33,001 research outputs found
Prestellar Core Formation, Evolution, and Accretion from Gravitational Fragmentation in Turbulent Converging Flows
We investigate prestellar core formation and accretion based on
three-dimensional hydrodynamic simulations. Our simulations represent local
pc regions within giant molecular clouds where a supersonic turbulent
flow converges, triggering star formation in the post-shock layer. We include
turbulence and self-gravity, applying sink particle techniques, and explore a
range of inflow Mach number . Two sets of cores are identified
and compared: -cores are identified of a time snapshot in each simulation,
representing dense structures in a single cloud map; -cores
are identified at their individual time of collapse, representing the initial
mass reservoir for accretion. We find that cores and filaments form and evolve
at the same time. At the stage of core collapse, there is a well-defined,
converged characteristic mass for isothermal fragmentation that is comparable
to the critical Bonner-Ebert mass at the post-shock pressure. The core mass
functions (CMFs) of -cores show a deficit of high-mass cores
() compared to the observed stellar initial mass function
(IMF). However, the CMFs of -cores are similar to the observed CMFs and
include many low-mass cores that are gravitationally stable. The difference
between -cores and -cores suggests that the full sample
from observed CMFs may not evolve into protostars. Individual sink particles
accrete at a roughly constant rate throughout the simulations, gaining one
-core mass per free-fall time even after the initial mass
reservoir is accreted. High-mass sinks gain proportionally more mass at late
times than low-mass sinks. There are outbursts in accretion rates, resulting
from clumpy density structures falling into the sinks
Implementation of Sink Particles in the Athena Code
We describe implementation and tests of sink particle algorithms in the
Eulerian grid-based code Athena. Introduction of sink particles enables
long-term evolution of systems in which localized collapse occurs, and it is
impractical (or unnecessary) to resolve the accretion shocks at the centers of
collapsing regions. We discuss similarities and differences of our methods
compared to other implementations of sink particles. Our criteria for sink
creation are motivated by the properties of the Larson-Penston collapse
solution. We use standard particle-mesh methods to compute particle and gas
gravity together. Accretion of mass and momenta onto sinks is computed using
fluxes returned by the Riemann solver. A series of tests based on previous
analytic and numerical collapse solutions is used to validate our method and
implementation. We demonstrate use of our code for applications with a
simulation of planar converging supersonic turbulent flow, in which multiple
cores form and collapse to create sinks; these sinks continue to interact and
accrete from their surroundings over several Myr.Comment: 39 pages, 14 figures, Accepted to ApJ
Towards a warped inflationary brane scanning
We present a detailed systematics for comparing warped brane inflation with
the observations, incorporating the effects of both moduli stabilization and
ultraviolet bulk physics. We explicitly construct an example of the inflaton
potential governing the motion of a mobile D3 brane in the entire warped
deformed conifold. This allows us to precisely identify the corresponding
scales of the cosmic microwave background. The effects due to bulk fluxes or
localized sources are parametrized using gauge/string duality. We next perform
some sample scannings to explore the parameter space of the complete potential,
and first demonstrate that without the bulk effects there can be large
degenerate sets of parameters with observationally consistent predictions. When
the bulk perturbations are included, however, the observational predictions are
generally spoiled. For them to remain consistent, the magnitudes of the bulk
effects need to be highly suppressed via fine tuning.Comment: (v1) 11 pages, 2 figures, 2 tables; (v2) more clarifications and
references added; (v3) 12 pages, more discussions, to appear in Physical
Review
Lyapunov Spectra in SU(2) Lattice Gauge Theory
We develop a method for calculating the Lyapunov characteristic exponents of
lattice gauge theories. The complete Lyapunov spectrum of SU(2) gauge theory is
obtained and Kolmogorov-Sinai entropy is calculated. Rapid convergence with
lattice size is found.Comment: 7pp, DUKE-TH-93-5
All-optical Imprinting of Geometric Phases onto Matter Waves
Traditional optical phase imprinting of matter waves is of a dynamical
nature. In this paper we show that both Abelian and non-Abelian geometric
phases can be optically imprinted onto matter waves, yielding a number of
interesting phenomena such as wavepacket re-directing and wavepacket splitting.
In addition to their fundamental interest, our results open up new
opportunities for robust optical control of matter waves.Comment: 5 pages, 2 figures, to appear in Phys. Rev.
A Pseudospectral Approach to High Index DAE Optimal Control Problems
Historically, solving optimal control problems with high index differential
algebraic equations (DAEs) has been considered extremely hard. Computational
experience with Runge-Kutta (RK) methods confirms the difficulties. High index
DAE problems occur quite naturally in many practical engineering applications.
Over the last two decades, a vast number of real-world problems have been
solved routinely using pseudospectral (PS) optimal control techniques. In view
of this, we solve a "provably hard," index-three problem using the PS method
implemented in DIDO, a state-of-the-art MATLAB optimal control toolbox. In
contrast to RK-type solution techniques, no laborious index-reduction process
was used to generate the PS solution. The PS solution is independently verified
and validated using standard industry practices. It turns out that proper PS
methods can indeed be used to "directly" solve high index DAE optimal control
problems. In view of this, it is proposed that a new theory of difficulty for
DAEs be put forth.Comment: 14 pages, 9 figure
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