Showing papers in "Astrophysics and Space Science in 1980"

Some Secondary Indications of Gravitational Collapse

Abstract: A stellar core becomes somewhat less massive due to neutrinos radiated away during its collapse in a neutron star or a black hole The paper deals with the hydrodynamic motion of stellar envelope induced by such a mass loss Depending on the structure of the outer stellar layers, the motion results either in ejection of an envelope with mass and energy proper for Nova outbursts; or nearly instantaneous excitation of strong pulsations of the star; or lastly in a slow slipping away of the whole stellar envelope These phenomena are of importance when more powerful events, like supernova outbursts presumably associated with gravitational collapse, are absent Such secondary indications of gravitational collapse are of special interest, since they may be a single observable manifestation (besides neutrinos and gravitational waves) of massive black hole formation

Extinction coefficients of amorphous carbon grains from 2100 Å to 340 μm

Abstract: Extinction measurements were made for three kinds of amorphous carbon grains in the range 2100 A-340 μm. Absolute values of extinction coefficients from different sources vary by almost constant factors. Wavelength dependences of extinction curves show a λ−1 or less steep fall off, against a λ−2 or steeper fall off of graphite grains. Small humps are found around 6.3, 8, 13.3 and 90 μm, although their origins are not yet clear. The infrared properties of amorphous carbon grains suggest that they could supply a significant amount of far-infrared emission noted in recent astronomical observations.

Origin of cosmic rays: Galactic models with halo

Abstract: Many years ago physical and radio-astronomical arguments and data led to the assumption that cosmic rays in the Galaxy (and probably in other galaxies) fill a more or less extended halo, but are not concentrated in the disk. It was not so long ago, however, that the existence of a radio-halo was discovered, in which the effective dimensions increase with a decrease in frequency. The frequency decrease occurs when relativistic electrons diffuse from the disk, losing energy due to bremsstrahlung and Compton scattering.

The influence of the plasma inhomogeneity on the critical velocity phenomenon.

Abstract: The phenomenon of a critical ionization velocity is discussed under the aspect of experimentally measured inhomogeneities. The difficulties inherent in homogeneous plasma models on the phenomenon are shown. A simple sheath model is presented which can be compared with the local plasma parameters measured in a rotating plasma device. The origin of the observed turbulent heating is attributed to a modified two-stream instability occurring in the sheath under discussion.

Quantum effects in cyclotron plasma absorption

Abstract: It is shown that X-ray radiation of neutron stars with magnetic fieldsB=1011–1013 G near cyclotron resonancesω=sωB(s=1,2,...) is deeply affected by such quantum effects as electron-positron vacuum polarization (significant at V=3×1028ne−1(B/BC4)≳1, whereBC=4.4×1013G), the quantizing character of the magnetic field (significant atV=3 x 1028ne−1(B/Bc)4≳1 whereBc=4.4 x 1013G), the non-harmonic character of the Landau levels, and the quantum recoil of electrons. The latter two factors shift the resonances by the frequency −s2ωB(B/2Bc)sin2ϑ, ϑ being the angle between the direction of radiation propagation and the magnetic field. IfV≪V0 (for ϑ∼1,V0∼β−1=(mc2/2T)1/2), the normal mode (NM) polarizations, as well as the absorption coefficientk1 of the extraordinary NM in the Doppler core of the first resonance (|ω−ω| ≲ωBβ cos ϑ), is only slightly affected by varyingb and/orV, whereas for the ordinary NM (at ϑ∼1)k2∼k1β2[b + (3 + tan2ϑ−2V)2]≪k1. For sufficiently largeb and/orV the quantum effects amplify resonant absorption of the ordinary NM atω∼ωB, with spin-flip transitions playing a major role atb≳1+V2. IfV∼V0, the coefficientsk1 andk2 in the Doppler core of the resonance are of the same order and acquire some peculiar features (shifts, intersections, etc.), with the NM polarizations depending sharply on ω and being strongly non-orthogonal. AtV≫V0,k2=k1(cos2 ϑ+B/2BC) and the polarizations are almost linear. Near high resonances (s≥2), as a rule,k1,2α(1 + b)s−1β2s−3 i.e., absorption increases withb due to replacement of the thermal energy of the transverse motion of electron,T, by the magnetic energyħωB. The above effects should be taken into account for an interpretation of observational data on X-ray pulsars (e.g., Her X-1) and other X-ray sources associated with neutron stars.

