1,202 research outputs found
Ipopv2: Photoionization of Ni XIV -- a test case
Several years ago, M. Asplund and coauthors (2004) proposed a revision of the
Solar composition. The use of this new prescription for Solar abundances in
standard stellar models generated a strong disagreement between the predictions
and the observations of Solar observables. Many claimed that the Standard Solar
Model (SSM) was faulty, and more specifically the opacities used in such
models. As a result, activities around the stellar opacities were boosted. New
experiments (J. Bailey at Sandia on Z-Pinch, The OPAC consortium at LULI200) to
measure directly absorbtion coefficients have been realized or are underway.
Several theoretical groups (CEA-OPAS, Los Alamos Nat. Lab., CEA-SCORCG, The
Opacity Project - The Iron Project (IPOPv2)) have started new sets of
calculations using different approaches and codes. While the new results seem
to confirm the good quality of the opacities used in SSM, it remains important
to improve and complement the data currently available. We present recent
results in the case of the photoionization cross sections for Ni XIV (Ni13+ )
from IPOPv2 and possible implications on stellar modelling.Comment: 10 pages, 3 figures, Conf. on New Advances in Stellar Physics: From
Microscopic to Macroscopic Processe
The relationship between andragogical and pedagogical orientation and the implications for adult learning
Current literature suggests that the relationship between andragogy and pedagogy is based on a continuum. This study found that the relationship of andragogical and pedagogical orientations, measured by the Students' Orientation Questionnaire, is more correctly represented as being orthogonal or at right angles to each other. Such an orthogonal relationship reflects the complexities involved in adult learning. This paper discusses the implications for both the learning process and for future research
Observation of thermally activated glassiness and memory dip in a-NbSi insulating thin films
We present electrical conductance measurements on amorphous NbSi insulating
thin films. These films display out-of equilibrium electronic features that are
markedly different from what has been reported so far in disordered insulators.
Like in the most studied systems (indium oxide and granular Al films), a slow
relaxation of the conductance is observed after a quench to liquid helium
temperature which gives rise to the growth of a memory dip in MOSFET devices.
But unlike in these systems, this memory dip and the related conductance
relaxations are still visible up to room temperature, with clear signatures of
a temperature dependent dynamics
Atomic data from the Iron Project.XLIII. Transition probabilities for Fe V
An extensive set of dipole-allowed, intercombination, and forbidden
transition probabilities for Fe V is presented. The Breit-Pauli R-matrix (BPRM)
method is used to calculate 1.46 x 10^6 oscillator strengths for the allowed
and intercombination E1 transitions among 3,865 fine-structure levels dominated
by configuration complexes with n <= 10 and l <= 9. These data are complemented
by an atomic structure configuration interaction (CI) calculation using the
SUPERSTRUCTURE program for 362 relativistic quadrupole (E2) and magnetic dipole
(M1) transitions among 65 low-lying levels dominated by the 3d^4 and 3d^ 4s
configurations. Procedures have been developed for the identification of the
large number of fine-structure levels and transitions obtained through the BPRM
calculations. The target ion Fe VI is represented by an eigenfunction expansion
of 19 fine-structure levels of 3d^3 and a set of correlation configurations. Fe
V bound levels are obtained with angular and spin symmetries SL\pi and J\pi of
the (e + Fe VI) system such that 2S+1 = 5,3,1, L <= 10, J <= 8 of even and odd
parities. The completeness of the calculated dataset is verified in terms of
all possible bound levels belonging to relevant LS terms and transitions in
correspondence with the LS terms. The fine-structure averaged relativistic
values are compared with previous Opacity Project LS coupling data and other
works. The 362 forbidden transition probabilities considerably extend the
available data for the E2 and M1 transtions, and are in good agreement with
those computed by Garstang for the 3d^4 transitions.Comment: 19 pages, 1 figure. This paper marks the beginning of a large-scale
effort of ab initio atomic calculations that should eventually lead to
re-calculation of accurate iron opacities. Astron. Astrophys. Suppl. Ser. (in
press
Ageing and relaxation times in disordered insulators
We focus on the slow relaxations observed in the conductance of disordered
insulators at low temperature (especially granular aluminum films). They
manifest themselves as a temporal logarithmic decrease of the conductance after
a quench from high temperatures and the concomitant appearance of a field
effect anomaly centered on the gate voltage maintained. We are first interested
in ageing effects, i.e. the age dependence of the dynamical properties of the
system. We stress that the formation of a second field effect anomaly at a
different gate voltage is not a "history free" logarithmic (lnt) process, but
departs from lnt in a way which encodes the system's age. The apparent
relaxation time distribution extracted from the observed relaxations is thus
not "constant" but evolves with time. We discuss what defines the age of the
system and what external perturbation out of equilibrium does or does not
rejuvenate it. We further discuss the problem of relaxation times and comment
on the commonly used "two dip" experimental protocol aimed at extracting
"characteristic times" for the glassy systems (granular aluminum, doped indium
oxide...). We show that it is inoperable for systems like granular Al and
probably highly doped InOx where it provides a trivial value only determined by
the experimental protocol. But in cases where different values are obtained
like in lightly doped InOx or some ultra thin metal films, potentially
interesting information can be obtained, possibly about the "short time"
dynamics of the different systems. Present ideas about the effect of doping on
the glassiness of disordered insulators may also have to be reconsidered.Comment: to appear in the proceedings of the 14th International Conference on
Transport and Interactions in Disordered Systems (TIDS14
OPserver: interactive online-computations of opacities and radiative accelerations
Codes to compute mean opacities and radiative accelerations for arbitrary
chemical mixtures using the Opacity Project recently revised data have been
restructured in a client--server architecture and transcribed as a subroutine
library. This implementation increases efficiency in stellar modelling where
element stratification due to diffusion processes is depth dependent, and thus
requires repeated fast opacity reestimates. Three user modes are provided to
fit different computing environments, namely a web browser, a local workstation
and a distributed grid.Comment: 5 pages, 1 figur
Updated opacities from the opacity project
Using the code autostructure, extensive calculations of inner-shell atomic data have been made for the chemical elements He, C, N, O, Ne, Na, Mg, Al, Si, S, Ar, Ca, Cr, Mn, Fe and Ni. The results are used to obtain updated opacities from the Opacity Project (OP). A number of other improvements on earlier work have also been included. Rosseland-mean opacities from the OP are compared with those from OPAL. Differences of 5-10 per cent occur. The OP gives the 'Z-bump', at log(T) 5.2, to be shifted to slightly higher temperatures. The opacities from the OP, as functions of temperature and density, are smoother than those from OPAL. The accuracy of the integrations used to obtain mean opacities can depend on the frequency mesh used. Tests involving variation of the numbers of frequency points show that for typical chemical mixtures the OP integrations are numerically correct to within 0.1 per cent. The accuracy of the interpolations used to obtain mean opacities for any required values of temperature and density depends on the temperature-density meshes used. Extensive tests show that, for all cases of practical interest, the OP interpolations give results correct to better than 1 per cent. Prior to a number of recent investigations which have indicated a need for downward revisions in the solar abundances of oxygen and other elements, there was good agreement between properties of the Sun deduced from helioseismology and from stellar evolution models calculated using OPAL opacities. The revisions destroy that agreement. In a recent paper, Bahcall et al. argue that the agreement would be restored if opacities for the regions of the Sun with 2 × 106T 5 × 106 K (0.7-0.4 R) were larger than those given by OPAL by about 10 per cent. In the region concerned, the present results from the OP do not differ from those of OPAL by more than 2.5 per cent
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