1,721,036 research outputs found
Planets in binaries: Formation and dynamical evolution
Full text linkBinary systems are very common among field stars, yet the vast majority of known exoplanets have been detected around single stars. While this relatively small number of planets in binaries is probably partly due to strong observational biases, there is, however, statistical evidence that planets are indeed less frequent in binaries with separations smaller than 100 au, strongly suggesting that the presence of a close-in companion star has an adverse effect on planet formation. It is indeed possible for the gravitational pull of the second star to affect all the different stages of planet formation, from proto-planetary disk formation to dust accumulation into planetesimals, to the accretion of these planetesimals into large planetary embryos and, eventually, the final growth of these embryos into planets. For the crucial planetesimal-accretion phase, the complex coupling between dynamical perturbations from the binary and friction due to gas in the proto-planetary disk suggests that planetesimal accretion might be hampered due to increased, accretion-hostile impact velocities. Likewise, the interplay between the binary's secular perturbations and mean motion resonances lead to unstable regions, where not only planet formation is inhibited, but where a massive body would be ejected from the system on a hyperbolic orbit. The amplitude of these two main effects is different for S-and P-type planets, so that a comparison between the two populations might outline the influence of the companion star on the planet formation process. Unfortunately, at present the two populations (circumstellar or circumbinary) are not known equally well and different biases and uncertainties prevent a quantitative comparison. We also highlight the long-term dynamical evolution of both S and P-type systems and focus on how these different evolutions influence the final architecture of planetary systems in binaries
Dynamics and Planet Formation in/Around Binaries
No full textThe extent to which planetesimal accretion is affected by the perturbing presence of a companion star is an important issue in the formation of planets in and around binary systems. In this chapter, we review this issue by concentrating on one crucial parameter: the distribution of encounter velocities within the planetesimal swarm. The evolution of this parameter is numerically explored accounting for the secular perturbations of the binary and the friction due to the very likely presence of gas in the disk. Maps of the average encounter velocity ⟨Δv⟩ between different size planetesimals are presented for a total of 120 stellar dynamical configurations obtained by different combinations of the binary semimajor axis a b and eccentricity e b . According to the different values of ⟨Δv⟩, 3 different planetesimal accumulation modes are identified: 1) in regions where ⟨Δv⟩ is comparable to that derived for planetesimal swarms around single-stars, "standard" accretion is likely, eventually via runaway growth, 2) in regions where ⟨Δv⟩ is larger than v ero , the threshold velocity above which all impacts are eroding, no accretion is possible and planet growth is stopped, 3) in between these two extremes, there is a large fraction of binary configurations where the increase in ⟨Δv⟩ is still below the erosion threshold. Planetesimal accumulation can still occur but it possibly proceeds at a slower rate than in the single-star case, following the so-called type II runaway growth mode
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
Planet formation: is it good or bad to have a stellar companion?
No full textPlanet formation in binary star systems is a complex issue due to thegravitational perturbations of the companion star. One of the crucialsteps of the core-accretion model is planetesimal accretion into large protoplanets which finally coalesce into planets.In a planetesimal swarm surrounding theprimary star, the average mutual impact velocitydetermines if larger bodies form or if thepopulation is grinded down to dust, halting the planetformation process. This velocity is stronglyinfluenced by the companion gravitational pull and by gas drag.The combined effect of these two forces may act in favour of or against planet formation, setting a lower or equal probability ofthe existence of extrasolar planets around single or binary stars.
Planetesimal accretion in binaries has been studied so far withtwo different approaches. N-body codes based on the assumptionthat the disk is axisymmetric are very cost-effective sincethey allow the study of the mutual relative velocity with limitedCPU usage. A large amount of planetesimal trajectories can becomputed making it possible to outline the regions around the star whereplanet formation is possible. The main limitation of the N-body codes is the axisymmetric assumption.The companion perturbations affect not only the planetesimal orbits,but also the gaseous disk, by forcing spiral density waves. In addition, the overallshape of the disk changes from circular to elliptic.
Hybrid codes have been recently developed which solvethe equations for the diskwith a hydrodynamical grid codeand use the computed gas densityand velocity vector to calculate an accurate value of the gas dragforce on the planetesimals. These codes are more complex andmay compute the trajectories of only a limited number of planetesimals
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
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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