Amorphous interstellar grains: Wavelength dependence of far-infrared emission efficiency

Abstract: It is shown theoretically that the emission efficiency of amorphous grains with radii smaller than ∼100 A has a λ-1-dependence in the wavelength region longer than ∼100 μ, whereas those of crystalline, metallic, and larger amorphous grains are proportional to λ-2 in far infrared. Astrophysical implications of the amorphous grains are discussed.

The system VV Ori and the consistency of photometric analysis of eclipsing binary light curves

Abstract: The extensive, homogeneous and automatically obtained OAO-2UV data on VV Ori at six different wavelengths, as presented by Eaton (1975), are subjected to detailed numerical analysis, and the extent of derivable information on this star from such photometry is critically assessed.

Stokes problem for infinite vertical plate with constant heat flux

Abstract: An exact solution for the Stokes problem for an infinite vertical plate has been derived on taking into account the constant heat flux at the plate. It has been observed that the velocity of the fluid increases with increasingt (time) orG (the Grashof number).

Curvature radiation of relativistic particles in the magnetosphere of pulsars: I: Theory

Abstract: The curvature radiation of charged relativistic particles in a dipole magnetic field is considered, taking into account the possibility that the emitted particles may reabsorb radiation. The calculations were carried out for emanating particles with both monoenergetic and power energy spectra. The dependence of the curvature radiation flow on both the frequency and angle between the magnetic axis and the line of sight is determined. The calculated results will be used in the second part of the paper to interprete the observable data on pulsars.

Power-series solutions of the Lane-Emden equation

Abstract: In this paper the problem of representing the numerical solutions of the Lane-Emden equation analytically by means of a convergent power series has been considered. Our results show that it is possible to represent the numerical solutions of the Lane-Emden equation by means of a power series which can be convergent in the whole interior of a polytropic model.

Types I and II supernovae and the neutrino mechanism of thermonuclear explosion of degenerate carbon-oxygen stellar cores

Abstract: The present work studies the hydrodynamic process of thermonuclear explosion of hydrostatic equilibrium, degenerate carbon-oxygen cores withM C=1.40M ⊙ with different values of central densityϱ c within the interval 2 × 109 <ϱ c < 3 × 1010 g cm−3. The initial temperature distribution has been determined by the preceding thermal stage of explosion. The calculations successively include the kinetics of thermonuclear burning, the kinetics of β-processes, and neutrino energy losses. By considering the neutrino mechanism of heating and carbon ignition we obtained in our numerical hydrodynamic calculations two characteristic versions of the development of an explosion: (a) at 2 × 109 <ϱ c < 9 × 109 g cm−3 there is disruption of the whole star with either complete or partial burning of the carbon and a 1050–1051 erg kinetic energy; and (b) at 9 × 109 <ϱ c < 3 × 1010 g cm−3 the stellar core collapses into a neutron star with partial outburst of the outer envelope with a smaller kinetic energy of 1049–1050 erg. The paper proposes and details a hypothesis (the scenario of supernovae and the formation of neutron stars) on the first version of explosion, corresponding to SNII, and on the second, supplemented by some mechanism of slow energy release into the envelope expelled from the newly formed neutron star, corresponding to SNI. On the basis of the proposed hypothesis a satisfactory agreement with the observed masses and energies of the supernovae envelope, their light curves and spectra, as well as with the data on their chemical composition has been obtained. For this agreement we must assume that type I pre-supernovae are almost bare compact carbon-oxygen stellar cores, and that type II presupernovae are red supergiants. It is most probable that the evolution of type I pre-supernovae occurs in close binaries while the evolution of type II pre-supernovae seems to be very similar to the evolution of a single star.

