315 research outputs found
Torsional birefringence in metric-affine Chern-Simons gravity: gravitational waves in late-time cosmology
In the context of the metric-affine Chern-Simons gravity endowed with
projective invariance, we derive analytical solutions for torsion and
nonmetricity in the homogeneous and isotropic cosmological case, described by a
flat Friedmann-Robertson-Walker metric. We describe in some details the general
properties of the cosmological solutions in the presence of a perfect fluid,
such as dynamical stability and the settling of big bounce points, and we
discuss the structure of some specific solutions reproducing de Sitter and
power law behaviours for the scale factor. Then, we focus on first-order
perturbations in the de Sitter scenario, and we study the propagation of
gravitational waves in the adiabatic limit, looking at tensor and scalar
polarizations. In particular, we find that metric tensor modes couple to
torsion tensor components, leading to the appearance, as in the metric version
of Chern-Simons gravity, of birefringence, described by different dispersion
relations for the left and right circularized polarization states. As a result,
the purely tensor part of torsion propagates like a wave, while nonmetricity
decouples and behaves like a harmonic oscillator. Finally, we discuss scalar
modes, outlining as they decay exponentially in time and do not propagate.Comment: References adde
Crack Front Waves and the dynamics of a rapidly moving crack
Crack front waves are localized waves that propagate along the leading edge
of a crack. They are generated by the interaction of a crack with a localized
material inhomogeneity. We show that front waves are nonlinear entities that
transport energy, generate surface structure and lead to localized velocity
fluctuations. Their existence locally imparts inertia, which is not
incorporated in current theories of fracture, to initially "massless" cracks.
This, coupled to crack instabilities, yields both inhomogeneity and scaling
behavior within fracture surface structure.Comment: Embedded Latex file including 4 figure
Elastic forces that do no work and the dynamics of fast cracks
Elastic singularities such as crack tips, when in motion through a medium
that is itself vibrating, are subject to forces orthogonal to the direction of
motion and thus impossible to determine by energy considerations alone. This
fact is used to propose a universal scenario, in which three dimensionality is
essential, for the dynamic instability of fast cracks in thin brittle
materials.Comment: 8 pages Latex, 1 Postscript figur
Arrested Cracks in Nonlinear Lattice Models of Brittle Fracture
We generalize lattice models of brittle fracture to arbitrary nonlinear force
laws and study the existence of arrested semi-infinite cracks. Unlike what is
seen in the discontinuous case studied to date, the range in driving
displacement for which these arrested cracks exist is very small. Also, our
results indicate that small changes in the vicinity of the crack tip can have
an extremely large effect on arrested cracks. Finally, we briefly discuss the
possible relevance of our findings to recent experiments.Comment: submitted to PRE, Rapid Communication
Submillimetre point sources from the Archeops experiment: Very Cold Clumps in the Galactic Plane
Archeops is a balloon-borne experiment, mainly designed to measure the Cosmic
Microwave Background (CMB) temperature anisotropies at high angular resolution
(~ 12 arcminutes). By-products of the mission are shallow sensitivity maps over
a large fraction of the sky (about 30 %) in the millimetre and submillimetre
range at 143, 217, 353 and 545 GHz. From these maps, we produce a catalog of
bright submillimetre point sources. We present in this paper the processing and
analysis of the Archeops point sources. Redundancy across detectors is the key
factor allowing to sort out glitches from genuine point sources in the 20
independent maps. We look at the properties of the most reliable point sources,
totalling 304. Fluxes range from 1 to 10,000 Jy (at the frequencies covering
143 to 545 GHz). All sources are either planets (2) or of galactic origin.
