6,153 research outputs found
Clinical Evaluation of the Peroral Cholangioscopy Using a New Videoscope
Peroral cholangioscopy (PCS) has been performed in 22 cases using XCHF-B200 (Olympus
Optical Co.) since June 1995 and in 77 cases using CHF-B20 (Olympus Optical Co.) after
EST from Jan. 1989. XCHF-B200 has a longer rigid portion of distal end and a smaller
channel diameter than CHF-B20. The successful rate of PCS using XCHF-B200 (82%) was
lower than that of CHF-B20 (89%). The vascular pattern and fine vertical groove of the bile
duct mucosa were shown more clearly on the photographs obtained with XCHF-B200 than
those obtained with CHF-B20. However, not enough biopsy specimens could be obtained
because the channel diameter of XCHF-B200 was rather small
Pair Instability Supernovae of Very Massive Population III Stars
Numerical studies of primordial star formation suggest that the first stars
in the universe may have been very massive. Stellar models indicate that
non-rotating Population III stars with initial masses of 140-260 Msun die as
highly energetic pair-instability supernovae. We present new two-dimensional
simulations of primordial pair-instability supernovae done with the CASTRO
code. Our simulations begin at earlier times than previous multidimensional
models, at the onset of core collapse, to capture any dynamical instabilities
that may be seeded by collapse and explosive burning. Such instabilities could
enhance explosive yields by mixing hot ash with fuel, thereby accelerating
nuclear burning, and affect the spectra of the supernova by dredging up heavy
elements from greater depths in the star at early times. Our grid of models
includes both blue supergiants and red supergiants over the range in progenitor
mass expected for these events. We find that fluid instabilities driven by
oxygen and helium burning arise at the upper and lower boundaries of the oxygen
shell 20 - 100 seconds after core bounce. Instabilities driven by
burning freeze out after the SN shock exits the helium core. As the shock later
propagates through the hydrogen envelope, a strong reverse shock forms that
drives the growth of Rayleigh--Taylor instabilities. In red supergiant
progenitors, the amplitudes of these instabilities are sufficient to mix the
supernova ejecta.Comment: 42 pages, 15 figures (accepted to ApJ
The Source Density And Observability Of Pair-Instability Supernovae From The First Stars
Theoretical models predict that some of the first stars ended their lives as extremely energetic pair-instability supernovae (PISNe). With energies approaching 10(53) erg, these supernovae are expected to be within the detection limits of the upcoming James Webb Space Telescope (JWST), allowing observational constraints to be placed on the properties of the first stars. We estimate the source density of PISNe using a semi-analytic halo mass function based approach, accounting for the effects of feedback from star formation on the PISN rate using cosmological simulations. We estimate an upper limit of similar to 0.2 PISNe per JWST field of view at any given time. Feedback can reduce this rate significantly, e. g., lowering it to as little as one PISN per 4000 JWST fields of view for the most pessimistic explosion models. We also find that the main obstacle to observing PISNe from the first stars is their scarcity, not their faintness; exposures longer than a few times 10(4) s will do little to increase the number of PISNe found. Given this, we suggest a mosaic style search strategy for detecting PISNe from the first stars. Even rather high-redshift PISNe are unlikely to be missed by moderate exposures, and a large number of pointings will be required to ensure a detection.NSF AST-0708795, AST-1009928NASA ATFP NNX09AJ33GAstronom
Masses for the Local Group and the Milky Way
We use the very large Millennium Simulation of the concordance CDM
cosmogony to calibrate the bias and error distribution of Timing Argument
estimators of the masses of the Local Group and of the Milky Way. From a large
number of isolated spiral-spiral pairs similar to the Milky Way/Andromeda
system, we find the interquartile range of the ratio of timing mass to true
mass to be a factor of 1.8, while the 5% and 95% points of the distribution of
this ratio are separated by a factor of 5.7. Here we define true mass as the
sum of the ``virial'' masses of the two dominant galaxies. For
current best values of the distance and approach velocity of Andromeda this
leads to a median likelihood estimate of the true mass of the Local Group of
5.27\times 10^{12}\msun, or , with an
interquartile range of and a 5% to 95% range of . Thus a 95% lower confidence limit on the true mass of the Local Group
is 1.81\times 10^{12}\msun. A timing estimate of the Milky Way's mass based
on the large recession velocity observed for the distant satellite Leo I works
equally well, although with larger systematic uncertainties. It gives an
estimated virial mass for the Milky Way of 2.43 \times 10^{12}\msun with a
95% lower confidence limit of 0.80 \times 10^{12}\msun.Comment: 11 pages, 6 figures, MNRAS accepted. Added a new discussion paragraph
and a new figure regarding the relative transverse velocity but conclusions
unchange
Viscoelastic gels of guar and xanthan gum mixtures provide long-term stabilization of iron micro- and nanoparticles
Iron micro- and nanoparticles used for groundwater remediation and medical applications are prone to fast aggregation and sedimentation. Diluted single biopolymer water solutions of guar gum (GG) or xanthan gum (XG) can stabilize these particles for few hours providing steric repulsion and by increasing the viscosity of the suspension. The goal of the study is to demonstrate that amending GG solutions with small amounts of XG (XG/GG weight ratio 1:19; 3 g/L of total biopolymer concentration) can significantly improve the capability of the biopolymer to stabilize highly concentrated iron micro- and nanoparticle suspensions. The synergistic effect between GG and XG generates a viscoelastic gel that can maintain 20 g/L iron particles suspended for over 24 h. This is attributed to (i) an increase in the static viscosity, (ii) a combined polymer structure the yield stress of which contrasts the downward stress exerted by the iron particles, and (iii) the adsorption of the polymers to the iron surface having an anchoring effect on the particles. The XG/GG viscoelastic gel is characterized by a marked shear thinning behavior. This property, coupled with the low biopolymer concentration, determines small viscosity values at high shear rates, facilitating the injection in porous media. Furthermore, the thermosensitivity of the soft elastic polymeric network promotes higher stability and longer storage times at low temperatures and rapid decrease of viscosity at higher temperatures. This feature can be exploited in order to improve the flowability and the delivery of the suspensions to the target as well as to effectively tune and control the release of the iron particle
Quasi-normal modes for doubly rotating black holes
Based on the work of Chen, L\"u and Pope, we derive expressions for the
dimensional metric for Kerr-(A)dS black holes with two independent
rotation parameters and all others set equal to zero: . The Klein-Gordon equation is then explicitly separated on this
background. For this separation results in a radial equation coupled
to two generalized spheroidal angular equations. We then develop a full
numerical approach that utilizes the Asymptotic Iteration Method (AIM) to find
radial Quasi-Normal Modes (QNMs) of doubly rotating flat Myers-Perry black
holes for slow rotations. We also develop perturbative expansions for the
angular quantum numbers in powers of the rotation parameters up to second
order.Comment: RevTeX 4-1, various figure
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