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Showing papers in "Astrophysics and Space Science in 2001"


Journal ArticleDOI
TL;DR: In this paper, it was shown that the incident magnetic energy flux is the driving factor for all these six radio emissions, and that it can be estimated in the same way in all cases.
Abstract: At least six intense nonthermal planetary radio emissions are known in our solar system: the auroral radio emissions from the Earth, Jupiter, Saturn, Uranus and Neptune, and the radio bursts from the Io-Jupiter flux tube. The former are thought to be driven by the solar wind flow pressure or energy flux on the magnetospheric cross-section, while the latter is a consequence of the Io-Jupiter electrodynamic interaction. Although in the solar wind, the flow ram pressure largely dominates the magnetic one, we suggest that the incident magnetic energy flux is the driving factor for all these six radio emissions, and that it can be estimated in the same way in all cases. Consequences for the possible radio emission from extrasolar planets are examined. ‘Hot Jupiters’, if they are magnetized, might possess a radio emission several orders of magnitude stronger than the Jovian one, detectable with large ground-based low-frequency arrays. On the other hand, ‘giants’ analogous to the Io-Jupiter interaction in the form of a pair star/hot-Jupiter are unlikely to produce intense radio emissions, unless the star is very strongly magnetized.

213 citations


Journal ArticleDOI
T. Blöcker1
TL;DR: In this paper, the evolution on the AGB and beyond is reviewed with respect to the origin of Wolf-Rayet central stars, focusing on thermal pulses due to their particular importance for the evolution of hydrogen deficient stars.
Abstract: The evolution on the AGB and beyond is reviewed with respect to the origin of Wolf-Rayet central stars. We focus on thermal pulses due to their particular importance for the evolution of hydrogen deficient stars. It is shown that overshoot applied to all convection regions is a key ingredient to model these objects leading to intershell abundances already close to the surface abundances of Wolf-Rayetcentral stars. In contrast to standard evolutionary calculations, overshoot models do show dredge up for very low envelope masses and efficient dredge up was found even during the post-AGB stage. Three thermal pulse scenarios for Wolf-Rayet central stars can nowbe distinguished: an AGB Final Thermal Pulse (AFTP) occurring at the very end of the AGB evolution, a Late Thermal Pulse (LTP) occurring during the post-AGBevolution when hydrogen burning is still on, and a Very Late Thermal Pulse (VLTP) occurring on the cooling branch when hydrogen burning has already ceased. All scenarios lead to hydrogen-deficient post-AGB stars with carbon and oxygen abundances as observed for Wolf-Rayet stars.Hydrogen is either diluted by dredge up (AFTP, LTP) or completely burnt (VLTP).

136 citations


Journal ArticleDOI
TL;DR: In this paper, a stellar evolution model of post-asymptotic giant branch (AGB) stars is presented. But, the model is not suitable for the case of stars with very small envelope masses.
Abstract: Recent advances in constructing stellar evolution models of hydrogen-deficient post-asymptotic giant branch (AGB) stars are presented. Hydrogen-deficiency can originate from mixing and subsequent convective burning of protons in the deeper layers during a thermal pulse on the post-AGB (VLTP). Dredge-up alone may also be responsible for hydrogen-deficiency of post-AGB stars. Models of the last thermal pulse on the AGB with very small envelope masses have shown efficient third dredge-up. The hydrogen content of the envelope is diluted sufficiently to produce H-deficient post-AGB stars (AFTP). Moreover, dredge-up alone may also cause H-deficiency during the Born-again phase (LTP). During the second AGB phase a convective envelope develops. A previously unknown lithium enrichment at the surface of Born-again stellar models may be used to distinguish between objects with different post-AGB evolution. The observed abundance ratios of C, O and He can be reproduced by all scenarios if an AGB starting model with inclusion of overshoot is used for the post-AGB model sequence. An appendix is devoted to the numerical methods for models of proton capture nucleosynthesis in the He-flash convection zone during a thermal pulse.

