1,721,041 research outputs found
Early neutron star evolution in high-mass X-ray binaries
No full textThe application of standard accretion theory to observations of X-ray binaries provides valuable insights into neutron star (NS) properties, such as their spin period and magnetic field. However, most studies concentrate on relatively old systems, where the NS is in its late propeller, accretor, or nearly spin equilibrium phase. Here, we use an analytic model from standard accretion theory to illustrate the evolution of high-mass X-ray binaries (HMXBs) early in their life. We show that a young NS is unlikely to be an accretor because of the long duration of ejector and propeller phases. We apply the model to the recently discovered ∼4000 yr old HMXB XMMU J051342.6−672412 and find that the system's NS, with a tentative spin period of 4.4 s, cannot be in the accretor phase and has a magnetic field B > a few × 10
13 G, which is comparable to the magnetic field of many older HMXBs and is much higher than the spin equilibrium inferred value of a few × 10
11 G. The observed X-ray luminosity could be the result of thermal emission from a young cooling magnetic NS or a small amount of accretion that can occur in the propeller phase.
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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
The impact of tides and mass transfer on the evolution of metal-poor massive binary stars
Get PDFStars more massive than 10Msun , although few in number compared to objects like our sun, play a very large role in the evolution of the universe. Through strong winds and supernovae they enrich the interstellar medium with heavy elements and provide mechanical feedback on galactic scales. Their large flux of ionizing photons may dominate the reionization of the universe. A thorough understanding of massive stellar evolution is then paramount to our understanding of the universe. In the last years, observations have established that binary interaction is a fundamental part of the evolution of massive stars, most of which will undergo mass exchange with a nearby companion. In addition, the recent detection of gravitational waves from the merger of two ∼ 30M black holes opens up exciting new possibilities to improve our understanding of the evolution of massive stars. To this purpose, we have extended the open-source stellar evolution code MESA to include the necessary physics to model binary systems. This new version of MESA allows us to explore in detail the parameter space of massive and very massive binary stars. The first problem we consider involves the evolution of very massive stars orbiting each other with periods of a few days. Owing to tidal locking, both components are fast rotators, which induces large scale mixing throughout their radiative envelopes. The stars then evolve chemically homogeneously, burning all their available nuclear fuel instead of developing a core envelope structure. This modifies their evolution dramatically: Instead of expanding significantly, these stars contract inside their Roche lobes to eventually form a pair of close black holes of similar mass that can merge in less than a Hubble time. For the design sensitivity of advanced LIGO, we expect ∼ 20 − 900 detections per year from this channel, with the large uncertainty arising from uncertainties in the chemical evolution of the universe. Owing to a range of very massive stars exploding as pair instability supernovae rather than forming a black hole, we expect to detect a gap in the distribution of black hole masses between 60M and 130M . Considering similar systems for which the secondary component is significantly less massive, we show that only the more massive star evolves homogeneously to become a black hole. On a longer timescale, the secondary expands and initiates mass transfer to the compact object, which makes it active as an X-ray source. Owing to their large black hole masses (in excess of 20M ), these X-ray binaries would have luminosities characteristic of those of observed ultra-luminous X-ray sources, which are difficult to explain through standard stellar evolution. The occurrence of pair-instability supernovae, just as was the case for binary black holes, produces a gap in black hole masses, which could be observable as a gap in the luminosity distribution of ultra-luminous sources. Our simulations show a strong metallicity dependence, and future X-ray surveys such as eROSITA could potentially test our claims. Finally, we modelled a large set of binary and single models of massive stars to study the population of objects in the LMC. Under extreme assumptions that minimize the contribution of binaries to our sample, we show that the large observed binary fraction inescapably implies a degeneracy between the predictions of rotational mixing in single stars and mass transfer in binaries. Binaries with low transfer efficiencies explain observed rapidly rotating stars with low nitrogen enrichment at their surfaces, but still undergo mixing and a slight enrichment in nitrogen. Accurate measurements of the abundances of rapid rotators could then provide valuable information on the efficiency of rotational mixing. Throughout this thesis we have extensively demonstrated the use of detailed stellar evolution models to cover the wide parameter space of single and binary stellar evolution in a statistically significant way. This new capability is certain to enable many new venues of research
Binary star population synthesis : Progenitors of gravitational wave driven mergers
Get PDFThe vast majority of all massive stars are born in binaries. The binarity of a star may influence its life drastically via a large variety of interactions with its companion star. Several astrophysical events, like the recently detected gravitational wave mergers, have a binary origin. To investigate the possible paths of binary evolution, a new stellar grid-based binary populations synthesis code ComBinE is developed. During binary evolution, many uncertain phases, including one or more mass-transfer phases, a common-envelope phase and kicks during a supernova explosion, have to be considered. The least understood among them is the common-envelope phase. A detailed analysis of the consequences of the conversion of available energy reservoirs into the ejection of the gas of the common envelope is performed at two metallicities, that of the Milky Way (Z = 0.0088) and that of the dwarf galaxy IZwicky18 (Z = 0.0002). The most crucial aspect is the bifurcation point which separates the remaining core from the lost material. ComBinE is applied to investigate a variety of parameters of the binary evolution phases and their effects on observable stages during the evolution leading to a final gravitational wave driven merger. The simulations performed with ComBinE are able to reproduce the observed Galactic double neutron star population with respect to their orbital parameters and, to some extend, their measured mass distributions. In addition, the simulations can reproduce the progenitor systems of all published merger events of double black hole mergers and the double neutron star merger, GW170817, for appropriate metallicities. Merger-rate densities in the local Universe are obtained from the simulations. Statistical uncertainties are discusses as well as predictions made for future observations and LIGO-Virgo detections. Finally, using ComBinE, the nature of the unseen companion star in the radio pulsar binary PSR J1755−2550 is probed to narrow the search window for the companion
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
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