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941.
942.
P. B. Jones 《Astrophysics and Space Science》1976,45(2):369-381
The time dependences of the inertia tensor and of a dissipative torque caused by the nonleptonic weak interaction have been investigated for a certain class of pulsars with no solid core. Early in the life of the pulsar, the angular velocity vector is predicted to move with respect to fixed body axes in such a way that it becomes perpendicular to the magnetic dipole moment. During this motion, the solid outer shell suffers plastic deformation so that the dipole moment becomes approximately collinear with a principal axis. After 104 or 105 yr, the dissipative torque is negligibly small compared with the electromagnetic torque, the Euler equations are those for a simple rigid body, and alignment of spin and dipole moment occurs. If the dipole moment discussed by Lyneet al. (1975) is interpreted as being equal to the component perpendicular to the spin, its secular decay is a natural property of this model and is not a consequence of field decay through electrical resistivity. 相似文献
943.
A detailed study of geochemical changes associated with the retrogressive metamorphism of granulite-facies gneisses of the Lewisian Complex of NW Scotland has been made, using nearly 250 gneisses analysed for 24 major, minor and trace elements. The gneiss samples have been divided into 3 groups: (1) granulite facies, (2) granulite facies retrogressed to amphibolite facies but remaining undeformed, and (3) retrogressed (amphibolite-facies) gneisses deformed in shear zones. Element distributions within these groups have been examined using correlation coefficients, and have been compared and tested for significance using Student's t and Fisher z statistics. It is shown that the process of retrogression involved considerable large-scale chemical equilibration. Major-element pairs show marked increases in correlation during retrogression, reflecting considerable reordering of elements into one or other of the main amphibolite-facies minerals: hornblende, plagioclase and (minor) biotite. These correlations are enhanced, but otherwise unchanged, in the deformed gneisses. The retrogressed gneisses have a much more constant Fe/Mg ratio and a more uniform plagioclase composition, while there is a strong correlation between Fe3+ and Fe2+ throughout the area studied. Trace elements, by contrast, mostly show a significant loss of correlation during retrogression, although Cr and Ni are exceptions. Retrogression occurred as a result of widespread introduction of hydrous fluids up vertical structures in the gneiss complex during the Early Proterozoic. These fluids allowed considerable metasomatic redistribution of elements within the complex as the whole-rock compositions adjusted to the new mineralogy. Throughout the North Atlantic Archaean Craton there is a close association between retrogression of high-grade gneisses and basic magmatism in the form of dyke swarms. It is suggested that the two may be connected, and that the fluids causing retrogression are mantle-derived. 相似文献
944.
945.
Eric M. Jones 《Icarus》1981,46(3):328-336
Monte Carlo calculations of the expansion of space-faring civilizations are presented for a wide range of values of the population growth coefficient (α) and emigration coefficient (γ). Even for the very low values proposed by Newman and Sagan (α = 10?4per year; γ = 10?8per year) the migration wavefront expands at 1.4 × 10?5 pc per year. Even with this low expansion velocity, such a civilization would fill the Galaxy in about 109 years. Filling times of the order of 60 million years seem probable. The wavefront velocity is approximated by , where is the average radial distance traveled, the average distance traveled, and υs the ship speed. This approximation was derived by Newman. 相似文献
946.
947.
948.
949.
950.
John D. Hadjidemetriou 《Astrophysics and Space Science》1969,3(1):31-45
The effects of non-isotropic ejection of mass from either component of a binary system on the orbital elements are studied, for the case of a small initial eccentricity of the relative orbit, when all the ejected mass falls on the other component. The problem is transformed to an equivalent two-body problem with isotropic variation of mass, plus a perturbing force which is a function of the intial conditions of ejection of the particles and their final, positions and velocities when they fall on the surface of the other star. The variation of the orbital elements are derived. It is shown that, to first-order terms in the eccentricity, the secular change of the semimajor axis is equal to the one corresponding to the case of zero initial eccentricity. On the contrary, the secular change of the eccentricity is smaller and it depends on the variations of mass ejection due to the finite eccentricity. 相似文献