22,415 research outputs found
Implementation of robust image artifact removal in SWarp through clipped mean stacking
We implement an algorithm for detecting and removing artifacts from
astronomical images by means of outlier rejection during stacking. Our method
is capable of addressing both small, highly significant artifacts such as
cosmic rays and, by applying a filtering technique to generate single frame
masks, larger area but lower surface brightness features such as secondary
(ghost) images of bright stars. In contrast to the common method of building a
median stack, the clipped or outlier-filtered mean stacked point-spread
function (PSF) is a linear combination of the single frame PSFs as long as the
latter are moderately homogeneous, a property of great importance for weak
lensing shape measurement or model fitting photometry. In addition, it has
superior noise properties, allowing a significant reduction in exposure time
compared to median stacking. We make publicly available a modified version of
SWarp that implements clipped mean stacking and software to generate single
frame masks from the list of outlier pixels.Comment: PASP accepted; software for download at
http://www.usm.uni-muenchen.de/~dgruen
Systematic Errors in Future Weak Lensing Surveys: Requirements and Prospects for Self-Calibration
We study the impact of systematic errors on planned weak lensing surveys and
compute the requirements on their contributions so that they are not a dominant
source of the cosmological parameter error budget. The generic types of error
we consider are multiplicative and additive errors in measurements of shear, as
well as photometric redshift errors. In general, more powerful surveys have
stronger systematic requirements. For example, for a SNAP-type survey the
multiplicative error in shear needs to be smaller than 1%(fsky/0.025)^{-1/2} of
the mean shear in any given redshift bin, while the centroids of photometric
redshift bins need to be known to better than 0.003(fsky/0.025)^{-1/2}. With
about a factor of two degradation in cosmological parameter errors, future
surveys can enter a self-calibration regime, where the mean systematic biases
are self-consistently determined from the survey and only higher-order moments
of the systematics contribute. Interestingly, once the power spectrum
measurements are combined with the bispectrum, the self-calibration regime in
the variation of the equation of state of dark energy w_a is attained with only
a 20-30% error degradation.Comment: 20 pages, 9 figures, to be submitted to MNRAS. Comments are welcom
Origin of the structural phase transition in Li7La3Zr2O12
Garnet-type Li7La3Zr2O12 (LLZO) is a solid electrolyte material with a
low-conductivity tetragonal and a high-conductivity cubic phase. Using
density-functional theory and variable cell shape molecular dynamics
simulations, we show that the tetragonal phase stability is dependent on a
simultaneous ordering of the Li ions on the Li sublattice and a
volume-preserving tetragonal distortion that relieves internal structural
strain. Supervalent doping introduces vacancies into the Li sublattice,
increasing the overall entropy and reducing the free energy gain from ordering,
eventually stabilizing the cubic phase. We show that the critical temperature
for cubic phase stability is lowered as Li vacancy concentration (dopant level)
is raised and that an activated hop of Li ions from one crystallographic site
to another always accompanies the transition. By identifying the relevant
mechanism and critical concentrations for achieving the high conductivity
phase, this work shows how targeted synthesis could be used to improve
electrolytic performance
Shrinking the western Pacific: psychiatric training for medical students from Micronesia.
