2,133 research outputs found
The influence of turbulence during magnetized core collapse and its consequences on low-mass star formation
[Abridged] Theoretical and numerical studies of star formation have shown
that magnetic field (B) has a strong influence on both disk formation and
fragmentation; even a relatively low B can prevent these processes. However,
very few studies investigated the combined effects of B and turbulence. We
study the effects of turbulence in magnetized core collapse, focusing on the
magnetic diffusion, the orientation of the angular momentum (J) of the
protostellar core, and on its consequences on disk formation, fragmentation and
outflows. We perform 3D, AMR, MHD simulations of magnetically supercritical
collapsing dense cores of 5 Msun using the MHD code RAMSES. A turbulent
velocity field is imposed as initial conditions, characterised by a Kolmogorov
power spectrum. Different levels of turbulence and magnetization are
investigated, as well as 3 realisations for the turbulent velocity field.
Magnetic diffusion, orientation of the rotation axis with respect to B,
transport of J, disk formation, fragmentation and outflows formation are
studied. The turbulent velocity field imposed as initial conditions contains a
non-zero J, responsible for a misalignment of the rotation axis. Turbulence is
also responsible for an effective turbulent diffusivity in the vicinity of the
core. Both effects are responsible for a significant decrease of the magnetic
braking, and facilitate the formation of early massive disks for not too high
magnetization. Fragmentation can occur even with mu ~ 5 at late time in
contrast with 1 Msun cores for which fragmentation is prevented for such values
of mu. Slow asymmetric outflows are launched. They carry a mass which is
comparable to the mass within the core. Because of misalignment and turbulent
diffusion, massive disk formation is possible though their mass and size are
still reduced compared to the hydrodynamical case. We find that for mu >= 5,
fragmentation can happen.Comment: 15 pages, 21 figures, submitted in A&
Promoting the use of reliable rate based transport protocols: the Chameleon protocol
Rate-based congestion control, such as TFRC, has not been designed to enable reliability. Indeed, the birth of TFRC protocol has resulted from the need for a congestion-controlled transport protocol in order to carry multimedia traffic. However, certain applications still prefer the use of UDP in order to implement their own congestion control on top of it. The present contribution proposes to design and validate a reliable rate-based protocol based on the combined use of TFRC, SACK and an adapted flow control. We argue that rate-based congestion control is a perfect alternative to window-based congestion control as most of today applications need to interact with the transport layer and should not be only limited to unreliable services. In this paper, we detail the implementation of a reliable rate-based protocol named Chameleon and bring out to the networking community an ns-2 implementation for evaluation purpose
Non-leptonic charmless Bc decays and their search at LHCb
We discuss the decay of Bc mesons into two light mesons (pi, K^(*), eta',
rho, omega, phi). All these decay channels come from a single type of diagram,
namely tree annihilation. This allows us to derive extremely simple SU(3)
relations among these processes. The size of annihilation contributions is an
important issue in B physics, and we provide two different estimates in the
case of non-leptonic charmless Bc decays, either a comparison with annihilation
decays of heavy-light mesons or a perturbative model inspired by QCD
factorisation. We finally discuss a possible search for these channels at LHCb.Comment: 22 pages, 1 figur
K-mouflage gravity models that pass Solar System and cosmological constraints
We show that Solar System tests can place very strong constraints on
K-mouflage models of gravity, which are coupled scalar field models with
nontrivial kinetic terms that screen the fifth force in regions of large
gravitational acceleration. In particular, the bounds on the anomalous
perihelion of the Moon imposes stringent restrictions on the K-mouflage
Lagrangian density, which can be met when the contributions of higher-order
operators in the static regime are sufficiently small. The bound on the rate of
change of the gravitational strength in the Solar System constrains the
coupling strength to be smaller than . These two bounds impose
tighter constraints than the results from the Cassini satellite and Big Bang
Nucleosynthesis. Despite the Solar System restrictions, we show that it is
possible to construct viable models with interesting cosmological predictions.
In particular, relative to -CDM, such models predict percent-level
deviations for the clustering of matter and the number density of dark matter
haloes. This makes these models predictive and testable by forthcoming
observational missions.Comment: 15 page
Optical BVRI Photometry of Common Proper Motion F/G/K+M Wide Separation Binaries
We present optical (BVRI) photometric measurements of a sample of 76 common
proper motion wide separation main sequence binary pairs. The pairs are
composed of a F-, G-, or K-type primary star and an M-type secondary. The
sample is selected from the revised NLTT catalog and the LSPM catalog. The
photometry is generally precise to 0.03 mag in all bands. We separate our
sample into two groups, dwarf candidates and subdwarf candidates, using the
reduced proper motion (RPM) diagram constructed with our improved photometry.
The M subdwarf candidates in general have larger colors than the M dwarf
candidates at a given color. This is consistent with an average
metallicity difference between the two groups, as predicted by the
PHOENIX/BT-Settl models. The improved photometry will be used as input into a
technique to determine the metallicities of the M-type stars.Comment: 26 pages, 8 figures, accepted for publication in A
On thermal effects in solid state lasers: the case of ytterbium-doped materials
A review of theoretical and experimental studies of thermal effects in
solid-state lasers is presented, with a special focus on diode-pumped
ytterbium-doped materials. A large part of this review provides however general
information applicable to any kind of solid-state laser. Our aim here is not to
make a list of the techniques that have been used to minimize thermal effects,
but instead to give an overview of the theoretical aspects underneath, and give
a state-of-the-art of the tools at the disposal of the laser scientist to
measure thermal effects. After a presentation of some general properties of
Yb-doped materials, we address the issue of evaluating the temperature map in
Yb-doped laser crystals, both theoretically and experimentally. This is the
first step before studying the complex problem of thermal lensing (part III).
We will focus on some newly discussed aspects, like the definition of the
thermo-optic coefficient: we will highlight some misleading interpretations of
thermal lensing experiments due to the use of the dn/dT parameter in a context
where it is not relevant. Part IV will be devoted to a state-of-the-art of
experimental techniques used to measure thermal lensing. Eventually, in part V,
we will give some concrete examples in Yb-doped materials, where their
peculiarities will be pointed out
Linear complex polarization propagator in a four-component Kohn-Sham framework.
International audienceAn algorithm for the solution of the linear response equation in the random phase approximation is presented. All entities including frequency arguments, matrices, and vectors, are assumed to be complex, and it represents the core equation solver needed in complex polarization propagator approaches where nonstimulated relaxation channels are taken into account. Stability and robustness of the algorithm are demonstrated in applications regarding visible, ultraviolet, and x-ray spectroscopies. An implementation of the algorithm at the level of four-component relativistic, noncollinear, density functional theory for imaginary (but not complex) frequency arguments has been achieved and is used to determine the electric dipole dispersion interaction coefficients for the rubidium and cesium dimers. Our best estimates for the C(6) coefficients of Rb(2) and Cs(2) are equal to 14.0x10(3) and 21.9x10(3) a.u., respectively
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