14,308 research outputs found
The "quasi-stable" lipid shelled microbubble in response to consecutive ultrasound pulses
Controlled microbubble stability upon exposure to consecutive ultrasound exposures is important for increased sensitivity in contrast enhanced ultrasound diagnostics and manipulation for localised drug release. An ultra high-speed camera operating at 13 × 10 6 frames per second is used to show that a physical instability in the encapsulating lipid shell can be promoted by ultrasound, causing loss of shell material that depends on the characteristics of the microbubble motion. This leads to well characterized disruption, and microbubbles follow an irreversible trajectory through the resonance peak, causing the evolution of specific microbubble spectral signatures. © 2012 American Institute of Physics
Structure of Disk Dominated Galaxies I. Bulge/Disk Parameters, Simulations, and Secular Evolution
(Abridged) A robust analysis of galaxy structural parameters, based on the
modeling of bulge and disk brightnesses in the BVRH bandpasses, is presented
for 121 face-on and moderately inclined late-type spirals. Each surface
brightness (SB) profile is decomposed into a sum of a generalized Sersic bulge
and an exponential disk. The reliability and limitations of our bulge-to-disk
(B/D) decompositions are tested with extensive simulations of galaxy brightness
profiles (1D) and images (2D). Galaxy types are divided into 3 classes
according to their SB profile shapes; Freeman Type-I and Type-II, and a third
``Transition'' class for galaxies whose profiles change from Type-II in the
optical to Type-I in the infrared. We discuss possible interpretations of
Freeman Type-II profiles. The Sersic bulge shape parameter for nearby Type-I
late-type spirals shows a range between n=0.1-2 but, on average, the underlying
surface density profile for the bulge and disk of these galaxies is adequately
described by a double-exponential distribution. We confirm a coupling between
the bulge and disk with a scale length ratio r_e/h=0.22+/-0.09, or
h_bulge/h_disk=0.13+/-0.06 for late-type spirals, in agreement with recent
N-body simulations of disk formation and models of secular evolution. This
ratio increases from ~0.20 for late-type spirals to ~0.24 for earlier types.
The similar scaling relations for early and late-type spirals suggest
comparable formation and/or evolution scenarios for disk galaxies of all Hubble
types.Comment: 78 pages with 23 embedded color figures + tables of galaxy structural
parameters. Accepted for publication in the Astrophysical Journal. The
interested reader is strongly encouraged to ignore some of the low res
figures within; instead, download the high resolution version from
http://www.astro.ubc.ca/people/courteau/public/macarthur02_disks.ps.g
Information Storage and Retrieval for Probe Storage using Optical Diffraction Patterns
A novel method for fast information retrieval from a probe storage device is
considered. It is shown that information can be stored and retrieved using the
optical diffraction patterns obtained by the illumination of a large array of
cantilevers by a monochromatic light source. In thermo-mechanical probe
storage, the information is stored as a sequence of indentations on the polymer
medium. To retrieve the information, the array of probes is actuated by
applying a bending force to the cantilevers. Probes positioned over
indentations experience deflection by the depth of the indentation, probes over
the flat media remain un-deflected. Thus the array of actuated probes can be
viewed as an irregular optical grating, which creates a data-dependent
diffraction pattern when illuminated by laser light. We develop a low
complexity modulation scheme, which allows the extraction of information stored
in the pattern of indentations on the media from Fourier coefficients of the
intensity of the diffraction pattern. We then derive a low-complexity maximum
likelihood sequence detection algorithm for retrieving the user information
from the Fourier coefficients. The derivation of both the modulation and the
detection schemes is based on the Fraunhofer formula for data-dependent
diffraction patterns. We show that for as long as the Fresnel number F<0.1, the
optimal channel detector derived from Fraunhofer diffraction theory does not
suffer any significant performance degradation.Comment: 14 pages, 11 figures. Version 2: minor misprints corrected,
experimental section expande
Ambipolar Drift Heating in Turbulent Molecular Clouds
Although thermal pressure is unimportant dynamically in most molecular gas,
the temperature is an important diagnostic of dynamical processes and physical
conditions. This is the first of two papers on thermal equilibrium in molecular
clouds. We present calculations of frictional heating by ion-neutral (or
ambipolar) drift in three-dimensional simulations of turbulent, magnetized
molecular clouds.
