833 research outputs found
The s Process: Nuclear Physics, Stellar Models, Observations
Nucleosynthesis in the s process takes place in the He burning layers of low
mass AGB stars and during the He and C burning phases of massive stars. The s
process contributes about half of the element abundances between Cu and Bi in
solar system material. Depending on stellar mass and metallicity the resulting
s-abundance patterns exhibit characteristic features, which provide
comprehensive information for our understanding of the stellar life cycle and
for the chemical evolution of galaxies. The rapidly growing body of detailed
abundance observations, in particular for AGB and post-AGB stars, for objects
in binary systems, and for the very faint metal-poor population represents
exciting challenges and constraints for stellar model calculations. Based on
updated and improved nuclear physics data for the s-process reaction network,
current models are aiming at ab initio solution for the stellar physics related
to convection and mixing processes. Progress in the intimately related areas of
observations, nuclear and atomic physics, and stellar modeling is reviewed and
the corresponding interplay is illustrated by the general abundance patterns of
the elements beyond iron and by the effect of sensitive branching points along
the s-process path. The strong variations of the s-process efficiency with
metallicity bear also interesting consequences for Galactic chemical evolution.Comment: 53 pages, 20 figures, 3 tables; Reviews of Modern Physics, accepte
s-Process Studies In the Light of New Experimental Cross Sections: Distribution of Neutron Fluences and r-Process Residuals
A best set of neutron-capture cross sections has been evaluated for the most important s-process isotopes. With this data base, s-process studies have been carried out using the traditional model which assumes a steady neutron flux and an exponential distribution of neutron irradiations. The calculated sigma-N curve is in excellent agreement with the empirical sigma-N-values of pure s-process nuclei. Simultaneously, good agreement is found between the difference of solar and s-process abundances and the abundances of pure r-process nuclei. The abundance pattern of the iron group elements where s-process results complement the abundances obtained from explosive nuclear burning is discussed. The results obtained from the traditional s-process model such as seed abundances, mean neutron irradiations, or neutron densities are compared to recent stellar model calculations which assume the He-burning shells of red giant stars as the site for the s-process
A generalized Cahn-Hilliard equation for biological applications
Recently we considered a stochastic discrete model which describes fronts of
cells invading a wound \cite{KSS}. In the model cells can move, proliferate,
and experience cell-cell adhesion. In this work we focus on a continuum
description of this phenomenon by means of a generalized Cahn-Hilliard equation
(GCH) with a proliferation term. As in the discrete model, there are two
interesting regimes. For subcritical adhesion, there are propagating "pulled"
fronts, similarly to those of Fisher-Kolmogorov equation. The problem of front
velocity selection is examined, and our theoretical predictions are in a good
agreement with a numerical solution of the GCH equation. For supercritical
adhesion, there is a nontrivial transient behavior, where density profile
exhibits a secondary peak. To analyze this regime, we investigated relaxation
dynamics for the Cahn-Hilliard equation without proliferation. We found that
the relaxation process exhibits self-similar behavior. The results of continuum
and discrete models are in a good agreement with each other for the different
regimes we analyzed.Comment: 11 figures, submitted to PR
Assessment of bulbar function in amyotrophic lateral sclerosis: validation of a self-report scale (Center for Neurologic Study Bulbar Function Scale).
BACKGROUND AND PURPOSE: Impaired bulbar functions of speech and swallowing are among the most serious consequences of amyotrophic lateral sclerosis (ALS). Despite this, clinical trials in ALS have rarely emphasized bulbar function as an endpoint. The rater-administered Amyotrophic Lateral Sclerosis Functional Rating Scale-Revised (ALSFRS-R) or various quality-of-life measures are commonly used to measure symptomatic benefit. Accordingly, we sought to evaluate the utility of measures specific to bulbar function in ALS.
METHODS: We assessed bulbar functions in 120 patients with ALS, with clinicians first making direct observations of the degree of speech, swallowing and salivation impairment in these subjects. Clinical diagnosis of bulbar impairment was then compared with ALSFRS-R scores, speech rate, time to swallow liquids and solids, and scores obtained when patients completed visual analog scales (VASs) and the newly-developed 21-question self-administered Center for Neurologic Study Bulbar Function Scale (CNS-BFS).
RESULTS: The CNS-BFS, ALSFRS-R, VAS and timed speech and swallowing were all concordant with clinician diagnosis. The self-report CNS-BFS and ALSFRS-R bulbar subscale best predicted clinician diagnosis with misclassification rates of 8% and 14% at the optimal cut-offs, respectively. In addition, the CNS-BFS speech and swallowing subscales outperformed both the bulbar component of the ALSFRS-R and speech and swallowing VASs when correlations were made between these scales and objective measures of timed reading and swallowing.
