On the state of ionization of the local interstellar medium

The abundance ratio of neutral hydrogen to helium, as deduced from interplanetary observations of Lyman-alpha and He 584 Å radiation by Mariner 10, is significantly lower than the cosmic abundance ratio of these elements, thus showing that the local interstellar medium (LISM) is partly ionized. The effect of various sources of ionization — especially electron impact and EUV photon flux — on hydrogen and helium is discussed. It is shown that the observational data on the temperature of the LISM, on the diffuse EUV flux and on the neutral and electron densities in the nearby interstellar medium (NISM) are not all compatible. However, if the diffuse EUV flux below 912 Å as deduced from the preliminary analysis of Voyager observations is not representative, then it is easy to reconcile all observations. In this case an important source of ionization of the LISM would be electron impact, yielding an ionization degree of about 50% for the hydrogen component.     

Interaction of a nonstationary ionization-shock front with an inhomogeneous interstellar medium

We consider the interaction of an ionization-shock front with isothermal waves in an H I region. Based on a two-front model in the linear approximation, we have detected a significant (approximately by an order of magnitude) periodic amplification of perturbations as they penetrate from the interstellar medium into an H II region. Numerical simulations have revealed that even when the shock-and ionization-front velocities differ and the relative density perturbations in the interstellar medium are ～10^?1, the variations in gas parameters near the ionization front are comparable to those observed at the boundaries of H II regions.     

Revised interstellar neutral helium/hydrogen density ratios and the interstellar UV-radiation field

The data deduced from the UV-spectroscope on theCopernicus satellite strongly suggest that the most important ionization source in interstellar space near the solar system is a UV radiation field originating from B-stars. Adopting this hypothesis, we have used the ionization state of several elements in the interstellar medium observed byCopernicus to determine the required radiation field. From this, the degree of ionization of elements that could not be observed byCopernicus is estimated.It is shown that this interpretation of thecopernicus data can be made consistent with neutral interstellar hydrogen densities inferred from extraterrestrial L observations and with electron densities deduced from pulsar dispersion measures. Furthermore, it is shown that the ratio of neutral interstellar helium to neutral interstellar hydrogen is likely to be 2 to 3 times as large as the cosmic abundance ratio of these elements. The possibility that this ratio is about 10 times as large, meaning equal interstellar neutral hydrogen and helium densities near the solar system, cannot be ruled out. It would, however, require an interstellar radiation temperature near 9000 K. A comparison of the intensity of the interplanetary back scattered He 584 Å and the H 1216 Å radiation would lead to a direct determination of this ratio provided the solar radiation at these lines is known.     

Interstellar matter and the location of the shock front

The initially supersonic flow of the solar wind passes through a magnetic shock front where its velocity is supposed to be reduced to subsonic values. The location of this shock front is primarily determined by the energy density of the external interstellar magnetic field and the momentum density of the solar wind plasma. Interstellar hydrogen penetrating into the heliosphere undergoes charge exchange processes with the solar wind protons and ionization processes by the solar EUV radiation. This results in an extraction of momentum from the solar wind plasma. Changes of the geometry and the location of the shock front due to this interaction are studied in detail and it is shown that the distance of the magnetic shock front from the Sun decreases from 200 to 80 AU for an increase of the interstellar hydrogen density from 0.1 to 1.0 cm⁻³. The geometry of the shock front is essentially spherical with a pronounced embayment in the direction opposite to the approach of interstellar matter which depends very much on the temperature of the interstellar gas. Due to the energy loss by the interaction with neutral matter the solar wind plasma reduces its velocity with increasing distance from the Sun. This modifies Parker's solution of a constant solar wind velocity.     

