2,922 research outputs found
Shape instabilities in vesicles: a phase-field model
A phase field model for dealing with shape instabilities in fluid membrane
vesicles is presented. This model takes into account the Canham-Helfrich
bending energy with spontaneous curvature. A dynamic equation for the
phase-field is also derived. With this model it is possible to see the vesicle
shape deformation dynamically, when some external agent instabilizes the
membrane, for instance, inducing an inhomogeneous spontaneous curvature. The
numerical scheme used is detailed and some stationary shapes are shown together
with a shape diagram for vesicles of spherical topology and no spontaneous
curvature, in agreement with known results
Sidebranching induced by external noise in solutal dendritic growth
We have studied sidebranching induced by fluctuations in dendritic growth.
The amplitude of sidebranching induced by internal (equilibrium) concentration
fluctuations in the case of solidification with solutal diffusion is computed.
This amplitude turns out to be significantly smaller than values reported in
previous experiments.The effects of other possible sources of fluctuations (of
an external origin)are examined by introducing non-conserved noise in a
phase-field model. This reproduces the characteristics of sidebranching found
in experiments. Results also show that sidebranching induced by external noise
is qualitatively similar to that of internal noise, and it is only
distinguished by its amplitude.Comment: 13 pages, 5 figure
Vesicle formation induced by thermal fluctuations
The process of fission and vesicle formation depends on the geometry of the
membrane that will split. For instance, a flat surface finds it difficult to
form vesicles because of the lack of curved regions where to start the process.
Here we show that vesicle formation can be promoted by temperature, by using a
membrane phase field model with Gaussian curvature. We find a phase transition
between fluctuating and vesiculation phases that depends on temperature,
spontaneous curvature, and the ratio between bending and Gaussian moduli. We
analysed the energy dynamical behaviour of these processes and found that the
main driving ingredient is the Gaussian energy term, although the curvature
energy term usually helps with the process as well. We also found that the
chemical potential can be used to investigate the temperature of the system.
Finally we address how temperature changes the condition for spontaneous
vesiculation for all geometries, making it happen in a wider range of values of
the Gaussian modulus.Comment: 31 pages, 10 figure
Multi-site observations of Delta Scuti stars 7 Aql and 8 Aql (a new Delta Scuti variable): The twelfth STEPHI campaign in 2003
We present an analysis of the pulsation behaviour of the Delta Scuti stars 7
Aql (HD 174532) and 8 Aql (HD 174589) -- a new variable star -- observed in the
framework of STEPHI XII campaign during 2003 June--July. 183 hours of high
precision photometry were acquired by using four-channel photometers at three
sites on three continents during 21 days. The light curves and amplitude
spectra were obtained following a classical scheme of multi-channel photometry.
Observations in different filters were also obtained and analyzed. Six and
three frequencies have been unambiguously detected above a 99% confidence level
in the range 0.090 mHz--0.300 mHz and 0.100 mHz-- 0.145 mHz in 7 Aql and 8 Aql
respectively. A comparison of observed and theoretical frequencies shows that 7
Aql and 8 Aql may oscillate with p modes of low radial orders, typical among
Delta Scuti stars. In terms of radial oscillations the range of 8 Aql goes from
n=1 to n=3 while for 7 Aql the range spans from n=4 to n=7. Non-radial
oscillations have to be present in both stars as well. The expected range of
excited modes according to a non adiabatic analysis goes from n=1 to n=6 in
both stars.Comment: 8 pages, 7 fugures, 5 tables, accepted for publication in
Astronomical Journa
Pulsational frequencies of the eclipsing delta-Scuti star HD 172189
The eclipsing delta-Scuti star HD 172189 is a probable member of the open
cluster IC 4756 and a promising candidate target for the CoRoT mission.
The detection of pulsation modes is the first step in the asteroseismological
study of the star. Further, the calculation of the orbital parameters of the
binary system allows us to make a dynamical determination of the mass of the
star, which works as an important constraint to test and calibrate the
asteroseismological models.
From a detailed frequency analysis of 210 hours of photometric data of HD
172189 obtained from the STEPHI XIII campaign we have identified six pulsation
frequencies with a confidence level of 99% and a seventh with a 65% confidence
level in the range between 100-300 uHz. In addiction, three eclipses were
observed during the campaign, allowing us to improve the determination of the
orbital period of the system.Comment: 6 pages, 7 figure
New multisite observations of Delta Scuti stars V624 Tauri and HD 23194
The preliminary results of STEPHI 2006 campaign are reported.Comment: To be published in Communications in Asteroseismolgy: Special Issu
Study of the D^0 \to pi^-pi^+pi^-pi^+ decay
Using data from the FOCUS (E831) experiment at Fermilab, we present new
measurements for the Cabibbo-suppressed decay mode . We measure the branching ratio .
An amplitude analysis has been performed, a first for this channel, in order to
determine the resonant substructure of this decay mode. The dominant component
is the decay , accounting for 60% of the decay rate.
The second most dominant contribution comes from the decay , with a fraction of 25%. We also study the
line shape and resonant substructure. Using the helicity formalism for the
angular distribution of the decay , we measure
a longitudinal polarization of %.Comment: 38 pages, 8 figures. accepted for publication in Physical Review
A Study of D0 --> K0(S) K0(S) X Decay Channels
Using data from the FOCUS experiment (FNAL-E831), we report on the decay of
mesons into final states containing more than one . We present
evidence for two Cabibbo favored decay modes, and
, and measure their combined branching fraction
relative to to be = 0.0106
0.0019 0.0010. Further, we report new measurements of
=
0.0179 0.0027 0.0026, = 0.0144 0.0032 0.0016,
and = 0.0208 0.0035 0.0021 where the first error is
statistical and the second is systematic.Comment: 11 pages, 3 figures, typos correcte
A Measurement of the Ds+ Lifetime
A high statistics measurement of the Ds+ lifetime from the Fermilab
fixed-target FOCUS photoproduction experiment is presented. We describe the
analysis of the two decay modes, Ds+ -> phi(1020)pi+ and Ds+ ->
\bar{K}*(892)0K+, used for the measurement. The measured lifetime is 507.4 +/-
5.5 (stat.) +/- 5.1 (syst.) fs using 8961 +/- 105 Ds+ -> phi(1020)pi+ and 4680
+/- 90 Ds+ -> \bar{K}*(892)0K+ decays. This is a significant improvement over
the present world average.Comment: 5 pages, 3 figures, 2 tables, submitted to PR
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