We show that ambipolar drift heating is a strong function of position in a
turbulent cloud, and its average value can be significantly larger than the
average cosmic ray heating rate. The volume averaged heating rate per unit
volume due to ambipolar drift, H_AD ~ |JxB|^2 ~ B^4/L_B^2, is found to depend
on the rms Alfvenic Mach number, M_A, and on the average field strength, as
H_AD ~ M_A^2^4. This implies that the typical scale of variation of the
magnetic field, L_B, is inversely proportional to M_A, which we also
demonstrate.Comment: 37 pages, 9 figures include
The spliceosome-activating complex: molecular mechanisms underlying the function of a pleiotropic regulator
Correct interpretation of the coding capacity of RNA polymerase II transcribed eukaryotic genes is determined by the recognition and removal of intronic sequences of pre-mRNAs by the spliceosome. Our current knowledge on dynamic assembly and subunit interactions of the spliceosome mostly derived from the characterization of yeast, Drosophila, and human spliceosomal complexes formed on model pre-mRNA templates in cell extracts. In addition to sequential structural rearrangements catalyzed by ATP-dependent DExH/D-box RNA helicases, catalytic activation of the spliceosome is critically dependent on its association with the NineTeen Complex (NTC) named after its core E3 ubiquitin ligase subunit PRP19. NTC, isolated recently from Arabidopsis, occurs in a complex with the essential RNA helicase and GTPase subunits of the U5 small nuclear RNA particle that are required for both transesterification reactions of splicing. A compilation of mass spectrometry data available on the composition of NTC and spliceosome complexes purified from different organisms indicates that about half of their conserved homologs are encoded by duplicated genes in Arabidopsis. Thus, while mutations of single genes encoding essential spliceosome and NTC components lead to cell death in other organisms, differential regulation of some of their functionally redundant Arabidopsis homologs permits the isolation of partial loss of function mutations. Non-lethal pleiotropic defects of these mutations provide a unique means for studying the roles of NTC in co-transcriptional assembly of the spliceosome and its crosstalk with DNA repair and cell death signaling pathways
Angle-resolved and core-level photoemission study of interfacing the topological insulator Bi1.5Sb0.5Te1.7Se1.3 with Ag, Nb and Fe
Interfaces between a bulk-insulating topological insulator (TI) and metallic
adatoms have been studied using high-resolution, angle-resolved and core-level
photoemission. Fe, Nb and Ag were evaporated onto Bi1.5Sb0.5Te1.7Se1.3 (BSTS)
surfaces both at room temperature and 38K. The coverage- and
temperature-dependence of the adsorption and interfacial formation process have
been investigated, highlighting the effects of the overlayer growth on the
occupied electronic structure of the TI. For all coverages at room temperature
and for those equivalent to less than 0.1 monolayer at low temperature all
three metals lead to a downward shift of the TI's bands with respect to the
Fermi level. At room temperature Ag appears to intercalate efficiently into the
van der Waals gap of BSTS, accompanied by low-level substitution of the Te/Se
atoms of the termination layer of the crystal. This Te/Se substitution with
silver increases significantly for low temperature adsorption, and can even
dominate the electrostatic environment of the Bi/Sb atoms in the BSTS
near-surface region. On the other hand, Fe and Nb evaporants remain close to
the termination layer of the crystal. On room temperature deposition, they
initially substitute isoelectronically for Bi as a function of coverage, before
substituting for Te/Se atoms. For low temperature deposition, Fe and Nb are too
immobile for substitution processes and show a behaviour consistent with
clustering on the surface. For both Ag and Fe/Nb, these differing adsorption
pathways leads to the qualitatively similar and remarkable behavior for low
temperature deposition that the chemical potential first moves upward (n-type
dopant behavior) and then downward (p-type behavior) on increasing coverage.Comment: 10 pages, 4 figures. In our Phys. Rev. B manuscript an error was made
in formulating the last sentence of the abstract that, unfortunately, was
missed in the page proofs. Version 2 on arxiv has the correct formulation of
this sentenc
On Star Formation and the Non-Existence of Dark Galaxies
We investigate whether a baryonic dark galaxy or `galaxy without stars' could
persist indefinitely in the local universe, while remaining stable against star
formation. To this end, a simple model has been constructed to determine the
equilibrium distribution and composition of a gaseous protogalactic disk.
Specifically, we determine the amount of gas that will transit to a Toomre
unstable cold phase via the H2 cooling channel in the presence of a UV--X-ray
cosmic background radiation field.
