4,928 research outputs found
Spectral analysis and abundances of the post-HB star HD 76431
HD76431 is a slow rotating post-HB star that shows an underabundance of
helium by 0.5 dex relative to the solar value. These observational facts
suggest that atomic diffusion could be active in its atmosphere. We have used
the MMT and Bok spectra to estimate the atmospheric parameters of the target
star using the model atmospheres and synthetic spectra calculated with TLUSTY
and SYNSPEC. The derived values of the effective temperature, surface gravity,
helium abundance are consistent with those obtained by Ramspeck et al. (2001b).
It appears that NLTE effect are not important for HD76431. We have used Stokes
I spectra from ESPaDOnS at CFHT to perform an abundance analysis and a search
for observational evidence of vertical stratification of the abundance of
certain elements. The results of our abundance analysis are in good agreement
with previously published data with respect to average abundances. Our
numerical simulations show that carbon and nitrogen reveal signatures of
vertical abundance stratification in the atmosphere of HD76431. It appears that
the carbon abundance increases toward the deeper atmospheric layers. Nitrogen
also shows a similar behaviour, but in deeper atmospheric layers we obtain a
significant dispersion for the estimates of its abundance. To our knowledge,
this is the first demonstration of vertical abundance stratification of metals
in a post-HB star and up to now it is the hottest star to show such
stratification features. We also report the detection of two SiIII and one
TiIII emission lines in the spectra of HD76431 that were not detected in
previous studies.Comment: 7 pages, 5 figures, accepted for publication in MNRA
Signatures of superconducting gap inhomogeneities in optical properties
Scanning tunneling spectroscopy applied to the high- cuprates has
revealed significant spatial inhomogeneity on the nanoscale. Regions on the
order of a coherence length in size show variations of the magnitude of the
superconducting gap of order or more. An important unresolved question
is whether or not these variations are also present in the bulk, and how they
influence superconducting properties. As many theories and data analyses for
high- superconductivity assume spatial homogeneity of the gap magnitude,
this is a pressing question. We consider the far-infrared optical conductivity
and evaluate, within an effective medium approximation, what signatures of
spatial variations in gap magnitude are present in various optical quantities.
In addition to the case of d-wave superconductivity, relevant to the high-
cuprates, we have also considered s-wave gap symmetry in order to provide
expected signatures of inhomogeneities for superconductors in general. While
signatures of gap inhomogeneities can be strongly manifested in s-wave
superconductors, we find that the far-infrared optical conductivity in d-wave
is robust against such inhomogeneity.Comment: 8 pages, 7 figure
Dynamics of spatially homogeneous solutions of the Einstein-Vlasov equations which are locally rotationally symmetric
The dynamics of a class of cosmological models with collisionless matter and
four Killing vectors is studied in detail and compared with that of
corresponding perfect fluid models. In many cases it is possible to identify
asymptotic states of the spacetimes near the singularity or in a phase of
unlimited expansion. Bianchi type II models show oscillatory behaviour near the
initial singularity which is, however, simpler than that of the mixmaster
model.Comment: 27 pages, 3 figures, LaTe
Magnetic Brane-worlds
We investigate brane-worlds with a pure magnetic field and a perfect fluid.