Unreddened stars and the two-phase model of the interstellar medium

We have computed two phase models of the interstellar medium, with cosmic rays and X-rays assumed to be the main ionizing agents, heating due to photoelectron ejection from the interstellar grains. We show that it is possible to have a hot and tenuous intercloud medium in pressure equilibrium with the interstellar clouds for a wide range of physical conditions, possibly existing in the interstellar space. The atomic and ionic line observations towards Sco are shown to be consistent with the origin of these lines in the intercloud medium for a range of values of the ionizing flux. It is suggested that the intercloud medium may be predominantly neutral, with ionization rates consistent with the limits imposed by molecular observations. The mean fractional ionization of the intercloud medium is 1%.On leave from Tata Institute of Fundamental Research, Bombay, India.     

Numerical simulation of unstable two-dimensional motions of a circumstellar shell

The growth of two-dimensional axisymmetric perturbations in the motion of a neutral shell formed in the interstellar medium when an ionization-shock front exits at the surface of a cloud is simulated numerically. The perturbations are assumed to emerge when the shock ahead of the ionization front reaches the cloud boundary. For long-wavelength perturbations, the accumulation of mass has been found to take place in radially oriented condensations in the shape of a rod pointed toward the star and widened at the opposite end as a result of instability. The shell fragmentation is accompanied by supersonic spouting of a hot plasma into a low-density medium. Flow nonstationarity is shown to affect significantly the gas density and velocity distributions both inside and in the immediate vicinity of the condensation. As one recedes from the ionization front, the density of charged particles changes only slightly, which is inconsistent with the power law of density decrease with increasing distance from the condensation center commonly used in interpreting observations.     

Fluctuations in the heliospheric hydrogen distribution induced by generalized and time-dependent interstellar boundary conditions

It is well known that the neutral component of the local interstellar medium can effectively pass through the plasma interface ahead of the solar system and can penetrate deeply into the inner heliosphere. Here we present a newly-developed theoretical approach to describe the distribution function of LISM neutral hydrogen in the heliosphere, also taking into account time-dependent solar and interstellar boundary conditions. For this purpose we start from a Boltzmann-Vlasov equation, Fourier-transformed with respect to space and time coordinates, in connection with correspondingly transformed solar radiation forces and ionization rates, and then arrive at semi-analytic solutions for the transformed hydrogen velocity distribution function. As interstellar boundary conditions we allow for very general, non-Maxwellian and time-dependent distribution functions to account for the case that some LISM turbulence patterns or nonlinear wave-like shock structures pass over the solar system. We consider this theoretical approach to be an ideal instrument for the synoptic interpretation of huge data samples on interplanetary Ly- resonance glow intensities registered from different celestial directions over extended periods of time. In addition we feel that the theoretical approach presented here, when applied to interplanetary resonance glow data, may permit the detection of genuine fluctuations in the local interstellar medium.     

On the ionization of interstellar magnesium

It has been shown that two concentric ionization zones of interstellar magnesium must exist around each star: internal, with a radius coinciding with that of the zone of hydrogen ionizationS _H; and external, with a radius greater thanS _H, by one order. Unlike interstellar hydrogen, interstellar magnesium is ionized throughout the Galaxy. It also transpires that the ionizing radiation of ordinary hot stars cannot provide for the observed high degree of ionization of interstellar magnesium. The discrepance can be eliminated by assuming the existence of circumstellar clouds or additional ionization sources of interstellar magnesium (X-ray background radiation, high-energy particles, etc.). Stars of the B5 and B0 class play the main role in the formation of ionization zones of interstellar magnesium; the contribution of O class stars is negligible (<1%).     

Weakly ionized cosmic gas: ionization and characterization

Since collective plasma behavior may determine important transport processes (e.g., plasma diffusion across a magnetic field) in certain cosmic environments, it is important to delineate the parameter space in which weakly ionized cosmic gases may be characterized as plasmas. In this short note, we do so. First, we use values for the ionization fraction given in the literature, wherein the ionization is generally assumed to be due primarily to ionization by cosmic rays. We also discuss an additional mechanism for ionization in such environments, namely, the photoelectric emission of electrons from cosmic dust grains in an interstellar FUV radiation field. Simple estimates suggest that under certain conditions this mechanism may dominate cosmic ray ionization, and possibly also the photoionization of metal atoms by the interstellar FUV field, and thereby lead to an enhanced ionization level.     

