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The abundance of 21-cm absorption lines seen in surveys at high latitudes can be translated into a line of sight abundance of clouds vs. column density using an empirical relationship between temperature and optical depth. As VLA surveys of 21-cm absorption at low latitudes are now becoming available, it is possible to study the variation of this function with galactic radius. It is interesting to compare the abundance of these diffuse atomic clouds (with temperatures of 50 to 100 K and masses of 1 to 10 M⊙) to the abundance of molecular clouds. To do the latter we must make assumptions about cloud cross-sections in order to convert the line of sight abundance of diffuse clouds into a number per unit volume, and to convert from cloud column density to mass. The spectrum of diffuse clouds matches fairly well the spectrum of molecular clouds, although observationally there is a gap of several orders of magnitude in cloud mass. Optical absorption studies also agree well with the 21-cm results for clouds of column density a few times 1020 M⊙.
In recent years, a revolution in astronomical position measurements has been taking place with the advent of modern space techniques. These new techniques, which supplement the traditional astrometric measurements, include laser ranging to the moon and artificial satellites, very-long-baseline interferometry (VLBI) of galactic and extra-galactic radio sources and spacecraft, radio tracking of satellites, and radar-ranging and spacecraft tracking during planetary encounters. Impressive accuracies have been achieved and further improvements are forthcoming. Each technique can be expected to establish its own reference frame which is derived from observations of a particular class of objects. The celestial and terrestrial coordinate systems are related through adopted constants and definitions. Contemporary astronomy has led to the development of three principal celestial coordinate systems: the optical frame (FK4/FK5) based on positions of galactic stars; the planetary/lunar ephemeris frame based on the major celestial bodies of the solar system; and the radio frame constructed from observations of extragalactic radio sources (quasars). Each frame is rotated with respect to others; furthermore, the optical frame offset is time variable. It is important that all frames be interconnected and unified. The optical frame is being connected to the radio frame by VLBI observations of radio emitting stars. The radio frame is being tied to the ephemeris frame in several ways – one is via differential VLBI measurements between quasars and planet-orbiting spacecraft.
A survey of the Milky Way disk and the Magellanic System at the wavelengths of the 21-cm atomic hydrogen (H i) line and three 18-cm lines of the OH molecule will be carried out with the Australian Square Kilometre Array Pathfinder telescope. The survey will study the distribution of H i emission and absorption with unprecedented angular and velocity resolution, as well as molecular line thermal emission, absorption, and maser lines. The area to be covered includes the Galactic plane (|b| < 10°) at all declinations south of δ = +40°, spanning longitudes 167° through 360°to 79° at b = 0°, plus the entire area of the Magellanic Stream and Clouds, a total of 13 020 deg2. The brightness temperature sensitivity will be very good, typically σT≃ 1 K at resolution 30 arcsec and 1 km s−1. The survey has a wide spectrum of scientific goals, from studies of galaxy evolution to star formation, with particular contributions to understanding stellar wind kinematics, the thermal phases of the interstellar medium, the interaction between gas in the disk and halo, and the dynamical and thermal states of gas at various positions along the Magellanic Stream.
The purpose of this study was to examine global epidemiological trends in human norovirus (NoV) outbreaks by transmission route and setting, and describe relationships between these characteristics, viral attack rates, and the occurrence of genogroup I (GI) or genogroup II (GII) strains in outbreaks. We analysed data from 902 reverse transcriptase–polymerase chain reaction-confirmed, human NoV outbreaks abstracted from a systematic review of articles published from 1993 to 2011 and indexed under the terms ‘norovirus’ and ‘outbreak’. Multivariate regression analyses demonstrated that foodservice and winter outbreaks were significantly associated with higher attack rates. Foodborne and waterborne outbreaks were associated with multiple strains (GI+GII). Waterborne outbreaks were significantly associated with GI strains, while healthcare-related and winter outbreaks were associated with GII strains. These results identify important trends for epidemic NoV detection, prevention, and control.
The existence of a classical solution to the initial boundary value problem for a semi-infinite extensible string is proved. The result is obtained by using a Galerkin procedure on a semi-infinite interval.
Large time behaviour of solutions to a damped quasi-linear wave equation are studied. Conditions are obtained which guarantee the global existence of a classical solution. The asymptotic behaviour of this solution is studied in the case of a unique equilibrium solution and in the case of multiple equilibria. The results are applied to various special examples.
Silicon-germanium (Si1−xGex:H) thin films have been prepared by plasma enhanced chemical vapor deposition of SiH4 and GeH4 and measured during growth using real time spectroscopic ellipsometry. A two-layer virtual interface analysis has been applied to study the structural evolution of Si:H films prepared in multistep processes utilizing alternating intermediate and low H2-dilution material layers, which have been designed to produce predominately amorphous films with a controlled distribution of microcrystalline particles. The compositional evolution of alloy-graded a-Si1−xGex:H has been studied as well using similar methods. In each case, depth profiles of microcrystalline content, fμc, or Ge content, x, have been extracted. Additionally, real time spectroscopic ellipsometry has been used to monitor post-deposition exposure of a-Si:H, a-Si1−xGex:H, and a-Ge:H films to a hydrogen plasma in situ in order to determine sub-surface amorphous film modification similar to that which would occur when a highly H2-diluted layer is deposited on a layer prepared with lower dilution. These analyses provide guidance for enhanced performance of Si:H based solar cells, through controlled bandgap grading using compositionally graded amorphous binary alloys (a-Si1−xGex:H) or the incorporation of controlled fractions of microcrystallites into bulk amorphous i-layer materials, and by providing a fundamental understanding of the modification of component layers during the deposition of subsequent layers in multilayer stacks.
The Global Magneto-Ionic Medium Survey (GMIMS) is a project to map the diffuse polarized emission over the entire sky, Northern and Southern hemispheres, from 300 MHz to 1.8 GHz. With an angular resolution of 30–60 arcmin and a frequency resolution of 1 MHz or better, GMIMS will provide the first spectro-polarimetric data set of the large-scale polarized emission over the entire sky, observed with single-dish telescopes. GMIMS will provide an invaluable resource for studies of the magneto-ionic medium of the Galaxy in the local disk, halo, and its transition.
We have subjected single-walled carbon nanotube materials (SWNTM's) to a variety of organic functionalization reactions. These reactions include radioactive photolabeling studies using diradical and nitrene sources, and treatment with dichlorocarbene and Birch reduction conditions. All of the reactions provide evidence for chemical attachment to the SWNTM's, but because of the impure nature of the staring materials, we are unable to ascertain the site of reaction. In the case of dichlorocarbene we are able to show the presence of chlorine in the SWNT bundles, but as a result of the large amount of amorphous carbon that is attached to the tube walls, we cannot distinguish between attachment of dichlorocarbene to the walls of the SWNT's and reaction with the amorphous carbon.
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