1,721,160 research outputs found
Magnetohydrodynamics and deep mixing in evolved stars. I. two- and three-dimensional analytical models for the asymptotic giant branch
No full textThe advection of thermonuclear ashes by magnetized domains emerging near the H shell was suggested to explain asymptotic giant branch (AGB) star abundances. Here we verify this idea quantitatively through exact MHD models. Starting with a simple two-dimensional (2D) geometry and in an inertia frame, we study plasma equilibria avoiding the complications of numerical simulations. We show that below the convective envelope of an AGB star, variable magnetic fields induce a natural expansion, permitted by the almost ideal MHD conditions, in which the radial velocity grows as the second power of the radius. We then study the convective envelope, where the complexity of macroturbulence allows only for a schematic analytical treatment. Here the radial velocity depends on the square root of the radius. We then verify the robustness of our results with 3D calculations for the velocity, showing that for both studied regions the solution previously found can be seen as a planar section of a more complex behavior, in which the average radial velocity retains the same dependency on the radius found in 2D. As a final check, we compare our results to approximate descriptions of buoyant magnetic structures. For realistic boundary conditions, the envelope crossing times are sufficient to disperse in the huge convective zone any material transported, suggesting magnetic advection as a promising mechanism for deep mixing. The mixing velocities are smaller than for convection but larger than for diffusion and adequate for extra mixing in red giants. © 2014. The American Astronomical Society
On the production of heavy neutron-rich isotopes during He and C burning in massive stars
No full textThe main purpose of this lecture is a re-examination of the yields of
s-elements by massive short-lived stars. He-burning conditions are
reviewed and results are presented for the nucleosynthesis in this
phase. The authors briefly analyse the possibility of n-captures during
carbon burning and discuss the contribution of massive stars to the
enrichment of the interstellar medium in s-elements
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
The puzzle of silicon, titanium, and magnesium anomalies in meteoritic silicon carbide grains
No full textAn astrophysical interpretation of the silicon, titanium, and magnesium
isotope anomalies measured in the mainstream population of single
silicon carbide (SiC) grains extracted from carbonaceous meteorites is
presented. The condensation site of the grains is envisaged in the cool
atmospheres of carbon stars. The Si isotope anomalies show a general
linear correlation between Si-29/Si-28 aand Si-30/Si-28, whose slope
cannot be explained only by s-processing in the He-burning shell and
dilution with material of solar composition from the envelope. We
suggest a multiple star scenario in which the SiC grains form in stellar
envelopes with slightly nonsolar initial Si isotope composition and
metallicities from one-half solar to solar. The initial Si abundances
are inferred from considerations of galactic chemical evolution,
coupling spectroscopic observations of abundances in stars of different
metal content with current predictions of stellar nucleosynthesis. The
isotopes Si-29 and Si-30 are assumed to be entirely produced by
short-lived massive stars exploding as supernovae, which also contribute
approximately 70% of the solar Si-28 abundance, the remaining
approximately 30% coming from long-lived stars, evolving in binary
systems, and leading to supernovae of Type Ia. More detailed
calculations of the Si isotopes yields from stars of various mass and
initial metallicity are, however, required, and a better understanding
of how the nucleosynthetic ejecta by supernovae are well homogenized
with the interstellar matter. Even the Ti isotope anomalies in SiC
grains cannot be explained as only an s-process signature. The linear
correlation shown by Ti and Si anomalies indicates that a similar
approach can be used to interpret the Ti anomalies as a mixture of a
pure s-component and of a variable nonsolar isotopic composition
initially present in the envelope of carbon stars. The question of the
large abundance of extinct Al-26 in many SiC grains is also considered.
We find that the production of Al-26 in the H shell of thermally pulsing
AGB stars, although followed by substantial consumption by neutron
captures during He thermal pulses, can account for the high Al-26/Al-27
ratios. The spread of carbon anomalies is interpreted as a consequence
of an initial spread of C-12/C-13 as observed in M stars and of the
subsequent enrichment in C-12 of the envelope during thermal pulses.
Finally, the nitrogen isotope anomalies are discussed
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