24,498 research outputs found
The Stagger-grid: A Grid of 3D Stellar Atmosphere Models - II. Horizontal and Temporal Averaging and Spectral Line Formation
We study the implications of averaging methods with different reference depth
scales for 3D hydrodynamical model atmospheres computed with the Stagger-code.
The temporally and spatially averaged (hereafter denoted as ) models are
explored in the light of local thermodynamic equilibrium (LTE) spectral line
formation by comparing spectrum calculations using full 3D atmosphere
structures with those from averages. We explore methods for computing mean
stratifications from the Stagger-grid time-dependent 3D radiative hydro-
dynamical atmosphere models by considering four different reference depth
scales (geometrical depth, column-mass density, and two optical depth scales).
Furthermore, we investigate the influence of alternative averages (logarithmic
or enforced hydrostatic equilibrium, flux-weighted temperatures). For the line
formation we compute curves of growth for Fe i and Fe ii lines in LTE . The
resulting stratifications for the four reference depth scales can be
considerably different. We find typically that in the upper atmosphere and in
the superadiabatic region just below the optical surface, where the temperature
and density fluctuations are highest, the differences become considerable and
increase for higher Teff, lower logg, and lower [Fe/H]. The differential
comparison of spectral line formation shows distinctive differences depending
on which model is applied. The averages over layers of constant
column-mass density yield the best mean representation for LTE line
formation, while the averages on layers at constant geometrical height are the
least appropriate. Unexpectedly, the usually preferred averages over layers of
constant optical depth are prone to the increasing interference of the reversed
granulation towards higher effective temperature, in particular at low
metallicity.Comment: Accepted for publication in A&A, 18 pages, 16 figure
Evidence for Partial Taylor Relaxation from Changes in Magnetic Geometry and Energy during a Solar Flare
Solar flares are powered by energy stored in the coronal magnetic field, a
portion of which is released when the field reconfigures into a lower energy
state. Investigation of sunspot magnetic field topology during flare activity
is useful to improve our understanding of flaring processes. Here we
investigate the deviation of the non-linear field configuration from that of
the linear and potential configurations, and study the free energy available
leading up to and after a flare. The evolution of the magnetic field in NOAA
region 10953 was examined using data from Hinode/SOT-SP, over a period of 12
hours leading up to and after a GOES B1.0 flare. Previous work on this region
found pre- and post-flare changes in photospheric vector magnetic field
parameters of flux elements outside the primary sunspot. 3D geometry was thus
investigated using potential, linear force-free, and non-linear force-free
field extrapolations in order to fully understand the evolution of the field
lines. Traced field line geometrical and footpoint orientation differences show
that the field does not completely relax to a fully potential or linear
force-free state after the flare. Magnetic and free magnetic energies increase
significantly ~ 6.5-2.5 hours before the flare by ~ 10^31 erg. After the flare,
the non-linear force-free magnetic energy and free magnetic energies decrease
but do not return to pre-flare 'quiet' values. The post-flare non-linear
force-free field configuration is closer (but not equal) to that of the linear
force-free field configuration than a potential one. However, the small degree
of similarity suggests that partial Taylor relaxation has occurred over a time
scale of ~ 3-4 hours.Comment: Accepted for Publication in Astronomy & Astrophysics. 11 pages, 11
figure
A lattice calculation of vector meson couplings to the vector and tensor currents using chirally improved fermions
We present a quenched lattice calculation of , the coupling of
vector mesons to the tensor current normalized by the vector meson decay
constant. The chirally improved lattice Dirac operator, which allows us to
reach small quark masses, is used. We put emphasis on analyzing the quark mass
dependence of and find only a rather weak dependence. Our
results at the and masses agree well with QCD sum rule
calculations and those from previous lattice studies.Comment: 6 pages, 3 figures, one sentence remove
Determination of the diffusion constant using phase-sensitive measurements
We apply a pulsed-light interferometer to measure both the intensity and the
phase of light that is transmitted through a strongly scattering disordered
material. From a single set of measurements we obtain the time-resolved
intensity, frequency correlations and statistical phase information
simultaneously. We compare several independent techniques of measuring the
diffusion constant for diffuse propagation of light. By comparing these
independent measurements, we obtain experimental proof of the consistency of
the diffusion model and corroborate phase statistics theory.Comment: 9 pages, 8 figures, submitted to Phys. Rev.
H-Alpha and Hard X-Ray Observations of a Two-Ribbon Flare Associated with a Filament Eruption
We perform a multi-wavelength study of a two-ribbon flare on 2002 September
29 and its associated filament eruption, observed simultaneously in the H-alpha
line by a ground-based imaging spectrograph and in hard X-rays by RHESSI. The
flare ribbons contain several H-alpha bright kernels that show different
evolutional behaviors. In particular, we find two kernels that may be the
footpoints of a loop. A single hard X-ray source appears to cover these two
kernels and to move across the magnetic neutral line. We explain this as a
result of the merging of two footpoint sources that show gradually asymmetric
emission owing to an asymmetric magnetic topology of the newly reconnected
loops. In one of the H-alpha kernels, we detect a continuum enhancement at the
visible wavelength. By checking its spatial and temporal relationship with the
hard X-ray emission, we ascribe it as being caused by electron beam
precipitation. In addition, we derive the line-of-sight velocity of the
filament plasma based on the Doppler shift of the filament-caused absorption in
the H-alpha blue wing. The filament shows rapid acceleration during the
impulsive phase. These observational features are in principal consistent with
the general scenario of the canonical two-ribbon flare model.Comment: 15 pages, 5 figures, accepted for publication in Ap
First results from 2+1 dynamical quark flavors on an anisotropic lattice: light-hadron spectroscopy and setting the strange-quark mass
We present the first light-hadron spectroscopy on a set of
dynamical, anisotropic lattices. A convenient set of coordinates that
parameterize the two-dimensional plane of light and strange-quark masses is
introduced. These coordinates are used to extrapolate data obtained at the
simulated values of the quark masses to the physical light and strange-quark
point. A measurement of the Sommer scale on these ensembles is made, and the
performance of the hybrid Monte Carlo algorithm used for generating the
ensembles is estimated.Comment: 24 pages. Hadron Spectrum Collaboratio
Chiral behavior of decay form factors in lattice QCD with exact chiral symmetry
We calculate the form factors of the semileptonic decays in
three-flavor lattice QCD, and study their chiral behavior as a function of the
momentum transfer and the Nambu-Goldstone boson masses. Chiral symmetry is
exactly preserved by using the overlap quark action, which enables us to
directly compare the lattice data with chiral perturbation theory (ChPT). We
generate gauge ensembles at a lattice spacing of 0.11fm with four pion masses
covering 290-540 MeV and a strange quark mass m_s close to its physical value.
By using the all-to-all quark propagator, we calculate the vector and scalar
form factors with high precision. Their dependence on m_s and the momentum
transfer is studied by using the reweighting technique and the twisted boundary
conditions for the quark fields. We compare the results for the semileptonic
form factors with ChPT at next-to-next-to leading order in detail. While many
low-energy constants appear at this order, we make use of our data of the light
meson electromagnetic form factors in order to control the chiral
extrapolation. We determine the normalization of the form factors as f_+(0) =
0.9636(36)(+57/-35), and observe reasonable agreement of their shape with
experiment.Comment: 34 pages, 13 figure
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