9,467 research outputs found
Xenon in Mercury-Manganese Stars
Previous studies of elemental abundances in Mercury-Manganese (HgMn) stars
have occasionally reported the presence of lines of the ionized rare noble gas
Xe II, especially in a few of the hottest stars with Teff ~ 13000--15000 K. A
new study of this element has been undertaken using observations from Lick
Observatory's Hamilton Echelle Spectrograph. In this work, the spectrum
synthesis program UCLSYN has been used to undertake abundance analysis assuming
LTE. We find that in the Smith & Dworetsky sample of HgMn stars, Xe is vastly
over-abundant in 21 of 22 HgMn stars studied, by factors of 3.1--4.8 dex. There
does not appear to be a significant correlation of Xe abundance with Teff. A
comparison sample of normal late B stars shows no sign of Xe II lines that
could be detected, consistent with the expected weakness of lines at normal
abundance. The main reason for the previous lack of widespread detection in
HgMn stars is probably due to the strongest lines being at longer wavelengths
than the photographic blue. The lines used in this work were 4603.03A, 4844.33A
and 5292.22A.Comment: 8 pages, 4 figures. Accepted by Monthly Notices of the Royal
Astronomical Society, 8 January 200
Instantaneous Pair Theory for High-Frequency Vibrational Energy Relaxation in Fluids
Notwithstanding the long and distinguished history of studies of vibrational
energy relaxation, exactly how it is that high frequency vibrations manage to
relax in a liquid remains somewhat of a mystery. Both experimental and
theoretical approaches seem to say that there is a natural frequency range
associated with intermolecular motions in liquids, typically spanning no more
than a few hundred cm^{-1}. Landau-Teller-like theories explain how a solvent
can absorb any vibrational energy within this "band", but how is it that
molecules can rid themselves of superfluous vibrational energies significantly
in excess of these values? We develop a theory for such processes based on the
idea that the crucial liquid motions are those that most rapidly modulate the
force on the vibrating coordinate -- and that by far the most important of
these motions are those involving what we have called the mutual nearest
neighbors of the vibrating solute. Specifically, we suggest that whenever there
is a single solvent molecule sufficiently close to the solute that the solvent
and solute are each other's nearest neighbors, then the instantaneous
scattering dynamics of the solute-solvent pair alone suffices to explain the
high frequency relaxation. The many-body features of the liquid only appear in
the guise of a purely equilibrium problem, that of finding the likelihood of
particularly effective solvent arrangements around the solute. These results
are tested numerically on model diatomic solutes dissolved in atomic fluids
(including the experimentally and theoretically interesting case of I_2 in Xe).
The instantaneous pair theory leads to results in quantitative agreement with
those obtained from far more laborious exact molecular dynamics simulations.Comment: 55 pages, 6 figures Scheduled to appear in J. Chem. Phys., Jan, 199
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
GM crops and gender issues
Correspondence in the December issue by Jonathan Gressel not only states that gender issues in rural settings have not been adequately addressed with respect to weed control biotech but also asserts that such technology can increase the quality of life of rural women in developing countries. Improved weed control is a labor-saving technology that can result in less employment in a labor surplus rural economy. Often in rural areas, wage income is the main source of income and an important determinant of the quality of life, particularly where employment opportunities are generally limited. Apart from soil preparation, planting and weeding, harvesting is also 'femanual' work that can generate more employment if yields are higher. Biotech can enhance the quality of life of women but only if the technology is associated with overall generation of rural employment
The nuclear radio structure of X-ray bright AGN
The physical nature of the X-ray/radio correlation of AGN is still an
unsolved question. High angular resolution observations are necessary to
disentangle the associated energy dynamics into nuclear and stellar components.
