共查询到20条相似文献，搜索用时 234 毫秒
1.
N. V. Kharchenko V. S. Kislyuk S. F. Rybka A. I. Yatsenko 《Astrophysics and Space Science》1991,177(1-2):65-71
The works of the Goloseevo Observatory (Kiev, U.S.S.R.) on the compilation of catalogues of absolute proper motions measured with respect to faint galaxies are discussed. Using these catalogues, some astrometric and stellar characteristics have been obtained. Particular attention is paid to the problem of improving the accuracy of stellar position and proper motion determinations. The optimum procedure for compiling the consolidated catalogue intended for the improvement of fundamental reference frame, kinematic characteristics of stars as well as for the solution of applied problems is developed. 相似文献
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Narrow-angle astrometry with long-baseline infrared interferometers can provide extremely high accuracies as required for indirect planet detection. Narrow-angle astrometric interferometry exploits the properties of atmospheric turbulence over fields smaller than the interferometer baseline divided by the atmospheric scale height. For such fields, accuracy is linear with star separation, and nearly inversely proportional to baseline length. To exploit these properties, the interferometer observes a relatively bright (< 13 magk) target in the near infrared at 2.2m, and uses phase referencing to find a reference star within the 2.2-m isoplanatic patch. With this technique faint references can be found for most targets. With baselines > 100 m, which also minimize photon-noise errors, and with careful control of systematic errors by using laser metrology, accuracies of tens of microarcseconds/hour should be possible.Paper presented at the Conference onPlanetary Systems: Formation, Evolution, and Detection held 7–10 December, 1992 at CalTech, Pasadena, California, U.S.A. 相似文献
4.
Transient radio sources are necessarily compact and usually are the locations of explosive or dynamic events, therefore offering unique opportunities for probing fundamental physics and astrophysics. In addition, short-duration transients are powerful probes of intervening media owing to dispersion, scattering and Faraday rotation that modify the signals. While radio astronomy has an impressive record obtaining high time resolution, usually it is achieved in quite narrow fields of view. Consequently, the dynamic radio sky is poorly sampled, in contrast to the situation in the X-ray and γ-ray bands. The SKA has the potential to change this situation, opening up new parameter space in the search for radio transients. We summarize the wide variety of known and hypothesized radio transients and demonstrate that the SKA offers considerable power in exploring this parameter space. Requirements on the SKA to search the parameter space include the abilities to: (1) make targeted searches using beam forming capability; (2) conduct blind, all-sky surveys with dense sampling of the frequency–time plane in wide fields; (3) sample the sky with multiple fields of view from spatially well-separated sites in order to discriminate celestial and terrestrial signals; (4) utilize as much of the SKAs aggregate collecting area as possible in blind surveys, thus requiring a centrally condensed configuration, and; (5) localize repeating transient sources to high angular precision, requiring a configuration with long baselines, thus requiring collecting area in both a centrally condensed “core” array and sufficient area on long baselines. 相似文献
5.
V. G. Turyshev 《Astronomy Letters》2009,35(4):215-234
We study the impact of relativistic gravitational deflection of light on the accuracy of future Space Interferometry Mission
(SIM). We estimate the deflection angles caused by the monopole, quadrupole and octupole components of gravitational fields
for a number of celestial bodies in the solar system. We observe that, in many cases, the magnitude of the corresponding effects
is significantly larger than the 1 μas accuracy expected from SIM. This fact argues for the development of a relativistic
observational model for the mission that would account for the influence of both static and time-varying effects of gravity
on light propagation. Results presented here are different from the ones obtained elsewhere by the fact that we specifically
account for the differential nature of the future SIM astrometric measurements. We also obtain an estimate for the accuracy
of possible determination of the Eddington’s parameter γ via SIM global astrometric campaign; we conclude that accuracy of ∼7 × 10−6 is achievable via measurements of deflection of light by solar gravity.
The article was translated by the authors. 相似文献
6.