107 citations


Journal ArticleDOI
TL;DR: In this paper, a collisional model based on the solution of the Fokker-Planck equation was used to determine the importance of different electron populations as a function of the radial distance.
Abstract: Electron velocity distribution functions (VDF) observed in the low speed solar wind flow are generally characterized by ‘core’ and ‘halo’ electrons. In the high speed solar wind, a third population of ‘strahl’ electrons is generally observed. New collisional models based on the solution of the Fokker-Planck equation can be used to determine the importance of the different electron populations as a function of the radial distance. Typical electron velocity distribution functions observed at 1 AU from the Sun are used as boundary conditions for the high speed solar wind and for the low speed solar wind. Taking into account the effects of external forces and Coulomb collisions with a background plasma, suprathermal tails are found to be present in the electron VDF at low altitudes in the corona when they exist at large radial distances.

64 citations


Journal ArticleDOI
TL;DR: In this article, the field equations in a scalar-tensor theory of gravitation were obtained with the aid of Friedmann-type metric, non-static plane symmetric metric and spatially homogeneous Bianchi type-III metric.
Abstract: Field equations in a scalar-tensor theory of gravitation proposed by Saezand Ballester (1985) are obtained with the aid of (i) Friedmann-type metric (ii) a non static plane symmetric metric and (iii) spatially homogeneous Bianchi type – III metric. Some cosmological models corresponding to perfect fluid and bulk viscous fluid are presented. Physical and kinematical properties of the models are also discussed.

63 citations


Book ChapterDOI
TL;DR: In this paper, an analysis and interpretation of reflected solar Lyman α intensity data acquired with the Hubble Space Telescope (HST) implies an equatorially confined atmosphere with SO2 column densities ∼ 1−2 × 1016 cm−2 Poleward of 30° the SO2 density must decrease sharply reaching an asymptotic polar value of < 1015 cm− 2 at 45° to achieve the observed 2 kR intensity peaks.
Abstract: An analysis and interpretation of reflected solar Lyman α intensity data acquired with the Hubble Space Telescope (HST) implies an equatorially confined atmosphere with SO2 column densities ∼ 1−2 × 1016 cm−2 Poleward of 30° the SO2 density must decrease sharply reaching an asymptotic polar value of < 1015 cm−2 at 45° to achieve the observed 2 kR intensity peaks The corresponding surface reflectivities must be either a constant 0047 for higher equatorial SO2 or a variable reflectivity of 0027 with lower SO2 densities at the equator increasing to a polar value of ∼ 005 The average residence time for an atmospheric SO2 molecule is ∼ 2–3 days for the canonical mass loading rate of the Io plasma torus = 1030 amu s−l With atomic hydrogen in the atmosphere and corona constrained by the HST observations, it is estimated that a pickup proton density ratio of 025–04% can be sustained by a supply of Io plasma torus protons neutralized in Io’s atmosphere/exosphere, if protons constitute 7% of the total torus ion density, which is close to the Chust et al (1999) pickup proton density ratio and under the widely quoted 10% proton content of the torus

63 citations


Journal ArticleDOI
TL;DR: In this paper, the authors summarize the results of non-LTE analyses of CSPN with wind and compare these results with the predictions of the new evolution models, open questions concerning the evolution of stars might be answered.
Abstract: So far, the evolution of post-AGB stars is not fully understood. In particular the formation of hydrogen-deficient and hydrogen-free Central Stars of Planetary Nebulae (CSPN) is unsettled. New evolution models, which allow for the consistent treatment of the physics of late thermalpulses, promise new insights to the formation of these stars. In this paper we summarize the results of non-LTE analyses of CSPN with wind. By comparing these results with the predictions of the new evolutionmodels, open questions concerning the evolution of the stars might be answered. In addition we discuss the driving mechanism of the winds of Wolf-Rayet CSPN. New models, which account for millions of iron lines, support the assumptions that these winds are driven by radiation.

