Showing papers in "Astronomy and Astrophysics in 1973"

Black holes in binary systems. Observational appearance

Abstract: The outward transfer of angular momentum of accreting matter can lead to the formation of a disk around the black hole The structure and radiation spectrum of the disk depends, in the main, on the rate of matter inflow \(\dot M\) into the disk at its external boundary Dependence on the efficiency of mechanisms of angular momentum transport (connected with the magnetic field and turbulence) is weaker If \(\dot M = {10^{ - 9}} - 3 \times {10^{ - 8}} {M_ \odot }/yr\), the disk around the black hole is a powerful source of X-radiation with hv ~ 1–10 keV and luminosity L ~ 1037–1038 erg s−1 If the flux of the accreting matter decreases, the effective temperature of radiation and the luminosity will drop At the same time when \(\dot M >{10^{ - 9}}{M_ \odot }y{r^{ - 1}}\), the optical luminosity of the disk exceeds the solar one The main contribution to the optical luminosity of the black hole is due to the re-radiation of that part of the X-ray and ultraviolet energy which is initially produced in the central high temperature regions of the disk and which is then absorbed by the low temperature outer regions The optical radiation spectrum of such objects must be saturated by the broad emission recombination and resonance lines Variability is connected with the character of the motion of the black hole and the gas flow in binary systems and possibly with eclipses For well defined conditions, the hard radiation can evaporate the gas This can counteract the matter inflow into the disk and lead to autoregulation of the accretion

Numerical Experiments on the Stability of Spherical Stellar Systems

Abstract: The concentric shell model is used to investigate numerically the stability of spherical steady-state stellar systems. Polytropic models with an isotropic velocity distribution are found to be stable almost down to the limiting index n=1/2. ‘Generalized polytropes’, with a distribution function depending on energy and angular momentum, show instability when n is low and the velocity distribution is radially elongated.

Molecular abundances in stellar atmospheres. II

Abstract: Chemical equilibria of 36 elements are solved for the physical conditions of cool stellar atmospheres It is found that the molecular species formed (monoxide, dioxide, halide etc) and the degree of molecular association, (ie the fraction of atoms locked in molecules) are well correlated with the position of each element both Group and atomic weight) in the periodic table In the atmospheres of carbon-rich stars, molecular formation is generally less important than in oxygen-rich atmospheres except for some carbon compounds The metal oxides in oxygen-rich atmospheres are genetally replaced by metal carbides, especially by dicarbides, in carbon-rich atmospheres The formation of carbides in carbon-rich atmospheres, however, is less effective than inat of the corresponding metal oxides in oxygen-rich atmospheres In carbon stars, it is shown that the Si/S ratio plays a critical role just like the C/O ratio (auth)

Comparison of rotation curves of different galaxy types

Abstract: Rotation curves extending to large radial distances are now available for 3 spiral galaxies, each of a different type. Differences in shape of the rotation curves indicate a mass distribution that is related to structural type and is in the same sense as the luminosity distribution for these galaxies. The shapes of the rotation curves at large radii indicate a significant amount of matter at these large distances and imply that spiral galaxies are larger than found from photometric measurements. (auth)

Approximate equation of state for stellar material

Abstract: A simple type of formula is described which approximates the Fermi-- Dirac integrals for electron density, pressure, and internal energy to about 0.1% or better over the entire range of their arguments; the accuracy depends on the chosen level of approximation. The method can be varied slightly to give thermodynamic quantities which, though only approximate, are entirely consistent thermodynamically li.e. satisfy Maxwell' s relations identically). The method is recommended for the rapid calculation of stellar interiors. A further simple formula is given for a very crude approximation to the effect of pressure ionization on the equation of state in stellar interiors. (auth)

