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1.
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2.
This paper deals with the question of whether elliptical galaxies can be considered as equilibrium systems (i.e., the gravitational+centrifugal potential is constant on the external surface). We find that equilibrium models such as Emden-Chandrasekhar polytropes and Roche polytropes withn=0 can account for the main part of observations relative to the ratio of maximum rotational velocity to central velocity dispersion in, elliptical systems. More complex models involving, for example, massive halos could lead to a more complete agreement. Models that are a good fit to the observed data are characterized by an inner component (where most of the mass is concentrated) and a low-density outer component. A comparison is performed between some theoretical density distributions and the density distribution observed by Younget al. (1978) in NGC 4473, but a number of limitations must be adopted. Alternative models, such as triaxial oblate non-equilibrium configurations with coaxial shells, involve a number of problems which are briefly discussed. We conclude that spheroidal oblate models describing elliptical glaxies cannot be ruled out until new analyses relative to more refined theoretical equilibrium models (involving, for example, massive halos) and more detailed observations are performed.  相似文献   

3.
多层湍流大气相位屏的数值模拟   总被引：1，自引：0，他引：1  
高分辨图像重建技术可以有效消除湍流大气的影响,重建目标的接近望远镜衍射极限的图像。但是当目标的视场角比较大时,大气—望远镜综合系统不再是线性空不变的,给图像重建带来新的困难,它是大气性质中的一个重要特性,对大气相位屏的模拟应该满足这一特性。提出了一种可以模拟等晕区效应的多层大气相位屏的数值模拟方法,这种方法可以应用于等晕区对高分辨统计重建方法的影响的研究中。利用该数值模拟方法模拟了5″和8″的双星的斑点图,用斑点干涉术对模拟的双星进行了重建,并与实测的双星的斑点干涉术重建结果进行了比较。比较结果表明该数值模拟方法所产生的等晕区效应与实际的等晕区效应相似。  相似文献   

4.
The non-axisymmetric features observed in the discs of dwarf novae in outburst are usually considered to be spiral shocks, which are the non-linear relatives of tidally excited waves. This interpretation suffers from a number of problems. For example, the natural site of wave excitation lies outside the Roche lobe, the disc must be especially hot, and most treatments of wave propagation do not take into account the vertical structure of the disc.
In this paper I construct a detailed semi-analytical model of the non-linear tidal distortion of a thin, three-dimensional accretion disc by a binary companion on a circular orbit. The analysis presented here allows for vertical motion and radiative energy transport, and introduces a simple model for the turbulent magnetic stress. The   m =2  inner vertical resonance has an important influence on the amplitude and phase of the tidal distortion. I show that the observed patterns find a natural explanation if the emission is associated with the tidally thickened sectors of the outer disc, which may be irradiated from the centre. According to this hypothesis, it may be possible to constrain the physical parameters of the disc through future observations.  相似文献   

5.
We investigate which practical constraints are imposed by foregrounds on the detection of the B-mode polarization generated by gravitational waves, in the case of experiments of the type currently being planned. As the B-mode signal is probably dominated by foregrounds at all frequencies, the detection of the cosmological component depends drastically on our ability to remove foregrounds. We provide an analytical expression with which to estimate the level of the residual polarization for Galactic foregrounds, according to the method employed for their subtraction. We interpret this result in terms of the lower limit of the tensor-to-scalar ratio r that allows us to disentangle the cosmological B-mode polarization from the foreground contribution. Polarized emission from extragalactic radio sources and gravitational lensing is also taken into account. As a first approach, we consider the ideal limit of an instrumental noise-free experiment: for full-sky coverage and a resolution of 1°, we obtain a limit of   r ∼ 10−4  . This value can be improved by high-resolution experiments and, in principle, there is no clear fundamental limit on the detectability of the polarization of gravitational waves. Our analysis is also applied to planned or hypothetical future polarization experiments, taking into account expected noise levels.  相似文献   

