1,721,182 research outputs found
Observational constraints on the progenitors of Type Ia Supernovae
No full textThis thesis investigates observational signatures and provides constraints on the progenitors of Type Ia Supernovae (SNe Ia), one of the most powerful cosmological probes, with a profound impact on understanding the evolution of the universe. By exploiting rich observational datasets, from powerful transient surveys, and extensively analysing them, we provide an insight into the long-standing progenitor problem.At first, we investigate the late-time evolution of the SN Ia light curve. Using the Palomar Transient Factory (PTF) observations of SN2011fe, added with external literature data, we achieve an unprecedented photometric coverage of the late-time light curve of a SN Ia, spanning from 200 to 1600 days after maximum, with remarkable temporal density. A combination of photometry and spectroscopy at these phases allows us to construct a pseudo-bolometric light curve and physical models that describe the behaviour of it are considered. Our main results are the explanation of the bolometric light curve by radioactive inputs of the 56Ni and 57Ni decay chains, with the estimated amount of 57Ni to be relatively large, indicating a high central density explosion environment. Moreover, our pseudo-bolometric light curve is not consistent with models that have complete trapping of the produced charged leptons, with models that allow for positron/electron escape or an infrared catastrophe (IRC) adequately describing the evolution. Finally, we studied a sample of 49 PTF SN Ia with late-time photometric coverage, finding no significant deviation on their light curve evolution compared to SN2011fe, and provide an upper limit on the hydrogen rich material at the site of the explosion to be ≤0.87 M".For the last part, we turn our focus to the spectroscopic class of 91T-like overluminous SNe Ia, by studying two members of this class: SN2014eg and SN2016hvl, for which we present their rich data set and provide physical parameters that describe them. Our results show that the highly reddened SN2014eg is a slightly less luminous 91T-like, with a 56Ni mass of 0.573 M", with indications of circumstellar material (CSM) at the site of the explosion, while SN2016hvl is a 91T-like with 0.767 M" mass of 56Ni. Finally, we study the host galaxies of the PTF 91T-like SNe Ia, and we find that they preferentially explode in active galaxies, with moderate to high star formation rates and a range in their stellar masses
K2 observations of SN 2018oh reveal a two-component rising light curve for a Type Ia supernova
No full textWe present an exquisite 30 minute cadence Kepler (K2) light curve of the Type Ia supernova (SN Ia) 2018oh (ASASSN-18bt), starting weeks before explosion, covering the moment of explosion and the subsequent rise, and continuing past peak brightness. These data are supplemented by multi-color Panoramic Survey Telescope (Pan-STARRS1) and Rapid Response System 1 and Cerro Tololo Inter-American Observatory 4 m Dark Energy Camera (CTIO 4-m DECam) observations obtained within hours of explosion. The K2 light curve has an unusual two-component shape, where the flux rises with a steep linear gradient for the first few days, followed by a quadratic rise as seen for typical supernovae (SNe) Ia. This "flux excess" relative to canonical SN Ia behavior is confirmed in our i-band light curve, and furthermore, SN 2018oh is especially blue during the early epochs. The flux excess peaks 2.14 ± 0.04 days after explosion, has a FWHM of 3.12 ± 0.04 days, a blackbody temperature of K, a peak luminosity of , and a total integrated energy of . We compare SN 2018oh to several models that may provide additional heating at early times, including collision with a companion and a shallow concentration of radioactive nickel. While all of these models generally reproduce the early K2 light curve shape, we slightly favor a companion interaction, at a distance of ~ based on our early color measurements, although the exact distance depends on the uncertain viewing angle. Additional confirmation of a companion interaction in future modeling and observations of SN 2018oh would provide strong support for a single-degenerate progenitor system
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Full text link“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
Full text linkWe 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
Full text linkWe 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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