Three-body recombination rates near a Feshbach resonance within a two-channel contact interaction model

We calculate the three-body recombination rate into a shallow dimer in a gas of cold bosonic atoms near a Feshbach resonance using a two-channel contact interaction model. The two-channel model naturally describes the variation of the scattering length through the Feshbach resonance and has a finite effective range. We confront the theory with the available experimental data and show that the two-channel model is able to quantitatively describe the existing data. The finite effective range leads to a reduction of the scaling factor between the recombination minima from the universal value of 22.7. The reduction is larger for larger effective ranges or, correspondingly, for narrower Feshbach resonances.Comment: 9 pages, 7 figure

Does presence of a mid-ocean ridge enhance biomass and biodiversity?

In contrast to generally sparse biological communities in open-ocean settings, seamounts and ridges are perceived as areas of elevated productivity and biodiversity capable of supporting commercial fisheries. We investigated the origin of this apparent biological enhancement over a segment of the North Mid-Atlantic Ridge (MAR) using sonar, corers, trawls, traps, and a remotely operated vehicle to survey habitat, biomass, and biodiversity. Satellite remote sensing provided information on flow patterns, thermal fronts, and primary production, while sediment traps measured export flux during 2007-2010. The MAR, 3,704,404 km 2 in area, accounts for 44.7% lower bathyal habitat (800-3500 m depth) in the North Atlantic and is dominated by fine soft sediment substrate (95% of area) on a series of flat terraces with intervening slopes either side of the ridge axis contributing to habitat heterogeneity. The MAR fauna comprises mainly species known from continental margins with no evidence of greater biodiversity. Primary production and export flux over the MAR were not enhanced compared with a nearby reference station over the Porcupine Abyssal Plain. Biomasses of benthic macrofauna and megafauna were similar to global averages at the same depths totalling an estimated 258.9 kt C over the entire lower bathyal north MAR. A hypothetical flat plain at 3500 m depth in place of the MAR would contain 85.6 kt C, implying an increase of 173.3 kt C attributable to the presence of the Ridge. This is approximately equal to 167 kt C of estimated pelagic biomass displaced by the volume of the MAR. There is no enhancement of biological productivity over the MAR; oceanic bathypelagic species are replaced by benthic fauna otherwise unable to survive in the mid ocean. We propose that globally sea floor elevation has no effect on deep sea biomass; pelagic plus benthic biomass is constant within a given surface productivity regime.Peer reviewe

The Bathypelagic Biome of the Atlantic Ocean: Character and Ecological Discreteness of the Fish Fauna

Recent global synthetic analyses have revealed that marine taxonomic inventories are far from complete, nowhere more so than in the deep-pelagic ocean. At over a billion km3, it is the largest biome on Earth, yet only a tiny fraction of the biogeographic records include the bathypelagic fauna. This data gap served as the impetus for recent deepwater surveys, many of which have altered our perceptions of pelagic ecosystems. Here we examine data from four deep-pelagic (0-5000+ m) sampling programs in the Atlantic (60°N-25°S) in order to assess the character of bathypelagic fish communities with respect to faunal distinctiveness and ecological connectivity. Regions studied include the Gulf of Mexico, Sargasso 702 Sea, eastern North/South Atlantic, and mid-North Atlantic. Quantitative analyses give contrasting pictures with respect to faunal composition and ecosystem operation. The discreteness of the bathypelagic zone is exhibited faunistically by the suite of ―holobathypelagic‖ species found only below 1000 m, most of which are highly modified morphologically. Geometric abundance class analyses reveal that the character of relative species abundance distributions between the meso- and bathypelagic zones is fundamentally dissimilar; the former exhibit a much higher proportion of common species, while the latter exhibit a much higher percentage of rarer species. From a community energetics perspective, however, the bathy- and mesopelagic zones are highly interconnected. Approximately 70% of fish species collected below 1000 m are also found in the mesopelagic zone, and in the far North Atlantic, are also found in the epipelagial. These species comprised 66 to \u3e90% of individuals collected below 1000 m in the regions sampled. In the mid-North Atlantic, these species contribute to the unexpected water-column biomass maximum observed between 1500-2300 m. Thus, the ―transient‖ taxa (primarily mesopelagic migrators and spanner taxa) add considerably to the ichthyofaunal diversity of the world ocean below 1000 m, and appear to be the vectors that support the diverse array of holobathypelagic fishes whose taxonomic composition is dominated by piscivores. Data from the four regions studied suggests that classic pelagic biogeographic boundaries do not apply to bathypelagic realm, as shared species are the rule rather than the exception. Last, cumulative species curves suggest we are far from understanding the true complexity of the bathypelagic zone

Birka uden frisere

Det har længe været anset for en historisk kendsgerning, at friserne i tiden fra før 600 og gennem vikingetiden har været det dominerende søfarts- og handelsfolk i Nordeuropa. Professor, dr. phil. Aksel E. Christensen tager her denne friserteori op til kritik og hævder, at den for Nordens vedkommende bygger på forkerte tolkninger af sproglige, arkæologiske og litterære vidnesbyrd og ikke lader sig opretholde. Tværtimod tyder alt på, at nordboerne har været Vesteuropa overlegne på økonomisk, teknisk og militært område

A Navier-Stokes Solver for Compressible Turbulent Flows on Quadtree and Octree Based Cartesian Grids

Cartesian grids represent a special extent in unstructured grid literature. They employ chiefly created algorithms to produce automatic meshing while simulating flows around complex geometries without considering shape of the bodies. In this article, firstly, it is intended to produce regionally developed Cartesian meshes for two dimensional and three dimensional, disordered geometries to provide solutions hierarchically. Secondly, accurate results for turbulent flows are developed by finite volume solver (GeULER-NaTURe) with both geometric and solution adaptations. As a result, a “hands-off” flow solver based on Cartesian grids as the preprocessor is performed using object-oriented programming. Spalart-Allmaras turbulence model added Reynolds Averaged Navier Stokes equations are solved for the flows around airfoils and wings. The solutions are validated and verified by one two dimensional and one three dimensional turbulent flow common test cases in literature. Both case studies disclose the efficaciousness of the developed codes and qualify in convergence and accuracy