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1.
Evgeny Griv 《Monthly notices of the Royal Astronomical Society》2006,365(3):1007-1011
The possibility that protoplanetary gaseous discs are dynamically unstable to axisymmetric and non-axisymmetric gravity perturbations (e.g. those produced by spontaneous disturbances) with characteristic scales larger than the vertical scale height is discussed analytically, using a local Wentzel–Kramers–Brillouin (WKB) approach. It is shown that such discs might be clumpy, and these gravitationally bound clumps may later collapse to become giant planets ('hot Jupiters'). The chief aim in this paper is to underscore a fact of vital importance for application in the planetary formation process: gravitationally unstable non-axisymmetric (spiral) perturbations can effectively transport both the angular momentum and the mass in a spatially inhomogeneous disc. 相似文献
2.
We describe gravitationalN-body simulations to investigate whether various non-Newtonian interactions between the stars of a system could explain the flat rotational curves which are characteristic of actual isolated spiral galaxies. It is shown that replacing the standard Newtonian interaction by the models of Sanders (1984), Kuhn and Kruglyak (1987) and Milgrom (1983), no massive halo (or dark matter) is required to produce the flat rotational curves of the systems under consideration. All models also generate the exponential surface mass density distribution which is in agreement with that observed in disk-shaped galaxies. In relation to the spiral structure of galaxies, we present the evidence that the non-Newtonian interactions can reproduce the multiple armed patterns in stellar disks without dark matter. 相似文献
3.
In this paper, the fourth in a series, we examine again one of the implications of the Lin‐Shu density‐wave theory, specifically, the noncircular systematic motion of the Galactic objects. Our previous investigation is extended by analyzing simultaneously both the line‐of‐sight and transversal velocities of a sample of open clusters for which velocities, distances and ages are available. The ordinary equations of the Oort‐Lindblad theory of galactic differential rotation are used. The minor effects caused by the two‐dimensional tightly‐wound density waves are also taken into account. The published data of 242 currently known optically visible clusters having distances r < 3 kpc from the Sun and ‐200 < z < 200 pc from the Galactic plane, and ages 2 × 108 < t < 2 × 109 yr are collected from Dias et al. (2014), excluding extremely far, high‐velocity, young and old objects in our fitting. The most noteworthy result is the fact that the parameters of Lin–Shu type density waves estimated from two independent line‐of‐sight and transversal along the Galactic longitude velocities are nearly equal. We argue that the resemblance of these Galactic wave structures is so remarkable that no doubt is felt as to the theory's truth with respect to these data. The results obtained allow us to conclude that several low‐m trailing density‐wave patterns with different number of spiral arms m (say, m = 1, 2, 3, and 4), pitch angles (about 5°, 8°, 11°, and 14°, respectively) and amplitudes of the perturbed gravitational potential may coexist in the Galaxy. The latter suggests the asymmetric multiarm, not well‐organized (“flocculent”) spiral structure of the system. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
4.
It is widely believed that electron dynamics in the shock front is essentially collisionless and determined by the quasistationary magnetic and electric fields in the shock. In thick shocks the electron motion is adiabatic: the magnetic moment is conserved throughout the shock and v21 ∝ B. In very thin shocks with large cross-shock potential (the last feature is typical for shocks with strong electron heating), electrons may become demagnetized (the magnetic moment is no longer conserved) and their motion may become nonadiabatic. We consider the case of substantial demagnetization in the shock profile with the small-scale internal structure. The dependence of electron dynamics and downstream distributions on the angle between the shock normal and upstream magnetic field and on the upstream electron temperature is analyzed. We show that demagnetization becomes significantly stronger with the increase of obliquity (decrease of the angle) which is related to the more substantial influence of the inhomogeneous parallel electric field. We also show that the demagnetization is stronger for lower upstream electron temperatures and becomes less noticeable for higher temperatures, in agreement with observations. We also show that demagnetization results, in general, in non-gyrotropic down-stream distributions. 相似文献
5.
Evgeny Griv Michael Gedalin David Eichler Chi Yuan 《Astrophysics and Space Science》2000,271(1):21-58
Linear kinetic theory is developed to describe collective oscillations (and their instabilities) propagating in a rapidly
rotating disk of stars, representing a highly flattened galaxy. The analysis is carried out for the special case of a self-gravitating,
infinitesimally thin, and spatially inhomogeneous system, taking into account the effects both of thermal movements of stars
and of gravitational encounters between stars and giant molecular clouds of an interstellar medium. The star–cloud encounters
are described with the use of the Landau collision integral. The dynamics of gravity perturbations with rare interparticle
encounters is considered. Such a disk is treated by employing the well elaborated mathematical formalisms from plasma perturbation
theory using normal-mode analysis. In particular, the method of solving the Boltzmann equation is applied by integration along
paths, neglecting the influence of star–cloud encounters on the distribution of stars in the zeroth-order approximation. We
are especially interested in important kinetic effects due to wave–star resonances, which we have little knowledge about.
