共查询到20条相似文献，搜索用时 31 毫秒
1.
A. F. Heavens T. D. Kitching L. Verde 《Monthly notices of the Royal Astronomical Society》2007,380(3):1029-1035
The Fisher matrix approach allows one to calculate in advance how well a given experiment will be able to estimate model parameters, and has been an invaluable tool in experimental design. In the same spirit, we present here a method to predict how well a given experiment can distinguish between different models, regardless of their parameters. From a Bayesian viewpoint, this involves computation of the Bayesian evidence. In this paper, we generalize the Fisher matrix approach from the context of parameter fitting to that of model testing, and show how the expected evidence can be computed under the same simplifying assumption of a Gaussian likelihood as the Fisher matrix approach for parameter estimation. With this 'Laplace approximation' all that is needed to compute the expected evidence is the Fisher matrix itself. We illustrate the method with a study of how well upcoming and planned experiments should perform at distinguishing between dark energy models and modified gravity theories. In particular, we consider the combination of 3D weak lensing, for which planned and proposed wide-field multiband imaging surveys will provide suitable data, and probes of the expansion history of the Universe, such as proposed supernova and baryonic acoustic oscillations surveys. We find that proposed large-scale weak-lensing surveys from space should be able readily to distinguish General Relativity from modified gravity models. 相似文献
2.
P. A. Rosen B. H. Wilde R. J. R. Williams J. M. Foster P. A. Keiter R. F. Coker T. S. Perry M. J. Taylor A. M. Khokhlov R. P. Drake G. R. Bennett D. B. Sinars R. B. Campbell 《Astrophysics and Space Science》2005,298(1-2):121-128
In recent years, we have carried out experiments at the University of Rochester’s Omega laser in which supersonic, dense-plasma
jets are formed by the interaction of strong shocks in a complex target assembly (Foster et al., Phys. Plasmas 9 (2002) 2251). We describe recent, significant extensions to this work, in which we consider scaling of the experiment, the
transition to turbulence, and astrophysical analogues. In new work at the Omega laser, we are developing an experiment in
which a jet is formed by laser ablation of a titanium foil mounted over a titanium washer with a central, cylindrical hole.
Some of the resulting shocked titanium expands, cools, and accelerates through the vacuum region (the hole in the washer)
and then enters a cylinder of low-density foam as a jet. We discuss the design of this new experiment and present preliminary
experimental data and results of simulations using AWE hydrocodes. In each case, the high Reynolds number of the jet suggests
that turbulence should develop, although this behaviour cannot be reliably modelled by present, resolution-limited simulations
(because of their low-numerical Reynolds number). 相似文献
3.
Thierry Lasserre 《Earth, Moon, and Planets》2006,99(1-4):275-284
The Double Chooz reactor neutrino experiment will be built in the forthcoming years. Eventhough not dedicated to geo-neutrino
detection, it is based on similar experimental methods. By pushing current technology to the limits an unprecedented precision
will be reached due to careful reduction and control of systematic errors below the percent level. The experience and technical
innovation achieved by this project could be valuable for future geo-neutrino experiments. After discussing the Double Chooz
detector design we focus on progress achieved on scintillating oils and compatible materials. 相似文献
4.
C. C. Kuranz R. P. Drake D. R. Leibrandt E. C. Harding H. F. Robey A. R. Miles B. E. Blue J. F. Hansen H. Louis M. Bono J. Knauer D. Arnett C. A. Meakin 《Astrophysics and Space Science》2005,298(1-2):9-16
Recent results from an ongoing series of Rayleigh-Taylor instability experiments being conducted on the Omega Laser are described.
