24,810 research outputs found
Microphysical, microchemical and adhesive properties of lunar material. 3: Gas interaction with lunar material
Knowledge of the reactivity of lunar material surfaces is important for understanding the effects of the lunar or space environment upon this material, particularly its nature, behavior and exposure history in comparison to terrestrial materials. Adsorptive properties are one of the important techniques for such studies. Gas adsorption measurements were made on an Apollo 12 ultrahigh vacuum-stored sample and Apollo 14 and 15 N2-stored samples. Surface area measurements were made on the latter two. Adsorbate gases used were N2, A, O2 and H2O. Krypton was used for the surface area determinations. Runs were made at room and liquid nitrogen temperature in volumetric and gravimetric systems. It was found that the adsorptive/desorptive behavior was in general significantly different from that of terrestrial materials of similar type and form. Specifically (1) the UHV-stored sample exhibited very high initial adsorption indicative of high surface reactivity, and (2) the N2-stored samples at room and liquid nitrogen temperatures showed that more gas was desorbed than introduced during adsorption, indicative of gas release from the samples. The high reactivity is a scribed cosmic ray track and solar wind damage
Localizing coalescing massive black hole binaries with gravitational waves
Massive black hole binary coalescences are prime targets for space-based
gravitational wave (GW) observatories such as {\it LISA}. GW measurements can
localize the position of a coalescing binary on the sky to an ellipse with a
major axis of a few tens of arcminutes to a few degrees, depending on source
redshift, and a minor axis which is times smaller. Neglecting weak
gravitational lensing, the GWs would also determine the source's luminosity
distance to better than percent accuracy for close sources, degrading to
several percent for more distant sources. Weak lensing cannot, in fact, be
neglected and is expected to limit the accuracy with which distances can be
fixed to errors no less than a few percent. Assuming a well-measured cosmology,
the source's redshift could be inferred with similar accuracy. GWs alone can
thus pinpoint a binary to a three-dimensional ``pixel'' which can help guide
searches for the hosts of these events. We examine the time evolution of this
pixel, studying it at merger and at several intervals before merger. One day
before merger, the major axis of the error ellipse is typically larger than its
final value by a factor of . The minor axis is larger by a factor
of , and, neglecting lensing, the error in the luminosity distance is
larger by a factor of . This large change over a short period of
time is due to spin-induced precession, which is strongest in the final days
before merger. The evolution is slower as we go back further in time. For , we find that GWs will localize a coalescing binary to within $\sim 10\
\mathrm{deg}^2$ as early as a month prior to merger and determine distance (and
hence redshift) to several percent.Comment: 30 pages, 10 figures, 5 tables. Version published in Ap
Development of advanced fabrication techniques for regeneratively cooled thrust chambers by the gas-pressure-bonding process Final report, 29 Jun. 1967 - 30 Apr. 1970
Production of regeneratively cooled rocket thrust chambers by removable tooling and subsequent hot isostatic pressing in gas autoclav
Microchemical, microphysical and adhesive properties of Apollo 11 and 12 Final report, 1 Aug. 1969 - 15 Mar. 1971
Gas exposure experiments of lunar soil with microchemical, microphysical, and adhesion analysi
Trends and Variability in Localized Precipitation Around Kibale National Park, Uganda, Africa
Our objective was to understand and describe local spatial and temporal variability in precipitation around Kibale National Park, a tropical forest area of high conservation value. Continental or regional-scale trends are often relied upon to make policy and management decisions, but these analyses are often at too coarse a resolution to capture important variability at a finer scale where management actions operate. Monthly rainfall data derived from ten long-term station records (1941-1975) were used to evaluate local spatiotemporal variability in seasonal and annual rainfall for the area surrounding Kibale National Park. The magnitude, direction and significance of trends in seasonal and annual rainfall within the area surrounding the park were identified using the Mann-Kendall trend test and Sen’s slope estimator. The standardized precipitation index was calculated at 3- and 12-month periods to identify areas of relative wetness or dryness. Analysis of annual trends and precipitation indices indicated that patterns in annual time series do not reflect the direction and magnitude of seasonal trends nor the spatial variability in intra-annual rainfall at the local scale. Significant negative trends in the seasonal long rains, following dry season and short rains were identified at stations west of Kibale, while significant positive trends in the seasonal short rains occurred at stations north of the park. Stations along the western park boundary tended to have more years in which the two dry seasons were abnormally dry than those stations located further from the park
