Nucleosynthesis of s-elements in zero-metal AGB stars

Contrary to previous expectations, recent evolutionary models of zero-metallicity stars show that the development of mixing episodes at the beginning of the AGB phase allows low- and intermediate-mass stars to experience thermal pulses. If these stars, like their metal-rich counterparts, also experience partial mixing of protons from the H-rich envelope into the C-rich layers at the time of the third dredge-up, an extensive neutron capture nucleosynthesis leads to the production of s-process nuclei up to Pb and Bi. Nucleosynthesis calculations based on stellar AGB models are performed assuming a parameterized H-abundance profile below the convective envelope at the time of the third dredge-up. Despite the absence of Fe-group elements, the large neutron flux resulting from the 13C(alpha,n)16O reaction leads to an efficient production of s-process elements starting from the neutron captures on the C-Ne isotopes. Provided partial mixing of protons takes place, it is shown that population III AGB stars should be enriched in s-process elementsand overall in Pb and Bi.Comment: 4 pages, 3 Postscript figures, uses aa.sty. Accepted for publication in A&A Letter

The Swallowing of Planets by Giant Stars

We present simulations of the accretion of a massive planet or brown dwarf by an AGB star. In our scenario, close planets will be engulfed by the star, spiral-in and be dissipated in the ``accretion region'' located at the bottom of the convective envelope of the star. The deposition of mass and chemical elements in this region will release a large amount of energy that will produce a significant expansion of the star. For high accretion rates, hot bottom burning is also activated. Finally, we present some observational signatures of the accretion of a planet/brown dwarf and we propose that this process may be responsible for the IR excess and high lithium abundance observed in 4-8% of single G and K giants.Comment: 4 pages, 1 figure, to appear in "Unsolved Problems in Stellar Evolution", ed. M. Livio, Cambridge University Press, in pres

Platelet interaction with bioactive lipids formed by mild oxidation of low-density lipoprotein

Oxidation of low-density lipoprotein (LDL) generates pro-inflammatory and pro-thrombotic mediators that play a crucial role in cardiovascular and inflammatory diseases. Mildly oxidized LDL (mox-LDL) and minimally modified LDL (mm-LDL) which escape the uptake of macrophage scavenger receptors accumulate in the atherosclerotic intima. Oxidatively modified LDL is also present within the electronegative LDL fraction in blood, which is elevated in patients at high risk for cardiovascular diseases. Mox-LDL and mm-LDL, but not native LDL are able to induce platelet shape change and aggregation. LDL oxidation generates lipids with platelet stimulatory properties such as lysophosphatidylcholine, certain oxidized phosphatidylcholine molecules, F-2-isoprostanes and lysophosphatidic acid (LPA). Mox-LDL and mm-LDL are like a Trojan horse carrying these biologically active lipids and attacking cells through activation of physiological receptors and signaling mechanisms. LPA has been identified as the lipid responsible for platelet stimulation by mox-LDL, mm-LDL and also mox-HDL. These lipoproteins activate platelets by stimulating G-protein coupled LPA receptors and a Rho/Rho kinase signaling pathway leading to platelet shape change and subsequent aggregation. LPA-mediated platelet activation might contribute to arterial thrombus formation after rupture of atherosclerotic plaques and to the increased blood thrombogenicity of patients with cardiovascular diseases. Copyright (c) 2006 S. Karger AG, Basel

Multidimensional hydrodynamic simulations of the hydrogen injection flash

The injection of hydrogen into the convection shell powered by helium burning during the core helium flash is commonly encountered during the evolution of metal-free and extremely metal-poor low-mass stars. With specifically designed multidimensional hydrodynamic simulations, we aim to prove that an entropy barrier is no obstacle for the growth of the helium-burning shell convection zone in the helium core of a metal-rich Pop I star, i.e. convection can penetrate into the hydrogen-rich layers for these stars, too. We further study whether this is also possible in one-dimensional stellar evolutionary calculations. Our hydrodynamical simulations show that the helium-burning shell convection zone in the helium core moves across the entropy barrier and reaches the hydrogen-rich layers. This leads to mixing of protons into the hotter layers of the core and to a rapid increase of the nuclear energy production at the upper edge of the helium-burning convection shell - the hydrogen injection flash. As a result a second convection zone appears in the hydrogen-rich layers. Contrary to 1D models, the entropy barrier separating the two convective shells from each other is largely permeable to chemical transport when allowing for multidimensional flow, and consequently, hydrogen is continuously mixed deep into the helium core. We find it difficult to achieve such a behavior in one-dimensional stellar evolutionary calculations.Comment: 8 pages, 8 figures - accepted for publication in Astronomy and Astrophysics. Animations related to the manuscript can be downloaded from http://www-astro.ulb.ac.be/~mocak/index.php/Main/AnimationsHeFlas