6.
Purpose of this paper is to clarify how Planetary Nebulae (PNe) are very interesting laboratories to study cosmic gas dynamics. I first recall the history of PNe which are generated from low and intermediate mass stars through successive mass loss processes starting in the Reg Giant phase of evolution and continuing also after the termination of the pulsed AGB phase, where most of the nebular mass is believed to be ejected. The correpondings tellar winds are the ingredients of the nebula. Their initial properties and subsequent mutual interactions, under the action of the evolving stellar radiation field, are responsible for the properties of the nebula. The observed structures of PNe are considered in detail. Larger scale macroscopic structures (MACS) are examined separately from quite smaller scale microscopic structures (MICS). The formation of MACS, at least in cases of round to moderately elliptical PNe, is shown to be reasonably well understood in terms of existing hydrodynamical models. Considering the kinematical behaviour, MICS can be separated into FLIERs (Fast Low Ionization Emitting Regions) and SLOWERs (slowly moving). Attention is focussed on FLIERs and on the proposed mechanisms to interpret them. Recent observations with the Hubble Space Telescope have provided us with a wealth of detailed (subarcsec) information on the nebular structures. The inner structure of FLIERs is here illustrated to consist of substructures of various shapes with an high degree of individually from object to object, also within the same PN. These new data call for deeper thoretical efforts to solve the problems of cosmic gas dynamics, posed by their observed properties. An ample account is given of the most relevant original works, in an effort to allow the non specialist reader to quickly become acquainted with the status of art in the various aspects of the subject. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

7.
Fresnel lenses offer the possibility of concentrating the flux of X-rays or gamma-rays flux falling on a geometric area of many square metres onto a focal point which need only be a millimetre or so in diameter (and which may even be very much smaller). They can do so with an efficiency that can approach 100%, and yet they are easily fabricated and have no special alignment requirements. Fresnel lenses can offer diffraction-limited angular resolution, even in a domain where that limit corresponds to less than a micro second of arc.Given all these highly desirable attributes, it is natural to ask why Fresnel gamma ray lenses are not already being used, or at least why there is not yet any mission that plans to use the technology. Possible reasons (apart from the obvious one that nobody thought of doing so) include the narrow bandwidth of simple Fresnel lenses, their very long focal length, and the problems of target finding. It is argued that none of these is a ‘show stopper’ and that this technique should be seriously considered for nuclear astrophysics.  相似文献   

8.
Data on the results of the analysis of the content of re-processed panorama of the VENERA-9 lander are presented. The panorama was transmitted historically for the first time from the surface of Venus in 1975. The low noise of the VENERA-9 data allowed allocating a large object of an unusual regular structure. Earlier, its fuzzy image was repeatedly cited in the literature being interpreted as a ??strange stone??. The complex shape and its other features suggest that the object may be a real habitant of the planet. It is not excluded that another similar object observed was damaged during the VENERA-9 landing. From the evidence of its movement and position of some other similar objects it is concluded that because of the limited energy capacity, the physical action of the Venusian fauna may be much slower than that of the Earth fauna. Another question considered is what sources of energy could be used by life in the conditions of the high temperature oxygenless atmosphere of the planet. It is natural to assume that, like on Earth, the Venusian fauna is heterotrophic and should be based on hypothetical flora, using photosynthesis (based on an unknown high temperature biophysical mechanism).  相似文献   

9.
We present VLA observations of the ( J , K )=(1, 1), (2, 2), (3, 3) and (4, 4) inversion transitions of NH3 toward the HW 2 object in Cepheus A, with 1-arcsec angular resolution. Emission is detected in the main hyperfine line of the first three transitions. The NH3(2, 2) emission shows a non-uniform 'ring' structure, which is more extended (3 arcsec) and intense than the emission seen in the (1, 1) and (3, 3) lines. A rotational temperature of ∼ 30–50 K and a lower limit to the mass of ∼ 1 ( X NH3/10−8)−1 M⊙ are derived for the ring structure. The spatio-kinematical distribution of the NH3 emission does not seem to be consistent with a simple circumstellar disc around the HW 2 thermal biconical radio jet. We suggest that it represents the remnant of the parental core from which both the inner 300-au (0.4 arcsec) disc, traced by the water maser spots previously found in the region, and the central object have formed. The complex velocity field of this core is probably produced from bound motions (similar to those of the inner disc) and from interaction with outflowing material.  相似文献   

