Dust grain dynamics in C-Type shock waves in molecular clouds

We investigate the role and behaviour of dust grains in C-type MHD shock waves in weakly ionized, dense molecular clouds. New shock models are presented for steady, oblique C-type shock structures with shock speed v_s=18km/s, n_H=10^5cm^-3, and a grain population represented by either a single grain species or a MRN grain size distribution. The grain size distribution is calculated using Gauss-Legendre weights and the integrals over the continuous distribution of grain sizes are represented by a number of grain bins. The grain population can then be thought of as a series of separate grain size classes, each with uniquely specified properties. The dynamics of each grain size class is different through the shock front, with the smaller grain classes remaining coupled to the magnetic field and larger grains becoming partially decoupled from the magnetic field due to collisions with the neutrals. The importance of grain charging is also demonstrated. The effects of the orientation of the pre-shock magnetic field B_0 are also considered. It is found that there are critical orientations for B_0 in which the shock is no longer C-type and the transition becomes C$^*$ or J-type. The degree of non-coplanarity of the shock solution depends upon the grain model chosen, as well as the orientation of B_0.Comment: 27 pages, 17 figures, submitted to MNRAS July 200

Equivalent beam modeling using numerical reduction techniques

Numerical procedures that can accomplish model reductions for space trusses were developed. Three techniques are presented that can be implemented using current capabilities within NASTRAN. The proposed techniques accomplish their model reductions numerically through use of NASTRAN structural analyses and as such are termed numerical in contrast to the previously developed analytical techniques. Numerical procedures are developed that permit reductions of large truss models containing full modeling detail of the truss and its joints. Three techniques are presented that accomplish these model reductions with various levels of structural accuracy. These numerical techniques are designated as equivalent beam, truss element reduction, and post-assembly reduction methods. These techniques are discussed in detail

Lifetimes and Sizes from Two-Particle Correlation Functions

We discuss the Yano-Koonin-Podgoretskii (YKP) parametrization of the two-particle correlation function for azimuthally symmetric expanding sources. We derive model-independent expressions for the YKP fit parameters and discuss their physical interpretation. We use them to evaluate the YKP fit parameters and their momentum dependence for a simple model for the emission function and propose new strategies for extracting the source lifetime. Longitudinal expansion of the source can be seen directly in the rapidity dependence of the Yano-Koonin velocity.Comment: 15 pages REVTEX, 2 figures included, submitted to Phys. Lett. B, Expanded discussion of disadvantages of standard HBT fit and of Fig.

Stability of optically-active charged excitons in quasi-two dimensional systems

A negatively charged quasi-two dimensional exciton ($X^-$) is solved exactly numerically in the presence of a uniform perpendicular B-field. Various quasi-two dimensional geometries are studied. The charge distribution of the $X^-$ parallel to the B-field is found to be crucial in determining the stability of the optically-active $X^-$ and hence its photoluminescence (PL) signature. The theory provides a quantitative explanation of recent experimental results obtained for a GaAs quantum well. Effects are found which cannot be described within a lowest Landau level approximation. PACS: 78.20.Ls 78.66.-w 73.20.DxComment: Minor revisions, mainly removal of the term trion in favour of the term charged-exciton to comply with Physical Review B. To be published as a Rapid Communication in Physical Review

Credit Constraints And Training After Job Loss

It is a widely held view that imperfect capital markets mean that individuals from poor backgrounds cannot borrow in order to finance educational investments. This view pervades policy formation, and is reflected in the fact that post-compulsory education processes in all countries involve considerable government intervention and large public subsidies. But are the existence of credit constraints an empirical reality? This paper uses unique data to take a new approach to this question. Specifically, the 1995 Canadian Out of Employment Panel (COEP) allows us to explore the financial resources and skill formation choices of a large number of recent job losers. This approach has several advantages, including: a direct test of the role of finances in determining training; the availability of considerable information concerning individual histories; and the fact that the unemployed are a particularly apposite group with which to explore the questions of credit constraints. We find that credit constraints do appear to limit the human capital investments of a significant minority of job seekers. In particular, controlling for a broad range of background characteristics (including past educational investments and labour market outcomes), the possession of liquid assets at the time of job loss is strongly associated with subsequent self-financed training. This basic finding is corroborated with several different kinds of evidence drawn from the survey. The data also allow us to make a rough estimate of the extent to which participation in training would have been increased, had no part of our sample been credit constrained.

The rheology of dense, polydisperse granular fluids under shear

The solution of the Enskog equation for the one-body velocity distribution of a moderately dense, arbitrary mixture of inelastic hard spheres undergoing planar shear flow is described. A generalization of the Grad moment method, implemented by means of a novel generating function technique, is used so as to avoid any assumptions concerning the size of the shear rate. The result is illustrated by using it to calculate the pressure, normal stresses and shear viscosity of a model polydisperse granular fluid in which grain size, mass and coefficient of restitution varies amoungst the grains. The results are compared to a numerical solution of the Enskog equation as well as molecular dynamics simulations. Most bulk properties are well described by the Enskog theory and it is shown that the generalized moment method is more accurate than the simple (Grad) moment method. However, the description of the distribution of temperatures in the mixture predicted by Enskog theory does not compare well to simulation, even at relatively modest densities.Comment: 8 postscript figures Replaced with new version correcting an error in the SME calculations and misc. small corrections. Second replacement with final correction of SME calculation

Hybrid Search: Effectively Combining Keywords and Semantic Searches

This paper describes hybrid search, a search method supporting both document and knowledge retrieval via the flexible combination of ontologybased search and keyword-based matching. Hybrid search smoothly copes with lack of semantic coverage of document content, which is one of the main limitations of current semantic search methods. In this paper we define hybrid search formally, discuss its compatibility with the current semantic trends and present a reference implementation: K-Search. We then show how the method outperforms both keyword-based search and pure semantic search in terms of precision and recall in a set of experiments performed on a collection of about 18.000 technical documents. Experiments carried out with professional users show that users understand the paradigm and consider it very powerful and reliable. K-Search has been ported to two applications released at Rolls-Royce plc for searching technical documentation about jet engines

On the scaling behaviour of cross-tie domain wall structures in patterned NiFe elements

The cross-tie domain wall structure in micrometre and sub-micrometre wide patterned elements of NiFe, and a thickness range of 30 to 70nm, has been studied by Lorentz microscopy. Whilst the basic geometry of the cross-tie repeat units remains unchanged, their density increases when the cross-tie length is constrained to be smaller than the value associated with a continuous film. This occurs when element widths are sufficiently narrow or when the wall is forced to move close to an edge under the action of an applied field. To a very good approximation the cross-tie density scales with the inverse of the distance between the main wall and the element edge. The experiments show that in confined structures, the wall constantly modifies its form and that the need to generate, and subsequently annihilate, extra vortex/anti-vortex pairs constitutes an additional source of hysteresis.Comment: 4 pages, 5 figures, accepted for publication in Europhysics Letters (EPL

A construction of integer-valued polynomials with prescribed sets of lengths of factorizations

For an arbitrary finite set S of natural numbers greater 1, we construct an integer-valued polynomial f, whose set of lengths in Int(Z) is S. The set of lengths of f is the set of all natural numbers n, such that f has a factorization as a product of n irreducibles in Int(Z)={g in Q[x] | g(Z) contained in Z}.Comment: To appear in Monatshefte f\"ur Mathematik; 11 page