The goal of these experiments is to study, in a controlled laboratory setting, the mixing that occurs at an unstable interface
subjected to an acceleration history similar to the explosion phase of a core-collapse supernova. In a supernova, the Reynolds
number characterizing this mixing is extremely large (Re > 1010) and is more than sufficient to produce a turbulent flow at the interface. In the laboratory experiment, by contrast, the
spatial scales are much smaller, but are still sufficiently large (Re > 105) to support a turbulent flow and therefore recreate the conditions relevant to the supernova problem. The data from these
experiments will be used to validate astrophysical codes as well as to better understand the transition to turbulence in such
high energy density systems. The experimental results to date using two-dimensional initial perturbations demonstrate a clear
visual transition from a well-ordered perturbation structure consisting of only a few modes to one with considerable modal
content. Analysis of these results, however, indicates that while a turbulent spectrum visually appears to be forming, the
layer has not yet reached the asymptotic growth rate characteristic of a fully turbulent layer. Recent advances in both target
fabrication and diagnostic techniques are discussed as well. These advances will allow for the study of well-controlled 3D
perturbations, increasing our ability to recreate the conditions occurring in the supernova. 相似文献
5.
We discuss the design of jet-driven, radiative-blast-wave experiments for a 10 kJ class pulsed laser facility. The astrophysical motivation is the fact that jets from Young Stellar Objects are typically radiative and that the resulting radiative bow shocks produce complex structure that is difficult to predict. To drive a radiative bow shock, the jet velocity must exceed the threshold for strong radiative effects. Using a 10 kJ class laser, it is possible to produce such a jet that can drive a radiative bow shock in gas that is dense enough to permit diagnosis by x-ray radiography. We describe the design and simulations of such experiments. The basic approach is to shock the jet material and then accelerate it through a collimating hole and into a Xe ambient medium. We identify issues that must be addressed through experimentation or further simulations in order to field successful experiments. 相似文献
6.
Numerical simulations of impacts involving porous bodies: II. Comparison with laboratory experiments
In this paper, we compare the outcome of high-velocity impact experiments on porous targets, composed of pumice, with the results of simulations by a 3D SPH hydrocode in which a porosity model has been implemented. The different populations of small bodies of our Solar System are believed to be composed, at least partially, of objects with a high degree of porosity. To describe the fragmentation of such porous objects, a different model is needed than that used for non-porous bodies. In the case of porous bodies, the impact process is not only driven by the presence of cracks which propagate when a stress threshold is reached, it is also influenced by the crushing of pores and compaction. Such processes can greatly affect the whole body's response to an impact. Therefore, another physical model is necessary to improve our understanding of the collisional process involving porous bodies. Such a model has been developed recently and introduced successfully in a 3D SPH hydrocode [Jutzi, M., Benz, W., Michel, P., 2008. Icarus 198, 242-255]. Basic tests have been performed which already showed that it is implemented in a consistent way and that theoretical solutions are well reproduced. However, its full validation requires that it is also capable of reproducing the results of real laboratory impact experiments. Here we present simulations of laboratory experiments on pumice targets for which several of the main material properties have been measured. We show that using the measured material properties and keeping the remaining free parameters fixed, our numerical model is able to reproduce the outcome of these experiments carried out under different impact conditions. This first complete validation of our model, which will be tested for other porous materials in the future, allows us to start addressing problems at larger scale related to small bodies of our Solar System, such as collisions in the Kuiper Belt or the formation of a family by the disruption of a porous parent body in the main asteroid belt. 相似文献
7.
Impacts into quartz sand: Crater formation,shock metamorphism,and ejecta distribution in laboratory experiments and numerical models 
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下载免费PDF全文 We investigated the ejection mechanics by a complementary approach of cratering experiments, including the microscopic analysis of material sampled from these experiments, and 2‐D numerical modeling of vertical impacts. The study is based on cratering experiments in quartz sand targets performed at the NASA Ames Vertical Gun Range. In these experiments, the preimpact location in the target and the final position of ejecta was determined by using color‐coded sand and a catcher system for the ejecta. The results were compared with numerical simulations of the cratering and ejection process to validate the iSALE shock physics code. In turn the models provide further details on the ejection velocities and angles. We quantify the general assumption that ejecta thickness decreases with distance according to a power‐law and that the relative proportion of shocked material in the ejecta increase with distance. We distinguish three types of shock metamorphic particles (1) melt particles, (2) shock lithified aggregates, and (3) shock‐comminuted grains. The agreement between experiment and model was excellent, which provides confidence that the models can predict ejection angles, velocities, and the degree of shock loading of material expelled from a crater accurately if impact parameters such as impact velocity, impactor size, and gravity are varied beyond the experimental limitations. This study is relevant for a quantitative assessment of impact gardening on planetary surfaces and the evolution of regolith layers on atmosphereless bodies. 相似文献
8.
