1,721,206 research outputs found
An empirical calibration of the mixing-length parameter alpha
No full textWe present an empirical calibration of the mixing-length free parameter alpha based on a homogeneous infrared database of 28 Galactic globular clusters spanning a wide metallicity range (-2.1
New Calculations of Evolution, Dredge-up and Nucleosynthesis for Low Mass Stars in the TP-AGB Phases
No full textNot Availabl
The fate of Li and Be in stars and in the laboratory
No full textWe connect the observed under-abundances of Li and Be in dwarfs, with recent results on nuclear cross sections at low energies: for collisions of protons with atomic or molecular targets, the measured cross sections seem too high with respect to extrapolations for bare nuclei. Phenomenologically, these anomalous nuclear interactions can be described in terms of an effective screening potential U-lab in the range of few hundred eV: in the presence of the electron cloud, nuclei become more transparent to each other as if the effective collision energy is aumented by U-lab. This implies that fusion cross sections are enlarged and at the same time elastic cross sections are lowered. If something similar occurs in stellar plasma, the nuclear burning temperatures are lowered, whereas diffusion processes are enhanced. We find that the observed Li and Be abundances in the Hyades and in the Sun can be reproduced for effective screening potentials of the plasma in the range of 600-700 eV, close to that found by experiments in the laboratory
Can extended mixing in red giants be attributed to magnetic mechanisms?
No full textThis work presents estimates of the effects that magnetic fields might have on
non-convective mixing phenomena in red giants of low mass. We discuss recent doubts on
the effectiveness of purely rotationally-induced mixing and an alternative idea is illustrated,
according to which kink modes of buoyant, toroidal magnetic flux tubes might guarantee
matter circulation. This occurs not through downward motions from the envelope (as assumed so far), but through different buoyancy efficiencies of tubes born at different depths
in the radiative zone, carrying upward material exposed to partial H burning. We adopt a
simple formalism to estimate the strength of the magnetic fields necessary to guarantee cool
bottom processes and the formation of the neutron source 13C , which drives slow neutron
captures in AGB stars. Our rough estimates do not allow final conclusions, but we find that
the required magnetic field strengths are in the range foreseen for the stages and zones of
interest. This tells us that the mechanisms here indicated are worth the (considerable) effort
of a full MHD treatment. For the moment, magnetic fields are to be seen as a promising
possibility for solving the mystery of red giant mixing
Influence of the 12C(α,γ)16O reaction rate on the evolution of a 15MO star. Nuclear Physics A688(2001)249
No full text
Influence of the 12C(α,γ)16O reaction rate on the evolution of a 15 M⊙ star
No full textIn this paper we present the evolution of various 15M(circle dot) stellar models of solar and subsolar chemical composition, calculated using different values for the C-12(alpha, gamma)O-16 reaction rate. This process influences the evolution of a star because it directly operates during the helium burning and it determines the final abundances of (12)G and O-16 left by this burning. Since this reaction works in a convective environment an analysis of its influence on the evolution of a star cannot be disentangled by the behavior of the convective core. Indeed the final C-12 and O-16 abundances largely depend on a delicate balance between the efficiency of this rate and the treatment of the convective core. We will show some tests which indicate quantitavely this interplay
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
- …
