1,353 research outputs found
On the Collision of Cosmic Superstrings
We study the formation of three-string junctions between (p,q)-cosmic
superstrings, and collisions between such strings and show that kinematic
constraints analogous to those found previously for collisions of Nambu-Goto
strings apply here too, with suitable modifications to take account of the
additional requirements of flux conservation. We examine in detail several
examples involving collisions between strings with low values of p and q, and
also examine the rates of growth or shrinkage of strings at a junction.
Finally, we briefly discuss the formation of junctions for strings in a warped
space, specifically with a Klebanov-Strassler throat, and show that similar
constraints still apply with changes to the parameters taking account of the
warping and the background flux.Comment: 13 pages, 2 figure
Measurement of Spectral Breaks in Pulsar Wind Nebulae with Millimeter-wave Interferometry
We have observed pulsar wind nebulae in the three supernova remnants
G11.2-0.3, G16.7+0.1, and G29.7-0.3 at 89 GHz with the
Berkeley-Illinois-Maryland Association Array, measuring total flux densities of
two of them for comparison with archival data at other frequencies . In
G16.7+0.1, we find a break in the spectrum of the PWN at ~26 GHz. In G29.7-0.3,
our data suggest a break in the integrated spectrum of the central nebula at
~55 GHz, lower than previously estimated. However, we have found spatial
structure in the spectrum of this nebula. The emission to the north of pulsar
J1846-0258 has a broken spectrum, with break frequency ~< 100 GHz, consistent
with a conventional pulsar-powered nebula. The emission to the south of the
pulsar has a near-power-law spectrum from radio to X-rays: this component may
be unrelated to the PWN, or may be evidence of asymmetries and/or time
evolution in the pulsar's energy output. We present 89 GHz images of each
remnant.Comment: 8 pages, including 7 eps figures. ApJ, in pres
An Automated Method for the Detection and Extraction of HI Self-Absorption in High-Resolution 21cm Line Surveys
We describe algorithms that detect 21cm line HI self-absorption (HISA) in
large data sets and extract it for analysis. Our search method identifies HISA
as spatially and spectrally confined dark HI features that appear as negative
residuals after removing larger-scale emission components with a modified CLEAN
algorithm. Adjacent HISA volume-pixels (voxels) are grouped into features in
(l,b,v) space, and the HI brightness of voxels outside the 3-D feature
boundaries is smoothly interpolated to estimate the absorption amplitude and
the unabsorbed HI emission brightness. The reliability and completeness of our
HISA detection scheme have been tested extensively with model data. We detect
most features over a wide range of sizes, linewidths, amplitudes, and
background levels, with poor detection only where the absorption brightness
temperature amplitude is weak, the absorption scale approaches that of the
correlated noise, or the background level is too faint for HISA to be
distinguished reliably from emission gaps. False detection rates are very low
in all parts of the parameter space except at sizes and amplitudes approaching
those of noise fluctuations. Absorption measurement biases introduced by the
method are generally small and appear to arise from cases of incomplete HISA
detection. This paper is the third in a series examining HISA at high angular
resolution. A companion paper (Paper II) uses our HISA search and extraction
method to investigate the cold atomic gas distribution in the Canadian Galactic
Plane Survey.Comment: 39 pages, including 14 figure pages; to appear in June 10 ApJ, volume
626; figure quality significantly reduced for astro-ph; for full resolution,
please see http://www.ras.ucalgary.ca/~gibson/hisa/cgps1_survey
The experimental gas-phase structures of 1,3,5-trisilylbenzene and hexasilylbenzene and the theoretical structures of all benzenes with three or more silyl substituents
The structures of 1,3,5-trisilylbenzene and hexasilylbenzene in the gas phase have been determined by electron diffraction, and that of 1,3,5-trisilylbenzene by X-ray crystallography. The structures of three trisilylbenzene isomers, three tetrasilylbenzenes, pentasilylbenzene and hexasilylbenzene have been computed, ab initio and using Density Functional Theory, at levels up to MP2/6-31G*. The primary effect of silyl substituents is to narrow the ring angle at the substituted carbon atoms. Steric interactions between silyl groups on neighbouring carbon atoms lead first to displacement of these groups away from one another, and then to displacement out of the ring plane, with alternate groups moving to opposite sides of the ring. In the extreme example, hexasilylbenzene, the SiCCSi dihedral angle is 17.8(8)°
Terahertz oscillations in an In<sub>0.53</sub>Ga<sub>0.47</sub>As submicron planar gunn diode
The length of the transit region of a Gunn diode determines the natural frequency at which it operates in fundamental mode â the shorter the device, the higher the frequency of operation. The long-held view on Gunn diode design is that for a functioning device the minimum length of the transit region is about 1.5ÎŒm, limiting the devices to fundamental mode operation at frequencies of roughly 60 GHz. Study of these devices by more advanced Monte Carlo techniques that simulate the ballistic transport and electron-phonon interactions that govern device behaviour, offers a new lower bound of 0.5ÎŒm, which is already being approached by the experimental evidence that has shown planar and vertical devices exhibiting Gunn operation at 600nm and 700nm, respectively. The paper presents results of the first ever THz submicron planar Gunn diode fabricated in In<sub>0.53</sub>Ga<sub>0.47</sub>A on an InP substrate, operating at a fundamental frequency above 300 GHz. Experimentally measured rf power of 28 ”W was obtained from a 600 nm long Ă120 ”m wide device. At this new length, operation in fundamental mode at much higher frequencies becomes possible â the Monte Carlo model used predicts power output at frequencies over 300 GHz
Molecular Tracers of the Central 12 pc of the Galactic Center
We have used the BIMA array to image the Galactic Center with a 19-pointing
mosaic in HCN(1-0), HCO+(1-0), and H 42-alpha emission with 5 km/s velocity
resolution and 13'' x 4'' angular resolution. The 5' field includes the
circumnuclear ring (CND) and parts of the 20 and 50 km/s clouds. HCN(1-0) and
HCO+ trace the CND and nearby giant molecular clouds while the H 42-alpha
emission traces the ionized gas in Sgr A West. We find that the CND has a
definite outer edge in HCN and HCO+ emission at ~45'' radius and appears to be
composed of two or three distinct streams of molecular gas rotating around the
nucleus. Outside the CND, HCN and HCO+ trace dense clumps of high-velocity gas
in addition to optically thick emission from the 20 and 50 km/s clouds. A
molecular ridge of compressed gas and dust, traced in NH3 emission and
self-absorbed HCN and HCO+, wraps around the eastern edge of Sgr A East. Just
inside this ridge are several arcs of gas which have been accelerated by the
impact of Sgr A East with the 50 km/s cloud. HCN and HCO+ emission trace the
extension of the northern arm of Sgr A West which appears to be an independent
stream of neutral and ionized gas and dust originating outside the CND. Broad
line widths and OH maser emission mark the intersection of the northern arm and
the CND. Comparison to previous NH3 and 1.2mm dust observations shows that HCN
and HCO+ preferentially trace the CND and are weaker tracers of the GMCs than
NH3 and dust. We discuss possible scenarios for the emission mechanisms and
environment at the Galactic center which could explain the differences in these
images.Comment: 24 pages, including 17 figures; to appear in The Astrophysical
Journa
Effect of deconfinement on resonant transport in quantum wires
The effect of deconfinement due to finite band offsets on transport through
quantum wires with two constrictions is investigated. It is shown that the
increase in resonance linewidth becomes increasingly important as the size is
reduced and ultimately places an upper limit on the energy (temperature) scale
for which resonances may be observed.Comment: 6 pages, 6 postscript files with figures; uses REVTe
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