1,363 research outputs found
On the appearance of hyperons in neutron stars
By employing a recently constructed hyperon-nucleon potential the equation of
state of \beta-equilibrated and charge neutral nucleonic matter is calculated.
The hyperon-nucleon potential is a low-momentum potential which is obtained
within a renormalization group framework. Based on the Hartree-Fock
approximation at zero temperature the densities at which hyperons appear in
neutron stars are estimated. For several different bare hyperon-nucleon
potentials and a wide range of nuclear matter parameters it is found that
hyperons in neutron stars are always present. These findings have profound
consequences for the mass and radius of neutron stars.Comment: 12 pages, 12 figures, RevTeX4; summary and conclusions are
strengthened, to appear in PR
Quantitative imaging of the 3-D distribution of cation adsorption sites in undisturbed soil
Several studies have shown that the distribution of cation
adsorption sites (CASs) is patchy at a millimetre to centimetre scale. Often,
larger concentrations of CASs in biopores or aggregate coatings have been
reported in the literature. This heterogeneity has implications on the
accessibility of CASs and may influence the performance of soil system models
that assume a spatially homogeneous distribution of CASs. In this study, we
present a new method to quantify the abundance and 3-D distribution of CASs in
undisturbed soil that allows for investigating CAS densities with distance to
the soil macropores. We used X-ray imaging with Ba<sup>2+</sup> as a contrast
agent. Ba<sup>2+</sup> has a high adsorption affinity to CASs and is widely used as
an index cation to measure the cation exchange capacity (CEC). Eight soil
cores (approx. 10 cm<sup>3</sup>) were sampled from three locations with
contrasting texture and organic matter contents. The CASs of our samples were
saturated with Ba<sup>2+</sup> in the laboratory using BaCl<sub>2</sub> (0.3 mol L<sup>−1</sup>). Afterwards, KCl (0.1 mol L<sup>−1</sup>) was used to rinse out Ba<sup>2+</sup>
ions that were not bound to CASs. Before and after this process the samples
were scanned using an industrial X-ray scanner. Ba<sup>2+</sup> bound to CASs was
then visualized in 3-D by the difference image technique. The resulting
difference images were interpreted as depicting the Ba<sup>2+</sup> bound to CASs
only. The X-ray image-derived CEC correlated significantly with results of
the commonly used ammonium acetate method to determine CEC in well-mixed
samples. The CEC of organic-matter-rich samples seemed to be systematically
overestimated and in the case of the clay-rich samples with less organic
matter the CEC seemed to be systematically underestimated. The results showed
that the distribution of the CASs varied spatially within most of our samples
down to a millimetre scale. There was no systematic relation between the
location of CASs and the soil macropore structure. We are convinced that the approach proposed here will strongly aid the development of more realistic
soil system models
Library of medium-resolution fiber optic echelle spectra of F, G, K, and M field dwarfs to giants stars
We present a library of Penn State Fiber Optic Echelle (FOE) observations of
a sample of field stars with spectral types F to M and luminosity classes V to
I. The spectral coverage is from 3800 AA to 10000 AA with nominal a resolving
power 12000. These spectra include many of the spectral lines most widely used
as optical and near-infrared indicators of chromospheric activity such as the
Balmer lines (H_alpha, H_beta), Ca II H & K, Mg I b triplet, Na I D_{1} and
D_{2}, He I D_{3}, and Ca II IRT lines. There are also a large number of
photospheric lines, which can also be affected by chromospheric activity, and
temperature sensitive photospheric features such as TiO bands. The spectra have
been compiled with the goal of providing a set of standards observed at medium
resolution. We have extensively used such data for the study of active
chromosphere stars by applying a spectral subtraction technique. However, the
data set presented here can also be utilized in a wide variety of ways ranging
from radial velocity templates to study of variable stars and stellar
population synthesis. This library can also be used for spectral classification
purposes and determination of atmospheric parameters (T_eff, log{g}, [Fe/H]). A
digital version of all the fully reduced spectra is available via ftp and the
World Wide Web (WWW) in FITS format.Comment: Latex file with 17 pages, 4 figures. Full postscript (text and
figures) available at http://www.ucm.es/info/Astrof/fgkmsl/FOEfgkmsl.html To
be published in ApJ
First High Contrast Imaging Using a Gaussian Aperture Pupil Mask
Placing a pupil mask with a gaussian aperture into the optical train of
current telescopes represents a way to attain high contrast imaging that
potentially improves contrast by orders of magnitude compared to current
techniques. We present here the first observations ever using a gaussian
aperture pupil mask (GAPM) on the Penn State near-IR Imager and Spectrograph
(PIRIS) at the Mt. Wilson 100 telescope. Two nearby stars were
observed, Eridani and Her A. A faint companion was detected
around Her A, confirming it as a proper motion companion. Furthermore,
the observed H and K magnitudes of the companion were used to constrain its
nature. No companions or faint structure were observed for Eridani.
We found that our observations with the GAPM achieved contrast levels similar
to our coronographic images, without blocking light from the central star. The
mask's performance also nearly reached sensitivities reported for other ground
based adaptive optics coronographs and deep HST images, but did not reach
theoretically predicted contrast levels. We outline ways that could improve the
performance of the GAPM by an order of magnitude or more.Comment: 8 pages, 4 figures, accepted by ApJ letter
Magnetic frustration in the spinel compounds Ge Co_2 O_4 and Ge Ni_2 O_4
In both spinel compounds GeCoO and GeNiO which order
antiferromagnetically (at and , ) with different Curie Weiss temperatures (=80.5 K and -15 K),
the usual magnetic frustration criterion is not fulfilled.
Using neutron powder diffraction and magnetization measurements up to 55 T,
both compounds are found with a close magnetic ground state at low temperature
and a similar magnetic behavior (but with a different energy scale), even
though spin anisotropy and first neighbor exchange interactions are quite
different. This magnetic behavior can be understood when considering the main
four magnetic exchange interactions. Frustration mechanisms are then
enlightened.Comment: submitted to Phys.Rev.B (2006
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