737 research outputs found
Fatigue failure of materials under broad band random vibrations
The fatigue life of material under multifactor influence of broad band random excitations has been investigated. Parameters which affect the fatigue life are postulated to be peak stress, variance of stress and the natural frequency of the system. Experimental data were processed by the hybrid computer. Based on the experimental results and regression analysis a best predicting model has been found. All values of the experimental fatigue lives are within the 95% confidence intervals of the predicting equation
Number operator-annihilation operator uncertainty as an alternative of the number-phase uncertainty relation
We consider a number operator-annihilation operator uncertainty as a well
behaved alternative to the number-phase uncertainty relation, and examine its
properties. We find a formulation in which the bound on the product of
uncertainties depends on the expectation value of the particle number. Thus,
while the bound is not a constant, it is a quantity that can easily be
controlled in many systems. The uncertainty relation is approximately saturated
by number-phase intelligent states. This allows us to define amplitude
squeezing, connecting coherent states to Fock states, without a reference to a
phase operator. We propose several setups for an experimental verification.Comment: 8 pages including 3 figures, revtex4; v2: typos corrected,
presentation improved; v3: presentation considerably extended; v4: published
versio
Completely Positive Quantum Dissipation
A completely positive master equation describing quantum dissipation for a
Brownian particle is derived starting from microphysical collisions, exploiting
a recently introduced approach to subdynamics of a macrosystem. The obtained
equation can be cast into Lindblad form with a single generator for each
Cartesian direction. Temperature dependent friction and diffusion coefficients
for both position and momentum are expressed in terms of the collision
cross-section.Comment: 8 pages, revtex, no figure
A classical appraisal of quantum definitions of non-Markovian dynamics
We consider the issue of non-Markovianity of a quantum dynamics starting from
a comparison with the classical definition of Markovian process. We point to
the fact that two sufficient but not necessary signatures of non-Markovianity
of a classical process find their natural quantum counterpart in recently
introduced measures of quantum non-Markovianity. This behavior is analyzed in
detail for quantum dynamics which can be built taking as input a class of
classical processes.Comment: 15 pages, 6 figures; to appear in J. Phys. B, Special Issue on "Loss
of coherence and memory effects in quantum dynamics
Argon Abundances in the Solar Neighborhood: Non-LTE Analysis of Orion Association B-type Stars
Argon abundances have been derived for a sample of B main-sequence stars in
the Orion association. The abundance calculations are based on NLTE metal
line-blanketed model atmospheres calculated with the NLTE code TLUSTY and an
updated and complete argon model atom. We derive an average argon abundance for
this young population of A(Ar) = 6.66 +- 0.06. While our result is in excellent
agreement with a recent analysis of the Orion nebula, it is significantly
higher than the currently recommended solar value which is based on abundance
measurements in the solar corona. Moreover, the derived argon abundances in the
Orion B stars agree very well with a measurement from a solar impulsive flare
during which unmodified solar photospheric material was brought to flare
conditions. We therefore argue that the argon abundances obtained independently
for both the Orion B stars and the Orion nebula are representative of the disk
abundance value in the solar neighborhood. The lower coronal abundance may
reflect a depletion related to the FIP effect. We propose a new reference value
for the abundance of argon in the solar neighborhood, A(Ar) = 6.63 +- 0.10,
corresponding to Ar/O = 0.009.Comment: to appear in Astrophysical Journal, 24 pages, 3 figures; minor
corrections after referee's repor
Test Particle in a Quantum Gas
A master equation with a Lindblad structure is derived, which describes the
interaction of a test particle with a macroscopic system and is expressed in
terms of the operator valued dynamic structure factor of the system. In the
case of a free Fermi or Bose gas the result is evaluated in the Brownian limit,
thus obtaining a single generator master equation for the description of
quantum Brownian motion in which the correction due to quantum statistics is
explicitly calculated. The friction coefficients for Boltzmann and Bose or
Fermi statistics are compared.Comment: 9 pages, revtex, no figure
Pumping up the [N I] nebular lines
The optical [N I] doublet near 5200 {\AA} is anomalously strong in a variety
of emission-line objects. We compute a detailed photoionization model and use
it to show that pumping by far-ultraviolet (FUV) stellar radiation previously
posited as a general explanation applies to the Orion Nebula (M42) and its
companion M43; but, it is unlikely to explain planetary nebulae and supernova
remnants. Our models establish that the observed nearly constant equivalent
