2,993 research outputs found

    Multidimensional hydrodynamic simulations of the hydrogen injection flash

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

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    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 0.01\sim 0.01 and 0.1M\sim 0.1 M_\odot. The calculations show that for high accretion rates (M˙acc=104Myr1\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 (M˙acc=105Myr1\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

    The Swallowing of Planets by Giant Stars

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    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

    Nucleosynthesis of s-elements in zero-metal AGB stars

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    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

    An Experimental Study of a Flat Slab Floor Reinforced with Welded Wire Fabric

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    Reinforced Concrete Reserach CouncilOffice of the Chief of Engineers, U.S. Army.General Services Administration, Public Buildings ServiceHeadquarters, U.S. Air Force. Contract AF 33(658)-47U.S. Navy, Engineering Division. Bureau of Yards and Docks. NBy 3763

    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

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

    Final Report

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

    Structure, Evolution and Nucleosynthesis of Primordial Stars

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    (abridge version) The evolution of population III stars (Z=0) is followed from the pre-main sequence phase up to the AGB phase for intermediate-mass stars and up to C ignition in more massive stars...We find that, thanks to the development of mixing episodes (carbon injections) at the beginning of the AGB phase, the carbon abundance of the 1, 1.5, 2, 3, 4 and 5Mo models is significantly increased in the envelope. This process then allows low- and intermediate-mass stars to achieve a ``standard'' thermally pulsing AGB phase... In the 7Mo model, the CNO envelope abundance following the second dredge-up is so large that the star does not experience the carbon injection episode and follows a more standard thermally pulsing AGB evolution. Our computations also indicate that, thanks to a small overshooting at the base of the convective envelope, the third dredge-up is already operating in stars with M >~1.5 Mo after a few pulses, and that by the end of our modeling, hot bottom burning is activated in stars more massive than ~ 2Mo. This evolutionary behavior suggests that primordial low- and intermediate stars could have been significant contributors to the production of primary 12C, 14N, and may have contributed to some extent to the production of Mg and Al and possibly s-elements (despite the lack of iron seeds) in the early universe.Comment: 26 pages, 13 figures, 6 tables, uses aastex. Accepted for publication in ApJ. Full postscript version available at http://www-astro.ulb.ac.be/~sies

    Age-Related Observations of Low Mass Pre-Main and Young Main Sequence Stars (Invited Review)

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    This overview summarizes the age dating methods available for young sub-solar mass stars. Pre-main sequence age diagnostics include the Hertzsprung-Russell (HR) diagram, spectroscopic surface gravity indicators, and lithium depletion; asteroseismology is also showing recent promise. Near and beyond the zero-age main sequence, rotation period or vsini and activity (coronal and chromospheric) diagnostics along with lithium depletion serve as age proxies. Other authors in this volume present more detail in each of the aforementioned areas. Herein, I focus on pre-main sequence HR diagrams and address the questions: Do empirical young cluster isochrones match theoretical isochrones? Do isochrones predict stellar ages consistent with those derived via other independent techniques? Do the observed apparent luminosity spreads at constant effective temperature correspond to true age spreads? While definitive answers to these questions are not provided, some methods of progression are outlined.Comment: to appear in IAU Symposium 258, "Ages of Stars", E.E. Mamajek, D.R. Soderblom, R.F.G. Wyse, ed
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