3,525 research outputs found
Integral throat entrance development, qualification and production for the Antares 3 nozzle
Although design analyses of a G-90 graphite integral throat entrance for the Antares 3 solid rocket motor nozzle indicated acceptable margins of safety, the nozzle throat insert suffered a thermostructural failure during the first development firing. Subsequent re-analysis using properties measured on material from the same billet as the nozzle throat insert showed negative margins. Carbon-carbon was investigated and found to result in large positive margins of safety. The G-90 graphite was replaced by SAI fast processed 4-D material which uses Hercules HM 10000 fiber as the reinforcement. Its construction allows powder filling of the interstices after preform fabrication which accelerates the densification process. Allied 15V coal tar pitch is then used to complete densification. The properties were extensively characterized on this material and six nozzles were subjected to demonstration, development and qualification firings
High-resolution spectroscopy of the R Coronae Borealis and Other Hydrogen Deficient Stars
High-resolution spectroscopy is a very important tool for studying stellar
physics, perhaps, particularly so for such enigmatic objects like the R Coronae
Borealis and related Hydrogen deficient stars that produce carbon dust in
addition to their peculiar abundances.
Examples of how high-resolution spectroscopy is used in the study of these
stars to address the two major puzzles are presented: (i) How are such rare
H-deficient stars created? and (ii) How and where are the obscuring soot clouds
produced around the R Coronae Borealis stars?Comment: 16 pages, 9 figures, Astrophysics and Space Science Proceedings,
Springer-Verlag, Berlin, 201
Regional differences in APD restitution can initiate wavebreak and re-entry in cardiac tissue: A computational study
Background
Regional differences in action potential duration (APD) restitution in the heart favour arrhythmias, but the mechanism is not well understood.
Methods
We simulated a 150 × 150 mm 2D sheet of cardiac ventricular tissue using a simplified computational model. We investigated wavebreak and re-entry initiated by an S1S2S3 stimulus protocol in tissue sheets with two regions, each with different APD restitution. The two regions had a different APD at short diastolic interval (DI), but similar APD at long DI. Simulations were performed twice; once with both regions having steep (slope > 1), and once with both regions having flat (slope < 1) APD restitution.
Results
Wavebreak and re-entry were readily initiated using the S1S2S3 protocol in tissue sheets with two regions having different APD restitution properties. Initiation occurred irrespective of whether the APD restitution slopes were steep or flat. With steep APD restitution, the range of S2S3 intervals resulting in wavebreak increased from 1 ms with S1S2 of 250 ms, to 75 ms (S1S2 180 ms). With flat APD restitution, the range of S2S3 intervals resulting in wavebreak increased from 1 ms (S1S2 250 ms), to 21 ms (S1S2 340 ms) and then 11 ms (S1S2 400 ms).
Conclusion
Regional differences in APD restitution are an arrhythmogenic substrate that can be concealed at normal heart rates. A premature stimulus produces regional differences in repolarisation, and a further premature stimulus can then result in wavebreak and initiate re-entry. This mechanism for initiating re-entry is independent of the steepness of the APD restitution curve
Formation of PAHs and Carbonaceous Solids in Gas-Phase Condensation Experiments
Carbonaceous grains represent a major component of cosmic dust. In order to
understand their formation pathways, they have been prepared in the laboratory
by gas-phase condensation reactions such as laser pyrolysis and laser ablation.
Our studies demonstrate that the temperature in the condensation zone
determines the formation pathway of carbonaceous particles. At temperatures
lower than 1700 K, the condensation by-products are mainly polycyclic aromatic
hydrocarbons (PAHs), that are also the precursors or building blocks for the
condensing soot grains. The low-temperature condensates contain PAH mixtures
that are mainly composed of volatile 3-5 ring systems. At condensation
temperatures higher than 3500 K, fullerene-like carbon grains and fullerene
compounds are formed. Fullerene fragments or complete fullerenes equip the
nucleating particles. Fullerenes can be identified as soluble components.