Radiative heat transfer in surface layers of neutron stars with a magnetic field

Abstract: Thermal conductivity due to Thomson scattering and free-free absorption of photons is numerically evaluated for a non-relativistic non-degenerate plasma in a magnetic field for a number of values ofb =ħωB/kT ⩾ 1000. In the case of pure scattering, simple fitting formulae are derived. Atb≫6, the magnetic field is shown to decrease (by about one order) the characteristic densities above which heat transfer is mainly determined by free-free transitions.

Numerical Simulations of Collisions in Keplerian Systems

Abstract: Computer simulations which were carried out for Keplerian collisional systems of 250 frictionless particles with a ratio of particle radius to mean semi-major axis of 0001, confirm the theoretically predicted evolution very well until the thickness of the system is a few times the particle radius and the mass-point approximation becomes invalidated Before this happens, the collisional contraction of denserregions can be observed The local dispersions of the perihelia and ascending nodes diminish if the local mean orbit is not too close to a circle with zero inclination When the mass-point approximation ceases to be valid, the system begins to expand, but with parameter values of our standard system this process is much slower than the simultaneously observed evolution toward grazing collisions which do not affect the orbital elements Therefore, such systems are not dispersed into the space If the ratio of particle radius to semi-major axis is larger, the expansion becomes faster and the contraction ceases earlier In late evolutionary phases the thickness of the system remains essentially constant At the end of the longest simulation (70 000 impacts) the centres of the particles were in a layer of thickness twice the radius of the particles The cross-section of the system is often wave-like or irregular and may even include detached parts with their own mean plane Accordingly the thickness as derived from the root-mean-square inclination of the whole system exceeds the true local thickness The local dispersion of eccentricities may also be considerably smaller than the root-mean-square eccentricity of the whole system

Notes on the central forcer n

Abstract: In this article we collect several results related to the classical problem of two-dimensional motion of a particle in the field of a central force proportional to a real power of the distancer. At first we generalize Whittaker's result of the fourteen powers ofr which lead to intergrability with elliptic functions. We enumerate six more general potentials, including Whittaker's fourteen potentials as particular cases (Sections 2 and 3). Next, we study the stability of the circular solutions, which are the singular solutions of the problem, in Whittaker's terminology. The stability index is computed as a function of the exponentn and its properties are explained, especially in terms of bifurcations with other families of ordinary periodic solutions (Sections 4, 5 and 7). In Section 6, the detailed solution of the inverse cube force problem is given in terms of an auxiliary variable which is similar to the eccentric anomaly of the Kepler problem. Finally, it is shown that the stable singular circular solutions of the central force problem generalize to stable singular elliptic solutions of the two-fixed-center problem. The stability and the bifurcations with other families of periodic solutions of the two-fixed-center problem are also described.

Hydromagnetic free convection flow in the stokes problem for a porous vertical limiting surface with constant suction

Abstract: This paper provides a comprehensive analysis of the effects of a uniform transverse magnetic field on the free-convection flow of a viscous incompressible and electrically conductive fluid (e.g., of a stellar atmosphere) past an impulsively started, infinite, porous, vertical limiting surface (e.g., of a star) with a constant suction. The magnetic Reynolds number is assumed small so that the induced magnetic field is considered negligible. Exact solution of the equations governing the flow is obtained in closed form with the help of the Laplace transform technique when the Prandtl numberP=1. Expressions are given for the velocity field, for the temperature field and for their related quantities. The results thus obtained are discussed quantitatively in the last section of this paper.

