2,747 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. MartĂ­nez-RodrĂ­guez 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(Âż, 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

    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

    Age determination in the snapper Lutjanus guttatus (Pisces, Lutjanidae) and investigation of fishery management strategies in the Pacific Coast of Guatemala

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    The rose-spotted snapper, Lutjanus guttatus, seems to be sequentially exploited by the shrimp trawler and the artisanal fleet along the Pacific Coast of Guatemala. The growth parameters of the snapper were estimated in the present study with basis on material collected in two fishing harbours. Other biological and fishery information was compiled to simulate stock dynamics with a yield-per-recruit model. This single-species model allowed the evaluation of possible outcomes of fleet interactions, in terms of relative yield, spawning stock development and profit. A large emphasis was put in the evaluation of skeletal structures, otoliths and vertebrae, to determine age. Vertebrae seem to be the best structure for ageing, as they revealed better ring definition, and resulted in more adequate growth patterns and plausible Von Bertalanffy growth parameters. The VBGF (total length-at-age) obtained with vertebrae was Lt = 66.4(1-exp(-0.13(t-0.03))). The age of first-capture (Tc) for the trawl and the artisanal fleets were estimated to be about 1 and 3 years, respectively. The age-dependent mortality was suggested to follow unimodal curves for the two fleets, owing to the patterns of selectivity of the gear and availability of the fish. The yield-per-recruit scenarios showed that for the suggested biological and fishery parameters, the highest yield of snapper is not being attained. Further, the model suggests that with the present fishing pattern high rates of fishing mortality could deplete the spawning stock. Implementation of size-selective practices that minimise the trawlers’ by-catch of L. guttatus, and an increase of the artisanal Tc (to above the maturation age, Tm, about 4.5 years) are likely to result in increased yield, total profit, and prevent recruitment overfishing

    Electronic health information exchange in underserved settings: examining initiatives in small physician practices & community health centers.

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    BackgroundHealth information exchange (HIE) is an important tool for improving efficiency and quality and is required for providers to meet Meaningful Use certification from the United States Centers for Medicare and Medicaid Services. However widespread adoption and use of HIE has been difficult to achieve, especially in settings such as smaller-sized physician practices and federally qualified health centers (FQHCs). We assess electronic data exchange activities and identify barriers and benefits to HIE participation in two underserved settings.MethodsWe conducted key-informant interviews with stakeholders at physician practices and health centers. Interviews were recorded, transcribed, and then coded in two waves: first using an open-coding approach and second using selective coding to identify themes that emerged across interviews, including barriers and facilitators to HIE adoption and use.ResultsWe interviewed 24 providers, administrators and office staff from 16 locations in two states. They identified barriers to HIE use at three levels-regional (e.g., lack of area-level exchanges; partner organizations), inter-organizational (e.g., strong relationships with exchange partners; achieving a critical mass of users), and intra-organizational (e.g., type of electronic medical record used; integration into organization's workflow). A major perceived benefit of HIE use was the improved care-coordination clinicians could provide to patients as a direct result of the HIE information. Utilization and perceived benefit of the exchange systems differed based on several practice- and clinic-level factors.ConclusionsThe adoption and use of HIE in underserved settings appears to be impeded by regional, inter-organizational, and intra-organizational factors and facilitated by perceived benefits largely at the intra-organizational level. Stakeholders should consider factors both internal and external to their organization, focusing efforts in changing modifiable factors and tailoring HIE efforts based on all three categories of factors. Collective action between organizations may be needed to address inter-organizational and regional barriers. In the interest of facilitating HIE adoption and use, the impact of interventions at various levels on improving the use of electronic health data exchange should be tested
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