5,794 research outputs found
Small angle neutron scattering contrast variation reveals heterogeneities of interactions in protein gels
The structure of model gluten protein gels prepared in ethanol/water is
investigated by small angle X-ray (SAXS) and neutrons (SANS) scattering. We
show that gluten gels display radically different SAXS and SANS profiles when
the solvent is (at least partially) deuterated. The detailed analysis of the
SANS signal as a function of the solvent deuteration demonstrates
heterogeneities of sample deuteration at different length scales. The
progressive exchange between the protons (H) of the proteins and the deuteriums
(D) of the solvent is inhomogeneous and 60 nm large zones that are enriched in
H are evidenced. In addition, at low protein concentration, in the sol state,
solvent deuteration induces a liquid/liquid phase separation. Complementary
biochemical and structure analyses show that the denser protein phase is more
protonated and specifically enriched in glutenin, the polymeric fraction of
gluten proteins. These findings suggest that the presence of H-rich zones in
gluten gels would arise from the preferential interaction of glutenin polymers
through a tight network of non-exchangeable intermolecular hydrogen bonds.Comment: Soft Matter, Royal Society of Chemistry, 201
Seismic study of stellar convective cores
It has been shown that a discontinuity in the derivatives of the sound speed
at the edge of the convective regions inside a star gives rise to a
characteristic oscillatory signal in the frequencies of stellar oscillations.
This oscillatory signal has been suggested as a means to study the base of the
outer convection zone in low mass stars and possibly the outer edge of the
convective core in high mass stars. Using stellar models we show that because
of a phenomenon similar to aliasing in Fourier transform, it may not be
possible to use this signal to detect the convective core. Nevertheless, it may
be possible to determine the size of convective cores using the frequency
separation \nu_{n+1,l}-\nu_{n,l}.Comment: Accepted for publication in A &
Numerical study of the temperature and porosity effects on the fracture propagation in a 2D network of elastic bonds
This article reports results concerning the fracture of a 2d triangular
lattice of atoms linked by springs. The lattice is submitted to controlled
strain tests and the influence of both porosity and temperature on failure is
investigated. The porosity is found on one hand to decrease the stiffness of
the material but on the other hand it increases the deformation sustained prior
to failure. Temperature is shown to control the ductility due to the presence
of cavities that grow and merge. The rough surfaces resulting from the
propagation of the crack exhibit self-affine properties with a roughness
exponent over a range of length scales which increases
with temperature. Large cavities also have rough walls which are found to be
fractal with a dimension, , which evolves with the distance from the crack
tip. For large distances, is found to be close to 1.5, and close to 1.0 for
cavities just before their coalescence with the main crack
Size Effect in Fracture: Roughening of Crack Surfaces and Asymptotic Analysis
Recently the scaling laws describing the roughness development of fracture
surfaces was proposed to be related to the macroscopic elastic energy released
during crack propagation [Mor00]. On this basis, an energy-based asymptotic
analysis allows to extend the link to the nominal strength of structures. We
show that a Family-Vicsek scaling leads to the classical size effect of linear
elastic fracture mechanics. On the contrary, in the case of an anomalous
scaling, there is a smooth transition from the case of no size effect, for
small structure sizes, to a power law size effect which appears weaker than the
linear elastic fracture mechanics one, in the case of large sizes. This
prediction is confirmed by fracture experiments on wood.Comment: 9 pages, 6 figures, accepted for publication in Physical Review
Power calculation for gravitational radiation: oversimplification and the importance of time scale
A simplified formula for gravitational-radiation power is examined. It is
shown to give completely erroneous answers in three situations, making it
useless even for rough estimates. It is emphasized that short timescales, as
well as fast speeds, make classical approximations to relativistic calculations
untenable.Comment: Three pages, no figures, accepted for publication in Astronomische
Nachrichte
First orbital solution for the non-thermal emitter Cyg OB2 #9
After the first detection of its binary nature, the spectroscopic monitoring
of the non-thermal radio emitter Cyg OB2 #9 (P=2.4yrs) has continued, doubling
