761 research outputs found
Matrix deformation of marls in a foreland fold-and-thrust belt: The example of the eastern Jaca basin, southern Pyrenees
In this study, we used the Anisotropy of Magnetic Susceptibility (AMS) to investigate the matrix strain record of two calcareous shale formations, the Eocene Larrès and Pamplona Marls, along the eastern Jaca foreland fold-and-thrust belt (Southern Pyrenees). More than 1000 unoriented fragments, collected from 62 sites along 4 sub-parallel sections in the footwall of the regional Oturia thrust and through local Yebra anticline, were measured. The analysis of the degree of anisotropy (P’) and shape parameter (T) allowed to identify four types of magnetic fabrics. Type II fabrics associated with poorly deformed rocks are characterized by a relatively high anisotropy and an oblate shape. In contrast, type III fabrics, associated with strongly fractured rocks are characterized by the lowest anisotropy and a triaxial shape. Type IV and type V fabrics are characterized by increasing anisotropy and shape parameters, and are associated with the development of a weak to a slaty cleavage in rocks. The distribution of the magnetic fabric is roughly similar along the four studied sections. In the footwall of the Oturia thrust, magnetic fabrics evolve from the type V to type II over a 1000 m-long interval. By contrast, the distribution of magnetic fabric is roughly symmetric across the Yebra anticline, evolving from a dominating type II fabric in both limbs to mixed type III-type V fabrics within the 1 km-large hinge zone. The succession of the magnetic fabrics is interpreted as recording various degrees of matrix strain in response to thrusting and folding. The correlation of magnetic fabrics between the four sections highlights some along-strike variations in the extension of fabric domains that are interpreted as reflecting the local influence of 2nd-order factors, such as the syn-tectonic sedimentation. Results are integrated within the tectono-sedimentary framework of the studied area to propose a model of matrix strain related to the tectonic and sedimentary evolution of a foreland fold and thrust belt
Probing Dark Matter
Recent novel observations have probed the baryonic fraction of the galactic
dark matter that has eluded astronomers for decades. Late in 1993, the MACHO
and EROS collaborations announced in this journal the detection of transient
and achromatic brightenings of a handful of stars in the Large Magellanic Cloud
that are best interpreted as gravitational microlensing by low-mass foreground
objects (MACHOS). This tantalized astronomers, for it implied that the
population of cool, compact objects these lenses represent could be the elusive
dark matter of our galactic halo. A year later in 1994, Sackett et al. reported
the discovery of a red halo in the galaxy NGC 5907 that seems to follow the
inferred radial distribution of its dark matter. This suggested that dwarf
stars could constitute its missing component. Since NGC 5907 is similar to the
Milky Way in type and radius, some surmised that the solution of the galactic
dark matter problem was an abundance of ordinary low-mass stars. Now Bahcall et
al., using the Wide-Field Camera of the recently repaired Hubble Space
Telescope, have dashed this hope.Comment: 3 pages, Plain TeX, no figures, published as a News and Views in
Nature 373, 191 (1995
Self-Lensing Models of the LMC
All of the proposed explanations for the microlensing events observed towards
the LMC have difficulties. One of these proposed explanations, LMC
self-lensing, which invokes ordinary LMC stars as the long sought-after lenses,
has recently gained considerable popularity as a possible solution to the
microlensing conundrum. In this paper, we carefully examine the set of LMC
self-lensing models. In particular, we review the pertinent observations made
of the LMC, and show how these observations place limits on such self-lensing
models. We find that, given current observational constraints, no purely LMC
disk models are capable of producing optical depths as large as that reported
in the MACHO collaboration 2-year analysis. Besides pure disk, we also consider
alternate geometries, and present a framework which encompasses the previous
studies of LMC self-lensing. We discuss which model parameters need to be
pushed in order for such models to succeed. For example, like previous workers,
we find that an LMC halo geometry may be able to explain the observed events.
However, since all known LMC tracer stellar populations exhibit disk-like
kinematics, such models will have difficulty being reconciled with
observations. For SMC self-lensing, we find predicted optical depths differing
from previous results, but more than sufficient to explain all observed SMC
microlensing. In contrast, for the LMC we find a self-lensing optical depth
contribution between 0.47e-8 and 7.84e-8, with 2.44e-8 being the value for the
set of LMC parameters most consistent with current observations.Comment: 20 pages, Latex, 14 figures, submitted to Ap
Possible Cosmological Implications of the Quark-Hadron Phase Transition
We study the quark-hadron phase transition within an effective model of QCD,
and find that in a reasonable range of the main parameters of the model, bodies
with quark content between and 10 solar masses can have been formed
in the early universe. In addition, we show that a significant amount of
entropy is released during the transition. This may imply the existence of a
higher baryon number density than what is usually expected at temperatures
above the QCD scale. The cosmological QCD transition may then provide a natural
way for decreasing the high baryon asymmetry created by an Affleck-Dine like
mechanism down to the value required by primordial nucleosynthesis.Comment: 19 pages, LaTeX, 5 Postscript figures included. Submitted to Journal
of Physics
The MACHO Project: Microlensing Results from 5.7 Years of LMC Observations
We report on our search for microlensing towards the Large Magellanic Cloud
(LMC). Analysis of 5.7 years of photometry on 11.9 million stars in the LMC
reveals 13 - 17 microlensing events. This is significantly more than the
2 to 4 events expected from lensing by known stellar populations. The
timescales (\that) of the events range from 34 to 230 days. We estimate the
microlensing optical depth towards the LMC from events with 2 < \that < 400
days to be 1.2 ^{+0.4}_ {-0.3} \ten{-7} 0.15 \msun 0.9 \msun$, depending on the halo model, and the total mass in MACHOs out
to 50 kpc is found to be 9+4-3 10^{10} msun, independent of the halo model.
