AzTEC millimetre survey of the COSMOS field – I. Data reduction and source catalogue

We present a 1.1 mm wavelength imaging survey covering 0.3 deg² in the COSMOS field. These data, obtained with the AzTEC continuum camera on the James Clerk Maxwell Telescope, were centred on a prominent large-scale structure overdensity which includes a rich X-ray cluster at z ≈ 0.73. A total of 50 mm-galaxy candidates, with a significance ranging from 3.5 to 8.5σ, are extracted from the central 0.15 deg² area which has a uniform sensitivity of ∼1.3 mJy beam⁻¹. 16 sources are detected with S/N ≥ 4.5, where the expected false-detection rate is zero, of which a surprisingly large number (9) have intrinsic (deboosted) fluxes ≥5 mJy at 1.1 mm. Assuming the emission is dominated by radiation from dust, heated by a massive population of young, optically obscured stars, then these bright AzTEC sources have far-infrared luminosities  >6 × 10¹² L_⊙  and star formation rates  >1100 M_⊙ yr⁻¹  . Two of these nine bright AzTEC sources are found towards the extreme peripheral region of the X-ray cluster, whilst the remainder are distributed across the larger scale overdensity. We describe the AzTEC data reduction pipeline, the source-extraction algorithm, and the characterization of the source catalogue, including the completeness, flux deboosting correction, false-detection rate and the source positional uncertainty, through an extensive set of Monte Carlo simulations. We conclude with a preliminary comparison, via a stacked analysis, of the overlapping MIPS 24-μm data and radio data with this AzTEC map of the COSMOS field.     

Cosmic-ray positrons: are there primary sources?

Galactic cosmic rays consist of primary and secondary particles. Primary cosmic rays are thought to be energized by first order Fermi acceleration processes at supernova shock fronts within our Galaxy. The cosmic rays that eventually reach the Earth from this source are mainly protons and atomic nuclei, but also include electrons. Secondary cosmic rays are created in collisions of primary particles with the diffuse interstellar gas. They are relatively rare but carry important information on the Galactic propagation of the primary particles. The secondary component includes a small fraction of antimatter particles, positrons and antiprotons. In addition, positrons and antiprotons may also come from unusual sources and possibly provide insight into new physics. For instance, the annihilation of heavy supersymmetric dark matter particles within the Galactic halo could lead to positrons or antiprotons with distinctive energy signatures. With the High-Energy Antimatter Telescope (HEAT) balloon-borne instrument, we have measured the abundances of positrons and electrons at energies between 1 and 50 GeV. The data suggest that indeed a small additional antimatter component may be present that cannot be explained by a purely secondary production mechanism. Here we describe the signature of the effect and discuss its possible origin.     

Grain‐size distributions on high‐energy sandy beaches and their relation to wave dissipation

Grain size and sorting represent two key parameters when characterizing sediments or modelling beach morphology and sediment transport. Traditionally, an average value for grain size or sorting is often assumed for the entire area, determined from only a few sediment samples, ignoring any spatial (or temporal) variability in sediment characteristics. This contribution uses a data set of physical surface sediment samples from 53 beach locations around the south‐west peninsula of the United Kingdom, in addition to bi‐monthly, high spatial resolution (mean 240 samples) digital grain‐size surveys from a high‐energy, oceanic, sandy beach (Perranporth, North Cornwall). Systematic spatial variations in grain size and sorting were consistently observed in the seaward direction across the intertidal zone of sandy beaches, with grain‐sizes coarsening and sorting improving by up to 51·7% and 64·3%, respectively. This variability was deterministically related to the time‐averaged, antecedent‐adjusted energy dissipated by breaking waves, with the observed maximum grain‐size and sorting values correlating with the location of peak wave energy dissipation (r² = 0·998, P < 0·01).     

