1,196 research outputs found
Chandra view of Kes 79: a nearly isothermal SNR with rich spatial structure
A 30 ks \chandra ACIS-I observation of Kes 79 reveals rich spatial
structures, including many filaments, three partial shells, a loop and a
``protrusion''. Most of them have corresponding radio features. Regardless of
the different results from two non-equilibrium ionization (NEI) codes,
temperatures of different parts of the remnant are all around 0.7 keV, which is
surprisingly constant for a remnant with such rich structure. If thermal
conduction is responsible for smoothing the temperature gradient, a lower limit
on the thermal conductivity of 1/10 of the Spitzer value can be derived.
Thus, thermal conduction may play an important role in the evolution of at
least some SNRs. No spectral signature of the ejecta is found, which suggests
the ejecta material has been well mixed with the ambient medium. From the
morphology and the spectral properties, we suggest the bright inner shell is a
wind-driven shell (WDS) overtaken by the blast wave (the outer shell) and
estimate the age of the remnant to be 6 kyr for the assumed dynamics.
Projection is also required to explain the complicated morphology of Kes 79.Comment: 12 pages, 6 figures (3 in color), ApJ, in press, April 20, 200
The Hystery Unit - A Short Term Memory Model for Computational Neurons
In this paper, a model of short term memory is introduced. This model is inspired by the transient behavior of neurons and magnetic storage as memory. The transient response of a neuron is hypothesized to be a combination of a pair of sigmoids, and a relation is drawn to the hysteresis loop characteristics of magnetic materials. A model is created as a composition of two coupled families of curves. Two theorems are derived regarding the asymptotic convergence behavior of the model. Another conjecture claims that the model retains full memory of all past unit step inputs
Fundamental Products and Autonomous Sets: An Algorithmic Approach
The main goal of this paper is to analyze an economy through its symmetric input-output table by using Graph Theory. After providing a definition of the characteristic digraph for the economy, the authors give some properties and algorithms to characterize and compute the fundamental products and, later, the autonomous sets of the studied economy. Six of these algorithms are explained in detail and formulated; the best choices are also implemented with the computational package Mathematica, compared in terms of efficiency, and applied to real input-output matrices from Greece
Adaptive Networks as a Model for Human Speech Development-Cluster Plots
This Technical Report contains all the cluster plots generated for the cluster analyses described in §9.7 of [1]. The Lance and William General Algorithm with complete linkages for hierarchical clustering analysis [2] is used. A brief description of the algorithm may be found in §9.7.1 of [1]. In the cluster plots the symbol -\u3e designates a letter-to-phoneme mapping. For example, c-\u3ek means the letter in the center of the window is c and is being mapped by the network into the phoneme /k/. Definitions of the phoneme symbols may be found on pp. 22-23 of [I]. A horizontal line designates a cluster. The length of a horizontal line has no significance. A vertical line designates the distance between the two clusters joining the top and bottom ends of the vertical line. The distance scale may be found at the beginning and the end of each plot. All plots have the same relative distance scale. Cluster analyses are performed after the 5th, 10th, 15th, 20th, and 25th passes through the English training database, and after the 2nd, 4th, 6th, 8th, and 10th passes through the Spanish training database. Cluster analyses for the second language trainings are performed similarly. Observations may be found in §9.7.2-§9.7.7 of [1
Efecto de diferentes tratamientos químicos en las características sorbentes de residuos derivados de la industria olivarera
En este trabajo se han caracterizado, mediante técnicas potenciométricas y análisis de IR, tres residuos de la industria olivarera (hueso de aceituna, alpeorujo y ramón del olivo) para su aplicación como biosorbentes de metales pesados antes y después de ser sometidos a diversos tratamientos químicos. Las titulaciones potenciométricas y los espectros de IR han mostrado que, tanto para el hueso de aceituna como para el ramón, el tratamiento con NaOH aumenta considerablemente la concentración de grupos activos y esto se ha manifestado en un aumento de la capacidad de bioadsorción de plomo. Así, en el caso del hueso y para una concentración inicial de Pb(II) de 80 mg/L, la capacidad de bioadsorción ha pasado de 3,54 mg/g para el hueso sin tratar a 7,32 mg/g para el hueso tratado. Para el alpeorujo, aunque la concentración de grupos activos es mayor en el sólido sin tratar o tratado con hexano, la capacidad de bioadsorciones mayor para el alpeorujo tratado con NaOH.En general, el tratamiento con HCl y con agua destilada ha eliminado los componentes solubles de los residuos, por lo que la concentración total de grupos activos es menor, siendo esta disminución muy notable en el alpeorujo y el ramón
Initial Ionization of Compressible Turbulence
We study the effects of the initial conditions of turbulent molecular clouds
on the ionization structure in newly formed H_{ii} regions, using
three-dimensional, photon-conserving radiative transfer in a pre-computed
density field from three-dimensional compressible turbulence. Our results show
that the initial density structure of the gas cloud can play an important role
in the resulting structure of the H_{ii} region. The propagation of the
ionization fronts, the shape of the resulting H_{ii} region, and the total mass
ionized depend on the properties of the turbulent density field. Cuts through
the ionized regions generally show ``butterfly'' shapes rather than spherical
ones, while emission measure maps are more spherical if the turbulence is
driven on scales small compared to the size of the H_{ii} region. The
ionization structure can be described by an effective clumping factor , where is number density of the gas. The larger
the value of , the less mass is ionized, and the more irregular the
H_{ii} region shapes. Because we do not follow dynamics, our results apply only
to the early stage of ionization when the speed of the ionization fronts
remains much larger than the sound speed of the ionized gas, or Alfv\'en speed
in magnetized clouds if it is larger, so that the dynamical effects can be
negligible.Comment: 9 pages, 10 figures, version with high quality color images can be
found in http://research.amnh.org/~yuexing/astro-ph/0407249.pd
Two Large HI Shells in the Outer Galaxy near l=279 degrees
As part of a survey of HI 21-cm emission in the Southern Milky Way, we have
detected two large shells in the interstellar neutral hydrogen near l=279 deg.
