46 research outputs found
Extended transition rates and lifetimes in Al I and Al II from systematic multiconfiguration calculations
Multiconfiguration Dirac-Hartree-Fock (MCDHF) and relativistic configuration
interaction (RCI) calculations were performed for 28 and 78 states in neutral
and singly ionized aluminium, respectively. In Al I, the configurations of
interest are for with to , as well as and
for . In Al II, the studied configurations are, besides the
ground configuration , with to and to , ,
, and . Valence and core-valence electron correlation
effects are systematically accounted for through large configuration state
function (CSF) expansions. Calculated excitation energies are found to be in
excellent agreement with experimental data from the NIST database. Lifetimes
and transition data for radiative electric dipole (E1) transitions are given
and compared with results from previous calculations and available
measurements, for both Al I and Al II. The computed lifetimes of Al I are in
very good agreement with the measured lifetimes in high-precision laser
spectroscopy experiments. The present calculations provide a substantial amount
of updated atomic data, including transition data in the infrared region. This
is particularly important since the new generation of telescopes are designed
for this region. There is a significant improvement in accuracy, in particular
for the more complex system of neutral Al I. The complete tables of transition
data are available
An algorithm for two-dimensional mesh generation based on the pinwheel tiling
We propose a new two-dimensional meshing algorithm called PINW able to
generate meshes that accurately approximate the distance between any two domain
points by paths composed only of cell edges. This technique is based on an
extension of pinwheel tilings proposed by Radin and Conway. We prove that the
algorithm produces triangles of bounded aspect ratio. This kind of mesh would
be useful in cohesive interface finite element modeling when the crack
propagation pathis an outcome of a simulation process.Comment: Short version appears in Proceedings of 2004 International Meshing
Roundtable at http://www.imr.sandia.go
Greek teachers' personal theory on writing at the elementary level : opportunity for innovation or defense mechanism?
In view of the importance of teachers' personal theories on their
education, teaching and in-service education, this study aims to examine Greek
elementary school teachers' personal theory on students writing. Forty-two
teachers of both genders, varying in years of experience, serving in urban and
sub urban schools, filled in a questionnaire. This was made up of closed and open-ended
questions concerning their knowledge, attitudes and assessment on (a)
conditions surrounding students' writing, (b) the actual teaching of writing in
their context, and (c) their evaluation of students' writing. The data was examined
through both qualitative and quantitative means. The findings indicated that
teachers see good writing not as a teachable skill, but as a gift; their role as only
that of a judge and not as a helper or a collaborator; students' writing as a
product and not a process and as 'knowledge telling' and 'knowledge
transforming'. A discussion of these findings in relation to changing teachers'
personal theory follows.peer-reviewe
Phylogeography of Aegean green toads (Bufo viridis subgroup): continental hybrid swarm vs. insular diversification with discovery of a new island endemic
BACKGROUND: Debated aspects in speciation research concern the amount of gene flow between incipient species under secondary contact and the modes by which post-zygotic isolation accumulates. Secondary contact zones of allopatric lineages, involving varying levels of divergence, provide natural settings for comparative studies, for which the Aegean (Eastern Mediterranean) geography offers unique scenarios. In Palearctic green toads (Bufo viridis subgroup or Bufotes), Plio-Pleistocene (~ 2.6 Mya) diverged species show a sharp transition without contemporary gene flow, while younger lineages, diverged in the Lower-Pleistocene (~ 1.9 Mya), admix over tens of kilometers. Here, we conducted a fine-scale multilocus phylogeographic analysis of continental and insular green toads from the Aegean, where a third pair of taxa, involving Mid-Pleistocene diverged (~ 1.5 Mya) mitochondrial lineages, earlier tentatively named viridis and variabilis, (co-)occurs. RESULTS: We discovered a new lineage, endemic to Naxos (Central Cyclades), while coastal islands and Crete feature weak genetic differentiation from the continent. In continental Greece, both lineages, viridis and variabilis, form a hybrid swarm, involving massive mitochondrial and nuclear admixture over hundreds of kilometers, without obvious selection against hybrids. CONCLUSIONS: The genetic signatures of insular Aegean toads appear governed by bathymetry and Quaternary sea level changes, resulting in long-term isolation (Central Cyclades: Naxos) and recent land-bridges (coastal islands). Conversely, Crete has been isolated since the end of the Messinian salinity crisis (5.3 My) and Cretan populations thus likely result from human-mediated colonization, at least since Antiquity, from Peloponnese and Anatolia. Comparisons of green toad hybrid zones support the idea that post-zygotic hybrid incompatibilities accumulate gradually over the genome. In this radiation, only one million years of divergence separate a scenario of complete reproductive isolation, from a secondary contact resulting in near panmixia
Time continuity in cohesive finite element modeling
We introduce the notion of time continuity for the analysis of cohesive zone interface finite element models. We focus on “initially rigid ” models in which an interface is inactive until the traction across it reaches a critical level. We argue that methods in this class are time discontinuous, unless special provision is made for the opposite. Time discontinuity leads to pitfalls in numerical implementations: oscillatory behavior, non-convergence in time and dependence on nonphysical regularization parameters. These problems arise at least partly from the attempt to extend uniaxial traction-displacement relationships to multiaxial loading. We also argue that any formulation of a time-continuous functional traction-displacement cohesive model entails encoding the value of the traction components at incipient softening into the model. We exhibit an example of such a model. Most of our numerical experiments concern explicit dynamics
An algorithm for two-dimensional mesh generation based on the pinwheel tiling ∗
We propose a new two-dimensional meshing algorithm called PINW able to generate meshes that accurately approximate the distance between any two domain points by paths composed only of cell edges. This technique is based on an extension of pinwheel tilings proposed by Radin and Conway. We prove that the algorithm produces triangles of bounded aspect ratio. This kind of mesh would be useful in cohesive interface finite element modeling when the crack propagation path is an outcome of a simulation process.
Extended theoretical transition data in C I - IV
Accurate atomic data are essential for opacity calculations and for abundance
analyses of the Sun and other stars. The aim of this work is to provide
accurate and extensive results of energy levels and transition data for C I -
IV. The Multiconfiguration Dirac-Hartree-Fock and relativistic configuration
interaction methods were used in the present work. To improve the quality of
the wave functions and reduce the relative differences between length and
velocity forms for transition data involving high Rydberg states, alternative
computational strategies were employed by imposing restrictions on the electron
substitutions when constructing the orbital basis for each atom and ion.
Transition data, e.g., weighted oscillator strengths and transition
probabilities, are given for radiative electric dipole (E1) transitions
involving levels up to 1s2s2p6s for C I, up to 1s2s7f for C II,
up to 1s2s7f for C III, and up to 1s8g for CIV. Using the difference
between the transition rates in length and velocity gauges as an internal
validation, the average uncertainties of all presented E1 transitions are
estimated to be 8.05%, 7.20%, 1.77%, and 0.28%, respectively, for C I - IV.
Extensive comparisons with available experimental and theoretical results are
performed and good agreement is observed for most of the transitions. In
addition, the C I data were employed in a reanalysis of the solar carbon
abundance. The new transition data give a line-by-line dispersion similar to
the one obtained when using transition data that are typically used in stellar
spectroscopic applications today.Comment: 21 pages, 4 figure