3,439 research outputs found
SN1991bg-like supernovae are a compelling source of most Galactic antimatter
The Milky Way Galaxy glows with the soft gamma ray emission resulting from
the annihilation of electron-positron pairs every
second. The origin of this vast quantity of antimatter and the peculiar
morphology of the 511keV gamma ray line resulting from this annihilation have
been the subject of debate for almost half a century. Most obvious positron
sources are associated with star forming regions and cannot explain the rate of
positron annihilation in the Galactic bulge, which last saw star formation some
ago, or else violate stringent constraints on the positron
injection energy. Radioactive decay of elements formed in core collapse
supernovae (CCSNe) and normal Type Ia supernovae (SNe Ia) could supply
positrons matching the injection energy constraints but the distribution of
such potential sources does not replicate the required morphology. We show that
a single class of peculiar thermonuclear supernova - SN1991bg-like supernovae
(SNe 91bg) - can supply the number and distribution of positrons we see
annihilating in the Galaxy through the decay of Ti synthesised in these
events. Such Ti production simultaneously addresses the observed
abundance of Ca, the Ti decay product, in solar system material.Comment: Accepted for publication in Proceedings of IAU Symposium 322: The
Multimessenger Astrophysics of the Galactic Center 4 page
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Modeling of Selective Area Laser Deposition Vapor Infiltration (SALDVI) of Silicon Carbide
Selective Area Laser Deposition Vapor Infiltration (SALDVI) is a developing
solid freeform fabrication (SFF) technique in which porous layers of powder are
densified by infiltrating the pore spaces with solid material deposited from a gas
precursor during laser heating. A 3D finite element model was developed that simulates
SALDVI of silicon carbide. The model predicts the laser input power and the distribution
of vapor deposited SiC within the powder bed as well as on the surface of the powder bed
(SALD). The model considers a moving Gaussian distribution laser beam, temperatureand porous-dependent thermal conductivity, specific heat and temperature-dependent
deposition rate. Furthermore, the model also includes closed-loop control of the laser
power to achieve a desired target processing temperature on the top surface of the power
bed. The simulation results agree fairly well with experimental data for simple
geometries and offer guidelines for further experimental studies of the SALDVI process.Mechanical Engineerin
Superior Real Estate Investment Performance: Enigma or Illusion? A Critical Review of the Literature
[Excerpt] The purpose of this paper is to critique the existing empirical evidence on the investment performance of real estate relative to alternative asset categories. The key issue which guides this review of the investment performance literature is whether abnormal real estate returns are merely an illusion which arises from the shortcomings associated with various real estate performance studies or are the result of an omission of more fundamental factors. We suggest that any superior return is a short-run phenomenon, because, according to capital market theory, all assets should exhibit similar risk and return characteristics in the long run. If real estate continues to possess superior performance in the long run, then this implies that fundamental factors have been omitted from the real estate pricing model. Moreover, we will propose that a world in which the capital asset pricing model holds might be compatible with the existing evidence, because most of the prior studies have focused on total risk rather than on systematic risk. l Consequently, all assets can plot on the security market line in equilibrium, given a CAPM world, regardless of whether one asset (portfolio) such as real estate dominates another asset (portfolio) such as stocks from a mean-variance perspective
Radio Synchrotron Emission from Secondary Leptons in the Vicinity of Sgr A*
A point-like source of ~TeV gamma-rays has recently been seen towards the
Galactic center by HESS and other air Cerenkov telescopes. In recent work
(Ballantyne et al. 2007), we demonstrated that these gamma-rays can be
attributed to high-energy protons that (i) are accelerated close to the event
horizon of the central black hole, Sgr A*, (ii) diffuse out to ~pc scales, and
(iii) finally interact to produce gamma-rays. The same hadronic collision
processes will necessarily lead to the creation of electrons and positrons.
Here we calculate the synchrotron emissivity of these secondary leptons in the
same magnetic field configuration through which the initiating protons have
been propagated in our model. We compare this emission with the observed ~GHz
radio spectrum of the inner few pc region which we have assembled from archival
data and new measurements we have made with the Australia Telescope Compact
Array. We find that our model predicts secondary synchrotron emission with a
steep slope consistent with the observations but with an overall normalization
that is too large by a factor of ~ 2. If we further constrain our theoretical
gamma-ray curve to obey the implicit EGRET upper limit on emission from this
region we predict radio emission that is consistent with observations, i.e.,
the hadronic model of gamma ray emission can, simultaneously and without
fine-tuning, also explain essentially all the diffuse radio emission detected
from the inner few pc of the Galaxy.Comment: 11 pages, 2 figures. Two references missing from published version
added and acknowledgements extende
SN1991bg-like supernovae are associated with old stellar populations
SN1991bg-like supernovae are a distinct subclass of thermonuclear supernovae
(SNe Ia). Their spectral and photometric peculiarities indicate their
progenitors and explosion mechanism differ from `normal' SNe Ia. One method of
determining information about supernova progenitors we cannot directly observe
is to observe the stellar population adjacent to the apparent supernova
explosion site to infer the distribution of stellar population ages and
metallicities. We obtain integral field observations and analyse the spectra
extracted from regions of projected radius about the
apparent SN explosion site for 11 91bg-like SNe in both early- and late-type
galaxies. We utilize full-spectrum spectral fitting to determine the ages and
metallicities of the stellar population within the aperture. We find that the
majority of the stellar populations that hosted 91bg-like supernovae have
little recent star formation. The ages of the stellar populations suggest that
that 91bg-like SN progenitors explode after delay times of ,
much longer than the typical delay time of normal SNe Ia, which peaks at .Comment: 12 pages, 3 figures, 3 tables, submitted to Publications of the
Astronomical Society of Australi
Hydrodynamic Coupling of Two Brownian Spheres to a Planar Surface
We describe direct imaging measurements of the collective and relative
diffusion of two colloidal spheres near a flat plate. The bounding surface
modifies the spheres' dynamics, even at separations of tens of radii. This
behavior is captured by a stokeslet analysis of fluid flow driven by the
spheres' and wall's no-slip boundary conditions. In particular, this analysis
reveals surprising asymmetry in the normal modes for pair diffusion near a flat
surface.Comment: 4 pages, 4 figure
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