The community morphology of skilled migration: the changing role of voluntary and community organisations (VCOs) in the grounding of British migrant identities in Paris (France)

Research into skilled migrant communities tends to emphasize the grounding of identities through transnationalism and mobility. Less research has been conducted into how skilled migrants actually ground their identities within the city through everyday social networks. The paper addresses this imbalance by examining the changing significance of British voluntary and community organisations (VCOs) in Paris, France. Combining qualitative and quantitative data, findings show local migrant networks to be important focal points for the British even though migrants’ lives may increasingly be transnational in orientation. The research also shows that in situ social networks are evolving. Specifically, traditional forms of elite expatriate communality are not as omnipotent as they once were, with the popularity of some high-profile British VCOs in Paris declining over recent decades. The paper argues that the link between social dynamism and communal morphology helps explain this shift: as British middle-class lifestyles have fragmented, and as the British migrant population in Paris has grown in scale and scope, so the range of VCOs and associated informal networks has grown. Whilst some skilled migrants still embed within traditional expatriate organisations, Britishness is increasingly grounded elsewhere; both within the city through alternative social networks, and located beyond the city through transnational ties.     

Modeling fluvial response to in‐stream woody vegetation: implications for stream corridor restoration

River restoration and bank stabilization programs often use vegetation for improving stream corridor habitat, aesthetic and function. Yet no study has examined the use of managed vegetation plantings to transform a straight, degraded stream corridor into an ecologically functional meandering channel. Experimental data collected using a distorted Froude‐scaled flume analysis show that channel expansion and widening, thalweg meandering and riffle and pool development are possible using discrete plantings of rigid, emergent vegetation, and the magnitudes of these adjustments depend on the shape of the vegetation zone and the density of the vegetation. These experimental results were verified and validated using a recently developed numerical model, and model output was then used to discuss mechanistically how rivers respond to the introduction of in‐stream woody vegetation. Finally, a hybrid method of meander design is proposed herein where managed vegetation plantings are used to trigger or force the desired morphologic response, transforming a straight, degraded reach into a more functional meandering corridor. It is envisioned that such numerical models could become the primary tool for designing future stream restoration programs involving vegetation and assessing the long‐term stability of such activities. Copyright © 2007 John Wiley & Sons, Ltd.     

Passive seismic control of cable-stayed bridges with damped resonant appendages

A study is conducted to investigate the effectiveness of attaching to cable-stayed bridges resonant appendages with a relatively small mass and a high damping ratio as a means to reduce their response to earthquake excitations. The study is based on a previously developed formulation that shows that the use of these appendages may increase the inherent damping of building structures and, as a result, may reduce their response to seismic disturbances. It includes numerical and experimental tests that are conducted to assess the validity of such a formulation for the case of cable-stayed bridges and the extent to which such appendages can reduce their seismic response. In the numerical study, an actual cable-stayed bridge is modelled with finite elements and analysed with and without the proposed appendages under different earthquake ground motions. Appendages with damping ratios of 10, 15, 20 and 30 per cent and masses that, respectively, represent 0.67, 1.5, 2.7 and 6.0 per cent of the total mass of the bridge are considered. In the experimental test, a 3.7 m long cable-stayed bridge and an appendage consisting of a small mass, a small spring and a small viscous damper are built and the bridge tested, without and with the appendage, on a pair of shaking tables which are set to reproduce ground acceleration records from past earthquakes. The damping ratio of the appendage in this test is 32 per cent and its mass represents about 8 per cent of the total mass of the bridge model. In the numerical test, it is found that the appendages reduce the longitudinal response of the bridge deck up to 88 per cent. Similarly, in the experimental test it is found that the appendage reduces the longitudinal bridge deck response by about 41 per cent. From these studies, it is concluded that the suggested appendages may indeed be effective in reducing the seismic response of cable-stayed bridges as they are for building structures.     