Dynamical Theory of Collisionless Relaxation

Abstract: In his theory of violent relaxation, Lynden-Bell gave a rigorous derivation of the equilibrium distribution, but only a qualitative discussion of the manner in which equilibrium is attained Here we present a fully explicit dynamical theory of collisionless relaxation towards Lynden-Bel equilibrium.

On the relativistic motion of the periastron in the eclipsing binary system DI Herculis

Abstract: An analysis of fifteen-year photoelectric observations of DI Herculis shows that the upper estimate for the relativistic part of apsidal motion in this binary system is almost three times lower than the theoretical value. The total observed angular velocity of apsidal rotation is 0.°0124 yr−1. The relativistic part is less than 0.°008 yr−1 instead of 0.°023 yr−1 required by the theory.

The radiation of a hot magnetized plasma accreted by degenerate stars with a strong magnetic field

Abstract: The absorption coefficients for extraordinary and ordinary electromagnetic modes are found for a tenuous hot magnetized plasma, taking into account the collisions between plasma particles and the scattering of photons. An approach is suggested which generalizes ‘collisionless’ and ‘cold-plasma’ approximations. The simple formulae obtained are valid both near, and at a distance from, the cyclotron harmonics. In particular, the ordinary mode is shown to have resonance at the cyclotron frequency. The number of noticeable reasonances of absorption coefficient at cyclotron harmonics is estimated for both modes. Using the coefficients obtained, the intensity, Stokes parameters and polarization of radiation of a homogeneous plasma slab are calculated for conditions which may be realized in the heated regions of accreted plasma in an AM Herculis-type system. The large difference between the absorption coefficient of extra-ordinary and ordinary modes near the cyclotron harmonics may result in the emission of the broad polarized continuum together with the narrow cyclotron lines. The polarization of these lines has a complicated spectral dependence. The results obtained are shown to be useful for explaining the main properties of AM Herculistype objects.

Hill's problem: Families of three-dimensional periodic orbits (part I)

Abstract: Starting from the vertical critical periodic orbits of the planar Hill's problem we computed five families of three-dimensional periodic orbits. It is found that their behaviour is qualitatively similar to that of the corresponding families of the restricted problem.

OJ 287: Polarization and photometric behaviour during 1971 76

Abstract: The results are given of polarimetric and photometric observations of BL Lacertae-type object OJ 287 for 1972–76. These, and all other data available from the literature, are used in a comparative analysis of polarimetric and photometric properties of the object. The variations of time-scales from several years to several hours are noted. The variability is caused by the flaring up and fading of separate sources (hot spots) of polarized (synchrotron) radiation. The existence of a preferable direction of polarization (θ0=80°) is an indication of a stable magnetic field. It may be used as an argument in favour of the single-body hypothesis of Lacertids. The behaviour of OJ 287 during a 6-day interval may be explained by intensity changes of a single source with constant parameters of polarization. It is found that, for this source,p=42.8%, θ0=101°. The night variations of brightnes and parameters of polarization limit the dimensions of the source responsible for this variability (R<-5×1014 cm) and enable us to estimate its degree of polarization (p≈50%).

Modulational instability of nonlinear waves in the relativistic plasma with account of the nonlinear Landau damping

Abstract: The modulational instability of the weakly nonlinear longitudinal Langmuir as well as the transverse electromagnetic waves, propagation in the relativistic plasma without the static fields is described. The nonlinear Schrodinger equation taking account of the nonlinear Landau damping for these waves has been derived by means of the relativistic Vlasov and Maxwell equations. The plasma with the weakly relativistic temperature and that with an ultrarelativistic one has been investigated. In the first case, for the electron-proton plasma with the temperature more than 2.3 KeV we found the regional change of the wave numbers for which the soliton of two types, subsonic and supersonic, can exist. The soliton of the transverse waves can exist when the group velocity of the waves is between the thermal velocity of the electron and ion and the length of the linear waves is less than 2πc/ωpi.

