7,955 research outputs found
Structural sensitivity analysis: Methods, applications, and needs
Some innovative techniques applicable to sensitivity analysis of discretized structural systems are reviewed. These techniques include a finite-difference step-size selection algorithm, a method for derivatives of iterative solutions, a Green's function technique for derivatives of transient response, a simultaneous calculation of temperatures and their derivatives, derivatives with respect to shape, and derivatives of optimum designs with respect to problem parameters. Computerized implementations of sensitivity analysis and applications of sensitivity derivatives are also discussed. Finally, some of the critical needs in the structural sensitivity area are indicated along with Langley plans for dealing with some of these needs
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
Extremely metal-poor stars from the SDSS
We give a progress report about the activities within the CIFIST Team related
to the search for extremely metal-poor stars in the Sloan Digital Sky Survey's
spectroscopic catalog. So far the search has provided 25 candidates with
metallicities around or smaller -3. For 15 candidates high resolution
spectroscopy with UVES at the VLT has confirmed their extremely metal-poor
status. Work is under way to extend the search to the SDSS's photometric
catalog by augmenting the SDSS photometry, and by gauging the capabilities of
X-shooter when going to significantly fainter targets.Comment: 6 pages, 6 figures, Proceedings paper of the conference "A stellar
journey: A symposium in celebration of Bengt Gustafsson's 65th birthday
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
Three gonadotropin-releasing hormone genes in one organism suggest novel roles for an ancient peptide.
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
The chemical abundance analysis of normal early A- and late B-type stars
Modern spectroscopy of early-type stars often aims at studying complex
physical phenomena. Comparatively less attention is paid to identifying and
studying the "normal" A- and B-type stars and testing how the basic atomic
parameters and standard spectral analysis allow one to fit the observations. We
wish to stablish whether the chemical composition of the solar photosphere can
be regarded as a reference for early A- and late B-type stars. We have obtained
optical high-resolution, high signal-to-noise ratio spectra of three slowly
rotating early-type stars (HD 145788, 21 Peg and pi Cet) that show no obvious
sign of chemical peculiarity, and performed a very accurate LTE abundance
analysis of up to 38 ions of 26 elements (for 21 Peg), using a vast amount of
spectral lines visible in the spectral region covered by our spectra. We
provide an exhaustive description of the abundance characteristics of the three
analysed stars with a critical review of the line parameters used to derive the
abundances. We compiled a table of atomic data for more than 1100 measured
lines that may be used in the future as a reference. The abundances we obtained
for He, C, Al, S, V, Cr, Mn, Fe, Ni, Sr, Y, and Zr are compatible with the
solar ones derived with recent 3D radiative-hydrodynamical simulations of the
solar photosphere. The abundances of the remaining studied elements show some
degree of discrepancy compared to the solar photosphere. Those of N, Na, Mg,
Si, Ca, Ti, and Nd may well be ascribed to non-LTE effects; for P, Cl, Sc and
Co, non-LTE effects are totally unknown; O, Ne, Ar, and Ba show discrepancies
that cannot be ascribed to non-LTE effects. The discrepancies obtained for O
(in two stars) and Ne agree with very recent non-LTE abundance analysis of
early B-type stars in the solar neighbourhood.Comment: Accepted for publication on Astronomy and Astrophysic
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
Simulations of the Alternating Access Mechanism of the Sodium Symporter Mhp1
AbstractSodium coupled cotransporters of the five-helix inverted repeat (5HIR) superfamily use an alternating access mechanism to transport a myriad of small molecules across the cell membrane. One of the primary steps in this mechanism is the conformational transition from a state poised to bind extracellular substrates to a state that is competent to deliver substrate to the cytoplasm. Here, we construct a coarse-grained model of the 5HIR benzylhydantoin transporter Mhp1 that incorporates experimental structures of the outward- and inward-open states to investigate the mechanism of this conformational change. Using the weighted ensemble path-sampling method, we rigorously sample the outward- to inward-facing transition path ensemble. The transition path ensemble reveals a heterogeneous set of pathways connecting the two states and identifies two modes of transport: one consistent with a strict alternating access mechanism and another where decoupling of the inner and outer gates causes the transient formation of a continuous permeation pathway through the transporter. We also show that the conformational switch between the outward- and inward-open states results from rigid body motions of the hash motif relative to the substrate bundle, supporting the rocking bundle hypothesis. Finally, our methodology provides the groundwork for more chemically detailed investigations of the alternating mechanism
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
- …