Longitude range is from 75 to 198 degrees. Some of the sources are associated
with well-known Lynds Nebulae and HII compact regions in the galactic plane. A
large fraction of the sources have an IRAS counterpart. Except for Jupiter,
Saturn, the Crab and Cas A, all sources show a dust-emission-like modified
blackbody emission spectrum. Temperatures cover a range from 7 to 27 K. For the
coldest sources (T<10 K), a steep nu^beta emissivity law is found with a
surprising beta ~ 3 to 4. An inverse relationship between T and beta is
observed. The number density of sources at 353 GHz with flux brighter than 100
Jy is of the order of 1 per degree of Galactic longitude. These sources will
provide a strong check for the calibration of the Planck HFI focal plane
geometry as a complement to planets. These very cold sources observed by
Archeops should be prime targets for mapping observations by the Akari and
Herschel space missions and ground--based observatories.Comment: Version matching the published article (English improved). Published
in Astron. Astrophys, 21 pages, 13 figures, 4 tables Full article (with
complete tables) can be retrieved at
http://www.archeops.org/Archeops_Publicatio
Time resolved particle dynamics in granular convection
We present an experimental study of the movement of individual particles in a
layer of vertically shaken granular material. High-speed imaging allows us to
investigate the motion of beads within one vibration period. This motion
consists mainly of vertical jumps, and a global ordered drift. The analysis of
the system movement as a whole reveals that the observed bifurcation in the
flight time is not adequately described by the Inelastic Bouncing Ball Model.
Near the bifurcation point, friction plays and important role, and the branches
of the bifurcation do not diverge as the control parameter is increased. We
quantify the friction of the beads against the walls, showing that this
interaction is the underlying mechanism responsible for the dynamics of the
flow observed near the lateral wall
Submillimeter dust emission of the M17 complex measured with PRONAOS
We map a 50' x 30' area in and around the M17 molecular complex with the
French submillimeter balloon-borne telescope PRONAOS, in order to better
understand the thermal emission of cosmic dust and the structure of the
interstellar medium. The PRONAOS-SPM instrument has an angular resolution of
about 3', corresponding to a size of 2 pc at the distance of this complex, and
a high sensitivity up to 0.8 MJy/sr. The observations are made in four wide
submillimeter bands corresponding to effective wavelengths of 200, 260, 360 and
580 um. Using an improved map-making method for PRONAOS data, we map the M17
complex and faint condensations near the dense warm core. We derive maps of
both the dust temperature and the spectral index, which vary over a wide range,
from about 10 K to 100 K for the temperature and from about 1 to 2.5 for the
spectral index. We show that these parameters are anticorrelated, the cold
areas (10-20 K) having a spectral index around 2, whereas the warm areas have a
spectral index between 1 and 1.5. We discuss possible causes of this effect,
and we propose an explanation involving intrinsic variations of the grain
properties. Indeed, to match the observed spectra with two dust components
having a spectral index equal to 2 leads to very large and unlikely amounts of
cold dust. We also give estimates of the column densities and masses of the
studied clumps. Three cold clumps (14-17 K) could be gravitationally unstable.Comment: 16 pages, 4 figures, accepted June 2002 in Astronomy & Astrophysic
Submillimeter Emission from Water in the W3 Region
We have mapped the submillimeter emission from the 1(10)-1(01) transition of
ortho-water in the W3 star-forming region. A 5'x5' map of the W3 IRS4 and W3
IRS5 region reveals strong water lines at half the positions in the map. The
relative strength of the Odin lines compared to previous observations by SWAS
suggests that we are seeing water emission from an extended region. Across much
of the map the lines are double-peaked, with an absorption feature at -39 km/s;
however, some positions in the map show a single strong line at -43 km/s. We
interpret the double-peaked lines as arising from optically thick,
self-absorbed water emission near the W3 IRS5, while the narrower blue-shifted
lines originate in emission near W3 IRS4. In this model, the unusual appearance
of the spectral lines across the map results from a coincidental agreement in
velocity between the emission near W3 IRS4 and the blue peak of the more
complex lines near W3 IRS5. The strength of the water lines near W3 IRS4
suggests we may be seeing water emission enhanced in a photon-dominated region.Comment: Accepted to A&A Letters as part of the special Odin issue; 4 page
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