131 research outputs found
An adjustable law of motion for relativistic spherical shells
A classical and a relativistic law of motion for an advancing shell are
deduced applying the thin layer approximation. A new parameter connected with
the quantity of absorbed matter in the expansion is introduced; this allows of
matching theory and observation.Comment: 15 pages, 10 figures and article in press; Central European Journal
of Physics 201
On the Truncated Pareto Distribution with applications
The Pareto probability distribution is widely applied in different fields
such us finance, physics, hydrology, geology and astronomy. This note deals
with an application of the Pareto distribution to astrophysics and more
precisely to the statistical analysis of mass of stars and of diameters of
asteroids. In particular a comparison between the usual Pareto distribution and
its truncated version is presented. Finally a possible physical mechanism that
produces Pareto tails for the distribution of the masses of stars is suggested.Comment: 10 pages 6 figure
A radio supernova remnant associated with the young pulsar J1119-6127
We report on Australia Telescope Compact Array observations in the direction
of the young high magnetic-field pulsar J1119-6127. In the resulting images we
identify a non-thermal radio shell of diameter 15', which we classify as a
previously uncatalogued young supernova remnant, G292.2-0.5. This supernova
remnant is positionally coincident with PSR J1119-6127, and we conclude that
the two objects are physically associated. No radio emission is detected from
any pulsar wind nebula (PWN) associated with the pulsar; our observed upper
limits are consistent with the expectation that high magnetic-field pulsars
produce radio nebulae which fade rapidly. This system suggests a possible
explanation for the lack of an associated radio pulsar and/or PWN in many
supernova remnants.Comment: 13 pages, 6 embedded eps figures. Accepted to Ap
Finding Galaxy Clusters using Voronoi Tessellations
We present an objective and automated procedure for detecting clusters of
galaxies in imaging galaxy surveys. Our Voronoi Galaxy Cluster Finder (VGCF)
uses galaxy positions and magnitudes to find clusters and determine their main
features: size, richness and contrast above the background. The VGCF uses the
Voronoi tessellation to evaluate the local density and to identify clusters as
significative density fluctuations above the background. The significance
threshold needs to be set by the user, but experimenting with different choices
is very easy since it does not require a whole new run of the algorithm. The
VGCF is non-parametric and does not smooth the data. As a consequence, clusters
are identified irrispective of their shape and their identification is only
slightly affected by border effects and by holes in the galaxy distribution on
the sky. The algorithm is fast, and automatically assigns members to
structures.Comment: 11 pages, 11 figures. It uses aa.cls (included). Accepted by A&
Models of diffusion of galactic Cosmic Rays from Super-bubbles
Super-bubbles are shells in the interstellar medium produced by the
simultaneous explosions of many supernova remnants. The solutions of the
mathematical diffusion and of the Fourier expansion in 1D, 2D and 3D were
deduced in order to describe the diffusion of nucleons from such structures.
The mean number of visits in the the case of the Levy flights in 1D was
computed with a Monte Carlo simulation. The diffusion of cosmic rays has its
physical explanation in the relativistic Larmor gyro-radius which is energy
dependent. The mathematical solution of the diffusion equation in 1D with
variable diffusion coefficient was computed. Variable diffusion coefficient
means magnetic field variable with the altitude from the Galactic plane. The
analytical solutions allow us to calibrate the code that describes the Monte
Carlo diffusion.
The maximum energy that can be extracted from the super-bubbles is deduced.
The concentration of cosmic rays is a function of the distance from the nearest
super-bubble and the selected energy. The interaction of the cosmic rays on the
target material allows us to trace the theoretical map of the diffuse Galactic
continuum gamma-rays. The streaming of the cosmic rays from the Gould Belts
that contains the sun at it's internal was described by a Monte Carlo
simulation. Nine new formulas are derived.Comment: 23 figures, 31 page
Time-Dependent Models for a decade of SN 1993J
A classical and a relativistic law of motion for a supernova remnant (SNR)
are deduced assuming an inverse power law behavior for the density of the
interstellar medium and applying the thin layer approximation. A third equation
of motion is found in the framework of relativistic hydrodynamics with
pressure, applying momentum conservation. These new formulas are calibrated
against a decade of observations of \snr. The existing knowledge of the
diffusive processes of ultrarelativistic electrons is reviewed in order to
explain the behavior of the `U' shaped profile of intensity versus distance
from the center of SN 1993J.Comment: 20 pages 19 figures, Accepted for pubblication in Astrophysics and
Space Science 201
Pennsylvanian-Early Triassic stratigraphy in the Alborz Mountains (Iran)
New fieldwork was carried out in the central and eastern Alborz, addressing the sedimentary succession from the Pennsylvanian to the Early Triassic. A regional synthesis is proposed, based on sedimentary analysis and a wide collection of new palaeontological data. The Moscovian Qezelqaleh Formation, deposited in a mixed coastal marine and alluvial setting, is present in a restricted area of the eastern Alborz, transgressing on the Lower Carboniferous Mobarak and Dozdehband formations. The late Gzhelian–early Sakmarian Dorud Group is instead distributed over most of the studied area, being absent only in a narrow belt to the SE. The Dorud Group is typically tripartite, with a terrigenous unit in the lower part (Toyeh Formation), a carbonate intermediate part (Emarat and Ghosnavi formations, the former particularly rich in fusulinids), and a terrigenous upper unit (Shah Zeid Formation), which however seems to be confined to the central Alborz. A major gap in sedimentation occurred before the deposition of the overlying Ruteh Limestone, a thick package of packstone–wackestone interpreted as a carbonate ramp of Middle Permian age (Wordian–Capitanian). The Ruteh Limestone is absent in the eastern part of the range, and everywhere ends with an emersion surface, that may be karstified or covered by a lateritic soil.
