16,306 research outputs found
The pressure distribution in thermally bistable turbulent flows
We present a systematic numerical study of the effect of turbulent velocity
fluctuations on the thermal pressure distribution in thermally bistable flows.
The simulations employ a random turbulent driving generated in Fourier space
rather than star-like heating. The turbulent fluctuations are characterized by
their rms Mach number M and the energy injection wavenumber, k_for. Our results
are consistent with the picture that as either of these parameters is
increased, the local ratio of turbulent crossing time to cooling time
decreases, causing transient structures in which the effective behavior is
intermediate between the thermal-equilibrium and adiabatic regimes. As a
result, the effective polytropic exponent gamma_ef ranges between ~0.2 to ~1.1.
The fraction of high-density zones with P>10^4 Kcm^-3 increases from roughly
0.1% at k_for=2 and M=0.5 to roughly 70% for k_for=16 and M=1.25. A preliminary
comparison with the pressure measurements of Jenkins (2004) favors our case
with M=0.5 and k_for=2. In all cases, the dynamic range of the pressure summed
over the entire density range, typically spans 3-4 orders of magnitude. The
total pressure histogram widens as the Mach number is increased, and develops
near-power-law tails at high (resp.low) pressures when gamma_ef<~ 0.5 (resp.
gamma_ef>~ 1), which occurs at k_for=2 (resp.k_for=16) in our simulations. The
opposite side of the pressure histogram decays rapidly, in an approx. lognormal
form. Our results show that turbulent advection alone can generate large
pressure scatters, with power-law high-P tails for large-scale driving, and
provide validation for approaches attempting to derive the shape of the
pressure histogram through a change of variable from the known form of the
density histogram, such as that performed by MacLow et al.(2004).Comment: to be published in Ap
Modeling two-language competition dynamics
During the last decade, much attention has been paid to language competition
in the complex systems community, that is, how the fractions of speakers of
several competing languages evolve in time. In this paper we review recent
advances in this direction and focus on three aspects. First we consider the
shift from two-state models to three state models that include the possibility
of bilingual individuals. The understanding of the role played by bilingualism
is essential in sociolinguistics. In particular, the question addressed is
whether bilingualism facilitates the coexistence of languages. Second, we will
analyze the effect of social interaction networks and physical barriers.
Finally, we will show how to analyze the issue of bilingualism from a game
theoretical perspective.Comment: 15 pages, 5 figures; published in the Special Issue of Advances in
Complex Systems "Language Dynamics
The Probability Distribution Function of Column Density in Molecular Clouds
(Abridged) We discuss the probability distribution function (PDF) of column
density resulting from density fields with lognormal PDFs, applicable to
isothermal gas (e.g., probably molecular clouds). We suggest that a
``decorrelation length'' can be defined as the distance over which the density
auto-correlation function has decayed to, for example, 10% of its zero-lag
value, so that the density ``events'' along a line of sight can be assumed to
be independent over distances larger than this, and the Central Limit Theorem
should be applicable. However, using random realizations of lognormal fields,
we show that the convergence to a Gaussian is extremely slow in the high-
density tail. Thus, the column density PDF is not expected to exhibit a unique
functional shape, but to transit instead from a lognormal to a Gaussian form as
the ratio of the column length to the decorrelation length increases.
Simultaneously, the PDF's variance decreases. For intermediate values of
, the column density PDF assumes a nearly exponential decay. We then
discuss the density power spectrum and the expected value of in actual
molecular clouds. Observationally, our results suggest that may be
inferred from the shape and width of the column density PDF in
optically-thin-line or extinction studies. Our results should also hold for gas
with finite-extent power-law underlying density PDFs, which should be
characteristic of the diffuse, non-isothermal neutral medium (temperatures
ranging from a few hundred to a few thousand degrees). Finally, we note that
for , the dynamic range in column density is small
( a factor of 10), but this is only an averaging effect, with no
implication on the dynamic range of the underlying density distribution.Comment: 13 pages, 7 figures (10 postscript files). Accepted in ApJ.
Eliminated implication that ratio of column length to correlation length
necessarily increases with resolution, and thus that 3D simulations are
unresolved. Added discussion of dependence of autocorrelation function with
parameters of the turbulenc
Dynamics of Multi-Player Games
We analyze the dynamics of competitions with a large number of players. In
our model, n players compete against each other and the winner is decided based
on the standings: in each competition, the mth ranked player wins. We solve for
the long time limit of the distribution of the number of wins for all n and m
and find three different scenarios. When the best player wins, the standings
are most competitive as there is one-tier with a clear differentiation between
strong and weak players. When an intermediate player wins, the standings are
two-tier with equally-strong players in the top tier and clearly-separated
players in the lower tier. When the worst player wins, the standings are least
competitive as there is one tier in which all of the players are equal. This
behavior is understood via scaling analysis of the nonlinear evolution
equations.Comment: 8 pages, 8 figure
A Very Large Array 3.6cm continuum survey of Galactic Wolf-Rayet stars
We report the results of a survey of radio continuum emission of Galactic
Wolf-Rayet stars north of declination -46 degrees. The observations were
obtained at 8.46 GHz (3.6cm) using the Very Large Array (VLA), with an angular
resolution of about 6 x 9 arcsec and typical rms noise of 0.04 mJy/beam. Our
survey of 34 WR stars resulted in 15 definite and 5 probable detections, 13 of
these for the first time at radio wavelengths. All detections are unresolved.
