Dielectronic Recombination in He+ Ions

This research was sponsored by the National Science Foundation Grant NSF PHY-931478

Experimental tests of slender reinforced concrete columns under combined axial load and lateral force

The use of high strength concrete (HSC) in columns has become more frequent since a substantial reduction of the cross-section is obtained, meaning that slenderness increases for the same axial load and length, producing higher second order effects. However, the experimental tests in the literature of reinforced concrete columns subjected to axial load and lateral force focus on shear span ratios, according to Eurocode 2 (2004), clause 5.6.3., (M/(V·h)) lower than 6.5. This gap in the literature limits technological development for the construction of these structural elements. This paper presents 44 experimental tests on reinforced concrete columns subjected to constant axial load and monotonic lateral force. The aim of this is to gain greater knowledge of the types of elements which will also be of use in calibrating the numerical models and validating the simplified methods. The test parameters are strength of concrete (normal- and high-strength concrete), shear span ratio, axial load level and longitudinal and transversal reinforcement ratios. The strength and deformation of the columns were studied, and an analysis of the simplified methods from Eurocode 2 (2004) and ACI-318 (2008) concluded that both are very conservative. © 2011 Elsevier Ltd.The authors wish to express their sincere gratitude to the Spanish Ministry of Science and Innovation for the help provided through project BIA2009-10207 and to the European Union for the financial support from Feder funds.Barrera Puerto, A.; Bonet Senach, JL.; Romero, ML.; Miguel Sosa, P. (2011). Experimental tests of slender reinforced concrete columns under combined axial load and lateral force. Engineering Structures. 33(12):3676-3689. https://doi.org/10.1016/j.engstruct.2011.08.003S36763689331

Enhanced carbon dioxide outgassing from the eastern equatorial Atlantic during the last glacial

Biological productivity and carbon export in the equatorial Atlantic are thought to have been dramatically higher during the last glacial period than during the Holocene. Here we reconstruct the pH and CO2 content of surface waters from the eastern equatorial Atlantic Ocean over the past ~30 k.y. using the boron isotope composition of Globigerinoides ruber (a mixed-layer–dwelling planktic foraminifera). Our new record, combined with previously published data, indicates that during the last glacial, in contrast to today, a strong west to east gradient existed in the extent of air:sea equilibrium with respect to ρCO2 (ΔρCO2), with the eastern equatorial Atlantic acting as a significant source of CO2 (+100 μatm) while the western Atlantic remained close to equilibrium (+25 μatm). This pattern suggests that a five- fold increase in the upwelling rate of deeper waters drove increased Atlantic productivity and large-scale regional cooling during the last glacial, but the higher than modern ΔρCO2 in the east indicates that export production did not keep up with enhanced upwelling of nutrients. However, the downstream decline of ΔρCO2 provides evidence that the unused nutrients from the east were eventually used for biologic carbon export, thereby effectively negating the impact of changes in upwelling on atmospheric CO2 levels. Our findings indicate that the equatorial Atlantic exerted a minimal role in contributing to lower glacial-age atmospheric CO2

The outer halos of elliptical galaxies

Recent progress is summarized on the determination of the density distributions of stars and dark matter, stellar kinematics, and stellar population properties, in the extended, low surface brightness halo regions of elliptical galaxies. With integral field absorption spectroscopy and with planetary nebulae as tracers, velocity dispersion and rotation profiles have been followed to ~4 and ~5-8 effective radii, respectively, and in M87 to the outer edge at ~150 kpc. The results are generally consistent with the known dichotomy of elliptical galaxy types, but some galaxies show more complex rotation profiles in their halos and there is a higher incidence of misalignments, indicating triaxiality. Dynamical models have shown a range of slopes for the total mass profiles, and that the inner dark matter densities in ellipticals are higher than in spiral galaxies, indicating earlier assembly redshifts. Analysis of the hot X-ray emitting gas in X-ray bright ellipticals and comparison with dynamical mass determinations indicates that non-thermal components to the pressure may be important in the inner ~10 kpc, and that the properties of these systems are closely related to their group environments. First results on the outer halo stellar population properties do not yet give a clear picture. In the halo of one bright galaxy, lower [alpha/Fe] abundances indicate longer star formation histories pointing towards late accretion of the halo. This is consistent with independent evidence for on-going accretion, and suggests a connection to the observed size evolution of elliptical galaxies with redshift.Comment: 8 pages. Invited review to appear in the proceedings of "Galaxies and their Masks" eds. Block, D.L., Freeman, K.C. & Puerari, I., 2010, Springer (New York

Effective flexural stiffness of slender reinforced concrete columns under axial forces and biaxial bending

Most of the design codes (ACI-318-2008 and Euro Code-2-2004) propose the moment magnifier method in order to take into account the second order effect to design slender reinforced concrete columns. The accuracy of this method depends on the effective flexural stiffness of the column. This paper proposes a new equation to obtain the effective stiffness EI of slender reinforced concrete columns. The expression is valid for any shape of cross-section, subjected to combined axial loads and biaxial bending, both for short-time and sustained loads, normal and high strength concretes, but it is only suitable for columns with equal effective buckling lengths in the two principal bending planes. The new equation extends the proposed EI equation in the "Biaxial bending moment magnifier method" by Bonet et al. (2004) [6], which is valid only for rectangular sections. The method was compared with 613 experimental tests from the literature and a good degree of accuracy was obtained. It was also compared with the design codes ACI-318 (08) and EC-2 (2004) improving the precision. The method is capable to verify and design with sufficient accuracy slender reinforced concrete columns in practical engineering design applications. © 2010 Elsevier Ltd.The authors wish to express their sincere gratitude to the Spanish "Ministerio de Ciencia e Innovacion" for help provided through projects BIA2008-03734 and BIA2009-10207 and to the European Community with the Feder funds.Bonet Senach, JL.; Romero, ML.; Miguel Sosa, P. (2011). Effective flexural stiffness of slender reinforced concrete columns under axial forces and biaxial bending. Engineering Structures. 33:881-893. doi:10.1016/j.engstruct.2010.12.009S8818933