Dynamical Models of Tailed Radio Sources in Clusters of Galaxies

Abstract: Two medels for the formation of tailed'' radio sources like 3C 129 are developed. Both models assume that the tails are the result of the rapid motion of an active galaxy through the relatively dense gas in a cluster of galaxies. The first model is a modification of current models of normal symmetrical double radio sources, with the effects of the motion of the parent galaxy and of the thermal pressure of the intracluster gas included. The second model assumes that the galaxy has a long, permanently attached, magnetospheric tail, similar to the geomagnetic tail, generated by the action of the intracluster gas flow on the intrinsic magnetic field of the galaxy. This tail is illuminated at intervals by outbursts of high energy electrons from the galaxy. An attempt is made to fit both models to 3C 129 to see if they give a good explanation of the observed characteristics of this source, and to determine the physical conditions in the intracluster gas. While both models give good fits to the shape of the tail, the magnetospheric model gives a better fit to the observed spectral indices, more » as well as relbes ia the interpretation of the polarization solving some difficulties in the interpretation of the polarization of the radio emission of 3C 129. The relation of the models to normal symmetric double radio sources, and to intermediate cases like 3C 465 is discussed. The derived values of the intracluster density and temperature are considered. (auth) « less

On the light curve and properties of type I supernovae.

Abstract: By the best fitting of all available light curves of Type I supernovae an average curve, representative of the class, was drawn. Due to the strong similarity of the light curves, the dispersion of the points was relatively small. From the analysis of the average curve some general properties of the SN-I were derived. The occurrence of Type I supernovae in different types of galaxies and the possibility of a further subdivision in two groups are discussed. (auth)

A high resolution neutral hydrogen study of the galaxy M51.

Abstract: Abstract : A 2 arc min synthesis study of the spiral galaxy M51 (NGC 5194) has been made in the lambda 21 cm emission line of neutral hydrogen. A morphological HI symmetry axis is found at a position angle of 22 degrees while the dynamically determined major axis is found at a position angle of -8 degrees. The velocities in the north-east outer spiral structure are found to be strongly non-circular. They agree in magnitude with predictions made by Toomre and Toomre for the tidal interaction between M51 and its companion galaxy NGC 5195. A possible continuation in neutral hydrogen of the outermost south-west spiral arm has been detected. M51 also exhibits a strong deficiency in HI surface density at its center. An upper limit of 10 billion solar mass units per beam area has been placed on the neutral hydrogen content of the companion galaxy NGC 5195. This, in turn, yields an upper limit of about 0.1 for the distance-independent ratio M(HI)/L(pg). (Author)

Lifetime measurements of optical levels for 6 elements of astrophysical interest

Abstract: The mean lifetimes of 25 optical levels of Mg I, Al l, Si I, Cr I, Co I, and Mn I were determined with the phase-shift method. The measured values can be used to calibrate previously determined scales of relative oscillator strengths. (auth)

Formation of Galaxies inside Clusters

Abstract: A model is presented for the formation of clusters of galaxies, after the decoupling of radiation from matter in an Einstein-DeSitter Universe. It is assumed that due to primeval turbulence in the Universe some spheroidal regions of space expand somewhat slower than others. These slow-expansion regions, which have a mass between 10/sup 45/ and 10/sup 47/ kg, eventually stop expanding and subsequently collapse to high density. If such a region is not exactly spherical its deviation from sphericity will increase throughout the evolution. The turbulence inside this protocluster eads to the formation of galaxies. In order to study this formation, the growth rate of perturbations of a collapsing prolate spheroid is determined. The protocluster is assumed to be prolate because then the density enhancement is much larger than with an oblate spheroid. The protoclusters collapse within about 10/sup 17/ s predominantly into highly elongated clusters. The time needed for a galaxy to cross such a cluster along its major axis is about 4 x-10/sup 17/ s so that many of these clusters have not yet reached virial equilibrium at the present time. (WDM)

The Beaming of Radiation from an Accreting Magnetic Neutron Star and the X-ray Pulsars

