823 research outputs found
On the Correlation Between Metallicity and the X-Shaped Morphology of the Milky Way Bulge
We demonstrate that failure to properly account for stellar evolution can
bias results in determinations of the spatial morphology of Galactic bulge
stars, focusing on the question of whether or not the X-shape is more
pronounced among the more metal-rich stars than among the metal-poor stars. We
argue that this trend, a result recently claimed by three separate groups, may
have been overestimated as it is relatively easier to detect a bimodality in
the distance distribution function at higher metallicities. This is due to
three factors. First, the intrinsic colour of red clump and red giant stars
vary with metallicity, at the level d(V-I)_{RC}/d\[M/H] ~ 0.25 mag dex^{-1},
and thus the ratio of red clump to red giant stars within a spectroscopic
sample will depend on the photometric selection of any investigation. Second,
the duration of ascent of the red giant branch goes down and the red clump
lifetime goes up as metallicity increases, which has the effect of increasing
the ratio of red clump to red giant stars by as much as ~33% over the range of
the bulge metallicity-distribution function. Finally, over the same metallicity
interval, the effective number of red giant branch bump stars is predicted to
increase by ~200%, and their presence becomes degenerate with the observational
parameters of the two red clumps, creating an illusory increase in
signal-to-noise for a second peak in the distance modulus distribution.Comment: 6 pages, 2 figures, accepted for publication in MNRA
A New Photometric Model of the Galactic Bar using Red Clump Giants
We present a study of the luminosity density distribution of the Galactic bar
using number counts of red clump giants (RCGs) from the OGLE-III survey. The
data were recently published by Nataf et al. (2013) for 9019 fields towards the
bulge and have RC stars over a viewing area of . The data include the number counts, mean distance modulus
(), dispersion in and full error matrix, from which we fit the data
with several tri-axial parametric models. We use the Markov Chain Monte Carlo
(MCMC) method to explore the parameter space and find that the best-fit model
is the model, with the distance to the GC is 8.13 kpc, the ratio of
semi-major and semi-minor bar axis scale lengths in the Galactic plane
, and vertical bar scale length , is (close to being prolate). The scale length of the stellar
density profile along the bar's major axis is 0.67 kpc and has an angle
of , slightly larger than the value obtained from a similar study
based on OGLE-II data. The number of estimated RC stars within the field of
view is , which is systematically lower than the observed
value. We subtract the smooth parametric model from the observed counts and
find that the residuals are consistent with the presence of an X-shaped
structure in the Galactic centre, the excess to the estimated mass content is
. We estimate the total mass of the bar is . Our results can be used as a key ingredient to construct new density
models of the Milky Way and will have implications on the predictions of the
optical depth to gravitational microlensing and the patterns of hydrodynamical
gas flow in the Milky Way.Comment: 15 pages, 6 figures, 4 tables. MNRAS accepte
What is responsible for thermal coupling in layered convection ?
