9,054 research outputs found
Erasable holographic medium using cis-trans isomerization
Photochemical process has been developed for recording of erasable holograms by utilizing reversible transformation of two isomers of molecule upon exposure to light. Hologram system records, reads, and erases in response to changes in refractive index of mixture of isomers
Requirements for multidisciplinary design of aerospace vehicles on high performance computers
The design of aerospace vehicles is becoming increasingly complex as the various contributing disciplines and physical components become more tightly coupled. This coupling leads to computational problems that will be tractable only if significant advances in high performance computing systems are made. Some of the modeling, algorithmic and software requirements generated by the design problem are discussed
Investigating Heating and Cooling in the BCS & B55 Cluster Samples
We study clusters in the BCS cluster sample which are observed by Chandra and
are more distant than redshift, z>0.1. We select from this subsample the
clusters which have both a short central cooling time and a central temperature
drop, and also those with a central radio source. Six of the clusters have
clear bubbles near the centre. We calculate the heating by these bubbles and
express it as the ratio r_heat/r_cool=1.34+/-0.20. This result is used to
calculate the average size of bubbles expected in all clusters with central
radio sources. In three cases the predicted bubble sizes approximately match
the observed radio lobe dimensions.
We combine this cluster sample with the B55 sample studied in earlier work to
increase the total sample size and redshift range. This extended sample
contains 71 clusters in the redshift range 0<z<0.4. The average distance out to
which the bubbles offset the X-ray cooling in the combined sample is at least
r_heat/r_cool=0.92+/-0.11. The distribution of central cooling times for the
combined sample shows no clusters with clear bubbles and t_cool>1.2Gyr. An
investigation of the evolution of cluster parameters within the redshift range
of the combined samples does not show any clear variation with redshift.Comment: 12 pages, 9 figures, accepted for publication in MNRA
The HgMn Binary Star Phi Herculis: Detection and Properties of the Secondary and Revision of the Elemental Abundances of the Primary
Observations of the Mercury-Manganese star Phi Herculis with the Navy
Prototype Optical Interferometer (NPOI) conclusively reveal the previously
unseen companion in this single-lined binary system. The NPOI data were used to
predict a spectral type of A8V for the secondary star Phi Her B. This
prediction was subsequently confirmed by spectroscopic observations obtained at
the Dominion Astrophysical Observatory. Phi Her B is rotating at 50 +/-3
km/sec, in contrast to the 8 km/sec lines of Phi Her A. Recognizing the lines
from the secondary permits one to separate them from those of the primary. The
abundance analysis of Phi Her A shows an abundance pattern similar to those of
other HgMn stars with Al being very underabundant and Sc, Cr, Mn, Zn, Ga, Sr,
Y, Zr, Ba, Ce, and Hg being very overabundant.Comment: Accepted to ApJ, 45 pages, 11 figure
Transversely Driven Charge Density Waves and Striped Phases of High-T Superconductors: The Current Effect Transistor
We show that a normal (single particle) current density {\em
transverse} to the ordering wavevector of a charge density
wave (CDW) has dramatic effects both above and {\em below} the CDW depinning
transition. It exponentially (in ) enhances CDW correlations, and
exponentially suppresses the longitudinal depinning field. The intermediate
longitudinal I-V relation also changes, acquiring a {\em linear} regime. We
propose a novel ``current effect transistor'' whose CDW channel is turned on by
a transverse current. Our results also have important implications for the
recently proposed ``striped phase'' of the high-T superconductors.Comment: change of title and minor corrections, 4 RevTeX pgs, to appear in
Phys. Rev. Lett., 81, 3711 (1998
A Photometric Metallicity Estimate of the Virgo Stellar Overdensity
We determine photometric metal abundance estimates for individual
main-sequence stars in the Virgo Overdensity (VOD), which covers almost 1000
deg^2 on the sky, based on a calibration of the metallicity sensitivity of
stellar isochrones in the gri filter passbands using field stars with
well-determined spectroscopic metal abundances. Despite the low precision of
the method for individual stars, we derive [Fe/H] = -2.0 +/-0.1 (internal)
+/-0.5 (systematic) for the metal abundance of the VOD from photometric
measurements of 0.7 million stars in the Northern Galactic hemisphere with
heliocentric distances from ~10 kpc to ~20 kpc. The metallicity of the VOD is
indistinguishable, within Delta [Fe/H] < 0.2, from that of field halo stars
covering the same distance range. This initial application suggests that the
SDSS gri passbands can be used to probe the properties of main-sequence stars
beyond ~10 kpc, complementing studies of nearby stars from more
metallicity-sensitive color indices that involve the u passband.Comment: 5 pages, 3 figures, Accepted for publication in ApJ Letter
Exact time evolution and master equations for the damped harmonic oscillator
Using the exact path integral solution for the damped harmonic oscillator it
is shown that in general there does not exist an exact dissipative Liouville
operator describing the dynamics of the oscillator for arbitrary initial bath
preparations. Exact non-stationary Liouville operators can be found only for
particular preparations. Three physically meaningful examples are examined. An
exact new master equation is derived for thermal initial conditions. Second,
the Liouville operator governing the time-evolution of equilibrium correlations
is obtained. Third, factorizing initial conditions are studied. Additionally,
one can show that there are approximate Liouville operators independent of the
initial preparation describing the long time dynamics under appropriate
conditions. The general form of these approximate master equations is derived
and the coefficients are determined for special cases of the bath spectral
density including the Ohmic, Drude and weak coupling cases. The connection with
earlier work is discussed.Comment: to be published in Phys. Rev.
The search for magnetic fields in mercury-manganese stars
We performed a highly sensitive search for magnetic fields on a large set of
HgMn stars. With the aid of a new polarimeter attached to the HARPS
spectrometer at the ESO 3.6m-telescope, we obtained high-quality circular
polarization spectra of 41 single and double HgMn stars. Using a multi-line
analysis technique on each star, we co-added information from hundreds of
spectral lines resulting in significantly greater sensitivity to the presence
of magnetic fields, including very weak fields. For the 47 individual objects
studied, including 6 components of SB2 systems, we do not detect any magnetic
fields at greater than the 3 sigma level. The lack of detection in the circular
polarization profiles indicates that if strong fields are present on these
stars, they must have complex surface topologies. For simple global fields, our
detection limits imply upper limits to the fields present of 2-10 Gauss in the
best cases. We conclude that HgMn stars lack large-scale magnetic fields,
typical for spotted magnetic Ap stars, sufficient to form and sustain the
chemical spots observed on HgMn stars. Our study confirms that in addition to
magnetically altered atomic diffusion, there exists another differentiation
mechanism operating in the atmospheres of late-B main sequence stars which can
compositional inhomogeneities on their surfaces.Comment: 12 pages, 8 figures, 2 table
Crossover from 2-dimensional to 1-dimensional collective pinning in NbSe3
We have fabricated NbSe structures with widths comparable to the
Fukuyama-Lee-Rice phase-coherence length. For samples already in the
2-dimensional pinning limit, we observe a crossover from 2-dimensional to
1-dimensional collective pinning when the crystal width is less than 1.6
m, corresponding to the phase-coherence length in this direction. Our
results show that surface pinning is negligible in our samples, and provide a
means to probe the dynamics of single domains giving access to a new regime in
charge-density wave physics.Comment: 4 pages, 2 figures, and 1 table. Accepted for publication in Physical
Review
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