Causes of ice age intensification across the Mid-Pleistocene Transition

During the Mid-Pleistocene Transition (MPT; 1,200–800 kya), Earth’s orbitally paced ice age cycles intensified, lengthened from ~40,000 (~40 ky) to ~100 ky, and became distinctly asymmetrical. Testing hypotheses that implicate changing atmospheric CO2 levels as a driver of the MPT has proven difficult with available observations. Here, we use orbitally resolved, boron isotope CO2 data to show that the glacial to interglacial CO2 difference increased from ~43 to ~75 μatm across the MPT, mainly because of lower glacial CO2 levels. Through carbon cycle modeling, we attribute this decline primarily to the initiation of substantive dust-borne iron fertilization of the Southern Ocean during peak glacial stages. We also observe a twofold steepening of the relationship between sea level and CO2-related climate forcing that is suggestive of a change in the dynamics that govern ice sheet stability, such as that expected from the removal of subglacial regolith or interhemispheric ice sheet phase-locking. We argue that neither ice sheet dynamics nor CO2 change in isolation can explain the MPT. Instead, we infer that the MPT was initiated by a change in ice sheet dynamics and that longer and deeper post-MPT ice ages were sustained by carbon cycle feedbacks related to dust fertilization of the Southern Ocean as a consequence of larger ice sheets.Research was supported by National Environmental Research Council (NERC) Studentship NE/I528626/1 (to T.B.C.); NERC Grant NE/P011381/1 (to T.B.C., M.P.H., G.L.F., E.J.R., and P.A.W.); NERC Fellowships NE/K00901X/1 (to M.P.H.), NE/I006346/1 (to G.L.F. and R.D.P), and NE/H006273/1 (to R.D.P.); Royal Society Wolfson Awards (to G.L.F. and P.A.W.); Australian Research Council Laureate Fellowship FL1201000050 (to E.J.R.); Swiss National Science Foundation Grant PP00P2-144811 (to S.L.J.); ETH Research Grant ETH-04 11-1 (to S.L.J.); European Research Council Consolidator Grant (ERC CoG) Grant 617462 (to H.P.); and NERC UK IODP Grant NE/F00141X/1 (to P.A.W.)

Measurement of (anti)deuteron and (anti)proton production in DIS at HERA

The first observation of (anti)deuterons in deep inelastic scattering at HERA has been made with the ZEUS detector at a centre-of-mass energy of 300--318 GeV using an integrated luminosity of 120 pb-1. The measurement was performed in the central rapidity region for transverse momentum per unit of mass in the range 0.3<p_T/M<0.7. The particle rates have been extracted and interpreted in terms of the coalescence model. The (anti)deuteron production yield is smaller than the (anti)proton yield by approximately three orders of magnitude, consistent with the world measurements.Comment: 26 pages, 9 figures, 5 tables, submitted to Nucl. Phys.

Observation of isolated high-E_T photons in deep inelastic scattering

First measurements of cross sections for isolated prompt photon production in deep inelastic ep scattering have been made using the ZEUS detector at the HERA electron-proton collider using an integrated luminosity of 121 pb^-1. A signal for isolated photons in the transverse energy and rapidity ranges 5 < E_T^gamma < 10 GeV and -0.7 < eta^gamma < 0.9 was observed for virtualities of the exchanged photon of Q^2 > 35 GeV^2. Cross sections are presented for inclusive prompt photons and for those accompanied by a single jet in the range E_T^jet \geq 6 GeV and -1.5 \leq eta^jet < 1.8. Calculations at order alpha^3alpha_s describe the data reasonably well.Comment: 16 pages, 5 figure

Photoproduction of D∗±D^{*\pm} mesons associated with a leading neutron

The photoproduction of $D^{*\pm} (2010)$ mesons associated with a leading neutron has been observed with the ZEUS detector in $ep$ collisions at HERA using an integrated luminosity of 80 pb$^{-1}$. The neutron carries a large fraction, {$x_L>0.2$}, of the incoming proton beam energy and is detected at very small production angles, {$\theta_n<0.8$ mrad}, an indication of peripheral scattering. The $D^*$ meson is centrally produced with pseudorapidity {$|\eta| 1.9$ GeV}, which is large compared to the average transverse momentum of the neutron of 0.22 GeV. The ratio of neutron-tagged to inclusive $D^*$ production is $8.85\pm 0.93({\rm stat.})^{+0.48}_{-0.61}({\rm syst.})\%$ in the photon-proton center-of-mass energy range {$130 <W<280$ GeV}. The data suggest that the presence of a hard scale enhances the fraction of events with a leading neutron in the final state.Comment: 28 pages, 4 figures, 2 table

Measurement of beauty production in deep inelastic scattering at HERA

The beauty production cross section for deep inelastic scattering events with at least one hard jet in the Breit frame together with a muon has been measured, for photon virtualities Q^2 > 2 GeV^2, with the ZEUS detector at HERA using integrated luminosity of 72 pb^-1. The total visible cross section is sigma_b-bbar (ep -> e jet mu X) = 40.9 +- 5.7 (stat.) +6.0 -4.4 (syst.) pb. The next-to-leading order QCD prediction lies about 2.5 standard deviations below the data. The differential cross sections are in general consistent with the NLO QCD predictions; however at low values of Q^2, Bjorken x, and muon transverse momentum, and high values of jet transverse energy and muon pseudorapidity, the prediction is about two standard deviations below the data.Comment: 18 pages, 4 figure