387 research outputs found
The winds and coronae of early-type stars
The properties of the winds of hot stars as derived from radio, ultraviolet and X-ray observations is presented. Special focus is given to observations that test line driven wind theory. In this theory the wind properties are determined by the stellar effective temperature and surface gravity, but not parameters that specify the mechanical or wave fluxes from the star. Alternate explanations of the X-ray emission from the early-type stars are discussed. Evidence is given for the presence of coronal zones at the base of the stellar winds
Quantum symmetries and the Weyl-Wigner product of group representations
In the usual formulation of quantum mechanics, groups of automorphisms of
quantum states have ray representations by unitary and antiunitary operators on
complex Hilbert space, in accordance with Wigner's Theorem. In the phase-space
formulation, they have real, true unitary representations in the space of
square-integrable functions on phase-space. Each such phase-space
representation is a Weyl-Wigner product of the corresponding Hilbert space
representation with its contragredient, and these can be recovered by
`factorising' the Weyl-Wigner product. However, not every real, unitary
representation on phase-space corresponds to a group of automorphisms, so not
every such representation is in the form of a Weyl-Wigner product and can be
factorised. The conditions under which this is possible are examined. Examples
are presented.Comment: Latex2e file, 37 page
Grain processes in massive star formation
Observational evidence suggests that stars greater than 100 M(solar) exist in the Galaxy and Large Magellanic Cloud (LMC), however classical star formation theory predicts stellar mass limits of only approx. 60 M(solar). A protostellar accretion flow consists of inflowing gas and dust. Grains are destroyed as they are near the central protostar creating a dust shell or cocoon. Radiation pressure acting on the grain can halt the inflow of material thereby limiting the amount of mass accumulated by the protostar. We first consider rather general constraints on the initial grain to gas ratio and mass accretion rates that permit inflow. We further constrain these results by constructing a numerical model. Radiative deceleration of grains and grain destruction processes are explicitly accounted for in an iterative solution of the radiation-hydrodynamic equations. Findings seem to suggest that star formation by spherical accretion requires rather extreme preconditioning of the grain and gas environment
Anomalous ionization seen in the spectra of B supergiants
An IUE survey of B supergiants has been conducted to study the persistence with spectral type of the ultraviolet resonance lines of N V, C IV and Si IV. N V is seen as late as B2.5Ia, C IV until B6Ia and Si IV throughout the range from B1.5 to B9. This is in fairly good agreement with the Auger ionization model of Cassinelli and Olson (1979). The terminal velocities are derived for the 20 stars in the sample and it is found that the ratio v(T)/v(esc) decreases monotonically with spectral type from the value of 3.0 that it has in the O spectral range to the value 1.0 at B9Ia
Dust in regions of massive star formation
It is suggested that protostars increase mass by accreting the surrounding gas and dust. Grains are destroyed as they near the central protostar creating a dust shell or cocoon. Radiation pressure acting on the grains can halt the inflow of material thereby limiting the amount of mass accumulated by the protostar. General constraints were considered on the initial dust-to-gas ratio and mass accretion rates that permit inflow. These results were constrained further by constructing a numerical model, including radiative deceleration on grains and grain destruction processes. Also the constraints on dust properties were investigated which allow the formation of massive stars. The obtained results seem to suggest that massive star formation requires rather extreme preconditioning of the grain and gas environment
Remote preparation of arbitrary ensembles and quantum bit commitment
The Hughston-Jozsa-Wootters theorem shows that any finite ensemble of quantum
states can be prepared "at a distance", and it has been used to demonstrate the
insecurity of all bit commitment protocols based on finite quantum systems
without superselection rules. In this paper, we prove a generalized HJW theorem
for arbitrary ensembles of states on a C*-algebra. We then use this result to
demonstrate the insecurity of bit commitment protocols based on infinite
quantum systems, and quantum systems with Abelian superselection rules.Comment: 21 pages, LaTeX. Version 2: Proofs expanded and made more
self-contained; added an example of a bit commitment protocol with continuous
ensemble
Analytical modeling of spacecraft power systems
Power system components are reviewed. Battery and solar array models are discussed. Shunt regulators, dc-dc converters, and cabling are also discussed
Study of solar array switching power management technology for space power system
This report documents work performed on the Solar Array Switching Power Management Study. Mission characteristics for three missions were defined to the depth necessary to determine their power management requirements. Solar array switching concepts which could satisfy the mission requirements were identified. The switching concepts were compared with a conventional buck regulator system for cost, weight and volume, reliability, efficiency and thermal control. Solar array switching provided significant advantages in all areas of comparison for the reviewed missions
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