1,280 research outputs found
Detection of arcs in Saturn's F ring during the 1995 Sun ring-plane crossing
Observations of the November 1995 Sun crossing of the Saturn's ring-plane
made with the 3.6m CFH telescope, using the UHAO adaptive optics system, are
presented here. We report the detection of four arcs located in the vicinity of
the F ring. They can be seen one day later in HST images. The combination of
both data sets gives accurate determinations of their orbits. Semi-major axes
range from 140020 km to 140080 km, with a mean of 140060 +- 60 km. This is
about 150 km smaller than previous estimates of the F ring radius from Voyager
1 and 2 data, but close to the orbit of another arc observed at the same epoch
in HST images.Comment: 8 pages, 3 figures, 1 table, To appear in A&A, for comments :
[email protected]
Bulk, rare earth and other trace elements in Apollo 14 and 15 and Luna 16 samples
The chemical abundances were measured by instrumental and radiochemical neutron activation analysis in a variety of lunar specimens. Apollo 14 soils are characterized by significant enrichments of Al2O3, Na2O and K2O and depletions of TiO2, FeO, MnO and Cr2O3 relative to Apollo 11 and to most of Apollo 12 soils. The uniform abundances in 14230 core tube soils and three other Apollo 14 soils indicate that the regolith is uniform to at least 22 cm depth and within approximately 200 m from the lunar module. Two Luna 16 breccias are similar in composition to Luna 16 soils. Four Apollo 15 soils (LM, STA 4, 9, and 9a) have variable compositions. Interelement correlations between MnO-FeO, Sc-FeO, V-Cr2O3 and K2O-Hf negate the hypothesis that howardite achondrites may be primitive lunar matter, argue against the fission hypothesis for the origin of the moon, and precludes any selective large scale volatilization of alkalies during lunar magmatic events
Validity of the Brunet-Derrida formula for the speed of pulled fronts with a cutoff
We establish rigorous upper and lower bounds for the speed of pulled fronts
with a cutoff. We show that the Brunet-Derrida formula corresponds to the
leading order expansion in the cut-off parameter of both the upper and lower
bounds. For sufficiently large cut-off parameter the Brunet-Derrida formula
lies outside the allowed band determined from the bounds. If nonlinearities are
neglected the upper and lower bounds coincide and are the exact linear speed
for all values of the cut-off parameter.Comment: 8 pages, 3 figure
Relaxation times for Hamiltonian systems
Usually, the relaxation times of a gas are estimated in the frame of the
Boltzmann equation. In this paper, instead, we deal with the relaxation problem
in the frame of the dynamical theory of Hamiltonian systems, in which the
definition itself of a relaxation time is an open question. We introduce a
lower bound for the relaxation time, and give a general theorem for estimating
it. Then we give an application to a concrete model of an interacting gas, in
which the lower bound turns out to be of the order of magnitude of the
relaxation times observed in dilute gases.Comment: 26 page
The Christiansen Effect in Saturn's narrow dusty rings and the spectral identification of clumps in the F ring
Stellar occultations by Saturn's rings observed with the Visual and Infrared
Mapping Spectrometer (VIMS) onboard the Cassini spacecraft reveal that dusty
features such as the F ring and the ringlets in the Encke and the Laplace Gaps
have distinctive infrared transmission spectra. These spectra show a narrow
optical depth minimum at wavelengths around 2.87 microns. This minimum is
likely due to the Christiansen Effect, a reduction in the extinction of small
particles when their (complex) refractive index is close to that of the
surrounding medium. Simple Mie-scattering models demonstrate that the strength
of this opacity dip is sensitive to the size distribution of particles between
1 and 100 microns across. Furthermore, the spatial resolution of the
occultation data is sufficient to reveal variations in the transmission spectra
within and among these rings. For example, in both the Encke Gap ringlets and F
ring, the opacity dip weakens with increasing local optical depth, which is
consistent with the larger particles being concentrated near the cores of these
rings. The strength of the opacity dip varies most dramatically within the F
ring; certain compact regions of enhanced optical depth lack an opacity dip and
therefore appear to have a greatly reduced fraction of grains in the few-micron
size range.Such spectrally-identifiable structures probably represent a subset
of the compact optically-thick clumps observed by other Cassini instruments.
