Local pulsars: A note on the birth-velocity distribution     

A. Blaauw  R. Ramachandran 《Journal of Astrophysics and Astronomy》1998,19(1-2):19-33

We explore a simple model for the representation of the observed distributions of the motions, and the characteristic ages of the local population of pulsars. The principal difference from earlier models is the introduction of a unique value,S, for the kick velocity with which pulsars are born. We consider separately the proper motion components in galactic longitude and latitude, and find that the distributions of the velocity components parallel and perpendicular to the galactic plane are represented satisfactorily byS = 200 km/sec, and leave no room for a significant fraction of much higher velocities. The successful proposition of a unique value for the kick velocity may provide an interesting tool in attempts to understand the physical process leading to the expulsion of the neutron star. This paper is an extended analysis of the talk presented by A. Blaauw at the Raman Research Institute on 20th February, 1996.  相似文献   

Ecological impact of urban stormwater runoff studied in experimental flumes: Population loss by drift and availability of refugial space     

Dietrich Borchardt  Bernhard Statzner 《Aquatic Sciences - Research Across Boundaries》1990,52(4):299-314

Urban stormwater runoff discharged through sewer systems into streams causes flush spills of water and pollutants in the receiving water. To make the right decisions in future plannings of the very costly rehabilitation of sewer systems, a solid ecological data base on the critical parameters of sewer overflows is badly needed. Therefore, we designed a laboratory flume which was operated in circular flow mode (to ensure adaptation of the test organisms) and in flow-through mode during the simulation of sewer overflows (to allow a proper evaluation of population loss by drift). Examples on the behaviour during the adaptation phase and the population loss during the exposure to flush spills of water and/or a mixture of sewage and clean water of a benthic invertebrate (Gammarus pulex) demonstrate the potential of the flume to identify critical parameters of sewer overflows at quasireal-world-conditions. We found clear evidence for synergetic effects since the exposure to high flow and sewage caused higher population loss ofGammarus than the sum of population loss at exposure to only high flow or only sewage. Population loss considerably depended on the availability of refugial space: if the interstices of the gravel in the flume were silted, this loss was higher than at open interstices. Only ten minutes of movement of the material forming the flume bottom reduced the population ofGammarus to about 60 or 50% of its initial size. Hence, our data strongly suggest that the characteristics of the receiving stream (refugial space, bed stability) play an important role for the potential ecological impact of a sewer overflow. Changes of stream morphology and/or creation of refugial space plus an appropriate technical solution for overflow treatment may be less costly and more effective than a large-scale technical project. Thus, the stream itself should be a major element in future management decisions.  相似文献   

压强梯度与前身星模型Ws15M⊙超新星爆发     

罗志全  刘门全  彭秋和 《天文学报》2008,49(1):12-17

针对前身星模型Ws15M⊙,采用程序WLYW89数值模拟了超新星瞬时爆发过程;数值结果表明,在不增大压强的前提下,适当调节坍缩和反弹阶段的压强梯度,超新星瞬时爆发是可以获得成功的.在有限的引力释放能量的情况下,压强梯度的调整增加了对流传能机制,可以缩短瞬时爆发所用的时间,提高爆发能量,使得瞬时强爆炸能够成功.  相似文献   

利用天基测角资料进行定轨的方法初探     

陈务深  甘庆波  掌静  陆本魁  马静远 《天文学报》2008,49(1):81-92

利用观测得到的测角资料确定目标星的轨道,具有非常重要的应用价值.为了解决该问题,首先给出了最优化数学模型STSM,其次,利用测角资料进行多项式拟合,给出求解最优化模型STSM所必需的初始值计算方案,再次,对STSM目标函数的性态进行了详细地分析,并据此给出了求解该优化问题的基本方法.最后,将上面的方法付诸实施,进行了大量的仿真计算,并对仿真结果进行了初步地分析.计算结果表明,在一定的精度和稀疏度的限制之下,根据方案使用天基测角数据确定目标星的轨道根数既可行又稳健.  相似文献   

Artificial viscosity in simulation of shock waves by smoothed particle hydrodynamics     

M. Nejad-Asghar  A. R. Khesali  J. Soltani 《Astrophysics and Space Science》2008,313(4):425-430

This paper reconsiders artificial viscosity in smoothed particle hydrodynamics in order to prevent interparticle penetration, unwanted heating, and unphysical solutions. The coefficients in the Monaghan’s standard artificial viscosity are considered as a time variable, and we propose a restriction on them such that we avoid the undesired effects in the subsonic regions. We use the shock formation in adiabatic and isothermal cases to study the ability of this modified artificial viscosity recipe. Our computer experiments show that the proposal appears to work and the accuracy of this restriction is acceptable.  相似文献   

The evolution of the galaxy red sequence in simulated clusters and groups     

A. D. Romeo  N. R. Napolitano  G. Covone  J. Sommer-Larsen  V. Antonuccio-Delogu †  M. Capaccioli 《Monthly notices of the Royal Astronomical Society》2008,389(1):13-26

