58,617 research outputs found
Line formation in solar granulation: II. The photospheric Fe abundance
The solar photospheric Fe abundance has been determined using realistic ab
initio 3D, time-dependent, hydrodynamical model atmospheres. The study is based
on the excellent agreement between the predicted and observed line profiles
directly rather than equivalent width, since the intrinsic Doppler broadening
from the convective motions and oscillations provide the necessary non-thermal
broadening. Thus, three of the four hotly debated parameters (equivalent
widths, microturbulence and damping enhancement factors) in the center of the
recent solar Fe abundance dispute regarding FeI lines no longer enter the
analysis, leaving the transition probabilities as the main uncertainty. Both
FeI (using the samples of lines of both the Oxford and Kiel studies) and FeII
lines have been investigated, which give consistent results: log FeI = 7.44 +-
0.05 and log FeII = 7.45 +- 0.10. Also the wings of strong FeI lines return
consistent abundances, log FeII = 7.42 +- 0.03, but due to the uncertainties
inherent in analyses of strong lines we give this determination lower weight
than the results from weak and intermediate strong lines. In view of the recent
slight downward revision of the meteoritic Fe abundance log Fe = 7.46 +- 0.01,
the agreement between the meteoritic and photospheric values is very good, thus
appearingly settling the debate over the photospheric Fe abundance from FeI
lines.Comment: Accepted for A&
Line formation in solar granulation: I. Fe line shapes, shifts and asymmetries
Realistic ab-initio 3D, radiative-hydrodynamical convection simulations of
the solar granulation have been applied to FeI and FeII line formation. In
contrast to classical analyses based on hydrostatic 1D model atmospheres the
procedure contains no adjustable free parameters but the treatment of the
numerical viscosity in the construction of the 3D, time-dependent,
inhomogeneous model atmosphere and the elemental abundance in the 3D spectral
synthesis. However, the numerical viscosity is introduced purely for numerical
stability purposes and is determined from standard hydrodynamical test cases
with no adjustments allowed to improve the agreement with the observational
constraints from the solar granulation. The non-thermal line broadening is
mainly provided by the Doppler shifts arising from the convective flows in the
solar photosphere and the solar oscillations. The almost perfect agreement
between the predicted temporally and spatially averaged line profiles for weak
Fe lines with the observed profiles and the absence of trends in derived
abundances with line strengths, seem to imply that the micro- and
macroturbulence concepts are obsolete in these 3D analyses. Furthermore, the
theoretical line asymmetries and shifts show a very satisfactory agreement with
observations with an accuracy of typically 50-100 m/s on an absolute velocity
scale. The remaining minor discrepancies point to how the convection
simulations can be refined further.Comment: Accepted for A&
The effects of numerical resolution on hydrodynamical surface convection simulations and spectral line formation
The computationally demanding nature of radiative-hydrodynamical simulations
of stellar surface convection warrants an investigation of the sensitivity of
the convective structure and spectral synthesis to the numerical resolution and
dimension of the simulations, which is presented here. With too coarse a
resolution the predicted spectral lines tend to be too narrow, reflecting
insufficient Doppler broadening from the convective motions, while at the
currently highest affordable resolution the line shapes have converged
essentially perfectly to the observed profiles. Similar conclusions are drawn
from the line asymmetries and shifts. In terms of abundances, weak FeI and FeII
lines show a very small dependence (~0.02 dex) while for intermediate strong
lines with significant non-thermal broadening the sensitivity increases (~0.10
dex). Problems arise when using 2D convection simulations to describe an
inherent 3D phenomenon, which translates to inaccurate atmospheric velocity
fields and temperature and pressure structures. In 2D the theoretical line
profiles tend to be too shallow and broad compared with the 3D calculations and
observations, in particular for intermediate strong lines. In terms of
abundances, the 2D results are systematically about 0.1 dex lower than for the
3D case for FeI lines. Furthermore, the predicted line asymmetries and shifts
are much inferior in 2D. Given these shortcomings and computing time
considerations it is better to use 3D simulations of even modest resolution
than high-resolution 2D simulations.Comment: Accepted for A&
Magnetohydrodynamic turbulence in warped accretion discs
Warped, precessing accretion discs appear in a range of astrophysical
systems, for instance the X-ray binary Her X-1 and in the active nucleus of
NGC4258. In a warped accretion disc there are horizontal pressure gradients
that drive an epicyclic motion. We have studied the interaction of this
epicyclic motion with the magnetohydrodynamic turbulence in numerical
simulations. We find that the turbulent stress acting on the epicyclic motion
is comparable in size to the stress that drives the accretion, however an
important ingredient in the damping of the epicyclic motion is its parametric
decay into inertial waves.Comment: to appear in the proceedings of the 20th Texas Symposium on
Relativistic Astrophysics, J. C. Wheeler & H. Martel (eds.
