1,721,022 research outputs found
Multi-wavelength accretion studies of cataclysmic variable stars
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159310.pdf (Publisher’s version ) (Open Access)RU Radboud Universiteit, 03 oktober 2016Promotores : Groot, P.J., Woudt, P.A. Co-promotor : Körding, E.G.iii, 148 p
MESA and NuGrid simulations of classical nova outbursts and nucleosynthesis
Get PDFClassical novae are the results of surface thermonuclear explosions of hydrogen accreted by white dwarfs (WDs) from their low-mass main-sequence or red-giant binary companions. Chemical composition analysis of their ejecta shows that nova outbursts occur on both carbon-oxygen (CO) and more massive oxygen-neon (ONe) WDs, and that there is cross-boundary mixing between the accreted envelope and underlying WD. We demonstrate that the state-of-the-art stellar evolution code MESA and post-processing nucleosynthesis tools of NuGrid can successfully be used for modeling of CO and ONe nova outbursts and nucleosynthesis. The convective boundary mixing (CBM) in our 1D numerical simulations is implemented using a diffusion coefficient that is exponentially decreasing with a distance below the bottom of the convective envelope. We show that this prescription produces maximum temperature evolution profiles and nucleosynthesis yields in good agreement with those obtained using the commonly adopted 1D nova model in which the CBM is mimicked by assuming that the accreted envelope has been pre-mixed with WD's material. In a previous paper, we have found that 3He can be produced in situ in solar-composition envelopes accreted with slow rates (Ṁ < 10–10 M⊙/yr) by cold (TWD<107 K) CO WDs, and that convection is triggered by 3He burning before the nova outburst in this case. Here, we confirm this result for ONe novae. Additionally, we find that the interplay between the 3He production and destruction in the solar-composition envelope accreted with an intermediate rate, e.g. Ṁ = 10–10 M⊙/yr, by the 1.15 M⊙ ONe WD with a relatively high initial central temperature, e.g. TWD =15×106 K, leads to the formation of a thick radiative buffer zone that separates the bottom of the convective envelope from the WD surface
MESA and NuGrid simulations of classical nova outbursts and nucleosynthesis
Get PDFClassical novae are the results of surface thermonuclear explosions of hydrogen accreted by white dwarfs (WDs) from their low-mass main-sequence or red-giant binary companions. Chemical composition analysis of their ejecta shows that nova outbursts occur on both carbon-oxygen (CO) and more massive oxygen-neon (ONe) WDs, and that there is cross-boundary mixing between the accreted envelope and underlying WD. We demonstrate that the state-of-the-art stellar evolution code MESA and post-processing nucleosynthesis tools of NuGrid can successfully be used for modeling of CO and ONe nova outbursts and nucleosynthesis. The convective boundary mixing (CBM) in our 1D numerical simulations is implemented using a diffusion coefficient that is exponentially decreasing with a distance below the bottom of the convective envelope. We show that this prescription produces maximum temperature evolution profiles and nucleosynthesis yields in good agreement with those obtained using the commonly adopted 1D nova model in which the CBM is mimicked by assuming that the accreted envelope has been pre-mixed with WD's material. In a previous paper, we have found that 3He can be produced in situ in solar-composition envelopes accreted with slow rates (Ṁ < 10–10 M⊙/yr) by cold (TWD<107 K) CO WDs, and that convection is triggered by 3He burning before the nova outburst in this case. Here, we confirm this result for ONe novae. Additionally, we find that the interplay between the 3He production and destruction in the solar-composition envelope accreted with an intermediate rate, e.g. Ṁ = 10–10 M⊙/yr, by the 1.15 M⊙ ONe WD with a relatively high initial central temperature, e.g. TWD =15×106 K, leads to the formation of a thick radiative buffer zone that separates the bottom of the convective envelope from the WD surface
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
The Search for Galactic Short-period Variable Stars in the Omega White Survey
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The Search for Galactic Short-period Variable Stars in the OmegaWhite Survey
* Author:
Sally Ann Macfarlane
* Abstract:
In this Thesis, I present details and results from the first 4.5 years of OmegaWhite (OW) Survey operations. The OW Survey is a wide-field, high-cadence synoptic survey, aiming to cover 400 square degrees along the Galactic plane. The survey has been specifically designed to target short-period variable star systems that exhibit periodic variations on timescales of minutes to under an hour, such as ultracompact binary star systems or rare pulsating stars. This Thesis consists of an Introductory chapter, four chapters based on the OW Survey papers, and a Summary & Conclusion chapter. In Chapter 2, the observations, observing strategy and processing pipeline are detailed, and results from the initial 26 square degrees are analysed. This analysis is extended to the first 4 years of OW operations in Chapter 3, where a study is made into selected individual systems and the various populations they belong to. This includes the bluest variables, which are our most likely UCB candidates. In Chapter 4, the photometric and spectroscopic follow-up observations of a number of variables identified in the OW survey are presented, and the light curve parameters determined during the OW processing pipeline are verified. Chapter 5 looks at the discovery of a variable "warm" carbon-enriched DQ white dwarf in the OW survey (only the second known) and, lastly, the main results from the first 4.5 years of OW operations and the future of the survey is discussed in the Summary & Conclusion chapter. Proefschrift Dissertatio
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
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
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