50 citations


Journal ArticleDOI
TL;DR: In this paper, the surface chemistry of hydrogen-deficient post-AGB stars can now be explained byevolutionary calculations involving a late He-shell flash, and a quantitative comparison of theoretical predictions with observed element abundances caneventually constrain details of this evolutionary scenario.
Abstract: According to latest results presented at this workshop, it appears that thesurface chemistry of hydrogen-deficient post-AGB stars can now be explained byevolutionary calculations involving a late He-shell flash. A quantitativecomparison of theoretical predictions with observed element abundances caneventually constrain details of this evolutionary scenario. I will emphasizehere the diverse chemical compositions observed in H-deficient post-AGB stars,which call for an explanation. The abundance patterns vary from almost purehelium atmospheres to hydrogen- and helium-free atmospheres, with a richchoice of compositions in between, involving rather different mixtures of H,He, and CNO.

45 citations


Journal ArticleDOI
TL;DR: In this paper, it was shown that the field equations are solvable for any arbitrary cosmic scale function, assuming a perfect fluid distribution of matter Bianchi type Vspace-time.
Abstract: Assuming a perfect fluid distribution of matter Bianchi type Vspace-time is considered and using a new generation techniqueit is shown that the field equations are solvable for anyarbitrary cosmic scale function. Solutions for particularforms of cosmic scale functions are obtained, and thegeometrical and physical properties of these solutions discussed.

37 citations


Book ChapterDOI
TL;DR: In this paper, the results of a high angular resolution (4″) multi-transition analysis of the molecular gas in M 82 were presented based on the two lowest transitions of 12CO and the ground transition of the rare isotopomeres 13CO and C18O measured with the PdBI, the BIMA array and the IRAM 30m telescope.
Abstract: We present the results of a high angular resolution (4″), multi-transition analysis of the molecular gas in M 82. The analysis is based on the two lowest transitions of 12CO and the ground transition of the rare isotopomeres 13CO and C18O measured with the PdBI, the BIMA array and the IRAM 30m telescope.

37 citations


Journal ArticleDOI
TL;DR: The major steps since van de Hulst's (1953) early hydrostatic models of the extended corona, until the most recent generations of kinetic models of coronal expansion and of the supersonic solar wind flows, are presented in this paper.
Abstract: The study of the solar corona has been strewn with great discoveries, surprises and controversies. The major steps since van de Hulst's (1953)and Chapman's (1957) early hydrostatic models of the extended corona, until the most recent generations of kinetic models of the coronal expansion and of the supersonic solar wind flows, are presented. These models are compared to in-situ observations. Progress in polar wind models went through a somewhat similar evolution that is outlined also. The advantages and limitations of the successive brands of solar wind and polar wind models are considered.

Journal ArticleDOI
TL;DR: In this article, the authors review some recent developments in the theory of stationary states in collisionless media that are very far from thermal equilibrium and show that it is possible to develop a macroscopic picture in the framework of statistical mechanics and thermodynamics.
Abstract: This paper reviews some recent developments in the theory of stationary states in collisionless media that are very far from thermal equilibrium. Such states may evolve under conditions when the binary collision time is much longer than any characteristic time of the processes under consideration. A typical example of such a system is collisionless turbulence in a plasma when the plasma evolves into a highly nonlinear state entirely dominated by wave generation, wave-wave and wave-particle interaction and generating a nearly stationary level of turbulence. Sometimes it is very difficult to describe the evolution of such a state. The present theory shows that it is nevertheless possible to develop a macroscopic picture in the framework of statistical mechanics and thermodynamics which allows for the macroscopic description of such states. This can be achieved introducing a control parameter κ. The equilibrium distribution which replaces the Maxwell-Boltzmann distribution is a generalized Lorentzian orκ-distribution. We sketch the underlying statistical mechanics and provide some arguments for the validity of this approach. On this level it is not possible to obtain a microscopic theory of κ, however, which must be constructed on the way of referring to the particular kind of turbulence. We note a number of unresolved problems.