Consequently, condensation products in cool and hot astrophysical environments
such as cool and hot AGB stars or Wolf Rayet stars should be different and
should have distinct spectral properties.Comment: 7 pages, 5 figure
On the metallicity dependence of crystalline silicates in oxygen-rich asymptotic giant branch stars and red supergiants
We investigate the occurrence of crystalline silicates in oxygen-rich evolved
stars across a range of metallicities and mass-loss rates. It has been
suggested that the crystalline silicate feature strength increases with
increasing mass-loss rate, implying a correlation between lattice structure and
wind density. To test this, we analyse Spitzer IRS and Infrared Space
Observatory SWS spectra of 217 oxygen-rich asymptotic giant branch stars and 98
red supergiants in the Milky Way, the Large and Small Magellanic Clouds and
Galactic globular clusters. These encompass a range of spectral morphologies
from the spectrally-rich which exhibit a wealth of crystalline and amorphous
silicate features to 'naked' (dust-free) stars. We combine spectroscopic and
photometric observations with the GRAMS grid of radiative transfer models to
derive (dust) mass-loss rates and temperature. We then measure the strength of
the crystalline silicate bands at 23, 28 and 33 microns. We detect crystalline
silicates in stars with dust mass-loss rates which span over 3 dex, down to
rates of ~10^-9 solar masses/year. Detections of crystalline silicates are more
prevalent in higher mass-loss rate objects, though the highest mass-loss rate
objects do not show the 23-micron feature, possibly due to the low temperature
of the forsterite grains or it may indicate that the 23-micron band is going
into absorption due to high column density. Furthermore, we detect a change in
the crystalline silicate mineralogy with metallicity, with enstatite seen
increasingly at low metallicity.Comment: Accepted for publication in MNRAS, 24 pages, 16 figure
Stellar evolution and nucleosynthesis of Post-AGB Stars
I discuss recent new models of post-Asymptotic Giant Branch stellar
evolution. These models aim to clarify the evolutionary origin and status of a
variety of hydrogen-deficient post-AGB stars such as central stars of planetary
nebulae of Wolf-Rayet spectral type, PG1159 stars or Sakurai's object. Starting
with AGB models with overshoot such stars can evolve through one of four
distinct channels. Each of these channels has typical abundance patterns
depending on the relative timing of the departure from the AGB and the
occurrence of the last thermal pulse. I discuss the responsible mechanisms and
observational counterparts.Comment: 9 pages, 1 figure, conference paper, workshop "Post-AGB objects
(proto-planetary nebulae) as a phase of stellar evolution", Jul 5-7, 2000,
Torun, Poland, to appear in Ap&S
The first data on the oxygen, carbon, and sulfur isotope composition of the Chelyabinsk meteorite
Element Abundance Determination in Hot Evolved Stars
The hydrogen-deficiency in extremely hot post-AGB stars of spectral class
PG1159 is probably caused by a (very) late helium-shell flash or a AGB final
thermal pulse that consumes the hydrogen envelope, exposing the usually-hidden
intershell region. Thus, the photospheric element abundances of these stars
allow us to draw conclusions about details of nuclear burning and mixing
processes in the precursor AGB stars. We compare predicted element abundances
to those determined by quantitative spectral analyses performed with advanced
non-LTE model atmospheres. A good qualitative and quantitative agreement is
found for many species (He, C, N, O, Ne, F, Si, Ar) but discrepancies for
others (P, S, Fe) point at shortcomings in stellar evolution models for AGB
stars. Almost all of the chemical trace elements in these hot stars can only be
identified in the UV spectral range. The Far Ultraviolet Spectroscopic Explorer
and the Hubble Space Telescope played a crucial role for this research.Comment: To appear in: Recent Advances in Spectroscopy: Theoretical,
Astrophysical, and Experimental Perspectives, Proceedings, Jan 28 - 31, 2009,
Kodaikanal, India (Springer
UBVJHKLM photometry and modeling of R Coronae Borealis
We present the results of UBVJHKLM photometry of R CrB spanning the period
from 1976 to 2001. Studies of the optical light curve have shown no evidence of
any stable harmonics in the variations of the stellar emission. In the L band
we found semi-regular oscillations with the two main periods of ~3.3 yr and
11.9 yr and the full amplitude of ~0.8 mag and ~0.6 mag, respectively. The
colors of the warm dust shell (resolved by Ohnaka et al. 2001) are found to be
remarkably stable in contrast to its brightness. This indicates that the inner
radius is a constant, time-independent characteristic of the dust shell. The
observed behavior of the IR light curve is mainly caused by the variation of
the optical thickness of the dust shell within the interval \tau(V)= 0.2-0.4.
Anticorrelated changes of the optical brightness (in particular with P ~ 3.3
yr) have not been found. Their absence suggests that the stellar wind of R CrB
deviates from spherical symmetry. The light curves suggest that the stellar
wind is variable. The variability of the stellar wind and the creation of dust
clouds may be caused by some kind of activity on the stellar surface. With some
time lag, periods of increased mass-loss cause an increase in the dust
formation rate at the inner boundary of the extended dust shell and an increase
in its IR brightness. We have derived the following parameters of the dust
shell (at mean brightness) by radiative transfer modeling: inner dust shell
radius r_in ~ 110 R_*, temperature T_dust(r_in) ~ 860 K, dust density
\rho_dust(r_in) ~ 1.1x10^{-20} g cm^-3, optical depth \tau(V) ~ 0.32 at 0.55
micron, mean dust formation rate [dM/dt]_dust ~ 3.1x10^-9 M_sun / yr, mass-loss
rate [dM/dt]_gas ~ 2.1x10^-7 M_sun / yr, size of the amorphous carbon grains
<(~) 0.01 micron, and B-V ~ -0.28.Comment: 9 pages, 6 figures, accepted for publication in A&
Tumor-derived exosomes confer antigen-specific immunosuppression in a murine delayed-type hypersensitivity model
Exosomes are endosome-derived small membrane vesicles that are secreted by most cell types including tumor cells. Tumor-derived exosomes usually contain tumor antigens and have been used as a source of tumor antigens to stimulate anti-tumor immune responses. However, many reports also suggest that tumor-derived exosomes can facilitate tumor immune evasion through different mechanisms, most of which are antigen-independent. In the present study we used a mouse model of delayed-type hypersensitivity (DTH) and demonstrated that local administration of tumor-derived exosomes carrying the model antigen chicken ovalbumin (OVA) resulted in the suppression of DTH response in an antigen-specific manner. Analysis of exosome trafficking demonstrated that following local injection, tumor-derived exosomes were internalized by CD11c+ cells and transported to the draining LN. Exosome-mediated DTH suppression is associated with increased mRNA levels of TGF-β1 and IL-4 in the draining LN. The tumor-derived exosomes examined were also found to inhibit DC maturation. Taken together, our results suggest a role for tumor-derived exosomes in inducing tumor antigen-specific immunosuppression, possibly by modulating the function of APCs. © 2011 Yang et al
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