A probability of encounter with interstellar comets and the likelihood of their existence

A theory of the probability of encounter of the Sun with an interstellar comet at a distance comparable to the Earth-Sun distance is formulated, and a general expression is derived establishing the relationship among the influx rate of interstellar comets, the perihelion distance, the space density of the comets, the Maxwellian distribution of comet velocities in the interstellar cloud, and the cloud's systematic velocity relative to the Sun. The fact that no comet with a strongly hyperbolic orbit has so far been observed is used to determine an upper limit of 6 × 10^?4 solar masses per cubic parsec (4 × 10^?26 gcm^?3) for the space density of interstellar comets. The theoretical distribution of semimajor axes of interstellar comets is derived to show that a strong hyperbolic excess must be present in the orbits of a majority of interstellar comets regardless of the dynamical characteristics of the comet cloud, except when the cloud is moving along with the Sun and the distribution of individual velocities has a very low dispersion. This case, however, implies a possibility of capture by the Sun and thus becomes a problem of an Oort-type cloud.     

Charge transfer reactions at interfaces between neutral gas and plasma: Dynamical effects and X‐ray emission

Charge‐transfer is the main process linking neutrals and charged particles in the interaction regions of neutral (or partly ionized) gas with a plasma. In this paper we illustrate the importance of charge‐transfer with respect to the dynamics and the structure of neutral gas‐plasma interfaces. We consider the following phenomena: (1) the heliospheric interface ‐ region where the solar wind plasma interacts with the partly‐ionized local interstellar medium (LISM) and (2) neutral interstellar clouds embedded in a hot, tenuous plasma such as the million degree gas that fills the so‐called “Local Bubble”. In (1), we discuss several effects in the outer heliosphere caused by charge exchange of interstellar neutral atoms and plasma protons. In (2) we describe the role of charge exchange in the formation of a transition region between the cloud and the surrounding plasma based on a two‐component model of the cloud‐plasma interaction. In the model the cloud consists of relatively cold and dense atomic hydrogen gas, surrounded by hot, low density, fully ionized plasma. We discuss the structure of the cloud‐plasma interface and the effect of charge exchange on the lifetime of interstellar clouds. Charge transfer between neutral atoms and minor ions in the plasma produces X‐ray emission. Assuming standard abundances of minor ions in the hot gas surrounding the cold interstellar cloud, we estimate the X‐ray emissivity consecutive to the charge transfer reactions. Our model shows that the charge‐transfer X‐ray emission from the neutral cloud‐plasma interface may be comparable to the diffuse thermal X‐ray emission from the million degree gas cavity itself (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the angular momentum in star formation

We discuss the rotation of interstellar clouds which are in a stage immediately before star formation. Cloud collisions seem to be the principal cause of the observed rotation of interstellar clouds. The rotational motion of the clouds is strongly influenced by turbulence.Theories dealing with the resolution of the angular momentum problem in star formation are classified into five major groups. We develop the old idea that the angular momentum of an interstellar cloud passes during star formation into the angular momentum of double star systems and/or circumstellar clouds.It is suggested that a rotating gas cloud contracts into a ring-like structure which fragments into self-gravitating subcondensations. By collisions and gas accretion these subcondensations accrete into binary systems surrounded by circumstellar clouds. Using some rough approximations we find analytical expressions for the semi-major axis of the binary system and for the density of the circumstellar clouds as a function of the initial density and of the initial angular velocity of an interstellar cloud. The obtained values are well within the observational limits.     

Polyatomic molecules,molecular ions,and radicals of astrophysical interest,i

This paper has been devoted to basic molecular studies of polyatomics. A critical analysis of the 356 available references in literature has been made to select 98 polyatomic molecules, molecular ions, and radicals containing three, four, and five atoms of astrophysical significance. The results have been arranged in a text-cum-tabular form. The compilation contains various information for each molecule, such as the spectral region, transition levels, astrophysical objects, where the respective molecules have been detected (say, comet, meteorite, Sun, planet, star, interstellar matter, interstellar cloud, molecular cloud, interstellar space, Galaxy, etc.).A few important areas of active research in laboratory astrophysics have also been identified in this article: laboratory astrophysics, molecular cloud chemistry, isotopic abundance, planetary and cometary atmospheres through satellites. Seventy-five new polyatomic molecules (containing three, four, and five atoms) of astrophysical significance have also been listed.Astrophysics and Space Science Review Paper I     

Masses of interstellar clouds

Equilibrium masses of slowly rotating interstellar gas clouds have been calculated using an equation of state for the interstellar gas developed by Penston and Brown. The rotation is found to increase the equilibrium masses, but still the cloud masses are not as large as indicated by other considerations.     

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.     