The center velocities are +36 and +59 km/s, which puts the shells at kinematic
distances of 7 and 10 kpc. The larger shell is about 610 pc in diameter and
very empty, with density contrast of at least 15 between the middle and the
shell walls. It has expansion velocity of about 20 km/s and swept up mass of
several million solar masses. The energy indicated by the expansion may be as
high as 2.4 X 10^53 ergs. We estimate its age to be 15 to 20 million years. The
smaller shell has diameter of about 400 pc, expansion velocity about 10 km/s
and swept up mass of about 10^6 solar masses.
Morphologically both regions appear to be shells, with high density regions
mostly surrounding the voids, although the first appears to have channels of
low density which connect with the halo above and below the HI layer. They lie
on the edge of the Carina arm, which suggests that they may be expanding
horizontally into the interarm region as well as vertically out of the disk. If
this interpretation is correct, this is the first detection of an HI chimney
which has blown out of both sides of the disk.Comment: 21 pages, 14 jpeg figures, accepted for publication in A
Cinética de la biosorción de Cr(VI) con hueso de aceituna
En este trabajo se ha estudiado la cinética de biosorción de cromo(VI) usando hueso de aceituna como sólido sorbente.Los resultados muestran que el Cr(VI) es en parte retirado por el hueso y en parte reducido a Cr(III) y que, a partir de un tiempo de contacto próximo a 300 minutos, el porcentaje retirado de Cr(VI) y Cr total se mantiene constante, lo que indica que no se produce ninguno de los dos procesos. Así mismo, la capacidad de eliminación de Cr (VI) que tiene el hueso de aceituna disminuye con el aumento del pH, siendo este descenso mucho más acusado a pH>2. Para el Cr total los valores de la capacidad de eliminación resultan inferiores a los obtenidos para Cr(VI), lo que pone de manifiesto el efecto dela reducción de Cr(VI) a Cr(III). Los resultados experimentalesse han ajustado a los modelos de pseudo-primer orden, pseudo-segundo orden, Elovich y difusión intraparticular, siendo el modelo de pseudo-segundo orden el que mejor reproduce la cinética del proceso en todas las condiciones experimentales utilizadas. Finalmente, el efecto de la temperatura en la cinética del proceso muestra que, para Cr(VI), la capacidad de biosorción y la constante cinética aumentan cuando la temperatura se eleva de 25 a 60 ºC, permaneciendo ambos parámetros prácticamente constantes cuando la temperaturaaumenta hasta 80 ºC
Supernova 1996cr: SN 1987A's Wild Cousin?
We report on new VLT optical spectroscopic and multi-wavelength archival
observations of SN1996cr, a previously identified ULX known as Circinus Galaxy
X-2. Our optical spectrum confirms SN1996cr as a bona fide type IIn SN, while
archival imaging isolates its explosion date to between 1995-02-28 and
1996-03-16. SN1996cr is one of the closest SNe (~3.8 Mpc) in the last several
decades and in terms of flux ranks among the brightest radio and X-ray SNe ever
detected. The wealth of optical, X-ray, and radio observations that exist for
this source provide relatively detailed constraints on its post-explosion
expansion and progenitor history, including an preliminary angular size
constaint from VLBI. The archival X-ray and radio data imply that the
progenitor of SN1996cr evacuated a large cavity just prior to exploding: the
blast wave likely expanded for ~1-2 yrs before eventually striking the dense
circumstellar material which surrounds SN1996cr. The X-ray and radio emission,
which trace the progenitor mass-loss rate, have respectively risen by a factor
of ~2 and remained roughly constant over the past 7 yr. This behavior is
reminiscent of the late rise of SN1987A, but 1000 times more luminous and much
more rapid to onset. Complex Oxygen line emission in the optical spectrum
further hints at a possible concentric shell or ring-like structure. The
discovery of SN1996cr suggests that a substantial fraction of the closest SNe
observed in the last several decades have occurred in wind-blown bubbles. An
Interplanetary Network position allows us to reject a tentative GRB association
with BATSE 4B960202. [Abridged]Comment: 25 pages with tables, 12 figures (color), accepted to ApJ, comments
welcome; v2 - updated to reflect the subsequent rejection of our tentative
GRB association based on a revised error region from the Interplanetary
Network (thanks to Kevin Hurley) and include a few additional references; v3
- corrected some errors in Tables 7 and
Superbubble evolution including the star-forming clouds: Is it possible to reconcile LMC observations with model predictions?
Here we present a possible solution to the apparent discrepancy between the
observed properties of LMC bubbles and the standard, constant density bubble
model. A two-dimensional model of a wind-driven bubble expanding from a
flattened giant molecular cloud is examined. We conclude that the expansion
velocities derived from spherically symmetric models are not always applicable
to elongated young bubbles seen almost face-on due to the LMC orientation. In
addition, an observational test to differentiate between spherical and
elongated bubbles seen face-on is discussed.Comment: 25 pages, 7 figures, accepted to ApJ (September, 1999 issue
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