10.
A simple but sufficiently accurate method for calculating an accretion disk structure is developed. The detailed analysis of the accretion disk is fulfilled by using this method. The effect of turbulent heat transfer on the disk structure is taken into account along with the effect of radiative transfer. The turbulent heat transfer is shown to play an important role, and may be even predominant in the inner disk region. The dependence of temperatureT and density on thez-coordinate is found. Simple analytic expressions are proposed for the run of the density in all the disk zones. It is shown that the inner disk region is convectively stable. The main parameters of all the zones are derived. The geometry of regular motions is studied; the regular hydrodynamical flows are found to appear directed both toward and outside the central object. A detailed comparison is made with the results of other authors.  相似文献   

11.
Some problems in determining the orbits of inner satellites associated with the complex behavior of the target function, which is strongly ravine and which possesses multiple minima in the case of the satellite orbit is determined based on fragmentary observations distributed over a rather long time interval, are studied. These peculiarities of the inverse problems are considered by the example of the dynamics of the inner Jupiter satellites: Amalthea, Thebe, Adrastea, and Metis. Numerical models of the satellite motions whose parameters were determined based on ground-based observations available at the moment to date have been constructed. A composite approach has been proposed for the effective search for minima of the target function. The approach allows one to obtain the respective evaluations of the orbital parameters only for several tens of iterations even in the case of very rough initial approximations. If two groups of observations are available (Adrastea), a formal minimization of the target function is shown to give a solution set, which is the best solution from the point of view of representation of the orbital motion, which is impossible to choose. Other estimates are given characterizing the specific nature of the inverse problems.  相似文献   

12.
We present an extension of the formalism recently proposed by Pepper and Gaudi to evaluate the yield of transit surveys in homogeneous stellar systems, incorporating the impact of correlated noise on transit time-scales on the detectability of transits, and simultaneously incorporating the magnitude limits imposed by the need for radial velocity (RV) follow-up of transit candidates. New expressions are derived for the different contributions to the noise budget on transit time-scales and the least-squares detection statistic for box-shaped transits, and their behaviour as a function of stellar mass is re-examined. Correlated noise that is constant with apparent stellar magnitude implies a steep decrease in detection probability at the high -mass end which, when considered jointly with the RV requirements, can severely limit the potential of otherwise promising surveys in star clusters. However, we find that small-aperture, wide-field surveys may detect hot Neptunes whose RV signal can be measured with present-day instrumentation in very nearby (<100 pc) clusters.  相似文献   

13.
The effectiveness of equal power spectrum filtering in image restoration is analyzed via digital simulation. We start with a gray-scale map of concentric rings with different grey values. To obtain a dirty image, the original image is first transformed via a linear space-invariant system. Then, white noise is mixed into the image. The equal power spectrum filter is then used for the image restoration. The signal to noise ratio (SNR) and a defined steepness of the steps, together with the visual impression, are accepted to characterize the performance of the image restoration. It is concluded that a satisfactory restoration can be realized when the SNR > 15. When SNR < 5, the image is completely un-restorable. The effect of system's transfer function on image restoration is also discussed. The effectiveness of the presented model of equal power spectrum filter is confirmed by applying it to the restoration of an actual astronomical image. Finally, a typical application of the equal power spectrum filter is given.  相似文献   

14.
Abstract— This paper explores two processes, sweeping secular resonance (Ward, 1981) and gas drag (Lecar and Franklin, 1997), at work during the dispersal of the solar nebula. we have two aims not previously considered for the two mechanisms: (1) to explain the likely depletion, by a factor of 1000 or so, of the rocky material in the inner belt (2.0 < a < 3.2 AU); (2) to introduce a means for providing—or contributing to—the dispersion in semimajor axis of the various asteroidal taxonomic classes. We suggest that large asteroids with birthplaces separated by an astronomical unit or more can be finally deposited, owing to drag, at the same semimajor axis. For example, we find that bodies with radii up to 100 km can be transferred by gas drag from the outer belt (a > 3.3 AU) well into the inner one, and that an object already in the inner belt as large or even larger than Vesta (r = 250 km)—thought to be the parent body of many meteorites—can be inwardly displaced by as much as an astronomical unit if the nebula dispersal times lie close to 105 years. For such times, a large fraction of the inner belt's primordial mass can be ejected, with most of it passing into the inner solar system.  相似文献   