Chemical abundances in cold, dark interstellar clouds

The Sun may well have formed in the type of interstellar cloud currently referred to as a cold, dark cloud. We present current tabulations of the totality of known interstellar molecules and of the subset which have been identified in cold clouds. Molecular abundances are given for two such clouds which show interesting chemical differences in spite of strong physical similarities, Taurus Molecular Cloud 1 (TMC-1) and Lynd's 134N (L134N, also referred to as L183). These regions may be at different evolutionary stages.     

Polyatomic molecules,molecular ions,and radicals of astrophysical interest,II

This paper has been devoted to a basic molecular studies of polyatomics. A critical analysis of the 70 available references in literature has been made to select 37 polyatomic molecules, molecular ions, and radicals containing six, seven, eight, nine, ten, eleven, twelve, and thirteen atoms of astrophysical significance. The results have been arranged in a text-cum-tabular form. The compilation contains various information for each molecule, such as the spectral region, transition levels, astrophysical objects where the respective molecules have been detected (say, comet, meteorite, Sun, planet, star, interstellar matter, interstellar cloud, molecular cloud, interstellar space, Galaxy, etc.). Two-hundred fourteen new polyatomic molecules (containing 6, 7, 8, 9, and 10 atoms) of astrophysical significance have also been listed.Astrophysics and Space Science Review Paper II.     

The dynamical effects of supernovae on the interstellar medium

An interstellar cloud suddenly overrun by a supernova blast wave experiences a very rapid increase in boundary pressure. A shock wave propagates into the cloud. As a preliminary investigation, the propagation of spherical shock waves in an adiabatic medium is studied numerically.     

Diffuse interstellar band formation in dense clouds

Measurements of the strengths of the diffuse interstellar bands at 4430, 5780 and 5797 Å show that the bands tend to be week with respect to extinction in dense interstellar clouds. Data on 10 stars in the ? Ophiuchi cloud complex show further that the diffuse band-producing efficiency of the grains decreases systematically with increasing grain size. It is concluded that the diffuse bands are not formed in the mantles which accrete on the grains in interstellar clouds, but that they could be produced in the cores of grains or in some molecular species.     

Nonstationary Phenomena in the Solar Wind and InterstellarMedium Interaction

We discuss the results of numerical modeling of the solar wind with the inhomogeneous interstellar medium. The density of the plasma component in the interstellar cloud is supposed to be space periodic. The interaction pattern is shown to be highly unsteady with hydrodynamic instabilities developing on the side portion of the heliopause. This revised version was published online in July 2006 with corrections to the Cover Date.     

Three components of 3–4 μm absorption bands

We present 2–4 μm spectra of six infrared sources for which the extinction is not purely interstellar, but is dominated by circumstellar or molecular cloud dust. In all cases the absorption bands differ from the interstellar case, though a component of the interstellar absorption is often present. We also present an improved absorption spectrum for the galactic centre source IRS 7, correcting spurious features in our previous spectrum. Three independent components can be identified: (i) The interstellar component, probably of organic origin, and itself not necessarily invariant; (ii) The symmetrical water ice feature at 3.06 μm, found most commonly in molecular clouds; (iii) A component at 3.53 μm possibly identified with solid formaldehyde grains, and seen only in molecular clouds because of its weakness. Other absorption components appear to be unrelated to those in the 3–4 μm region, most notably the 10 μm absorption found in oxygen-rich giants and the interstellar medium, and presumable inorganic in nature. Our observations include the first detection of water ice absorption in a source in the ρ Oph dark cloud. Biological materials provide the best fit to the interstellar case, but do not presently account for the distinct 3.53 μm component. We stress the need for further laboratory experiments using simpler organic materials.     

The formation of heavy hydrocarbons in molecular clouds

Laboratory data on the conversion of solid methane into large hydrocarbons by particle radiation are used to estimate the fraction of interstellar carbon converted by this process into refractory form. We find that the maximum fraction of carbon that can be converted into refractory form during the life of a dense core within an interstellar cloud is in the range of 1–5 per cent. The implication of this result is that the conversion of enough carbon into refractory form to contribute significantly to interstellar extinction requires the frequent passage of material into and out of dense cores. If so, then interstellar clouds must exist for at least 10 Myr. However, these conclusions should be regarded as preliminary until confirmed by further laboratory studies of the particle irradiation of complex ice mixtures.     