The kinetic effects are introduced via a minor drift motion of stars which is computed from the equations of stellar motion
in an unperturbed central force field of a galaxy. The dispersion laws for two main branches of disk's oscillations, that
is the classical Jeans branch and an additional gradient branch, are deduced. The resonant Landau-type instabilities of hydrodynamically
stable Jeans and gradient gravity perturbations is considered to be a long-term generating mechanism for propagating density
waves, thereby leading to spiral-like and/or ring-like patterns in the flat galaxies.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
6.
7.
Natural analogue investigations aim to understand key phenomena and processes in natural systems related to those expected to occur in radioactive waste repositories. One of the key applications of natural analogue studies has been the possibility to test the geochemical models to be used to describe the migration of radionuclides in a future radioactive waste repository system. To this end, several geochemical modelling testing exercises (commonly denoted as blind predictive modelling, BPM) have formed an integral part of these studies over the last decade.
We have reviewed, discussed and compared the results obtained from geochemical modelling BPM exercises carried out within six natural analogue studies: Poços de Caldas, Cigar Lake, Maqarin, El Berrocal, Oklo and Palmottu. To make this comparison meaningful, we present the main geochemical characteristics of each site in order to highlight the most relevant mineralogical and hydrochemical differences. The elements selected for discussion are: Sr, Ba, Sn, Pb, Se, Ni, Zn, REEs, Th and U. We have based our discussion on the results obtained from the calculated aqueous speciation as well as by comparing solubility calculations with the actually observed concentrations.
Results can be differentiated into two categories of elemental behaviour:
1. those elements like Th and U under reducing conditions that can be fairly well described by assuming solubility control exerted by pure solid phases as their oxyhydroxides;
2. elements such as Sr, Zn, REEs and U under oxidising conditions for which the association to major geochemical components of the system must be considered in order to explain their concentrations in groundwaters.
Additionally, we discuss the main improvements made to the thermodynamic databases and the geochemical calculation methodologies due to the BPM exercises. Furthermore, the most important characterisation geochemical data needed to complete predictive solubility and speciation calculations are identified. 相似文献
8.
Edward Liverts Evgeny Griv David Eichler Michael Gedalin 《Astrophysics and Space Science》2000,274(1-2):315-320
An improved linear stability theory of small-amplitude oscillations of a self-gravitating, infinitesimally thin gaseous disk
of spiral galaxies has been developed by Bertin, Lau, Lin, Mark, Morozov, Polyachenko, and others in the approximation of
moderately tightly wound gravity perturbations. In this regime, the generalized Lin–Shu type dispersion relation was also
found by including higher order terms in the small parameter 1/kr for wavenumber k and radius r. It was shown that in the differentially rotating disks for nonaxisymmetric (spiral)perturbations Toomre's modified critical
Q-parameter is larger than the standard one: the fact that the spiral perturbations in the nonuniformly rotating system are
more unstable than the axisymmetric ones is taken into account in this modified local stability criterion. We use hydrodynamical
simulations to test the validity of the modified local criterion.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
9.
Detrital zircon U–Pb provenance and palaeogeography of Triassic rift basins in the Marrakech High Atlas 
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下载免费PDF全文 The Marrakech High Atlas contains some of the best exposures of the Triassic early‐rift strata related to Atlantic opening in NW Africa. We present the first detrital zircon U–Pb data of five Triassic redbed samples from the Tizi n'Test basin to quantify sediment provenance, transport and dispersal patterns during early rifting. These U–Pb ages document dominant sediment sourcing from the south, the Anti‐Atlas domain, with very limited to absent input from the Variscan Meseta domain to the north. This combined with stratigraphic and thermochronologic information points to a highly asymmetric palaeogeography during Triassic rifting. Furthermore, the occurrence of Archaean detrital zircon grains in Triassic sandstone, likely recycled from the Reguibat shield, suggests the presence of a fully developed regional drainage system with rivers and catchments reaching hundreds of kilometres into the hinterland of the rift flank. 相似文献
10.
On the basis of their orbital elements, present-day Kuiper belt objects can be grouped into distinct dynamical classes: classical, resonant and scattered ones. Jiang & Yeh have proposed gas-drag-induced resonant capture in a protostellar disc analogous to the primordial solar nebula as a mechanism able to explain the dominant 3:2 resonant population observed in Kuiper belt objects. de la Fuente Marcos & de la Fuente Marcos further investigated the drag-induced mechanism numerically. Our significant contribution is a hydrodynamic theory derivation of results obtained in the Jiang & Yeh and de la Fuente Marcos & de la Fuente Marcos numerical simulations. 相似文献