7,417 research outputs found
Extreme ultraviolet and X-ray spectroheliograph for OSO-H
A complex scientific instrument was designed, fabricated, tested, and calibrated for launch onboard OSO-H. This instrument consisted of four spectroheliographs and an X-ray polarimeter. The instrument is designed to study solar radiation at selected wavelengths in the X-ray and the extreme ultraviolet ranges, make observations at the H-alpha wavelength, and measure the degree of polarization of X-ray emissions
Variational discrete variable representation for excitons on a lattice
We construct numerical basis function sets on a lattice, whose spatial
extension is scalable from single lattice sites to the continuum limit. They
allow us to compute small and large bound states with comparable, moderate
effort. Adopting concepts of discrete variable representations, a diagonal form
of the potential term is achieved through a unitary transformation to Gaussian
quadrature points. Thereby the computational effort in three dimensions scales
as the fourth instead of the sixth power of the number of basis functions along
each axis, such that it is reduced by two orders of magnitude in realistic
examples. As an improvement over standard discrete variable representations,
our construction preserves the variational principle. It allows for the
calculation of binding energies, wave functions, and excitation spectra. We use
this technique to study central-cell corrections for excitons beyond the
continuum approximation. A discussion of the mass and spectrum of the yellow
exciton series in the cuprous oxide, which does not follow the hydrogenic
Rydberg series of Mott-Wannier excitons, is given on the basis of a simple
lattice model.Comment: 12 pages, 7 figures. Final version as publishe
The Geometry and Ionization Structure of the Wind in the Eclipsing Nova-like Variables RW Tri and UX UMa
The UV spectra of nova-like variables are dominated by emission from the
accretion disk, modified by scattering in a wind emanating from the disk. Here
we model the spectra of RW Tri and UX UMa, the only two eclipsing nova-likes
which have been observed with the Hubble Space Telescope in the
far-ultraviolet, in an attempt to constrain the geometry and the ionization
structure of their winds. Using our Monte Carlo radiative transfer code we
computed spectra for simply-parameterized axisymmetric biconical outflow models
and were able to find plausible models for both systems. These reproduce the
primary UV resonance lines - N V, Si IV, and C IV - in the observed spectra in
and out of eclipse. The distribution of these ions in the wind models is
similar in both cases as is the extent of the primary scattering regions in
which these lines are formed. The inferred mass loss rates are 6% to 8% of the
mass accretion rates for the systems. We discuss the implication of our point
models for our understanding of accretion disk winds in cataclysmic variables.Comment: 13 pages, 15 figures and 4 tables. Published in Ap
Flight-test evaluation of two electronic display formats for approach to landing under instrument conditions
The results of a flight evaluation of two electronic display formats for the approach to landing under instrument conditions are presented. The evaluation was conducted for a base-line electronic display format and for the same format with runway symbology and track information added. The evaluation was conducted during 3 deg, manual straight-in approaches with and without initial localizer offsets. Flight path tracking performance data and pilot subjective comments were examined with regard to the pilot's ability to capture and maintain localizer and glide slope by using both display formats
The Compact UV Nucleus of M33
The most luminous X-ray source in the Local Group is associated with the
nucleus of M33. This source, M33 X-8, appears modulated by ~20% over a ~106 day
period, making it unlikely that the combined emission from unresolved sources
could explain the otherwise persistent ~1e39 erg/s X-ray flux (Dubus et al.
1997, Hernquist et al. 1991). We present here high resolution UV imaging of the
nucleus with the Planetary Camera of the HST undertaken in order to search for
the counterpart to X-8. The nucleus is bluer and more compact than at longer
wavelength images but it is still extended with half of its 3e38 erg/s UV
luminosity coming from the inner 0.14". We cannot distinguish between a
concentrated blue population and emission from a single object.Comment: 3 figures, accepted for publication in ApJ Letter
A limit on the detectability of the energy scale of inflation
We show that the polarization of the cosmic microwave background can be used
to detect gravity waves from inflation if the energy scale of inflation is
above 3.2 times 10^15 GeV. These gravity waves generate polarization patterns
with a curl, whereas (to first order in perturbation theory) density
perturbations do not. The limiting ``noise'' arises from the second--order
generation of curl from density perturbations, or rather residuals from its
subtraction. We calculate optimal sky coverage and detectability limits as a
function of detector sensitivity and observing time.Comment: 4 pages, 3 figures, submitted to PR
Water and energy footprint of irrigated agriculture in the Mediterranean region
Irrigated agriculture constitutes the largest consumer of freshwater in the Mediterranean region and provides a major source of income and employment for rural livelihoods. However, increasing droughts and water scarcity have highlighted concerns regarding the environmental sustainability of agriculture in the region. An integrated assessment combining a gridded water balance model with a geodatabase and GIS has been developed and used to assess the water demand and energy footprint of irrigated production in the region. Modelled outputs were linked with crop yield and water resources data to estimate water (m3 kg−1) and energy (CO2 kg−1) productivity and identify vulnerable areas or 'hotspots'. For a selected key crops in the region, irrigation accounts for 61 km3 yr−1 of water abstraction and 1.78 Gt CO2 emissions yr−1, with most emissions from sunflower (73 kg CO2/t) and cotton (60 kg CO2/t) production. Wheat is a major strategic crop in the region and was estimated to have a water productivity of 1000 t Mm−3 and emissions of 31 kg CO2/t. Irrigation modernization would save around 8 km3 of water but would correspondingly increase CO2 emissions by around +135%. Shifting from rain-fed to irrigated production would increase irrigation demand to 166 km3 yr−1 (+137%) whilst CO2 emissions would rise by +270%. The study has major policy implications for understanding the water–energy–food nexus in the region and the trade-offs between strategies to save water, reduce CO2 emissions and/or intensify food production
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