190 research outputs found
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
Space power distribution system technology. Volume 2: Autonomous power management
Electrical power subsystem requirements, power management system functional requirements, algorithms, power management subsystem, hardware development, and trade studies and analyses are discussed
Space power distribution system technology. Volume 1: Reference EPS design
The multihundred kilowatt electrical power aspects of a mannable space platform in low Earth orbit is analyzed from a cost and technology viewpoint. At the projected orbital altitudes, Shuttle launch and servicing are technically and economically viable. Power generation is specified as photovoltaic consistent with projected planning. The cost models and trades are based upon a zero interest rate (the government taxes concurrently as required), constant dollars (1980), and costs derived in the first half of 1980. Space platform utilization of up to 30 years is evaluated to fully understand the impact of resupply and replacement as satellite missions are extended. Such lifetimes are potentially realizable with Shuttle servicing capability and are economically desirable
A dilemma in representing observables in quantum mechanics
There are self-adjoint operators which determine both spectral and
semispectral measures. These measures have very different commutativity and
covariance properties. This fact poses a serious question on the physical
meaning of such a self-adjoint operator and its associated operator measures.Comment: 10 page
Stagnation and Infall of Dense Clumps in the Stellar Wind of tau Scorpii
Observations of the B0.2V star tau Scorpii have revealed unusual stellar wind
characteristics: red-shifted absorption in the far-ultraviolet O VI resonance
doublet up to +250 km/s, and extremely hard X-ray emission implying gas at
temperatures in excess of 10^7 K. We describe a phenomenological model to
explain these properties. We assume the wind of tau Sco consists of two
components: ambient gas in which denser clumps are embedded. The clumps are
optically thick in the UV resonance lines primarily responsible for
accelerating the ambient wind. The reduced acceleration causes the clumps to
slow and even infall, all the while being confined by the ram pressure of the
outflowing ambient wind. We calculate detailed trajectories of the clumps in
the ambient stellar wind, accounting for a line radiation driving force and the
momentum deposited by the ambient wind in the form of drag. We show these
clumps will fall back towards the star with velocities of several hundred
km/sec for a broad range of initial conditions. The infalling clumps produce
X-ray emitting plasmas with temperatures in excess of (1-6)x10^7 K in bow
shocks at their leading edge. The infalling material explains the peculiar
red-shifted absorption wings seen in the O VI doublet. The required mass loss
in clumps is 3% - 30% ofthe total mass loss rate. The model developed here can
be generally applied to line-driven outflows with clumps or density
irregularities. (Abstract Abridged)Comment: To appear in the ApJ (1 May 2000). 24 pages, including 6 embedded
figure
Neon Abundances from a Spitzer/IRS Survey of Wolf-Rayet Stars
We report on neon abundances derived from {\it Spitzer} high resolution
spectral data of eight Wolf-Rayet (WR) stars using the forbidden line of
[\ion{Ne}{3}] 15.56 microns. Our targets include four WN stars of subtypes
4--7, and four WC stars of subtypes 4--7. We derive ion fraction abundances
of Ne^{2+} for the winds of each star. The ion fraction abundance is a
product of the ionization fraction in stage i and the abundance by
number of element E relative to all nuclei. Values generally
consistent with solar are obtained for the WN stars, and values in excess of
solar are obtained for the WC stars.Comment: to appear in Astrophysical Journa
Galilei invariant theories. I. Constructions of indecomposable finite-dimensional representations of the homogeneous Galilei group: directly and via contractions
All indecomposable finite-dimensional representations of the homogeneous
Galilei group which when restricted to the rotation subgroup are decomposed to
spin 0, 1/2 and 1 representations are constructed and classified. These
representations are also obtained via contractions of the corresponding
representations of the Lorentz group. Finally the obtained representations are
used to derive a general Pauli anomalous interaction term and Darwin and
spin-orbit couplings of a Galilean particle interacting with an external
electric field.Comment: 23 pages, 2 table
An XMM-Newton view of the young open cluster NGC 6231 -- II. The OB star population
In this second paper, we pursue the analysis of the 180 ks XMM-Newton
campaign towards the young open cluster NGC 6231 and we focus on its rich OB
star population. We present a literature-based census of the OB stars in the
field of view with more than one hundred objects, among which 30% can be
associated with an X-ray source. All the O-type stars are detected in the X-ray
domain as soft and reasonably strong emitters. In the 0.5-10.0 keV band, their
X-ray luminosities scale with their bolometric luminosities as . Such a scaling law holds in
the soft (0.5-1.0 keV) and intermediate (1.0-2.5 keV) bands but breaks down in
the hard band. While the two colliding wind binaries in our sample clearly
deviate from this scheme, the remaining O-type objects show a very limited
dispersion (40% or 20% according to whether `cool' dwarfs are included or not),
much smaller than that obtained from previous studies. At our detection
threshold and within our sample, the sole identified mechanism that produces
significant modulations in the O star X-ray emission is related to wind
interaction. The intrinsic X-ray emission of non-peculiar O-type stars seems
thus constant for a given star and the level of its X-ray emission is
accurately related to the its luminosity or, equivalently, to its wind
properties. Among B-type stars, the detection rate is only about 25% in the
sub-type range B0-B4 and remains mostly uniform throughout the different
sub-populations while it drops significantly at later sub-types. The associated
X-ray spectra are harder than those of O-type stars. Our analysis points
towards the detected emission being associated with a physical PMS companion
>... [see paper for the complete abstract]Comment: 21 pages, 14 figures, Table 2 and Figs 2 to 5 will be available
through the CDS only, accepted for publication by MNRAS, Fig 1 not included
in the present preprint because of size limitation
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