M. T. Crosta A. Vecchiato F. de Felice M. G. Lattanzi B. Bucciarelli 《Celestial Mechanics and Dynamical Astronomy》2003,87(1-2):209-218
In this article we outline the structure of a general relativistic astrometric model which has been developed to deduce the position and proper motion of stars from 1 µarcsecond optical observations made by an astrometric satellite orbiting around the Sun. The basic assumption of our model is that the Solar System is the only source of gravity, hence we show how we modeled the satellite observations in a many-body perturbative approach limiting ourselves to the order of accuracy of (v/c)2. The microarcsecond observing scenario outlined is that for the GAIA astrometric mission. 相似文献
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M. E. VanHoosier J.-D. F. Bartoe G. E. Brueckner D. K. Prinz J. W. Cook 《Solar physics》1981,74(2):521-530
There exists a growing need to improve the accuracy of measurement of the absolute solar flux within the wavelength range 120–400 nm. Although full-disk solar fluxes and variations thereof in the 120–400 nm region are required to model the solar atmosphere, current increased interest in the measurements arises from their importance in modeling the terrestrial atmosphere. We describe the Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) experiment under development at the Naval Research Laboratory (NRL) for flight aboard the Space Shuttle and the Upper Atmospheric Research Satellite (UARS). SUSIM will monitor the solar flux in the 120–400 nm region with high precision, using an in-flight calibration system to reduce absolute error to < 10%, and error relative to the 400 nm continuum to < 1%.Proceedings of the 14th ESLAB Symposium on Physics of Solar Variations, 16–19 September 1980, Scheveningen, The Netherlands. 相似文献
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C.L. Carilli S. Furlanetto F. Briggs M. Jarvis S. Rawlings H. Falcke 《New Astronomy Reviews》2004,48(11-12):1029
The epoch of reionization (EoR) sets a fundamental benchmark in cosmic structure formation, corresponding to the formation of the first luminous objects that act to ionize the neutral intergalactic medium (IGM). Recent observations at near-IR and radio wavelengths imply that we are finally probing into this key epoch of galaxy formation at z 6. The Square Kilometer Array (SKA) will provide critical insight into the EoR, in a number of ways. First, the ability of the SKA to image the neutral IGM in 21-cm emission is a truly unique probe of the process of reionization, and is recognized as the next necessary and fundamental step in our study of the evolution of large scale structure and cosmic reionization. Second, study of HI 21-cm absorption toward the first radio loud objects probes small to intermediate scale structure in the neutral ‘cosmic web’, as well as HI in the first collapsed structures (proto-disks and mini-halos). And third, the incomparable sensitivity of the SKA allows for the study of the molecular gas, dust, and star formation activity in the first galaxies, as well as the radio continuum emission from the first accreting massive black holes. Such objects will be obscured at optical wavelengths due to absorption by the neutral IGM. 相似文献
11.
For infinitesimal, homologous perturbations, stability analysis has found the solar radiative interior thermally stable. It is considered for the first time here whether stability is preserved when finite amplitude nonhomologous perturbations are present. We argue that local heated regions may develop in the solar core due to magnetic instabilities. Simple numerical estimations are derived for the timescales of the decay of these events and, when heated bubbles are generated that rise towards the surface, of their rising motion. These estimations suggest that the solar core is in a metastable state. For more detailed analysis, we developed a numerical code to solve the differential equation system. Our calculations determined the conditions of metastability and the evolution of timescales. We obtained two principal results. One of them shows that small amplitude heating events (with energy surplus Qo < 1026 ergs) contribute to subtle but long-lifetime heat waves and give the solar interior a persistently oscillating character. Interestingly, the slow decay of heat waves may make their accumulation possible and so their overlapping may contribute to the development of an intermittent, individual, local process of bubble generation, which may also be generated directly by stronger (Qo > 1026 ergs) heating events. Our second principal result is that for heated regions with ΔT/T ≥ 10−4 and radius 105–106cm, the generated bubbles may travel distances larger than their linear size. We point out to some possible observable consequences of the obtained results. 相似文献
12.
We present an investigation of the differences between quasi-instantaneous stellar proper motions from the Hipparcos catalogue and long-term proper motions determined by combining Hipparcos and the Astrographic Catalogue. Our study is based on a sample of about 12000 stars of visual magnitude from 7 to 10 in two declination zones on the northern and equatorial sky. The distribution of the proper-motion differences shows an excess of large deviations. This is caused by the influence of orbital motion of unresolved binary systems. The proper-motion deviations provide statistical evidence for 360 astrometric binaries in the investigated zones, corresponding to about 2400 such binaries in the entire Hipparcos catalogue, in addition to those already known. In order to check whether the observed deviations are compatible with standard assumptions on the basic parameters of binary stars, we model the impact of orbital motion on the observed proper motions in a Monte Carlo simulation. We show that the simulation yields an acceptable approximation of the observations, if a binary frequency between 70% and 100% is assumed, i.e.if most of the stars in the sample are assumed to have a companion. Thus Hipparcos astrometric binaries confirm that the frequency of non-single stars among field stars is very high. We also investigate the influence of the mass function for the secondary component on the result of the simulation. A constant mass function and mass functions with moderate increase towards low masses lead to results, which are compatible with the observed proper-motion effects. A high preponderance of very-low-mass or substellar companions as produced, for example, by a M—1 power law is not in agreement with the frequency of proper-motion deviations in our sample of stars. 相似文献
13.
