6,961 research outputs found
Phonon Properties of Knbo3 and Ktao3 from First-Principles Calculations
The frequencies of transverse-optical phonons in KNbO and
KTaO are calculated in the frozen-phonon scheme making use of the
full-potential linearized muffin-tin orbital method. The calculated frequencies
in the cubic phase of KNbO and in the tetragonal ferroelectric phase are in
good agreement with experimental data. For KTaO, the effect of lattice
volume was found to be substantial on the frequency of the soft mode, but
rather small on the relative displacement patterns of atoms in all three modes
of the symmetry. The TO frequencies in KTaO are found to be of the
order of, but somehow higher than, the corresponding frequencies in cubic
KNbO.Comment: 8 pages + 1 LaTeX figure, Revtex 3.0, SISSA-CM-94-00
T-PHOT version 2.0: improved algorithms for background subtraction, local convolution, kernel registration, and new options
We present the new release v2.0 of T-PHOT, a publicly available software
package developed to perform PSF-matched, prior-based, multiwavelength
deconfusion photometry of extragalactic fields. New features included in the
code are presented and discussed: background estimation, fitting using position
dependent kernels, flux prioring, diagnostical statistics on the residual
image, exclusion of selected sources from the model and residual images,
individual registration of fitted objects. These new options improve on the
performance of the code, allowing for more accurate results and providing
useful aids for diagnostics.Comment: 7 pages, 8 figure
Thermodynamic Scaling of the Viscosity of Van Der Waals, H-Bonded, and Ionic Liquids
Viscosities and their temperature, T, and volume, V, dependences are reported
for 7 molecular liquids and polymers. In combination with literature viscosity
data for 5 other liquids, we show that the superpositioning of relaxation times
for various glass-forming materials when expressed as a function of TV^g, where
the exponent g is a material constant, can be extended to the viscosity. The
latter is usually measured to higher temperatures than the corresponding
relaxation times, demonstrating the validity of the thermodynamic scaling
throughout the supercooled and higher T regimes. The value of g for a given
liquid principally reflects the magnitude of the intermolecular forces (e.g.,
steepness of the repulsive potential); thus, we find decreasing g in going from
van der Waals fluids to ionic liquids. For strongly H-bonded materials, such as
low molecular weight polypropylene glycol and water, the superpositioning
fails, due to the non-trivial change of chemical structure (degree of
H-bonding) with thermodynamic conditions.Comment: 16 pages 7 figure
A direct empirical proof of the existence of dark matter
We present new weak lensing observations of 1E0657-558 (z=0.296), a unique
cluster merger, that enable a direct detection of dark matter, independent of
assumptions regarding the nature of the gravitational force law. Due to the
collision of two clusters, the dissipationless stellar component and the
fluid-like X-ray emitting plasma are spatially segregated. By using both
wide-field ground based images and HST/ACS images of the cluster cores, we
create gravitational lensing maps which show that the gravitational potential
does not trace the plasma distribution, the dominant baryonic mass component,
but rather approximately traces the distribution of galaxies. An 8-sigma
significance spatial offset of the center of the total mass from the center of
the baryonic mass peaks cannot be explained with an alteration of the
gravitational force law, and thus proves that the majority of the matter in the
system is unseen.Comment: Accepted for publication in ApJ
Statistical mechanics of RNA folding: importance of alphabet size
We construct a minimalist model of RNA secondary-structure formation and use
it to study the mapping from sequence to structure. There are strong,
qualitative differences between two-letter and four or six-letter alphabets.
With only two kinds of bases, there are many alternate folding configurations,
yielding thermodynamically stable ground-states only for a small set of
structures of high designability, i.e., total number of associated sequences.
In contrast, sequences made from four bases, as found in nature, or six bases
have far fewer competing folding configurations, resulting in a much greater
average stability of the ground state.Comment: 7 figures; uses revtex
Reply
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/135604/1/hep28794.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/135604/2/hep28794_am.pd
Strong and weak lensing united III: Measuring the mass distribution of the merging galaxy cluster 1E0657-56
The galaxy cluster 1E0657-56 (z = 0.296) is remarkably well-suited for
addressing outstanding issues in both galaxy evolution and fundamental physics.
