4,659 research outputs found
Mean encounter times for cell adhesion in hydrodynamic flow: analytical progress by dimensional reduction
For a cell moving in hydrodynamic flow above a wall, translational and
rotational degrees of freedom are coupled by the Stokes equation. In addition,
there is a close coupling of convection and diffusion due to the
position-dependent mobility. These couplings render calculation of the mean
encounter time between cell surface receptors and ligands on the substrate very
difficult. Here we show for a two-dimensional model system how analytical
progress can be achieved by treating motion in the vertical direction by an
effective reaction term in the mean first passage time equation for the
rotational degree of freedom. The strength of this reaction term can either be
estimated from equilibrium considerations or used as a fit parameter. Our
analytical results are confirmed by computer simulations and allow to assess
the relative roles of convection and diffusion for different scaling regimes of
interest.Comment: Reftex, postscript figures include
Gaia FGK Benchmark Stars: Effective temperatures and surface gravities
Large Galactic stellar surveys and new generations of stellar atmosphere
models and spectral line formation computations need to be subjected to careful
calibration and validation and to benchmark tests. We focus on cool stars and
aim at establishing a sample of 34 Gaia FGK Benchmark Stars with a range of
different metallicities. The goal was to determine the effective temperature
and the surface gravity independently from spectroscopy and atmospheric models
as far as possible. Fundamental determinations of Teff and logg were obtained
in a systematic way from a compilation of angular diameter measurements and
bolometric fluxes, and from a homogeneous mass determination based on stellar
evolution models. The derived parameters were compared to recent spectroscopic
and photometric determinations and to gravity estimates based on seismic data.
Most of the adopted diameter measurements have formal uncertainties around 1%,
which translate into uncertainties in effective temperature of 0.5%. The
measurements of bolometric flux seem to be accurate to 5% or better, which
contributes about 1% or less to the uncertainties in effective temperature. The
comparisons of parameter determinations with the literature show in general
good agreements with a few exceptions, most notably for the coolest stars and
for metal-poor stars. The sample consists of 29 FGK-type stars and 5 M giants.
Among the FGK stars, 21 have reliable parameters suitable for testing,
validation, or calibration purposes. For four stars, future adjustments of the
fundamental Teff are required, and for five stars the logg determination needs
to be improved. Future extensions of the sample of Gaia FGK Benchmark Stars are
required to fill gaps in parameter space, and we include a list of suggested
candidates.Comment: Accepted by A&A; 34 pages (printer format), 14 tables, 13 figures;
language correcte
Efficiency of initiating cell adhesion in hydrodynamic flow
We theoretically investigate the efficiency of initial binding between a
receptor-coated sphere and a ligand-coated wall in linear shear flow. The mean
first passage time for binding decreases monotonically with increasing shear
rate. Above a saturation threshold of the order of a few 100 receptor patches,
the binding efficiency is enhanced only weakly by increasing their number and
size, but strongly by increasing their height. This explains why white blood
cells in the blood flow adhere through receptor patches localized to the tips
of microvilli, and why malaria-infected red blood cells form elevated receptor
patches (knobs).Comment: 4 pages, Revtex, 4 Postscript figures included, to appear in PR
Reduction of Guided Acoustic Wave Brillouin Scattering in Photonic Crystal Fibers
Guided Acoustic Wave Brillouin Scattering (GAWBS) generates phase and
polarization noise of light propagating in glass fibers. This excess noise
affects the performance of various experiments operating at the quantum noise
limit. We experimentally demonstrate the reduction of GAWBS noise in a photonic
crystal fiber in a broad frequency range using cavity sound dynamics. We
compare the noise spectrum to the one of a standard fiber and observe a 10-fold
noise reduction in the frequency range up to 200 MHz. Based on our measurement
results as well as on numerical simulations we establish a model for the
reduction of GAWBS noise in photonic crystal fibers.Comment: 4 pages, 7 figures; added numerical simulations, added reference
Mean first passage times for bond formation for a Brownian particle in linear shear flow above a wall
Motivated by cell adhesion in hydrodynamic flow, here we study bond formation
between a spherical Brownian particle in linear shear flow carrying receptors
for ligands covering the boundary wall. We derive the appropriate Langevin
equation which includes multiplicative noise due to position-dependent mobility
functions resulting from the Stokes equation. We present a numerical scheme
which allows to simulate it with high accuracy for all model parameters,
including shear rate and three parameters describing receptor geometry
(distance, size and height of the receptor patches). In the case of homogeneous
coating, the mean first passage time problem can be solved exactly. In the case
of position-resolved receptor-ligand binding, we identify different scaling
regimes and discuss their biological relevance.Comment: final version after minor revision
The Hamburg/ESO R-process Enhanced Star survey (HERES) IV. Detailed abundance analysis and age dating of the strongly r-process enhanced stars CS 29491-069 and HE 1219-0312
We report on a detailed abundance analysis of two strongly r-process
enhanced, very metal-poor stars newly discovered in the HERES project, CS
29491-069 ([Fe/H]=-2.51, [r/Fe]=+1.1) and HE 1219-0312 ([Fe/H]=-2.96,
[r/Fe]=+1.5). The analysis is based on high-quality VLT/UVES spectra and MARCS
model atmospheres. We detect lines of 15 heavy elements in the spectrum of CS
29491-069, and 18 in HE 1219-0312; in both cases including the Th II 4019 {\AA}
line. The heavy-element abundance patterns of these two stars are mostly
well-matched to scaled solar residual abundances not formed by the s-process.
We also compare the observed pattern with recent high-entropy wind (HEW)
calculations, which assume core-collapse supernovae of massive stars as the
astrophysical environment for the r-process, and find good agreement for most
lanthanides. The abundance ratios of the lighter elements strontium, yttrium,
and zirconium, which are presumably not formed by the main r-process, are
reproduced well by the model. Radioactive dating for CS 29491-069 with the
observed thorium and rare-earth element abundance pairs results in an average
age of 9.5 Gyr, when based on solar r-process residuals, and 17.6 Gyr, when
using HEW model predictions. Chronometry seems to fail in the case of HE
1219-0312, resulting in a negative age due to its high thorium abundance. HE
1219-0312 could therefore exhibit an overabundance of the heaviest elements,
which is sometimes called an "actinide boost"
Towards higher precision and operational use of optical homodyne tomograms
We present the results of an operational use of experimentally measured
optical tomograms to determine state characteristics (purity) avoiding any
reconstruction of quasiprobabilities. We also develop a natural way how to
estimate the errors (including both statistical and systematic ones) by an
analysis of the experimental data themselves. Precision of the experiment can
be increased by postselecting the data with minimal (systematic) errors. We
demonstrate those techniques by considering coherent and photon-added coherent
states measured via the time-domain improved homodyne detection. The
operational use and precision of the data allowed us to check for the first
time purity-dependent uncertainty relations and uncertainty relations for
Shannon and R\'{e}nyi entropies.Comment: 11 pages, 6 figures, 1 table, some results are extende
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