Abstract: As a result of accretion, a neutron star with a strong magnetic field H ~ 10/sub 10/ to 10/sup 12/ Gauss may exhibit characteristics of an x-ray pulsar with a knife radiation pattern. The infalling gas is channeled to the magnetic pole regions where its kinetic energy is released as it strikes the star surface. The cyclotron radiation at high gyrofrequency harmonics (lying in the x-ray range) determines the rate of energy removal from the hot plasma (kT/sub e~/ 10 to 100 keV). The net harmonic radiation spectrum resembles the spectrum of bremsstrahlung radiation from an optically thin plasma, but with a lower temperature. Radiation at high harmonics is characterized by a strong beaming prpendicular to the magnetic field lines of force. However the situation is also possible, when the radiating electrons are relativistic and move at small pitch angles to the magnetic field. In this case the Doppler-shifted cyclotron radiation at nu << &nn/sub H/ is directed along magnetic field, forming the pencil diagram. ( auth)

A model to explain alternate period changes in Algol-like binaries.

Abstract: Following the ideas of Smak (1972) on period changes in U Gem systems, a model is developed to explain the alternate period changes in Algol-like binaries. The model is based on the hypothesis that sudden bursts of mass are transferred to the hot star as a result of an instability probably of Bath's (1972) type. These bursts cause angular momentum to be stored temporarily as rotation in the outer 10/sup -1/ M/sub sub solar / or less of the hot star (thereby decreasing the period) and then to go back to the orbit as increased turbulence lets the friction time scale catch up with the dynamical time scale of the mass transfer (thereby increasing the period). Equations are given for the two phases of this cycle (period decrease and increase) and also for the long-term average period increase. These equations are used to analyze a segment of the O-C diagram of U Cep (Svechnikov, 1955) and derive the Mass flow rate of each burst occurring within this segment. The sum of all the mass bursts seems to be greater than the rate of Mass transfer implied by the longterm parabolic period increase. The implications of this discrepancy are discussed, andmore » further observations which can test our model are suggested. (auth)« less

Water masers in a protostellar gas cloud

Abstract: Population inversion of the 6/sub 16/-5/sub 23/ transition of water molecules is a natural consequence of the breakdown of thermal equilibrium in the outer layers of a dense gas cloud Approaching the surface of the cloud from below, radiation of different lines starts to escape at different depths Energy is continuously pumped into the lines since the molecules are coupled to the reservoir of kinetic energy of the gas by collisions with hydrogen molecules In support of this picture the transfer of line radiation in a gas containing model water molecules with thirty-one pure rotational levels was calculated Two simple approximations of H2-H20 collisional cross sections were used: (A) The cross sections were assumed to be proportional to the square of the matrix elements of the dipole transitions; (B) All cross sections were assumed to be equal to the geometrical cross section, still following dipole selection rules Population inversion of the 6/sub 16/-5/sub 23/ transition occurred for both collisional models since the breakdown of thermial equilibrium is governed by the line opacities The required amplification could only be produced in case (A) The level populations of some other- transitions of water become also inverted One particular class of molecular masermore » sources could be produced by limb brightening'' in the cocoon'' around new born O stars This may explain the observed coincidence with bright knots'' in the continuum radiation of H II regions (auth)« less

Delta Scuti stars

Abstract: The extensive observational and theoretical data on sigma Scuti variables (published and previously unpublished) are collected and analyzed. Properties of pulsation are discussed, and relationships between sigma Scuti stars and other types of pulsators and nonvariable stars are established. (auth)

Manifold of galaxies. Galaxies with known dynamical parameters

Abstract: For the sample of galaxies with known rotation curves the following independent integral properties were compiled: morphological type, photometric radius, radius and velocity of the maximum of the rotation curve, absolute luminosity, color, mass of neutral hydrogen The main result of a component analysis is the presence of two significant dimensions in the sample The relationships between the different variables indicate that the star formation rate is not a function of interstellar density alone (auth)