Velocity of the local interstellar medium relative to the sun from interstellar helium flux measurements onboard the Ulysses and IBEX spacecraft

The question of determining the relative velocity of the local interstellar medium (LISM) based on direct interstellar helium flux measurements in the Solar system is considered. Such measurements were made onboard the Ulysses spacecraft in 1990–2007 at a distance of 2–5 AU from the Sun and have been made from 2009 to the present day onboard the Interstellar Boundary Explorer (IBEX) spacecraft at the Earth’s orbit. Recent works on analyzing the IBEX measurements have shown that the LISM velocity relative to the Sun determined from the IBEX data differs in magnitude (by ≈3 km s^?1) and direction (by ≈4°) from the LISM velocity obtained previously by Witte based on Ulysses measurements. We have modeled the Ulysses data (including the 2007 data that have not been considered previously by anybody) by taking into account various LISM velocity vectors and compare our numerical simulations with experimental data. The LISM velocity vector derived from the IBEX data is shown to contradict the Ulysses data in the position of the measured interstellar helium flux maximum on the sky map. In addition, the position of the flux maximum is shown to be determined exclusively by the LISM velocity vector and to be independent of other model parameters (the LISM temperature and ionization rate). This means that the Ulysses data (including the 2007 data obtained only two years before the IBEX measurements) cannot be explained in terms of the existing models with the LISM velocity vector from the IBEX data. Possible reasons for the detected contradictions are discussed.     

Synchrotron X-ray emission from supernova remnants. Exponential cut-off in the electron spectrum

We present a model which describes the evolution of the energy spectrum of relativistic electrons in supernova remnants, with radiation losses of electrons taken into account. The model can be used to calculate the synchrotron X-ray emission from supernova remnants in the uniform interstellar medium and in the uniform interstellar magnetic field. The importance of various factors in the variations of spatial distributions of nonthermal electrons and their synchrotron emissive capacity is demonstrated. We analyze the errors which arise in the magnetic field strength when it is estimated with the use of the models which ignore the detailed pattern of the evolution of the magnetic field and the electron spectrum behind the shock front in the remnant. The evolution of synchrotron emission spectrum and the ratio between the synchrotron radio and X-ray fluxes from supernova remnants are calculated.     

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.     

Interstellar water and interstellar ice

There are two ways that water ice can form in the interstellar medium: H₂O molecules can form in the gas phase and then freeze out onto dust grain surfaces, or O and OH can be converted at the surfaces of grains to form H₂O, which is then retained. Bergin et al. (1998) have recently shown that shocks passing through interstellar clouds sufficiently frequently can make the first method effective. However, we present results from a similar chemical model which indicate that this requires significant optical shielding because of the high ionization fraction in regions exposedto a high UV flux. We deduce, therefore, that grain surface reactions probably represent the main source of H₂O ice on lines of sight with visual extinction up to about 6 magnitudes to an embedded source or 12 magnitudes to a background object. This revised version was published online in July 2006 with corrections to the Cover Date.     

Three-Dimensional Particle Acceleration in Electromagnetic Dominated Outflows with Background Plasma and Clump

The effect of background plasma on particle acceleration via Poynting fluxes is studied in 3D PIC simulation of electron-positron and electron-ion plasmas. When a strongly magnetized ejecta at the center expands to low-temperature electron-positron ambient plasma background and a low-density clump, electromagnetic wave front accelerates particles in the background and clump, and captures them in the Ponderomotive potential well. We do not observe any instability, and the momentum distributions of background and clump form a power law of slope close to −1.5 with a sharp peak in the middle. When an ejecta expands to the ion-electron interstellar medium (ISM), the acceleration via Poynting flux is severely damped due to the charge separation.     