We present MERLIN/EVN 18cm observations of 13 X-raying AGN. The sample consists
of Seyfert 1, Narrow Line Seyfert 1, and LINER-like galaxies. We find that for
all objects the radio emission is unresolved and that the radio luminosities
and brightness temperatures are too high for star formation to play an
important role. This indicates that the radio emission in these sources is
closely connected to processes that occur in the vicinity of the central
massive black hole, also where the X-ray emission is believed to originate in.Comment: 5 pages, 3 figures, submitted to "The Universe under the Microscope -
Astrophysics at High Angular Resolution", Bad Honnef, German
Uses of zeta regularization in QFT with boundary conditions: a cosmo-topological Casimir effect
Zeta regularization has proven to be a powerful and reliable tool for the
regularization of the vacuum energy density in ideal situations. With the
Hadamard complement, it has been shown to provide finite (and meaningful)
answers too in more involved cases, as when imposing physical boundary
conditions (BCs) in two-- and higher--dimensional surfaces (being able to
mimic, in a very convenient way, other {\it ad hoc} cut-offs, as non-zero
depths). What we have considered is the {\it additional} contribution to the cc
coming from the non-trivial topology of space or from specific boundary
conditions imposed on braneworld models (kind of cosmological Casimir effects).
Assuming someone will be able to prove (some day) that the ground value of the
cc is zero, as many had suspected until very recently, we will then be left
with this incremental value coming from the topology or BCs. We show that this
value can have the correct order of magnitude in a number of quite reasonable
models involving small and large compactified scales and/or brane BCs, and
supergravitons.Comment: 9 pages, 1 figure, Talk given at the Seventh International Workshop
Quantum Field Theory under the Influence of External Conditions, QFEXT'05,
Barcelona, September 5-9, 200
The nuclear radio structure of X-ray bright AGN
The physical nature of the X-ray/radio correlation of AGN is still an
unsolved question. High angular resolution observations are necessary to
disentangle the associated energy dynamics into nuclear and stellar components.
We present MERLIN/EVN 18cm observations of 13 X-raying AGN. The sample consists
of Seyfert 1, Narrow Line Seyfert 1, and LINER-like galaxies. We find that for
all objects the radio emission is unresolved and that the radio luminosities
and brightness temperatures are too high for star formation to play an
important role. This indicates that the radio emission in these sources is
closely connected to processes that occur in the vicinity of the central
massive black hole, also where the X-ray emission is believed to originate in.Comment: 5 pages, 3 figures, submitted to "The Universe under the Microscope -
Astrophysics at High Angular Resolution", Bad Honnef, German
The nuclear radio structure of X-ray bright AGN
The physical nature of the X-ray/radio correlation of AGN is still an
unsolved question. High angular resolution observations are necessary to
disentangle the associated energy dynamics into nuclear and stellar components.
We present MERLIN/EVN 18cm observations of 13 X-raying AGN. The sample consists
of Seyfert 1, Narrow Line Seyfert 1, and LINER-like galaxies. We find that for
all objects the radio emission is unresolved and that the radio luminosities
and brightness temperatures are too high for star formation to play an
important role. This indicates that the radio emission in these sources is
closely connected to processes that occur in the vicinity of the central
massive black hole, also where the X-ray emission is believed to originate in.Comment: 5 pages, 3 figures, submitted to "The Universe under the Microscope -
Astrophysics at High Angular Resolution", Bad Honnef, German
Temporally ordered collective creep and dynamic transition in the charge-density-wave conductor NbSe3
We have observed an unusual form of creep at low temperatures in the
charge-density-wave (CDW) conductor NbSe. This creep develops when CDW
motion becomes limited by thermally-activated phase advance past individual
impurities, demonstrating the importance of local pinning and related
short-length-scale dynamics. Unlike in vortex lattices, elastic collective
dynamics on longer length scales results in temporally ordered motion and a
finite threshold field. A first-order dynamic phase transition from creep to
high-velocity sliding produces "switching" in the velocity-field
characteristic.Comment: 4 pages, 4 eps figures; minor clarifications To be published in Phys.
Rev. Let
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
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