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We calculate the orbital evolution of binary systems where the primary star is an evolved red giant branch (RGB) star, while the secondary star is a low-mass main sequence (MS) star or a brown dwarf. The evolution starts with a tidal interaction that causes the secondary to spiral-in. Than either a common envelope (CE) is formed in a very short time, or alternatively the system reaches synchronization and the spiraling-in process substantially slows down. Some of the latter systems later enter a CE phase. We find that for a large range of system parameters, binary systems reach stable synchronized orbits before the onset of a CE phase. Such stable synchronized orbits allow the RGB star to lose mass prior to the onset of the CE phase. Even after the secondary enters the giant envelope, the rotational velocity is high enough to cause an enhanced mass-loss rate. Our results imply that it is crucial to include the pre-CE evolution when studying the outcome of the CE phase. We find that many more systems survive the CE phase than would be the case if these preceding spin-up and mass-loss phases had not been taken into account. Although we have made the calculations for RGB stars, the results have implications for other evolved stars that interact with close companions. 相似文献
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The simplifying assumption is often made, that when two fluids (whether miscible or immiscible) occupy the void space of a porous medium, they are separated by a sharp interface. Examples are the phreatic surface (between air and water) and the interface between fresh and salt water in a coastal aquifer. The orientation of such a sharp interface as it crosses a surface of discontinuity between media of different permeabilities, and as it intersects an impervious boundary, is shown to depend not only on the fluid and porous media properties, but also on the direction and rate of motion of the interface. Thus, advancing and retreating interfaces intersect boundaries of discontinuity in permeability at different angles. 相似文献
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Employing the principles of continuum mechanics and a volumetric averaging approach to the derivation of the macroscopic balance equation of an extensive quantity of a fluid phase in a porous medium, the paper derives a macroscopic expression for the dispersive flux that appears in the latter as a result of averaging. It is shown that the dispersive flux obeys a Fickian type law, i.e., it is proportional to the macroscopic density gradient of the considered extensive quantity. The nature of the coefficient of dispersion that appears in the expression of the dispersive flux is analyzed and interpreted. 相似文献
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Jacob Bear 《Advances in water resources》1977,1(1):15-23
A general methodology is presented for describing transport phenomena in porous media at a macroscopic level. Then, these macroscopic balance equations are integrated (or averaged) along the vertical for confined, leaky and phreatic aquifers.The results are employed to derive (averaged) aquifer equations for the flow of water and of a solute (hydrodynamic dispersion). It is shown that in all cases, the resulting equation is identical to that derived on the basis of an assumption of horizontal flow (the Dupuit assumption).Macrodispersion, occurring at the aquifer level, is discussed and appropriate coefficients are proposed. 相似文献
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Wang XS Neuman SP Strack OD Verruijt A Jamali M Seymour B Bear J Cheng AH 《Ground water》2011,49(2):133-42; discussion 142-3
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L. I. Elpiner S. A. Bear I. S. Zektser R. K. Klige A. E. Shapovalov 《Water Resources》2007,34(3):340-349
The paper presents the results of analysis of present-day scientific data on the dynamics of global hydroclimatic processes (with the establishment of long-term variations in the global water exchange processes) and the features of water abundance in territories under the effect of various natural and anthropogenic factors. Possible transformations of the hydrogeological situation under global climate changes are considered. The basic principles of the concept regarding the assessment of the effect of global hydroclimatic changes on the population health are presented. Medical-environmental data on the consequences of water-triggered environmental crises, taken as analogous natural models, are analyzed. The processes and regularities in the development of human pathology under the conditions of higher and lower water abundance in a territory are identified. The succession, time stages, and phases of the development of infection, parasitic, noninfectious pathologies are determined. Cause-and-effect relationships were established between the observed pathology and the effect of a complex of changing natural, anthropogenic, and social-environmental factors. Methodological approaches to the assessment of medical-environmental consequences of distinct changes in the water abundance in the territories are described. The possible use of subsurface water sources as a reserve of domestic water supply under emergencies are considered. 相似文献
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