共查询到20条相似文献，搜索用时 46 毫秒
1.
The problem of escape of planetary atmospheres is discussed taking into account the effects of planet's rotation and planet's
orbital momentum. The expressions for the rates of loss of mass and angular momentum per unit area and the total rates of
loss of mass and angular momentum for the whole planetary surface are obtained. These expressions are, then, applied to study
the escape of the constituents of the atmospheres of Mars and Venus.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
2.
We have examined the wind momentum problem of O-type stars in the Galaxy. It is shown that the discrepancy between theoretical
and empirical mass loss rates and terminal velocities can be reversed by using recently updated values of force multiplier
parameters. With these new values, the momentum problem found by former investigators is reversed so that there now appears
to be more than enough radiation force in order to accelerate the stellar winds of a sample of Galactic O-type stars.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
3.
Moncy V. John 《Astrophysics and Space Science》2010,330(1):7-11
Marginal likelihoods for the cosmic expansion rates are evaluated using the ‘Constitution’ data of 397 supernovas, thereby
updating the results in some previous works. Even when beginning with a very strong prior probability that favors an accelerated
expansion, we obtain a marginal likelihood for the deceleration parameter q
0 peaked around zero in the spatially flat case. It is also found that the new data significantly constrains the cosmographic
expansion rates, when compared to the previous analyses. These results may strongly depend on the Gaussian prior probability
distribution chosen for the Hubble parameter represented by h, with h=0.68±0.06. This and similar priors for other expansion rates were deduced from previous data. Here again we perform the Bayesian
model-independent analysis in which the scale factor is expanded into a Taylor series in time about the present epoch. Unlike
such Taylor expansions in terms of redshift, this approach has no convergence problem. 相似文献
4.
Abstract— We reviewed here the recent development on the mathematical formulation of closure temperature of a cooling geochronological system, which permits direct retrieval of cooling rate from cooling age when the diffusion parameters, grain size and initial temperature are known. This formulation is used to show how the cooling rate can be retrieved by comparing the core and bulk age of a mineral determined by a single decay system. The cooling rates of seven H chondrites of the metamorphic types H4, H5 and H6 were retrieved from the available data on the Pb‐Pb model ages of the phosphates and the diffusion kinetic data of Pb in apatite. The results are in excellent agreement with the metallographic cooling rates and show an inverse relation with the metamorphic grade of these chondrites. We also addressed the problem of ～90 Ma younger Sm‐Nd mineral isochron age, defined by orthopyroxene, phosphate and plagioclase, of the Morristown mesosiderite compared to the Pb‐Pb age of the Estherville mesosiderite. It is shown that this younger age could have been a consequence of resetting during cooling instead of an “impulsive heating” event, as suggested earlier. 相似文献
5.
A near equality between the nodal rates of suitably defined Trojan orbits and Jupiter represents an important type of a secular resonance. This case is realized by the model Sun-Jupiter-Saturn-Trojan, referred to the invariable plane. A second theoretical example is based on the elliptic three-body problem Sun-Jupiter-Trojan, where the vanishing nodal rate of a special Trojan orbit and the vanishing rate of Jupiter's longitude of perihelion define a secular resonance.We investigate the perturbations in the asteroidal inclinations and the nodes and consider the possibility of a libration. 相似文献
6.
Internal layers in ice masses can be detected with ice-penetrating radar. In a flowing ice mass, each horizon represents a past surface that has been subsequently buried by accumulation, and strained by ice flow. These layers retain information about relative spatial patterns of accumulation and ablation (mass balance). Internal layers are necessary to accurately infer mass-balance patterns because the ice-surface shape only weakly reflects spatial variations in mass balance. Additional rate-controlling information, such as the layer age, the ice temperature, or the ice-grain sizes and ice-crystal fabric, can be used to infer the absolute rate of mass balance. To infer mass balance from the shapes of internal layers, we solve an inverse problem. The solution to the inverse problem is the best set or sets of unknown boundary conditions or initial conditions that, when used in our calculation of ice-surface elevation and internal-layer shape, generate appropriate predictions of observations that are available. We also show that internal layers can be used to infer martian paleo-surface topography from a past era of ice flow, even though the topography may have been largely altered by subsequent erosion. We have successfully inferred accumulation rates and surface topography from internal layers in Antarctica. Using synthetic data, we demonstrate the ability of this method to solve the corresponding inverse problem to infer accumulation and ablation rates, as well as the surface topography, for martian ice. If past ice flow has affected the shapes of martian internal layers, this method is necessary to infer the spatial pattern and rate of mass balance. 相似文献
7.
George B. Field 《Astrophysics and Space Science》1975,38(1):167-190
The fate of interstellar gas heated by energetic radiation and far from the galactic plane is considered. It is shown that for plausible heating rates a very hot corona forms, separated from the cooler gas in the disk by a conductive boundary layer. For heating rates exceeding a critical value a galactic wind is set up, as first shown by Mathews and Baker for elliptical galaxies. Such winds may fill the intracluster space in clusters of galaxies; further heating may drive a cluster wind, as shown by Yahil and Ostriker. It is believed that the flow problem of intracluster gas in the presence of intercluster gas is not yet adequately solved. A variety of observations at X-ray and ultraviolet wavelengths are needed to resolve these questions. 相似文献
8.