R.J.R. Williams D. Ward-Thompson A.P. Whitworth 《Monthly notices of the Royal Astronomical Society》2001,327(3):788-798
We present hydrodynamical simulations of the formation, structure and evolution of photoionized columns, with parameters based on those observed in the Eagle Nebula. On the basis of these simulations we argue that there is no unequivocal evidence that the dense neutral clumps at heads of the columns were cores in the pre-existing molecular cloud. In our simulations, a variety of initial conditions leads to the formation and maintenance of near-equilibrium columns. Therefore, it is likely that narrow columns will often occur in regions with large-scale inhomogeneities, but that observations of such columns can tell us little about the processes by which they formed. The manner in which the columns in our simulations develop suggests that their evolution may result in extended sequences of radiation-induced star formation. 相似文献
9.
An international project is underway to design and build a radiotelescope with an effective collecting area two orders of magnitude greaterthan the largest existing instruments. One of the many scientific goals of thisinstrument will be the investigation of the extragalactic radio sourcepopulation at flux densities two to three orders of magnitude fainter thanthe limits of existing observations. We present simulations of the radiosky at 1.4 GHz down to a flux density limit of 0.1 Jy usingextrapolations of known radio luminosity functions for two differentpopulation scenarios. The resulting simulations confirm that a resolutionof 0'
.1 is necessary to avoid formal confusion, but sourceblending may still dominate if the intrinsic size of such faint sourcesis larger than a few kiloparsecs. 相似文献
10.
11.
A. Ciardi S. V. Lebedev A. Frank E. G. Blackman D. J. Ampleford C. A. Jennings J. P. Chittenden T. Lery S. N. Bland S. C. Bott G. N. Hall J. Rapley F. A. Suzuki Vidal A. Marocchino 《Astrophysics and Space Science》2007,307(1-3):17-22
Jets and outflows are thought to be an integral part of accretion phenomena and are associated with a large variety of objects. In these systems, the interaction of magnetic fields with an accretion disk and/or a magnetized central object is thought to be responsible for the acceleration and collimation of plasma into jets and wider angle flows. In this paper we present three-dimensional MHD simulations of magnetically driven, radiatively cooled laboratory jets that are produced on the MAGPIE experimental facility. The general outflow structure comprises an expanding magnetic cavity which is collimated by the pressure of an extended plasma background medium, and a magnetically confined jet which develops within the magnetic cavity. Although this structure is intrinsically transient and instabilities in the jet and disruption of the magnetic cavity ultimately lead to its break-up, a well collimated, “knotty” jet still emerges from the system; such clumpy morphology is reminiscent of that observed in many astrophysical jets. The possible introduction in the experiments of angular momentum and axial magnetic field will also be discussed. 相似文献
12.
《天文和天体物理学研究(英文版)》2021,(10)
We present a conceptual design study of external calibrators in the 21 cm experiment towards detecting the globally averaged radiation of the epoch of reionization(EoR). Employment of external calibrator instead of internal calibrator commonly used in current EoR experiments allows removing instrumental effects such as beam pattern, receiver gain and instability of the system if the conventional three-position switch measurements are implemented in a short time interval. Furthermore, in the new design the antenna system is placed in an underground anechoic chamber with an open/closing ceiling to maximally reduce the environmental effect such as RFI and ground radiation/reflection. It appears that three of the four external calibrators proposed in this paper, including two indoor artificial transmitters and one outdoor celestial radiation(the Galactic polarization), fail to meet our purpose. Diurnal motion of the Galactic diffuse emission turns out to be the most probable source as an external calibrator, for which we have discussed the observational strategy and the algorithm of extracting the EoR signal. 相似文献
13.