15.
白仲瑞  张垒  叶中付 《天文学报》2007,48(4):507-514
天光是天体观测中的一种重要噪声源.减天光问题是制约多目标光纤光谱观测深度的重要因素.主分量分析(PCA)是统计学的一种分析方法,它可以用来寻找各个天光谱之间的关系,以进一步获得目标光谱中含有的天光成分.为了研究LAMOST的减天光方法,用SDSS的一组原始观测数据进行了仿真实验,实验结果表明,采用PCA方法比SDSS处理程序能够更有效地减天光.最后对PCA方法在LAMOST中的应用前景进行了展望.  相似文献   

16.
Paradoxical properties of the KIC 8462852 object discovered in the course the Kepler mission are considered. It has been shown that the assumptions about the nature of the object as a swarm of cometary bodies, fragments resulting from catastrophic collisions of asteroids, or the KIC 8462852b exoplanet meet serious problems and even contradict the Kepler laws, if the eclipsing object is considered as a physical body orbiting a central star. According the energy and other requirements, the hypothetical orbit of KIC 8462852b does not meet the Dyson sphere conception either. In the paper, we used the materials of the study by Boyajian et al. (2015) and the subsequent publications on this theme.  相似文献   

17.
The Monte-Carlo simulation of the neutral interstellar atoms penetration into the solar system is considered. The ionization and charge exchange processes with due regard for the plasma interface structure as well as the gravitational force and the radiation pressure are taken into account. The Monte-Carlo scheme with splitting of atom trajectories is developed, and its high efficiency is shown against the imitation Monte-Carlo scheme. The main algorithms of the splitting scheme and the possible applications of the suggested method are described. In particular calculation of the mass, momentum and energy sources appearing in the plasma gasdynamical equations is considered. The gasdynamical approach for these terms is shown to be inconsistent with the Monte-Carlo results.  相似文献   

18.
We discuss (n+1)-dimensional dynamical wormholes in an evolving cosmological background with a throat expanding with time. These solutions are examined in the general relativity framework. A linear relation between diagonal elements of an anisotropic energy-momentum tensor is used to obtain the solutions. The energy-momentum tensor elements approach the vacuum case when we are far from the central object for one class of solutions. Finally, we discuss the energy-momentum tensor which supports this geometry, taking into account the energy conditions.  相似文献   

19.
Reconstruction of objects by direct demodulation   总被引：3，自引：0，他引：3  
High resolution reconstruction of complicated objects from incomplete and noisy data can be achieved by solving modulation equations iteratively under physical constraints. This direct demodulation method is a powerful technique for dealing with inverse problem in general case. Spectral and image restorations and computerized tomography are only particular cases of general demodulation. It is possible to reconstruct an object in higher dimensional space from observations by a simple lower dimensional instrument through direct demodulation. Our simulations show that wide field and high resolution images of space hard X-rays and soft-rays can be obtained by a collimated non-position-sensitive detector without coded aperture masks.  相似文献   

20.
We present a careful analysis of the point-source detection limit of the AKARI All-Sky Survey in the WIDE-S 90-μm band near the North Ecliptic Pole (NEP). Timeline analysis is used to detect IRAS ( Infrared Astronomy Satellite ) sources and then a conversion factor is derived to transform the peak timeline signal to the interpolated 90-μm flux of a source. Combined with a robust noise measurement, the point-source flux detection limit at signal-to-noise ratio  (S/N) > 5  for a single detector row is  1.1 ± 0.1 Jy  which corresponds to a point-source detection limit of the survey of ∼0.4 Jy.
Wavelet transform offers a multiscale representation of the Time Series Data ( tsd ). We calculate the continuous wavelet transform of the tsd and then search for significant wavelet coefficients considered as potential source detections. To discriminate real sources from spurious or moving objects, only sources with confirmation are selected. In our multiscale analysis, IRAS sources selected above 4σ can be identified as the only real sources at the Point Source Scales. We also investigate the correlation between the non- IRAS sources detected in timeline analysis and cirrus emission using wavelet transform and contour plots of wavelet power spectrum. It is shown that the non- IRAS sources are most likely to be caused by excessive noise over a large range of spatial scales rather than real extended structures such as cirrus clouds.  相似文献   

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