Journal ArticleDOI
TL;DR: In this paper, the authors explore the feasibility of engineering such a migration over a long time period using gravitational assists to (in effect)transfer orbital energy from Jupiter to the Earth, and thereby enlarge the orbital radius of Earth.
Abstract: The Sun's gradual brightening will seriously compromise the Earth'sbiosphere within ∼ 109 years If Earth's orbit migrates outward,however, the biosphere could remain intact over the entiremain-sequence lifetime of the Sun In this paper, we explore thefeasibility of engineering such a migration over a long timeperiod The basic mechanism uses gravitational assists to (in effect)transfer orbital energy from Jupiter to the Earth, and therebyenlarges the orbital radius of Earth This transfer is accomplishedby a suitable intermediate body, either a Kuiper Belt object or a mainbelt asteroid The object first encounters Earth during an inward passon its initial highly elliptical orbit of large (∼ 300 AU)semimajor axis The encounter transfers energy from the object to theEarth in standard gravity-assist fashion by passing close to theleading limb of the planet The resulting outbound trajectory of theobject must cross the orbit of Jupiter; with proper timing, theoutbound object encounters Jupiter and picks up the energy it lost toEarth With small corrections to the trajectory, or additionalplanetary encounters (eg, with Saturn), the object can repeat thisprocess over many encounters To maintain its present flux of solarenergy, the Earth must experience roughly one encounter every 6000years (for an object mass of 1022 g) We develop the details ofthis scheme and discuss its ramifications

Journal ArticleDOI
TL;DR: In this paper, the effect of oblateness of the primary on the maximum distance of Hill stable direct ''moon'' orbits is investigated, and the equilibrium points and zero velocity curves are discussed.
Abstract: We introduce a new version of Hill's problem to include the effect of oblateness of the primaries, and briefly discuss its equilibrium points and zero velocity curves. As a first application we use this to study Hill stability of direct orbits around the small primary. This can be employed to study the stability of a planet's moon perturbed by an oblate Sun, or of a star's planet perturbed by a distant disk-shaped galaxy. Oblateness of the `Sun' is found to decrese the maximum distance of Hill stable direct `moon' orbits.

Journal ArticleDOI
TL;DR: In this paper, a new model of the Cartwheel galaxy is presented, based on the nearly central inclined collision of a rigid companion galaxy with a self-gravitating disc containing both stars and gas.
Abstract: A new model of the Cartwheel galaxy is presented, based on the nearly central inclined collision of a rigid companion galaxy with a self-gravitating disc containing both stars and gas. We assume that the intruder is the most distant companion (G3 in J. Higdon's notation). Two rings and a transient network of gaseous spokes develop after the passage of the companion. The expansion velocity of the outer ring is in agreement with the one observed for the atomic gas. At the end of the simulation, the position and velocity of the companion agree with those of G3.


Journal ArticleDOI
TL;DR: A review of planetary nebula (PN) WCcentral stars and their nebulae can be found in this paper, where the authors discuss the discrepancies between nebularabundances derived from collisionally excited lines and derived from optical recombination lines.
Abstract: We review elemental abundances derived for planetary nebula (PN) WCcentral stars and for their nebulae. Uncertainties in the abundances of[WC] stars are still too large to enable an abundance sequenceto be constructed. In particular it is not clear why the hotter [WCE]stars have C and O abundances which are systematically lower than those oftheir supposed precursors, the [WCL] stars. This abundance differencecould be real or it may be due to unaccounted-for systematic effects inthe analyses. Hydrogen might not be present in [WC] star winds asoriginallysuggested, since broad pedestals observed at the base of nebular lines canplausibly be attributed to high velocity nebular components. It isrecommended that stellar abundance analyses should be carried out withnon-LTE model codes, although recombination line analyses can provideuseful insights. In particular, C II dielectronic recombinationlines provide a unique means to determine electron temperatures in cool[WC] star winds. We then compare the abundances found for PNe which have [WC] central starswith those that do not. Numerous abundance analyses of PNe have beenpublished, but comparisons based on non-uniform samples and methods arelikely to lack reliability. Nebular C/H ratios, which might be expected todistinguish between PNe around H-poor and H-rich stars, are rather similarfor the two groups, with only a small tendency towards larger values fornebulae around H-deficient stars. Nebular abundances should be obtainedwith photoionization models using the best-fitting non-LTE modelatmosphere for the central star as the input. Heavy-metal line blanketingstill needs to be taken into consideration when modeling the central star,as its omission can significantly affect the ionizing fluxes as well asthe abundance determinations. We discuss the discrepancies between nebularabundances derived from collisionally excited lines and thosederived from optical recombination lines, a phenomenon that may havelinks with the presence of H-deficient central stars.

