Showing papers in "Astrophysics and Space Science in 1981"

Fundamental Stellar Parameters Derived from the Evolutionary Tracks

Abstract: The surface gravities and radii of stars are calculated for different MK spectral types using the masses of stars determined from their evolutionary tracks in the HR diagram and the most reliable values of effective temperatures and absolute bolometric magnitudes. MK spectral types are calibrated in absolute visual magnitudes using the studies ofM v published since 1965. The calibration of MK types in temperatures is based on the newest investigations including the results both from the ultraviolet and the infrared. The obtained masses, gravities, and the mass-luminosity relationship show reasonable agreement with independent observational data.

Production spectrum of gamma rays in interstellar space through neutral pion decay

Abstract: A simple representation is obtained of the observed invariant cross section for the production of neutral pions in proton-proton collisions. Using this representation, the differential and integral production spectra of gamma rays in the galaxy are calculated from interactions of cosmic ray nuclei with interstellar gas. It is shown that the uncertainties in deducing interstellar proton spectrum by demodulating the observed spectrum have only a limited effect on the gamma ray spectrum. Also determined is the gamma ray production spectrum through bremsstrahlung process for a typical interstellar electron spectrum derived from the radio spectrum in the galaxy.

Recent results from the gamma-ray burst studies in the KONUS experiment

Abstract: Observations of 85 gamma bursts by the KONUS instruments on the Venera 11 and Venera 12 spacecraft in the period September 1978 to May 1979 inclusive have provided proof of a galactic localization of the gamma-burst sources based on an analysis of the logN-logS plot and the revealed anisotropy in the angular distribution of sources over the celestial sphere. Evaluation of the energy released in the sources yields 1040–1041 erg. There apparently exist several types of gamma bursts differing in time profile, duration and shape of their energy spectrum. In some cases, extensive evolution of the energy spectrum is observed during a burst. The discovery of a flaring X-ray pulsar in Dorado has provided the first observational evidence for a connection of gamma bursts with neutron stars. Repeated short bursts from this source have revealed for the first time the recurrent features of this phenomenon. Repeated bursts have been detected from one more source in the short burst class. The data obtained thus far impose a number of restrictions on the applicability of many theoretical suggestions concerning the nature of the gamma bursts. The most plausible model for the gamma-burst source appears to be a binary with a neutron star with strongly non-stationary accretion involving, possibly, non-stationary thermonuclear fusion of matter falling onto the surface of a degenerate star.

Frequency shift in Fourier analysis

Abstract: Error analysis of the frequency determination by conventional Fourier method was performed. Taking into consideration only equidistant data, estimates were obtained on the accuracy of the frequencies deduced by analysing a noise-free signal containing not more than two sinusoidal components with closely spaced frequencies. An important consequence of these results was the empirical distribution function of the frequencies for signals contaminated by noise and containing sufficiently distant frequencies.

Compton scattering of relativistic electrons in compact X-ray sources

Abstract: The problem of single Compton scattering is considered and the resulting spectrum, angular distribution and polarization of scattered photons in a general case are obtained The inverse Compton scattering (ICS) for arbitrary energies of electronsE and photons ω0 is investigated in detail In the case of isotropically-distributed initial photons and relativistic electrons, a strong rise of the scattered spectrum near the upper edge takes place, starting from the values of the characteristic parameterb≡4Eω0≳10 (in units of mc2=1) The energy-loss rate of relativistic electrons due to ICS is calculated It is shown that the relativistic electrons of the energiesE≳100 MeV, when scattering on the X-rays with ω0~10KeV, transmit the dominant part of their energy to the photons which fall after scattering into the energy range of the electrons (≳100 MeV)

Sequential explosions of supernovae in an ob association and formation of a superbubble

Abstract: From the standpoint of view that the early type stars are formed sequentially at an OB association, it is expected that the supernova explosions will also occur sequentially. We study the expansion law of a supernova remnant, which is formed by sequential explosions of supernovae. The superbubbles and supershells with the radii 200∼1000 pc are naturally explained by this model. Assuming that the sequential explosion of supernovae occurs at every OB association, we deduce the star formation rate in our Galaxy.