CONCLUSIONS: Based on these findings and its relative ease of administration, we conclude that the CNS-BFS is a useful metric for assessing bulbar function in patients with ALS
School performance in Australia: is there a role for quasi-markets?
Recent changes to the organisation of Australia's education system have raised the possibility of implementing wide-ranging market reforms. In this article we discuss the scope for introducing reforms similar to the United Kingdom's 'quasi-market' model. We discuss the role of school league tables in providing signals and incentives in a quasi-market. Specifically, we compare a range of unadjusted and model-based league tables of primary school performance in Queensland's public education system. These comparisons indicate that model-based tables which account for socio-economic status and student intake quality vary significantly from the unadjusted tables
Quantum feedback control of a superconducting qubit: Persistent Rabi oscillations
The act of measurement bridges the quantum and classical worlds by projecting
a superposition of possible states into a single, albeit probabilistic,
outcome. The time-scale of this "instantaneous" process can be stretched using
weak measurements so that it takes the form of a gradual random walk towards a
final state. Remarkably, the interim measurement record is sufficient to
continuously track and steer the quantum state using feedback. We monitor the
dynamics of a resonantly driven quantum two-level system -- a superconducting
quantum bit --using a near-noiseless parametric amplifier. The high-fidelity
measurement output is used to actively stabilize the phase of Rabi
oscillations, enabling them to persist indefinitely. This new functionality
shows promise for fighting decoherence and defines a path for continuous
quantum error correction.Comment: Manuscript: 5 Pages and 3 figures ; Supplementary Information: 9
pages and 3 figure
Correlated radio:X-ray emission in the hard states of Galactic microquasars
We present results of our study of correlated radio and X-ray emission in two
black hole candidates and Galactic microquasars GRS 1915+105 and Cygnus X-1 in
their steady long term hard states, along with Cygnus X-3 (using data obtained
from RXTE-ASM, CGRO-BATSE and GBI). We detect a pivotal behavior in the X-ray
spectrum of GRS 1915+105, correlated to the radio emission. Similar to the
results obtained for Cygnus X-3, the flux of X-rays softer than the pivoting
point correlates with the radio emission, while the corresponding harder X-ray
flux anti-correlates with both the radio and the softer X-ray emission, in this
state. We examine all the previously reported correlations of X-ray properties
with the radio emission in Galactic microquasars and argue that these are
consistent with a general picture where a spectral pivoting is a common feature
in these sources with the shape of the spectrum determining the flux of radio
emission, during the hard states. We also detect a general monotonic increase
in the radio emission of these sources with the soft X-ray emission spanning
about 5 orders of magnitude. We qualitatively explain these findings with a Two
Component Advective Flow model where the location of a boundary layer between
the thin disk and the Comptonizing region determines the spectral shape and
also the amount of outflow.Comment: Minor typos corrected in matter as well as figures. 12 pages, 5
figures, to appear in ApJ, August 2003 (vol. 593
The lady vanishes: what's missing from the stem cell debate
Most opponents of somatic cell nuclear transfer and embryonic stem cell technologies base their arguments on the twin assertions that the embryo is either a human being or a potential human being, and that it is wrong to destroy a human being or potential human being in order to produce stem cell lines. Proponentsâ justifications of stem cell research are more varied, but not enough to escape the charge of obsession with the status of the embryo. What unites the two warring sides in âthe stem cell warsâ is that women are equally invisible to both: âthe lady vanishesâ. Yet the only legitimate property in the body is that which women possess in their reproductive tissue and the products of their reproductive labour. By drawing on the accepted characterisation in law of property as a bundle of rights, and on a Hegelian model of contract as mutual recognition, we can lessen the impact of the tendency to regard women and their eggs as merely receptacles and womenâs reproductive labour as unimportant
Initial/boundary-value problems of tumor growth within a host tissue
This paper concerns multiphase models of tumor growth in interaction with a
surrounding tissue, taking into account also the interplay with diffusible
nutrients feeding the cells. Models specialize in nonlinear systems of possibly
degenerate parabolic equations, which include phenomenological terms related to
specific cell functions. The paper discusses general modeling guidelines for
such terms, as well as for initial and boundary conditions, aiming at both
biological consistency and mathematical robustness of the resulting problems.
Particularly, it addresses some qualitative properties such as a priori
nonnegativity, boundedness, and uniqueness of the solutions. Existence of the
solutions is studied in the one-dimensional time-independent case.Comment: 30 pages, 5 figure
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