Journal ArticleDOI
TL;DR: The status of planetary nebulae with Wolf-Rayet type central stars (WR] PN) remains one of the most important problems inthe investigation of planetary NEB as mentioned in this paper.
Abstract: The status of planetary nebulae with Wolf-Rayet type central stars([WR] PN) remains one of the most important problems inthe investigation of planetary nebulae. We cannot claim to understand theevolution of low and intermediate-mass stars without answering the questionhow [WR] PN are created.Analyzing the statistical properties of the whole population of [WR] PNand comparing them to other planetary nebulae (non-[WR] PN) bringsimportant information on their origin and evolutionary status. In thisarticle I will summarize our results of this type of studies and show whatlimits they put on the possible evolutionary routes of [WR] PN creation.

Book ChapterDOI
TL;DR: There are many space plasma physics problems that are both major and unsolved, there are other problems for which the categorization of solved or unsolved depends on one's point of view, and there are still other problems that cannot be made although the basic physics is known as discussed by the authors.
Abstract: There are many space plasma physics problems that are both major and unsolved, there are other problems for which the categorization of solved or unsolved depends on one’s point of view, and there are still other problems that are well understood but unsolved in the sense that quantitative predictions cannot be made although the basic physics is known. The following discussion will, of necessity, be limited and selective. The nature of the Alfvenic turbulence in the solar wind remains a major unsolved mystery: Why is the power spectrum of this anisotropic, compressible, magnetofluid often Kolmogoroff-like, with a power spectral index close to the −5/3 value characteristic of normal fluids? What is the three-dimensional symmetry of the turbulence? Are the magnetic fields quasi-two-dimensional and stochastic, or have they been highly refracted by small velocity shears? What is the origin of the — 1 slope of the energy containing scales? What is the relationship between the turbulent fields and the diffusion coefficients for energetic particle transport parallel and perpendicular to the ambient magnetic field? A general problem in turbulence research is the relationship between the fluid approximation and the kinetic physics that describes the dissipation and damping of fluctuations. There is still much to learn about solar flares, coronal mass ejections and magnetospheric substorms. Another major puzzle is how to quantitatively describe the interaction of the solar wind with the interstellar medium; a problem probably not amenable to solution using fluid equations.

Journal ArticleDOI
TL;DR: In this article, the effect of dust charge variation on small amplitude dust acoustic wave was studied and a Korteweg-de Vries (KdV) equation with positive or negative damping term depending on the wave velocity and the ring parameters was proposed.
Abstract: Nonlinear properties of small amplitude dust acoustic waves, incorporatingboth the ion inertial effect and dust drift effect have been studiedThe effect of dust charge variation is also incorporated It is seen thatdue to the dust charge variation, a Korteweg-de Vries (KdV) equationwith positive or negative damping term depending on the wave velocityand the ring parameters governes the nonlinear dust acoustic wave It isseen that the damping or growth arises due to the assumption that dusthydrodynamical time scale is much smaller than that of the dust chargingscale This assumption is valid only for planetary rings such as Saturn'sF, G and E rings Numerical investigations reveal thatall the three rings in F, G and E, dust acoustic solitary wave admits both negative and positive potentials Instability arises from the available freeenergy of drift motion of dust grains only for the wave with wave velocity λ

Journal ArticleDOI
TL;DR: In this article, Infrared Space Observatory (ISO) Short Wavelength Spectrometer (SWS) observations for 16Wolf-Rayet planetary nebulae (PNe) in the range from 24 to165 μm with the aim of analyzing the dust features present in this group of objects.
Abstract: We present Infrared Space Observatory (ISO)Short Wavelength Spectrometer (SWS) observations for 16Wolf–Rayet ([WR]) planetary nebulae (PNe) in the range from 24 to165 μm with the aim of analyzing the dust features present inthis group of objects We have found that Policyclic AromaticHydrocarbon (PAH) molecular bands are present in most of the observed[WR] planetary nebulae with clear exception for K 2–16 among latetype [WC] stars

