3,067 research outputs found
Atomic data from the Iron Project. LXIV. Radiative transition rates and collision strengths for Ca II
This work reports radiative transition rates and electron impact excitation
rate coefficients for levels of the n= 3, 4, 5, 6, 7, 8 configurations of Ca
II. The radiative data were computed using the Thomas-Fermi-Dirac central
potential method in the frozen core approximation and includes the polarization
interaction between the valence electron and the core using a model potential.
This method allows for configuration interactions (CI) and relativistic effects
in the Breit-Pauli formalism. Collision strengths in LS-coupling were
calculated in the close coupling approximation with the R-matrix method. Then,
fine structure collision strengths were obtained by means of the
intermediate-coupling frame transformation (ICFT) method which accounts for
spin-orbit coupling effects. We present extensive comparisons with the most
recent calculations and measurements for Ca II as well as a comparison between
the core polarization results and the "unpolarized" values. We find that core
polarization affects the computed lifetimes by up to 20%. Our results are in
very close agreement with recent measurements for the lifetimes of metastable
levels. The present collision strengths were integrated over a Maxwellian
distribution of electron energies and the resulting effective collision
strengths are given for a wide range of temperatures. Our effective collision
strengths for the resonance transitions are within ~11% from previous values
derived from experimental measurements, but disagree with latter computations
using the distorted wave approximation.Comment: 8 pages, 4 figures. The atomic data from this work, including energy
levels, A-values, and effective collision strengths, is available in
electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr
(130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A
Productivity improvement, considering legal conditions and Just In Time principles in the mixed-model Sequencing problem
A new mathematical model to solve the Mixed-Model Sequencing Problem with Work overload minimization is formulated. The model incorporates productive, social and legal aspects in order to move the theory problem closer to the actual industrial environments. Specifically, there are considered the variation of work pace of workers throughout the workday to increase the completed work; the conditions of occupancy level of workers imposed by the collective agreements; and the idea of keeping constant the production mix through the sequence leading both to a balance between the required workloads at stations and regular consumption of components. Indeed, by means of a case study linked to Nissan, a gain of over 98% is achieved in terms of regular cumulative production and required work, while performing the 100% of required work and following legal restrictions of operators’ saturation.Postprint (published version
Electron-Ion Recombination Rate Coefficients and Photoionization Cross Sections for Astrophysically Abundant Elements VI. Ni II
We present the first detailed ab initio quantum mechanical calculations for
total and state-specific recombination rate coefficients for e + Ni III --> Ni
II. These rates are obtained using a unified treatment for total electron-ion
recombination that treats the nonresonant radiative recombination and the
resonant dielectronic recombination in a self-consistent unified manner in the
close coupling approximation. Large-scale calculations are carried out using a
49-state wavefunction expansion from core configurations 3d^8, 3d^74s, and
3d^64p that permits the inclusion of prominent dipole allowed core transitions.
These extensive calculations for the recombination rates of Ni II required
hundreds of CPU hours on the Cray T90. The total recombination rate
coefficients are provided for a wide range of temperature. The state-specific
recombination rates for 532 bound states of doublet and quartet symmetries, and
the corresponding photoionization cross sections for leaving the core in the
ground state, are presented. Present total recombination rate coefficients
differ considerably from the currently used data in astrophysical models.Comment: ApJ Suppl. (submitted), 4 figure
[TiII] and [NiII] emission from the strontium filament of eta Carinae
We study the nature of the [TiII] and [NiII] emission from the so-called
strontium filament found in the ejecta of eta Carinae. To this purpose we
employ multilevel models of the TiII and NiII systems which are used to
investigate the physical condition of the filament and the excitation
mechanisms of the observed lines. For the TiII ion, for which no atomic data
was previously available, we carry out ab initio calculations of radiative
transition rates and electron impact excitation rate coefficients. It is found
that the observed spectrum is consistent with the lines being excited in a
mostly neutral region with an electron density of the order of cm
and a temperature around 6000 K. In analyzing three observations with different
slit orientations recorded between March~2000 and November~2001 we find line
ratios that change among various observations, in a way consistent with changes
of up to an order of magnitude in the strength of the continuum radiation
field. These changes result from different samplings of the extended filament,
due to the different slit orientations used for each observation, and yield
clues on the spatial extent and optical depth of the filament. The observed
emission indicates a large Ti/Ni abundance ratio relative to solar abundances.
