599 research outputs found
New constraints on a triaxial model of the Galaxy
We determine the values of parameters of an N-body model for the Galaxy
developed by Fux via comparison with an unbiased, homogeneous sample of OH/IR
stars. Via Monte-Carlo simulation, we find the plausibilities of the
best-fitting models, as well as their errors. The parameters that are
constrained best by these projected data are the total mass of the model and
the viewing angle of the central Bar, although the distribution of the latter
has multiple maxima. The best model has a viewing angle of 44 degrees,
semi-major axis of 2.5 kpc, a bar mass of 1.7E10 solar masses and a tangential
velocity of the local standard of rest of 171 km/s . We argue that the lower
values that are commonly found from stellar data for the viewing angle (around
25 degrees) arise when too few coordinates are available, when the longitude
range is too narrow or when low latitudes are excluded from the fit. The new
constraints on the viewing angle of the galactic Bar from stellar line-of-sight
velocities decrease further the ability of the Bar's distribution to account
for the observed micro-lensing optical depth toward Baade's window : our model
reproduces only half the observed value. The signal of triaxiality diminishes
quickly with increasing latitude, fading within approximately one scaleheight.
This suggests that Baade's window is not a very appropriate region to sample
Bar properties.Comment: 10 pages, 8 figures, TeX, accepted for publication in MNRA
The Effect of the Outer Lindblad Resonance of the Galactic Bar on the Local Stellar Velocity Distribution
Hydro-dynamical modeling of the inner Galaxy suggest that the radius of the
outer Lindblad resonance (OLR) of the Galactic bar lies in the vicinity of the
Sun. How does this resonance affect the distribution function in the outer
parts of a barred disk, and can we identify any effect of the resonance in the
velocity distribution f(v) actually observed in the solar neighborhood? To
answer these questions, detailed simulations of f(v) in the outer parts of an
exponential stellar disks with nearly flat rotation curves and a rotating
central bar have been performed. For a model resembling the old stellar disk,
the OLR causes a distinct feature in f(v) over a significant fraction of the
outer disk. For positions <2kpc outside the OLR radius and at bar angles of
\~10-70 degrees, f(v) inhibits a bi-modality between the low-velocity stars
moving like the local standard of rest (LSR) and a secondary mode of stars
predominantly moving outward and rotating more slowly than the LSR.
Such a bi-modality is indeed present in f(v) inferred from the Hipparcos data
for late-type stars in the solar neighborhood. If one interpretes this observed
bi-modality as induced by the OLR -- and there are hardly any viable
alternatives -- then one is forced to deduce that the OLR radius is slightly
smaller than Ro. Moreover, by a quantitative comparison of the observed with
the simulated distributions one finds that the pattern speed of the bar is
1.85+/-0.15 times the local circular frequency, where the error is dominated by
the uncertainty in bar angle and local circular speed.
