Direct Detection of Galactic Halo Dark Matter

The Milky Way Galaxy contains a large, spherical component which is believed to harbor a substantial amount of unseen matter. Recent observations indirectly suggest that as much as half of this ``dark matter'' may be in the form of old, very cool white dwarfs, the remnants of an ancient population of stars as old as the Galaxy itself. We conducted a survey to find faint, cool white dwarfs with large space velocities, indicative of their membership in the Galaxy's spherical halo component. The survey reveals a substantial, directly observed population of old white dwarfs, too faint to be seen in previous surveys. This newly discovered population accounts for at least 2% of the halo dark matter. It provides a natural explanation for the indirect observations, and represents a direct detection of Galactic halo dark matter.Comment: 13 pages, 4 figures, 1 table. Note added after Science Express online publication: This text reflects the correction of a few typographical errors in the online version of the table. It also includes the new constraint on the calculation of d_max which accounts for the fact that the survey could not have detected stars with proper motions below 0.33 arcseconds per year. Published online at ScienceExpress www.sciencemag.org 22 March 2001; 10.1126/science.1059954; To appear in Science 27 April 200

Ichthyological Bulletin of the J.L.B. Smith Institute of Ichthyology; No. 37

Lethrinops argentea Ahl, L. lethrinus (Gunther) and L. leptodon Regan are re-described and illustrated. An additional new species, L. longipinnis, sharing with the above the possession a steeply sloping, wedge-shaped snout, a slender lower pharyngeal bone and few (9—14) gillrakers is described. The status of L. lunaris Trewavas is discussed.Rhodes University Libraries (Digitisation

Ichthyological Bulletin of the J.L.B. Smith Institute of Ichthyology; No. 36

Haplochromis micrentodon Regan is transferred to the genus Lethrinops and redescribed. Two new species of Lethrinops which, like the above, are characterised by the possession of lower pharyngeal bones bearing close pavements of slender blunt-tipped teeth are described.Rhodes University Libraries (Digitisation

Ichthyological Bulletin of the J.L.B. Smith Institute of Ichthyology; No. 38

The group of species considered all have short moderate snouts and a group of slightly to strongly enlarged teeth on the lower pharyngeal bone, the anterior and lateral teeth of which are biscupid, with the posterior cusp turned forwards. Lethrinops parvidens Trewavas, L. aurita (Regan), L. macrophthalmus (Boulenger), L. macrochir (Regan), L.longimanus Trewavas and L. macracanthus Trewavas are re-described, and L. mylodon n. sp. is described, the latter being divided into two geographically seperated sub-species. While L. longimanus and L. macracanthus co-exist with very slight overlap of morphological characters in the south of Lake Malawi, an apparently intermediate population is reported from another part of the Lake.Rhodes University Libraries (Digitisation

Diagnostic work-up of patients presenting in primary care with lower abdominal symptoms:which faecal test and triage strategy should be used?

Bowel endoscopy referrals from primary care have increased steadily over recent years. However, most patients do not have significant colorectal disease (SCD). Therefore, strategies to select those who would benefit most from endoscopy are of current interest. A recent study developed a multivariable diagnostic model for SCD with routine clinical information, extended with quantitative faecal calprotectin (f-C) point-of-care (POC) testing and/or qualitative POC faecal immunochemical test (FIT) for haemoglobin (f-Hb) results. This study used POC tests for both f-C and f-Hb; however, POC tests have many disadvantages and there are several reasons why quantitative measurements of f-Hb are advantageous. Quantitative faecal immunochemical tests have been used very successfully in triage of patients presenting in primary care as a rule-out test. Studies have compared f-C and f-Hb in this clinical context and consider that f-C is not required in diagnosis. A single quantitative f-Hb result, without any clinical information, could be sufficient to decide whom to refer for endoscopy and, because of the significant overlap of symptoms in those with and without SCD, could be the primary investigation performed. Please see related article: http://bmcmedicine.biomedcentral.com/articles/10.1186/s12916-016-0684-5