I2PA, U-prove, and Idemix: An Evaluation of Memory Usage and Computing Time Efficiency in an IoT Context
The Internet of Things (IoT), in spite of its innumerable advantages, brings
many challenges namely issues about users' privacy preservation and constraints
about lightweight cryptography. Lightweight cryptography is of capital
importance since IoT devices are qualified to be resource-constrained. To
address these challenges, several Attribute-Based Credentials (ABC) schemes
have been designed including I2PA, U-prove, and Idemix. Even though these
schemes have very strong cryptographic bases, their performance in
resource-constrained devices is a question that deserves special attention.
This paper aims to conduct a performance evaluation of these schemes on
issuance and verification protocols regarding memory usage and computing time.
Recorded results show that both I2PA and U-prove present very interesting
results regarding memory usage and computing time while Idemix presents very
low performance with regard to computing time
Influence of gallic and tannic acids on enzymatic activity and growth of Pectobacterium chrysanthemi (Dickeya chrysanthemi bv. chrysanthemi)
The effect of phenolic acids (gallic and tannic acids) on growth of Pectobacterium chrysanhemi, and its protease and pectate lyase activities was tested. The results obtained showed a significant inhibitingeffect of the tannic and gallic acids on the growth of this strain. The growth rate decreases in the presence of 400 g/ml for gallic acid and 100 ìg/ml fortannic acid. The enzymatic activity retardationwas observed with the two phenolic compounds as well but the rate of inhibition varied from one compound to another. The highest antimicrobial potentials and the highest effect on enzymatic activities were observed with the tannic acid at 200 g/ml, which inhibited 91% of the tested microorganisms, and 88% of pectate lyase activity
Fracture of complex metallic alloys: An atomistic study of model systems
Molecular dynamics simulations of crack propagation are performed for two
extreme cases of complex metallic alloys (CMAs): In a model quasicrystal the
structure is determined by clusters of atoms, whereas the model C15 Laves phase
is a simple periodic stacking of a unit cell. The simulations reveal that the
basic building units of the structures also govern their fracture behaviour.
Atoms in the Laves phase play a comparable role to the clusters in the
quasicrystal. Although the latter are not rigid units, they have to be regarded
as significant physical entities.Comment: 6 pages, 4 figures, for associated avi file, see
http://www.itap.physik.uni-stuttgart.de/~frohmut/MOVIES/C15.LJ.011.100.av
An Introduction to Superconducting Qubits and Circuit Quantum Electrodynamics
A subset of the concepts of circuit quantum electrodynamics are reviewed as a
reference to the Axion Dark Matter Experiment (ADMX) community as part of the
proceedings of the 2nd Workshop on Microwave Cavities and Detectors for Axion
Research. The classical Lagrangians and Hamiltonians for an LC circuit are
discussed along with black box circuit quantization methods for a weakly
anharmonic qubit coupled to a resonator or cavity
An Integro-Differential Equation of the Fractional Form: Cauchy Problem and Solution
Producción CientÃficaWe solve the Cauchy problem defined by the fractional partial differential
equation [∂tt − κD]u = 0, with D the pseudo-differential Riesz operator of first
order, and certain initial conditions. The
solution of the Cauchy problem resulting from the substitution of the Gaussian pulse
u(x, 0) by the Dirac delta distribution ϕ(x) = μδ(x) is obtained as corollary.MINECO grant MTM2014-57129-C2-1-P
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