Journal ArticleDOI
TL;DR: In this paper, some physical processes in dust plasmas are discussed, with an emphasis on applications to dust in space and in laboratory plasms, and Coulomb crystals of positively charged dust.
Abstract: Ionized gases containing fine (μm to sub-μm sized) charged dust grains, referred to as dusty plasmas, occur in diverse cosmic and laboratory environments. Dust occurs in many space and astrophysical environments, including planetary rings, comets, the Earth's ionosphere, and interstellar molecular clouds. Dust also occurs in laboratory plasmas, including processing plasmas, and crystallized dusty plasmas. Charged dust can lead to various effects in a plasma. In this review, some physical processes in dusty plasmas are discussed, with an emphasis on applications to dusty plasmas in space. This includes theoretical work on several wave instabilities, the role of dust as an electron source, and Coulomb crystals of positively charged dust.

Journal ArticleDOI
TL;DR: In this article, the authors used the optical-infrared B vs I vs I -H color-color diagram to determine the ages of stars in bulges of large, early-type spiral galaxies.
Abstract: We have used the optical-infrared B — I vs I — H color-color diagram to determine the ages of stars in bulges of large, early-type spiral galaxies While the centers are found to be so dusty that no reliable ages can be determined in 95 ± 5% of the sample, the colors at 1 bulge effective radius are remarkably uniform Applying the stellar population models of Vazdekis et al(1996) we find that galactic bulges of types S0 to Sb are old, with a spread in age of at most 2 Gyr The fact that the colors are similar to those of bright ellipticals in the Coma cluster suggests that these bulges formed at the same time There are three galaxies of type She, which are clearly younger than the rest of the sample

Book ChapterDOI
TL;DR: In this article, a quasi-thermal noise spectroscopy (QSS) was used for plasma diagnostics in the solar wind and the Earth's plasmasphere.
Abstract: We present the method of plasma diagnostics by quasi-thermal noise spectroscopy and show examples of application in the solar wind and the Earth’s plasmasphere. Using only an electric antenna and a radio receiver, diagnostics of various space environments (magnetized or not) can be obtained in situ. Because of its accuracy, this technique can be used to cross-check other plasma sensors.

Journal ArticleDOI
TL;DR: In this paper, the authors estimate the uncertainty associated with the use of different stellar evolution prescriptions for the fitting functions and compare their models to those of other authors computed with the same fitting functions, and find that the uncertainties introduced by different fitting functions can be appreciably larger than the error affecting the observational data.
Abstract: The study of line strengths in the spectra of early-type galaxies has proved to be a powerful tool for investigating the age and the metallicity of these systems. When computing models for spectrophotometric narrow-band indices, index calibrations as functions of the relevant stellar parameters (e.g. temperature, gravity and metal content) are used. Thus synthetic indices depend upon these calibrations (called fitting functions), as well as on the stellar evolution ingredients adopted. All these inputs suffer from uncertainties, which impact on the derived value for the indices. In this paper we address this problem quantitatively. We compute synthetic Mg2, Fe 5270, Fe 5335 and Hβ indices for simple stellar populations (SSPs) of various ages and metallicities, under different prescriptions for the fitting functions. This allows us to estimate the impact of the uncertainties in the fitting functions. By comparing our models to those of other authors computed with the same fitting functions, we estimate the uncertainty associated with the use of different stellar evolution prescriptions. It is found that the modelling of the horizontal branch impacts particularly on Fe and Hβ. In the range of parameters explored, the uncertainties introduced by the use of different fitting functions can be appreciably larger than the error affecting the observational data. This typically occurs at high Zfor the metallic line strengths, and at low Z for the Hβindex.

