1,721,171 research outputs found
The thermonuclear explosion of chandrasekhar mass white dwarfs
No full textThe flame born in the deep interior of a white dwarf that becomes a Type Ia supernova is subject to several instabilities. We briefly review these instabilities and the corresponding flame acceleration. We discuss the conditions necessary for each of the currently proposed explosion mechanisms and the attendant uncertainties. A grid of critical masses for detonation in the range - g cm is calculated and its sensitivity to composition explored. Prompt detonations are physically improbable and appear unlikely on observational grounds. Simple deflagrations require some means of boosting the flame speed beyond what currently exists in the literature. 'Active turbulent combustion' and multi-point ignition are presented as two plausible ways of doing this. A deflagration that moves at the 'Sharp-Wheeler' speed, , is calculated in one dimension and shows that a healthy explosion is possible in a simple deflagration if the front moves with the speed of the fastest floating bubbles. The relevance of the transition to the 'distributed burning regime' is discussed for delayed detonations. No model emerges without difficulties, but detonation in the distributed regime is plausible, will produce intermediate mass elements, and warrants further study
Off-center deflagrations in Chandrasekhar mass type Ia supernova models
No full textA series of two dimensional numerical simulations of explosive nuclear burning is presented for white dwarfs near the Chandraskhar mass. We assume that the burning begins as a slow deflagration front at or near the center of the star, and continues until the density in the burning regions has declined to about 10 g cm, where the flame is essentially extinguished. We employ a novel numerical representation of the turbulent flame brush based upon ideas previously developed for modelling laboratory combustion and explore in some detail the sensitivity of the outcome to the manner in which burning is initiated. In particular, we simulate 1) a centrally ignited deflagration, 2) off-center ignition at a single ``point", and 3) simultaneous off-center ignition at five ``points". We find that the amount of $^{56} that is produced and other observable properties depend sensitively upon how the fuel is ignited
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Multi-spot ignition in type Ia supernova models
No full textWe present a systematic survey of the capabilities of type Ia
supernova explosion models starting from a number of flame seeds
distributed around the center of the white dwarf star. To this end
we greatly improved the resolution of the numerical simulations in
the initial stages. This novel numerical approach facilitates a
detailed study of multi-spot ignition scenarios with up to hundreds
of ignition sparks. Two-dimensional simulations are shown to be
inappropriate to study the effects of initial flame
configurations. Based on a set of three-dimensional models, we
conclude that multi-spot ignition
scenarios may improve type Ia supernova models towards
better agreement with observations. The achievable effect
reaches a maximum at a limited number of flame ignition kernels as
shown by the numerical models and corroborated by a simple
dimensional analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Distributed burning in Type Ia supernovae: A statistical approach
No full textWe present a statistical model which shows the influence of turbulence on a thermonuclear flame propagating in C + O white dwarf matter. Based on a Monte Carlo description of turbulence, it provides a method for investigating the physics in the so-called distributed burning regime. Using this method, we perform numerical simulations of turbulent flames and show that in this particular regime the flamelet model for the turbulent flame velocity loses its validity. In fact, at high turbulent intensities, burning in the distributed regime can lead to a deceleration of the turbulent flame and thus induce a competing process to turbulent effects that cause a higher flame speed. It is also shown that in dense C + O matter turbulent heat transport is described adequately by the Peclet number rather than by the Reynolds number, which means that flame propagation is decoupled from small-scale turbulence. Finally, at the onset of our results we argue that the available turbulent energy in an exploding C + O white dwarf is probably too low to make a deflagration-to-detonation transition possible
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