7,619 research outputs found
HARM: A Numerical Scheme for General Relativistic Magnetohydrodynamics
We describe a conservative, shock-capturing scheme for evolving the equations
of general relativistic magnetohydrodynamics. The fluxes are calculated using
the Harten, Lax, and van Leer scheme. A variant of constrained transport,
proposed earlier by T\'oth, is used to maintain a divergence free magnetic
field. Only the covariant form of the metric in a coordinate basis is required
to specify the geometry. We describe code performance on a full suite of test
problems in both special and general relativity. On smooth flows we show that
it converges at second order. We conclude by showing some results from the
evolution of a magnetized torus near a rotating black hole.Comment: 38 pages, 18 figures, submitted to Ap
Stellar Winds on the Main-Sequence I: Wind Model
Aims: We develop a method for estimating the properties of stellar winds for
low-mass main-sequence stars between masses of 0.4 and 1.1 solar masses at a
range of distances from the star.
Methods: We use 1D thermal pressure driven hydrodynamic wind models run using
the Versatile Advection Code. Using in situ measurements of the solar wind, we
produce models for the slow and fast components of the solar wind. We consider
two radically different methods for scaling the base temperature of the wind to
other stars: in Model A, we assume that wind temperatures are fundamentally
linked to coronal temperatures, and in Model B, we assume that the sound speed
at the base of the wind is a fixed fraction of the escape velocity. In Paper II
of this series, we use observationally constrained rotational evolution models
to derive wind mass loss rates.
Results: Our model for the solar wind provides an excellent description of
the real solar wind far from the solar surface, but is unrealistic within the
solar corona. We run a grid of 1200 wind models to derive relations for the
wind properties as a function of stellar mass, radius, and wind temperature.
Using these results, we explore how wind properties depend on stellar mass and
rotation.
Conclusions: Based on our two assumptions about the scaling of the wind
temperature, we argue that there is still significant uncertainty in how these
properties should be determined. Resolution of this uncertainty will probably
require both the application of solar wind physics to other stars and detailed
observational constraints on the properties of stellar winds. In the final
section of this paper, we give step by step instructions for how to apply our
results to calculate the stellar wind conditions far from the stellar surface.Comment: 24 pages, 13 figures, 2 tables, Accepted for publication in A&
ANALYSIS OF THE PERFORMANCE OF AN UWB-BASED COOPERATIVE POSITIONING FOR DIFFERENT CAR PLATOON CONFIGURATIONS
Abstract. The increasing interest in autonomous vehicles motivates the researches aiming at developing reliable positioning system also in conditions challenging for the Global Navigation Satellite Systems (GNSS), such as in urban canyons, tunnels, under quite dense vegetation. The uso of Ultra Wide-Band (UWB) systems is among the quite well known methods for providing reasonable positioning results without exploiting GNSS. UWB systems are typically used indoors, however their use can be of interest also outdoors, in particular when the need is to ensure good positioning results over a quite small area. This paper investigates the use of UWB systems for positioning in the case of terrestrial vehicles, and, more specifically, it focuses on checking the influence of car platoon configurations on the performance of an UWB cooperative positioning system. In the considered tests, where a high percentage of UWB communications was successful, the obtained results show that the car configuration can have a quite remarkable impact on the positioning performance, doubling the obtained median error
Magnetic Effects Change Our View of the Heliosheath
There is currently a controversy as to whether Voyager 1 has already crossed
the Termination Shock, the first boundary of the Heliosphere. The region
between the Termination Shock and the Heliopause, the Helisheath, is one of the
most unknown regions theoretically. In the Heliosheath magnetic effects are
crucial, as the solar magnetic field is compressed at the Termination Shock by
the slowing flow. Recently, our simulations showed that the Heliosheath
presents remarkable dynamics, with turbulent flows and the presence of a jet
flow at the current sheet that is unstable due to magnetohydrodynamic
instabilities \cite{opher,opher1}. In this paper we review these recent
results, and present an additional simulation with constant neutral atom
background. In this case the jet is still present but with reduced intensity.
