1,721,123 research outputs found
The evolution of CNO isotopes: the impact of massive stellar rotators
Full text linkChemical abundances and abundance ratios measured in galaxies provide precious information about the mechanisms, modes, and time-scales of the assembly of cosmic structures. Yet, the nucleogenesis and chemical evolution of elements heavier than helium are dictated mostly by the physics of the stars and the shape of the stellar mass spectrum. In particular, estimates of CNO isotopic abundances in the hot, dusty media of high-redshift starburst galaxies offer a unique glimpse into the shape of the stellar initial mass function (IMF) in extreme environments that cannot be accessed with direct observations (star counts). Underlying uncertainties in stellar evolution and nucleosynthesis theory, however, may hurt our chances of getting a firm grasp of the IMF in these galaxies. In this work, we adopt new yields for massive stars, covering different initial rotational velocities. First, we implement the new yield set in a well-tested chemical evolution model for the Milky Way. The calibrated model is then adapted to the specific case of a prototype submillimetre galaxy (SMG). We show that, if the formation of fast-rotating stars is favoured in the turbulent medium of violently star-forming galaxies irrespective of metallicity, the IMF needs to be skewed towards high-mass stars in order to explain the CNO isotopic ratios observed in SMGs. If, instead, stellar rotation becomes negligible beyond a given metallicity threshold, as is the case for our own Galaxy, there is no need to invoke a top-heavy IMF in starbursts
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
Deep radio imaging of the UKIDSS Ultra Deep Survey field : the nature of the faint radio population, and the star-formation history of the Universe
Full text linkThe centrepiece of this thesis is a deep, new, high-resolution 1.4-GHz image
covering the United Kingdom Infrared (IR) Telescope IR Deep Sky Survey (UKIDSS)
Ultra Deep Survey (UDS) legacy field. Deep pseudo-continuum observations were
made using the Very Large Array, prior to its recent upgrade, in its A, B and DnC
configurations. The resulting mosaic has a full-width-at-half-maximum synthesised
beam width of ≈ 1.7 arcsec and a point-source sensitivity of ≈ 60μJy (6σ ) across
the central 0.6 deg2, while conserving flux from sources of extended emission. The
full image covers 1.3 deg2. I also present a catalogue containing over 1,000 radio
emitters, having chosen the 6-σ threshold by maximising the number of radio sources
with secure optical/near-IR counterparts.
Most of the sources in the catalogue (≈ 90 per cent) lie in the sub-mJy flux density
regime. Deep, complementary data covering a wide range of wavelengths was used to
explore this faint radio population, whose nature remains controversial. It was found
that 53 per cent of the sample comprise active galactic nuclei (AGN). AGN dominate
at & 0.2mJy and remain a significant population down to 0.1mJy; at lower fluxes –
the so-called μJy radio population – star-forming galaxies become dominant.
The radio sample presented here was also matched to Hubble Space Telescope
imaging of the UDS field (which is part of the Cosmic Assembly Near-IR Deep
Extragalactic Legacy Survey – CANDELS) to classify the faint radio population
morphologically. These classifications were done using the Gini–M20 method. It was
found that a low fraction of AGN and SFGs are undergoing interactions and mergers,
33 ± 9 and 13 ± 7 per cent, respectively. The merger fraction does not appear to have
evolved significantly since z ∼ 3. This suggests that mergers have played a relatively
minor role in the assembly of galaxies and super-massive black holes – certainly less
significant than previously thought.
Finally, I present a study of cosmic star-formation activity as a function of stellar mass and redshift, exploiting panchromatic stacking. Mid-IR–through–radio images,
including new data from Herschel, are stacked at the positions of a K-selected (i.e.
an approximately mass-selected) sample in the UDS field. Specific star-formation
rates (SSFR, i.e. star-formation rate per stellar mass, or the rate at which a galaxy is
converting its gas into stars) were derived from UDS radio luminosities measured here
and stellar masses from the literature. The SSFR was found to be poorly correlated
with stellar mass; it decreases with decreasing redshift; at a given mass, SSFR rises
with redshift. These results indicate that at early epochs, galaxies were forming stars
more efficiently and at a higher rate
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