4,144 research outputs found

    SNR-calibrated Type Ia supernova models

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    Current Type Ia supernova (SN Ia) models can reproduce most visible+IR + UV observations. In the X-ray band, the determination of elemental abundance ratios in supernova remnants (SNRs) through their spectra has reached enough precision to constrain SN Ia models. Martínez-Rodríguez et al have shown that the Ca/S mass ratio in SNRs cannot be reproduced with the standard nuclear reaction rates for a wide variety of SN Ia models, and suggested that the 12C+16O reaction rate could be overestimated by a factor as high as ten. We show that the same Ca/S ratio can be obtained by simultaneously varying the rates of the reactions 12C + 16O, 12C + 12C, 16O + 16O, and 16O(¿, a)12C within the reported uncertainties. We also show that the yields of the main products of SN Ia nucleosynthesis do not depend on the details of which rates are modified, but can be parametrized by an observational quantity such as Ca/S. Using this SNR-calibrated approach, we then proceed to compute a new set of SN Ia models and nucleosynthesis for both Chandrasekhar and sub-Chandrasekhar mass progenitors with a 1D hydrodynamics and nucleosynthesis code. We discuss the nucleosynthesis of the models as a function of progenitor metallicity, mass, and deflagration-to-detonation transition density. The yields of each model are almost independent on the reaction rates modified for a common Ca/S ratio.Peer ReviewedPostprint (author's final draft

    Central galaxies in different environments: Do they have similar properties?

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    We perform an exhaustive comparison among central galaxies from SDSS catalogs in different local environments at 0.01<=z<=0.08. The central galaxies are separated into two categories: group centrals (host halos containing satellites) and field centrals (host halos without satellites). From the latter, we select other two subsamples: isolated centrals and bright field centrals, both with the same magnitude limit. The stellar mass (Ms) distributions of the field and group central galaxies are different, which explains why in general the field central galaxies are mainly located in the blue cloud/star forming regions, whereas the group central galaxies are strongly biased to the red sequence/passive regions. The isolated centrals occupy the same regions as the bright field centrals since both populations have similar Ms distributions. At parity of Ms, the color and specific star formation rate (sSFR) distributions of the samples are similar, specially between field and group centrals. Furthermore, we find that the stellar-to-halo mass (Ms-Mh) relation of isolated galaxies does not depend on the color, sSFR and morphological type. For systems without satellites, the Ms-Mh relation steepens at high halo masses compared to group centrals, which is a consequence of assuming a one-to-one relation between group total stellar mass and halo mass. Under the same assumption, the scatter around the Ms-Mh relation of centrals with satellites increases with halo mass. Our results suggest that the mass growth of central galaxies is mostly driven by the halo mass, with environment and mergers playing a secondary role.Comment: 17 pages, 11 figures after last Referee's report. Accepted for publication in Ap

    Alzheimer's disease and HIV associated dementia related genes: I. location and function.

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    Alzheimer's disease (AD), the most common cause of dementia, has few clinical similarities to HIV-1-associated dementia (HAD). However, genes were identified related among these dementias. Discovering correlations between gene function, expression, and structure in the human genome continues to aid in understanding the similarities between pathogenesis of these two dementing disorders. The current work attempts to identify relationships between these dementias in spite of their clinical differences, based on genomic structure, function, and expression. In this comparative study, the NCBI Entrez Genome Database is used to detect these relationships. This approach serves as a model for future diagnosis and treatment in the clinical arena as well as suggesting parallel pathways of disease mechanisms. Identifying a correlation among expression, structure, and function of genes involved in pathogenesis of these dementing disorders, may assist to understand better their interaction with each other and the human genome

    Cost and benefits of using best management practices to control non-point sources of pollution under environmental and economic uncertainty

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    The economy of northwest Arkansas, including the Lincoln Lake watershed (a sub-watershed of the Illinois River), relies greatly upon livestock and poultry production. The supply of production by-products is increasingly under scrutiny as one of the potential sources of water pollution in the region. In light of the recent economic crisis, methodologies that help producers to evaluate the environmental and economic impacts of several practices before implementing them may be a cost-effective means of increasing BMP adoption. This study uses stochastic dominance techniques to evaluate, environmentally and economically, ten best management practices (BMPs) combinations to lessen water pollution in the Lincoln Lake watershed. All BMP combinations analyzed were effective in reducing total phosphorous (TP) losses. However, six combinations also decreased net returns (NR) when compared to a baseline. This suggests that including BMPs in the bermudagrass production systems may lead to increased NR risk. Without additional incentives, producers will not likely implement these BMP combinations regardless of their TP reduction benefits. Although, as expected, rankings of BMP combinations in terms of TP or NR differed from each other, four scenarios established that environmental and economic goals are not necessarily conflicting; they may be complementary. Additionally, this analysis revealed that producers’ risk preferences did not matter when selecting among the top-four BMP combinations but it could be a factor for other less preferred scenarios.nonpoint pollution, watershed, best management practices, risk analysis, stochastic dominance, Environmental Economics and Policy, Risk and Uncertainty, Q25, Q53,

    Nariai--Bertotti--Robinson spacetimes as a building material for one-way wormholes with horizons, but without singularity

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    We discuss the problem of wormholes from the viewpoint of gluing together two Reissner--Nordstr\"om-type universes while putting between them a segment of the Nariai-type world (in both cases there are also present electromagnetic fields as well as the cosmological constant). Such a toy wormhole represents an example of one-way topological communication free from causal paradoxes, though involving a travel to next spacetime sheet since one has to cross at least a pair of horizons through which the spacetimes' junction occurs. We also consider the use of thin shells in these constructions. Such a ``material'' for wormholes we choose taking into account specific properties of the Nariai--Bertotti--Robinson spacetimes.Comment: 5 pages, a talk delivered at the 11th Marcel Grossmann Meeting (2006
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