Collisions, Cosmic Radiation and the Colors of the Trojan Asteroids

The Trojan asteroids orbit about the Lagrangian points of Jupiter and the residence times about their present location are very long for most of them. If these bodies originated in the outer Solar System, they should be mainly composed of water ice, but, in contrast with comets, all the volatiles close to the surface would have been lost long ago. Irrespective of the rotation period, and hence the surface temperature and ice sublimation rate, a dust layer exists always on the surface. We show that the timescale for resurfacing the entire surface of the Trojan asteroids is similar to that of the flattening of the red spectrum of the new dust by solar-proton irradiation. This, if the cut-off radius of the size distribution of the impacting objects is between 1mm and 1m and its slope is -3, for the entire size-range. Therefore, the surfaces of most Trojan asteroids should be composed mainly of unirradiated dust.Comment: In press in Icaru

Particle breakage criteria in discrete-element modelling

Previous work by the authors, using the discrete element method (DEM) has used the octahedral shear stress within a sphere together with a Weibull distribution of strengths and a size effect on average strength, to determine whether fracture occurs or not. This leads to fractal particle size distributions and a normal compression line which are consistent with experimental data. However there is no agreement in the literature as to what the fracture criterion should be and as yet it is not clear whether other criteria could lead to the correct evolution of voids ratio and particle size distribution under increasing stress. Various possibilities for the criterion have been studied in detail here to ascertain whether these other criteria may give the correct behaviour under normal compression. The use of the major principal stress within a particle, the mean stress, and the stress calculated from the maximum contact force on a particle are each investigated as alternatives to the octahedral shear stress. Only the criterion based on the maximum contact force is shown to give behaviour observed experimentally and the simulations shed further insight into the micro mechanics of normal compression

Atomic spectral-product representations of molecular electronic structure: metric matrices and atomic-product composition of molecular eigenfunctions

Recent progress is reported in development of ab initio computational methods for the electronic structures of molecules employing the many-electron eigenstates of constituent atoms in spectral-product forms. The approach provides a universal atomic-product description of the electronic structure of matter as an alternative to more commonly employed valence-bond- or molecular-orbital-based representations. The Hamiltonian matrix in this representation is seen to comprise a sum over atomic energies and a pairwise sum over Coulombic interaction terms that depend only on the separations of the individual atomic pairs. Overall electron antisymmetry can be enforced by unitary transformation when appropriate, rather than as a possibly encumbering or unnecessary global constraint. The matrix representative of the antisymmetrizer in the spectral-product basis, which is equivalent to the metric matrix of the corresponding explicitly antisymmetric basis, provides the required transformation to antisymmetric or linearly independent states after Hamiltonian evaluation. Particular attention is focused in the present report on properties of the metric matrix and on the atomic-product compositions of molecular eigenstates as described in the spectral-product representations. Illustrative calculations are reported for simple but prototypically important diatomic (H_2, CH) and triatomic (H_3, CH_2) molecules employing algorithms and computer codes devised recently for this purpose. This particular implementation of the approach combines Slater-orbital-based one- and two-electron integral evaluations, valence-bond constructions of standard tableau functions and matrices, and transformations to atomic eigenstate-product representations. The calculated metric matrices and corresponding potential energy surfaces obtained in this way elucidate a number of aspects of the spectral-product development, including the nature of closure in the representation, the general redundancy or linear dependence of its explicitly antisymmetrized form, the convergence of the apparently disparate atomic-product and explicitly antisymmetrized atomic-product forms to a common invariant subspace, and the nature of a chemical bonding descriptor provided by the atomic-product compositions of molecular eigenstates. Concluding remarks indicate additional studies in progress and the prognosis for performing atomic spectral-product calculations more generally and efficiently

Modifications of comet materials by the sublimation process: Results from simulation experiments

An active comet like comet Halley loses by sublimation a surface layer of the order of 1 m thickness per perihelion passage. In situ measurements show that water ice is the main constituent which contributes to the gas emission although even more volatile species (CO, NH3, CH4, CO2 etc.) have been identified. Dust particles which were embedded in the ices are carried by the sublimating gases. Measurements of the chemical composition of cometary grains indicate that they are composed of silicates of approximate chondritic composition and refractory carbonaceous material. Comet simulation experiments show that significant modifications of cometary materials occur due to sublimation process in near surface layers which have to be taken into account in order to derive the original state of the material

Theoretical Spectra and Atmospheres of Extrasolar Giant Planets

We present a comprehensive theory of the spectra and atmospheres of irradiated extrasolar giant planets. We explore the dependences on stellar type, orbital distance, cloud characteristics, planet mass, and surface gravity. Phase-averaged spectra for specific known extrasolar giant planets that span a wide range of the relevant parameters are calculated, plotted, and discussed. The connection between atmospheric composition and emergent spectrum is explored in detail. Furthermore, we calculate the effect of stellar insolation on brown dwarfs. We review a variety of representative observational techniques and programs for their potential for direct detection, in light of our theoretical expectations, and we calculate planet-to-star flux ratios as a function of wavelength. Our results suggest which spectral features are most diagnostic of giant planet atmospheres and reveal the best bands in which to image planets of whatever physical or orbital characteristics.Comment: 47 pages, plus 36 postscript figures; with minor revisions, accepted to the Astrophysical Journal, May 10, 2003 issu