Interacting Hofstadter spectrum of atoms in an artificial gauge field
Motivated by experimental advances in the synthesis of gauge potentials for
ultracold atoms, we consider the superfluid phase of interacting bosons on a
square lattice in the presence of a magnetic field. We show that superfluid
order implies spatial symmetry breaking, and predict clear signatures of
many-body effects in time-of-flight measurements. By developing a Bogoliubov
expansion based on the exact Hofstadter spectrum, we find the dispersion of the
quasiparticle modes within the superfluid phase, and describe the consequences
for Bragg spectroscopy measurements. The theory also provides an estimate of
the critical interaction strength at the transition to the Mott insulator
phase.Comment: 4+ pages, 2 figures; v2: published versio
Population pressure and global markets drive a decade of forest cover change in Africa\u27s Albertine Rift
Africa\u27s Albertine Rift region faces a juxtaposition of rapid human population growth and protected areas, making it one of the world\u27s most vulnerable biodiversity hotspots. Using satellite-derived estimates of forest cover change, we examined national socioeconomic, demographic, agricultural production, and local demographic and geographic variables, to assess multilevel forces driving local forest cover loss and gain outside protected areas during the first decade of this century. Because the processes that drive forest cover loss and gain are expected to be different, and both are of interest, we constructed models of significant change in each direction. Although rates of forest cover change varied by country, national population change was the strongest driver of forest loss for all countries – with a population doubling predicted to cause 2.06% annual cover loss, while doubling tea production predicted to cause 1.90%. The rate of forest cover gain was associated positively with increased production of the local staple crop cassava, but negatively with local population density and meat production, suggesting production drivers at multiple levels affect reforestation. We found a small but significant decrease in loss rate as distance from protected areas increased, supporting studies suggesting higher rates of landscape change near protected areas. While local population density mitigated the rate of forest cover gain, loss was also correlated with lower local population density, an apparent paradox, but consistent with findings that larger scale forces outweigh local drivers of deforestation. This implicates demographic and market forces at national and international scales as critical drivers of change, calling into question the necessary scales of forest protection policy in this biodiversity hotspot. Using a satellite derived estimate of forest cover change for both loss and gain added a dynamic component to more traditionally static and unidirectional studies, significantly improving our understanding of landscape processes and drivers at work
Longitudinal photocurrent spectroscopy of a single GaAs/AlGaAs v-groove quantum wire
Modulation-doped GaAs v-groove quantum wires (QWRs) have been fabricated with
novel electrical contacts made to two-dimensional electron-gas (2DEG)
reservoirs. Here, we present longitudinal photocurrent (photoconductivity/PC)
spectroscopy measurements of a single QWR. We clearly observe conductance in
the ground-state one-dimensional subbands; in addition, a highly
temperature-dependent response is seen from other structures within the
v-groove. The latter phenomenon is attributed to the effects of structural
topography and localization on carrier relaxation. The results of
power-dependent PC measurements suggest that the QWR behaves as a series of
weakly interacting localized states, at low temperatures
A Catalog of MIPSGAL Disk and Ring Sources
We present a catalog of 416 extended, resolved, disk- and ring-like objects
as detected in the MIPSGAL 24 micron survey of the Galactic plane. This catalog
is the result of a search in the MIPSGAL image data for generally circularly
symmetric, extended "bubbles" without prior knowledge or expectation of their
physical nature. Most of the objects have no extended counterpart at 8 or 70
micron, with less than 20% detections at each wavelength. For the 54 objects
with central point sources, the sources are nearly always seen in all IRAC
bands. About 70 objects (16%) have been previously identified, with another 35
listed as IRAS sources. Among the identified objects, those with central
sources are mostly listed as emission-line stars, but with other source types
including supernova remnants, luminous blue variables, and planetary nebulae.
The 57 identified objects (of 362) without central sources are nearly all PNe
(~90%).which suggests that a large fraction of the 300+ unidentified objects in
this category are also PNe. These identifications suggest that this is
primarily a catalog of evolved stars. Also included in the catalog are two
filamentary objects that are almost certainly SNRs, and ten unusual compact
extended objects discovered in the search. Two of these show remarkable spiral
structure at both 8 and 24 micron. These are likely background galaxies
previously hidden by the intervening Galactic plane
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