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81.
A statistical analysis of possible clumping (not periodicity) of emission line redshifts of QSO's shows the available data to be compatible with random fluctuations of a smooth, nonclumped distribution. This result is demonstrated with Monte Carlo simulations as well as with the Kolmogorov-Smirnov test. It is in complete disagreement with the analysis by Varshni, which is shown to be incorrect. 相似文献
82.
83.
Probhas Ray Chaudhuri 《Astrophysics and Space Science》1971,12(2):325-328
The spectroscopic determination of the helium abundance in the oldest stars of the galaxy is supported by the theory of stellar evolution when neutrino emission is considered according to the photon-neutrino coupling theory, if it is assumed that the population II stars started their life with a low surface helium content.Receipt delayed by postal strike in Great Britain. 相似文献
84.
Summary. The response of the Earth to an earthquake is a transient that is effectively zero several days after the event. A recording of the event, of finite duration in time, has a Fourier spectrum that is an entire, or integral, analytic function of frequency. We present a very simple procedure for computing the Fourier spectrum as a function of complex frequency; the analytically continued spectrum. By investigating the properties of the analytically continued spectrum we show how to extract high- Q modes, how to estimate Q either from the amplitude or from the width of a resonance function, and how to improve the resolution of splitting to the theoretical maximum. Examples of these procedures, using observed data, are presented. 相似文献
85.
A sediment quality triad (SQT) assessment was made of the impact of copper mine tailings disposal on littoral meiofaunal assemblages in the Atacama region of northern Chile. This situation is unusual in that the disposal is direct into the high-energy coastal system and not via a river estuary or other low-energy environment. This situation also allows for the examination of the impact of copper mine tailings in the absence of confounding effects from other pollutants. The three components of the SQT were: 1. an analysis of the bioavailable metals in both the sedimentary porewater and the adjacent seawater, 2. a microcosm bioassay of both sediments and seawaters using meiofaunal assemblages, and 3. quantitative field samples of the meiofaunal assemblages. Twelve study sites with varying degrees of impact were used, including three reference sites. The study identified that both the meiofaunal assemblage densities and taxa diversities decrease with increasing levels of bioavailable copper, that the Foraminifera and Harpacticoida are sensitive to copper, and that otoplanid Turbellaria are often characteristic of impacted sites; tailings also have both chemical and physical impacts on the environment. In some cases the physical impact of tailings is more important in excluding some organisms e.g. the interstitial polychaete, Saccocirrus sonomacus, from a site than is their chemical impact. 相似文献
86.
Ole B. Andersen Gary D. Egbert Svetlana Y. Erofeeva Richard D. Ray 《Ocean Dynamics》2006,56(5-6):416-429
Overtides and compound tides are generated by nonlinear mechanisms operative primarily in shallow waters. Their presence complicates
tidal analysis owing to the multitude of new constituents and their possible frequency overlap with astronomical tides. The
science of nonlinear tides was greatly advanced by the pioneering researches of Christian Le Provost who employed analytical
theory, physical modeling, and numerical modeling in many extensive studies, especially of the tides of the English Channel.
Le Provost’s complementary work with satellite altimetry motivates our attempts to merge these two interests. After a brief
review, we describe initial steps toward the assimilation of altimetry into models of nonlinear tides via generalized inverse
methods. A series of barotropic inverse solutions is computed for the M tide over the northwest European Shelf. Future applications of altimetry to regions with fewer in situ measurements will
require improved understanding of error covariance models because these control the tradeoffs between fitting hydrodynamics
and data, a delicate issue in coastal regions. While M can now be robustly determined along the Topex/Poseidon satellite ground tracks, many other compound tides face serious aliasing
problems.
In memory of Christian Le Provost 相似文献
87.
Strategies to mitigate aliasing of loading signals while estimating GPS frame parameters 总被引:7,自引:5,他引:2
Xavier?CollilieuxEmail author Tonie?van?Dam Jim?Ray David?Coulot Laurent?Métivier Zuheir?Altamimi 《Journal of Geodesy》2012,86(1):1-14
Although GNSS techniques are theoretically sensitive to the Earth center of mass, it is often preferable to remove intrinsic
origin and scale information from the estimated station positions since they are known to be affected by systematic errors.
This is usually done by estimating the parameters of a linearized similarity transformation which relates the quasi-instantaneous
frames to a long-term frame such as the International Terrestrial Reference Frame (ITRF). It is well known that non-linear
station motions can partially alias into these parameters. We discuss in this paper some procedures that may allow reducing
these aliasing effects in the case of the GPS techniques. The options include the use of well-distributed sub-networks for
the frame transformation estimation, the use of site loading corrections, a modification of the stochastic model by downweighting
heights, or the joint estimation of the low degrees of the deformation field. We confirm that the standard approach consisting
of estimating the transformation over the whole network is particularly harmful for the loading signals if the network is
not well distributed. Downweighting the height component, using a uniform sub-network, or estimating the deformation field
perform similarly in drastically reducing the amplitude of the aliasing effect. The application of these methods to reprocessed
GPS terrestrial frames permits an assessment of the level of agreement between GPS and our loading model, which is found to
be about 1.5 mm WRMS in height and 0.8 mm WRMS in the horizontal at the annual frequency. Aliased loading signals are not
the main source of discrepancies between loading displacement models and GPS position time series. 相似文献
88.
Jayanta K. Bhattacharjee Atri Bhattacharya Tapas K. Das Arnab K. Ray 《Monthly notices of the Royal Astronomical Society》2009,398(2):841-852
In a novel approach to studying viscous accretion flows, viscosity has been introduced as a perturbative effect, involving a first-order correction in the α-viscosity parameter. This method reduces the problem of solving a second-order non-linear differential equation (Navier–Stokes equation) to that of an effective first-order equation. Viscosity breaks down the invariance of the equilibrium conditions for stationary inflow and outflow solutions, and distinguishes accretion from wind. Under a dynamical systems classification, the only feasible critical points of this 'quasi-viscous' flow are saddle points and spirals. On large spatial scales of the disc, where a linearized and radially propagating time-dependent perturbation is known to cause a secular instability, the velocity evolution equation of the quasi-viscous flow has been transformed to bear a formal closeness with Schrödinger's equation with a repulsive potential. Compatible with the transport of angular momentum to the outer regions of the disc, a viscosity-limited length-scale has been defined for the full spatial extent over which the accretion process would be viable. 相似文献
89.
90.
Manjari Bagchi Rachid Ouyed Jan Staff Subharthi Ray Mira Dey † Jishnu Dey ‡ 《Monthly notices of the Royal Astronomical Society》2008,387(1):115-120
We study the effects of temperature on strange stars. It is found that the maximum mass of the star decreases with the increase of temperature, as at high temperatures the equations of state become softer. Moreover, if the temperature of a strange star increases, keeping its baryon number fixed, its gravitational mass increases and its radius decreases. This leads to a limiting temperature, where it turns into a black hole. These features are the result of a combined effect of the change of gluon mass and the quark distribution with temperature. We report on a new type of radial oscillation of strange stars, driven by what we call 'chromothermal' instability. We also discuss the relevance of our findings in the astrophysics of core collapse supernovae and gamma-ray bursts. 相似文献