These variations in the ring's particle size distribution can provide new
insights into the processes of grain aggregation, disruption and transport
within dusty rings. For example, the unusual spectral properties of the F-ring
clumps could perhaps be ascribed to small grains adhering onto the surface of
larger particles in regions of anomalously low velocity dispersion.Comment: 42 pages, 15 figures, accepted for publication in Icarus. A few small
typographical errors fixed to match correction in proof
Experimental observation of extreme multistability in an electronic system of two coupled R\"{o}ssler oscillators
We report the first experimental observation of extreme multistability in a
controlled laboratory investigation. Extreme multistability arises when
infinitely many attractors coexist for the same set of system parameters. The
behavior was predicted earlier on theoretical grounds, supported by numerical
studies of models of two coupled identical or nearly identical systems. We
construct and couple two analog circuits based on a modified coupled
R\"{o}ssler system and demonstrate the occurrence of extreme multistability
through a controlled switching to different attractor states purely through a
change in initial conditions for a fixed set of system parameters. Numerical
studies of the coupled model equations are in agreement with our experimental
findings.Comment: to be published in Phys. Rev.
On the Integrability, B\"Acklund Transformation and Symmetry Aspects of a Generalized Fisher Type Nonlinear Reaction-Diffusion Equation
The dynamics of nonlinear reaction-diffusion systems is dominated by the
onset of patterns and Fisher equation is considered to be a prototype of such
diffusive equations. Here we investigate the integrability properties of a
generalized Fisher equation in both (1+1) and (2+1) dimensions. A Painlev\'e
singularity structure analysis singles out a special case () as
integrable. More interestingly, a B\"acklund transformation is shown to give
rise to a linearizing transformation for the integrable case. A Lie symmetry
analysis again separates out the same case as the integrable one and
hence we report several physically interesting solutions via similarity
reductions. Thus we give a group theoretical interpretation for the system
under study. Explicit and numerical solutions for specific cases of
nonintegrable systems are also given. In particular, the system is found to
exhibit different types of travelling wave solutions and patterns, static
structures and localized structures. Besides the Lie symmetry analysis,
nonclassical and generalized conditional symmetry analysis are also carried
out.Comment: 30 pages, 10 figures, to appear in Int. J. Bifur. Chaos (2004
On a Conjecture of Goriely for the Speed of Fronts of the Reaction--Diffusion Equation
In a recent paper Goriely considers the one--dimensional scalar
reaction--diffusion equation with a polynomial reaction
term and conjectures the existence of a relation between a global
resonance of the hamiltonian system and the asymptotic
speed of propagation of fronts of the reaction diffusion equation. Based on
this conjecture an explicit expression for the speed of the front is given. We
give a counterexample to this conjecture and conclude that additional
restrictions should be placed on the reaction terms for which it may hold.Comment: 9 pages Revtex plus 4 postcript figure
Does diabetes mellitus influence pathologic complete response and tumor downstaging after neoadjuvant chemoradiation for esophageal and gastroesophageal cancer? A two-institution report.
BACKGROUND: Esophageal carcinoma is an aggressive disease that is often treated with neoadjuvant therapy followed by surgical resection. Diabetes mellitus (DM) has been associated with reduced efficacy of chemoradiation (CRT) in other gastrointestinal cancers. The goal of this study was to determine if DM affects response to neoadjuvant CRT in the management of gastroesophageal carcinoma.
METHODS: We retrospectively reviewed the esophageal cancer patient databases and subsequently analyzed those patients who received neoadjuvant CRT followed by surgical resection at two institutions, Thomas Jefferson University (TJUH) and Fox Chase Cancer Center (FCCC). Comparative analyses of rates of pathologic complete response rate (pCR) and pathologic downstaging in DM patients versus non-DM patients was performed.
RESULTS: Two hundred sixty patients were included in the study; 36 patients had DM and 224 were non-diabetics. The average age of the patients was 61 years (range 24-84 years). The overall pCR was 26%. The pCR rate was 19% and 27% for patients with DM and without DM, respectively (P = 0.31). Pathologic downstaging occurred in 39% of study patients, including of 33% of DM patients and 40% of non-DM patients (P = 0.42).
CONCLUSIONS: Although the current analysis does not demonstrate a significant reduction in pCR rates or pathologic downstaging in patients with DM, the observed trend suggests that a potential difference may be observed with a larger patient population. Further studies are warranted to evaluate the influence of DM on the effectiveness of neoadjuvant CRT in esophageal cancer
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