It is suggested that the observed high Ti/Ni ratio in gas is caused by dust-gas
fractionation processes and does not reflect the absolute Ti/Ni ratio in the
ejecta of \etacar. We study the condensation chemistry of Ti, Ni and Fe within
the filament and suggest that the observed gas phase overabundance of TiComment: 14 paginas, 12 figure
Radiative transition rates and collision strengths for Si II
Aims. This work reports radiative transition rates and electron impact
excitation collision strengths for levels of the 3s23p, 3s3p2, 3s24s, and 3s23d
configurations of Siii. Methods. The radiative data were computed using the
Thomas-Fermi-Dirac-Amaldi central potential, but with the modifications
introduced by Bautista (2008) that account for the effects of electron-electron
interactions. We also introduce new schemes for the optimization of the
variational parameters of the potential. Additional calculations were carried
out with the Relativistic Hartree-Fock and the multiconfiguration Dirac-Fock
methods. Collision strengths in LS-coupling were calculated in the close
coupling approximation with the R-matrix method. Then, fine structure collision
strengths were obtained by means of the intermediate-coupling frame
transformation (ICFT) method which accounts for spin-orbit coupling effects.
Results. We present extensive comparisons between the results of different
approximations and with the most recent calculations and experiment available
in the literature. From these comparisons we derive a recommended set of gf-
values and radiative transition rates with their corresponding estimated
uncertainties. We also study the effects of different approximations in the
representation of the target ion on the electron-impact collision strengths.
Our most accurate set of collision strengths were integrated over a Maxwellian
distribution of electron energies and the resulting effective collision
strengths are given for a wide range of temperatures. Our results present
significant differences from recent calculations with the B-spline
non-orthogonal R-matrix method. We discuss the sources of the differences.Comment: 6 figures, 5 tables within text, 2 electronic table
Research assessment under debate: disentangling the interest around the DORA declaration on Twitter
Much debate has been around the misapplication of metrics in research
assessment. As a result of this concern, the Declaration on Research Assessment
(DORA) was launched, an initiative that caused opposing viewpoints. However,
the discussion topics about DORA have not been formally identified, especially
in participatory environments outside the scholarly communication process, such
as social networks. This paper contributes to that end by analyzing 20,717
DORA-related tweets published from 2015 to 2022. The results show an increasing
volume of tweets, mainly promotional and informative, but with limited
participation of users, either commenting or engaging with the tweets,
generating a scarcely polarized conversation driven primarily by a few DORA
promoters. While a varied list of discussion topics is found (especially "Open
science and research assessment," "Academics career assessment & innovation,"
and "Journal Impact Factor"), the DORA debate appears as part of broader
conversations (research evaluation, open science). Further studies are needed
to check whether these results are restricted to Twitter or reveal more general
patterns. The findings might interest the different evaluators and evaluated
agents regarding their interests and concerns around the reforms in the
research evaluation
Atomic Processes in Planetary Nebulae and H II Regions
Spectroscopic studies of Planetary Nebulae (PNe) and H {\sc ii} regions have
driven much development in atomic physics. In the last few years the
combination of a generation of powerful observatories, the development of ever
more sophisticated spectral modeling codes, and large efforts on mass
production of high quality atomic data have led to important progress in our
understanding of the atomic spectra of such astronomical objects. In this paper
I review such progress, including evaluations of atomic data by comparisons
with nebular spectra, detection of spectral lines from most iron-peak elements
and n-capture elements, observations of hyperfine emission lines and analysis
of isotopic abundances, fluorescent processes, and new techniques for
diagnosing physical conditions based on recombination spectra. The review is
directed toward atomic physicists and spectroscopists trying to establish the
current status of the atomic data and models and to know the main standing
issues.Comment: 9 pages, 1 figur
Elliptic flow in proton-proton collisions at 7 TeV
The angular correlations measured in proton-proton collisions at 7 TeV are
decomposed into contributions from back to back emission and elliptic flow.
Modeling the dominant term in the correlation functions as a momentum
conservation effect or as an effect of the initial transverse velocity of the
source, the remaining elliptic flow component can be estimated. The elliptic
flow coefficient extracted from the CMS Collaboration data is 0.04-0.08. No
additional small-angle, ridge-like correlations are needed to explain the
experimental data
- …