All but one of the models are predicted to become unstable to star formation.
Moreover, we find that all our model objects would be detectable via HI line
emission, even in the case that star formation is potentially avoided. These
results are consistent with the non-detection of isolated extragalactic HI
clouds with no optical counterpart (galaxies without stars) by HIPASS.
Additionally, where star formation is predicted to occur, we determine the
minimum interstellar radiation field required to restore gravothermal
stability, which we then relate to a minimum global star formation rate. This
leads to the prediction of a previously undocumented relation between HI mass
and star formation rate that is observed for a wide variety of dwarf galaxies
in the HI mass range 10^8--10^10 M_sun. The existence of such a relation
strongly supports the notion that the well observed population of dwarf
galaxies represent the minimum rates of self-regulating star formation in the
universe. (Barely abridged)Comment: 19 pages, 8 figures, TeX using emulateapj.cls, v2 accepted for
publication in ApJ (16/8/5) with one figure deleted and a number of minor
clarifying revision
Emergent AdS3 and BTZ Black Hole from Weakly Interacting Hot 2d CFT
We investigate emergent holography of weakly coupled two-dimensional
hyperK\"ahler sigma model on cotangent bundle of (N-1)-dimensional complex
projective space at zero and finite temperature. The sigma model is motivated
by the spacetime conformal field theory dual to the near-horizon geometry of Q1
D1-brane bound to Q5 D5-brane wrapped on four-torus times circle, where N =
Q1*Q5. The sigma model admits nontrivial instanton for all N greater than or
equal to 2, which serves as a local probe of emergent holographic spacetime. We
define emergent geometry of the spacetime as that of instanton moduli space via
Hitchin's information metric. At zero temperature, we find that emergent
geometry is AdS3. At finite temperature, time-periodic instanton is mappable to
zero temperature instanton via conformal transformation. Utilizing the
transformation, we show that emergent geometry is precisely that of the
non-extremal, non-rotating BTZ black hole.Comment: 12 pages, no figure, JHEP.cls; v2. typos correcte
Effective temperature and Gilbert damping of a current-driven localized spin
Starting from a model that consists of a semiclassical spin coupled to two
leads we present a microscopic derivation of the Langevin equation for the
direction of the spin. For slowly-changing direction it takes on the form of
the stochastic Landau-Lifschitz-Gilbert equation. We give expressions for the
Gilbert damping parameter and the strength of the fluctuations, including their
bias-voltage dependence. At nonzero bias-voltage the fluctuations and damping
are not related by the fluctuation-dissipation theorem. We find, however, that
in the low-frequency limit it is possible to introduce a voltage-dependent
effective temperature that characterizes the fluctuations in the direction of
the spin, and its transport-steady-state probability distribution function.Comment: 8 pages, 2 figures. v2: published versio
IK-FA, a new heuristic inverse kinematics solver using firefly algorithm
In this paper, a heuristic method based on Firefly Algorithm is proposed for inverse kinematics problems in articulated robotics. The proposal is called, IK-FA. Solving inverse kinematics, IK, consists in finding a set of joint-positions allowing a specific point of the system to achieve a target position. In IK-FA, the Fireflies positions are assumed to be a possible solution for joints elementary motions. For a robotic system with a known forward kinematic model, IK-Fireflies, is used to generate iteratively a set of joint motions, then the forward kinematic model of the system is used to compute the relative Cartesian positions of a specific end-segment, and to compare it to the needed target position. This is a heuristic approach for solving inverse kinematics without computing the inverse model. IK-FA tends to minimize the distance to a target position, the fitness function could be established as the distance between the obtained forward positions and the desired one, it is subject to minimization. In this paper IK-FA is tested over a 3 links articulated planar system, the evaluation is based on statistical analysis of the convergence and the solution quality for 100 tests. The impact of key FA parameters is also investigated with a focus on the impact of the number of fireflies, the impact of the maximum iteration number and also the impact of (a, ß, ¿, d) parameters. For a given set of valuable parameters, the heuristic converges to a static fitness value within a fix maximum number of iterations. IK-FA has a fair convergence time, for the tested configuration, the average was about 2.3394 × 10-3 seconds with a position error fitness around 3.116 × 10-8 for 100 tests. The algorithm showed also evidence of robustness over the target position, since for all conducted tests with a random target position IK-FA achieved a solution with a position error lower or equal to 5.4722 × 10-9.Peer ReviewedPostprint (author's final draft
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