24,132 research outputs found
Automatic continuum analysis of reflectance spectra
A continuum algorithm based on a Segmented Upper Hull method (SUH) is described. An upper hull is performed on segments of a spectrum defined by local minima and maxima. The segments making a complete spectrum are then combined. The definition of the upper hull allows the continuum to be both concave and/or convex, adapting to the shape of the spectrum. The method performs multiple passes on a spectrum by segmenting each local maximum to minimum and performing an upper hull. The algorithm naturally adapts to the widths of absorption features, so that all features are found, including the nature of doublets, triplets, etc. The algorithm is also reasonably fast on common minicomputers so that it might be applied to the large data sets from imaging spectrometers
Causes of spurious features in spectral reflectance data
Several techniques are becoming common in the analysis of imaging spectrometer data that can lead to spurious absorption features or to changes in the position, width, and shape of actual absorption features. It is a common practice to calibrate AIS or other imaging spectrometer data by averaging each pixel along the flight line. The average is used to calibrate the spectral data by dividing the spectrum at each pixel by the average. If some pixels in the data set contain an absorption, then the average will also show an absorption. Some AIS data has had problems with wavelength stability from one scan line to the next which can produce spurious features with some analysis methods. If a pixel has a spectrum with an absorption having a different position or width than the spectrum used in a ratio, then the ratio can produce a spurious absorption at a different position and width than the true absorption feature. An average spectrum ratioed to each pixel will produce band shifts, and changes in width or shape. If continuum removal is performed by substraction rather than division, band positions can also be shifted
Rotating Metal Band Target for Pion Production at Muon Colliders and Neutrino Factories
A conceptual design is presented for a high power pion production target for
muon colliders and neutrino factories that is based around a rotating metal
band.Comment: 28 pages, 16 figures; to be published in Phys. Rev. ST Accel. Beam
New spectral observations of Callisto and leading/trailing hemisphere distinctions
In December 1989 and January 1990, new observations of the leading and trailing edges of Callisto were made from the NASA Infrared Telescope Facility on Mauna Kea in Hawaii. Using the Cool Grating Array Spectrometer, spectral coverage was obtained from 1.89 to 2.46 microns and from 2.8 to 4.2 microns for both the leading and trailing hemispheres. In addition, spectral coverage of the leading hemisphere was obtained from 1.30 to 2.55 microns and from 4.2 to 4.8 microns. Interpretations of the data are given
On closed rotating worlds
A new solution for the stationary closed world with rigid rotation is
obtained for the spinning fluid source. It is found that the spin and vorticity
are locally balanced. This model qualitatively shows that the local rotation of
the cosmological matter can be indeed related to the global cosmic vorticity,
provided the total angular momentum of the closed world is vanishing.Comment: 10 pages, Revtex, to appear in Phys. Rev. D6
Geometric phases and Wannier functions of Bloch electrons in 1-dimension
We present a formal expression for Wannier functions of composite bands of
1-D Bloch electrons in terms of parallel-transported Bloch functions and their
non-Abelian geometric phases. Spatial decay properties of these Wannier
functions are studied in the case of simple bands of 1-D model insulator and
metal. Within first-principles density functional theory, we illustrate the
formalism through the construction of Wannier functions of polyethylene and
polyacetylene.Comment: 4 pages, 4 figure
Differential constraints and exact solutions of nonlinear diffusion equations
The differential constraints are applied to obtain explicit solutions of
nonlinear diffusion equations. Certain linear determining equations with
parameters are used to find such differential constraints. They generalize the
determining equations used in the search for classical Lie symmetries
On the role of magnetic reconnection in jet/accretion disk systems
The most accepted model for jet production is based on the
magneto-centrifugal acceleration out off an accretion disk that surrounds the
central source (Blandford & Payne, 1982). This scenario, however, does not
explain, e.g., the quasi-periodic ejection phenomena often observed in
different astrophysical jet classes. de Gouveia Dal Pino & Lazarian (2005)
(hereafter GDPL) have proposed that the large scale superluminal ejections
observed in microquasars during radio flare events could be produced by violent
magnetic reconnection (MR) episodes. Here, we extend this model to other
accretion disk systems, namely: active galactic nuclei (AGNs) and young stellar
objects (YSOs), and also discuss its role on jet heating and particle
acceleration.Comment: To be published in the IAU Highlights of Astronomy, Volume 15, XXVII
IAU General Assembly, August 2009, Ian F. Corbett et al., eds., 201
Evidence for ammonium-bearing minerals in Ceres
Evidence for ammonium-bearing minerals was found on the surface of the largest asteroid Ceres. The presence of ammonium-bearing clays suggests that Ceres has experienced a period of alteration by substantial amounts of an ammonium-bearing fluid. The presence of the ammonium-bearing clays does not preclude Ceres maintaining a volatile inventory in the core or in a volatile-rich zone at some distance below the surface. Telescopic observations of Ceres, using the 3.0 meter NASA Infrared telescope facility prompted this reevaluation of its surface mineralogy
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