A Method for Estimating Field-Scale Mass Transfer Rate Parameters and Assessing Aquifer Cleanup Times

A general method for estimating ground-water solute mass transfer rate parameters from field test data is presented. The method entails matching solute concentration and hydraulic head data collected during the recovery phase of a pumping test through application of a simulation-regression technique. Estimation of hydraulic conductivity and mass transfer rate parameter values is performed by fitting model simulations to the data. Parameter estimates are utilized to assess cleanup times for pump-and-treat aquifer remediation scenarios. Uncertainty in the cleanup time estimate is evaluated using statistical information obtained with the parameter estimation technique. Application of the method is demonstrated using a hypothetical ground-water flow and solute transport system. Simulations of field testing, parameter estimation, and remedial time frames are performed to evaluate the usefulness of the method. Sets of random noise that signify potential field and laboratory measurement errors are combined with the hypothetical data to provide rigorous testing of the method. Field tests are simulated using ranges of values for data noise, the mass transfer rate parameters, the test pumping rates, and the duration of recovery monitoring to evaluate their respective influence on parameter and cleanup time estimates. The demonstration indicates the method is capable of yielding accurate estimates of the solute mass transfer rate parameters. When the parameter values for the hypothetical system are well estimated, cleanup time predictions are shown to be more accurate than when calculated using the local equilibrium assumption.     

Fractal analysis of long-range paleoclimatic data: Oxygen isotope record of pacific core V28-239

R/S analysis of the oxygen isotope curve of Pacific core V28-239 yields a fractal dimension of 1.22. This value is considered to characterize global climatic change over the last 2 million years as expressed by changing O¹⁸ ratios and confirms that climatic variations are characterized by long-term persistence. The fractal dimension of 1.22 compares favorably with the approximate fractal dimension of 1.26 for annual precipitation records for nine major cities in the United States. Although the precipitation and oxygen isotope data are measured in different physical units and recorded at different time scales, fractal analysis allows for a mathematical comparison of the two phenomena. Additionally, since the fractal dimensions of the oxygen isotope and precipitation records are similar, it is implied that such fractal dimensions are characteristic of climate change over the spectral range of 10 to 10⁶ years. Given this temperature curves based on fractal parameters of long-term O¹⁸ data could be constructed which would allow examination of characteristics of temperature variation over tens and hundreds of years. Such studies may allow the establishment of limits on natural temperature variation and document the persistence of temperature trends through time. If these trends and limits can be resolved, long-range climatic prediction is feasible.     

The hydrological effects of fire in South African mountain catchments

Streamflow and its storm-flow elements in four catchments were analyzed by the paired catchment method for a response to fire. Prior to burning two of the catchments were vegetated with over-mature fynbos (the indigenous scrub vegetation of the southwestern Cape, South Africa), one was afforested with Pinus radiata and the fourth with Eucalyptus fastigata. One of the fynbos catchments was burned in a prescribed fire in the late dry season. The other catchments burned in wildfires.

Neither of the fynbos catchments showed a change in storm-flow. Annual total flow increases of around 16% were in agreement with model predictions, being related to the reductions in transpiration and interception. The manner of streamflow generation appeared to have remained unaltered despite the presence of some water repellency in the soils and consequent overland flow on some steep midslope sites.

The two timber plantation catchments experienced large and significant increases in storm-flows and soil losses, while total flow increased by 12% in the pine catchment and decreased marginally in the eucalypt catchment. The pattern of the storm-flow increases was similar in both cases. After fire, storm hydrographs were higher and steeper though their duration was little changed. The respective first year increases in the pine and eucalypt catchments were 290% and 1110% for peak discharge, 201% and 92% for quick-flow volume, and 242% and 319% for storm response ratio. These fire effects are considered to be due to changes in storm-flow generation consistent with an increased delivery of overland flow (surface runoff) to the stream channel. This was caused, in part, by reduced infiltration resulting from water repellency in the soils of the burned catchments. Overall the hydrological effects of fire are related to numerous interactive factors, including the degree of soil heating, the vegetation type and soil properties.