Radio spectra of pulsars

Abstract: The interpretation of the average spectra of pulsar radiation is given It is shown that the observed relationship between the part of the pulsar kinetic energy, which is transformed into radio emission, and the basic period can explain the lack of very long period pulsars The statistical relationships between the characteristic frequencies rule out effective operating of the Melrose's mechanism (ALAE) in pulsar magnetospheres Two possible reasons of the high-frequency cut-off are analyzed The values of the energy of emitting electrons are obtained (γ=ɛ/mec2∼103)

Baade-Wesselink and related techniques for mode discrimination in nonradial stellar pulsations

Abstract: Linearized nonradial expressions for light, colour and radial velocity changes are employed to examine the Baade-Wesselink method as applied to the problem of nonradial mode identification in variable stars. It is demonstrated that the usual assumption of a single-valued relation between surface brightness changes and colour variations fails for spherical harmonics withl>1. A Baade-Wesselink expression taking this into account is then given. Using test models for different variable star regimes it is shown that discrimination betweenl=0, 1, and 2 is currently feasible provided adequate attention is paid to pressure effects and to the procedure used to obtain radial velocity data. For higherl, mode discrimination, while possible in principle, is constrained by the stringent requirements on radial velocity measurements and by cases of confusion with lowerl. A review of related but less observationally demanding possible mode discriminators is given. This demonstrates that techniques based on amplitude and phase information in light and colour data may provide reliable discrimination betweenl=0, 1, and 2 when pressure effects are allowed for. For these techniques discrimination of higherl is limited by inadequate model atmosphere limbdarkening information and by confusion with lowerl.

A diffraction limit on the gravitational lens effect

Abstract: The focussing of gravitational radiation by the interior and exterior gravitational field of a Newtonian gravitational lens is considered. A graphical method for determining the caustic structure of a Newtonian gravitational lens is presented and the caustic structure of a solar type gravitational lens is discussed. Estimates of the amplitude magnification in the caustic region indicate that waves with frequencies less than a critical cutoff frequency ω c are not amplified significantly. For a lens of massM this cutoff frequency is ω c ≈(10-1πM)-1; for the Sun ω c ≈104s-1.

Problems of low-mass binary evolution

Abstract: Some problems connected with low-mass binary evolution (from contact binaries to cataclysmic variables and origin of bursters) are considered Most attention is given to contact W UMa-stars and to (still unclear) scenarios where the angular momentum loss by magnetic braking may, at least partly, control the contact binary evolution

Rate of H2 formation on amorphous grains

Abstract: The rate of formation of molecular hydrogen from hydrogen atoms adsorbed on grains is analyzed, assuming that the grains are single crystals, polycrystalline or amorphous. On polycrystalline grains, and on graphite platelets, this rate could be orders of magnitude lower than on single crystal grains. The same is true for amorphous grains because there, at low temperatures, only atoms absorbed on neighboring sites can form molecules. Suitable formulae are derived and compared with the classical results for single crystal grains. Quantitative results are given for crystalline and amorphous ice, but with small changes these should also be valid for other solids. The rates for amorphous grains can approximate, within a factor of 10 or so, those for crystalline grains if the density of H atoms is high and the density of H2 molecules is low and only when the temperature of the grains satisfies a relation which for ice and graphite leads to a value in the proximity of 15–17 K. This maximum rate occurs only a degree or so above the temperature at which the grains are totally covered by an H2 layer and the reaction ceases. Furthermore, for a constant number density of grains, the rates on amorphous grains are second order while those on crystalline grains are first order. Both these circumstances predict amorphous grains to lead to H2 clouds with irregular and sharply delineated features in contrast to more uniform clouds formed on crystalline grains.

Synchrotron cooling and annihilation of an e/+/-e/-/ plasma - The radiation mechanism for the 5 March, 1979 transient

Abstract: Positron-electron pair radiation is examined as a mechanism that could be responsible for the impulsive phase emission of the March 5, 1979 transient. Synchrotron cooling and subsequent annihilation of the pairs can account for the energy spectrum, the very high brightness, and the approximately 0.4 MeV feature observed from this transient, whose source is likely to be a neutron star in the supernova remnant N49 in the Large Magellanic Cloud. In this model, the observed radiation is produced in the skin layer of a hot, radiation dominated pair atmosphere, probably confined to the vicinity of the neutron star by a strong magnetic field. The width of this layer is only about 0.1 mm. In this layer, approximately 10 to the 12th power generations of pairs are formed (by photon-photon collisions), cooled and annihilated during the approximately 0.15 sec duration of the impulsive phase. The very large burst energy implied by the distance of the Large Magellanic Cloud, and its very rapid release, are unsolved problems. Nonetheless, the possibility of neutron star vibrations, which could transport the energy coherently to the surface, heat the atmosphere mechanically to a hot, pair-producing temperature, and have a characteristic damping time roughly equal to the duration of the impulsive phase are addressed.

