Characterization of solar cells for space applications. Volume 12: Electrical characteristics of Solarex BSF, 2-ohm-cm, 50-micron solar cells (1978 pilot line) as a function of intensity, temperature, and irradiation

Electrical characteristics of Solarex back-surface-field, 2-ohm-cm, 50-micron N/P silicon solar cells are presented in graphical and tabular format as a function of solar illumination intensity, temperature, and irradiation

Characterization of solar cells for space applications. Volume 8: Electrical characteristics of Spectrolab BSF, BSR, textured 290-micron solar cells (K7) as a function of intensity, temperature and irradiation

A set of parametric data is presented on the Spectrolab textured, back-surface-field, back-surface-reflector solar cell which is a commercially available product

Characterization of solar cells for space applications. Volume 13: Electrical characteristics of Hughes LPE gallium arsenide solar cells as a function of intensity and temperature

Electrical characteristics of Hughes Liquid phase epitaxy, P/N gallium aluminum arsenide solar cells are presented in graphical and tabular format as a function of solar illumination intensity and temperature

Characterization of solar cells for space applications. Volume 11: Electrical characteristics of 2 ohm-cm, 228 micron wraparound solar cells as a function of intensity, temperature, and irradiation

Parametric characterization data on Spectrolab 2 by 4 cm, 2 ohm/cm, 228 micron thick wraparound cell, a candidate for the Solar Electric Propulsion Mission, are presented. These data consist of the electrical characteristics of the solar cell under a wide range of temperature and illumination intensity combinations of the type encountered in space applications

From the Circumnuclear Disk in the Galactic Center to thick, obscuring tori of AGNs

We compare three different models of clumpy gas disk and show that the Circumnuclear Disk (CND) in the Galactic Center and a putative, geometrically thick, obscuring torus are best explained by a collisional model consisting of quasi-stable, self-gravitating clouds. Kinetic energy of clouds is gained by mass inflow and dissipated in cloud collisions. The collisions give rise to a viscosity in a spatially averaged gas dynamical picture, which connects them to angular momentum transport and mass inflow. It is found that CND and torus share the same gas physics in our description, where the mass of clouds is 20 - 50 M_sun and their density is close to the limit of disruption by tidal shear. We show that the difference between a transparent CND and an obscuring torus is the gas mass and the velocity dispersion of the clouds. A change in gas supply and the dissipation of kinetic energy can turn a torus into a CND-like structure and vice versa. Any massive torus will naturally lead to sufficiently high mass accretion rates to feed a luminous AGN. For a geometrically thick torus to obscure the view to the center even super-Eddington accretions rates with respect to the central black hole are required.Comment: 9 pages, no figures. Accepted for publication in A&

Stochastic Cooling at the ESR

Stochastic precooling at the ESR storage ring of GSI will be used mainly for experiments with stored radioactive fragment beams. They arrive from the fragment separator with momentum spreads and emittances for which electron cooling is too slow. The installation of components at the ESR is now complete and first commissioning experiments have been performed. Both longitudinal and transverse stochastic cooling have been demonstrated. The paper gives a short account of the system architecture, and of the response of quarter-wave plates and superelectrodes at intermediate energies. The preparation of fragment beams suitable for subsequent electron cooling is discussed for the case that a mixture of different ion species is present in the cooler ring. Results of commissioning and future prospects are presented

High Proper Motion Stars in the Vicinity of Sgr A*: Evidence for a Supermassive Black Hole at the Center of Our Galaxy

Over a two year period (1995-1997), we have conducted a diffraction-limited imaging study at 2.2 microns of the inner 6"x6" of the Galaxy's central stellar cluster using the Keck 10-m telescope. The K band images obtained reveal a large population of faint stars. We use an unbiased approach for identifying and selecting stars to be included in this proper motion study, which results in a sample of 90 stars with brightness ranging from K=9-17 and velocities as large as 1,400+-100 km/sec. Compared to earlier work (Eckart et al. 1997; Genzel et al. 1997), the source confusion is reduced by a factor of 9, the number of stars with proper motion measurement in the central 25 arcsec^2 of our galaxy is doubled, and the accuracy of the velocity measurements in the central 1 arcsec^2 is improved by a factor of 4. The peaks of both the stellar surface density and the velocity dispersion are consistent with the position of the unusual radio source and blackhole candidate, Sgr A*, suggesting that Sgr A* is coincident (+-0."1) with the dynamical center of the Galaxy. As a function of distance from Sgr A*, the velocity dispersion displays a falloff well fit by Keplerian motion about a central dark mass of 2.6(+-0.2)x10^6 Mo confined to a volume of at most 10^-6 pc^3, consistent with earlier results. Although uncertainties in the measurements mathematically allow for the matter to be distributed over this volume as a cluster, no realistic cluster is physically tenable. Thus, independent of the presence of Sgr A*, the large inferred central density of at least 10^12 Mo/pc^3, which exceeds the volume-averaged mass densities found at the center of any other galaxy, leads us to the conclusion that our Galaxy harbors a massive central black hole.Comment: 19 pages, 8 figures, accepted for publications in the Astrophysical Journa

Turbulent viscosity in clumpy accretion disks II supernova driven turbulence in the Galaxy

An analytical model for a turbulent clumpy gas disk is presented where turbulence is maintained by the energy input due to supernovae. Expressions for the disk parameters, global filling factors, molecular fractions, and star formation rates are given as functions of the Toomre parameter $Q$, the ratio between the cloud size and the turbulent driving length scale $\delta$, the mass accretion rate within the disk $\dot{M}$, the constant of molecule formation $\alpha$, the disk radius, the angular velocity, and its radial derivative. Two different cases are investigated: a dominating stellar disk and a self-gravitating gas disk in $z$ direction. The turbulent driving wavelength is determined in a first approach by energy flux conservation, i.e. the supernovae energy input is transported by turbulence to smaller scales where it is dissipated. The results are compared to those of a fully gravitational model. For Q=1 and $\delta=1$ both models are consistent with each other. In a second approach the driving length scale is directly determined by the size of supernovae remnants. Both models are applied to the Galaxy and can reproduce its integrated and local gas properties. The influence of thermal and magnetic pressure on the disk structure is investigated. We infer $Q \sim 1$ and $\dot{M} \sim 0.05 - 0.1 M_{\odot} yr ^{-1}$ for the Galaxy.Comment: 15 pages with 10 figures. Accepted for publication in A&

A dynamic logic for every season

This paper introduces a method to build dynamic logics with a graded semantics. The construction is parametrized by a structure to support both the spaces of truth and of the domain of computations. Possible instantiations of the method range from classical assertional) dynamic logic to less common graded logics suitable to deal with programs whose transitional semantics exhibits fuzzy or weighted behaviour.This leads to the systematic derivation of program logics tailored to specific program classes