1,330 research outputs found
Single microtubules and small networks become significantly stiffer on short time-scales upon mechanical stimulation
The transfer of mechanical signals through cells is a complex phenomenon. To
uncover a new mechanotransduction pathway, we study the frequency-dependent
transport of mechanical stimuli by single microtubules and small networks in a
bottom-up approach using optically trapped beads as anchor points. We
interconnected microtubules to linear and triangular geometries to perform
micro-rheology by defined oscillations of the beads relative to each other. We
found a substantial stiffening of single filaments above a characteristic
transition frequency of 1-30 Hz depending on the filament's molecular
composition. Below this frequency, filament elasticity only depends on its
contour and persistence length. Interestingly, this elastic behavior is
transferable to small networks, where we found the surprising effect that
linear two filament connections act as transistor-like, angle dependent
momentum filters, whereas triangular networks act as stabilizing elements.
These observations implicate that cells can tune mechanical signals by temporal
and spatial filtering stronger and more flexibly than expected
Statistics of Gravitational Microlensing Magnification. I. Two-Dimensional Lens Distribution
(Abridged) In this paper we refine the theory of microlensing for a planar
distribution of point masses. We derive the macroimage magnification
distribution P(A) at high magnification (A-1 >> tau^2) for a low optical depth
(tau << 1) lens distribution by modeling the illumination pattern as a
superposition of the patterns due to individual ``point mass plus weak shear''
lenses. We show that a point mass plus weak shear lens produces an astroid-
shaped caustic and that the magnification cross-section obeys a simple scaling
property. By convolving this cross-section with the shear distribution, we
obtain a caustic-induced feature in P(A) which also exhibits a simple scaling
property. This feature results in a 20% enhancement in P(A) at A approx 2/tau.
In the low magnification (A-1 << 1) limit, the macroimage consists of a bright
primary image and a large number of faint secondary images formed close to each
of the point masses. Taking into account the correlations between the primary
and secondary images, we derive P(A) for low A. The low-A distribution has a
peak of amplitude ~ 1/tau^2 at A-1 ~ tau^2 and matches smoothly to the high-A
distribution. We combine the high- and low-A results and obtain a practical
semi-analytic expression for P(A). This semi-analytic distribution is in
qualitative agreement with previous numerical results, but the latter show
stronger caustic-induced features at moderate A for tau as small as 0.1. We
resolve this discrepancy by re-examining the criterion for low optical depth. A
simple argument shows that the fraction of caustics of individual lenses that
merge with those of their neighbors is approx 1-exp(-8 tau). For tau=0.1, the
fraction is surprisingly high: approx 55%. For the purpose of computing P(A) in
the manner we did, low optical depth corresponds to tau << 1/8.Comment: 35 pages, including 6 figures; uses AASTeX v4.0 macros; submitted to
Ap
Stochastic bias of colour-selected BAO tracers by joint clustering-weak lensing analysis
The baryon acoustic oscillation (BAO) feature in the two-point correlation
function of galaxies supplies a standard ruler to probe the expansion history
of the Universe. We study here several galaxy selection schemes, aiming at
building an emission-line galaxy (ELG) sample in the redshift range
, that would be suitable for future BAO studies, providing a highly
biased galaxy sample. We analyse the angular galaxy clustering of galaxy
selections at the redshifts 0.5, 0.7, 0.8, 1 and 1.2 and we combine this
analysis with a halo occupation distribution (HOD) model to derive the
properties of the haloes these galaxies inhabit, in particular the galaxy bias
on large scales. We also perform a weak lensing analysis (aperture statistics)
to extract the galaxy bias and the cross-correlation coefficient and compare to
the HOD prediction.
We apply this analysis on a data set composed of the photometry of the deep
co-addition on Sloan Digital Sky Survey (SDSS) Stripe 82 (225 deg), of
Canda-France-Hawai Telescope/Stripe 82 deep \emph{i}-band weak lensing survey
and of the {\it Wide-Field Infrared Survey Explorer}infrared photometric band
W1.