The equivalent width of the Hβ emission line and the evolution of the H ii regions

Abstract: The synthetic equivalent width (W Hβ) of the line Hβ in emission is obtained for Hii regions opaque to the Lyman photon flux, with embedded OB associations with different initial chemical compositions and initial mass functions. The variation ofW Hβ as a function of the evolution of the ionizing stars is analysed. The observations ofW Hβ for M33, M101, and M51 by Searle (1971) are discussed.

30 keV (n, γ) cross-sections from the nuclear statistical model

Abstract: Global input parameters for a Hauser-Feshbach calculation of astrophysical nuclear reaction rates are evaluated; wherever possible they are expressed as systematic functions of nucleon numbersN andZ rather than mass numberA. Average (n, γ) cross-sections at 30 keV are calculated for all stable nuclei and nuclei with lifetimes ≥ 1 yr, taking into account the effects of width fluctuations. Agreement with experimental results is generally obtained to within 70%, but systematic discrepancies arise for even-even target nuclei between neutron shell closuresN=50 andN=82. A σN curve for thes-process nuclei is in good agreement with theoretical predictions, except for a possible anomalous peak aroundA=180.

Spectrum of the Sunyaev-Zel'dovich effect for high electron temperatures

Abstract: We analyse in detail the spectral shape of the Sunyaev-Zel'dovich effect in the cosmic background radiation, taking into account the relativistic corrections for a hot electron gas We calculate the displacement of the zero-signal frequency, which is especially informative in a new method for measuring the millimetric temperature of the background radiation; we also present a simple analytical expression, to be used to fit the experimental data in spectral measurements of the effect in very hot cluster gases

The circular restricted four-body problem

Abstract: By generalizing the restricted three-body problem, we introduce the restricted four-body problem. We present a numerical study of this problem which includes a study of equilibrium points, regions of possible motion and periodic orbits. Our main motivation for introducing this problem is that it can be used as an intermediate step for a systematic exploration of the genral four-body problem. In an analogous way, one may introduce the restrictedN-body problem.

Additional results for unstable stratified toroidal magnetic fields in stars

Abstract: General conditions for adiabatic instability of weak axisymmetric toroidal magnetic fields are obtained. In particular all fields which haveHϕ=0 and ∂Hϕ2/∂δ>0 simultaneously are unstable. This includes all fields which exist near to the axis of symmetry and, indeed, any without an infinite current density anywhere. A detailed discussion is given of fields with an angular dependenceHϕ2∼|Pl (cos θ)|. These fields are dynamically unstable with respect to both axisymmetric and non-axisymmetric perturbations for all values of the azimuthal wave number |m|. The maximum growth rates are independent ofm and they are largest for the fields that are defined in the regions closest to the polar axis.

Secondary production of antiprotons in cosmic radiation

Abstract: A reliable representation to the invariant cross-section for the production of antiprotons ( $$\bar P$$ ) in inclusive reactions has been obtained, which fits the data extremely well from threshold to ISR energies. Using this, the production spectrum of $$\bar P$$ by cosmic ray interaction with interstellar gas is calculated and is compared with other existing calculations. The equilibrium spectrum $$\bar P$$ in the Galaxy has been derived from about 100 MeV to a few hundred GeV in kinetic energy using Leaky Box Model for the propagation of cosmic rays, by taking into account all energy loss processes. It is found that $$\bar P$$ /P ratio calculated here is very much smaller than the observed ratio. In view of the fact that this excess of observed $$\bar P$$ /P ratio is much larger than the measured upper limits on the fraction of antimatter in the form of antinuclei, the excess $$\bar P$$ /P ratio has not been attributed to the existence of antimatter. Instead, it is shown that Closed Galaxy Model for the propagation of cosmic rays predicts more antiprotons than observed. Therefore, it is suggested that if cosmic rays contain about 50% new component of local origin, the resulting Closed Galaxy Model explains well the observations. The present calculations also predict too small a flux of $$\bar P$$ below a few hundred MeV to make this energy region ideally suited to look for antiprotons of primordial origin.