Further study, e.g., including neutrals and the tilt of the solar rotation from
the magnetic axis, is required before we can definitively address how the
Heliosheath behaves. Already we can say that this region presents remarkable
dynamics, with turbulent flows, indicating that the Heliosheath might be very
different from what we previously thought.Comment: 6 pages, 5 figures, to appear in IGPP 3rd Annual International
Astrophysics Conference, "PHYSICS OF THE OUTER HELIOSPHERE
Crystallization in suspensions of hard spheres: A Monte Carlo and Molecular Dynamics simulation study
The crystallization of a metastable melt is one of the most important non
equilibrium phenomena in condensed matter physics, and hard sphere colloidal
model systems have been used for several decades to investigate this process by
experimental observation and computer simulation. Nevertheless, there is still
an unexplained discrepancy between simulation data and experimental nucleation
rate densities. In this paper we examine the nucleation process in hard spheres
using molecular dynamics and Monte Carlo simulation. We show that the
crystallization process is mediated by precursors of low orientational
bond-order and that our simulation data fairly match the experimental data
sets
Quantum dynamics, dissipation, and asymmetry effects in quantum dot arrays
We study the role of dissipation and structural defects on the time evolution
of quantum dot arrays with mobile charges under external driving fields. These
structures, proposed as quantum dot cellular automata, exhibit interesting
quantum dynamics which we describe in terms of equations of motion for the
density matrix. Using an open system approach, we study the role of asymmetries
and the microscopic electron-phonon interaction on the general dynamical
behavior of the charge distribution (polarization) of such systems. We find
that the system response to the driving field is improved at low temperatures
(and/or weak phonon coupling), before deteriorating as temperature and
asymmetry increase. In addition to the study of the time evolution of
polarization, we explore the linear entropy of the system in order to gain
further insights into the competition between coherent evolution and
dissipative processes.Comment: 11pages,9 figures(eps), submitted to PR
The Puzzle of the Flyby Anomaly
Close planetary flybys are frequently employed as a technique to place
spacecraft on extreme solar system trajectories that would otherwise require
much larger booster vehicles or may not even be feasible when relying solely on
chemical propulsion. The theoretical description of the flybys, referred to as
gravity assists, is well established. However, there seems to be a lack of
understanding of the physical processes occurring during these dynamical
events. Radio-metric tracking data received from a number of spacecraft that
experienced an Earth gravity assist indicate the presence of an unexpected
energy change that happened during the flyby and cannot be explained by the
standard methods of modern astrodynamics. This puzzling behavior of several
spacecraft has become known as the flyby anomaly. We present the summary of the
recent anomalous observations and discuss possible ways to resolve this puzzle.Comment: 6 pages, 1 figure. Accepted for publication by Space Science Review
Locating Depots for Capacitated Vehicle Routing
We study a location-routing problem in the context of capacitated vehicle
routing. The input is a set of demand locations in a metric space and a fleet
of k vehicles each of capacity Q. The objective is to locate k depots, one for
each vehicle, and compute routes for the vehicles so that all demands are
satisfied and the total cost is minimized. Our main result is a constant-factor
approximation algorithm for this problem. To achieve this result, we reduce to
the k-median-forest problem, which generalizes both k-median and minimum
spanning tree, and which might be of independent interest. We give a
(3+c)-approximation algorithm for k-median-forest, which leads to a
(12+c)-approximation algorithm for the above location-routing problem, for any
constant c>0. The algorithm for k-median-forest is just t-swap local search,
and we prove that it has locality gap 3+2/t; this generalizes the corresponding
result known for k-median. Finally we consider the "non-uniform"
k-median-forest problem which has different cost functions for the MST and
k-median parts. We show that the locality gap for this problem is unbounded
even under multi-swaps, which contrasts with the uniform case. Nevertheless, we
obtain a constant-factor approximation algorithm, using an LP based approach.Comment: 12 pages, 1 figur
A mutation in caspase-9 decreases the expression of BAFFR and ICOS in patients with immunodeficiency and lymphoproliferation
Lymphocyte apoptosis is mainly induced by either death receptor-dependent activation of caspase-8 or mitochondria-dependent activation of caspase-9. Mutations in caspase-8 lead to autoimmunity/lymphoproliferation and immunodeficiency. This work describes a heterozygous H237P mutation in caspase-9 that can lead to similar disorders. H237P mutation was detected in two patients: Pt1 with autoimmunity/lymphoproliferation, severe hypogammaglobulinemia and Pt2 with mild hypogammaglobulinemia and Burkitt lymphoma. Their lymphocytes displayed defective caspase-9 activity and decreased apoptotic and activation responses. Transfection experiments showed that mutant caspase-9 display defective enzyme and proapoptotic activities and a dominant-negative effect on wild-type caspase-9. Ex vivo analysis of the patients' lymphocytes and in vitro transfection experiments showed that the expression of mutant caspase-9 correlated with a downregulation of BAFFR (B-cell-activating factor belonging to the TNF family (BAFF) receptor) in B cells and ICOS (inducible T-cell costimulator) in T cells. Both patients carried a second inherited heterozygous mutation missing in the relatives carrying H237P: Pt1 in the transmembrane activator and CAML interactor (TACI) gene (S144X) and Pt2 in the perforin (PRF1) gene (N252S). Both mutations have been previously associated with immunodeficiencies in homozygosis or compound heterozygosis. Taken together, these data suggest that caspase-9 mutations may predispose to immunodeficiency by cooperating with other genetic factors, possibly by downregulating the expression of BAFFR and ICO
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