Also other, less prominent but still significant, features in the observed
f(v) resemble properties of the simulated velocity distributions, in particular
a ripple caused by orbits trapped in the outer 1:1 resonance.Comment: 14 pages, 10 figures (Fig.2 in full resolution available upon
request), accepted for publication in A
Dualâregulated expression of C/EBPâα and BMPâ2 enables differential differentiation of C2C12 cells into adipocytes and osteoblasts
CCAAT/enhancerâbinding proteins (C/EBPs) as well as bone morphogenic proteins (BMPs) play essential roles in mammalian cell differentiation in shaping adipogenic and osteoblastic lineages in particular. Recent evidence suggested that adipocytes and osteoblasts share a common mesenchymal precursor cell phenotype. Yet, the molecular details underlying the decision of adipocyte versus osteoblast differentiation as well as the involvement of C/EBPs and BMPs remains elusive. We have engineered C2C12 cells for dualâregulated expression of human C/EBPâα and BMPâ2 to enable independent transcription control of both differentiation factors using clinically licensed antibiotics of the streptogramin (pristinamycin) and tetracycline (tetracycline) classes. Differential as well as coordinated expression of C/EBPâα and BMPâ2 revealed that (i) C/EBPâα may differentiate C2C12 myoblasts into adipocytes as well as osteoblasts, (ii) BMPâ2 prevents myotube differentiation, (iii) is incompetent in differentiating C2C12 into osteoblasts and (iv) even decreases C/EBPâα's osteoblastâspecific differentiation potential but (v) cooperates with C/EBPâα on adipocyte differentiation, (vi) osteoblast formation occurs at low C/EBPâα levels while adipocyteâspecific differentiation requires maximum C/EBPâα expression and that (vii) BMPâ2 may bias the C/EBPâαâmediated adipocyte versus osteoblast differentiation switch towards fat cell formation. Dualâregulated expression technology enabled precise insight into combinatorial effects of two key differentiation factors involved in adipocyte/osteoblast lineage control which could be implemented in rational reprogramming of multipotent cells into desired cell phenotypes tailored for gene therapy and tissue engineerin
The Pattern Speed of the Galactic Bar
Most late-type stars in the solar neighborhood have velocities similar to the
local standard of rest (LSR), but there is a clearly separated secondary
component corresponding to a slower rotation and a mean outward motion.
Detailed simulations of the response of a stellar disk to a central bar show
that such a bi-modality is expected from outer-Lindblad resonant scattering.
When constraining the run of the rotation curve by the proper motion of Sgr A*
and the terminal gas velocities, the value observed for the rotation velocity
separating the two components results in a value of (53+/-3)km/s/kpc for the
pattern speed of the bar, only weakly dependent on the precise values for Ro
and bar angle phi.Comment: 5 pages LaTeX, 2 Figs, accepted for publication in ApJ Letter
HIV-Infektion : Update 2009 fĂŒr HausĂ€rzte. Teil 1
Quintessenz: In der Schweiz werden jÀhrlich >750 HIV-Infektionen neu diagnostiziert. Die Ansteckung geschieht via Blut oder Sex, und nicht bei alltÀglichen
sozialen Kontakten. Die CD4-Lymphozytenzahl ist bestens etabliert zur Messung der Immunsuppression. Die grösste Gefahr opportunistischer Infekte droht bei CD4-Werten <200/ÎŒl. Dank antiretroviraler Therapie (ART) sollten viele HIV-infizierte Patienten in der Schweiz eine quasi normale Lebenserwartung haben. Momentan ist eine ART bei CD4-Werten <350/ÎŒl indiziert. Die HIV-Diagnose wird auch in der Schweiz hĂ€ufig zu spĂ€t gestellt. HausĂ€rzte spielen bei der frĂŒhzeitigen Diagnosestellung (grosszĂŒgiges Anbieten von HIV-Tests) und bei der PrĂ€vention von Immundefizienz und
opportunistischen Komplikationen eine entscheidende Rolle.
