66 research outputs found
New hypersonic facility capability at NASA Lewis Research Center
Four facility activities are underway at NASA Lewis Research Center to develop new hypersonic propulsion test capability. Two of these efforts consist of upgrades to existing operational facilities. The other two activities will reactivate facilities that have been in a standby condition for over 15 years. These four activities are discussed and the new test facilities NASA Lewis will have in place to support evolving high speed research programs are described
PROBLEMATIQUE DE LA CONSERVATION DES FORMATIONS VEGETALES RESIDUELLES A BATOUFAM DANS LES HAUTES TERRES DE L’OUEST CAMEROUN
Deal with the dual economic and social crisis in the 1980s, many of the Third World farming communities have been able to develop agro-pastoral systems and forestry in order to stamp rural poverty. The case study of Batoufam situated in the western highlands of Cameroon is an edifying illustration. Studies based on surveys and remote sensing data show a dynamic that leads to a gallery forest snacking, isolated groves and alpine meadows in favor of the habitat, communication channels and farms. Despite the increased rate of afforestation in the soils of the range of 20 to 30 percent, the future of these residual plant formations remains concerned because of population growth and anthropogenic pressures on the lands.Keywords: Plant formations, isolated groves, western highlands, Cameroon
Event-horizon-scale structure in the supermassive black hole candidate at the Galactic Centre
The cores of most galaxies are thought to harbour supermassive black holes,
which power galactic nuclei by converting the gravitational energy of accreting
matter into radiation (ref 1). Sagittarius A*, the compact source of radio,
infrared and X-ray emission at the centre of the Milky Way, is the closest
example of this phenomenon, with an estimated black hole mass that is 4 million
times that of the Sun (refs. 2,3). A long-standing astronomical goal is to
resolve structures in the innermost accretion flow surrounding Sgr A* where
strong gravitational fields will distort the appearance of radiation emitted
near the black hole. Radio observations at wavelengths of 3.5 mm and 7 mm have
detected intrinsic structure in Sgr A*, but the spatial resolution of
observations at these wavelengths is limited by interstellar scattering (refs.
4-7). Here we report observations at a wavelength of 1.3 mm that set a size of
37 (+16, -10; 3-sigma) microarcseconds on the intrinsic diameter of Sgr A*.
This is less than the expected apparent size of the event horizon of the
presumed black hole, suggesting that the bulk of SgrA* emission may not be not
centred on the black hole, but arises in the surrounding accretion flow.Comment: 12 pages including 2 figure
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Persistent Asymmetric Structure of Sagittarius A* on Event Horizon Scales
The Galactic Center black hole Sagittarius A* (Sgr A*) is a prime observing target for the Event Horizon Telescope (EHT), which can resolve the 1.3 mm emission from this source on angular scales comparable to that of the general relativistic shadow. Previous EHT observations have used visibility amplitudes to infer the morphology of the millimeter-wavelength emission. Potentially much richer source information is contained in the phases. We report on 1.3 mm phase information on Sgr A* obtained with the EHT on a total of 13 observing nights over 4 years. Closure phases, the sum of visibility phases along a closed triangle of interferometer baselines, are used because they are robust against phase corruptions introduced by instrumentation and the rapidly variable atmosphere. The median closure phase on a triangle including telescopes in California, Hawaii, and Arizona is nonzero. This result conclusively demonstrates that the millimeter emission is asymmetric on scales of a few Schwarzschild radii and can be used to break 180-degree rotational ambiguities inherent from amplitude data alone. The stability of the sign of the closure phase over most observing nights indicates persistent asymmetry in the image of Sgr A* that is not obscured by refraction due to interstellar electrons along the line of sight.Astronom
Significance of MAGIC and the Japanese ambassador to Berlin: (III) the months of growing certainty
Synthesis of (2R,5R)-5-Chloropipecolic Acid via a Homochiral Acyliminium Ion Intermediate
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