384 research outputs found
Space construction system analysis. Part 2: Construction analysis
The construction methods specific to the end to end construction process for building the ETVP in low Earth orbit, using the space shuttle orbiter as a construction base, are analyzed. The analyses concerned three missions required to build the basic platform. The first mission involved performing the fabrication of beams in space and assembling the beams into a basic structural framework. The second mission was to install the forward support structure and aft support structure, the forward assembly, and a TT&C antenna. The third mission plan was to complete the construction of the platform and activate it to begin operations in low Earth orbit. The integration of the activities for each mission is described along with the construction requirements and construction logic
Global existence for semilinear reaction-diffusion systems on evolving domains
We present global existence results for solutions of reaction-diffusion
systems on evolving domains. Global existence results for a class of
reaction-diffusion systems on fixed domains are extended to the same systems
posed on spatially linear isotropically evolving domains. The results hold
without any assumptions on the sign of the growth rate. The analysis is valid
for many systems that commonly arise in the theory of pattern formation. We
present numerical results illustrating our theoretical findings.Comment: 24 pages, 3 figure
The temperature dependency of Wolf-Rayet-type mass loss: An exploratory study for winds launched by the hot iron bump
CONTEXT: The mass loss of He-burning stars, which are partially or completely
stripped of their outer hydrogen envelope, is a catalyst of the cosmic matter
cycle and decisive ingredient of massive star evolution. Yet, its theoretical
fundament is only starting to emerge with major dependencies still to be
uncovered.
AIMS: A temperature or radius dependence is usually not included in
descriptions for the mass loss of classical Wolf-Rayet (cWR) stars, despite
being crucial for other hot star wind domains. We thus aim to determine whether
such a dependency will also be necessary for a comprehensive description of
mass loss in the cWR regime.
METHODS: Sequences of dynamically consistent atmosphere models were
calculated with the hydrodynamic branch of the PoWR code along the temperature
domain, using different choices for luminosity, mass, and surface abundances.
For the first time, we allowed nonmonotonic velocity fields when solving the
equation of motion. The resulting velocity structures were then interpolated
for the comoving-frame radiative transfer, ensuring that the main wind
characteristics were preserved.
RESULTS: We find a strong dependence of the mass-loss rate with the
temperature of the critical/sonic point which mainly reflects the different
radii and resulting gravitational accelerations. Moreover, we obtain a relation
between the observed effective temperature and the transformed mass-loss rate
which seems to be largely independent of the underlying stellar parameters. The
relation shifts for different clumping factors in the outer wind. Below a
characteristic value of -4.5, the slope of this relation changes and the winds
become transparent for He II ionizing photons.
CONCLUSIONS: The mass loss of cWR stars is a high-dimensional problem but
also shows inherent scalings which can be used to obtain an approximation of
the observed effective temperature. (...)Comment: 16 pages + 5 page appendix, 17+9 figures, 3+2 tables. Accepted for
publication in A&
Co-occurrence of resonant activation and noise-enhanced stability in a model of cancer growth in the presence of immune response
We investigate a stochastic version of a simple enzymatic reaction which
follows the generic Michaelis-Menten kinetics. At sufficiently high
concentrations of reacting species, the molecular fluctuations can be
approximated as a realization of a Brownian dynamics for which the model
reaction kinetics takes on the form of a stochastic differential equation.
After eliminating a fast kinetics, the model can be rephrased into a form of a
one-dimensional overdamped Langevin equation. We discuss physical aspects of
environmental noises acting in such a reduced system, pointing out the
possibility of coexistence of dynamical regimes where noise-enhanced stability
and resonant activation phenomena can be observed together.Comment: 18 pages, 11 figures, published in Physical Review E 74, 041904
(2006
Magnetic nanoparticles with bulk-like properties
The magnetic behavior of nanoparticles synthesized either by
high-temperature decomposition of an organic iron precursor or low-temperature
co-precipitation in aqueous conditions, is compared. Transmission electron
microscopy, X-ray absorption spectroscopy, X-ray magnetic circular dichroism
and magnetization measurements show that nanoparticles synthesized by thermal
decomposition display high crystal quality and bulk-like magnetic and
electronic properties, while nanoparticles synthesized by co-precipitation show
much poorer crystallinity and particle-like phenomenology, including reduced
magnetization, high closure fields and shifted hysteresis loops. The key role
of the crystal quality is thus suggested since particle-like behavior for
particles larger than about 5 nm is only observed when they are structurally
defective. These conclusions are supported by Monte Carlo simulations. It is
also shown that thermal decomposition is capable of producing nanoparticles
that, after further stabilization in physiological conditions, are suitable for
biomedical applications such as magnetic resonance imaging or bio-distribution
studies.Comment: Review article. 20 pages, 7 figures. Invited contribution to 55th MMM
Conference 2010 (Atlanta), Session HG-01. To be published in J. Appl. Phy
Close-up of primary and secondary asteroseismic CoRoT targets and the ground-based follow-up observations
To optimise the science results of the asteroseismic part of the CoRoT
satellite mission a complementary simultaneous ground-based observational
campaign is organised for selected CoRoT targets. The observations include both
high-resolution spectroscopic and multi-colour photometric data. We present the
preliminary results of the analysis of the ground-based observations of three
targets. A line-profile analysis of 216 high-resolution FEROS spectra of the
delta Sct star HD 50844 reveals more than ten pulsation frequencies in the
frequency range 5-18 c/d, including possibly one radial fundamental mode (6.92
c/d). Based on more than 600 multi-colour photometric datapoints of the beta
Cep star HD180642, spanning about three years and obtained with different
telescopes and different instruments, we confirm the presence of a dominant
radial mode nu1=5.48695 c/d, and detect also its first two harmonics. We find
evidence for a second mode nu2=0.3017 c/d, possibly a g-mode, and indications
for two more frequencies in the 7-8 c/d domain. From Stromgren photometry we
find evidence for the hybrid delta Sct/gamma Dor character of the F0 star HD
44195, as frequencies near 3 c/d and 21 c/d are detected simultaneously in the
different filters.Comment: 7 pages, 6 figures, HELAS II International Conference
"Helioseismology, Asteroseismology and MHD Connections", 2008, J.Phys.: Conf.
