182 research outputs found
Thin layer shearing of a highly plastic clay
International audienceShearing tests with a thin layer of clay between filter slabs render possible large and cyclic deformations with drainage. In the pressure range of 100 kPa they serve to validated visco-hypoplastic constitutive relations. This theory is also confirmed by tests with up to 14 MPa and super-imposed anti-plane cycles. After this kind of seismic disturbance the clay stabilizes if the ratio of permanent stresses is undercritical. Otherwise a spontaneous acceleration occurs after a delay. This could help to understand critical phenomena with clay smears in faults
Near-infrared Spectroscopy Monitoring of the Collateral Network Prior to, During, and After Thoracoabdominal Aortic Repair: A Pilot Study
ObjectiveThe aim of this study was to evaluate the feasibility of non-invasive monitoring of the paraspinous collateral network (CN) oxygenation prior to, during, and after thoracoabdominal aortic repair in a clinical series.MethodsNear-infrared spectroscopy optodes were positioned bilaterally—over the thoracic and lumbar paraspinous vasculature—to transcutaneously monitor muscle oxygenation of the CN in 20 patients (age: 66 ± 10 years; men = 11) between September 2010 and April 2012; 15 had open thoracoabdominal aortic repair (Crawford II and III), three had thoracic endovascular aortic repair (TEVAR; Crawford I), and two had a hybrid repair (Crawford II). CN oxygenation was continuously recorded until 48 hours postoperatively.ResultsHospital mortality was 5% (n = 1), 15% suffered ischemic spinal cord injury (SCI). Mean thoracic CN oxygenation saturation was 75.5 ± 8% prior to anesthesia (=baseline) without significant variations throughout the procedure (during non-pulsatile cooling on cardiopulmonary bypass and with aortic cross-clamping; range = 70.6–79.5%). Lumbar CN oxygenation (LbS) dropped significantly after proximal aortic cross-clamping to a minimum after 11.7 ± 4 minutes (74 ± 13% of baseline), but fully recovered after restoration of pulsatile flow to 98.5% of baseline. During TEVAR, stent-graft deployment did not significantly affect LbS. Three patients developed relevant SCI (paraplegia n = 1/paraparesis n = 2). In these patients LbS reduction after aortic cross-clamping was significantly lower compared with patients who did not experience SCI (p = .041).ConclusionsNon-invasive monitoring of CN oxygenation prior to, during, and after thoracoabdominal aortic repair is feasible. Lumbar CN oxygenation levels directly respond to compromise of aortic blood circulation
Hubble Space Telescope Astrometry of the Procyon System
The nearby star Procyon is a visual binary containing the F5 IV-V subgiant
Procyon A, orbited in a 40.84 yr period by the faint DQZ white dwarf Procyon B.
Using images obtained over two decades with the Hubble Space Telescope, and
historical measurements back to the 19th century, we have determined precise
orbital elements. Combined with measurements of the parallax and the motion of
the A component, these elements yield dynamical masses of 1.478 +/- 0.012 Msun
and 0.592 +/- 0.006 Msun for A and B, respectively.
The mass of Procyon A agrees well with theoretical predictions based on
asteroseismology and its temperature and luminosity. Use of a standard
core-overshoot model agrees best for a surprisingly high amount of core
overshoot. Under these modeling assumptions, Procyon A's age is ~2.7 Gyr.
Procyon B's location in the H-R diagram is in excellent agreement with
theoretical cooling tracks for white dwarfs of its dynamical mass. Its position
in the mass-radius plane is also consistent with theory, assuming a
carbon-oxygen core and a helium-dominated atmosphere. Its progenitor's mass was
1.9-2.2 Msun, depending on its amount of core overshoot.
