The low-redshift evolution of the Lyman-β forest

The low-redshift evolution of the intergalactic medium is investigated using hydrodynamic cosmological simulations. The assumed cosmological model is a critical density cold dark matter universe. The imposed uniform background of ionizing radiation has the amplitude, shape and redshift evolution as computed from the observed quasar luminosity function by Haardt &38; Madau. We have analysed simulated Lyman-α spectra using Voigt-profile fitting, mimicking the procedure with which quasar spectra are analysed. Our simulations reproduce the observed evolution of the number of Lyman-α absorption lines over the whole observed interval of z  = 0.5 to 4. In particular, our simulations show that the decrease in the rate of evolution of Lyman-α absorption lines at z  ≤ 2, as observed by the Hubble Space Telescope , can be explained by the steep decline in the photoionizing background resulting from the rapid decline in quasar numbers at low redshift.     

Conceptual model of recharge to southeastern Badain Jaran Desert groundwater and lakes from environmental tracers

Sources of groundwater recharge to the Badain Jaran Desert in China have been investigated using geochemical and isotopic techniques. Stable isotope compositions (δ¹⁸O and δ²H) of shallow groundwater and surface water from oasis lakes evolve from a starting composition considerably depleted compared to local unsaturated zone moisture, confirming inferences from chloride mass balance that direct infiltration of precipitation is not a volumetrically important source of recharge to the shallow aquifer in the study area. Shallow phreatic and deeper confined groundwater bodies appear unconnected based on chemical composition and radiocarbon activities. Hydrogeologic evidence points toward a bordering mountain range (Yabulai) as a likely recharge zone, which is consistent with tracer results. A mean residence time in the range of 1–2 ka for the desert’s southeastern margin is inferred from radiocarbon. These results reveal that some replenishment to the desert aquifer is occurring but at a rate much lower than previously suggested, which is relevant for water resources planning in this ecologically sensitive area.     

Ambient and cold‐temperature infrared spectra and XRD patterns of ammoniated phyllosilicates and carbonaceous chondrite meteorites relevant to Ceres and other solar system bodies

Mg‐phyllosilicate‐bearing, dark surface materials on the dwarf planet Ceres have NH₄‐bearing materials, indicated by a distinctive 3.06 μm absorption feature. To constrain the identity of the Ceres NH₄‐carrier phase(s), we ammoniated ground particulates of candidate materials to compare their spectral properties to infrared data acquired by Dawn's Visible and Infrared (VIR) imaging spectrometer. We treated Mg‐, Fe‐, and Al‐smectite clay minerals; Mg‐serpentines; Mg‐chlorite; and a suite of carbonaceous meteorites with NH₄‐acetate to exchange ammonium. Serpentines and chlorites showed no evidence for ammoniation, as expected due to their lack of exchangeable interlayer sites. Most smectites showed evidence for ammoniation by incorporation of NH₄⁺ into their interlayers, resulting in the appearance of absorptions from 3.02 to 3.08 μm. Meteorite samples tested had weak absorptions between 3.0 and 3.1 μm but showed little clear evidence for enhancement upon ammoniation, likely due to the high proportion of serpentine and other minerals relative to expandable smectite phases or to NH₄⁺ complexing with organics or other constituents. The wavelength position of the smectite NH₄ absorption showed no variation between IR spectra acquired under dry‐air purge at 25 °C and under vacuum at 25 °C to ?180 °C. Collectively, data from the smectite samples show that the precise center wavelength of the characteristic ~3.05 μm v₃ absorption in NH₄ is variable and is likely related to the degree of hydrogen bonding of NH₄‐H₂O complexes. Comparison with Dawn VIR spectra indicates that the hypothesis of Mg‐saponite as the ammonium carrier phase is the simplest explanation for observed data, and that Ceres dark materials may be like Cold Bokkeveld or Tagish Lake but with proportionally more Mg‐smectite.     

Empirical Relations Between Aquifer Geohydraulic–Geoelectric Properties Derived from Surficial Resistivity Measurements in Parts of Akwa Ibom State,Southern Nigeria

Natural Resources Research - Aquifer geohydraulic properties such as hydraulic conductivity and transmissivity are very vital parameters in groundwater resource management and exploitation. In this...     

Inelastic velocity ratio

Knowledge of maximum velocity is essential for the design of structures and especially those with supplementary dampers. Although the nonlinear time history analysis leads to reliable estimation of actual velocities, it seems to be complicated for the everyday engineering practice due to the increased computational cost. This paper proposes an alternative for single‐degree‐of‐freedom (SDOF) systems to estimate the actual velocity in a straightforward and effective manner. More specifically, this study examines the inelastic velocity ratio (IVR), i.e., the ratio of the maximum inelastic to the maximum elastic velocity of an SDOF system, the knowledge of which allows the computation of maximum inelastic velocity directly from the corresponding elastic counterpart. The proposed method is general and can be applied to both conventional structures and structures with supplementary dampers. Extensive parametric studies are conducted to obtain expressions for IVR in terms of the period of vibration, viscous damping ratio, force reduction factor, and soil class. Copyright © 2012 John Wiley & Sons, Ltd.     

Doubly-symmetric motions in the elliptic problem

Poincaré's continuation method is applied to the elliptic restricted problem for the computation of four families of doubly symmetric three-dimensional periodic orbits emanating from similar orbits corresponding to zero eccentricity. The results are given in four tables and the orbits are characterized in regard to their stability.     

The thermal history of the intergalactic medium

At redshifts z ≳2, most of the baryons reside in the smooth intergalactic medium which is responsible for the low column density Ly α forest. This photoheated gas follows a tight temperature–density relation which introduces a cut-off in the distribution of widths of the Ly α absorption lines ( b -parameters) as a function of column density. We have measured this cut-off in a sample of nine high-resolution, high signal-to-noise ratio quasar spectra and determined the thermal evolution of the intergalactic medium in the redshift range 2.0–4.5. At a redshift z ∼3, the temperature at the mean density shows a peak and the gas becomes nearly isothermal. We interpret this as evidence for the reionization of He  ii .     

Stochastic seismic sliding of rigid mass supported through non-symmetric friction

The sliding behaviour of a rigid mass supported on a randomly vibrating foundation through a non-symmetric Coulomb-friction contact is studied both analytically and by numerical simulation. The analysis is based on a stationary solution of the associated Fokker-Planck equation, and makes use of equivalent linearization and of a suitable decomposition of the non-zero mean non-stationary sliding process. It is shown that the analytical results yield several exact asymptotic expansions for both small and large values of time. An extensive Monte Carlo type numerical simulation study produces non-stationary response statistics which are in very good accord with the analytical results. Furthermore, it is found that Gumbel's Extreme Value Distribution reproduces with remarkable accuracy the observed cumulative frequency of maximum slip displacement. The results of this paper may find application in seismic design of embankment dams, earth retaining walls and base ‘isolation’ systems.