47,556 research outputs found
Thermionic performance of a variable-gap cesium diminiode with a 110-single-crystal-tungsten emitter and a polycrystalline-niobium collector
Results from tests of the first variable-gap diminiode at an initial interelectrode spacing of 0.23 millimeter indicate sharply defined, relatively low ultimate power points. This characteristic supports the value of the diminiode as a well-controlled tool for thermionic-conversion research and development
Observations of ozone production in a dissipating tropical convective cell during TC4
From 13 July–9 August 2007, 25 ozonesondes were launched from Las Tablas, Panama as part of the Tropical Composition, Cloud, and Climate Coupling (TC4) mission. On 5 August, a strong convective cell formed in the Gulf of Panama. World Wide Lightning Location Network (WWLLN) data indicated 563 flashes (09:00–17:00 UTC) in the Gulf. NO2 data from the Ozone Monitoring Instrument (OMI) show enhancements, suggesting lightning production of NOx. At 15:05 UTC, an ozonesonde ascended into the southern edge of the now dissipating convective cell as it moved west across the Azuero Peninsula. The balloon oscillated from 2.5–5.1 km five times (15:12–17:00 UTC), providing a unique examination of ozone (O3) photochemistry on the edge of a convective cell. Ozone increased at a rate of 1.6–4.6 ppbv/hr between the first and last ascent, resulting cell wide in an increase of (2.1–2.5)×106 moles of O3. This estimate agrees to within a factor of two of our estimates of photochemical lightning O3 production from the WWLLN flashes, from the radar-inferred lightning flash data, and from the OMI NO2 data (1.2, 1.0, and 1.7×106 moles, respectively), though all estimates have large uncertainties. Examination of DC-8 in situ and lidar O3 data gathered around the Gulf that day suggests 70–97% of the O3 change occurred in 2.5–5.1 km layer. A photochemical box model initialized with nearby TC4 aircraft trace gas data suggests these O3 production rates are possible with our present understanding of photochemistry
Computer acquired performance data from a chemically vapor-deposited-rhenium, niobium planar diode
Performance data from a chemically vapor-deposited-rhenium, niobium thermionic converter are presented. The planar converter has a guard-ringed collector and a nominal fixed spacing of 0.25 mm (10 mils). The data were obtained by using a computerized acquisition system and are available on request to one of the authors on microfiche as individual and composite parametric current, voltage curves. The parameters are the temperatures of the emitter T sub E collector T sub C, and cesium reservoir T sub R. The composite plots have constant T sub E and varying T sub C or T sub R, or both. Current, voltage envelopes having constant T sub E with and without fixed T sub C appear in the present report. The diode was tested at increments between 1600 and 2000 K for the emitter Hohlraum, 800 to 1100 K for the collector, and 540 and 650 K for the reservoir. A total of 312 current, voltage curves were obtained in the present performance evaluation. Current, voltage envelopes from three rhenium emitter converters evaluated in the present program are also given. The data are compared at commom emitter Hohlraum temperatures
IRS 16SW - A New Comoving Group of Young Stars in the Central Parsec of the Milky Way
One of the most perplexing problems associated with the supermassive black
hole at the center of our Galaxy is the origin of the young stars in its close
vicinity. Using proper motion measurements and stellar number density counts
based on 9 years of diffraction-limited K(2.2 micron)-band speckle imaging at
the W. M. Keck 10-meter telescopes, we have identified a new comoving group of
stars, which we call the IRS 16SW comoving group, located 1.9" (0.08 pc, in
projection) from the central black hole. Four of the five members of this
comoving group have been spectroscopically identified as massive young stars,
specifically He I emission-line stars and OBN stars. This is the second young
comoving group within the central parsec of the Milky Way to be recognized and
is the closest, by a factor of 2, in projection to the central black hole.
These comoving groups may be the surviving cores of massive infalling star
clusters that are undergoing disruption in the strong tidal field of the
central supermassive black hole.Comment: 10 pages, 1 figure, accepted for ApJL, uses emulateap
Charged Vacuum Bubble Stability
A type of scenario is considered where electrically charged vacuum bubbles,
formed from degenerate or nearly degenerate vacuua separated by a thin domain
wall, are cosmologically produced due to the breaking of a discrete symmetry,
with the bubble charge arising from fermions residing within the domain wall.
