1,469 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
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
Spectroscopic investigation of a 'Virgin of Sorrows' canvas painting : a multi-method approach
Dynamical Expansion of Ionization and Dissociation Front around a Massive Star. II. On the Generality of Triggered Star Formation
We analyze the dynamical expansion of the HII region, photodissociation
region, and the swept-up shell, solving the UV- and FUV-radiative transfer, the
thermal and chemical processes in the time-dependent hydrodynamics code.
Following our previous paper, we investigate the time evolutions with various
ambient number densities and central stars. Our calculations show that basic
evolution is qualitatively similar among our models with different parameters.
The molecular gas is finally accumulated in the shell, and the gravitational
fragmentation of the shell is generally expected. The quantitative differences
among models are well understood with analytic scaling relations. The detailed
physical and chemical structure of the shell is mainly determined by the
incident FUV flux and the column density of the shell, which also follow the
scaling relations. The time of shell-fragmentation, and the mass of the
gathered molecular gas are sensitive tothe ambient number density. In the case
of the lower number density, the shell-fragmentation occurs over a longer
timescale, and the accumulated molecular gas is more massive. The variations
with different central stars are more moderate. The time of the
shell-fragmentation differs by a factor of several with the various stars of
M_* = 12-101 M_sun. According to our numerical results, we conclude that the
expanding HII region should be an efficient trigger for star formation in
molecular clouds if the mass of the ambient molecular material is large enough.Comment: 49 pages, including 17 figures ; Accepted for publication in Ap
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
Supernova Remnant Evolution in Wind Bubbles: A Closer Look at Kes 27
Massive Stars (> 8 solar masses) lose mass in the form of strong winds. These
winds accumulate around the star, forming wind-blown bubbles. When the star
explodes as a supernova (SN), the resulting shock wave expands within this
wind-blown bubble, rather than the interstellar medium. The properties of the
resulting remnant, its dynamics and kinematics, the morphology, and the
resulting evolution, are shaped by the structure and properties of the
wind-blown bubble. In this article we focus on Kes 27, a supernova remnant
(SNR) that has been proposed by Chen et al (2008) to be evolving in a
wind-blown bubble, explore its properties, and investigate whether the
properties could be ascribed to evolution of a SNR in a wind-blown bubble. Our
initial model does not support this conclusion, due to the fact that the
reflected shock is expanding into much lower densities.Comment: 5 pages, 3 figures. Revised version submitted to High Energy Density
Physics. To be published in a special issue of the proceedings of the 2012
HEDLA conferenc
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