Silicon chemistry in dense clouds

Abstract: The equilibrium chemistry of silicon in dense interstellar clouds is discussed in terms of both gas phase and grain surface reactions. Unless the metal depletion is very large, the gas phase scheme tends to over-produce SiO and/or SiS when compared to the observations of Sgr B2. The scheme also predicts SiC to be an abundant form of silicon. There is a great need for relevant laboratory data on the reactions used here—of the 35 rate coefficients adopted in the scheme, only three have been measured in the laboratory. Reactions between positively charged gas phase ions and small grains can lead to the formation of SiO and SiS. This type of reaction seems to offer a simple explanation for the observed differences between sulphur and silicon chemistry in dense clouds.

Close binary systems before and after mass transfer: A comparison of observations and theory

Abstract: From a search through the literature 174 close binaries with known absolute dimensions have been sampled. Distinction is made between systems before and after mass exchange. Mass, period and mass ratio distributions and relations of the group of ‘unevolved’ binaries (i.e., prior to mass exchange) are transformed into corresponding distributions and relations of evolved binaries. The transformations are based upon theM1f=g(M1f) relation derived from an extended set of published theoretical computations of the evolution of close binaries. From this relation the following characteristics of the system after mass exchange are computed:M1f,M2f (andqf),Pf. Five different modes of mass transfer were applied for the computation of the values ofPf andM2f. The variation of the period was calculated using the formalism given by Vanbeverenet al. (1979). The results are compared to the observations of binary systems after mass exchange, and are discussed together with an analysis of the effect of several selection effects present in the distributions. The main conclusion is that, during mass exchange in close binaries, more than 50% of the mass is lost to the system in the process of transfer, removing a large amount of angular momentum.

Laboratory Spectra of Protosilicates and the Interstellar Silicate Absorption Bands

Abstract: Infrared spectra of submicrometre-sized particles of several protosilicates have been used to derive mass absorption coefficients. These data have been applied in order to describe the infrared radiation from the source OH 26.5+0.6 in terms of a simple model. A good fit of the 10 μm absorption feature could be achieved, but some discrepancies remain in the 20 μm region. The reasons for the weakness of the 20 μm band in infrared sources are discussed.

The primeval hadron: Origin of stars, galaxies and astronomical universe

Abstract: The relationship between mass and angular momentum for the known cosmic objects has been examined, and it is shown that they are described by the generalized Regge-like dependence of the formJ=(m/mp)1+1/nħ wheren=2 for galaxies, their clusters and superclusters, andn=3 for asteroids, planets and stars. It offers the possibility, that Ambartsumian's superdense proto-matter has hadronic nature. This allows us to give a realistic and quantitave explanation with a minimum number of arbitrary assumptions for the origin of cosmic angular momenta, cosmic magnetic fields and offers the framework for other cosmogonic implications. This approach incorporates in a natural way the fundamental quantum-mechanical parameters ħ andmp, besides the classical parametersG andc, and allows us to derive simple expressions for masses and spins of cosmic objects through fundamental constants, some of which coincide with Eddington-Dirac's ‘Large Number’ relations.