Rotational modes in a slowly and uniformly rotating star

Abstract: A perturbation method is derived forr-modes in a slowly and uniformly rotating star. In contrast to previous studies, the perturbation of the gravitational potential is included in the perturbation method. On the assumption that the effects of the centrifugal force are taken into account in the equilibrium model up to the second order in the angular velocity, an eigenvalue problem of sixth-order in the radial coordinate is derived that allows one to determine the zeroth-order toroidal displacement field and the third-order term in the expansion of the eigenfrequency. Furthermore, another eigenvalue problem is derived that governs the first-order toroidal displacement field and the fourth-order term in the expansion of the eigenfrequency. This second eigenvalue problem is also of the sixth-order in the radial coordinate. It is shown that the third-order term in the expansion of the eigenfrequency is real, and that the fourth-order term is zero.

Free convection and mass transfer effects on the hydro-magnetic oscillatory flow past an infinite vertical porous plate

Abstract: Two-dimensional unsteady free convection and mass transfer, flow of an incompressible viscous dissipative and electrically conducting fluid, past an infinite, vertical porous plate, is considered, when the flow, is subjected in the action of uniform transverse magnetic field. The magnetic Reynolds number is taken to be small enough so that the induced magnetic field is negligible. The solution of the problem is obtained in the form of power series of Eckert numberE, which is very small for incompressible fluids. Analytical expressions for the velocity field and temperature field are given, as well as for the skin friction and the rate of heat transfer for the case of the mean steady flow and for the unsteady one. The influence of the magnetic parameter,M, modified Grashof numberG c , Schmidt numberS c and frequency ω, on the flow field, is discussed with the help of graphs, when the plate is being cooled, by the free convection currents (G r ,E>0), or heated (G r ,E<0). A comparative study with hydrodynamic case (M=0) and the hydromagnetic one (M≠0) is also made whenever necessary.

Generation of a d.c. field by nonlinear electromagnetic waves in relativistic plasmas

Abstract: A finite amplitude linearly polarized electromagnetic wave propagating in a relativistic plasma, is found to generate the longitudinal d.c. as well as the oscillating electric field at the second harmonic. In a plasma consisting of only electrons and positrons, these fields cannot be generated.

Influence of cosmic rays on propagation of long magneto hydrodynamic waves

Abstract: The influence of relativistic particles on the dispersion properties of a cosmic plasma are considered. Use is made of the equation of magneto-hydrodynamics for the thermal plasma and the diffusion equation for cosmic rays. The mechanism of dissipation of the waves due to the diffusion of cosmic rays is shown to be predominant for magneto acoustic waves in the interstellar medium.

A dynamical model of type i supernova atmosphere with the velocity gradient

Abstract: A compact structure of a low-mass Type I presupernovae is assumed to be an essential feature of the hydrodynamical problem dealing with the supernova Type I (SNI) envelope outbursts. This structure is characterized by a degenerate carbon-oxygen core, which suffers a thermonuclear explosion of carbon fuel (M 0≃1.40M ⊙), and by a compact lowmass envelope (M e ≲0.1M ⊙) with external radiusR e≃109 cm. The parameters, of this hydrostatic envelope are specified and then, for a relatively small explosion energy, ofW 0≃(2–10)×1049 erg, hydrodynamic problem of the envelope ejection is solved numerically. This energy comes from neutrino-induced detonative carbon burning. The resulting structure of the SNI atmosphere expanding with the velocity gradient can be employed for an interpretation of the observed SNI spectra. In accordance with our previous papers, the SNI light curves are considered to occur due to an additional slow (with time-scale 106–107 s) release of the bulk of the SNI energy,W≃1051, erg. The slow energy release does not, however, affect the structure of the outermost expanding layers of the envelope which are responsible for the SNI spectra. A short (Δt≃10−2 s) burst of soft (2–10 keV) X-rays with total radiated energy of about 1040 erg is found to appear 10–20 days before the SNI optical maximum.