M. A. Barucci R. Casacchia M. Fulchignoni R. Burchi A. Dipaoloantonio C. Giuliani L. Milano F. Scaltriti V. Zappala 《Earth, Moon, and Planets》1982,27(4):387-395
We report laboratory simulations of asteroid observations obtained using a specially designed apparatus SAM (the acronym stands for System for Asteroid Models).The aim of the experiment was to assess the geometrical and physical characteristics of an asteroidal body which may affect the shape of its light curve; and, in particular, the parameters which define asteroid orientation with respect to the observer, and the shape and surface morphology of the asteroids.The design and operation of the SAM is described in some detail and the first results obtained with models having a regular shape are presented and discussed.Istituto di Astrofisica Spaziale del CNR 相似文献
14.
We present simulations of interferometers in Earth orbit and on the lunar surface to guide the design and optimization of space-based ultra-long wavelength missions, such as those pioneered by China’s Chang’e Program. We choose parameters and present simulations using simulated data to identify inter-dependencies and constraints on science and engineering parameters. A regolith model is created for the lunar surface array simulation, and the results show that the lunar regolith will have an undesirable effect on the observations. We estimate data transmission requirements,calculate sensitivities for both cases, and discuss the trade-off between brightness temperature sensitivity and angular resolution for the Earth orbit array case. 相似文献
15.
D. Sutton B. R. Johnson M. L. Brown P. Cabella P. G. Ferreira K. M. Smith 《Monthly notices of the Royal Astronomical Society》2009,393(3):894-910
Map making presents a significant computational challenge to the next generation of kilopixel cosmic microwave background polarization experiments. Years worth of time ordered data (TOD) from thousands of detectors will need to be compressed into maps of the T , Q and U Stokes parameters. Fundamental to the science goal of these experiments, the observation of B modes, is the ability to control noise and systematics. In this paper, we consider an alternative to the maximum likelihood method, called destriping , where the noise is modelled as a set of discrete offset functions and then subtracted from the time stream. We compare our destriping code (Descart: the DEStriping CARTographer) to a full maximum likelihood mapmaker, applying them to 200 Monte Carlo simulations of TOD from a ground-based, partial-sky polarization modulation experiment. In these simulations, the noise is dominated by either detector or atmospheric 1/ f noise. Using prior information of the power spectrum of this noise, we produce destriped maps of T , Q and U which are negligibly different from optimal. The method does not filter the signal or bias the E- or B-mode power spectra. Depending on the length of the destriping baseline, the method delivers between five and 22 times improvement in computation time over the maximum likelihood algorithm. We find that, for the specific case of single detector maps, it is essential to destripe the atmospheric 1/ f in order to detect B modes, even though the Q and U signals are modulated by a half-wave plate spinning at 5 Hz. 相似文献
16.
A series of experiments is underway using the Omega laser to examine radiative shocks of astrophysical relevance. In these
experiments, the laser accelerates a thin layer of low-Z material, which drives a strong shock into xenon gas. One-dimensional numerical simulations using the HYADES radiation hydrodynamics
code predict that radiation cooling will cause the shocked xenon to collapse spatially, producing a thin layer of high density
(i.e., a collapsed shock). Preliminary experimental results show a less opaque layer of shocked xenon than would be expected
assuming that all the xenon accumulates in the layer and that the X-ray source is a pure Kα source. However, neither of these
assumptions is strictly correct. Here we explore whether radial mass and/or energy transport may be significant to the dynamics
of the system. We report the results of two-dimensional numerical simulations using the ZEUS-2D astrophysical fluid dynamics
code. Particular attention is given to the simulation method. 相似文献
17.
R. J. de Meijer F. D. Smit F. D. Brooks R. W. Fearick H. J. Wörtche F. Mantovani 《Earth, Moon, and Planets》2006,99(1-4):193-206
The programme Earth AntineutRino TomograpHy (EARTH) proposes to build ten underground facilities each hosting a telescope.
Each telescope consists of many detector modules, to map the radiogenic heat sources deep in the interior of the Earth by
utilising direction sensitive geoneutrino detection. Recent hypotheses target the core-mantle boundary (CMB) as a major source
of natural radionuclides and therefore of radiogenic heat. A typical scale of the processes that take place at the CMB is
about 200 km. To observe these processes from the surface requires an angular resolution of about 3°. EARTH aims at creating
a high-resolution 3D-map of the radiogenic heat sources in the Earth’s interior. It will thereby contribute to a better understanding
of a number of geophysical phenomena observed at the Earth’s surface. This condition requires a completely different approach
from the monolithic detector systems as e.g. KamLAND.