Understanding 26Al Emission from Cygnus

The COMPTEL observations of the galactic 1.809 MeV emission attributed to the radioactive decay of ²⁶Al have confirmed the diffuse nature of this interstellar emission line. One of the most significant features of the reconstructed intensity pattern is a flux enhancement towards Cygnus. This region is fairly young and contains a wealth of massive stars, most of them grouped in the Cygnus OB associations. Multi-frequency model fitting strongly supports the hypothesis of massive stars and their descendent supernovae being the dominant sources of interstellar ²⁶Al as observed by COMPTEL. Massive stars and supernovae are known to impart a large amount of kinetic energy into their surroundings causing shock regions and large cavities in the ISM. In addition, a significant fraction of the electro-magnetic radiation of these stars is emitted in the EUV regime leading to photoionisation of the surrounding medium. We applied a population synthesis model in combination with an 1D model of expanding superbubbles to the Cygnus OB associations. Besides the expected 1.809 MeV flux and the γ-ray line intensity due to interstellar ⁶⁰Fe we compute the sizes and expansion parameters of the expected HI-structures and the free–free emission intensities due to the photoionizing radiation from massive stars within this region of the sky. We discuss our present understanding of the Cygnus region with respect to the massive star census. Our model assigns about 70% of the 1.809 MeV intensity to six known OB associations, about 20% to known isolated sources and roughly 10% to an unknown diffuse component.     

On the effect of fluctuating interstellar density on observed cosmic-ray intensities

The effects of a fluctuating interstellar density on the measured flux of cosmic-ray particles are examined within the framework of stochastic processes. The dispersion of the flux is given as a function of the characteristics of the interstellar medium. Both discrete cloud and fluctuating density field models are developed.     

The ATCA intraday variability survey of extragalactic radio sources

We present the results of an Australia Telescope Compact Array (ATCA) survey for intraday variability (IDV) of the total and polarized flux densities of 118 compact, flat-spectrum, extragalactic radio sources from the Parkes 2.7-GHz Survey. A total of 22 total flux density IDV sources were discovered and 15 sources were found to show IDV of their polarized flux density. We discuss the statistical properties of the IDV sources, including the distribution of source modulation indices, and the dependence of the variability amplitude on source spectral index and on Galactic position. We suggest interstellar scintillation (ISS) in the Galactic interstellar medium as the most likely mechanism for IDV. Even so, the inferred high brightness temperatures cannot be easily explained.     

Near-infrared and optical emission-line structure of the Keyhole Nebula in NGC 3372

Narrow-band infrared and optical images of the Keyhole Nebula in NGC 3372 reveal which structures are caused by extinction, and show the underlying morphology of photoionized and shock-excited gas. Dark clouds conspire with ionized gas to create the apparent keyhole shape, which is prominent at blue wavelengths and less apparent in the infrared. The  Pa β /H α   line ratio shows the spatial distribution of foreground extinction. The wavelength dependence of this extinction indicates a reddening law with   R ≈4.8  , different from the normal interstellar medium. This confirms previous estimates of reddening toward the Carina Nebula determined from stellar photometry, and reveals that the anomalous extinction is patchy and within the H  ii region. The morphology of the ionized gas is different from the extinction clouds; it shows an edge-on ionization front running NE to SW, with a limb-brightened indentation that forms the upper outline of the keyhole shape. A fast polar wind from η Carinae may have punctured the ionization front, since the indentation is directly along a projection of the polar axis of the star. This is supported by the morphology of shock-excited gas revealed by a high  [S  ii ]/H α   ratio. High-excitation gas emitting [O  iii ] and He  i has a smoother distribution. Molecular clumps in the region are also discussed.     

Interstellar pickup protons and solar wind heating in the outer heliosphere

The ionization of hydrogen atoms that penetrate into the heliosphere from the interstellar medium gives rise to a peculiar population of energetic protons (interstellar pickup protons) in the solar wind. The short-wavelength Alfvènic turbulence in the outer heliosphere is entirely attributable to the source associated with the instability of the initial anisotropic pickup proton velocity distribution. The bulk of the generated turbulent energy is subsequently absorbed by the pickup protons themselves through the cyclotron-resonance particle-wave interaction, and only an insignificant fraction of this energy can be transferred to the solar wind protons and heat them up.