The Effects of Pattern Loadings on Reinforced Concrete Floor Slabs

Reinforced Concrete Reserach CouncilOffice of the Chief of Engineers, U.S. Army.General Services Administration, Public Buildings ServiceHeadquarters, U.S. Air Force, Directorate of Civil Engineering.U.S. Navy, Engineering Division. Bureau of Yards and Docks. NBy 3763

Final Report

AASHO Road TestHighway Research BoardNational Academy of Sciences - National Research Counci

Issued as a Part of Progress Report No. 14 of The Investigation of Prestressed Reinforced Concrete for Highway Bridges; Project IHR-10, Illinois Cooperative Highway Research Program

The Bureau of Public Roads. U.S. Department of CommerceThe Division of Highways. State of Illinois

The Accretion of Brown Dwarfs and Planets by Giant Stars -- I. AGB Stars

We study the response of the structure of an asymptotic giant branch (AGB) star to the accretion of a brown dwarf or planet in its interior. In particular, we examine the case in which the brown dwarf spirals-in, and the accreted matter is deposited at the base of the convective envelope and in the thin radiative shell surrounding the hydrogen burning shell. In our spherically symmetric simulations, we explore the effects of different accretion rates and we follow two scenarios in which the amounts of injected mass are equal to $\sim 0.01$ and $\sim 0.1 M_\odot$. The calculations show that for high accretion rates ($\dot M_{acc} = 10^{-4} M_\odot yr^{-1}$), the considerable release of accretion energy produces a substantial expansion of the star and gives rise to hot bottom burning at the base of the convective envelope. For somewhat lower accretion rates ($\dot M_{acc} = 10^{-5} M_\odot yr^{-1}$), the accretion luminosity represents only a small fraction of the stellar luminosity, and as a result of the increase in mass (and concomitantly of the gravitational force), the star contracts. Our simulations also indicate that the triggering of thermal pulses is delayed (accelerated) if mass is injected at a slower (faster) rate. We analyze the effects of this accretion process on the surface chemical abundances and show that chemical modifications are mainly the result of deposition of fresh material rather than of active nucleosynthesis. Finally, we suggest that the accretion of brown dwarfs and planets can induce the ejection of shells around giant stars, increase their surface lithium abundance and lead to significant spin-up. The combination of these features is frequently observed among G and K giant stars.Comment: 11 pages, 9 Postscript figures, to be published in the MNRAS. see also http://www-laog.obs.ujf-grenoble.fr/~sies

Binary evolution using the theory of osculating orbits: conservative Algol evolution

Our aim is to calculate the evolution of Algol binaries within the framework of the osculating orbital theory, which considers the perturbing forces acting on the orbit of each star arising from mass exchange via Roche lobe overflow (RLOF). The scheme is compared to results calculated from a `classical' prescription. Using our stellar binary evolution code BINSTAR, we calculate the orbital evolution of Algol binaries undergoing case A and case B mass transfer, by applying the osculating scheme. The velocities of the ejected and accreted material are evaluated by solving the restricted three-body equations of motion, within the ballistic approximation. This allows us to determine the change of linear momentum of each star, and the gravitational force applied by the mass transfer stream. Torques applied on the stellar spins by tides and mass transfer are also considered. Using the osculating formalism gives shorter post-mass transfer orbital periods typically by a factor of 4 compared to the classical scheme, owing to the gravitational force applied onto the stars by the mass transfer stream. Additionally, during the rapid phase of mass exchange, the donor star is spun down on a timescale shorter than the tidal synchronization timescale, leading to sub-synchronous rotation. Consequently, between 15 and 20 per cent of the material leaving the inner-Lagrangian point is accreted back onto the donor (so-called `self-accretion'), further enhancing orbital shrinkage. Self-accretion, and the sink of orbital angular momentum which mass transfer provides, may potentially lead to more contact binaries. Even though Algols are mainly considered, the osculating prescription is applicable to all types of interacting binaries, including those with eccentric orbits.Comment: A&A in press. Minor typos correcte

Analysis of Clamped Square Plates Containing Openings with Stiffened Edges

National Science Foundation Grant No. G-657