1,720,965 research outputs found
Nucleosynthesis of Light-Element Isotopes in Evolved Stars Experiencing Extended Mixing
No full textWe present computations of nucleosynthesis in red giants and Asymptotic Giant Branch (AGB) stars of Population I experiencing extended mixing. The assumed physical cause for mass transport is the buoyancy of magnetized structures, according to recent suggestions. The peculiar property of such a mechanism is to allow for both fast and slow mixing phenomena, as required for reproducing the spread in Li abundances displayed by red giants and as discussed in an accompanying paper. We explore here the effects of this kind of mass transport on CNO and intermediate-mass nuclei and compare the results with the available evidence from evolved red giants and from the isotopic composition of presolar grains of AGB origin. It is found that a good general accord exists between predictions and measurements; in this framework we also show which type of observational data best constrains the various parameters. We conclude that magnetic buoyancy, allowing for mixing at rather different speeds, can be an interesting scenario to explore for explaining together the abundances of CNO nuclei and of Li
A NEW SOLAR FLUORINE ABUNDANCE AND A FLUORINE DETERMINATION IN THE TWO OPEN CLUSTERS M67 AND NGC 6404
No full textA Revision of the solar fluorine abundanc
s-Processing in AGB Stars Revisited. I. Does the Main Component Constrain the Neutron Sourve in the 13C-pocket
No full textNeutrons for -processing at are mainly provided by \ctanb in AGB stars, requiring some proton penetration below the envelope so far assumed to be of small mass ( \ms). However, models with rotation suggested that there excessive N would hamper -processing. On the other hand, -element abundances in Galaxies require \ct-rich layers more extended in mass. We present new calculations for clarifying the above issues, aiming at understanding if the solar composition can help in fixing the extension of the \ctb ``pocket''. We show: i) that mixing ``from bottom to top'' (like in magnetic buoyancy or other forced mechanisms) might in principle form a \ctb reservoir much larger than assumed so far; ii) that stellar models at a suitable metallicity, using a similarly extended pocket would reproduce the main -component as accurately as before; iii) that with the extended pocket the previously envisaged contributions from an unknown nucleosynthesis process () would no longer be required. The new scheme also fulfils the requirements of C-star luminosities. Consisting of a few large neutron exposures, it would imply a large production of nuclei below ; in particular, Sr would be fully synthesized by AGB stars, while Sr, Y and Zr would be contributed more efficiently by the new model. We suggest some tests, which would probably say a final word on the real extension of the \ctb pocket
S-PROCESSING in AGB STARS REVISITED. II. ENHANCED 13C PRODUCTION THROUGH MHD-INDUCED MIXING
No full textSlow neutron captures are responsible for the production of about 50% of elements heavier than iron, mainly occurring during the asymptotic giant branch phase of low-mass stars (1 lsim; M/Mo ≲3), where the main neutron source is the 13C(α, n)16O reaction. This last reaction is activated from locally produced 13C, formed by partial mixing of hydrogen into the He-rich layers. We present here the first attempt to describe a physical mechanism for the formation of the 13C reservoir, studying the mass circulation induced by magnetic buoyancy without adding new free parameters to those already involved in stellar modeling. Our approach represents the application to the stellar layers relevant for s-processing of recent exact analytical 2D and 3D models for magneto-hydrodynamic processes at the base of convective envelopes in evolved stars in order to promote downflows of envelope material for mass conservation during the occurrence of a dredge-up phenomenon. We find that the proton penetration is characterized by small concentrations, but is extended over a large fractional mass of the He-layers, thus producing 13C reservoirs of several 10-3 Mo. The ensuing 13C-enriched zone has an almost flat profile, while only a limited production of 14N occurs. In order to verify the effects of our new findings we show how the abundances of the main s-component nuclei can be accounted for in solar proportions and how our large 13C-reservoir allows us to solve a few so far unexplained features in the abundance distribution of post-AGB objects
Can the Main Component of the s-Process in AGB Stars Constrain the C-13-Pocket Formation?
No full textThe s-process main component is mainly produced in low-mass AGB stars by the C-13(alpha, n)O-16 reaction, requiring proton injection from the envelope. The C-13 pocket was typically assumed to involve a small mass (= 4 x 10(-3) M-circle dot). We speculated that mixing driven by magnetic buoyancy (as in Maiorca et al. 2012; Trippella et al. 2014, or other forced mechanisms "from bottom to top") can form a C-13 reservoir larger than assumed so far, covering most of the He-rich layers. We present new calculations (Trippella et al. 2014) aimed at understanding if the solar composition helps to constrain the C-13-pocket extension. Stellar models at a fixed metallicity, based on a large C-13 reservoir, reproduce the main s-component as accurately as before and don't require any nuclear contribution from an unknown nucleosynthesis processes (LEPP). These models also avoid problems of mixing at the envelope border and meet requirements from C-star luminosities. A large production of nuclei below A = 90 is expected, so that (86)'Sr-87 may be fully synthesized by AGB stars, while Sr-88, Y-89 and Zr-94 are contributed more efficiently than before
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
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