In 1989 the Hawaii State Hospital became the primary site for clinical teaching of psychiatry to students of the Pacific Basin Medical Officer Training Program, a program designed to train clinicians for the western Pacific. The psychiatry clerkship was developed to provide practical training in psychiatry to clinicians who will practice in Micronesia. Challenges encountered by the educators, including transcultural issues, are discussed. Interventions found to be effective in resolving these challenges are described
Slice Stretching Effects for Maximal Slicing of a Schwarzschild Black Hole
Slice stretching effects such as slice sucking and slice wrapping arise when
foliating the extended Schwarzschild spacetime with maximal slices. For
arbitrary spatial coordinates these effects can be quantified in the context of
boundary conditions where the lapse arises as a linear combination of odd and
even lapse. Favorable boundary conditions are then derived which make the
overall slice stretching occur late in numerical simulations. Allowing the
lapse to become negative, this requirement leads to lapse functions which
approach at late times the odd lapse corresponding to the static Schwarzschild
metric. Demanding in addition that a numerically favorable lapse remains
non-negative, as result the average of odd and even lapse is obtained. At late
times the lapse with zero gradient at the puncture arising for the puncture
evolution is precisely of this form. Finally, analytic arguments are given on
how slice stretching effects can be avoided. Here the excision technique and
the working mechanism of the shift function are studied in detail.Comment: 16 pages, 4 figures, revised version including a study on how slice
stretching can be avoided by using excision and/or shift
Molecular collisions. 15 - Classical limit of the generalized phase shift treatment of rotational excitation - Atom-rigid rotor
Generalized phase shift approach to problem of rotationally inelastic molecular collision
Weak Lensing Determination of the Mass in Galaxy Halos
We detect the weak gravitational lensing distortion of 450,000 background
galaxies (20<R<23) by 790 foreground galaxies (R<18) selected from the Las
Campanas Redshift Survey (LCRS). This is the first detection of weak lensing by
field galaxies of known redshift, and as such permits us to reconstruct the
shear profile of the typical field galaxy halo in absolute physical units
(modulo H_0), and to investigate the dependence of halo mass upon galaxy
luminosity. This is also the first galaxy-galaxy lensing study for which the
calibration errors are negligible. Within a projected radius of 200 \hkpc, the
shear profile is consistent with an isothermal profile with circular velocity
164+-20 km/s for an L* galaxy, consistent with typical disk rotation at this
luminosity. This halo mass normalization, combined with the halo profile
derived by Fischer et al (2000) from lensing analysis SDSS data, places a lower
limit of (2.7+-0.6) x 10^{12}h^{-1} solar masses on the mass of an L* galaxy
halo, in good agreement with satellite galaxy studies. Given the known
luminosity function of LCRS galaxies, and the assumption that for galaxies, we determine that the mass within 260\hkpc of normal
galaxies contributes to the density of the Universe (for
) or for . These lensing data suggest
that (95% CL), only marginally in agreement with the usual
Faber-Jackson or Tully-Fisher scaling. This is the most
complete direct inventory of the matter content of the Universe to date.Comment: 18 pages, incl. 3 figures. Submitted to ApJ 6/7/00, still no response
from the referee after four months
Tight-binding study of structure and vibrations of amorphous silicon
We present a tight-binding calculation that, for the first time, accurately
describes the structural, vibrational and elastic properties of amorphous
silicon. We compute the interatomic force constants and find an unphysical
feature of the Stillinger-Weber empirical potential that correlates with a much
noted error in the radial distribution function associated with that potential.
We also find that the intrinsic first peak of the radial distribution function
is asymmetric, contrary to usual assumptions made in the analysis of
diffraction data. We use our results for the normal mode frequencies and
polarization vectors to obtain the zero-point broadening effect on the radial
distribution function, enabling us to directly compare theory and a high
resolution x-ray diffraction experiment
Deriving Boltzmann Equations from Kadanoff-Baym Equations in Curved Space-Time
To calculate the baryon asymmetry in the baryogenesis via leptogenesis
scenario one usually uses Boltzmann equations with transition amplitudes
computed in vacuum. However, the hot and dense medium and, potentially, the
expansion of the universe can affect the collision terms and hence the
generated asymmetry. In this paper we derive the Boltzmann equation in the
curved space-time from (first-principle) Kadanoff-Baym equations. As one
expects from general considerations, the derived equations are covariant
generalizations of the corresponding equations in Minkowski space-time. We find
that, after the necessary approximations have been performed, only the
left-hand side of the Boltzmann equation depends on the space-time metric. The
amplitudes in the collision term on the right--hand side are independent of the
metric, which justifies earlier calculations where this has been assumed
implicitly. At tree level, the matrix elements coincide with those computed in
vacuum. However, the loop contributions involve additional integrals over the
the distribution function.Comment: 14 pages, 5 figures, extended discussion of the constraint equations
and the solution for the spectral functio
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