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641.
642.
用Voronoi图描述空间方向关系的理论依据 总被引：8，自引：3，他引：8
基于可视点，可视链，可视区域等概念，分析了影响空间方向判断的两个因子：两目标群凸壳的直径与可视区域，证明了用Voronoi图描述空间方向关系的可行性。 相似文献
643.
对Cu-Mo-Sn-S四元系500-640℃的相关系进行研究,并对相关的Cu-Sn-S三元系做了进一步研究与测定。实验表明,500℃时Cu-Sn-S系的稳定的三元化合物有Cu_4SnS_4、Cu_4SnS_6(或Cu_(9.75)Sn_(2.17)S_(13))、Cu_5Sn_2S_7(或Cu_(4.90)Sn_2S_(7.01))、Cu_2SnS_3和Cu_2Sn_3S_7(或Cu_2Sn_(3.26)S_(7.51))。这些三元化合物,除Cu_4SnS_4外,都与硫形成双变关系。 Cu-Mo-Sn-S四元系在500℃时相关系主要表现为MoS_2与Cu_4SnS_6、Cu_5Sn_2S_7、Cu_2SnS_3和Cu_2Sn_3S_7等各相的双变关系,以及金属与Cu_4SnS_4、Cu_5Sn_2S_7、Cu_2SnS_3和Cu_2Sn_3S_7的双变关系。 以不同起始原料,不同温度和方法合成Cu-Mo-Sn-S四元系中唯一的四元化合物——硫钼锡铜矿未获得成功。 相似文献
644.
水热法合成的表面增生水晶的晶体习性与天然水晶近似,但包体性质、偏光镜干涉色、晶体表面特征等方面有明显区别,藉此可作鉴别。 相似文献
645.
646.
Outlier Detection for Compositional Data Using Robust Methods 总被引：6，自引：2，他引：4
Outlier detection based on the Mahalanobis distance (MD) requires an appropriate transformation in case of compositional data.
For the family of logratio transformations (additive, centered and isometric logratio transformation) it is shown that the
MDs based on classical estimates are invariant to these transformations, and that the MDs based on affine equivariant estimators
of location and covariance are the same for additive and isometric logratio transformation. Moreover, for 3-dimensional compositions
the data structure can be visualized by contour lines. In higher dimension the MDs of closed and opened data give an impression
of the multivariate data behavior. 相似文献
647.
Luca Casagrande Chris Flynn Laura Portinari Leo Girardi Raul Jimenez 《Monthly notices of the Royal Astronomical Society》2007,382(4):1516-1540
We use nearby K dwarf stars to measure the helium-to-metal enrichment ratio Δ Y /Δ Z , a diagnostic of the chemical history of the solar neighbourhood. Our sample of K dwarfs has homogeneously determined effective temperatures, bolometric luminosities and metallicities, allowing us to fit each star to the appropriate stellar isochrone and determine its helium content indirectly. We use a newly computed set of Padova isochrones which cover a wide range of helium and metal content.
Our theoretical isochrones have been checked against a congruous set of main-sequence binaries with accurately measured masses, to discuss and validate their range of applicability. We find that the stellar masses deduced from the isochrones are usually in excellent agreement with empirical measurements. Good agreement is also found with empirical mass-luminosity relations.
Despite fitting the masses of the stars very well, we find that anomalously low helium content (lower than primordial helium) is required to fit the luminosities and temperatures of the metal-poor K dwarfs, while more conventional values of the helium content are derived for the stars around solar metallicity.
We have investigated the effect of diffusion in stellar models and the assumption of local thermodynamic equilibrium (LTE) in deriving metallicities. Neither of these is able to resolve the low-helium problem alone and only marginally if the cumulated effects are included, unless we assume a mixing-length which is strongly decreasing with metallicity. Further work in stellar models is urgently needed.
The helium-to-metal enrichment ratio is found to be Δ Y /Δ Z = 2.1 ± 0.9 around and above solar metallicity, consistent with previous studies, whereas open problems still remain at the lowest metallicities. Finally, we determine the helium content for a set of planetary host stars. 相似文献
Our theoretical isochrones have been checked against a congruous set of main-sequence binaries with accurately measured masses, to discuss and validate their range of applicability. We find that the stellar masses deduced from the isochrones are usually in excellent agreement with empirical measurements. Good agreement is also found with empirical mass-luminosity relations.
Despite fitting the masses of the stars very well, we find that anomalously low helium content (lower than primordial helium) is required to fit the luminosities and temperatures of the metal-poor K dwarfs, while more conventional values of the helium content are derived for the stars around solar metallicity.
We have investigated the effect of diffusion in stellar models and the assumption of local thermodynamic equilibrium (LTE) in deriving metallicities. Neither of these is able to resolve the low-helium problem alone and only marginally if the cumulated effects are included, unless we assume a mixing-length which is strongly decreasing with metallicity. Further work in stellar models is urgently needed.
The helium-to-metal enrichment ratio is found to be Δ Y /Δ Z = 2.1 ± 0.9 around and above solar metallicity, consistent with previous studies, whereas open problems still remain at the lowest metallicities. Finally, we determine the helium content for a set of planetary host stars. 相似文献
648.
649.
A. L. Tadross 《Monthly notices of the Royal Astronomical Society》2008,389(1):285-291
The main astrophysical parameters of 24 previously unstudied open clusters of Berkeley catalogue are presented here. JHK near-infrared (Two Micron All Sky Survey) photometry and the proper motions astrometry of Naval Observatory Merged Astrometric Dataset (NOMAD) are used. The clusters' centres, angular diameters, ages, distances and colour excesses for these clusters are estimated for the first time. 相似文献
650.
基于场论分析的建筑物群的移位 总被引：9，自引：2，他引：7
在多边形群目标综合中,移位是一种旨在解决空间邻近冲突的重要操作.以建筑物群在街道拓宽后产生移位操作为例,提出一种基于场论分析的移位方法,认为街区块多边形边界的收缩产生向街区块内部逐步传递并衰减的作用力,从而促使建筑物多边形的空间位置移动,借助于物理学场论中的"等势线"模型来表达这一移位现象.基于Delaunay三角网建立了类似于Voronoi图的建筑物群剖分结构,用于表达移位场模型的"等距离关系曲线".在移位场中目标的运动方向与运动距离由矢量和运算及梯度衰减函数计算完成.算法思想在一地图综合软件系统中已实现. 相似文献