1,479 research outputs found
Symplectic algorithm for constant-pressure molecular dynamics using a Nose-Poincare thermostat
We present a new algorithm for isothermal-isobaric molecular-dynamics
simulation. The method uses an extended Hamiltonian with an Andersen piston
combined with the Nos'e-Poincar'e thermostat, recently developed by Bond,
Leimkuhler and Laird [J. Comp. Phys., 151, (1999)]. This
Nos'e-Poincar'e-Andersen (NPA) formulation has advantages over the
Nos'e-Hoover-Andersen approach in that the NPA is Hamiltonian and can take
advantage of symplectic integration schemes, which lead to enhanced stability
for long-time simulations. The equations of motion are integrated using a
Generalized Leapfrog Algorithm and the method is easy to implement, symplectic,
explicit and time reversible. To demonstrate the stability of the method we
show results for test simulations using a model for aluminum.Comment: 7 page
Refractive index of a transparent liquid measured with a concave mirror
This paper describes the spherical concave mirror method for measuring the
index of refraction of transparent liquids. We derived the refractive index
equation using Snell's law and the small-angle approximation. We also verified
the validity of this method using the traditional spherical mirror and
thin-lens Gaussian equations.Comment: IOPart, 8 pages, 4 figure
Visualizing the logistic map with a microcontroller
The logistic map is one of the simplest nonlinear dynamical systems that
clearly exhibit the route to chaos. In this paper, we explored the evolution of
the logistic map using an open-source microcontroller connected to an array of
light emitting diodes (LEDs). We divided the one-dimensional interval
into ten equal parts, and associated and LED to each segment. Every time an
iteration took place a corresponding LED turned on indicating the value
returned by the logistic map. By changing some initial conditions of the
system, we observed the transition from order to chaos exhibited by the map.Comment: LaTeX, 6 pages, 3 figures, 1 listin
Two-Temperature Intracluster Medium in Merging Clusters of Galaxies
We investigate the evolution of intracluster medium during a cluster merger,
explicitly considering the relaxation process between the ions and electrons by
N-body and hydrodynamical simulations. When two subclusters collide each other,
a bow shock is formed between the centers of two substructures and propagate in
both directions along the collision axis. The shock primarily heats the ions
because the kinetic energy of an ion entering the shock is larger than that of
an electron by the ratio of masses. In the post-shock region the energy is
transported from the ions to electrons via Coulomb coupling. However, since the
energy exchange timescale depends both on the gas density and temperature,
distribution of electron temperature becomes more complex than that of the
plasma mean temperature, especially in the expanding phase. After the collision
of two subclusters, gas outflow occurs not only along the collision axis but
also in its perpendicular direction. The gas which is originally located in the
central part of the subclusters moves both in the parallel and perpendicular
directions. Since the equilibrium timescale of the gas along these directions
is relatively short, temperature difference between ions and electrons is
larger in the directions tilted by the angles of with respect to
the collision axis. The electron temperature could be significantly lower that
the plasma mean temperature by at most. The significance of our
results in the interpretation of X-ray observations is briefly discussed.Comment: 20 pages, 11 figures, Accepted for publication in Ap
Disk galaxies with broken luminosity profiles from cosmological simulations
We present SPH cosmological simulations of the formation of three disk
galaxies with a detailed treatment of chemical evolution and cooling. The
resulting galaxies have properties compatible with observations: relatively
high disk-to-total ratios, thin stellar disks and good agreement with the
Tully-Fisher and the luminosity-size relations. They present a break in the
luminosity profile at 3.0 +- 0.5 disk scale lengths, while showing an
exponential mass profile without any apparent breaks, in line with recent
observational results. Since the stellar mass profile is exponential, only
differences in the stellar populations can be the cause of the luminosity
break. Although we find a cutoff for the star formation rate imposed by a
density threshold in our star formation model, it does not coincide with the
luminosity break and is located at 4.3 +- 0.4 disk scale lengths, with star
formation going on between both radii. The color profiles and the age profiles
are "U-shaped", with the minimum for both profiles located approximately at the
break radius. The SFR to stellar mass ratio increases until the break,
explaining the coincidence of the break with the minimum of the age profile.