Laboratory experiments have been conducted on convection in a layered system. The system consists in two liquid layers of equal thickness. The liquids are immiscible : the upper one is silicon oil, and the lower one is glycerol. The structure of convection has been analysed, and data obtained both on the temperature field and the velocity field. It is shown that the coupling between the two convecting systems in « thermal », i.e. convection cells are superposed with uprising currents above uprisings. This result is surprising because it contradicts numerical experiments recently obtained for layered convection. These find « mechanical » coupling (cells are superposed but turn in opposite senses) to be the stable mode for the conditions we tried to reproduce in the laboratory. Several tests have been conducted in order to isolate the phenomenon which is responsible for the discrepancy between the two types of analyses. A tentative mechanism is proposed : it involves an equivalent interfacial longitudinal viscosity, whose origin is not yet clearly understood
Protected quantum computation with multiple resonators in ultrastrong coupling circuit QED
We investigate theoretically the dynamical behavior of a qubit obtained with
the two ground eigenstates of an ultrastrong coupling circuit-QED system
consisting of a finite number of Josephson fluxonium atoms inductively coupled
to a transmission line resonator. We show an universal set of quantum gates by
using multiple transmission line resonators (each resonator represents a single
qubit). We discuss the intrinsic 'anisotropic' nature of noise sources for
fluxonium artificial atoms. Through a master equation treatment with colored
noise and manylevel dynamics, we prove that, for a general class of anisotropic
noise sources, the coherence time of the qubit and the fidelity of the quantum
operations can be dramatically improved in an optimal regime of ultrastrong
coupling, where the ground state is an entangled photonic 'cat' state.Comment: Added results with N = 3,4,5 Josephson atoms and different anisotropy
ratios for the decoherence channels in the new figures 2 and
Strong-field dynamo action in rapidly rotating convection with no inertia
The earth's magnetic field is generated by dynamo action driven by convection in the outer core. For numerical reasons, inertial and viscous forces play an important role in geodynamo models; however, the primary dynamical balance in the earth's core is believed to be between buoyancy, Coriolis, and magnetic forces. The hope has been that by setting the Ekman number to be as small as computationally feasible, an asymptotic regime would be reached in which the correct force balance is achieved. However, recent analyses of geodynamo models suggest that the desired balance has still not yet been attained. Here we adopt a complementary approach consisting of a model of rapidly rotating convection in which inertial forces are neglected from the outset. Within this framework we are able to construct a branch of solutions in which the dynamo generates a strong magnetic field that satisfies the expected force balance. The resulting strongly magnetized convection is dramatically different from the corresponding solutions in which the field is weak
Simulation of Laser Propagation in a Plasma with a Frequency Wave Equation
The aim of this work is to perform numerical simulations of the propagation
of a laser in a plasma. At each time step, one has to solve a Helmholtz
equation in a domain which consists in some hundreds of millions of cells. To
solve this huge linear system, one uses a iterative Krylov method with a
preconditioning by a separable matrix. The corresponding linear system is
solved with a block cyclic reduction method. Some enlightments on the parallel
implementation are also given. Lastly, numerical results are presented
including some features concerning the scalability of the numerical method on a
parallel architecture
D010 Mesenchymal stem cells protect cardiomyocytes from reperfusion injury through a paracrine activation of the PI3 kinase pathway
ObjectivesPrevious data suggest that implantation of mesenchymal stem cells (MSCs) improves heart function after myocardial infarction. We investigated whether protection afforded by MSCs might involve a paracrine activation of the PI3 kinase pathway in reperfused cardiomyocytes.MethodMSCs and neonatal rat cardiomyocytes (NRCs) were isolated and cultured separately. NRCs (2.106) were subjected to 5 hours of ischemia followed by 16 hours of reperfusion. At the time of reperfusion, NRCs (n=8-14/group) received either fresh medium (control group), or the following treatments: MSCs (2.105 MSCs in fresh medium), conditioned SN (MSCs supernatant alone (i.e. without MSCs) obtained after 8 hours of serum deprived culture), [conditioned SN + LY294002] (15 microM of LY294002 a specifi c inhibitor of PI3K), [conditioned SN + Wortmannin] (100 nM of wortmannin, a non specifi c inhibitor of PI3K), or CsA (200 nM in fresh medium) a potent inhibitor of the mitochondrial permeability transition pore. Cell death was assessed by LDH release in NRCs supernatant at the end of reperfusion.ResultsAs expected, LDH activity was dramatically reduced by CsA, averaging 4 % of control values. LDH activity was signifi cantly reduced by MSCs alone and by conditioned SN, averaging 29 % and 12 % of control value, respectively. Both LY294002 and wortmannin signifi cantly attenuated conditioned SN induced protection.Conclusionour data suggest that MSCs can protect NRCs from reperfusion injury through a paracrine activation of the PI3K pathway
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