i-process Nucleosynthesis and Mass Retention Efficiency in He-shell Flash Evolution of Rapidly Accreting White Dwarfs

© 2017. The American Astronomical Society. All rights reserved. Based on stellar evolution simulations, we demonstrate that rapidly accreting white dwarfs (WDs) in close binary systems are an astrophysical site for the intermediate neutron-capture process. During recurrent and very strong He-shell flashes in the stable H-burning accretion regime H-rich material enters the He-shell flash convection zone. 12 C(p, γ) 13 N reactions release enough energy to potentially impact convection, and i process is activated through the 13 C(α, n) 16 O reaction. The H-ingestion flash may not cause a split of the convection zone as it was seen in simulations of He-shell flashes in post-AGB and low-Z asymptotic giant branch (AGB) stars. We estimate that for the production of first-peak heavy elements this site can be of similar importance for galactic chemical evolution as the s-process production by low-mass AGB stars. The He-shell flashes result in the expansion and, ultimately, ejection of the accreted and then i-process enriched material, via super-Eddington-luminosity winds or Roche-lobe overflow. The WD models do not retain any significant amount of the accreted mass, with a He retention efficiency of ≲ 10% depending on mass and convective boundary mixing assumptions. This makes the evolutionary path of such systems to supernova Ia explosion highly unlikely

A produção do sujeito de altas habilidades em matemática e a inclusão escolar na educação contemporânea

Understanding high abilities in Mathematics as practices that amidst power-knowledge relations identify, compare, and classify the student-subjects as persons with high abilities, this article aims to discuss how power-language games produce these subject-forms in contemporary school. Our analytical-conceptual production was based on the thoughts of M. Foucault and related authors in relation to language, discursive practices, power relations, and forms of government. Following these studies, the inclusive educational policy is analyzed, emphasizing its effects on how conducts and performances are handled in the context of neoliberalism, as well as its effects on contemporary school in Brazil. Thus, high abilities constitute no mere update of nomenclature or adjustment of vocabulary, but refer to a game of power and classification that produces, in the school space, the student-subject with high abilities based on norms, processes, and purposes related to behavior and performanceEntendendo as altas habilidades em Matemática como práticas que, em meio a relações de poder-saber, identificam, comparam e classificam os sujeitos-alunos como sendo portadores de altas habilidades, este artigo tem por objetivo discutir como jogos de poder-linguagem produzem essas formas-sujeito na escola contemporânea. A produção analítico-conceitual tomou como base os pensamentos de M. Foucault e autores afins, no que se refere às discussões sobre linguagem, práticas discursivas, relações de poder e formas de governo. Na esteira desses estudos, pensa-se a política educacional inclusiva, destacando seus efeitos nas maneiras de se orientarem condutas e performances no âmbito do neoliberalismo e seus efeitos na escola contemporânea no Brasil. Assim, as altas habilidades não constituem apenas uma atualização de nomenclatura ou uma mera acomodação de vocabulário, mas diz respeito a todo um jogo de poder e classificação que no espaço escolar produz "o sujeito-aluno portador de altas habilidades" a partir de normas, processos e finalidades comportamentais e performativas

Uncertainties in Galactic Chemical Evolution Models

We use a simple one-zone galactic chemical evolution model to quantify the uncertainties generated by the input parameters in numerical predictions for a galaxy with properties similar to those of the Milky Way. We compiled several studies from the literature to gather the current constraints for our simulations regarding the typical value and uncertainty of the following seven basic parameters: the lower and upper mass limits of the stellar initial mass function (IMF), the slope of the high-mass end of the stellar IMF, the slope of the delay-time distribution function of Type Ia supernovae (SNe Ia), the number of SNe Ia per M ⊙ formed, the total stellar mass formed, and the final mass of gas. We derived a probability distribution function to express the range of likely values for every parameter, which were then included in a Monte Carlo code to run several hundred simulations with randomly selected input parameters. This approach enables us to analyze the predicted chemical evolution of 16 elements in a statistical manner by identifying the most probable solutions, along with their 68% and 95% confidence levels. Our results show that the overall uncertainties are shaped by several input parameters that individually contribute at different metallicities, and thus at different galactic ages. The level of uncertainty then depends on the metallicity and is different from one element to another. Among the seven input parameters considered in this work, the slope of the IMF and the number of SNe Ia are currently the two main sources of uncertainty. The thicknesses of the uncertainty bands bounded by the 68% and 95% confidence levels are generally within 0.3 and 0.6 dex, respectively. When looking at the evolution of individual elements as a function of galactic age instead of metallicity, those same thicknesses range from 0.1 to 0.6 dex for the 68% confidence levels and from 0.3 to 1.0 dex for the 95% confidence levels. The uncertainty in our chemical evolution model does not include uncertainties relating to stellar yields, star formation and merger histories, and modeling assumptions

JINA-NuGrid Galactic Chemical Evolution Pipeline

Galactic chemical evolution is a topic that involves nuclear physics, stellar evolution, galaxy evolution, observation, and cosmology. Continuous communication and feedback between these fields is a key component in improving our understanding of how galaxies form and how elements are created and recycled in galaxies and intergalactic space. In this proceedings, we present the current state of the JINA-NuGrid chemical evolution pipeline. It is designed to probe the impact of nuclear astrophysics uncertainties on galactic chemical evolution, to improve our knowledges regarding the origin of the elements in a cosmological context, and to create the required interdisciplinary connections.Comment: 3 pages, 2 figures, submitted to JPS Conference Proceedings, Nuclei in the Cosmos XI

Growth and surface alloying of Fe on Pt(997)

The growth of ultra-thin layers of Fe on the vicinal Pt(997) surface is studied by thermal energy He atom scattering (TEAS) and Auger electron spectroscopy (AES) in the temperature range between 175K and 800K. We find three distinct regimes of qualitatively different growth type: Below 450K the formation of a smooth first monolayer, at and above 600K the onset of bulk alloy formation, and at intermediate temperature 500K - 550K the formation of a surface alloy. Monatomic Fe rows are observed to decorate the substrate steps between 175K and 500K. The importance of the high step density is discussed with respect to the promotion of smooth layer growth and with respect to the alloying process and its kinetics

The i-process and CEMP-r/s stars

© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. We investigate whether the anomalous elemental abundance patterns in some of the C-enhanced metal-poor-r/s (CEMP-r/s) stars are consistent with predictions of nucleosynthesis yields from the i-process, a neutron-capture regime at neutron densities intermediate between those typical for the slow (s) and rapid (r) processes. Conditions necessary for the i-process are expected to be met at multiple stellar sites, such as the He-core and He-shell flashes in low-metallicity low-mass stars, super-AGB and post-AGB stars, as well as low-metallicity massive stars. We have found that single-exposure one-zone simulations of the i-process reproduce the abundance patterns in some of the CEMP-r/s stars much better than the model that assumes a superposition of yields from s and r-process sources. Our previous study of nuclear data uncertainties relevant to the i-process revealed that they could have a significant impact on the i-process yields obtained in our idealized one-zone calculations, leading, for example, to ∼ 0:7dex uncertainty in our predicted [Ba/La] ratio. Recent 3D hydrodynamic simulations of convection driven by a He-shell flash in post-AGB Sakurai's object have discovered a new mode of non-radial instabilities: the Global Oscillation of Shell H-ingestion. This has demonstrated that spherically symmetric stellar evolution simulations cannot be used to accurately model physical conditions for the i-process