P Bei HIV-Neudiagnose soll der Patient einem HIV-Spezialisten zugewiesen werden, um weitere AbklĂ€rungen durchzufĂŒhren und die Indikation zum ART-Beginn zu stellen
A method for comparing discrete kinematic data and N-body simulations
This paper describes a method for quantitatively comparing an N-body model
with a sample of discrete kinematic data. The comparison has two stages: (i)
finding the optimum scaling and orientation of the model relative to the data;
and (ii) calculating a goodness of fit, and hence assessing the plausibility of
the model in vew of the data. The method derives from considering the data and
model both as samples from some underlying binned distribution function, and
applying probability theory arguments. As an example, I consider a published
N-body model for the Galactic bulge and disc, and fictitious l,b,v
measurements, and recover (with error estimates) the spatial and velocity
scales of the model and the orientation of the bar. The fictitious data are
actually derived from the model by assuming the mass scale and the solar
position, but their size and extent mimics a recent survey of OH/IR stars. The
results indicate that mass of the bulge and our viewing angle of the bar are
usefully estimable from current surveys.Comment: To appear in A
Formation of Nuclear Spirals in Barred Galaxies
We have performed smoothed particle hydrodynamics (SPH) simulations for the
response of the gaseous disk to the imposed moderately strong non-axisymmetric
potentials. The model galaxies are composed of the three stellar components
(disk, bulge and bar) and two dark ones (supermassive black hole and halo)
whose gravitational potentials are assumed to be invariant in time in the frame
corotating with the bar. We found that the torques alone generated by the
moderately strong bar that gives the maximum of tangential-to-radial force
ratio as are not sufficient to drive the gas
particles close to the center due to the barrier imposed by the inner Lindblad
resonances (ILRs). In order to transport the gas particles towards the nucleus
( pc), a central supermassive black hole (SMBH) and high sound speed of
the gas are required to be present. The former is required to remove the inner
inner Lindblad resonance (IILR) that prevents gas inflow close to the nucleus,
while the latter provides favourable conditions for the gas particles to lose
their angular momentum and to spiral in. Our models that have no IILR show the
trailing nuclear spirals whose innermost parts reach close to the center in a
curling way when the gas sound speed is km s. They
resemble the symmetric two-armed nuclear spirals observed in the central
kiloparsec of spiral galaxies. We found that the symmetric two-armed nuclear
spirals are formed by the hydrodynamic spiral shocks caused by the
gravitational torque of the bar in the presence of a central SMBH that can
remove IILR when the sound speed of gas is high enough to drive a large amount
of gas inflow deep inside the ILR. However, the detailed morphology of nuclear
spirals depends on the sound speed of gas.Comment: 38 pages, 10 figures, accepted for publication in Ap
An inner ring and the micro lensing toward the Bulge
All current Bulge-Disk models for the inner Galaxy fall short of reproducing
self-consistently the observed micro-lensing optical depth by a factor of two
(). We show that the least mass-consuming way to increase the
optical depth is to add density roughly half-way the observer and the highest
micro-lensing-source density. We present evidence for the existence of such a
density structure in the Galaxy: an inner ring, a standard feature of barred
galaxies. Judging from data on similar rings in external galaxies, an inner
ring can contribute more than 50% of a pure Bulge-Disk model to the
micro-lensing optical depth. We may thus eliminate the need for a small viewing
angle of the Bar. The influence of an inner ring on the event-duration
distribution, for realistic viewing angles, would be to increase the fraction
of long-duration events toward Baade's window. The longest events are expected
toward the negative-longitude tangent point at -22\degr . A properly
sampled event-duration distribution toward this tangent point would provide
essential information about viewing angle and elongation of the over-all
density distribution in the inner Galaxy.Comment: 9 pages, 7(15) figs, LaTeX, AJ (accepted
Optimizing performance of quantum operations with non-Markovian decoherence: the tortoise or the hare?
The interaction between a quantum system and its environment limits our
ability to control it and perform quantum operations on it. We present an
efficient method to find optimal controls for quantum systems coupled to
non-Markovian environments, by using the process tensor to compute the gradient
of an objective function. We consider state transfer for a driven two-level
system coupled to a bosonic environment, and characterize performance in terms
of speed and fidelity. We thus determine the best achievable fidelity as a
function of process duration. We show there is a trade-off between speed and
fidelity, and that slower processes can have higher fidelity by exploiting
non-Markovian effects.Comment: 6 pages, 3 figure
Development and validation of a risk score for chronic kidney disease in HIV infection using prospective cohort data from the D:A:D study.
Chronic kidney disease (CKD) is a major health issue for HIV-positive individuals, associated with increased morbidity and mortality. Development and implementation of a risk score model for CKD would allow comparison of the risks and benefits of adding potentially nephrotoxic antiretrovirals to a treatment regimen and would identify those at greatest risk of CKD. The aims of this study were to develop a simple, externally validated, and widely applicable long-term risk score model for CKD in HIV-positive individuals that can guide decision making in clinical practice
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