These results are marginally consistent with our previous results, but are
lower by about a factor of two. Besides a larger data set, this work also
includes an improved efficiency determination, improved likelihood analysis,
and more thorough testing of systematic errors, especially with respect to the
treatment of potential backgrounds to microlensing, such as supernovae in
galaxies behind the LMC. [Abridged]Comment: 53 pages, Latex with 12 postscript figures, submitted to Ap
Dark halo baryons not in ancient halo white dwarfs
Having ruled out the possibility that stellar objects are the main
contributor of the dark matter embedding galaxies, microlensing experiments
cannot exclude the hypothesis that a significant fraction of the Milky Way dark
halo might be made of MACHOs with masses in the range 0.5-0.8 \msun. Ancient
white dwarfs are generally considered the most plausible candidates for such
MACHOs. We report the results of a search for such white dwarfs in a proper
motion survey covering a 0.16 sqd field at three epochs at high galactic
latitude, and 0.938 sqd at two epochs at intermediate galactic latitude (VIRMOS
survey), using the CFH telescope. Both surveys are complete to I = 23, with
detection efficiency fading to 0 at I = 24.2. Proper motion data are suitable
to separate unambiguously halo white dwarfs identified by belonging to a non
rotating system. No candidates were found within the colour-magnitude-proper
motion volume where such objects can be safely discriminated from any standard
population as well as from possible artefacts. In the same volume, we estimate
the maximum white dwarf halo fraction compatible with this observation at
different significance levels if the halo is at least 14 gigayears old and
under different ad hoc initial mass functions. Our data alone rules out a halo
fraction greater than 14% at 95% confidence level. Combined with two previous
investigations exploring comparable volumes pushes the limit below 4 % (95%
confidence level) or below 1.3% (64% confidence), this implies that if baryonic
dark matter is present in galaxy halos, it is not, or it is only marginally in
the form of faint hydrogen white dwarfs.Comment: accepted in Astronomy and Astrophysics (19-05-2004
Development and evaluation of the Walking Estimated-Limitation Calculated by History questionnaire in patients with claudication
BACKGROUND: The Walking Impairment Questionnaire (WIQ) is used to estimate walking impairment in patients with peripheral artery disease; however, it faces frequent errors when self-completed and is complex to score. We aimed to validate an alternative, easily scored four-item tool, the Walking Estimated-Limitation Calculated by History (WELCH) questionnaire. METHODS: The WIQ and WELCH were prospectively tested in five centers. We studied 434 patients, among which 298 had a treadmill test (3.2 km/h; 10% slope) to determine their maximum walking time (MWT), and 30 were seen twice during the study period. RESULTS: After self-completion, we found at least one error in 177 WIQ (40.8%; 95% confidence interval [CI], 36.3%-45.5%) vs 56 WELCH (12.9%; 95% CI, 10.1%-16.4%) questionnaires (P < .0001). When scoring only questionnaires without missing or duplicate answers, 267 WIQ (61.5%; 95% CI, 56.9%-66.0%) vs 393 WELCH (90.6%; 95% CI, 87.4%-93.0%) questionnaires could be scored (P < .001). The median MWT was 233 seconds (interquartile range, 133-654 seconds) for the 298 patients who had a treadmill test. When the 296 patients who had both questionnaire scores available were studied, no difference was found between the Pearson r coefficient of correlation of the WIQ (r = 0.615) and the WELCH (r = 0.653) with MWT (P = .211). In the 30 patients who completed the WELCH twice, correlation was r = 0.839 (P < .001) between the two scores in 22 nonrevascularized patients, and the area under the receiver-operating characteristic curve was 0.830 +/- 0.105 (P < .01) to discriminate the eight revascularized from the 22 nonrevascularized patients. CONCLUSIONS: The WELCH questionnaire is a simple tool to estimate walking limitation in patients with suspected peripheral artery disease. It is easily scored by mental calculation. It may help to standardize the estimation of walking limitation in routine clinical practice
Gravitational Lensing by Dark Matter Caustics
Dark matter caustics have specific density profiles and, therefore, precisely
calculable gravitational lensing properties. We present a formalism which
simplifies the relevant calculations, and apply it to four specific cases. In
the first three, the line of sight is tangent to a smooth caustic surface. The
curvature of the surface at the tangent point is positive, negative or zero. In
the fourth case the line of sight passes near a cusp. For each we derive the
map between the image and source planes. In some cases, a point source has
multiple images and experiences infinite magnification when the images merge.
Unfortunately, for the dark matter caustics expected in realistic galactic halo
models, the angular resolution required to resolve the multiple images is not
presently achievable. A more promising approach aims to observe the distortions
caused by dark matter caustics in the images of extended sources such as radio
jets.Comment: 36 pages, 11 figure
Cosmological constraints on primordial black holes produced in the near-critical gravitational collapse
The mass function of primordial black holes created through the near-critical
gravitational collapse is calculated in a manner fairly independent of the
statistical distribution of underlying density fluctuation, assuming that it
has a sharp peak on a specific scale. Comparing it with various cosmological
constraints on their mass spectrum, some newly excluded range is found in the
volume fraction of the region collapsing into black holes as a function of the
horizon mass.Comment: 9 pages. Typos corrected. To appear in Physical Review
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