This paper presents, for such telescopes, the boundary conditions set by physics, the estimated count rates, and the first
initial results from Monte-Carlo simulations and laboratory experiments. The Monte-Carlo simulations indicate that the large
volume telescope should consist of detector modules each comprising a very large number of detector units, with a cross section
of roughly a few square centimetres. The signature of an antineutrino event will be a double pulse event. One pulse arises
from the slowing down of the emitted positron, the other from the neutron capture. In laboratory experiments small sized,
10B-loaded liquid scintillation detectors were investigated as candidates for direction sensitive, low-energy antineutrino detection. 相似文献
18.
对中俄联合火星星-星电离层掩星技术体制进行了分析和介绍,采用三维射线追踪方法对电离层掩星事件的电波观测值进行了模拟计算,并利用模拟的掩星观测数据进行了电子密度廓线反演,结果说明仿真算法可靠.利用仿真的方法,分别对掩星电波相位观测误差和卫星轨道误差等带来的反演误差进行了个例计算和分析,结果得到:5%周的相位测量误差对白天电离层掩星探测结果的影响可以忽略,而夜间电子密度测量的绝对误差小于4×108 m-3;卫星轨道误差对掩星的主要影响是导致电离层高度抬升或下降.结果表明,中俄联合火星电离层掩星探测技术体制先进,可望获得高精度的电子密度廓线;其技术体制也可以用于月球电离层环境的探测. 相似文献
19.
Experimental petrology of the Tissint meteorite: Redox estimates,crystallization curves,and evaluation of petrogenetic models 下载免费PDF全文
下载免费PDF全文 Tissint is an olivine‐phyric shergottite from an incompatible element depleted Martian mantle source. Oxythermobarometry applied to Tissint mineral phases demonstrates that the Tissint magma underwent an increase in oxygen fugacity, from ~3.5 log units below the quarz‐fayalite‐magnetite (QFM ) buffer during the early stages of crystallization, to QFM ?1.4 during the latter stages. This is the first time that such an oxidation event has been observed in a depleted shergottite. The reason for the oxidation event is unclear; however, calculations using the MELTS thermodynamic model suggest that auto‐oxidation is insufficient to cause more than ~1 log unit of oxidation, and therefore an external oxidation mechanism—such as oxidation by degassing—is required. If volatiles are responsible for the oxidation, then it indicates that volatiles are not exclusively tied to the enriched Martian mantle reservoir. A series of experiments using the Tissint parental magma were carried out under fixed (isothermal) or variable (cooling rate) temperature control, and at either reducing (QFM ?3.2) or oxidizing (QFM ?1) redox conditions. The observed liquid line of descent supports a potential genetic relationship between basaltic shergottites and olivine‐phyric shergottites. A peritectic relation where olivine is resorbed to form pyroxene is favored by increased oxygen fugacity; if oxidation during crystallization is more common than presently believed, it may explain why olivine is typically anhedral in olivine‐phyric shergottites. Results from a cooling‐rate experiment in which the oxygen fugacity was changed during the latter stages of crystallization resulted in olivine with a Cr compositional profile consistent with oxidized isothermal experiments, despite forming primarily under reducing conditions. A similar profile is observed in Tissint olivines, consistent with its redox history. Our results provide insights into the potential influence of oxidation events on the compositional zoning of minor or trace elements in olivine in olivine‐phyric basalts. 相似文献
20.
针对FAST的天文观测要求,对其天文观测软件进行了设计与开发。首先介绍了FAST天文观测的原理,对天文观测软件进行了需求分析。而后提出了馈源天文运动轨迹规划算法,并进行了仿真。针对其轨迹要求给出了控制方法,对天文观测控制软件进行了设计与实现。最后通过现场实地实验,验证了本文所提的算法与软件的可行性。 相似文献