2,262 research outputs found

    Production of 92Nb, 92Mo, and 146Sm in the gamma-process in SNIa

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    The knowledge of the production of extinct radioactivities like 92Nb and 146Sm by photodisintegration processes in ccSN and SNIa models is essential for interpreting abundances in meteoritic material and for Galactic Chemical Evolution (GCE). The 92Mo/92Nb and 146Sm/144Sm ratios provide constraints for GCE and production sites. We present results for SNIa with emphasis on nuclear uncertainties.Comment: 6 pages, 4 figures, Proceedings of the 13th Symposium on Nuclei in the Cosmos (NIC XIII), July 2014, Debrecen, Hungar

    Stellar evolution of massive stars at very low metallicities

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    Recently, measurements of abundances in extremely metal poor (EMP) stars have brought new constraints on stellar evolution models. In an attempt to explain the origin of the abundances observed, we computed pre--supernova evolution models, explosion models and the related nucleosynthesis. In this paper, we start by presenting the pre-SN models of rotating single stars with metallicities ranging from solar metallicity down to almost metal free. We then review key processes in core-collapse and bounce, before we integrate them in a simplistic parameterization for 3D MHD models, which are well underway and allow one to follow the evolution of the magnetic fields during collapse and bounce. Finally, we present explosive nucleosynthesis results including neutrino interactions with matter, which are calculated using the outputs of the explosion models. The main results of the pre-SN models are the following. First, primary nitrogen is produced in large amount in models with an initial metallicity Z=10−8Z=10^{-8}. Second, at the same metallicity of Z=10−8Z=10^{-8} and for models with an initial mass larger than about 60 Mo, rotating models may experience heavy mass loss (up to more than half of the initial mass of the star). The chemical composition of these winds can qualitatively reproduce the abundance patterns observed at the surface of carbon-rich EMP stars. Explosive nucleosynthesis including neutrino-matter interactions produce improved abundances for iron group elements, in particular for scandium and zinc. It also opens the way to a new neutrino and proton rich process (ν\nup-process) able to contribute to the nucleosynthesis of elements with A > 64. (Abridged)Comment: 29 pages, 10 figures, Reviews of Modern Astronomy 19, proceedings for 79th Annual Scientific Meeting of the Deutsche Astronomische Gesellschaft 200

    The s process in massive stars at low metallicity. Effect of primary N14 from fast rotating stars

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    The goal of this paper is to analyze the impact of a primary neutron source on the s-process nucleosynthesis in massive stars at halo metallicity. Recent stellar models including rotation at very low metallicity predict a strong production of primary N14. Part of the nitrogen produced in the H-burning shell diffuses by rotational mixing into the He core where it is converted to Ne22 providing additional neutrons for the s process. We present nucleosynthesis calculations for a 25 Msun star at [Fe/H] = -3, -4, where in the convective core He-burning about 0.8 % in mass is made of primary Ne22. The usual weak s-process shape is changed by the additional neutron source with a peak between Sr and Ba, where the s-process yields increase by orders of magnitude with respect to the yields obtained without rotation. Iron seeds are fully consumed and the maximum production of Sr, Y and Zr is reached. On the other hand, the s-process efficiency beyond Sr and the ratio Sr/Ba are strongly affected by the amount of Ne22 and by nuclear uncertainties, first of all by the Ne22(alpha,n)Mg25 reaction. Finally, assuming that Ne22 is primary in the considered metallicity range, the s-process efficiency decreases with metallicity due to the effect of the major neutron poisons Mg25 and Ne22. This work represents a first step towards the study of primary neutron source effect in fast rotating massive stars, and its implications are discussed in the light of spectroscopic observations of heavy elements at halo metallicity.Comment: Accepted for publication in ApJ Letters, 11 pages, 2 figures, 1 tabl

    Certain General Anesthetics Used in Pediatrics Hinder Neurological Development in Infants

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    General anesthetics act by either blocking N-methyl-d-aspartate (NMDA) receptors or over stimulating γ-aminobutyric acid (GABA) receptors.1–3 The actions of these receptors are responsible for the anesthetized state and are also crucial in the neurological development of infants.2,4–8 Animal studies, although limited, provide vital information about general anesthesia’s neurotoxicity its hindrance of neurological development. Exposure to general anesthesia can severely hinder proper neuronal migration, synaptogenesis, and can drastically increase neuronal apoptosis in infant animals.9–16 General anesthetics are more neurotoxic to infant animals in combination compared to individually.10,16,17 Additionally, multiple exposures to general anesthesia tend to have compounding deleterious effects on neurological development in infant animals.12 It is likely that repeated exposure and combinational exposure to general anesthetics are the most detrimental to neurological development. Many retrospective studies on human infants show a correlation between exposure to general anesthesia and an increased risk of neurodevelopment disorders.18–25 However, these studies are statistically limited due to confounding factors. These confounding factors are the reason direct evidence of the neurotoxic effects of general anesthetics has been so elusive. In vitro human stem cell models provide an ethical alternative to clinical studies. However, clinical trials are necessary and are the most promising methods for obtaining direct evidence of the deleterious effects of general anesthesia on the developing human brain. Different methods used in clinical trials on infants help to minimalize ethical dilemmas, increase recruitment rates, and maximize safety and expediency. Specifically, this paper will evaluate the efficacy and safety of different methods used in clinical trials and will propose how clinical trials can be designed for future studies

    Boron depletion in 9 to 15 M(circle dot) stars with rotation

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    The treatment of mixing is still one of the major uncertainties in stellar evolution models. One open question is how well the prescriptions for rotational mixing describe the real effects. We tested the mixing prescriptions included in the Geneva stellar evolution code (GENEC) by following the evolution of surface abundances of light isotopes in massive stars, such as boron and nitrogen. We followed 9, 12 and 15 M(O) models with rotation from the zero age main sequence up to the end of He burning. The calculations show the expected behaviour with faster depletion of boton for faster rotating stars and more massive stars. The mixing at the surface is more efficient, than predicted by prescriptions used in other codes and reproduces the majority of observations very well However two observed stars with strong boron depletion but, no nitrogen enhancement still can not be explained and let the question open whether additional mixing processes are acting in these massive star

    Abundance Uncertainties Obtained With the PizBuin Framework For Monte Carlo Reaction Rate Variations

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    Uncertainties in nucleosynthesis models originating from uncertainties in astrophysical reaction rates were estimated in a Monte Carlo variation procedure. Thousands of rates were simultaneously varied within individual, temperature-dependent errors to calculate their combined effect on final abundances. After a presentation of the method, results from application to three different nucleosynthesis processes are shown: the γ\gamma-process and the s-process in massive stars, and the main s-process in AGB stars (preliminary results). Thermal excitation of nuclei in the stellar plasma and the combined action of several reactions increase the final uncertainties above the level of the experimental errors. The total uncertainty, on the other hand, remains within a factor of two even in processes involving a large number of unmeasured rates, with some notable exceptions for nuclides whose production is spread over several stellar layers and for s-process branchings.Comment: 8 pages, 4 figures; Proceedings of OMEG 2017, Daejeon, Korea, June 27-30, 2017; to appear in AIP Conf. Pro
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