodynamics (MHD) simulation of an interstellar medium swept-up by a blast wave. We analyse the diffusion of CRs at a length scale of order a few pc in our simulated SNR, and find the diffusion of CRs is precisely described by a Bohm diffusion, which is required for efficient acceleration at least for particles with energies above 30 TeV for a realistic interstellar medium. Although we find the possibility of a superdiffusive process (travel distance ∝ t^0.75) in our simulations, its effect on CR diffusion at the length scale of the turbulence in the SNR is limited.     

Test of the hypothesis of circular motion of interstellar clouds

The study of the motion of interstellar clouds by direct dynamical methods is hindered by the lack of a complete set of initial conditions of the motion. The hypothesis of the circular motion of interstellar matter of various composition is tested on the basis of the catalog of Brand and Blitz. A modification of the method proposed by Edmondson and Hoerner, which was successfully used for a statistical investigation of the shapes of the orbits of globular clusters, is used. It is established that interstellar clouds move in nearly circular orbits with small deviations, so that data on the spiral structure of the Galaxy obtained by radio-astronomical methods should be considered as close to reality. Translated from Astrofizika, Vol. 43, No. 2, pp. 239–246, April-June, 2000.