11,770 research outputs found
Influence of disordered porous media in the anomalous properties of a simple water model
The thermodynamic, dynamic and structural behavior of a water-like system
confined in a matrix is analyzed for increasing confining geometries. The
liquid is modeled by a two dimensional associating lattice gas model that
exhibits density and diffusion anomalies, in similarity to the anomalies
present in liquid water. The matrix is a triangular lattice in which fixed
obstacles impose restrictions to the occupation of the particles. We show that
obstacules shortens all lines, including the phase coexistence, the critical
and the anomalous lines. The inclusion of a very dense matrix not only suppress
the anomalies but also the liquid-liquid critical point
Thermodynamic and Dynamic Anomalies for Dumbbell Molecules Interacting with a Repulsive Ramp-Like Potential
Using collision driven discrete molecular dynamics (DMD), we investigate the
thermodynamics and dynamics of systems of 500 dumbbell molecules interacting by
a purely repulsive ramp-like discretized potential, consisting of steps of
equal size. We compare the behavior of the two systems, with and steps. Each system exhibits both thermodynamic and dynamic anomalies, a
density maximum and the translational and rotational mobilities show anomalous
behavior. Starting with very dense systems and decreasing the density, both
mobilities first increase, reache a maximum, then decrease, reache a minimum,
and finally increase; this behavior is similar to the behavior of SPC/E water.
The regions in the pressure-temperature plane of translational and rotational
mobility anomalies depend strongly on . The product of the translational
diffusion coefficient and the orientational correlation time increases with
temperature, in contrast with the behavior of most liquids
Modeling the input history of programs for improved instruction-memory performance
When a program is loaded into memory for execution, the relative position of
its basic blocks is crucial, since loading basic blocks that are unlikely to be
executed first places them high in the instruction-memory hierarchy only to be
dislodged as the execution goes on. In this paper we study the use of Bayesian
networks as models of the input history of a program. The main point is the
creation of a probabilistic model that persists as the program is run on
different inputs and at each new input refines its own parameters in order to
reflect the program's input history more accurately. As the model is thus
tuned, it causes basic blocks to be reordered so that, upon arrival of the next
input for execution, loading the basic blocks into memory automatically takes
into account the input history of the program. We report on extensive
experiments, whose results demonstrate the efficacy of the overall approach in
progressively lowering the execution times of a program on identical inputs
placed randomly in a sequence of varied inputs. We provide results on selected
SPEC CINT2000 programs and also evaluate our approach as compared to the gcc
level-3 optimization and to Pettis-Hansen reordering
The evolution of the Sun's birth cluster and the search for the solar siblings with Gaia
We use self-consistent numerical simulations of the evolution and disruption
of the Sun's birth cluster in the Milky Way potential to investigate the
present-day phase space distribution of the Sun's siblings. The simulations
include the gravitational N-body forces within the cluster and the effects of
stellar evolution on the cluster population. In addition the gravitational
forces due to the Milky Way potential are accounted for in a self-consistent
manner. Our aim is to understand how the astrometric and radial velocity data
from the Gaia mission can be used to pre-select solar sibling candidates. We
vary the initial conditions of the Sun's birth cluster, as well as the
parameters of the Galactic potential. We show that the disruption time-scales
of the cluster are insensitive to the details of the non-axisymmetric
components of the Milky Way model and we make predictions, averaged over the
different simulated possibilities, about the number of solar siblings that
should appear in surveys such as Gaia or GALAH. We find a large variety of
present-day phase space distributions of solar siblings, which depend on the
cluster initial conditions and the Milky Way model parameters. We show that
nevertheless robust predictions can be made about the location of the solar
siblings in the space of parallaxes (), proper motions () and
radial velocities (). By calculating the ratio of the number of
simulated solar siblings to that of the number of stars in a model Galactic
disk, we find that this ratio is above 0.5 in the region given by: mas, masyr, and kms. Selecting stars from this region should increase the probability
of success in identifying solar siblings through follow up observations
[Abridged].Comment: 13 pages, 7 figures. Accepted for publication in MNRA
Searching for Very High Energy Emission from Pulsars Using the High Altitude Water Cherenkov (HAWC) Observatory
There are currently over 160 known gamma-ray pulsars. While most of them are
detected only from space, at least two are now seen also from the ground. MAGIC
and VERITAS have measured the gamma ray pulsed emission of the Crab pulsar up
to hundreds of GeV and more recently MAGIC has reported emission at
TeV. Furthermore, in the Southern Hemisphere, H.E.S.S. has detected the Vela
pulsar above 30 GeV. In addition, non-pulsed TeV emission coincident with
pulsars has been detected by many groups, including the Milagro Collaboration.