George Gatewood Lee Breakiron Ronald Goebel Steven Kipp Jane Russell John Stein 《Icarus》1980,41(2):205-231
Extensive testing suggests that astrometric techniques can be employed to detect and study virtually any planetary system that may exist within 40 light years (12.5 parsec) of the Sun. Following the conclusion of Paper I [G. Gatewood, Icarus27 (1976), 1–12], the astrometric group at the Allegheny Observatory began an intensive survey of 20 nearby stars to detect the nonlinear variations in their motion that planetary systems would induce. Several tests conducted to further our understanding of the limitations of this survey indicated that the photographic detector itself is responsible for the majority of the random error. A new photoelectric detector has been designed and a simplified prototype of it successfully tested. The new detector is expected to be able to utilize virtually all of the astrometric information transmitted through the Earth's atmosphere. This is sufficient to determine relative positions to within an accuracy of approximately 1 milliarcsec/hr. Such precisions exceed the design capabilities of the best existing astrometric telescopes, thus a feasibility study has been conducted for the design of an improved instrument. The study concludes that a new ground-based telescope and the new detector combined should be able to study stars as faint as the 17th magnitude with an annual accuracy of a few tenths of a milliarcsecond. However, to obtain the ultimate accuracy possible from current technology, we must place an astrometric system above the Earth's atmosphere. A space-borne instrument utilizing the new detector would in theory have sufficient accuracy to detect any Earth-like planet orbiting any of the several hundred stars nearest the Sun. 相似文献
14.
The space telescope Search for Terrestrial Exo-Planets (STEP) employed a method of sub-pixel technology which ensures that the astrometric accuracy of the telescope on the focal plane is at the order of 1 μas. This kind of astrometric precision is promising to detect the earth-like planets beyond the solar system. In this paper, we analyze the influence of some key factors, including the errors in the stellar proper motion, parallax, the optical center of the system, and the velocity and position of the satellite, on the detection of exoplanets. We propose a relative angular distance method to evaluate the non-linear terms in the variation of star-pair's angular distance caused by the possibly existing exoplanet. This method could avoid the direct influence of measuring errors of the position and proper motion of the reference stars. Supposing that there are eight reference stars and a target star with a planet system in the same field of view, we simulate their five-year observational data, and use the least square method to get the parameters of the planet orbit. Our results show that the method is robust to detect terrestrial planets based on the 1 μas precision of STEP. 相似文献
15.
S. Rawlings F.B. Abdalla S.L. Bridle C.A. Blake C.M. Baugh L.J. Greenhill J.M. van der Hulst 《New Astronomy Reviews》2004,48(11-12):1013
The present-day Universe is seemingly dominated by dark energy and dark matter, but mapping the normal (baryonic) content remains vital for both astrophysics – understanding how galaxies form – and astro-particle physics – inferring properties of the dark components.The Square Kilometer Array (SKA) will provide the only means of studying the cosmic evolution of neutral hydrogen (HI) which, alongside information on star formation from the radio continuum, is needed to understand how stars formed from gas within dark-matter over-densities and the rôles of gas accretion and galaxy merging.‘All hemisphere’ HI redshift surveys to z 1.5 are feasible with wide-field-of-view realizations of the SKA and, by measuring the galaxy power spectrum in exquisite detail, will allow the first precise studies of the equation-of-state of dark energy. The SKA will be capable of other uniquely powerful cosmological studies including the measurement of the dark-matter power spectrum using weak gravitational lensing, and the precise measurement of H0 using extragalactic water masers.The SKA is likely to become the premier dark-energy-measuring machine, bringing breakthroughs in cosmology beyond those likely to be made possible by combining CMB (e.g. Planck), optical (e.g. LSST, SNAP) and other early-21st-century datasets. 相似文献
16.
N. Razavi-Ghods E. de Lera Acedo A. El-Makadema P. Alexander A. Brown 《Experimental Astronomy》2012,33(1):141-155
The new generation of radio telescopes, such as the proposed Square Kilometer Array (SKA) and the Low-Frequency Array (LOFAR) rely heavily on the use of very large phased aperture arrays operating over wide band-widths at frequency ranges up to approximately 1.4?GHz. The SKA in particular will include aperture arrays consisting of many thousands of elements per station providing un-paralleled survey speeds. Currently two different arrays (from nominally 70?MHz to 450?MHz and from 400?MHz to 1.4?GHz) are being studied for inclusion within the overall SKA configuration. In this paper we aim to analyze the array contribution to system temperature for a number of regular and irregular planar antenna array configurations which are possible geometries for the low-frequency SKA (sparse disconnected arrays). We focus on the sub-500?MHz band where the real sky contribution to system temperature (T sys ) is highly significant and dominants the overall system noise temperature. We compute the sky noise contribution to T sys by simulating the far field response of a number of SKA stations and then convolve that with the sky brightness temperature distribution from the Haslam 408?MHz survey which is then scaled to observations at 100?MHz. Our analysis of array temperature is carried out by assuming observations of three cold regions above and below the Galactic plane. The results show the advantages of regular arrays when sampled at the Nyquist rate as well as their disadvantages in the form of grating lobes when under-sampled in comparison to non-regular arrays. 相似文献
17.
