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231.
We present predictions for the numbers of ultracool dwarfs in the Galactic disc population that could be detected by the WFCAM/UKIDSS Large Area Survey and Ultra Deep Survey. Simulated samples of objects are created with masses and ages drawn from different mass functions and birthrates. Each object is then given absolute magnitudes in different passbands based on empirically derived bolometric correction versus effective temperature relationships (or model predictions for Y dwarfs). These are then combined with simulated space positions, velocities and photometric errors to yield observables such as apparent magnitudes and proper motions. Such observables are then passed through the survey selection mechanism to yield histograms in colour. This technique also produces predictions for the proper motion histograms for ultracool dwarfs and estimated numbers for the as yet undetected Y dwarfs. Finally, it is shown that these techniques could be used to constrain the ultra-low-mass mass function and birthrate of the Galactic disc population. 相似文献
232.
N. Callegari Jr S. Ferraz-Mello T. A. Michtchenko 《Celestial Mechanics and Dynamical Astronomy》2006,94(4):381-397
This paper considers the dynamics of two planets, as the planets B and C of the pulsar PSR B1257+12, near a 3/2 mean-motion resonance. A two-degrees-of-freedom model, in the framework of the general three-body planar problem, is used and the solutions are analyzed through surfaces of section and Fourier techniques in the full phase space of the system.On the possibility of existence of a fourth planet in distant orbit, see Wolszczan et al., 2000 相似文献
233.
Theoretical predictions of an accretion model of star formation in galactic clusters, published quite long ago, which is found
to have some relevance with the current picture of star formation have been tested with observation of seven clusters of young
and intermediate ages. It is found that the agreement between the theory and observation is very good. 相似文献
234.
Z. Q. Qu C. L. Xu X. Y. Zhang X. L. Yan C. L. Jin 《Monthly notices of the Royal Astronomical Society》2006,370(4):1790-1796
The influence of magnetic fields on the energy level populations of atoms is firstly studied by analysing the Stokes profiles of Fe i 6302.5 forming in the solar magnetized atmosphere, with the aid of a departure factor defined to evaluate the deviation from the normal Boltzmann distribution without a magnetic field. This factor is directly related to the ratio of line-source function to the continuum one. The relationship between the departure and the magnetic field reveals an effect that the magnetic field induces an exponential increase in the level population of the lower level of Fe i 6302.5 (Landé factor g = 2.5 ) with the field strength. This indicates that the magnetic field can cause the redistribution of populations of those levels whose Landé factors are non-zero. Therefore, this effect should be included in the calculation of the statistical equilibrium. Secondly, an experiment utilizing the Hg5461 line in the laboratory on the Earth is carried out to reveal that the exponential relation is independent of variations in temperature, and the excitation is completely magneto-induced. Finally, the exponential relation is explained by taking account of the magnetic energy in the Boltzmann distribution. 相似文献
235.
For a Hamiltonian that can be separated into N+1(N\geq 2) integrable parts, four algorithms can be built for a symplectic integrator. This research compares these algorithms for the
first and second order integrators. We found that they have similar local truncation errors represented by error Hamiltonian
but rather different numerical stability. When the computation of the main part of the Hamiltonian, H
0, is not expensive, we recommend to use S
* type algorithm, which cuts the calculation of the H
0 system into several small time steps as Malhotra(1991) did. As to the order of the N+1 parts in one step calculation, we found that from the large to small would get a slower error accumulation.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
236.
237.
The mean state of the tropical atmosphere is important as the nature of the coupling between the ocean and the atmosphere
depends nonlinearly on the basic state of the coupled system. The simulation of the annual cycle of the tropical surface wind
stress by 17 atmospheric general circulation models (AGCMs) is examined and intercompared. The models considered were part
of the Atmospheric Model Intercomparison Project (AMIP) and were integrated with observed sea surface temperature (SST) for
the decade 1979–1988. Several measures have been devised to intercompare the performance of the 17 models on global tropical
as well as regional scales. Within the limits of observational uncertainties, the models under examination simulate realistic
tropical area-averaged zonal and meridional annual mean stresses. This is a noteworthy improvement over older generation low
resolution models which were noted for their simulation of surface stresses considerably weaker than the observations. The
models also simulate realistic magnitudes of the spatial distribution of the annual mean surface stress field and are seen
to reproduce realistically its observed spatial pattern. Similar features are observed in the simulations of the annual variance
field. The models perform well over almost all the tropical regions apart from a few. Of these, the simulations over Somali
are interesting. Over this region, the models are seen to underestimate the annual mean zonal and meridional stresses. There
is also wide variance between the different models in simulating these quantities. Large model-to-model variations were also
seen in the simulations of the annual mean meridional stress field over equatorial Indian Ocean, south central Pacific, north
east Pacific and equatorial eastern Pacific oceans. It is shown that the systematic errors in simulating the surface winds
are related to the systematic errors in simulating the Inter-Tropical Convergence Zone (ITCZ) in its location and intensity.
Weaker than observed annual mean southwesterlies simulated by most models over Somali is due to weaker than observed southwesterlies
during the Northern Hemisphere summer. This is related to the weaker than observed land precipitation simulated by most models
during the Northern Hemisphere summer. The diversity in simulation of the surface wind over Somali and equatorial Indian ocean
is related to the diversity of AGCMs in simulating the precipitation zones in these regions.
Received: 2 August 1996 / Accepted: 7 February 1997 相似文献
238.
E. Tóth F. Deak C. S. Gyurkócza Z. S. Kasztovszky R. Kuczi G. Marx B. Nagy S. Oberstedt L. Sajó-Bohus C. S. Sükösd G. Toth N. Vajda 《Environmental Geology》1997,31(1-2):123-127
A steady radon exhalation is assumed in most publications. In a village of North-East Hungary, however, high radon concentrations
have been measured, differing strongly in neighbouring houses and varying in time, due to the interplay of geochemical phenomena.
Received: 20 November 1995 · Accepted: 18 June 1996 相似文献
239.
240.
张众服 《大地测量与地球动力学》1996,(4)
利用三峡地区多年的地形变观测资料,分析了该区的地形变特征。结果表明,各断层活动性甚微或基本不活动,区域地壳相对稳定 相似文献