These GeV-TeV observations open the possibility of searching for
very-high-energy (VHE, > 100GeV) pulsations from gamma-rays pulsars in the HAWC
field of view.Comment: Presented at the 34th International Cosmic Ray Conference (ICRC2015),
The Hague, The Netherlands. See arXiv:1508.03327 for all HAWC contribution
The Stellar Content of Obscured Galactic Giant HII Regions. VI: W51A
We present K-band spectra of newly born OB stars in the obscured Galactic
giant H II region W51A and ~ 0.8'' angular resolution images in the J, H and
K_S-bands. Four objects have been spectroscopically classified as O-type stars.
The mean spectroscopic parallax of the four stars gives a distance of 2.0 \pm
0.3 kpc (error in the mean), significantly smaller than the radio recombination
line kinematic value of 5.5 kpc or the values derived from maser propermotion
observations (6--8 kpc). The number of Lyman continuum photons from the
contribution of all massive stars (NLyc ~ 1.5 x 10^{50} s^{-1}) is in good
agreement with that inferred from radio recombination lines (NLyc = 1.3 x
10^{50} s^{-1}) after accounting for the smaller distance derived here.
We present analysis of archival high angular resolution images (NAOS CONICA
at VLT and T-ReCS at Gemini) of the compact region W51 IRS2. The K_S--band
images resolve the infrared source IRS~2 indicating that it is a very young
compact HII region. Sources IRS2E was resolved into compact cluster (within 660
AU of projected distance) of 3 objects, but one of them is just bright extended
emission. W51d1 and W51d2 were identified with compact clusters of 3 objects
(maybe 4 in the case of W51d1) each one. Although IRS~2E is the brightest
source in the K-band and at 12.6 \micron, it is not clearly associated with a
radio continuum source. Our spectrum of IRS~2E shows, similar to previous work,
strong emission in Br and HeI, as well as three forbidden emission
lines of FeIII and emission lines of molecular hydrogen (H_2) marking it as a
massive young stellar object.Comment: 31 pages and 9 figures, submitted to A
Expectations For an Interferometric Sunyaev-Zel'dovich Effect Survey for Galaxy Clusters
Non-targeted surveys for galaxy clusters using the Sunyaev-Zel'dovich effect
(SZE) will yield valuable information on both cosmology and evolution of the
intra-cluster medium (ICM). The redshift distribution of detected clusters will
constrain cosmology, while the properties of the discovered clusters will be
important for studies of the ICM and galaxy formation. Estimating survey yields
requires a detailed model for both cluster properties and the survey strategy.
We address this by making mock observations of galaxy clusters in cosmological
hydrodynamical simulations. The mock observatory consists of an interferometric
array of ten 2.5 m diameter telescopes, operating at a central frequency of 30
GHz with a bandwidth of 8 GHz. We find that clusters with a mass above will be detected at any redshift, with the
exact limit showing a very modest redshift dependence. Using a Press-Schechter
prescription for evolving the number densities of clusters with redshift, we
determine that such a survey should find hundreds of galaxy clusters per year,
many at high redshifts and relatively low mass -- an important regime uniquely
accessible to SZE surveys. Currently favored cosmological models predict
roughly 25 clusters per square degree.Comment: revised to match published versio
Diffusion Enhancement in Core-softened fluid confined in nanotubes
We study the effect of confinement in the dynamical behavior of a
core-softened fluid. The fluid is modeled as a two length scales potential.
This potential in the bulk reproduces the anomalous behavior observed in the
density and in the diffusion of liquid water. A series of Molecular
Dynamics simulations for this two length scales fluid confined in a nanotube
were performed. We obtain that the diffusion coefficient increases with the
increase of the nanotube radius for wide channels as expected for normal
fluids. However, for narrow channels, the confinement shows an enhancement in
the diffusion coefficient when the nanotube radius decreases. This behavior,
observed for water, is explained in the framework of the two length scales
potential.Comment: 17 pages, 8 figures, accept for publication at J. Chem. Phy
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