866 research outputs found
Modeling lithium rich carbon stars in the Large Magellanic Cloud: an independent distance indicator ?
We present the first quantitative results explaining the presence in the
Large Magellanic Cloud of some asymptotic giant branch stars that share the
properties of lithium rich carbon stars. A self-consistent description of
time-dependent mixing, overshooting, and nuclear burning was required. We
identify a narrow range of masses and luminosities for this peculiar stars.
Comparison of these models with the luminosities of the few Li-rich C stars in
the Large Magellanic Cloud provides an independent distance indicator for the
LMCComment: 7 pages, 2 figure
Thermo-mechanic-electrical coupling in phospholipid monolayers near the critical point
Lipid monolayers have been shown to represent a powerful tool in studying
mechanical and thermodynamic properties of lipid membranes as well as their
interaction with proteins. Using Einstein's theory of fluctuations we here
demonstrate, that an experimentally derived linear relationship both between
transition entropy S and area A as well as between transition entropy and
charge q implies a linear relationships between compressibility \kappa_T, heat
capacity c_\pi, thermal expansion coefficient \alpha_T and electric capacity
CT. We demonstrate that these couplings have strong predictive power as they
allow calculating electrical and thermal properties from mechanical
measurements. The precision of the prediction increases as the critical point
TC is approached
Flexible Lipid Bilayers in Implicit Solvent
A minimalist simulation model for lipid bilayers is presented. Each lipid is
represented by a flexible chain of beads in implicit solvent. The hydrophobic
effect is mimicked through an intermolecular pair potential localized at the
``water''/hydrocarbon tail interface. This potential guarantees realistic
interfacial tensions for lipids in a bilayer geometry. Lipids self assemble
into bilayer structures that display fluidity and elastic properties consistent
with experimental model membrane systems. Varying molecular flexibility allows
for tuning of elastic moduli and area/molecule over a range of values seen in
experimental systems.Comment: 5 pages, 5 figure
Phase ordering and shape deformation of two-phase membranes
Within a coupled-field Ginzburg-Landau model we study analytically phase
separation and accompanying shape deformation on a two-phase elastic membrane
in simple geometries such as cylinders, spheres and tori. Using an exact
periodic domain wall solution we solve for the shape and phase ordering field,
and estimate the degree of deformation of the membrane. The results are
pertinent to a preferential phase separation in regions of differing curvature
on a variety of vesicles.Comment: 4 pages, submitted to PR
Thinking strategically about assessment
Drawing upon the literature on strategy formulation in organisations, this paper argues for a focus on strategy as process. It relates this to the need to think strategically about assessment, a need engendered by resource pressures, developments in learning and the demands of external stakeholders. It is argued that in practice assessment strategies are often formed at the level of practice, but that this produces contradiction and confusion at higher levels. Such tensions cannot be managed away, but they can be reflected on and mitigated. The paper suggests a framework for the construction of assessment strategies at different levels of an institution. However, the main conclusion is that the process of constructing such strategies should be an opportunity for learning and reflection, rather than one of compliance
Radial distribution function of semiflexible polymers
We calculate the distribution function of the end--to--end distance of a
semiflexible polymer with large bending rigidity. This quantity is directly
observable in experiments on single semiflexible polymers (e.g., DNA, actin)
and relevant to their interpretation. It is also an important starting point
for analyzing the behavior of more complex systems such as networks and
solutions of semiflexible polymers. To estimate the validity of the obtained
analytical expressions, we also determine the distribution function numerically
using Monte Carlo simulation and find good quantitative agreement.Comment: RevTeX, 4 pages, 1 figure. Also available at
http://www.cip.physik.tu-muenchen.de/tumphy/d/T34/Mitarbeiter/frey.htm
Mass-luminosity relation for FGK main sequence stars: metallicity and age contributions
The stellar mass-luminosity relation (MLR) is one of the most famous
empirical "laws", discovered in the beginning of the 20th century. MLR is still
used to estimate stellar masses for nearby stars, particularly for those that
are not binary systems, hence the mass cannot be derived directly from the
observations. It's well known that the MLR has a statistical dispersion which
cannot be explained exclusively due to the observational errors in luminosity
(or mass). It is an intrinsic dispersion caused by the differences in age and
chemical composition from star to star. In this work we discuss the impact of
age and metallicity on the MLR. Using the recent data on mass, luminosity,
metallicity, and age for 26 FGK stars (all members of binary systems, with
observational mass-errors <= 3%), including the Sun, we derive the MLR taking
into account, separately, mass-luminosity, mass-luminosity-metallicity, and
mass-luminosity-metallicity-age. Our results show that the inclusion of age and
metallicity in the MLR, for FGK stars, improves the individual mass estimation
by 5% to 15%.Comment: 7 pages, 4 figures, 1 table, accepted in Astrophysics and Space
Scienc
Orientational order on curved surfaces - the high temperature region
We study orientational order, subject to thermal fluctuations, on a fixed
curved surface. We derive, in particular, the average density of zeros of
Gaussian distributed vector fields on a closed Riemannian manifold. Results are
compared with the density of disclination charges obtained from a Coulomb gas
model. Our model describes the disordered state of two dimensional objects with
orientational degrees of freedom, such as vector ordering in Langmuir
monolayers and lipid bilayers above the hexatic to fluid transition.Comment: final version, 13 Pages, 2 figures, uses iopart.cl
Our Sun. IV. The Standard Model and Helioseismology: Consequences of Uncertainties in Input Physics and in Observed Solar Parameters
Helioseismology provides a powerful tool to explore the deep interior of the
Sun: for example, the adiabatic sound speed can be inferred with an accuracy of
a few parts in 10,000. This has become a serious challenge to theoretical
models of the Sun. Therefore, we have undertaken a self-consistent, systematic
study of sources of uncertainties in the standard solar model, which must be
understood before the helioseismic observations can be used as constraints on
theory. We find that the largest uncertainty in the sound speed in the solar
interior, namely, 3 parts in 1000, arises from uncertainties in the observed
photospheric abundances of the elements; uncertainties of 1 part in 1000 arise
from (1) the 4% uncertainty in the OPAL opacities, (2) the 5% uncertainty in
the basic pp nuclear reaction rate, (3) the 15% uncertainty in the diffusion
constants for the gravitational settling of helium, and (4) the 50%
uncertainties in diffusion constants for the heavier elements. (Other
investigators have shown that similar uncertainties arise from uncertainties in
the interior equation of state and in rotation-induced turbulent mixing.) The
predicted pre-main-sequence solar lithium depletion is a factor of order 20 (an
order of magnitude larger than that predicted by earlier models that neglected
gravitational settling and used older opacities), and is uncertain by a factor
of 2. The predicted neutrino capture rate is uncertain by 30% for the Cl-37
experiment and by 3% for the Ga-71 experiments (not including uncertainties in
the capture cross sections), while the B-8 neutrino flux is uncertain by 30%.Comment: LaTeX, 38 pages (including 8 figures); ApJ, in press. Added
figures/color figurea available at
http://www.cita.utoronto.ca/~boothroy/sun4.htm
Force-Extension Relation and Plateau Modulus for Wormlike Chains
We derive the linear force-extension relation for a wormlike chain of
arbitrary stiffness including entropy elasticity, bending and thermodynamic
buckling. From this we infer the plateau modulus of an isotropic
entangled solution of wormlike chains. The entanglement length is
expressed in terms of the characteristic network parameters for three different
scaling regimes in the entangled phase. The entanglement transition and the
concentration dependence of are analyzed. Finally we compare our findings
with experimental data.Comment: 5 pages, 1 eps-figure, to appear in PR
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