Time variations in flux are confirmed in the cases of WR98a, WR104, WR105 and
WR125. WR79a and WR89 are also variable in flux and we suspect they are also
non-thermal emitters. Thus, of our sample 20-30% of the detected stars are
non-thermal emiters. Average mass loss rates determinations obtained excluding
definite and suspected non-thermal cases give similar values for WN (all
subtypes) and WC5-7 stars, while a lower value was obtained for WC8-9 stars.
Uncertainties in stellar distances largely contribute to the observed scatter
in mass loss rates. Upper limits to the mass loss rates were obtained in cases
of undetected sources or for sources which probably show additional non-thermal
emission.Comment: 21 pages, Latex, 21 postscript figures, to be published in The
Astronomical Journal, May 200
Formation and Collapse of Quiescent Cloud Cores Induced by Dynamic Compressions
(Abridged) We present numerical hydrodynamical simulations of the formation,
evolution and gravitational collapse of isothermal molecular cloud cores. A
compressive wave is set up in a constant sub-Jeans density distribution of
radius r = 1 pc. As the wave travels through the simulation grid, a
shock-bounded spherical shell is formed. The inner shock of this shell reaches
and bounces off the center, leaving behind a central core with an initially
almost uniform density distribution, surrounded by an envelope consisting of
the material in the shock-bounded shell, with a power-law density profile that
at late times approaches a logarithmic slope of -2 even in non-collapsing
cases. The resulting density structure resembles a quiescent core of radius <
0.1 pc, with a Bonnor-Ebert-like (BE-like) profile, although it has significant
dynamical differences: it is initially non-self-gravitating and confined by the
ram pressure of the infalling material, and consequently, growing continuously
in mass and size. With the appropriate parameters, the core mass eventually
reaches an effective Jeans mass, at which time the core begins to collapse.
Thus, there is necessarily a time delay between the appearance of the core and
the onset of its collapse, but this is not due to the dissipation of its
internal turbulence as it is often believed. These results suggest that
pre-stellar cores may approximate Bonnor-Ebert structures which are however of
variable mass and may or may not experience gravitational collapse, in
qualitative agreement with the large observed frequency of cores with BE-like
profiles.Comment: Accepted for publication in ApJ. Associated mpeg files can be found
in http://www.astrosmo.unam.mx/~g.gomez/publica.htm
Quarantine generated phase transition in epidemic spreading
We study the critical effect of quarantine on the propagation of epidemics on
an adaptive network of social contacts. For this purpose, we analyze the
susceptible-infected-recovered (SIR) model in the presence of quarantine, where
susceptible individuals protect themselves by disconnecting their links to
infected neighbors with probability w, and reconnecting them to other
susceptible individuals chosen at random. Starting from a single infected
individual, we show by an analytical approach and simulations that there is a
phase transition at a critical rewiring (quarantine) threshold w_c separating a
phase (w<w_c) where the disease reaches a large fraction of the population,
from a phase (w >= w_c) where the disease does not spread out. We find that in
our model the topology of the network strongly affects the size of the
propagation, and that w_c increases with the mean degree and heterogeneity of
the network. We also find that w_c is reduced if we perform a preferential
rewiring, in which the rewiring probability is proportional to the degree of
infected nodes.Comment: 13 pages, 6 figure
Coevolution of Glauber-like Ising dynamics on typical networks
We consider coevolution of site status and link structures from two different
initial networks: a one dimensional Ising chain and a scale free network. The
dynamics is governed by a preassigned stability parameter , and a rewiring
factor , that determines whether the Ising spin at the chosen site flips
or whether the node gets rewired to another node in the system. This dynamics
has also been studied with Ising spins distributed randomly among nodes which
lie on a network with preferential attachment. We have observed the steady
state average stability and magnetisation for both kinds of systems to have an
idea about the effect of initial network topology. Although the average
stability shows almost similar behaviour, the magnetisation depends on the
initial condition we start from. Apart from the local dynamics, the global
effect on the dynamics has also been studied. These parameters show interesting
variations for different values of and , which helps in determining
the steady-state condition for a given substrate.Comment: 8 pages, 10 figure
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