1,720,969 research outputs found
Pulsating star research from Antarctica
Get PDFThis invited talk discusses the pulsating star research from the heart of Antarctica and the scientific polar challenges in the extreme environment of Antarctica, and how the new polar technology could cope with unresolved stellar pulsation enigmas and evolutionary properties challenges towards an understanding of the mysteries of the Universe.
PAIX, the first robotic photometer Antarctica program, has been successfully launched during the polar night 2007. This ongoing program gives a new insight to cope with unresolved stellar enigmas and stellar oscillation challenges with a great opportunity to benefit from an access to the best astronomical site on Earth, Dome C. PAIX achieves astrophysical measurement time-series of stellar fields, challenging photometry from space. A continuous and an uninterrupted series of multi-color photometric observations has been collected each polar night – 150 days – without regular interruption, Earth’s rotation effect. PAIX shows the first light curve from Antarctica and first step for the astronomy in Antarctica giving new insights in remote polar observing runs and robotic instruments towards a new technology
Detection of Gravity Modes in RR Lyrae Stars
No full textInternational audienceAbstract We report the detection of gravity modes in RR Lyrae stars. Thanks to Photometer AntarctIca eXtinction (PAIX), the first Antarctic polar photometer. Unprecedented and uninterrupted UBVRI time-series photometric ground-based data are collected during 150 days from the highest plateau of Antarctica. PAIX light-curve analyses reveal an even richer power spectrum with mixed modes in RR Lyrae stars. The nonlinear nature of several dominant peaks, showing lower and higher frequencies, occurs around the dominant fundamental radial pressure mode. These lower frequencies and harmonics linearly interact with the dominant fundamental radial pressure mode and its second and third overtone pressure modes, as well. Half-integer frequencies are also detected, likewise side-peak structures, demonstrating that HH Puppis is a bona-fide Blazhko star. Fourier correlations are used to derive underlying physical characteristics for HH Puppis. The most striking finding is the direct detection of gravity waves. We interpret the excitation mechanism of gravity waves in RR Lyrae stars by the penetrative convection-driving mechanism. We demonstrate that RR Lyrae stars’ pulsation is excited by several distinct mechanisms, and hence RR Lyrae stars are simultaneously g- mode and p- mode pulsators. Our discoveries make RR Lyrae stars very challenging stellar objects, and provide their potential to undergo at the same time g and p modes toward an advancement of the theory of stellar evolution and a better understanding of the universe
A study of RR Lyrae stars pulsation and Blazhko effect
No full textCette thèse propose une nouvelle théorie de l’effet Blazhko. Elle apporte également une justification théorique à certaines observations encore inexpliquées : la décroissance des harmoniques de la courbe de lumière ; l’asymétrie des enveloppes et des lobes ainsi que la variation des temps de montée et de descente dans le cas de l’effet Blazhko ; la synchronisation des couches et l’existence des trois types d’étoiles RR Lyrae : RRab, RRc et RRd. La première partie présente une étude extensive d’une étoile RR Lyrae avec effet Blazhko : S Arae. Nous débutons par une définition rigoureuse de l’analyse harmonique des courbes de lumière. Cette démarche, encore jamais entreprise en astéroséismologie, permet une meilleure interprétation des habituels résultats des études photométriques. Nous poursuivons avec une présentation du programme d’analyse fréquentielle que nous avons développé : PDM13. Nous établissons ensuite le spectre fréquentiel de la courbe de lumière de S Arae et, à la présentation des résultats de cette étude, nous apportons une démonstration mathématique à deux observations communément effectuées : la décroissance des harmoniques et l’asymétrie de modulation. Dans la deuxième partie, après un rappel des mécanismes d’oscillation, nous présentons une nouvelle modélisation discrète, et non linéaire, des équations de pulsations. Celle-ci nous permettra d’expliquer les phénomènes de synchronisation, mais surtout, conduira à une nouvelle théorie du phénomène Blazhko fondée sur l’existence de solitons, que nous étayerons par des premiers résultats, obtenus grâce à l’utilisation d’un nouvel outil : la transformée en ondelettesIn this thesis we develop a new theory of the Blazhko effect. We also provide a theoretical justification to the following commonly observed facts: the light curve harmonics decrease; the asymetry of envelopes and sidelobes ; the synchronization of layers and the mode selection. The first part of this thesis is dedicated to the extensive study of a RR Lyrae star presenting the Blazhko effect: S Arae. Firstly, a rigorous definition of harmonic analysis applied to light curves is given. This work, which has never been undertaken in the asteroseismology field, up to now, allows a better interpretation of the usual results of photometric studies. We carry on with the presentation of a software that we have developed, dedicated to frequency analysis: PDM13. After that, we perform the usual analysis of the frequency spectrum of the light curve, which we complete we two rigorous demonstrations of commonly observed facts: the harmonics decrease and the asymmetry induced by modulation. We complete it with a study of the parameters which vary during a Blazhko cycle, on which we will capitalize to understand this modulation effect. The second part begins with a reminder of the basic perturbed and linearized equations of stellar pulsation together with the oscillation mechanisms. We pursue this presentation with a non linear and discrete formalism that we have developed for these equations. This formalism will allow us to underline the importance of synchronization in those stars, but, most of all, it will lead us to a new theory of the Blazhko effect, based on solitons, which will be supported by a new results obtained with the wavelet transform metho
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
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