Interfacial instabilities driven by self-gravity: first numerical results

The evolution of the interstellar medium (ISM) is driven by a variety of phenomena, including turbulence, shearing flows, magnetic fields and the thermal properties of the gas. Among the most important forces at work is self-gravity, which ultimately drives protostellar collapse. As part of an ongoing study of instabilities in the ISM, Hunter, Whitaker & Lovelace have discovered another process driven by self-gravity: the instability of an interface of discontinuous density. Theory predicts that this self-gravity driven interfacial instability persists in the static limit and in the absence of a constant background acceleration. Disturbances to a density interface are found to grow on a time-scale of the order of the free-fall time, even when the perturbation wavelength is much less than the Jeans length. Here we present the first numerical simulations of this instability. The theoretical growth rate is confirmed and the non-linear morphology displayed. The self-gravity interfacial instability is shown to be fundamentally different from the Rayleigh–Taylor instability, although both exhibit similar morphologies under the condition of a high density contrast, such as is commonly found in the ISM. Such instabilities are a possible mechanism by which observed features, such as the pillars of gas seen near the boundaries of interstellar clouds, are formed.     

The origin of spatial intermittency in the galaxy distribution

The dynamical equations describing the evolution of a self-gravitating fluid can be rewritten in the form of a Schrödinger equation coupled to a Poisson equation determining the gravitational potential. This approach has a number of interesting features, many of which were pointed out in a seminal paper by Widrow & Kaiser. In particular we show that this approach yields an elegant reformulation of an idea of Jones concerning the origin of lognormal intermittency in the galaxy distribution.     

Problems, Connections and Expectations of Asteroseismology: a Summary of the Workshop

The workshop took place at the beginning of what promises tobe a golden age of asteroseismology.Ground-based instrumentation is finally reaching a level of stabilitywhich allows detailed investigations of solar-like oscillations in atleast bright, slowly rotating main-sequence stars.Very extensive results are expected from the coming space missions,including data on a broad range of stars from the Eddington mission.The observational situation is therefore extremely promising.To make full use of these promises, major efforts are requiredtowards the efficient utilization of the data, through the developmentof techniques for the analysis and interpretation of the data.A broad range of topics related to these issues is discussed in the presentproceedings. Here I review some of the relevant problems,relate the asteroseismic investigations to broader areas of astrophysics and consider briefly the basis for our great expectations for the developmentof the field.     

Experiments with CP stars

Described here is a preliminary and tentative application of a method for fine-classification of stars, selected for studies of galactic structure. The present investigation is based on about 100 stars within the approximate spectral type range B6-A5, for which both spectra andUBV photometry have been obtained. The motivation of the project is the following: For the study of galactic fine-structure it is essential to use as many members as possible of stellar agglomerations of various type for a statistical treatment of the material. A-type stars are fairly numerous and reasonably bright, but if all chemically peculiar stars, fast rotators or multiple systems have to be omitted there is generally too little left for a relevant investigation. Here we perform some experiments in order to find a method for fine-classification of A stars, both normal and chemically peculiar, within the framework of the M K system. In this connection it is desirable to reduce the present multitude of CP classifications to a manageable number. It is shown here that, independenttly of the definition of the degree of peculiarity, there is no sharp borderline separating the CP stars from the ‘normal’ ones. Also it is found that spectral classification can on an average be performed with almost the same accuracy for CP stars as for the ‘normal’ ones provided cases of extreme peculiarity are avoided. Based on observations collected at European Southern Observatory, La Silla, Chile.     

Spline-surface interpolation, spatial filtering, and trend surfaces for geological mapped variables

Spline surfaces are interpolated for top of the Dundee Limestone of the central Michigan Basin, USA. The requirement of gridded data render spline functions inappropriate tools for representing many types of geological mapped data. Comparisons are drawn with maps for the same Michigan data based on trend surfaces and spatial filtering.     

景德镇陶瓷作坊的文化生态景观研究

根据自然—经济—文化相互联系及物质与能量转换的关系,通过实地考察景德镇生态资源,考证现存的大型陶瓷作坊,分析相关的历史文献和考古资料,运用手工绘图、测量数据,从文化地理学的角度论证了陶瓷作坊应严格遵循自然规律,因地制宜、因材施艺、资源循环利用;建筑材料、结构功能、生产工序、工艺布局具有科学性、合理性、严谨性。指出景德镇陶瓷工匠创造了采光、通风、温度、湿度宜人的生产环境,陶瓷作坊达到了结构、功能、效率、风格的完美统一,堪称世界人与自然和谐相处的经典之作,揭示作坊景观形成机理。     

雷击地闪密度与雷暴日数的关系分析

根据美国卫星观测得到的我国各省区年平均总闪电资料并计算出各省区的年平均地闪密度(以下简称地闪密度);根据全国各省区220个站点的年平均雷暴日资料,按照<建筑物防雷设计规范>GB50057-94(2000年版)(以下简称<规范>)公式计算出全国各省区的地闪密度.比较两种方法得出的地闪密度,发现存在较大的差异,且不同区域差异变化很大.用单位雷暴日地闪密度概念,分析了我国各地单位雷暴日地闪密度的差异及其原因,指出单位雷暴日地闪密度能粗略反映与我国气候特征相适应的地闪密度的分布状况.进一步分析湖州市2007年人工观测和闪电定位仪观测的闪电资料,得到相同的结论.从而提出<规范>规定的地闪密度计算公式存在较大误差,应对其进行进一步的修订和完善.