1,720,961 research outputs found
Ultraviolet Radiative Feedback on High-Redshift Protogalaxies
No full textWe use three-dimensional hydrodynamic simulations to investigate the effects of a transient photoionizing UV flux on the collapse and cooling of pregalactic clouds. These clouds have masses in the range 105-10 7 M⊙, form at high redshifts (z ≳18), and are assumed to lie within the short-lived cosmological H II regions around the first generation of stars. In addition, we study the combined effects of this transient UV flux and a persistent Lyman-Werner (LW) background (at photon energies below 13.6 eV) from distant sources. In the absence of a LW background, we find that a critical specific intensity of JUV ∼ 0.1 × 10-21 ergs s-1 cm-2 Hz-1 sr -1 demarcates a transition from net negative to positive feedback for the halo population. A weaker UV flux stimulates subsequent star formation inside the fossil H II regions, by enhancing the H2 molecule abundance. A stronger UV flux significantly delays star formation by reducing the gas density, and increasing the cooling time, at the centers of collapsing halos. At a fixed JUV the sign of the feedback also depends strongly on the density of the gas at the time of UV illumination. Regardless of whether the feedback is positive or negative, we find that once the UV flux is turned off, its impact starts to diminish after ∼30% of the Hubble time. In the more realistic case when a LW background is present, with JLW ≳ 0.01 × 10-21 ergs s-1 cm-2 Hz -1 sr-1, strong suppression persists down to the lowest redshift (z = 18) in our simulations. Finally, we find evidence that heating and photoevaporation by the transient UV flux render the ∼ 106 M⊙ halos inside fossil H II regions more vulnerable to subsequent H2 photodissociation by a LW background. © 2006. The American Astronomical Society. All rights reserved
Relic HII regions and radiative feedback at high redshifts
Full text linkUltraviolet (UV) radiation from early astrophysical sources could have a large impact on subsequent star formation in nearby protogalaxies, and in general on the progress of cosmological reionization. Theoretical arguments based on the absence of metals in the early Universe suggest that the first stars were likely massive, bright, yet short-lived, with lifetimes of a few million years. Here we study the radiative feedback arising from such stars using hydrodynamical simulations with transient UV backgrounds (UVBs) and persistent Lyman-Werner backgrounds (LWBs) of varying intensity. We extend our prior work in Mesinger et al., by studying a more typical region whose protogalaxies form at lower redshifts, z ∼ 13-20, in the epoch likely preceding the bulk of reionization. We confirm our previous results that feedback in the relic H ii regions resulting from such transient radiation is itself transient. Feedback effects dwindle away after ∼30 per cent of the Hubble time, and the same critical specific intensity of JUV ∼ 0.1 × 10-21 erg s-1 cm-2 Hz-1 sr-1 separates positive and negative feedback regimes. This suggests that overall feedback is fairly insensitive to the large-scale environment, overdensity and redshift-dependent halo parameters, and can accurately be modelled in this regime with just the intensity of the impinging UVB. Additionally, we discover a second episode of eventual positive feedback in haloes which have not yet collapsed when their progenitor regions were exposed to the transient UVB. When exposed to the transient UVB, this gas suffers relatively little density depletion but a significant enhancement of the molecular hydrogen abundance, thus resulting in net positive feedback. This eventual positive feedback appears in all runs, regardless of the strength of the UVB. However, this feedback regime is very sensitive to the presence of Lyman-Werner radiation, and notable effects disappear under fairly modest background intensities of JLW ≳ 10-3 × 10 -21 erg s-1 cm-2 Hz-1 sr -1, assuming the region is optically thin for LW photons. Nevertheless, when exposed to the same LWB, haloes inside relic H ii regions always have a higher H2 abundance and shorter cooling times than haloes outside relic H ii regions, allowing gas to cool faster once it finally begins to collapse on to the halo. We conclude that UV radiative feedback in relic H ii regions, although a complicated process, seems unlikely to have a major impact on the progress of cosmological reionization, provided that present estimates of the lifetime and luminosity of a Population III star are accurate. More likely is that the build-up of the LWB ultimately governs the feedback strength until a persistent UV background can be established. © 2009 RAS
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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