The Late Permian transgression was directed southwards in the central Alborz, where marine facies (Nesen Formation) are more common. Time-equivalent alluvial fans with marsh intercalations and lateritic soils (Qeshlaq Formation) are present in the east. Towards the end of the Permian most of the Alborz emerged, the marine facies being restricted to a small area on the Caspian side of the central Alborz. There, the Permo-Triassic boundary interval is somewhat similar to the Abadeh–Shahreza belt in central Iran, and contains oolites, flat microbialites and domal stromatolites, forming the base of the Elikah Formation. The P–T boundary is established on the basis of conodonts, small foraminifera and stable isotope data. The development of the lower and middle part of the Elikah Formation, still Early Triassic in age, contains vermicular bioturbated mudstone/wackestone, and anachronostic-facies-like gastropod oolites and flat pebble conglomerates.
Three major factors control the sedimentary evolution. The succession is in phase with global sea-level curve in the Moscovian and from the Middle Permian upwards. It is out of phase around the Carboniferous–Permian boundary, when the Dorud Group was deposited during a global lowstand of sealevel. When the global deglaciation started in the Sakmarian, sedimentation stopped in the Alborz and the area emerged. Therefore, there is a consistent geodynamic control. From the Middle Permian upwards, passive margin conditions control the sedimentary evolution of the basin, which had its depocentre(s) to the north. Climate also had a significant role, as the Alborz drifted quickly northwards with other central Iran blocks towards the Turan active margin. It passed from a southern latitude through the aridity belt in the Middle Permian, across the equatorial humid belt in the Late Permian and reached the northern arid tropical belt in the Triassic
A functional approach to estimation of the parameters of generalized negative binomial and gamma distributions
The generalized negative binomial distribution (GNB) is a new flexible family
of discrete distributions that are mixed Poisson laws with the mixing
generalized gamma (GG) distributions. This family of discrete distributions is
very wide and embraces Poisson distributions, negative binomial distributions,
Sichel distributions, Weibull--Poisson distributions and many other types of
distributions supplying descriptive statistics with many flexible models. These
distributions seem to be very promising for the statistical description of many
real phenomena. GG distributions are widely applied in signal and image
processing and other practical problems. The statistical estimation of the
parameters of GNB and GG distributions is quite complicated. To find estimates,
the methods of moments or maximum likelihood can be used as well as two-stage
grid EM-algorithms. The paper presents a methodology based on the search for
the best distribution using the minimization of -distances and
-metrics for GNB and GG distributions, respectively. This approach, first,
allows to obtain parameter estimates without using grid methods and solving
systems of nonlinear equations and, second, yields not point estimates as the
methods of moments or maximum likelihood do, but the estimate for the density
function. In other words, within this approach the set of decisions is not a
Euclidean space, but a functional space.Comment: 13 pages, 6 figures, The XXI International Conference on Distributed
Computer and Communication Networks: Control, Computation, Communications
(DCCN 2018
Floating Patches of HCN at the Surface of Their Aqueous Solutions - Can They Make "HCN World" Plausible?
The liquid/vapor interface of the aqueous solutions of HCN of different concentrations has been investigated using molecular dynamics simulation and intrinsic surface analysis. Although HCN is fully miscible with water, strong interfacial adsorption of HCN is observed at the surface of its aqueous solutions, and, at the liquid surface, the HCN molecules tend to be located even at the outer edge of the surface layer. It turns out that in dilute systems the HCN concentration can be about an order of magnitude larger in the surface layer than in the bulk liquid phase. Furthermore, HCN molecules show a strong lateral self-association behavior at the liquid surface, forming thus floating HCN patches at the surface of their aqueous solutions. Moreover, HCN molecules are staying, on average, an order of magnitude longer at the liquid surface than water molecules, and this behavior is more pronounced at smaller HCN concentrations. Because of this enhanced dynamical stability, the floating HCN patches can provide excellent spots for polymerization of HCN, which can be the key step in the prebiotic synthesis of partially water-soluble adenine. All of these findings make the hypothesis of "HCN world" more plausible
Properties of the Liquid-Vapor Interface of Acetone-Water Mixtures. A Computer Simulation and ITIM Analysis Study
Molecular dynamics simulations of the liquid-vapor interface of acetone-water mixtures of different compositions, covering the entire composition range have been performed on the canonical (N, V, T) ensemble at 298 K, using a model combination that excellently describes the mixing properties of these compounds. The properties of the intrinsic liquid surfaces have been analyzed in terms of the Identification of the Truly Interfacial Molecules (ITIM) method. Thus, the composition, width, roughness, and separation of the subsurface molecular layers, as well as self-association, orientation, and dynamics of exchange with the bulk phase of the surface molecules have been analyzed in detail. Our results show that acetone molecules are strongly adsorbed at the liquid surface, and this adsorption extends to several molecular layers. Like molecules in the surface layer are found to form relatively large lateral self-associates. The effect of the vicinity of the vapor phase on a number of properties of the liquid phase vanishes beyond the first molecular layer, with the second subsurface layer already part of the bulk liquid phase in these respects. The orientational preferences of the surface molecules are governed primarily by the dipole-dipole interaction of the neighboring acetone molecules, and hydrogen bonding interaction of the neighboring acetone-water pairs. (Figure Presented). © 2015 American Chemical Society
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