The analysis on the SDSS-III/constant mass galaxies selection at is
in agreement with previous studies on the tracer, moreover we measure its
cross-correlation coefficient . For the higher redshift bins, we
confirm the trends that the brightest galaxy populations selected are strongly
biased (), but we are limited by current data sets depth to derive
precise values of the galaxy bias. A survey using such tracers of the mass
field will guarantee a high significance detection of the BAO.Comment: 17 pages, 15 figures, submitted to MNRA
Interface-mediated ferroelectric patterning and Mn valency in nano-structured PbTiO <sub>3</sub> /La <sub>0.7</sub> Sr <sub>0.3</sub> MnO <sub>3</sub>
International audienceWe employed a multitechnique approach using piezo-force response microscopy and photoemission microscopy to investigate a self-organizing polarization domain pattern in PbTiO3/La0.7Sr0.3MnO3 (PTO/LSMO) nanostructures. The polarization is correlated with the nanostructure morphology as well as with the thickness and Mn valence of the LSMO template layer. On the LSMO dots, the PTO is upwards polarized, whereas outside the nanodots, the polarization appears both strain and interface roughness dependent. The results suggest that the electronic structure and strain of the PTO/LSMO interface contribute to determining the internal bias of the ferroelectric layer
The Time-Domain Spectroscopic Survey: Understanding the Optically Variable Sky with SEQUELS in SDSS-III
The Time-Domain Spectroscopic Survey (TDSS) is an SDSS-IV eBOSS subproject
primarily aimed at obtaining identification spectra of ~220,000
optically-variable objects systematically selected from SDSS/Pan-STARRS1
multi-epoch imaging. We present a preview of the science enabled by TDSS, based
on TDSS spectra taken over ~320 deg^2 of sky as part of the SEQUELS survey in
SDSS-III, which is in part a pilot survey for eBOSS in SDSS-IV. Using the
15,746 TDSS-selected single-epoch spectra of photometrically variable objects
in SEQUELS, we determine the demographics of our variability-selected sample,
and investigate the unique spectral characteristics inherent in samples
selected by variability. We show that variability-based selection of quasars
complements color-based selection by selecting additional redder quasars, and
mitigates redshift biases to produce a smooth quasar redshift distribution over
a wide range of redshifts. The resulting quasar sample contains systematically
higher fractions of blazars and broad absorption line quasars than from
color-selected samples. Similarly, we show that M-dwarfs in the TDSS-selected
stellar sample have systematically higher chromospheric active fractions than
the underlying M-dwarf population, based on their H-alpha emission. TDSS also
contains a large number of RR Lyrae and eclipsing binary stars with
main-sequence colors, including a few composite-spectrum binaries. Finally, our
visual inspection of TDSS spectra uncovers a significant number of peculiar
spectra, and we highlight a few cases of these interesting objects. With a
factor of ~15 more spectra, the main TDSS survey in SDSS-IV will leverage the
lessons learned from these early results for a variety of time-domain science
applications.Comment: 17 pages, 14 figures, submitted to Ap
Strong Lensing Analysis of A1689 from Deep Advanced Camera Images
We analyse deep multi-colour Advanced Camera images of the largest known
gravitational lens, A1689. Radial and tangential arcs delineate the critical
curves in unprecedented detail and many small counter-images are found near the
center of mass. We construct a flexible light deflection field to predict the
appearance and positions of counter-images. The model is refined as new
counter-images are identified and incorporated to improve the model, yielding a
total of 106 images of 30 multiply lensed background galaxies, spanning a wide
redshift range, 1.0z5.5. The resulting mass map is more circular in
projection than the clumpy distribution of cluster galaxies and the light is
more concentrated than the mass within . The projected mass profile
flattens steadily towards the center with a shallow mean slope of
, over the observed range,
r, matching well an NFW profile, but with a relatively high
concentration, . A softened isothermal profile
(\arcs) is not conclusively excluded, illustrating that
lensing constrains only projected quantities. Regarding cosmology, we clearly
detect the purely geometric increase of bend-angles with redshift. The
dependence on the cosmological parameters is weak due to the proximity of
A1689, , constraining the locus, .