We extend earlier work to brane-worlds, and find new properties of the Bianchi
type I brane-world. We find new asymptotic behaviours on approach to the
singularity and classify the critical points of the dynamical phase space. It
is known that the Einstein equations for the magnetic Bianchi type I models are
in general oscillatory and are believed to be chaotic, but in the brane-world
model this chaotic behaviour does not seem to be possible.Comment: 21 pages, 3 ps figures; To appear in CQ
The Hot Horizontal-Branch Stars in NGC288 - Effects of Diffusion and Stratification on Their Atmospheric Parameters*
Context. NGC288 is a globular cluster with a well developed blue horizontal branch covering the so-called u-jump which indicates the onset of diffusion. It is therefore well suited to study the effects of diffusion in blue horizontal branch (HB) stars. Aims. We compare observed abundances to predictions from stellar evolution models calculated with diffusion and from stratified atmospheric models. We verify the effect of using stratified model spectra to derive atmospheric parameters. In addition we investigate the nature of the overluminous blue HB stars around the u-jump. Methods. We define a new photometric index sz from uvby measurements that is gravity sensitive between 8 000K and 12 000 K. Using medium-resolution spectra and Stroemgren photometry we determine atmospheric parameters (Teff, logg) and abundances for the blue HB stars. We use both homogeneous and stratified model spectra for our spectroscopic analyses. Results. The atmospheric parameters and masses of the hot HB stars in NGC288 show a behaviour seen also in other clusters for temperatures between 9 000K and 14 000 K. Outside this temperature range, however, they follow rather the results found for such stars in (omega)Cen. The abundances derived from our observations are for most elements (except He and P) within the abundance range expected from evolutionary models that include the effects of atomic diffusion and assume a surface mixed mass of 10(exp 7) M. The abundances predicted by stratified model atmospheres are generally significantly more extreme than observed, except for Mg. The use of stratified model spectra to determine effective temperatures, surface gravities and masses moves the hotter stars to a closer agreement with canonical evolutionary predictions. Conclusions. Our results show definite promise towards solving the long-standing issue of surface gravity and mass discrepancies for hot HB stars, but there is still much work needed to arrive at a self-consistent solution
Global dynamics of the mixmaster model
The asymptotic behaviour of vacuum Bianchi models of class A near the initial
singularity is studied, in an effort to confirm the standard picture arising
from heuristic and numerical approaches by mathematical proofs. It is shown
that for solutions of types other than VIII and IX the singularity is velocity
dominated and that the Kretschmann scalar is unbounded there, except in the
explicitly known cases where the spacetime can be smoothly extended through a
Cauchy horizon. For types VIII and IX it is shown that there are at most two
possibilities for the evolution. When the first possibility is realized, and if
the spacetime is not one of the explicitly known solutions which can be
smoothly extended through a Cauchy horizon, then there are infinitely many
oscillations near the singularity and the Kretschmann scalar is unbounded
there. The second possibility remains mysterious and it is left open whether it
ever occurs. It is also shown that any finite sequence of distinct points
generated by iterating the Belinskii-Khalatnikov-Lifschitz mapping can be
realized approximately by a solution of the vacuum Einstein equations of
Bianchi type IX.Comment: 16 page
Bosonic D-branes at finite temperature with an external field
Bosonic boundary states at finite temperature are constructed as solutions of
boundary conditions at for bosonic open strings with a constant gauge
field coupled to the boundary. The construction is done in the
framework of thermo field dynamics where a thermal Bogoliubov transformation
maps states and operators to finite temperature. Boundary states are given in
terms of states from the direct product space between the Fock space of the
closed string and another identical copy of it. By analogy with zero
temperature, the boundary states heve the interpretation of -brane at
finite temperature. The boundary conditions admit two different solutions. The
entropy of the closed string in a -brane state is computed and analysed. It
is interpreted as the entropy of the -brane at finite temperature.Comment: 21 pages, Latex, revised version with minor corrections and
references added, to be published in Phys. Rev.
New Algorithm for Mixmaster Dynamics
We present a new numerical algorithm for evolving the Mixmaster spacetimes.
By using symplectic integration techniques to take advantage of the exact Taub
solution for the scattering between asymptotic Kasner regimes, we evolve these
spacetimes with higher accuracy using much larger time steps than previously
possible. The longer Mixmaster evolution thus allowed enables detailed
comparison with the Belinskii, Khalatnikov, Lifshitz (BKL) approximate
Mixmaster dynamics. In particular, we show that errors between the BKL
prediction and the measured parameters early in the simulation can be
eliminated by relaxing the BKL assumptions to yield an improved map. The
improved map has different predictions for vacuum Bianchi Type IX and magnetic
Bianchi Type VI Mixmaster models which are clearly matched in the
simulation.Comment: 12 pages, Revtex, 4 eps figure
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