Clairaut coordinates and the vibrational stability of distorted stars

Abstract: The aim of the present paper will be to generalize the concept of the ‘Roche coordinates’, introduced previously by the author (see Kopal, 1969, 1970, 1971) for a treatment of dynamical phenomena in close binary systems, to ‘Clairaut's coordinates’ in which the Roche potential of a rotating dipole is replaced by the actual potential of configurations of finite density concentration and arbitrary structure. By virtue of an identification of the potential with the radial coordinate of our three-dimensional system, the Roche and Clairaut coordinates are both bound to be curvilinear if the star in question departs from spherical form. However, unlike Roche coordinates, the Clairaut coordinates introduced in this paper will not be required to constitute an orthogonal system; and, as a result of the freedom so preserved, their angular variables will be identified with the angles θ and ϕ of spherical polars. Such an adoption entails advantages and disadvantages. In the orthodox Roche system, the radial coordinate (i.e., the potential ξ) is given to us in a closed form; but their angular variables η and ζ must, in general, be obtained by an integration of partial differential equations constituting the orthogonality conditions. On the other hand for the Clairaut (non-orthogonal) system of coordinates no such integration is necessary — and, in fact, the angular variables can be adopted at will. However, their radial coordinate (i.e., the potential of a star of arbitrary structure and distortion) is no longer available in a closed form and must be constructed by a sequence of successive approximations — a process initiated in the 18th century by Clairaut (1743), which can be developed to any desired accuracy. As is well known, investigations of the stability of self-gravitating configurations of arbitrary internal structure must be conducted on the basis of fundamental equations of stellar hydrodynamics, which for small oscillations can be reduced to linear forms. In Section 2 the explicit form of these fundamental equations will be set up in Clairaut's coordinates and linearized in Section 3 to the case of small oscillations, while in Section 4 a critical comparison of the Clairaut and Roche coordinates will be made. However their application to rotating stars will be the subject of subsequent papers.

Can elliptical galaxies be equilibrium systems

Abstract: This paper deals with the question of whether elliptical galaxies can be considered as equilibrium systems (i.e., the gravitational+centrifugal potential is constant on the external surface). We find that equilibrium models such as Emden-Chandrasekhar polytropes and Roche polytropes withn=0 can account for the main part of observations relative to the ratio of maximum rotational velocity to central velocity dispersion in, elliptical systems. More complex models involving, for example, massive halos could lead to a more complete agreement. Models that are a good fit to the observed data are characterized by an inner component (where most of the mass is concentrated) and a low-density outer component. A comparison is performed between some theoretical density distributions and the density distribution observed by Younget al. (1978) in NGC 4473, but a number of limitations must be adopted. Alternative models, such as triaxial oblate non-equilibrium configurations with coaxial shells, involve a number of problems which are briefly discussed. We conclude that spheroidal oblate models describing elliptical glaxies cannot be ruled out until new analyses relative to more refined theoretical equilibrium models (involving, for example, massive halos) and more detailed observations are performed.

Absorption Effects due to Intergalactic Long Whiskers of Pyrolytic Graphite and the Cosmic Microwave Background

Abstract: Earlier (Rana, 1979, 1980) it was shown that an intergalactic medium containing natural graphite whiskers could not adequately thermalize the ambient radiations to generate 3 K microwave backgroud. In the present paper we have carried out a similar investigation with whiskers of pyrolytic graphite. Provided the abundance is about 10−34 g cm−3, the model is capable of thermalizing the background. Some of the observational consequences have been studied with reference to extragalactic astronomy and quasistellar objects. No conflicting evidence has been found so far.

Axially symmetric explosion in magnetogasdynamics.

Abstract: A point explosion in a spheroid with axially symmetric exponential density distribution is investigated by generalizing the method of Laumbach and Probstein to include the effects of a magnetic field. It is shown that the shock velocity decreases and tends to zero. Also, the elongation of the shock envelope along the axis of symmetry is much reduced and the blowout of the shock wave is removed on account of the magnetic field.

Large Amplitude Wave-Trains of Cosmic-Ray Intensity

Abstract: The large amplitude wave-trains of cosmic-ray intensity observed during June, July and August, 1973, were analysed. These events exhibit the same characteristics as the event of May, 1973. During these days the phase of the enhanced diurnal anisotropy is shifted to a point earlier than either the corotation direction or the anti-garden-hose direction. For this analysis we used data from high- and middle-latitude neutron monitors and from the satellites HEOS-2, IMP-7 and IMP-8. The diurnal variation of these days is well understood in terms of a radially outward convective vector and a field-aligned inward diffusive vector yielding a diurnal anisotropy vector along about 1600 h in space.