Beyond the break we find a steep decline in the gas density and, consequently,
a decline in the SFR and redder colors. We show that most stars (64-78%) in the
outer disk originate in the inner disk and afterwards migrate there. Such
stellar migrations are likely the main origin of the U-shaped age profile and,
therefore, of the luminosity break.Comment: 8 pages, 4 figures. Accepted by ApJ
Large-scale gas dynamics in the adhesion model: Implications for the two-phase massive galaxy formation scenario
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2011 RAS © 2011 The AuthorsPublished by Oxford University Press on behalf of the Royal Astronomical Society. All rights reservedWe have studied the mass assembly and star formation histories of massive galaxies identified at low redshift in different cosmological hydrodynamical simulations. To this end, we have carried out a detailed follow-up backwards in time of their constituent mass elements (sampled by particles) of different types. After that, the configurations they depict at progressively higher zs were carefully analysed. The analyses show that these histories share common generic patterns, irrespective of particular circumstances. In any case, however, the results we have found are different depending on the particle type. The most outstanding differences follow. We have found that by z⌠3.5-6, mass elements identified as stellar particles at z= 0 exhibit a gaseous cosmic-web-like morphology with scales of âŒ1 physical Mpc, where the densest mass elements have already turned into stars by z⌠6. These settings are in fact the densest pieces of the cosmic web, where no hot particles show up, and dynamically organized as a hierarchy of flow convergence regions (FCRs), that is, attraction basins for mass flows. At high z FCRs undergo fast contractive deformations with very low angular momentum, shrinking them violently. Indeed, by z⌠1 most of the gaseous or stellar mass they contain shows up as bound to a massive elliptical-like object at their centres, with typical half-mass radii of rmass star⌠2-3kpc. After this, a second phase comes about where the mass assembly rate is much slower and characterized by mergers involving angular momentum. On the other hand, mass elements identified at the diffuse hot coronae surrounding massive galaxies at z= 0 do not display a clear web-like morphology at any z. Diffuse gas is heated when FCRs go through contractive deformations. Most of this gas remains hot and with low density throughout the evolution. To shed light on the physical foundations of the behaviour revealed by our analyses (i.e. a two-phase formation process with different implications for diffuse or shocked mass elements), as well as on their possible observational implications, these patterns have been confronted with some generic properties of singular flows as described by the adhesion model (i.e. potential character of the velocity field, singular versus regular points, dressing, locality when a spectrum of perturbations is implemented). We have found that the common patterns the simulations show can be interpreted as a natural consequence of flow properties that, moreover, could explain different generic observational results from massive galaxies or their samples. We briefly discuss some of themThis work was partially supported by the DGES (Spain) through the grants AYA2009-12792-C03-02 and AYA2009-12792- C03-03 from the PNAyA, as well as by the regional Madrid V PRICIT programme through the ASTROMADRID network (CAM S2009/ESP-1496
Ellipticals at z=0 from Self-Consistent Hydrodynamical Simulations: Clues on Age Effects in their Stellar Populations
We present results of a study of the stellar age distributions in the sample
of elliptical-like objects (ELOs) identified at z=0 in four simulations
operating in the context of a concordance cosmological model. The simulations
show that the formation of most stars in each ELO of the sample is a
consequence of violent dynamical events, either fast multiclump collapse at
high z, or mergers at lower z. This second way can explain the age spread as
well as the dynamical peculiarities observed in some ellipticals, but its
relative weight is never dominant and decreases as the ELO mass at the halo
scale, , increases, to such an extent that some recent mergers
contributing an important fraction to the total ELO mass can possibly
contribute only a small fraction of new born stars. More massive objects have
older means and narrower spreads in their stellar age distributions than less
massive ones. The ELO sample shows also a tight correlation between
and the central stellar l.o.s. velocity dispersion, . This gives
a trend of the means and spreads of ELO stellar populations with
that is consistent, even quantitatively, with the age effects observationally
detected in the stellar populations of elliptical galaxies. Therefore, these
effects can be explained as the observational manifestation of the intrinsic
correlations found in the ELO sample between and the properties of
the stellar age distribution, on the one hand, and and
, on the other hand. These correlations hint, for the first time,
at a possible way to reconcile age effects in ellipticals, and, particularly,
the increase of ratios with , with the
hierarchical clustering paradigm.Comment: 13 pages, 2 figures, accepted for publication in Astrophysical
Journal Letter
Hyperextended Scalar-Tensor Gravity
We study a general Scalar-Tensor Theory with an arbitrary coupling funtion
but also an arbitrary dependence of the ``gravitational
constant'' in the cases in which either one of them, or both, do not
admit an analytical inverse, as in the hyperextended inflationary scenario. We
present the full set of field equations and study their cosmological behavior.
We show that different scalar-tensor theories can be grouped in classes with
the same solution for the scalar field.Comment: latex file, To appear in Physical Review
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