12,825 research outputs found
Investigating the interstellar dust through the Fe K-edge
The chemical and physical properties of interstellar dust in the densest
regions of the Galaxy are still not well understood. X-rays provide a powerful
probe since they can penetrate gas and dust over a wide range of column
densities (up to ). The interaction (scattering and
absorption) with the medium imprints spectral signatures that reflect the
individual atoms which constitute the gas, molecule, or solid. In this work we
investigate the ability of high resolution X-ray spectroscopy to probe the
properties of cosmic grains containing iron. Although iron is heavily depleted
into interstellar dust, the nature of the Fe-bearing grains is still largely
uncertain. In our analysis we use iron K-edge synchrotron data of minerals
likely present in the ISM dust taken at the European Synchrotron Radiation
Facility. We explore the prospects of determining the chemical composition and
the size of astrophysical dust in the Galactic centre and in molecular clouds
with future X-ray missions. The energy resolution and the effective area of the
present X-ray telescopes are not sufficient to detect and study the Fe K-edge,
even for bright X-ray sources. From the analysis of the extinction cross
sections of our dust models implemented in the spectral fitting program SPEX,
the Fe K-edge is promising for investigating both the chemistry and the size
distribution of the interstellar dust. We find that the chemical composition
regulates the X-ray absorption fine structures in the post edge region, whereas
the scattering feature in the pre-edge is sensitive to the mean grain size.
Finally, we note that the Fe K-edge is insensitive to other dust properties,
such as the porosity and the geometry of the dust.Comment: 11 pages, 10 figures. Accepted for publication in Astronomy and
Astrophysic
Scaling of polymers in aligned rods
We study the behavior of self avoiding polymers in a background of vertically
aligned rods that are either frozen into random positions or free to move
horizontally. We find that in both cases the polymer chains are highly
elongated, with vertical and horizontal size exponents that differ by a factor
of 3. Though these results are different than previous predictions, our results
are confirmed by detailed computer simulations.Comment: 4 pages, 4 figure
Dust absorption and scattering in the silicon K-edge
The composition and properties of interstellar silicate dust are not well
understood. In X-rays, interstellar dust can be studied in detail by making use
of the fine structure features in the Si K-edge. The features in the Si K-edge
offer a range of possibilities to study silicon-bearing dust, such as
investigating the crystallinity, abundance, and the chemical composition along
a given line of sight. We present newly acquired laboratory measurements of the
silicon K-edge of several silicate-compounds that complement our measurements
from our earlier pilot study. The resulting dust extinction profiles serve as
templates for the interstellar extinction that we observe. The extinction
profiles were used to model the interstellar dust in the dense environments of
the Galaxy. The laboratory measurements, taken at the Soleil synchrotron
facility in Paris, were adapted for astrophysical data analysis and implemented
in the SPEX spectral fitting program. The models were used to fit the spectra
of nine low-mass X-ray binaries located in the Galactic center neighborhood in
order to determine the dust properties along those lines of sight. Most lines
of sight can be fit well by amorphous olivine. We also established upper limits
on the amount of crystalline material that the modeling allows. We obtained
values of the total silicon abundance, silicon dust abundance, and depletion
along each of the sightlines. We find a possible gradient of
dex/kpc for the total silicon abundance versus the Galactocentric distance. We
do not find a relation between the depletion and the extinction along the line
of sight.Comment: 18 pages, 16 figures. Accepted for publication in Astronomy and
Astrophysic
A low-energy perspective on the minimal left-right symmetric model
We perform a global analysis of the low-energy phenomenology of the minimal
left-right symmetric model (mLRSM) with parity symmetry. We match the mLRSM to
the Standard Model Effective Field Theory Lagrangian at the left-right-symmetry
breaking scale and perform a comprehensive fit to low-energy data including
mesonic, neutron, and nuclear -decay processes, and CP-even and -odd processes in the bottom and strange sectors, and electric
dipole moments (EDMs) of nucleons, nuclei, and atoms. We fit the
Cabibbo-Kobayashi-Maskawa and mLRSM parameters simultaneously and determine a
lower bound on the mass of the right-handed boson. In models where a
Peccei-Quinn mechanism provides a solution to the strong CP problem, we obtain
TeV at C.L. which can be significantly improved
with next-generation EDM experiments. In the -symmetric mLRSM without a
Peccei-Quinn mechanism we obtain a more stringent constraint TeV at C.L., which is difficult to improve with low-energy
measurements alone. In all cases, the additional scalar fields of the mLRSM are
required to be a few times heavier than the right-handed gauge bosons. We
consider a recent discrepancy in tests of first-row unitarity of the CKM
matrix. We find that, while TeV-scale bosons can alleviate some of the
tension found in the determinations, a solution to the discrepancy
is disfavored when taking into account other low-energy observables within the
mLRSM.Comment: 42 pages plus appendices. Published versio
A deformed QRPA formalism for single and two-neutrino double beta decay
We use a deformed QRPA formalism to describe simultaneously the energy
distributions of the single beta Gamow-Teller strength and the two-neutrino
double beta decay matrix elements. Calculations are performed in a series of
double beta decay partners with A = 48, 76, 82, 96, 100, 116, 128, 130, 136 and
150, using deformed Woods-Saxon potentials and deformed Skyrme Hartree-Fock
mean fields. The formalism includes a quasiparticle deformed basis and residual
spin-isospin forces in the particle-hole and particle-particle channels. We
discuss the sensitivity of the parent and daughter Gamow-Teller strength
distributions in single beta decay, as well as the sensitivity of the double
beta decay matrix elements to the deformed mean field and to the residual
interactions. Nuclear deformation is found to be a mechanism of suppression of
the two-neutrino double beta decay. The double beta decay matrix elements are
found to have maximum values for about equal deformations of parent and
daughter nuclei. They decrease rapidly when differences in deformations
increase. We remark the importance of a proper simultaneous description of both
double beta decay and single Gamow-Teller strength distributions. Finally, we
conclude that for further progress in the field it would be useful to improve
and complete the experimental information on the studied Gamow-Teller strengths
and nuclear deformations.Comment: 33 pages, 19 figures. To be published in Phys. Rev.
Inclusion of non-spherical components of the Pauli blocking operator in (p,p') reactions
We present the first calculations of proton elastic and inelastic scattering
in which the Pauli blocking operator contains the leading non-spherical
components as well as the usual spherical (angle-averaged) part. We develop a
formalism for including the contributions to the effective nucleon-nucleon
interaction from the resulting new G-matrix elements that extend the usual
two-nucleon spin structure and may not conserve angular momentum. We explore
the consequences of parity conservation, time reversal invariance, and
nucleon-nucleon antisymmetrization for the new effective interaction. Changes
to the calculated cross section and spin observables are small in the energy
range from 100 to 200 MeV.Comment: 24 pages, 4 figures, to be published in Physical Review
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