39,456 research outputs found
Precessing Jets and Molecular Outflows: A 3-D Numerical Study
We present 3-D numerical hydrodynamical simulations of precessing supersonic
heavy jets to explore how well they serve as a model for generating molecular
outflows from Young Stellar Objects. The dynamics are studied with a number of
high resolution simulations on a Cartesian grid (128x128x128 zones) using a
high order finite difference method. A range of cone angles and precession
rates were included in the study. Two higher resolution runs (256x256x256
zones) were made for comparison in order to confirm numerical convergence of
global flow characteristics. Morphological, kinematical and dynamical
characteristics of precessing jets are described and compared to important
properties of straight jets and also to observations of YSOs. In order to
examine the robustness of precessing jets as a mean to produce molecular
outflows around Young Stellar Objects, ``synthetic observations'' of the
momentum distributions of the simulated precessing jets are compared to
observations of molecular outflows. It is found that precessing jets match
better the morphology, highly forward driven momentum and momentum
distributions along the long axis of molecular outflows than do wind-driven or
straight jet-driven flow models.Comment: Accepted by ApJ, 31 pages, using aasms.sty, Also available in
postscript with figures via a gzipped tar file at
ftp://s1.msi.umn.edu/pub/afrank/3DJet/3DJet.tar.gz . For information contact
[email protected]
Stochastic Generation of Particle Structures with Controlled Degree of Heterogeneity
The recently developed void expansion method (VEM) allows for an efficient
generation of porous packings of spherical particles over a wide range of
volume fractions. The method is based on a random placement of the structural
particles under addition of much smaller "void-particles" whose radii are
repeatedly increased during the void expansion. Thereby, they rearrange the
structural particles until formation of a dense particle packing and introduce
local heterogeneities in the structure. In this paper, microstructures with
volume fractions between 0.4 and 0.6 produced by VEM are analyzed with respect
to their degree of heterogeneity (DOH). In particular, the influence of the
void- to structural particle number ratio, which constitutes a principal
VEM-parameter, on the DOH is studied. The DOH is quantified using the pore size
distribution, the Voronoi volume distribution and the density-fluctuation
method in conjunction with fit functions or integral measures. This analysis
has revealed that for volume fractions between 0.4 and 0.55 the void-particle
number allows for a quasi-continuous adjustment of the DOH. Additionally, the
DOH-range of VEM-generated microstructures with a volume fraction of 0.4 is
compared to the range covered by microstructures generated using previous
Brownian dynamics simulations, which represent the structure of coagulated
colloidal suspensions. Both sets of microstructures cover similarly broad and
overlapping DOH-ranges, which allows concluding that VEM is an efficient method
to stochastically reproduce colloidal microstructures with varying DOH.Comment: 10 pages, 7 figure
Microstructures and Mechanical Properties of Dense Particle Gels: Microstructural Characterization
The macroscopic mechanical properties of densely packed coagulated colloidal
particle gels strongly depend on the local arrangement of the powder particles
on length scales of a few particle diameters. Heterogeneous microstructures
exhibit up to one order of magnitude higher elastic properties and yield
strengths than their homogeneous counterparts. The microstructures of these
gels are analyzed by the straight path method quantifying quasi-linear particle
arrangements of particles. They show similar characteristics than force chains
bearing the mechanical load in granular material. Applying this concept to gels
revealed that heterogeneous colloidal microstructures show a significantly
higher straight paths density and exhibit longer straight paths than their
homogeneous counterparts.Comment: 7 pages, 9 figure
A profit model for spread trading with an application to energy futures
This paper proposes a profit model for spread trading by focusing on the stochastic movement of the price spread and its first hitting time probability density. The model is general in that it can be used for any financial instrument. The advantage of the model is that the profit from the trades can be easily calculated if the first hitting time probability density of the stochastic process is given. We then modify the profit model for a particular market, the energy futures market. It is shown that energy futures spreads are modeled by using a meanreverting process. Since the first hitting time probability density of a mean-reverting process is approximately known, the profit model for energy futures price spreads is given in a computable way by using the parameters of the process. Finally, we provide empirical evidence for spread trades of energy futures by employing historical prices of energy futures (WTI crude oil, heating oil, and natural gas futures) traded on the New York Mercantile Exchange. The results suggest that natural gas futures trading may be more profitable than WTI crude oil and heating oil due to its high volatility in addition to its long-term mean reversion, which offers supportive evidence of the model prediction. --futures spread trading,energy futures markets,mean-reverting process,first hitting,time probability density,profit model,WTI crude oil,heating oil,natural gas
The case of PSR J1911-5958A in the outskirts of NGC 6752: signature of a black hole binary in the cluster core?
We have investigated different scenarios for the origin of the binary
millisecond pulsar PSR J1911-5958A in NGC 6752, the most distant pulsar
discovered from the core of a globular cluster to date. The hypothesis that it
results from a truly primordial binary born in the halo calls for
accretion-induced collapse and negligible recoil speed at the moment of neutron
star formation. Scattering or exchange interactions off cluster stars are not
consistent with both the observed orbital period and its offset position. We
show that a binary system of two black holes with (unequal) masses in the range
of 3-100 solar masses can live in NGC 6752 until present time and can have
propelled PSR J1911-5958A into an eccentric peripheral orbit during the last ~1
Gyr.Comment: Accepted by ApJ Letter. 5 pages, 1 figure, 1 tabl
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