N -body/hydrodynamical simulations of the formation and evolution of galaxy groups and clusters in a Λ cold dark matter (ΛCDM) cosmology are used in order to follow the building-up of the colour–magnitude relation in two clusters and in 12 groups. We have found that galaxies, starting from the more massive, move to the red sequence (RS) as they get aged over times and eventually set upon a 'dead sequence' (DS) once they have stopped their bulk star formation activity. Fainter galaxies keep having significant star formation out to very recent epochs and lie broader around the RS. Environment plays a role as galaxies in groups and cluster outskirts hold star formation activity longer than the central cluster regions. However, galaxies experiencing infall from the outskirts to the central parts keep star formation on until they settle on to the DS of the core galaxies. Merging contributes to mass assembly until z ∼ 1, after which major events only involve the brightest cluster galaxies.
The emerging scenario is that the evolution of the colour–magnitude properties of galaxies within the hierarchical framework is mainly driven by star formation activity during dark matter haloes assembly. Galaxies progressively quenching their star formation settle to a very sharp 'red and dead' sequence, which turns out to be universal, its slope and scatter being almost independent of the redshift (since at least z ∼ 1.5) and environment.
Differently from the DS, the operatively defined RS evolves more evidently with z , the epoch when it changes its slope being closely corresponding to that at which the passive galaxies population takes over the star-forming one: this goes from z ≃ 1 in clusters down to 0.4 in normal groups.  相似文献   

Principal Components and Independent Component Analysis of Solar and Space Data     

A.?C.?CadavidEmail author  J.?K.?Lawrence  A.?Ruzmaikin 《Solar physics》2008,248(2):247-261

Principal components analysis (PCA) and independent component analysis (ICA) are used to identify global patterns in solar and space data. PCA seeks orthogonal modes of the two-point correlation matrix constructed from a data set. It permits the identification of structures that remain coherent and correlated or that recur throughout a time series. ICA seeks for maximally independent modes and takes into account all order correlations of the data. We apply PCA to the interplanetary magnetic field polarity near 1 AU and to the 3.25R _⊙ source-surface fields in the solar corona. The rotations of the two-sector structures of these systems vary together to high accuracy during the active interval of solar cycle 23. We then use PCA and ICA to hunt for preferred longitudes in northern hemisphere Carrington maps of magnetic fields.  相似文献   

Sixth- and eighth-order Hermite integrator for N-body simulations     

Keigo Nitadori  Junichiro Makino 《New Astronomy》2008,13(7):498-507

We present sixth- and eighth-order Hermite integrators for astrophysical N-body simulations, which use the derivatives of accelerations up to second-order (snap) and third-order (crackle). These schemes do not require previous values for the corrector, and require only one previous value to construct the predictor. Thus, they are fairly easy to implement. The additional cost of the calculation of the higher-order derivatives is not very high. Even for the eighth-order scheme, the number of floating-point operations for force calculation is only about two times larger than that for traditional fourth-order Hermite scheme. The sixth-order scheme is better than the traditional fourth-order scheme for most cases. When the required accuracy is very high, the eighth-order one is the best. These high-order schemes have several practical advantages. For example, they allow a larger number of particles to be integrated in parallel than the fourth-order scheme does, resulting in higher execution efficiency in both general-purpose parallel computers and GRAPE systems.  相似文献   

瞬变电磁法在地下水勘查中的应用综述   总被引：13，自引：3，他引：10  

张保祥  刘春华 《地球物理学进展》2004,19(3):537-542

水是地球上最重要的天然资源,供人类需要的淡水有95％以上取自地下水,地下水在人类经济活动中发挥着重要的作用．传统的地球物理方法在地下水勘查工作中得到应用的同时,新的更先进的物探方法不断涌现．近年来,瞬变电磁法在国内外的应用得到迅速发展,在地下水勘查领域的应用也得到拓展．与其它电探方法相比,利用瞬变电磁法进行地下水勘查具有明显优势。  相似文献   

Development of an -based spectral analysis system for the coded-aperture hard X-ray balloon payload EXITE2     

P. F. Bloser  Y. Chou  J. E. Grindlay  T. Narita  G. Monnelly 《Astroparticle Physics》2002,17(4):856

We present the spectral analysis system for the second-generation energetic X-ray imaging telescope experiment (EXITE2) balloon payload. EXITE2 is an imaging hard X-ray telescope using a coded-aperture mask and a NaI/CsI phoswich detector operating in the energy range 20–600 keV. The instrument was flown on a high-altitude scientific balloon from Ft. Sumner, NM on 7–8 May, 1997. We describe the details of the EXITE2 spectral analysis system, with emphasis on those aspects peculiar to coded-aperture instruments. In particular, we have made our analysis compatible with the standard X-ray spectral fitting package by generating a response matrix in the appropriate format including all the effects of a coded-aperture system. The use of , which may be a first for coded-aperture data, permits great flexibility in the fitting of spectral models. The additional effects of our phoswich system, or any other detector-specific considerations, may be easily included as well. We test our spectral analysis using observations of the Crab Nebula, and find that the EXITE2 Crab spectrum is consistent with those recorded by previous instruments operating in this energy range.  相似文献