Albedo and Reflection Spectra of Extrasolar Giant Planets

We generate theoretical albedo and reflection spectra for a full range of extrasolar giant planet (EGP) models, from Jovian to 51-Pegasi class objects. Our albedo modeling utilizes the latest atomic and molecular cross sections, a Mie theory treatment of extinction by condensates, a variety of particle size distributions, and an extension of the Feautrier radiative transfer method which allows for a general treatment of the scattering phase function. We find that due to qualitative similarities in the compositions and spectra of objects within each of five broad effective temperature ranges, it is natural to establish five representative EGP albedo classes: a ``Jovian'' class (T$_{\rm eff} \lesssim 150$ K; Class I) with tropospheric ammonia clouds, a ``water cloud'' class (T$_{\rm eff} \sim 250$ K; Class II) primarily affected by condensed H$_2$O, a ``clear'' class (T$_{\rm eff} \gtrsim 350$ K; Class III) which lacks clouds, and two high-temperature classes: Class IV (900 K $\lesssim$ T$_{\rm{eff}}$ $\lesssim$ 1500 K) for which alkali metal absorption predominates, and Class V (T$_{\rm{eff}}$ $\gtrsim$ 1500 K and/or low surface gravity ($\lesssim$ 10$^3$ cm s$^{-2}$)) for which a high silicate layer shields a significant fraction of the incident radiation from alkali metal and molecular absorption. The resonance lines of sodium and potassium are expected to be salient features in the reflection spectra of Class III, IV, and V objects. We derive Bond albedos and effective temperatures for the full set of known EGPs and explore the possible effects of non-equilibrium condensed products of photolysis above or within principal cloud decks. As in Jupiter, such species can lower the UV/blue albedo substantially, even if present in relatively small mixing ratios.Comment: revised LaTeX manuscript accepted to Ap.J.; also available at http://jupiter.as.arizona.edu/~burrows/paper

DEM of triaxial tests on crushable sand

This paper presents simulations of high-pressure triaxial shear tests on a crushable sand. The discrete element method is used, featuring a large number of particles and avoiding the use of agglomerates. The triaxial model features a flexible membrane, therefore allowing realistic deformation, and a simple breakage mechanism is implemented using the octahedral shear stress induced in the particles. The simulations show that particle crushing is essential to replicate the realistic behaviour of sand (in particular the volumetric contraction) in high-pressure shear tests. The general effects of crushing during shear are explored, including its effects on critical states, and the influence of particle strength and confining pressure on the degree of crushing are discussed

Review: ‘Gimme five’: future challenges in multiple sclerosis. ECTRIMS Lecture 2009

This article is based on the ECTRIMS lecture given at the 25th ECTRIMS meeting which was held in Düsseldorf, Germany, from 9 to 12 September 2009. Five challenges have been identified: (1) safeguarding the principles of medical ethics; (2) optimizing the risk/benefit ratio; (3) bridging the gap between multiple sclerosis and experimental autoimmune encephalitis; (4) promoting neuroprotection and repair; and (5) tailoring multiple sclerosis therapy to the individual patient. Each of these challenges will be discussed and placed in the context of current research into the pathogenesis and treatment of multiple sclerosis

Passagierdampfer als Hilfskreuzer: kurze Geschichte der Entwicklung einer Schiffsgattung

To date there is no cohesive historical account of the international development of the auxiliary cruiser, and the scattered investigations of various single aspects of this ship category do not add up to a comprehensive survey. These were the circumstances discovered by the author as, upon request of the editor of this publication, he undertook to provide an introduction to the report by Captain Meyer of the only voyage of the "Kaiser Wilhelm der Grosse" as an auxiliary cruiser (see the following article). The first task was thus to gain an overall impression of this history. "Auxiliary cruisers", as defined here, were passenger steamships kept ready for battle by large shipping companies in countries with significant naval power. These ships were technically equipped for the installation of weapons, and their conversion could be carried out within a few days. The auxiliary cruiser was first to put into service by the Russians, who felt threatened by the British Royal Navy after the Russian-Turkish Balkan War. At that time a newly established shipping company, later called the "Russian Voluntary Fleet", bought three steamships from the Hamburg-America Line and had them converted for combat purposes. By the turn of the century, this originally Russian idea had been adopted by all of the world's Iarge navies . This brief study also touches on the various attempt worldwide to incorporate the legal status of the ship type "auxiliary cruiser" into international public and military law. Because of geopolitically based conflicts of interest between the naval powers, these endeavours were only partially successful. ln conclusion the author examines the auxiliary cruiser concept of the Imperial German Navy, whose goal it was to interfere with and impair the enemy's overseas commerce. The express Iiners originally designated for this purpose, however, were for various reasons inconceivably ill-suited to it. As early as 1915 the German Navy proceeded to commission cargo vessels as auxiliary cruisers, as they were inconspicuous and easier to supply. This was then the mode of operation employed by the navy during World War II

DEM of triaxial tests on crushable cemented sand

Using the discrete element method, triaxial simulations of cemented sand consisting of crushable particles are presented. The triaxial model used features a flexible membrane, allowing realistic deformation to occur, and cementation is modelled using inter-particle bonds. The effects of particle crushing are explored, as is the influence of cementation on the behaviour of the soil. An insight to the effects that cementation has on the degree of crushing is presented