The observation of the solar irradiance and its variations,challenging space metrology 总被引：1，自引：0，他引：1
The problems associated with the accurate determination of the total and spectral irradiances of the Sun are discussed. It is estimated that an ultimate accuracy of the order of 2 to 5 × 10–4 should be aimed at and be feasible for total solar irradiance measurements made with second generation objectively characterised absolute radiometers.Proceedings of the 14th ESLAB Symposium on Physics of Solar Variations, 16–19 September 1980, Scheveningen, The Netherlands. 相似文献
18.
S. Johnston R. Taylor M. Bailes N. Bartel C. Baugh M. Bietenholz C. Blake R. Braun J. Brown S. Chatterjee J. Darling A. Deller R. Dodson P. Edwards R. Ekers S. Ellingsen I. Feain B. Gaensler M. Haverkorn G. Hobbs A. Hopkins C. Jackson C. James G. Joncas V. Kaspi V. Kilborn B. Koribalski R. Kothes T. Landecker A. Lenc J. Lovell J.-P. Macquart R. Manchester D. Matthews N. McClure-Griffiths R. Norris U.-L. Pen C. Phillips C. Power R. Protheroe E. Sadler B. Schmidt I. Stairs L. Staveley-Smith J. Stil S. Tingay A. Tzioumis M. Walker J. Wall M. Wolleben 《Experimental Astronomy》2008,22(3):151-273
19.
Summary TheIAU Symposium No. 61 (Perth, 1973, “New Problems in Astrometry’) makes clear that radiosources will play an important role for astrometry
in view of the linkage of reference systems.
The purposes developed later were to link the various systems from the earth and the solar system to the most inertial possible
one.
Extragalactic radiosources are the most reliable objects but they are faint. So, intermediate objects are needed. Among them
the radiostars, optical objects having radioemission at a certain level and for given frequencies have a fundamental and key
position.
Most of such radiostars are double or multiple ‘binaries’ as it can be seen from the list of reference stars issued by the
Working Group of Commission 24, Working Group created in 1978. The last issue of the core list of radiostars was presented
by Ch. de Vegt (Chairman of the WG) on the occasion of the last General Assembly of the IAU (November 1985).
A recent study of the system α Scorpii (Antares) has shown that the data concerning the magnitude and the spectral type are
not accurate enough, according to the accuracy obtained for the astrometric position (±0
.
s
003 for an astrolabe campaign in right ascension).
There is also a need for better knowledge about the orbits. In the case of the astrolabe observations, if the distance between
the components in <3″ the astrometric position concerns the photocenter and accurate physical properties are needed for comparison
with the radio position.
Better astrophysical properties are strongly needed for all radiostars to be used as references, during the time the astrometrists
have to improve the accuracy of their measurements both in optical and radio fields.
Presented by S. Débarbat. 相似文献
20.
The Air Force/NASA Solar Mass Ejection Imager (SMEI) will provide two-dimensional images of the sky in visible light with high (0.1%) photometric precision, and unprecedented sky coverage and cadence. To optimize the information available from these images they must be interpreted in three dimensions. We have developed a Computer Assisted Tomography (CAT) technique that fits a three-dimensional kinematic heliospheric model to remotely-sensed Thomson scattering observations. This technique is designed specifically to determine the corotating background solar wind component from data provided by instruments like SMEI. Here, we present results from this technique applied to the Helios spacecraft photometer observations. The tomography program iterates to a least-squares solution of observed brightnesses using solar rotation, spacecraft motion and solar wind outflow to provide perspective views of each point in space covered by the observations. The corotational tomography described here is essentially the same as used by Jackson et al. (1998) for the analysis of interplanetary scintillation (IPS) observations. While IPS observations are related indirectly to the solar wind density through an assumed (and uncertain) relationship between small-scale density fluctuations and density, Thomson scattering physics is more straightforward, i.e., the observed brightness depends linearly on the solar wind density everywhere in the heliosphere. Consequently, Thomson scattering tomography can use a more direct density-convergence criterion to match observed Helios photometer brightness to brightness calculated from the model density. The general similarities between results based on IPS and Thomson scattering tomography validate both techniques and confirm that both observe the same type of solar wind structures. We show results for Carrington rotation 1653 near solar minimum. We find that longitudinally segmented dense structures corotate with the Sun and emanate from near the solar equator. We discuss the locations of these dense structures with respect to the heliospheric current sheet and regions of activity on the solar surface. 相似文献