For a simple spherically symmetric mean‐field dynamo model we investigate the possibility of determining the radial dependence of the coefficient α. Growth rates for different magnetic field modes are assumed to be known by measurement. An evolutionary strategy (ES) is used for the solution of the inverse problem. Numerically, we find quite different α‐profiles giving nearly the same eigenvalues. The ES is also applied to find functions α(r) yielding zero growth rates for the lowest four magnetic field modes. Additionally, a slight modification of the ES is utilized for an “energetic” optimization of α2‐dynamos. The consequences of our findings for inverse dynamo theory and for the design of future dynamo experiments are discussed. 相似文献
9.
10.
Mario G. Abadi Richard G. Bower Julio F. Navarro 《Monthly notices of the Royal Astronomical Society》2000,314(4):759-767
We describe similarity solutions that characterize the collapse of collisional gas on to scale-free perturbations in an Einstein–de Sitter universe. We consider the effects of radiative cooling and derive self-similar solutions under the assumption that the cooling function is a power law of density and temperature, Λ( T , ρ )∝ ρ 3/2 T . We use these results to test the ability of smooth particle hydrodynamics (SPH) techniques to follow the collapse and accretion of shocked, rapidly cooling gas in a cosmological context. Our SPH code reproduces the analytical results very well in cases that include or exclude radiative cooling. No substantial deviations from the predicted central mass accretion rates or from the temperature, density and velocity profiles are observed in well-resolved regions inside the shock radius. This test problem lends support to the reliability of SPH techniques to model the complex process of galaxy formation. 相似文献
11.
The problem of contrastreaming instability in collisionless anisotropic plasmas is investigated both for electrostatic and electromagnetic perturbations propagatingalong the ambient magnetic field. Both electron-electron and ion-ion streams are considered. It is found that the electromagnetic instability may, under certain conditions, be characterised by larger growth rates than the electrostatic instability. 相似文献
12.
Translationally hot O(3P) atoms are produced in the atmosphere of Earth by photolysis of O2 and O3 and quenching of O(1D). A rigorous kinetic theory analysis of this problem is developed and compared with the approach previously employed by Logan and McElroy [Planet. Space Sci.25, 117 (1977)]. It is shown that the kinetic theory employed by the previous workers is somewhat deficient. With the line-of-centers cross-section, the rates of reactions of the translationally hot O(3P) atoms with other atmospheric gases are calculated and found to be in some instances many orders of magnitude larger than the equilibrium rates. Though the non-equilibrium reaction rates with O(3P) are substantially increased, they are still not competitive with the corresponding reaction rates with O(1D). 相似文献
13.
Kenneth R. Meyer 《Celestial Mechanics and Dynamical Astronomy》1981,23(1):69-81
This paper shows that there exist two families of periodic solutions of the restrictedN-body problem which are close to large circular orbits of the Kepler problem. These solutions are shown to be of general elliptic type and hence are stable. If the restricted problem admits a symmetry, then there are symmetric periodic solutions which are close to large elliptic orbits of the Kepler problem. 相似文献
14.
Guzik Joyce A. Neuforge-Verheecke Corinne Young A. Cody Epstein Richard I. Poulin Francis M. Schissel Jason R. 《Solar physics》2001,200(1-2):305-321
We summarize the physical input and assumptions commonly adopted in modern standard solar models that also produce good agreement with solar oscillation frequencies. We discuss two motivations for considering non-standard models: the solar neutrino problem and surface lithium abundance problem. We begin to explore the potential for mixed core models to solve the neutrino problem, and compare the structure, neutrino flux, and oscillation frequency predictions for several models in which the inner 25% of the radius is homogenized, taking into account the effects of non-local equilibrium abundances of 3He. The results for the neutrino flux and helioseismic predictions are far from satisfactory, but such models have the potential to reduce the predicted 7Be/8B neutrino flux ratio, and further studies are warranted. Finally, we discuss how much the neutrino problem can be alleviated in the framework of the standard solar model by using reaction rates, abundances and neutrino capture cross-sections at the limits of their uncertainties, while still satisfying the constraints of helioseismology. 相似文献
15.
MIT's Lincoln Laboratory has developed a computer driven, rapidly slewing (4° s–1), electro-optical (3 resolution) telescope. This enables the rapid measurement of angles and instantaneous angular rates for artificial satellites. The simultaneous acquisition of angles and angular rates constitutes a new initial orbit problem which has been solved. Three different methods of solution are presented including an exact, analytical one. Numerical tests on six widely different satellite orbits indicate that the topocentric distance can be determined to better than 1% (and usually as well as 0.1%) for most satellites after a 5–10 min observation interval.The views and conclusion contained in this document are those of the contractor and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the United States Government.This work was sponsored by the Department of the Air Force. 相似文献
16.