We present a reconstruction of the mass distribution from both strong and weak
gravitational lensing data. Multi-color, high-resolution HST ACS images allow
detection of many more arc candidates than were previously known, especially
around the subcluster. Using the known redshift of one of the multiply imaged
systems, we determine the remaining source redshifts using the predictive power
of the strong lens model. Combining this information with shape measurements of
"weakly" lensed sources, we derive a high-resolution, absolutely-calibrated
mass map, using no assumptions regarding the physical properties of the
underlying cluster potential. This map provides the best available
quantification of the total mass of the central part of the cluster. We also
confirm the result from Clowe et al. (2004,2006a).Comment: Accepted for publication in ApJ; Version with full-resolution figures
available at
http://www.slac.stanford.edu/~marusa/Work/bradac_strong_weak_III.pd
Optical Line Emission from z6.8 Sources with Deep Constraints on Ly Visibility
We analyze a sample of -dropout galaxies in the CANDELS GOODS South and
UDS fields that have been targeted by a dedicated spectroscopic campaign aimed
at detecting their Ly line. Deep IRAC observations at 3.6 and 4.5
m are used to determine the strength of optical emission lines affecting
these bands at z6.5-6.9 in order to i) investigate possible physical
differences between Ly emitting and non-emitting sources; ii) constrain
the escape fraction of ionizing photons; iii) provide an estimate of the
specific star-formation rate at high redshifts. We find evidence of strong
[OIII]+H emission in the average (stacked) SEDs of galaxies both with
and without Ly emission. The blue IRAC [3.6]-[4.5] color of the stack
with detected Ly line can be converted into a rest-frame equivalent
width EW([OIII]+H)=1500 assuming a flat intrinsic
stellar continuum. This strong optical line emission enables a first estimate
of f20% on the escape fraction of ionizing photons from
Ly detected objects. The objects with no Ly line show less
extreme EW([OIII]+H)=520 suggesting different
physical conditions of the HII regions with respect to Ly-emitting
ones, or a larger f. The latter case is consistent with a combined
evolution of f and the neutral hydrogen fraction as an explanation of
the lack of bright Ly emission at z6. A lower limit on the specific
star formation rate, SSFR9.1 for galaxies at these redshifts can be derived from the
spectroscopically confirmed sample.Comment: 10 pages, 7 figures; The Astrophysical Journal in press; matched to
the published versio
Faint AGNs at z>4 in the CANDELS GOODS-S field: looking for contributors to the reionization of the Universe
In order to derive the AGN contribution to the cosmological ionizing
emissivity we have selected faint AGN candidates at in the CANDELS
GOODS-South field which is one of the deepest fields with extensive
multiwavelength coverage from Chandra, HST, Spitzer and various groundbased
telescopes. We have adopted a relatively novel criterion. As a first step high
redshift galaxies are selected in the NIR band down to very faint levels
() using reliable photometric redshifts. This corresponds at to
a selection criterion based on the galaxy rest-frame UV flux. AGN candidates
are then picked up from this parent sample if they show X-ray fluxes above a
threshold of cgs (0.5-2 keV). We have found 22 AGN
candidates at and we have derived the first estimate of the UV luminosity
function in the redshift interval and absolute magnitude interval
typical of local Seyfert galaxies. The
faint end of the derived luminosity function is about two/four magnitudes
fainter at than that derived from previous UV surveys. We have then
estimated ionizing emissivities and hydrogen photoionization rates in the same
redshift interval under reasonable assumptions and after discussion of possible
caveats, the most important being the large uncertainties involved in the
estimate of photometric redshift for sources with featureless, almost power-law
SEDs and/or low average escape fraction of ionizing photons from the AGN host
galaxies. We argue that, under reasonable evaluations of possible biases, the
probed AGN population can produce at photoionization rates consistent
with that required to keep highly ionized the intergalactic medium observed in
the Lyman- forest of high redshift QSO spectra, providing an important
contribution to the cosmic reionization.Comment: 15 pages, 8 figures, A&A accepted, updated figure 6, corrected typo
in table 3, updated reference
Análise de amostras de carne bovina por tomografia de ressonância magnética nuclear para quantificação do marmoreiro.
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