Showing papers in "Astronomy and Astrophysics in 1977"

Expressions for the precession quantities based upon the IAU /1976/ system of astronomical constants

Abstract: The structure of the expressions usually employed in calculating the effects of precession is examined, and a method is outlined for revising the expressions to account for changes in the fundamental astronomical constants It is shown that the basic set of parameters, upon which depend the lengthy polynomials for computing the mean obliquity of data and the elements of the precession matrix, consists of the mean obliquity, the speed of general precession in longitude at a fixed epoch, and the system of planetary masses Special attention is given to the motion of the ecliptic pole, formulations for a basic epoch as well as an arbitrary epoch, and ecliptic motion relative to the basic epoch Numerical precession quantities at epoch J20000 (JED 24515450) are presented which result from the revision of astronomical constants adopted at the XVI General Assembly of the IAU

Large-scale inhomogeneity of the Universe - Spherically symmetric models

Abstract: Dust-filled Tolman-Bondi (1934, 1947) spherically symmetric cosmological models are studied with application to superclustering of galaxies Asymptotic expressions are given for the density and curvature contrast, valid near the singularity and at long times The density profile of infalling material is expressed in terms of a single arbitrary function (the distribution of total energy per unit mass) Measurement of the distribution of excess galaxy counts and deviations from the Hubble flow in the vicinity of a supercluster could provide a determination of the cosmological deceleration parameter

Disk accretion onto a black hole at subcritical luminosity.

Abstract: The influence of radiation pressure on the structure of accretion disk is considered when the total luminosity L approaches the Eddington limit L/sub c/ The motion of particles in the disk radiation field and gravitational field of a nonrotating black hole was investigated It is shown that the disk accretion is destroyed when L approximately equals (06 + 10)L/sub c/ Matter outflow from the central parts of the disk to infinity then sets in It is concluded that the luminosity cannot significantly exceed the Eddington limit It is shown that for L greater than or approximately equal to 01 L/sub c/ the plasma in upper layers of the central region of the disk is heated up to temperatures approximately 10/sup 9/ K, and the disk becomes thicker as compared with the standard theory The radiative force can generate magnetic fields B approximately 100 Gs Convection is the main energy transfer mechanism along Z-coordinate in the central parts of the disk The convection generates an acoustic flux which dissipates in the upper, optically thin layers of the disk and heats them The comptonization of soft photons going from lower layers to the hot upper layers and variable accretion rate may explainmore » the spectrum and variations of X-ray emission of the Cyg X-1« less

Some optical and kinetic properties of the nearby interstellar gas.

Abstract: A model for resonant scattering of sunlight by the nearby interstellar gas is developed which takes into account the modification of a Maxwell-Boltzmann velocity distribution by a central force. While the formalism applies to both hydrogen and helium, the latter is emphasized. Theoretical line profiles and integrated emission rates for helium are compared with those of approximate models and with existing observations. As a result of this analysis, an upward revision of the bulk flow velocity of interstellar helium relative to the solar system, previously deduced from He 584-A observations, is required. Velocities of 15-20 km/s, temperatures of the order of 5000-10,000 K, and densities of the order of 0.008-0.02 per cu cm are sufficient to explain the (helium) observations. However, higher-precision measurements are required in order to place more stringent limits on the gas parameters. Calculations of the flux of interstellar helium atoms impinging on earth's upper atmosphere show that the resulting impact ionization is small compared with terrestrial processes.

Theory of motion of Jupiter's Galilean satellites

Abstract: Final results are presented for a theory enabling one to calculate the positions of the Galilean satellites and their partial derivatives. Extensive use of algebraic manipulation software on a digital computer is made to generate the final expressions. The theory is, in effect, a revitalization of Sampson's (1921) theory. Algebraic and mathematical errors existing in Sampson's work are removed, some neglected effects due to solar interactions and the 3-7 commensurability are introduced, allowance is made for nonzero amplitude and phase of the free libration, and the final results are expressed as analytic functions of variations in 49 arbitrary constants of integration and physical parameters. The theory is constructed in a manner which readily allows for future revision, and analytic expressions are provided for the partial derivatives with respect to the 49 parameters.