1,720,955 research outputs found
The influence of the monomer shape in the first stage of dust growth in the protoplanetary disk
Full text linkContext. Laboratory experiments have shown that the initial stage of the dust growth
in the protoplanetary disk starts with low-velocity hit-and-stick collisions
between sub-micron grains, before the relative impact energy becomes large
enough to cause the morphological restructuring of the forming aggregates.
The results of these collisions are loose aggregates characterized by
fractal dimensions ≤2.
Numerical studies generally model this early stage with colliding clusters made of
monodisperse spherical monomers.
Aims. In this paper we aimed to investigate how a more complex representation
of the monodisperse monomers structure influences the morphology of
the final aggregate particles in terms of fractal dimension, porosity, cross-section, and
friction time. This study had also the purpose of testing
the validity of the current fractal models in representing irregular particles.
Methods. We used three kinds of hit-and-stick aggregation methods: two particle-cluster
aggregations producing compact
and extremely loose aggregates, respectively, and simplified cluster-cluster
aggregation as intermediate class in terms of fluffyness and porosity.
In our measures, we used two kinds of monomer shapes: spheres
and elongated prolate ellipsoids with axis ratio 3:1 resembling the ones used to model
interstellar dust grains.
Results. We found that the monomer shape has little influence (≲2%) on the fractal dimension
of the final aggregate, once large particles are taken into account,
independently on the morphological class. Elongated monomers produce
structures with larger fractal dimension in the limit of small particles.
For loose objects the influence on the porosity is even smaller, whereas
elongated monomers give particles with ~5% higher porosity in the case of
large compact fractals.
Regardless the ellipsoidal monomer used has, by construction, a cross-section ~26% larger
than the one of the volume-equivalent sphere, the difference in cross-section per unit mass is decreasing
when aggregates are considered. Clusters made by elliptical monomers result much coupled
to the surrounding gas than objects made by spherical monomers belonging to the
same morphological class. Differences in the
friction time are estimated to be below 15%.
We conclude that the monomer shape influence on the morphology of the final
particles during the hit-and-stick aggregation step is much lower than the
differences due to considering different morphological classes in modeling and the uncertainties produced
in a realistic accretion scenario, where a wide distribution of morphologies is expected.
Present models of irregular particles as fractals made by spherical monomers can therefore
also represent well more complex shapes, before the restructuring phase is taken into account
Crossing barriers in planetesimal formation: The growth of mm-dust aggregates with large constituent grains
No full textCollisions of mm-size dust aggregates play a crucial role in the early phases of planet formation. It is for example currently unclear whether there is a bouncing barrier where millimeter aggregates no longer grow by sticking. We developed a laboratory setup that allowed us to observe collisions of dust aggregates levitating at mbar pressures and elevated temperatures of 800 K. We report on collisions between basalt dust aggregates of from 0.3 to 5 mm in size at velocities between 0.1 and 15 cm/s. Individual grains are smaller than 25 μm in size. We find that for all impact energies in the studied range sticking occurs at a probability of 32.1 ± 2.5% on average. In general, the sticking probability decreases with increasing impact parameter. The sticking probability increases with energy density (impact energy per contact area). We also observe collisions of aggregates that were formed by a previous sticking of two larger aggregates. Partners of these aggregates can be detached by a second collision with a probability of on average 19.8 ± 4.0%. The measured accretion efficiencies are remarkably high compared to other experimental results. We attribute this to the relatively large dust grains used in our experiments, which make aggregates more susceptible to restructuring and energy dissipation. Collisional hardening by compaction might not occur as the aggregates are already very compact with only 54 ± 1% porosity. The disassembly of previously grown aggregates in collisions might stall further aggregate growth. However, owing to the levitation technique and the limited data statistics, no conclusive statement about this aspect can yet be given. We find that the detachment efficiency decreases with increasing velocities and accretion dominates in the higher velocity range. For high accretion efficiencies, our experiments suggest that continued growth in the mm-range with larger constituent grains would be a viable way to produce larger aggregates, which might in turn form the seeds to proceed to growing planetesimals
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
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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