Journal ArticleDOI
TL;DR: A number of late [WC] stars have unique infrared properties, not found among the non-[WC] planetary nebulae, and together define a class of IR]-WC stars as discussed by the authors.
Abstract: A number of late [WC] stars have unique infrared properties, not foundamong the non-[WC] planetary nebulae, and together define a class of IR-[WC]stars. They have unusual IRAS colours, resembling stars in theearliest post-AGB evolution and possibly related to PAH formation.Most or all show a double chemistry, with both a neutral (molecular)oxygen-rich and an inner carbon-rich region. Their dense nebulae indicaterecent evolution from the AGB, suggesting a fatal-thermal-pulse (FTP)scenario. Although both the colours and the stellar characteristicspredict fast evolution, it is shown that this phase must last for104 yr. The morphologies of the nebulae are discussed. Forone object in Sgr, the progenitor mass (1.3 M⊙) is known.The stellar temperatures of the IR-[WC] stars appear much higher inlow metallicity systems (LMC, Sgr). This may be indicative of anextended `pseudo' photosphere. It is proposed that re-accretion ofejected gas may slow down the post-AGB evolution and so extend the lifetime of the IR-[WC] stars.

Journal ArticleDOI
TL;DR: In this article, the effects of gravitational lensing in a modified generalrelativity flat cosmological model were investigated and the predicted lensing probability for low matter densities was larger than in the Einstein-deSitter model but considerably less than in constant-Λ models.
Abstract: We investigate the effects of gravitational lensing in a modified generalrelativity flat cosmological model. We show that the predicted lensingprobability for low matter densities is larger than in the Einstein-deSitter model but considerably less than in constant-Λ models. Wealso find that the optical depth at low density rises with the sourcered-shift more slowly than in constant-Λ models and that the lensdistribution peaks at lower values of the lens red-shift.

Journal ArticleDOI
TL;DR: In this article, the authors derived matter collineations for some static spherically symmetricspacetimes and compared the results with Killing, Ricci and Curvaturesymmetries.
Abstract: We derive matter collineations for some static spherically symmetricspacetimes and compare the results with Killing, Ricci and Curvaturesymmetries We conclude that matter and Ricci collineations are not,in general, the same

Journal ArticleDOI
Bo Peng, Rendong Nan, Yan Su, Y. Qiu, L. Zhu, W. Zhu 
TL;DR: A Five hundred meter Aperture Spherical Telescope (FAST) is proposed to be built in the unique karst area of southwest China, and will act, in a sense, as a prototype for the Square Kilometer Array (SKA) as discussed by the authors.
Abstract: A Five hundred meter Aperture Spherical Telescope (FAST) is proposed to be built in the unique karst area of southwest China, and will act, in a sense, as a prototype for the Square Kilometer Array (SKA). It will be over twice as large as the Arecibo telescope coupled with much wider sky coverage. Some results from site surveys for such a SKA concept are briefly reported. Technically, FAST is not simply a copy of the existing Arecibo telescope but has rather a number of innovations. Firstly, the proposed main spherical reflector, by conforming to a paraboloid of revolution in real time through actuated active control, enables the realization of both wide bandwidth and full polarization capability while using standard feed design. Secondly, a feed support system which integrates optical, mechanical and electronic technologies will effectively reduce the cost of the support structure and control system. Pre-research on FAST has become a key project in the CAS.

Journal ArticleDOI
TL;DR: In this paper, the authors present the basic features and preliminary results of the interface between their spectrophotometric model GRASIL (which calculates galactic SEDs from the UV to the submm with a detailed computation of dust extinction and thermal reemission) with the semi-analytical galaxy formation model GALFORM (which computes galaxy formation and evolution in the hierarchical scenario, providing the star formation history as an input to our model).
Abstract: We present the basic features and preliminary results of the interface between our spectrophotometric model GRASIL (which calculates galactic SEDs from the UV to the submm with a detailed computation of dust extinction and thermal reemission) with the semi-analytical galaxy formation model GALFORM (which computes galaxy formation and evolution in the hierarchical scenario, providing the star formation history as an input to our model). With these two models we are able to synthesize simulated samples of a few thousand galaxies for statistical studies of galaxy properties to investigate galaxy formation and evolution. There is good agreement with the available SED and luminosity function data.