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Nucleosynthesis in accretion disks around black holes
Get PDFNucleosynthesis is the mechanism which produces new elements in nuclear reactions. Nuclear reaction rates are highly temperature dependant, and nuclear reactions take place in very hot environments. Current theories predict that the light elements such as hydrogen and helium were produced during the Big Bang. On the other hand, the core of stars produce heavier elements through nuclear fusion. These new elements are then released to the interstellar medium through stellar winds, enriching the gas which will form later generations of stars. Black hole accretion disks also can contain material of high temperatures generated by high accretion rates, allowing nuclear fusion to take place. Nucleosynthesis products can be expelled in winds driven by a super-Eddington accretion, and enrich the interstellar medium. I wrote a computer program 1 integrating a nuclear burning network in a black hole accretion disk for various ranges of black hole mass and accretion rates. I found that the accretion rate needed for nucleosynthesis to take place increases with the black hole mass. The highest temperatures are located in the inner disk, and the black hole event horizon increases linearly with its mass, preventing the disk from attaining high temperatures. For a stellar mass black hole, highly super-Eddington accretion rates allow nuclear burning and powerful winds. Such accretion rates can be supplied by unstable mass transfer during the disruption of a white dwarf. The alpha chain reactions, involving captures of helium nuclei, structure the disk composition radially, with isotope abundances dominating at specific radii. Assuming a given fraction of the disk material is expelled in winds due to Super-Eddington accretion, and knowing the rate at which such events happen in the Galaxy allowed me to compute upper limits of the contribution of accretion disks to the interstellar medium enrichment. Comparing this production to combined stellar yields from stars, I find that black hole–white dwarf accretion disks produce at most 10 −4 times the amount of the same elements that stars produce. This result shows that such a small contribution can be neglected to the overall content of the Galaxy. But the nucleosynthesis involved in general may perhaps play a role in observing these systems, for example a light curve emitted by radioactive elements produced in these short-lived black hole accretion disks.La nucléosynthèse dans les disques d'accrétion autour de trous noirs. "Nous sommes faits de poussières d'étoiles", comme le décrit Hubert Reeves, reprenant, dans son magnifique ouvrage sur la synthèse des éléments dans l'Univers, l'idée de Carl Sagan. Cela signifie que les éléments qui nous compensent ajourd'hui, carbone, oxygène, calcium, etc., ont été fabriqués à partir de l'hydrogène et de l'hélium hérités du Big Bang, dans les denses coeurs brûlants des étoiles. Mais est-ce là la fin de l'histoire ? La synthèse des éléments nécessite des conditions extrêmes de températures et densités. Ces conditions sont atteignables dans les disques formés par le le gas qui se fait avaler par les trous noirs, objets si denses que même la lumière ne peut s'en échapper. Du gas orbitant un trou noir chauffe par friction, permettant à des réactions nucléaires d'avoir lieu. Si une partie des éléments produits peu s'échapper, à l'aide de vents par exemple, alors peut-être que les disques d'accrétion autour des trous noirs jouent un rôle dans la synthèse des éléments nous composant. Seulement, les températures ne sont suffisantes que dans de rares conditions qui mettent en jeu la destruction d'objets compacts, tels que les naines blanches et les étoiles à neutrons, qui sont respectivement les restes de vies d'étoiles légères, et massives. Le travail décrit dans ce mémoire se concentre sur les réactions nucléaires dans les disques d'accrétion provenant de ma destructions d'une naine blanche par un trou noir. Comme ces événements sont très rares, leur contribution à la synthèse des éléments qui nous cmposent aujourd'hui est négligeable comparée à tout ce que les nombreuses étoiles de notre galaxie produisent. En revanche, le cas des étoiles à neutrons n'a pas dit son dernier mot et est toujours un sujet de recherche actuel. Quoi qu'il en soit, les cas les plus prometteurs de synthèse des éléments dans les disques d'accrétion autour des trous noirs mettent en jeu des étoiles, donc il est bien vrai, nous sommes des poussières d'étoiles.Nukleosynthes i ackretionsskrivor kring svarta hål. Hur bildas atomerna som bygger upp oss människor - till exempel atomer av kol, syre eller kalcium? Den idag gâllande teorin är att lättare atomer såsom väte, helium och litium skapades vid "Big Bang", medan en supernova från en döende stjärna producerade och frigjorde bland annat kol, syre, kalcium och järn. Det som stjärnnor och Big Bang har gemensamt i sin förmåga att fungera som kärnreaktorer är att de utmärks av väldigt hög värme och en hög densitet. Ett svart hål som muycket snabbt fångar gaser från en omgivande ansamlingsskiva, som vi kallar ackretionsskiva, skapar också mycket hög värme och hög densitet. Detta tillstånd har också alla fôrutsättningar att fungera som en kärnreactor och att skapa nya tyngre grundåmnen. I mitt examenarsbete har jag undersökt kärnreaktioner i sådana ackretionsskivor kring svarta hål. Temperaturen och densiteten blir tillråckligt hög endast om infångningen av gaser sker mycket hastigt. Detta kan inträffa om ansamlingsskivan kommer från en vit dvärg, vilken ökar i storlek när den kastat ut massa. Jag har skrivit ett program som beräknar hur mycket av nua tyngre atomer som skapas och frigörs. Jämfört med stjärnor är produktionen av nya grundämnen betydligt mindre förekommande. Eftersom en vit dvärg utgör slutprodukten från en död stjärna, kan vi alltid säga att vi människor har erhållit våra byggstenar från stjärnstoft
Forward Modelling the Secular Evolution of the Milky Way Disk
Get PDFWe know precisely the position of the Sun in our Galaxy. Yet, like for most stars,
we cannot tell where it was born. Stars undergo dynamical memory loss: their
orbits evolve, because the Milky Way, like many galaxies, has non-axisymmetric
structures (e.g. bar, spirals) that shuffle stellar orbits. My thesis quantifies the
strength of that process to answer: How (much) do stars change orbit? Can we still
infer their birth places, to constrain the formation of the Milky Way disk? I have
combined data from the large stellar surveys APOGEE and Gaia, and developed
a method to extract the information they contain on the Galactic disk evolution. I
forward-modelled the formation of the stellar disk, the stars’ elemental abundances
and their subsquent orbital diffusion, which then informs us about their birth radii
through ’weak chemical tagging’. I have found that stars can change orbits by large
amounts, and most of this evolution is cold (the orbits stay near- circular). Secular
evolution determines how the Milky Way disk is structued. If the Milky Way is
typical this explain what drives disk galaxies in general to their typical exponential
disk density profiles
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
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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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