the number of available spectra of the star. Since the discovery paper of 2008,
a second periastron passage has occurred in February 2009. Using a variety of
techniques, the radial velocities could be estimated and a first, preliminary
orbital solution was derived from the HeI5876 line. The mass ratio appears
close to unity and the eccentricity is large, 0.7--0.75. X-ray data from 2004
and 2007 are also analyzed in quest of peculiarities linked to binarity. The
observations reveal no large overluminosity nor strong hardness, but it must be
noted that the high-energy data were taken after the periastron passage, at a
time where colliding wind emission may be low. Some unusual X-ray variability
is however detected, with a 10% flux decrease between 2004 and 2007. To clarify
their origin and find a more obvious signature of the wind-wind collision,
additional data, taken at periastron and close to it, are needed.Comment: 15 pages, 4 figures, accepted by Ap
B fields in OB stars (BOB): low-resolution FORS2 spectropolarimetry of the first sample of 50 massive stars
Within the context of the collaboration "B fields in OB stars (BOB)", we used
the FORS2 low-resolution spectropolarimeter to search for a magnetic field in
50 massive stars, including two reference magnetic massive stars. Because of
the many controversies of magnetic field detections obtained with the FORS
instruments, we derived the magnetic field values with two completely
independent reduction and analysis pipelines. We compare and discuss the
results obtained from the two pipelines. We obtained a general good agreement,
indicating that most of the discrepancies on magnetic field detections reported
in the literature are caused by the interpretation of the significance of the
results (i.e., 3-4 sigma detections considered as genuine, or not), instead of
by significant differences in the derived magnetic field values. By combining
our results with past FORS1 measurements of HD46328, we improve the estimate of
the stellar rotation period, obtaining P = 2.17950+/-0.00009 days. For
HD125823, our FORS2 measurements do not fit the available magnetic field model,
based on magnetic field values obtained 30 years ago. We repeatedly detect a
magnetic field for the O9.7V star HD54879, the HD164492C massive binary, and
the He-rich star CPD -57 3509. We obtain a magnetic field detection rate of
6+/-4%, while by considering only the apparently slow rotators we derive a
detection rate of 8+/-5%, both comparable with what was previously reported by
other similar surveys. We are left with the intriguing result that, although
the large majority of magnetic massive stars is rotating slowly, our detection
rate is not a strong function of the stellar rotational velocity.Comment: 20 pages, 10 figures, 4 tables; accepted for publication on Astronomy
& Astrophysic
Seismic detection of acoustic sharp features in the CoRoT target HD49933
The technique of determining the acoustic location of layers of sharp changes
in the sound speed inside a star from the oscillatory signal in its frequencies
is applied on a solar-type star, the CoRoT target, HD49933. We are able to
determine the acoustic depth of the second helium ionisation zone of HD49933 to
be 794 +55/-68 seconds. The acoustic depth of the base of the convective zone
is found to be 1855 +173/-412 seconds where the large error bars reflect the
ambiguity in the result, which is difficult to determine with present precision
on the frequencies because of the intrinsically weak nature of the signal. The
positions of both these layers are consistent with those in a representative
stellar model of HD49933.Comment: Accepted for publication in Astronomy & Astrophysic
Using seismic targets as benchmarks for spectroscopic analyses of cool stars
The frequency of maximum oscillation power measured in dwarfs and giants exhibiting solar-like pulsations provides a precise, and potentially accurate, inference of the stellar surface gravity. An extensive comparison for about 40 well-studied pulsating stars with gravities derived by traditional means (ionization balance, pressure-sensitive spectral features or location with respect to evolutionary tracks) supports the validity of this technique and reveals an overall remarkable agreement with mean differences not exceeding 0.05 dex (although with a dispersion of up to ~0.2 dex). It is argued that interpolation in theoretical isochrones may be the most precise way of estimating the gravity by traditional means in nearby dwarfs. The use of seismic targets as benchmarks in the context of forthcoming large-scale surveys (such as the follow up of the Gaia mission) is briefly discussed.Peer reviewe
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