This consistency with standard cosmology provides independent support for our
model, because the redshift information is not required to derive an accurate
mass map. Similarly, the relative fluxes of the multiple images are reproduced
well by our best fitting lens model.Comment: Accepted by ApJ. For high quality figures see
http://wise-obs.tau.ac.il/~kerens/A168
Coherent multi-flavour spin dynamics in a fermionic quantum gas
Microscopic spin interaction processes are fundamental for global static and
dynamical magnetic properties of many-body systems. Quantum gases as pure and
well isolated systems offer intriguing possibilities to study basic magnetic
processes including non-equilibrium dynamics. Here, we report on the
realization of a well-controlled fermionic spinor gas in an optical lattice
with tunable effective spin ranging from 1/2 to 9/2. We observe long-lived
intrinsic spin oscillations and investigate the transition from two-body to
many-body dynamics. The latter results in a spin-interaction driven melting of
a band insulator. Via an external magnetic field we control the system's
dimensionality and tune the spin oscillations in and out of resonance. Our
results open new routes to study quantum magnetism of fermionic particles
beyond conventional spin 1/2 systems.Comment: 9 pages, 5 figure
Evaluation of the Prescribing Skills Assessment implementation, performance and medical student experience in Australia and New Zealand
Aims: The UK Prescribing Safety Assessment was modified for use in Australia and New Zealand (ANZ) as the Prescribing Skills Assessment (PSA). We investigated the implementation, student performance and acceptability of the ANZ PSA for final-year medical students. Methods:This study used a mixed-method approach involving student data (n = 6440) for 2017–2019 (PSA overall score and 8 domain subscores). Data were also aggregated by medical school and included student evaluation survey results. Quantitative data were analysed using descriptive and multivariate analyses. The pass rate was established by a modified Angoff method. Thematic analyses of open-ended survey comments were conducted. Results:The average pass rate was slightly higher in 2017 (89%) which used a different examination to 2018 (85%) and 2019 (86%). Little difference was identified between schools for the PSA overall performance or domain subscores. There was low intercorrelation between subscores. Most students provided positive feedback about the PSA regarding the interface and clarity of questions, but an average of 35% reported insufficient time for completion. Further, 70% on average felt unprepared by their school curricula for the PSA, which is in part explained by the low prescribing experience; 69% reported completing ≤10 prescriptions during training. Conclusion:The ANZ PSA was associated with high pass rates and acceptability, although student preparedness was highlighted as a concern for further investigation. We demonstrate how a collaboration of medical schools can adapt a medical education assessment resource (UK PSA) as a means for fulfilling an unmet need.</p
The Carnegie Supernova Project: First Near-Infrared Hubble Diagram to z~0.7
The Carnegie Supernova Project (CSP) is designed to measure the luminosity
distance for Type Ia supernovae (SNe Ia) as a function of redshift, and to set
observational constraints on the dark energy contribution to the total energy
content of the Universe. The CSP differs from other projects to date in its
goal of providing an I-band {rest-frame} Hubble diagram. Here we present the
first results from near-infrared (NIR) observations obtained using the Magellan
Baade telescope for SNe Ia with 0.1 < z < 0.7. We combine these results with
those from the low-redshift CSP at z <0.1 (Folatelli et al. 2009). We present
light curves and an I-band Hubble diagram for this first sample of 35 SNe Ia
and we compare these data to 21 new SNe Ia at low redshift. These data support
the conclusion that the expansion of the Universe is accelerating. When
combined with independent results from baryon acoustic oscillations (Eisenstein
et al. 2005), these data yield Omega_m = 0.27 +/- 0.0 (statistical), and
Omega_DE = 0.76 +/- 0.13 (statistical) +/- 0.09 (systematic), for the matter
and dark energy densities, respectively. If we parameterize the data in terms
of an equation of state, w, assume a flat geometry, and combine with baryon
acoustic oscillations, we find that w = -1.05 +/- 0.13 (statistical) +/- 0.09
(systematic). The largest source of systematic uncertainty on w arises from
uncertainties in the photometric calibration, signaling the importance of
securing more accurate photometric calibrations for future supernova cosmology
programs. Finally, we conclude that either the dust affecting the luminosities
of SNe Ia has a different extinction law (R_V = 1.8) than that in the Milky Way
(where R_V = 3.1), or that there is an additional intrinsic color term with
luminosity for SNe Ia independent of the decline rate.Comment: 44 pages, 23 figures, 9 tables; Accepted for publication in the
Astrophysical Journa
Taxation of Risky Investment and Paradoxical Investor Behavior
Under uncertainty and irreversibility, real option-based models are widely accepted for assessing investment projects. So far the existing post-tax analyses do not provide a general analytical description of investor reactions towards profit tax rate changes. This paper sets out to fill part of the void. We implement a simple tax system and focus on risky capital market investment and an option to wait. Taxes affect risk-free and risky capital market investment asymmetrically and hence cause distortions. We analytically identify a set of neutral tax rates (tax regimes) that preserve the critical post-tax investment threshold in case of tax rate changes as well as general normal and paradoxical settings. Unlike for other tax paradoxa neither depreciation rules nor loss offset restrictions are responsible for the observed paradoxical reaction. Identifying normal and paradoxical tax regimes can be regarded as a first step to a generalized description of tax effects under uncertainty, both for individual project evaluation as well as for understanding tax effects on an aggregate level
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