Dixon's approach to describe the dynamics of extended bodies in metric theories of gravity is elaborated. The exact, general relation between the center-of-mass 4-velocity and the 4-momentum is derived. Quasirigid bodies are defined, and their equations of motion are shown to be determinate for a given metric. Multipole approximations are considered, and the physical meaning of quasirigidity is investigated by establishing an approximate connection with continuum mechanics. G. Dixon has laid the foundations of an exact dynamics of extended bodies in metric theories of gravity. The purposes of this paper are (1) to extend Dixon's general theory by elaborating the center-of-mass description of arbitrary bodies, (2) to restrict the center-of-mass description to what we shall call quasirigid bodies, (3) to consider some properties of multipole approximations to the general theory, and (4) to establish a connection between continuum mechanics and the theory of quasirigid bodies.After a review of those definitions and results of Dixon's theory on which this work is based we derive in Section 2 a formula for the relativistic center-of-197
The ``Century Survey'' (CS hereafter) is a complete redshift survey of a
1$^\circ$-wide strip. It covers 0.03 steradians to a limiting m$_R$ = 16.13.
The survey is 98.4% complete and contains 1762 galaxies. Large-scale features
in the survey are qualitatively similar to those in other surveys: there are
large voids surrounded or nearly surrounded by thin dense regions which are
sections of structures like (and including) the Great Wall.
The survey crosses the classical Corona Borealis supercluster. The galaxy
density enhancement associated with this system extends for $\simgreat 100
h^{-1}$ Mpc (the Hubble constant is H$_0 = 100h$ km s$^{-1}$Mpc$^{-1}$).
The Schechter (1976) luminosity function parameters for the CS are: $M^*_{CS}
= -20.73 ^{+0.17}_{-0.18}$, $\alpha_{CS} = -1.17 ^{+0.19}_{-0.19}$, and
$\phi^*_{CS} = 0.0250\pm0.0061$ Mpc$^{-3}$mag$^{-1}$. In concert with the ESO
Key Program (\cite{vet97}; \cite{zuc97}) and the AUTOFIB (\cite{ell96})
surveys, the CS indicates that the absolute normalization of the luminosity
function exceeds estimates based on shallower and/or sparser surveys.Comment: 19 pages, 3 figures, 1 plate. The plate is substituted by a color
lineplot in the preprint. AASTeX. To appear in tje December 1997 Astronomical
Journa
Abstract.We investigated the long-term light variations of six young irregular variables, all of them UX Orionis stars or related objects. For this aim we determined new photographic magnitudes for the five stars VX Cas, BH Cep, BO Cep, SV Cep, and RZ Psc and made use of similar data for WW Vul published independently. The magnitudes were derived from Argelander brightness estimates performed on plates of the Harvard College Observatory and Sonneberg Observatory plate collections. The data cover a period of about 100 years and are available at the Centre des Données Stellaires, Strasbourg. The new data were supplemented by multicolour photoelectric measurements taken from the literature and visual estimates collected by the AAVSO. The resulting lightcurves are displayed and discussed. Algol-like minima are observed with all the stars. Their properties are discussed under the presumption that they are caused by circumstellar dust clouds orbiting the stars. Parameters of the circumstellar dust shells are derived from modelling the observed infrared excess radiation and compared with constraints from the light variations.
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