Stability issues associated with wall tension, fermion gas, and Coulombic
effects for such configurations are examined. The stability of a bubble depends
upon parameters such as the symmetry breaking scale and the fermion coupling. A
dominance of either the Fermi gas or the Coulomb contribution may be realized
under certain conditions, depending upon parameter values.Comment: 16 pages,revtex; accepted for publication in Phys.Rev.
Coal-shale interface detection system
A coal-shale interface detection system for use with coal cutting equipment consists of a reciprocating hammer on which an accelerometer is mounted to measure the impact of the hammer as it penetrates the ceiling or floor surface of a mine. A pair of reflectometers simultaneously view the same surface. The outputs of the accelerometer and reflectometers are detected and jointly registered to determine when an interface between coal and shale is being cut through
Galactic Center Youth: Orbits and Origins of the Young Stars in the Central Parsec
We present new proper motions for the massive, young stars at the Galactic Center, based on 10 years of diffraction limited data from the Keck telescopes. Our proper motion measurements now have uncertainties of only 1-2 km/s and allow us to explore the origin of the young stars that reside within the sphere of inflience of the supermassive black hole whose strong tidal forces make this region inhospitable for star formation. Their presence, however, may be explained either by in situ star formation in an accretion disk or as the remnants of a massive stellar cluster which spiraled in via dynamical friction. Earlier stellar velocity vectors were used to postulate that all the young stars resided in two counter-rotating stellar disks, which is consistent with both of the above formation scenarios. Our precise proper motions allow us, for the frst time, to determine the orbital parameters of each individual star and thereby to test the hypothesis that the massive stars reside in two stellar disks. Of the 26 young stars in this study that were previously proposed to lie on the inner, clockwise disk, we find that nearly all exhibit orbital constraints consistent with such a disk. On the other hand, of the 7 stars in this study previously proposed to lie in the outer, less well-defhed counter-clockwise disk, 6 exhibit inclinations that are inconsistent with such a disk, bringing into question the existence of the outer disk. Furthermore, for stars in the inner disk that have eccentricity constraints, we find several that have lower limits to the eccentricity of more than 0.4, implying highly eccentric orbits. This stands in contrast to simple accretion disk formation scenarios which typically predict predominantly circular orbits
Adaptive Optics Near-Infrared Spectroscopy of the Sgr A* Cluster
We present K-band ~ 2600 spectroscopy of five stars
(K ~ 14 - 16 mag) within 0.''5 of Sgr A*, the radio source associated with the
compact massive object suspected to be a 2.6 x 10 \msun black hole at the
center of our Galaxy. High spatial resolution of ~ 0.''09, and good strehl
ratios of ~ 0.2 achieved with adaptive optics on the 10-meter Keck telescope
make it possible to measure moderate-resolution spectra of these stars
individually for the first time. Two stars (S0-17 and S0-18) are identified as
late-type stars by the detection of CO bandhead absorption in their spectra.
Their absolute K magnitudes and CO bandhead absorption strengths are consistent
with early K giants. Three stars (S0-1, S0-2, and S0-16), with r
0.0075 pc (~ 0.''2) from Sgr A*, lack CO bandhead absorption, confirming the
results of earlier lower spectral and lower spatial resolution observations
that the majority of the stars in the Sgr A* Cluster are early-type stars. The
absolute K magnitudes of the early-type stars suggest that they are late O -
early B main sequence stars of ages 20 Myr. The presence of young stars in
the Sgr A* Cluster, so close to the central supermassive black hole, poses the
intriguing problem of how these stars could have formed, or could have been
brought, within its strong tidal field.Comment: 19 pages, 8 figures, 2 tables. Accepted for publication in Ap
Data management study, volume 5. Appendix K - Contractor data package data management /DM/ Final report
Contractor data management system for Voyager projec
Gauged Dimension Bubbles
Some of the peculiar electrodynamical effects associated with gauged
``dimension bubbles'' are presented. Such bubbles, which effectively enclose a
region of 5d spacetime, can arise from a 5d theory with a compact extra
dimension. Bubbles with thin domain walls can be stabilized against total
collapse by the entrapment of light charged scalar bosons inside the bubble,
extending the idea of a neutral dimension bubble to accommodate the case of a
gauged U(1) symmetry. Using a dielectric approach to the 4d dilaton-Maxwell
theory, it is seen that the bubble wall is almost totally opaque to photons,
leading to a new stabilization mechanism due to trapped photons. Photon
dominated bubbles very slowly shrink, resulting in a temperature increase
inside the bubble. At some critical temperature, however, these bubbles
explode, with a release of radiation.Comment: 14 pages, no figures; to appear in Phys.Rev.
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