E. Gaztañaga A. Romeo J. Barriga E. Elizalde 《Monthly notices of the Royal Astronomical Society》2001,320(1):12-20
We address the problem of encoding and compressing data dominated by noise. Information is decomposed into 'reference' sequences plus arrays containing noisy differences susceptible to being described by a known probability distribution. One can then give reliable estimates of the optimal compression rates by estimating the corresponding Shannon entropy. As a working example, this idea is applied to an idealized model of the cosmic microwave background (CMB) data on board the Planck satellite. Data reduction is a critical issue in space missions because the total information that can be downloaded to Earth is sometimes limited by telemetry allocation. Similar limitations might arise in remotely operated ground based telescopes. This download-rate limitation could reduce the amount of diagnostics sent on the stability of the instruments and, as a consequence, curb the final sensitivity of the scientific signal. Our proposal for Planck consists of taking differences of consecutive circles at a given sky pointing. To a good approximation, these differences could be made independent of the external signal, so that they are dominated by thermal (white) instrumental noise, which is simpler to model than the sky signal. Similar approaches can be found in other individual applications. Generic simulations and analytical predictions show that high compression rates, can be obtained with minor or zero loss of sensitivity. Possible effects of digital distortion are also analysed. The proposed scheme is flexible and reliable enough to be optimized in relation to other critical aspects of the corresponding application. For Planck , this study constitutes an important step towards a more realistic modelling of the final sensitivity of the CMB temperature anisotropy maps. 相似文献
17.
18.
J. S. B. Wyithe R. L. Webster E. L. Turner 《Monthly notices of the Royal Astronomical Society》2000,312(4):843-852
We present a method for computing the probability distribution of microlensed light-curve derivatives both in the case of a static lens with a transverse velocity, and in the case of microlensing that is produced through stellar proper motions. The distributions are closely related in form, and can be considered equivalent after appropriate scaling of the input transverse velocity. The comparison of the distributions in this manner provides a consistent way to consider the relative contribution to microlensing (both large and small fluctuations) of the two classes of motion, a problem that is otherwise an extremely expensive computational exercise. We find that the relative contribution of stellar proper motions to the microlensing rate is independent of the mass function assumed for the microlenses, but is a function of optical depth and shear. We find that stellar proper motions produce a higher overall microlensing rate than a transverse velocity of the same magnitude. This effect becomes more pronounced at higher optical depth. With the introduction of shear, the relative rates of microlensing become dependent on the direction of the transverse velocity. This may have important consequences in the case of quadruply lensed quasars such as Q2237+0305, where the alignment of the shear vector with the source trajectory varies between images. 相似文献
19.
Gamow-Teller (GT) transitions play a preeminent role in the collapse of stellar core in the stages leading to a Type-II supernova. The microscopically calculated GT strength distributions from ground and excited states are used for the calculation of weak decay rates for the core-collapse supernova dynamics and for probing the concomitant nucleosynthesis problem. The B(GT) strength for 57Zn is calculated in the domain of proton-neutron Quasiparticle Random Phase Approximation (pn-QRPA) theory. No experimental insertions were made (as usually made in other pn-QRPA calculations of B(GT) strength function) to check the performance of the model for proton-rich nuclei. The calculated B(GT) strength distribution is in good agreement with measurements and shows differences with the earlier reported shell model calculation. The pn-QRPA model reproduced the measured low-lying strength for 57Zn better in comparison to the KB3G interaction used in the large-scale shell model calculation. The stellar weak rates are sensitive to the location and structure of these low-lying states in daughter 57Cu. The structure of 57Cu plays a sumptuous role in the nucleosynthesis of proton-rich nuclei. The primary mechanism for producing such nuclei is the rp-process and is believed to be important in the dynamics of the collapsing supermassive stars. Small changes in the binding and excitation energies can lead to significant modifications of the predictions for the synthesis of proton rich isotopes. The ?? +-decay and electron capture (EC) rates on 57Zn are compared to the seminal work of Fuller, Fowler and Newman (FFN). The pn-QRPA calculated ?? +-decay rates are generally in good agreement with the FFN calculation. However at high stellar temperatures the calculated ?? +-decay rates are almost half of FFN rates. On the other hand, for rp-process conditions, the calculated electron capture (?? +-decay) rates are bigger than FFN rates by more than a factor 2 (1.5) and may have interesting astrophysical consequences. 相似文献
20.
随着500 m口径球面射电望远镜(Five-hundred-meter Aperture Spherical radio Telescope, FAST)等大型射电望远镜的建设和使用,脉冲星巡天数据进入PB时代.为解决如此大量高速采样的标量数据挖掘问题,促进新天文现象的发现,提出一种基于无监督聚类的脉冲星候选体筛选方案.该方案采用基于密度层次、划分方法的混合聚类算法,结合MapReduce/Spark并行计算模型和基于滑动窗口的分组策略,进而提高大量候选体信号筛选的效率.通过在脉冲星数据集HTRU2 (High Time Resolution Universe)上的对比实验,结果表明该算法能取得较高的精确度和召回率,分别是0.946和0.905,并且当并行节点足够时,该算法的时间复杂度相比串行执行明显下降.可见,该方法为脉冲星观测大数据的分析挖掘提供一种可行思路. 相似文献