Several astrophysical puzzles remain. In the progenitor system, the stars at
periastron were separated by only ~5 AU, which might have led to tidal
interactions and even mass transfer; yet there is no direct evidence that these
have occurred. Moreover the orbital eccentricity has remained high (~0.40). The
mass of Procyon B is somewhat lower than anticipated from the
initial-to-final-mass relation seen in open clusters. The presence of heavy
elements in its atmosphere requires ongoing accretion, but the place of origin
is uncertain.Comment: Accepted by Astrophysical Journa
Galaxy Distances in the Nearby Universe: Corrections For Peculiar Motions
By correcting the redshift--dependent distances for peculiar motions through
a number of peculiar velocity field models, we recover the true distances of a
wide, all-sky sample of nearby galaxies (~ 6400 galaxies with velocities
cz<5500 km/s), which is complete up to the blue magnitude B=14 mag. Relying on
catalogs of galaxy groups, we treat ~2700 objects as members of galaxy groups
and the remaining objects as field galaxies.
We model the peculiar velocity field using: i) a cluster dipole
reconstruction scheme; ii) a multi--attractor model fitted to the Mark II and
Mark III catalogs of galaxy peculiar velocities. According to Mark III data the
Great Attractor has a smaller influence on local dynamics than previously
believed, whereas the Perseus-Pisces and Shapley superclusters acquire a
specific dynamical role. Remarkably, the Shapley structure, which is found to
account for nearly half the peculiar motion of the Local Group, is placed by
Mark III data closer to the zone of avoidance with respect to its optical
position.
Our multi--attractor model based on Mark III data favors a cosmological
density parameter Omega ~ 0.5 (irrespective of a biasing factor of order
unity). Differences among distance estimates are less pronounced in the ~ 2000
- 4000 km/s distance range than at larger or smaller distances. In the last
regions these differences have a serious impact on the 3D maps of the galaxy
distribution and on the local galaxy density --- on small scales.Comment: 24 pages including (9 eps figures and 7 tables). Figures 1,2,3,4 are
available only upon request. Accepted by Ap
The anisotropy of granular materials
The effect of the anisotropy on the elastoplastic response of two dimensional
packed samples of polygons is investigated here, using molecular dynamics
simulation. We show a correlation between fabric coefficients, characterizing
the anisotropy of the granular skeleton, and the anisotropy of the elastic
response. We also study the anisotropy induced by shearing on the subnetwork of
the sliding contacts. This anisotropy provides an explanation to some features
of the plastic deformation of granular media.Comment: Submitted to PR
Development of Stresses in Cohesionless Poured Sand
The pressure distribution beneath a conical sandpile, created by pouring sand
from a point source onto a rough rigid support, shows a pronounced minimum
below the apex (`the dip'). Recent work of the authors has attempted to explain
this phenomenon by invoking local rules for stress propagation that depend on
the local geometry, and hence on the construction history, of the medium. We
discuss the fundamental difference between such approaches, which lead to
hyperbolic differential equations, and elastoplastic models, for which the
equations are elliptic within any elastic zones present .... This displacement
field appears to be either ill-defined, or defined relative to a reference
state whose physical existence is in doubt. Insofar as their predictions depend
on physical factors unknown and outside experimental control, such
elastoplastic models predict that the observations should be intrinsically
irreproducible .... Our hyperbolic models are based instead on a physical
picture of the material, in which (a) the load is supported by a skeletal
network of force chains ("stress paths") whose geometry depends on construction
history; (b) this network is `fragile' or marginally stable, in a sense that we
define. .... We point out that our hyperbolic models can nonetheless be
reconciled with elastoplastic ideas by taking the limit of an extremely
anisotropic yield condition.Comment: 25 pages, latex RS.tex with rspublic.sty, 7 figures in Rsfig.ps.