width of [N I] with respect to the dust-scattered stellar continuum depends
primarily on three factors: the FUV to visual-band flux ratio of the stellar
population; the optical properties of the dust; and the line broadening where
the pumping occurs. In contrast, the intensity ratio [N I]/H{\beta} depends
primarily on the FUV to extreme-ultraviolet ratio, which varies strongly with
the spectral type of the exciting star. This is consistent with the observed
difference of a factor of five between M42 and M43, which are excited by an O7
and B0.5 star respectively. We derive a non-thermal broadening of order 5 km/s
for the [N I] pumping zone and show that the broadening mechanism must be
different from the large-scale turbulent motions that have been suggested to
explain the line-widths in this H II region. A mechanism is required that
operates at scales of a few astronomical units, which may be driven by thermal
instabilities of neutral gas in the range 1000 to 3000 K. In an appendix, we
describe how collisional and radiative processes are treated in the detailed
model N I atom now included in the Cloudy plasma code.Comment: ApJ in press. 8 pages of main paper plus 11 pages of appendices, with
13 figures and 12 table
The Ultraviolet Spectrum and Physical Properties of the Mass Donor Star in HD 226868 = Cygnus X-1
We present an examination of high resolution, ultraviolet spectroscopy from
Hubble Space Telescope of the photospheric spectrum of the O-supergiant in the
massive X-ray binary HD 226868 = Cyg X-1. We analyzed this and ground-based
optical spectra to determine the effective temperature and gravity of the O9.7
Iab supergiant. Using non-local thermodynamic equilibrium (non-LTE), line
blanketed, plane parallel models from the TLUSTY grid, we obtain T_eff = 28.0
+/- 2.5kK and log g > 3.00 +/- 0.25, both lower than in previous studies. The
optical spectrum is best fit with models that have enriched He and N
abundances. We fit the model spectral energy distribution for this temperature
and gravity to the UV, optical, and IR fluxes to determine the angular size of
and extinction towards the binary. The angular size then yields relations for
the stellar radius and luminosity as a function of distance. By assuming that
the supergiant rotates synchronously with the orbit, we can use the radius -
distance relation to find mass estimates for both the supergiant and black hole
as a function of the distance and the ratio of stellar to Roche radius. Fits of
the orbital light curve yield an additional constraint that limits the
solutions in the mass plane. Our results indicate masses of 23^{+8}_{-6} M_sun
for the supergiant and 11^{+5}_{-3} M_sun for the black hole.Comment: ApJ in pres
Quantum theory: the role of microsystems and macrosystems
We stress the notion of statistical experiment, which is mandatory for
quantum mechanics, and recall Ludwig's foundation of quantum mechanics, which
provides the most general framework to deal with statistical experiments giving
evidence for particles. In this approach particles appear as interaction
carriers between preparation and registration apparatuses. We further briefly
point out the more modern and versatile formalism of quantum theory, stressing
the relevance of probabilistic concepts in its formulation. At last we discuss
the role of macrosystems, focusing on quantum field theory for their
description and introducing for them objective state parameters.Comment: 12 pages. For special issue of J.Phys.A, "The Quantum Universe", on
the occasion of 70th birthday of Professor Giancarlo Ghirard
Dust and Stellar Populations in the Large Magellanic Cloud
We present an analysis of line-of-sight extinction measurements obtained
using data from the Magellanic Clouds Photometric Survey, which provides
4-filter photometry for millions of stars in the Large Magellanic Cloud. We
find that visual extinctions are typically larger by several tenths of a
magnitude for stars with effective temperatures > 12000 K, than for stars with
effective temperatures between 5500 K and 6500 K. Several repercussions of this
population-dependent extinction are discussed. In particular, LMC distance
measurements that utilize old stellar populations (such as red clump stars),
but use extinctions derived from OB stars, may be biased low.
Population-dependent extinction affects the interpretation of color-magnitude
diagrams and results in an effective absorption law that is steeper than that
intrinsic to the dust for unresolved stellar systems. We further explore the
relation between the stellar populations and dust by comparing our extinction
map to the 100mu image of the region and identifying potential heating sources
of the dust. We conclude that 100mu flux should be used with caution as a star
formation tracer, particularly for studies of star formation within galaxies.
Finally, we reproduce the observed extinction variation between the hot and
cold stellar populations with a simple model of the distribution of the stars
and dust where the scaleheight of the cooler stars is >> than that of the dust
(which is twice that of the OB stars). (Abridged Abstract)Comment: Accepted for publication in AJ (scheduled for Dec. 1999). 31 pgs
(including Figures
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