Ser. 118, 01207
Differential expression of lncRNAs during the HIV replication cycle: an underestimated layer in the HIV-host interplay.
Studying the effects of HIV infection on the host transcriptome has typically focused on protein-coding genes. However, recent advances in the field of RNA sequencing revealed that long non-coding RNAs (lncRNAs) add an extensive additional layer to the cell's molecular network. Here, we performed transcriptome profiling throughout a primary HIV infection in vitro to investigate lncRNA expression at the different HIV replication cycle processes (reverse transcription, integration and particle production). Subsequently, guilt-by-association, transcription factor and co-expression analysis were performed to infer biological roles for the lncRNAs identified in the HIV-host interplay. Many lncRNAs were suggested to play a role in mechanisms relying on proteasomal and ubiquitination pathways, apoptosis, DNA damage responses and cell cycle regulation. Through transcription factor binding analysis, we found that lncRNAs display a distinct transcriptional regulation profile as compared to protein coding mRNAs, suggesting that mRNAs and lncRNAs are independently modulated. In addition, we identified five differentially expressed lncRNA-mRNA pairs with mRNA involvement in HIV pathogenesis with possible cis regulatory lncRNAs that control nearby mRNA expression and function. Altogether, the present study demonstrates that lncRNAs add a new dimension to the HIV-host interplay and should be further investigated as they may represent targets for controlling HIV replication
Collective pulsational velocity broadening due to gravity modes as a physical explanation for macroturbulence in hot massive stars
We aimed at finding a physical explanation for the occurrence of
macroturbulence in the atmospheres of hot massive stars, a phenomenon found in
observations since more than a decade but yet unexplained. We computed time
series of line profiles for evolved massive stars broadened by rotation and by
hundreds of low-amplitude nonradial gravity-mode pulsations which are predicted
to be excited for evolved massive stars. In general, line profiles based on
macrotubulent broadening can mimic those subject to pulsational broadening. In
several cases, though, good fits require macroturbulent velocities that pass
the speed of sound for realistic pulsation amplitudes. Moreover, we find that
the rotation velocity can be seriously underestimated by using a simple
parameter description for macroturbulence rather than an appropriate
pulsational model description to fit the line profiles. We conclude that
macroturbulence is a likely signature of the collective effect of pulsations.
We provide line diagnostics and their typical values to decide whether or not
pulsational broadening is present in observed line profiles, as well as a
procedure to avoid an inaccurate estimation of the rotation velocity.Comment: 12 pages, 9 figures, accepted for publication by Astronomy and
Astrophysic
Tobacco\u27s Minor Alkaloids: Effects on Place Conditioning and Nucleus Accumbens Dopamine Release in Adult and Adolescent Rats
Tobacco products are some of the most commonly used psychoactive drugs worldwide. Besides nicotine, alkaloids in tobacco include cotinine, myosmine, and anatabine. Scientific investigation of these constituents and their contribution to tobacco dependence is less well developed than for nicotine. The present study evaluated the nucleus accumbens dopamine-releasing properties and rewarding and/or aversive properties of nicotine (0.2-0.8 mg/kg), cotinine (0.5-5.0 mg/kg), anatabine (0.5-5.0 mg/kg), and myosmine (5.0-20.0 mg/kg) through in vivo microdialysis and place conditioning, respectively, in adult and adolescent male rats. Nicotine increased dopamine release at both ages, and anatabine and myosmine increased dopamine release in adults, but not adolescents. The dopamine release results were not related to place conditioning, as nicotine and cotinine had no effect on place conditioning, whereas anatabine and myosmine produced aversion in both ages. While the nucleus accumbens shell is hypothesized to play a role in strengthening drug-context associations following initiation of drug use, it may have little involvement in the motivational effects of tobacco constituents once these associations have been acquired. Effects of myosmine and anatabine on dopamine release may require a fully developed dopamine system, since no effects of these tobacco alkaloids were observed during adolescence. In summary, while anatabine and myosmine-induced dopamine release in nucleus accumbens may play a role in tobacco dependence in adults, the nature of that role remains to be elucidated
- …