A realistic model of the galaxy

Abstract: A three-component mass model of the Galaxy consisting of a modified exponential disk, a spherical inner bulge and a massive outer corona has been derived that attempts to give a valid description of observed features over a wide range of galactic distances. The new model shows the steep rise of the circular velocity close to the galactic centre with a first peak of the rotational velocity at approximatelyr=350 pc and a deep minimum aroundr=2 kpc. Outside the solar circle the rotation-velocity stays reasonably constant, requiring the addition of a massive dark corona. A third feature of this model is a variable disk-thickness like that of an equilibrium disk with constant velocity dispersion in thez-direction. A two-step procedure deriving the potential field has been used: First, a model consisting of a bulge, a massive corona and a disk of infinitesimal thickness has been fitted to rotation curve data, and the potential field was then computed using numerically performed Hankel-transforms. In a second step this field was then modified following Vandervoort's (1970a) procedure in order to make it to agree with the potential of a specified thick disk model. Both the Oort constantsA andB of the model agree with recent observed values, and the local mass density is identical to Oort's value ρ0=0.15M ⊙ pc−3. The same is true forK z in the solar neighbourhood. A discussion of Oort's curve forK z in the light of the evidence given by the model even results in an observed value of ρc=0.0019M ⊙ pc−3 for the local mass density of the corona compared to ρc M ⊙ pc−3 in the model. If the model data are inserted into a relation given by the galactic warp theory by Bertin and Mark (1980), we find a good agreement with empirical data from Henderson (1979). The scale size of the disk can be checked by a comparison with the logarithmic density gradient of disk material as given by Blaauw (1965) and Oort (1960, 1965), and it is shown that the visual aspect of equilibrium disk models seen from the outside does not contradict the well-known appearence of edge-on spirals.

Electrostatic double layers and a plasma evacuation process.

Abstract: An evacuation process due to the growth of current driven instabilties in a plasma is discussed. The process, which leads to localized extreme density reductions, is related to the formation of electrostatic double layers. The initial linear phase is treated using the superposition of unstable plasma waves. In the long wavelength, non-dispersive limit, a density dip — which is initially present as a small disturbance — grows rapidly and remains localized in the plasma. The process works for a variety of plasma conditions provided a certain current density is exceeded. For a particular choice of plasma parameters the non-linear development is followed, by solving the coupled Vlasov-Poisson equations by finite difference methods. The evacuation process is found to work even more effectively in the non-linear phase and leads to an extreme density reduction within the dip. It is suggested that the growth of such structures produces weak points within the plasma that can lead to the formation of double layers.

Beta decay rates forS-process studies

Abstract: The rates for a variety of beta decay processes have been determined as a function of temperature for nuclei which can participate in thes-process production of heavy elements, occurring in the presence of the22Ne(α,n)25 Mg neutron source operating in the convective helium shells of thermally pulsing stars. Specifically: calculated half-lives are presented for electron emission, positron emission, and electron capture over the temperature range 108–109 K.

The extinction coefficients in mid- and farinfrared of silicate and iron-oxide minerals of interest for astronomical observations

Abstract: Extinction measurements were made for some silicate and iron-oxide mineral grains in mid- and far-infrared region. For far-infrared region, high temperature magnesium silicates such as olivine and pyroxenes show the absorption spectra of steep dependence as λ−3 (λ being the wavelength) with some peak structure, but the spectrum of magnetite shows λ−1 dependence.

Decametric survey of discrete sources

Abstract: The recent data of the decametric survey carried out in Grakovo with the UTR-2 radio telescope and the data of a number of higher frequency surveys have been used for constructing the spectra of 147 discrete sources in the frequency range 12.6–1400 MHz. The analysis of the spectra shows that the mean value of the spectral index for galaxies in an interval of declinations −2° to −13° is somewhat higher than for those with declinations from −2° to +20°, while the mean spectral indices of quasars and unidentified sources for these two declination ranges coincide quite well.

Morphological study of the time histories of gamma-ray bursts

Abstract: Time histories of gamma-ray bursts have revealed a great diversity, both in total duration and in the details of fine time structure. Results of the gamma-ray experiment on the Helios-2 spacecraft have suggested some similarities in time profiles of events. The possibility of some characteristic 'separation times' in events may also be indicated.