Ichthyological Bulletin of the J.L.B. Smith Institute of Ichthyology; No. 39

Part 1: The nature of the barriers separating the Lake Malawi and Zambezi fish faunas: The nature of the barrier separating the ‘Lake Malawi’ from the ‘Lower Zambezi’ fish faunas was investigated by electrofishing the 145 km stretch of the Shire River surrounding the Murchison cataracts. The study shows that the lowermost element of the cataracts, the Kapachira Falls, is an absolute physical barrier to upstream movement of ‘Lower Zambezi’ species. The barrier to downstream movement by ‘Lake Malawi’ species is largely ecological and has two components, the fluviatile nature of the Upper and Middle Shire and especially the torrential nature of the cataracts, which are unsuitable for lacustrine species, and competition with the ‘Lower Zambezi’ fauna of the Lower Shire by the few species which succeed in negotiating the cataracts.Part 2: An annotated checklist of the fish fauna of the River Shire south of Kapachira Falls, Malawi: The Shire River drains Lake Malawi and is a major tributary of the Lower Zambezi River. Sixty-one species of fish have been recorded in the lower reaches which are separated from the lower Shire basin by the Kapachira Falls and from the ‘East Coast River’ fauna present in the Upper Ruo River by the Zoa Falls. The fauna is essentially similar to that of the Lower Zambezi, although some Lake Malawi endemics occur sporadically in the upper part of the flood plain.Rhodes University Libraries (Digitisation

Why Effective Medium Theory Fails in Granular Materials

Experimentally it is known that the bulk modulus, K, and shear modulus, \mu, of a granular assembly of elastic spheres increase with pressure, p, faster than the p^1/3 law predicted by effective medium theory (EMT) based on Hertz-Mindlin contact forces. To understand the origin of these discrepancies, we perform numerical simulations of granular aggregates under compression. We show that EMT can describe the moduli pressure dependence if one includes the increasing number of grain-grain contacts with p. Most important, the affine assumption (which underlies EMT), is found to be valid for K(p) but breakdown seriously for \mu(p). This explains why the experimental and numerical values of \mu(p) are much smaller than the EMT predictions.Comment: 4 pages, 5 figures, http://polymer.bu.edu/~hmaks

Granular Packings: Nonlinear elasticity, sound propagation and collective relaxation dynamics

Experiments on isotropic compression of a granular assembly of spheres show that the shear and bulk moduli vary with the confining pressure faster than the 1/3 power law predicted by Hertz-Mindlin effective medium theories (EMT) of contact elasticity. Moreover, the ratio between the moduli is found to be larger than the prediction of the elastic theory by a constant value. The understanding of these discrepancies has been a longstanding question in the field of granular matter. Here we perform a test of the applicability of elasticity theory to granular materials. We perform sound propagation experiments, numerical simulations and theoretical studies to understand the elastic response of a deforming granular assembly of soft spheres under isotropic loading. Our results for the behavior of the elastic moduli of the system agree very well with experiments. We show that the elasticity partially describes the experimental and numerical results for a system under compressional loads. However, it drastically fails for systems under shear perturbations, particularly for packings without tangential forces and friction. Our work indicates that a correct treatment should include not only the purely elastic response but also collective relaxation mechanisms related to structural disorder and nonaffine motion of grains.Comment: 21 pages, 13 figure

Pressure dependence of the sound velocity in a 2D lattice of Hertz-Mindlin balls: a mean field description

We study the dependence on the external pressure $P$ of the velocities $v_{L,T}$ of long wavelength sound waves in a confined 2D h.c.p. lattice of 3D elastic frictional balls interacting via one-sided Hertz-Mindlin contact forces, whose diameters exhibit mild dispersion. The presence of an underlying long range order enables us to build an effective medium description which incorporates the radial fluctuations of the contact forces acting on a single site. Due to the non linearity of Hertz elasticity, self-consistency results in a highly non-linear differential equation for the "equation of state" linking the effective stiffness of the array with the applied pressure, from which sound velocities are then obtained. The results are in excellent agreement with existing experimental results and simulations in the high and intermediate pressure regimes. It emerges from the analysis that the departure of $v_{L}(P)$ from the ideal $P^{1/6}$ Hertz behavior must be attributed primarily to the fluctuations of the stress field, rather than to the pressure dependence of the number of contacts