14,327 research outputs found
Extragalactic H 2 regions in the UV: Implications for primeval galaxies and quasars
Three extragalactic regions of rapid star formation with red shifts great enough to separate the L alpha region from geocoronal L alpha were observed with the IUE satellite. Only the low metal abundance object had detectable L alpha emission. L alpha is therefore expected to be weak or absent in collapsed primeval galaxies. The detected object has a L alpha H beta identical to that of quasars
Misconceptions About General Relativity in Theoretical Black Hole Astrophysics
The fundamental role played by black holes in our study of microquasars,
gamma ray bursts, and the outflows from active galactic nuclei requires an
appreciation for, and at times some in-depth analysis of, curved spacetime. We
highlight misconceptions surrounding the notion of coordinate transformation in
general relativity as applied to metrics for rotating black holes that are
beginning to increasingly appear in the literature. We emphasize that there is
no coordinate transformation that can turn the metric of a rotating spacetime
into that for a Schwarzschild spacetime, or more generally, that no coordinate
transformation exists that can diagonalize the metric for a rotating spacetime.
We caution against the notion of "local" coordinate transformation, which is
often incorrectly associated with a global analysis of the spacetime.Comment: MNRAS accepte
Structure and thermodynamics of platelet dispersions
Various properties of fluids consisting of platelike particles differ from
the corresponding ones of fluids consisting of spherical particles because
interactions between platelets depend on their mutual orientations. One of the
main issues in this topic is to understand how structural properties of such
fluids depend on factors such as the shape of the platelets, the size
polydispersity, the orientational order, and the platelet number density. A
statistical mechanics approach to the problem is natural and in the last few
years there has been a lot of work on the study of properties of platelet
fluids. In this contribution some recent theoretical developments in the field
are discussed and experimental investigations are described.Comment: 23 pages, 18 figure
Silicon dendritic web material
The development of a low cost and reliable contact system for solar cells and the fabrication of several solar cell modules using ultrasonic bonding for the interconnection of cells and ethylene vinyl acetate as the potting material for module encapsulation are examined. The cells in the modules were made from dendritic web silicon. To reduce cost, the electroplated layer of silver was replaced with an electroplated layer of copper. The modules that were fabricated used the evaporated Ti, Pd, Ag and electroplated Cu (TiPdAg/Cu) system. Adherence of Ni to Si is improved if a nickel silicide can be formed by heat treatment. The effectiveness of Ni as a diffusion barrier to Cu and the ease with which nickel silicide is formed is discussed. The fabrication of three modules using dendritic web silicon and employing ultrasonic bonding for interconnecting calls and ethylene vinyl acetate as the potting material is examined
Response of single junction GaAs/GaAs and GaAs/Ge solar cells to multiple doses of 1 MeV electrons
A comparison of the radiation tolerance of MOCVD-grown GaAs cells and GaAs/Ge cells was undertaken using 1 MeV electrons. The GaAs/Ge cells are somewhat more tolerant of 1 MeV electron irradiation and more responsive to annealing than are the GaAs/GaAs cells examined in this study. However, both types of cells suffer a greater degradation in efficiency than has been observed in other recent studies. The reason for this is not certain, but it may be associated with an emitter thickness which appears to be greater than desired. The deep level transient spectroscopy (DLTS) spectra following irradiation are not significantly different for the GaAs/Ge and the GaAs/GaAs cells, with each having just two peaks. The annealing behavior of these peaks is also similar in the two samples examined. It appears that no penalty in radiation tolerance, and perhaps some benefit, is associated with fabricating MOCVD GaAs cells on Ge substrates rather than GaAs substrates
A Magnetically-Switched, Rotating Black Hole Model For the Production of Extragalactic Radio Jets and the Fanaroff and Riley Class Division
A model is presented in which both Fanaroff and Riley class I and II
extragalactic jets are produced by magnetized accretion disk coronae in the
ergospheres of rotating black holes. While the jets are produced in the
accretion disk itself, the output power still is an increasing function of the
black hole angular momentum. For high enough spin, the black hole triggers the
magnetic switch, producing highly-relativistic, kinetic-energy-dominated jets
instead of Poynting-flux-dominated ones for lower spin. The coronal mass
densities needed to trigger the switch at the observed FR break power are quite
small (), implying that the source of the jet material
may be either a pair plasma or very tenuous electron-proton corona, not the
main accretion disk itself.
The model explains the differences in morphology and Mach number between FR I
and II sources and the observed trend for massive galaxies to undergo the FR
I/II transition at higher radio power. It also is consistent with the energy
content of extended radio lobes and explains why, because of black hole
spindown, the space density of FR II sources should evolve more rapidly than
that of FR I sources.
If the present model is correct, then the ensemble average speed of
parsec-scale jets in sources distinguished by their FR I morphology (not
luminosity) should be distinctly slower than that for sources with FR II
morphology. The model also suggests the existence of a population of
high-redshift, sub-mJy FR I and II radio sources associated with spiral or
pre-spiral galaxies that flared once when their black holes were formed but
were never again re-kindled by mergers.Comment: 14 pages, 2 figures, final version to appear in Sept Ap
Multi-Dimensional Astrophysical Structural and Dynamical Analysis I. Development of a Nonlinear Finite Element Approach
A new field of numerical astrophysics is introduced which addresses the
solution of large, multidimensional structural or slowly-evolving problems
(rotating stars, interacting binaries, thick advective accretion disks, four
dimensional spacetimes, etc.). The technique employed is the Finite Element
Method (FEM), commonly used to solve engineering structural problems. The
approach developed herein has the following key features:
1. The computational mesh can extend into the time dimension, as well as
space, perhaps only a few cells, or throughout spacetime.
2. Virtually all equations describing the astrophysics of continuous media,
including the field equations, can be written in a compact form similar to that
routinely solved by most engineering finite element codes.
3. The transformations that occur naturally in the four-dimensional FEM
possess both coordinate and boost features, such that
(a) although the computational mesh may have a complex, non-analytic,
curvilinear structure, the physical equations still can be written in a simple
coordinate system independent of the mesh geometry.
(b) if the mesh has a complex flow velocity with respect to coordinate space,
the transformations will form the proper arbitrary Lagrangian- Eulerian
advective derivatives automatically.
4. The complex difference equations on the arbitrary curvilinear grid are
generated automatically from encoded differential equations.
This first paper concentrates on developing a robust and widely-applicable
set of techniques using the nonlinear FEM and presents some examples.Comment: 28 pages, 9 figures; added integral boundary conditions, allowing
very rapidly-rotating stars; accepted for publication in Ap.
Development of high efficiency solar cells on silicon web
Web base material is being improved with a goal toward obtaining solar cell efficiencies in excess of 18% (AM1). Carrier loss mechanisms in web silicon was investigated, techniques were developed to reduce carrier recombination in the web, and web cells were fabricated using effective surface passivation. The effect of stress on web cell performance was also investigated
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