279 research outputs found
Latin American Economic Integration: An Overview of Trade and Investment Developments in ANCOM, CACM, and LAFTA
This article examines different approaches towards greater economic cooperation among Latin American countries that have been taken
The Chemical Compositions of the Type II Cepheids -- The BL Her and W Vir Variables
Abundance analyses from high-resolution optical spectra are presented for 19
Type II Cepheids in the Galactic field. The sample includes both short-period
(BL Her) and long-period (W Vir) stars. This is the first extensive abundance
analysis of these variables. The C, N, and O abundances with similar spreads
for the BL Her and W Vir show evidence for an atmosphere contaminated with
-process and CN-cycling products. A notable anomaly of the BL Her
stars is an overabundance of Na by a factor of about five relative to their
presumed initial abundances. This overabundance is not seen in the W Vir stars.
The abundance anomalies running from mild to extreme in W Vir stars but not
seen in the BL Her stars are attributed to dust-gas separation that provides an
atmosphere deficient in elements of high condensation temperature, notably Al,
Ca, Sc, Ti, and -process elements. Such anomalies have previously been seen
among RV Tau stars which represent a long-period extension of the variability
enjoyed by the Type II Cepheids. Comments are offered on how the contrasting
abundance anomalies of BL Her and W Vir stars may be explained in terms of the
stars' evolution from the blue horizontal branch.Comment: 41 pages including 11 figures and 4 tables; Accepted for publication
in Ap
Effects of a Supermassive Black Hole Binary on a Nuclear Gas Disk
We study influence of a galactic central supermassive black hole (SMBH)
binary on gas dynamics and star formation activity in a nuclear gas disk by
making three-dimensional Tree+SPH simulations. Due to orbital motions of SMBHs,
there are various resonances between gas motion and the SMBH binary motion. We
have shown that these resonances create some characteristic structures of gas
in the nuclear gas disk, for examples, gas elongated or filament structures,
formation of gaseous spiral arms, and small gas disks around SMBHs. In these
gaseous dense regions, active star formations are induced. As the result, many
star burst regions are formed in the nuclear region.Comment: 19 pages, 11 figures, accepted for publication in Ap
Hydrodynamic Processes in Young Binary Systems as a Source of Cyclic Variations of Circumstellar Extinction
Hydrodynamic models of a young binary system accreting matter from the
remnants of a protostellar cloud have been calculated by the SPH method. It is
shown that periodic variations in column density in projection onto the primary
component take place at low inclinations of the binary plane to the line of
sight. They can result in periodic extinction variations. Three periodic
components can exist in general case. The first component has a period equal to
the orbital one and is attributable to the streams of matter penetrating into
the inner regions of the binary. The second component has a period that is a
factor of 5-8 longer than the orbital one and is related to the density waves
generated in a circumbinary (CB) disk. The third, longest period is
attributable to the precession of the inner asymmetric region of CB disk. The
relationship between the amplitudes of these cycles depends on the model
parameters as well as on the inclination and orientation of the binary in
space. We show that at a dust-to-gas ratio of 1:100 and and a mass extinction
coefficient of 250 cm g, the amplitude of the brightness variations
of the primary component in the V-band can reach at a mass accretion rate
onto the binary components of yr and a
inclination of the binary plane to the line of sight. We discuss possible
applications of the model to pre-main-sequence stars.Comment: 13 pages, 6 figures, published in Astronomy Letters (v.33, 2007
Quasars: What turns them off?
(Abridged) We explore the idea that the anti-hierarchical turn-off observed
in the quasar population arises from self-regulating feedback, via an outflow
mechanism. Using a detailed hydrodynamic simulation we calculate the luminosity
function of quasars down to a redshift of z=1 in a large, cosmologically
representative volume. Outflows are included explicitly by tracking halo
mergers and driving shocks into the surrounding intergalactic medium. Our
results are in excellent agreement with measurements of the spatial
distribution of quasars, and we detect an intriguing excess of galaxy-quasar
pairs at very short separations. We also reproduce the anti-hierarchical
turnoff in the quasar luminosity function, however, the magnitude of the
turn-off falls short of that observed as well as that predicted by analogous
semi-analytic models. The difference can be traced to the treatment of gas
heating within galaxies. The simulated galaxy cluster L_X-T relationship is
close to that observed for z~1 clusters, but the simulated galaxy groups at z=1
are significantly perturbed by quasar outflows, suggesting that measurements of
X-ray emission in high-redshift groups could well be a "smoking gun" for the
AGN heating hypothesis.Comment: 16 pages, 11 figures, submitted to ApJ, comments welcome
Information field dynamics for simulation scheme construction
Information field dynamics (IFD) is introduced here as a framework to derive
numerical schemes for the simulation of physical and other fields without
assuming a particular sub-grid structure as many schemes do. IFD constructs an
ensemble of non-parametric sub-grid field configurations from the combination
of the data in computer memory, representing constraints on possible field
configurations, and prior assumptions on the sub-grid field statistics. Each of
these field configurations can formally be evolved to a later moment since any
differential operator of the dynamics can act on fields living in continuous
space. However, these virtually evolved fields need again a representation by
data in computer memory. The maximum entropy principle of information theory
guides the construction of updated datasets via entropic matching, optimally
representing these field configurations at the later time. The field dynamics
thereby become represented by a finite set of evolution equations for the data
that can be solved numerically. The sub-grid dynamics is treated within an
auxiliary analytic consideration and the resulting scheme acts solely on the
data space. It should provide a more accurate description of the physical field
dynamics than simulation schemes constructed ad-hoc, due to the more rigorous
accounting of sub-grid physics and the space discretization process.