Philosophical Transactions A, Royal Society, submitted 02/9
V474 Car: A Rare Halo RS CVn Binary in Retrograde Galactic Orbit
We report the discovery that the star V474 Car is an extremely active, high
velocity halo RS CVn system. The star was originally identified as a possible
pre-main sequence star in Carina, given its enhanced stellar activity, rapid
rotation (10.3 days), enhanced Li, and absolute magnitude that places it above
the main sequence. However, its extreme radial velocity (264 km s)
suggested that this system was unlike any previously known pre-MS system. Our
detailed spectroscopic analysis of echelle spectra taken with the CTIO 4-m
finds that V474 Car is both a spectroscopic binary with orbital period similar
to the photometric rotation period, and metal poor ([Fe/H] 0.99). The
star's Galactic orbit is extremely eccentric (e 0.93) with
perigalacticon of only 0.3 kpc of the Galactic center - and its
eccentricity and smallness of its perigalacticon are only surpassed by
0.05%, of local F/G-type field stars. The observed characteristics are
consistent with V474 Car being a high velocity, metal poor, tidally-locked
chromospherically active binary (CAB), i.e.\ a halo RS CVn binary, and one of
only a few such specimens known.Comment: Accepted to Astronomical Journa
Large scale diffuse light in the Coma cluster: a multi-scale approach
We have obtained wide field images of the Coma cluster in the B, V, R and I
bands with the CFH12K camera at CFHT. In order to search for large scale
diffuse emission, we have applied to these images an iterative multi scale
wavelet analysis and reconstruction technique which allowed to model all the
sources (stars and galaxies) and subtract them from the original images. We
found various concentrations of diffuse emission present in the central zone
around the central galaxies NGC4874 and NGC4889. We characterize the positions,
sizes and colors of these concentrations. Some sources do not seem to have
strong star formation, while another one probably exhibits spiral-like color.
One possible origin for the star forming diffuse emission sources is that in
the region of the two main galaxies NGC4874 and NGC4889 spiral galaxies have
recently been disrupted and star formation is still active in the dispersed
material. We also use the characteristics of the sources of diffuse emission to
trace the cluster dynamics. A scenario in which the group around NGC 4874 is
moving north is consistent with our data.Comment: 11 pages, accepted in A&A, jpg figure
The Nearby Optical Galaxy Sample: The Local Galaxy Luminosity Function
In this paper we derive the galaxy luminosity function from the Nearby
Optical Galaxy (NOG) sample, which is a nearly complete, magnitude-limited
(B<14 mag), all-sky sample of nearby optical galaxies (~6400 galaxies with
cz<5500 km/s). For this local sample, we use galaxy distance estimates based on
different peculiar velocity models. Therefore, the derivation of the luminosity
function is carried out using the locations of field and grouped galaxies in
real distance space.
The local field galaxy luminosity function in the B system is well described
by a Schechter function. The exact values of the Schechter parameters slightly
depend on the adopted peculiar velocity field models.
The shape of the luminosity function of spiral galaxies does not differ
significantly from that of E-S0 galaxies. On the other hand, the late-type
spirals and irregulars have a very steeply rising luminosity function towards
the faint end, whereas the ellipticals appreciably decrease in number towards
low luminosities.
The presence of galaxy systems in the NOG sample does not affect
significantly the field galaxy luminosity function, since environmental effects
on the total luminosity function appear to be marginal.Comment: 35 pages including 7 figures and 4 tables. Accepted for publication
in Ap
Cool core remnants in galaxy clusters
X ray clusters are conventionally divided into two classes: "cool core" (CC)
and "non cool core" (NCC) objects, on the basis of the observational properties
of their central regions. Recent results have shown that the cluster population
is bimodal (Cavagnolo et al. 2009). We want to understand whether the observed
distribution of clusters is due to a primordial division into two distinct
classes rather than to differences in how these systems evolve across cosmic
time. We systematically search the ICM of NCC clusters in a subsample of the
B55 flux limited sample of clusters for regions which have some characteristics
typical of cool cores, namely low entropy gas and high metal abundance We find
that most NCC clusters in our sample host regions reminiscent of CC, i. e.
characterized by relative low entropy gas (albeit not as low as in CC systems)
and a metal abundance excess. We have dubbed these structures "cool core
remnants", since we interpret them as what remains of a cool core after a
heating event (AGN giant outbursts in a few cases and more commonly mergers).
We infer that most NCC clusters have undergone a cool core phase during their
life. The fact that most cool core remnants are found in dynamically active
objects provides strong support to scenarios where cluster core properties are
not fixed "ab initio" but evolve across cosmic time.Comment: Accepted for publication in Astronomy & Astrophysics. Version with
full resolution figures available at:
http://www.iasf-milano.inaf.it/~rossetti/public/CCR/rossetti.pd
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