Tidal gravitational forces: The infall of “new” comets and comet showers

The tidal gravitational field of the Galaxy directed into the galactic plane changes the angular momentum of comets in the Oort cloud. For comet orbits with semimajor axis greater than 2 × 10⁴ AU, the change of angular momentum in one orbit is sufficient to bring comets from the Oort cloud into the visible region, causing the infall of “new” comets. The limiting size orbit is weakly dependent on the angle between the major axis of the comet orbit and the galactic plane. The flux of comets into the inner Solar System caused by the galactic tidal field will be continuous and nearly isotropic. This effect appears to exclude any determination of the trajectories of passing stars by analysis of the angular distribution of new comets. The production of intense comet showers by the tidal field of a solar companion or of an interstellar cloud is considered. We show that the direction of a solar companion cannot be found from the present distribution of observable comets. The frequency of comet showers induced by encounters with interstellar clouds is found to be much lower than that from passing stars, and the tidal fields of interstellar clouds are not strong enough to cause comet showers of sufficient intensity to result in Earth impacts.     

Mass-Stripping Analysis of an Interstellar Cloud by a Supernova Shock

The interaction of supernova shocks and interstellar clouds is an important astrophysical phenomenon since it can result in stellar and planetary formation. Our experiments attempt to simulate this mass-loading as it occurs when a shock passes through interstellar clouds. We drive a strong shock using the Omega laser (∼5kJ)into a foam-filled cylinder with an embedded Al sphere(diameterD=120 μm) simulating an interstellar cloud. The density ratio between Al and foamis∼9. We have previously reported on the interaction between shock and cloud, the ensuing Kelvin-Helmholtz and Widnall instabilities, and the rapid stripping of all mass from the cloud. We now present a theory that explains the rapid mass-stripping. The theory combines (1) the integral momentum equations for a viscous boundary layer, (2) the equations for a potential flow past a sphere, (3) Spalding's law of the wall for turbulent boundary layers, and (4) the skin friction coefficient for a turbulent boundary layer on a flat plate. The theory gives as its final result the mass stripped from a sphere in a turbulent high Reynolds number flow, and it agrees very well with our experimental observations.     

Polarization in the young cluster NGC 6611: circumstellar, interstellar, or ... both?

We obtained linear polarization observations of 82 A/B-type stars in the young cluster NGC 6611, in order to probe the circumstellar material and to search for any evidence of intracluster or interstellar material that could also contribute to the polarization. We found linear polarization values that reach up to 14%. We consider the distribution of the polarization, its position angle, correlations with extinction and membership probability, polarization variability and wavelength distribution to identify the origin of the polarization toward NGC 6611. The polarization is found to be dominated by interstellar polarization, although some stars also have some circumstellar polarization. There is no evidence for intracluster dust. Rather, the dust must be located in a low density cloud toward the general line of sight to NGC 6611 and in front of it. The depth of that cloud along the line of sight increases slowly from the south–east to the north–west. The cloud is threaded by a very uniform magnetic field.     

Wave motions in magnetic interstellar medium

This communication considers the continuum approach modelling of large-scale dynamics of a nonconducting interstellar medium capable of sustaining long-ranged filamentary agglomeration of tiny superparamagnetic grains suspended in a dense molecular cloud. The filamentary ordering of permanently magnetized grains, oriented in the direction of the regular galactic field threading the cloud, is thought of as an effect of soft magnetic solidification of a nonconducting gas-dust substance imparting to the interstellar material the mechanical features of single-axis magnetoelastic insulators. With this physical picture in mind, we set up macroscopic equations to study the dissipative-free motions of superparamagnetic gas-dust nonionized matter in terms of continuum mechanics magnetoelastic materials. Particular attention is given to oscillatory behavior in the regime of strong magnetization-flow coupling. The most remarkable inference of this model is that nonconducting magnetically polarized interstellar medium can transmit perturbations by transverse waves of magnetization which can be regarded as a counterpart of Alfvén waves generic to cosmic dusty plasma. Published in Astrofizika, Vol. 43. No. 3, pp. 405-410, July–September, 2000.     

Vibrationally excited molecular hydrogen in circumstellar clouds and the interstellar CH+ abundance

The production of CH⁺ in dense interstellar clouds under intense UV irradiation is discussed. A model applicable to the cloud towards the star 20 Tau is described.     

Possible mechanism for limiting grain sizes

It is suggested that grains are driven out of interstellar clouds by galactic radiation pressure and it is shown that the corresponding sizes which they can achieve within the cloud due to condensation is in reasonable agreement with observations.     

Collisionless Slowing Down of Nova and Supernova Shells in Magnetized Interstellar Medium

The collisionless interaction of an expanding plasma cloud with a magnetized background plasma is examined in the framework of a 3D kinetic-hydrodynamic model. The slowing down of a hydrogen cloud is studied for high Alfven-Mach numbers and magneto-laminar interaction parameters. A particle-in-cell method is used to study the dynamics of the magnetic field, plasma cloud, background plasma, and collisionless shock wave generated by the intense particle flux. A numerical simulation is consistent with the nonstationary interactions between the plasma shells formed during nova and supernova explosions and the interstellar plasma medium.