Assimilation of measurement data into an IFD simulation is conceptually
straightforward since measurement and simulation data can just be merged. The
IFD approach is illustrated using the example of a coarsely discretized
representation of a thermally excited classical Klein-Gordon field. This should
pave the way towards the construction of schemes for more complex systems like
turbulent hydrodynamics.Comment: 19 pages, 3 color figures, accepted by Phys. Rev.
NeXSPheRIO results on azimuthal anisotropy in Au-Au collisions at 200A GeV
In this work, we present the results obtained by the hydrodynamic code
NeXSPheRIO on anisotropic flows. In our calculation, we made use of
event-by-event fluctuating initial conditions, and chemical freeze-out was
explicitly implemented. We studied directed flow, elliptic flow and forth
harmonic coefficient for various hadrons at different centrality windows for
Au+Au collisions at 200 AGeV. The results are discussed and compared with
experimental data from RHIC.Comment: 6 pages and 6 figures, sqm2008 contributio
The Effects of Gas Dynamics, Cooling, Star Formation, and Numerical Resolution in Simulations of Cluster Formation
We present the analysis of a suite of simulations of a Virgo mass galaxy
cluster. Undertaken within the framework of standard cold dark matter
cosmology, these simulations were performed at differing resolutions and with
increasingly complex physical processes, with the goal of identifying the
effects of each on the evolution of the cluster. We focus on the cluster at the
present epoch and examine properties including the radial distributions of
density, temperature, entropy and velocity. We also map `observable' projected
properties such as the surface mass density, X-ray surface brightness and SZ
signature. We identify significant differences between the simulations, which
highlights the need for caution when comparing numerical simulations to
observations of galaxy clusters. While resolution affects the inner density
profile in dark matter simulations, the addition of a gaseous component,
especially one that cools and forms stars, affects the entire cluster. We
conclude that both resolution and included physical processes play an important
role in simulating the formation and evolution of galaxy clusters. Therefore,
physical inferences drawn from simulations that do not include a gaseous
component that can cool and form stars present a poor representation of
reality. (Abridged)Comment: Accepted for publication in the Astrophysical Journal. Several
changes from previous version, including new materia
High Resolution Spectroscopy of the high galactic latitude RV Tauri star CE Virginis
Analysis of the surface composition of the suspected cool RV Tauri star CE
Vir shows no systematic trend in depletions of elements with respect to
condensation temperature. However, there is a significant depletion of the
elements with respect to the first ionization potential of the element. The
derived Li abundance of log (Li) = 1.50.2 indicates production
of Li in the star. Near infrared colours indicate sporadic dust formation close
to the photosphere.Comment: 12 pages, including 8 pages: Accepted for publication in MNRA
Hydrodynamic Simulations of Propagating Warps and Bending Waves In Accretion Discs
We present the results of a study of propagating warp or bending waves in
accretion discs. Three dimensional hydrodynamic simulations were performed
using SPH, and the results of these are compared with calculations based on the
linear theory of warped discs. We consider primarily the physical regime in
which the dimensionless viscosity parameter `alpha' < H/r, the disc aspect
ratio, so that bending waves are expected to propagate. We also present
calculations in which `alpha' > H/r, where the warps are expected to behave
diffusively. Small amplitude perturbations are studied in both Keplerian and
slightly non Keplerian discs, and we find that the SPH results can be
reasonably well fitted by those of the linear theory. The main results of these
calculations are: (1) the warp in Keplerian discs when `alpha' < H/r propagates
with little dispersion and damps at a rate expected from estimates of the code
viscosity, (2) warps evolve diffusively when `alpha' > H/r, (3) the non
Keplerian discs exhibit a substantially more dispersive behaviour of the warps.
Initially imposed higher amplitude nonlinear warping disturbances were studied
in Keplerian discs. The results indicate that nonlinear warps can lead to the
formation of shocks, and that the evolution of the warp becomes less wave-like
and more diffusive in character. This work is relevant to the study of the
warped accretion discs that may occur around Kerr black holes or in misaligned
binary systems. The results indicate that SPH can accurately model the
hydrodynamics of warped discs, even when using rather modest numbers of
particles.Comment: 14 pages, 9 figures, to appear in MNRA
- …