7,741 research outputs found
External field dependence of the correlation lengths in the three-dimensional O(4) model
We investigate numerically the transverse and longitudinal correlation
lengths of the three-dimensional O(4) model as a function of the external field
H. In the low-temperature phase we verify explicitly the H^{-1/2}-dependence of
the transverse correlation length, which is expected due to the Goldstone modes
of the model. On the critical line we find the universal amplitude ratio xi^c_T
/ xi^c_L = 1.99(1). From our data we derive the universal scaling function for
the transverse correlation length. The H-dependencies of the correlation
lengths in the high temperature phase are discussed and shown to be in accord
with the scaling functions.Comment: 3 pages, 4 figures, Lattice2003(higgs) contribution, espcrc2.st
Numerical equation of state and other scaling functions from an improved three-dimensional Ising model
We study an improved three-dimensional Ising model with external magnetic
field near the critical point by Monte Carlo simulations. From our data we
determine numerically the universal scaling functions of the magnetization,
that is the equation of state, of the susceptibility and of the correlation
length. In order to normalize the scaling functions we calculate the critical
amplitudes of the three observables on the critical line, the phase boundary
and the critical isochore. These amplitudes lead to the universal ratios
C^+/C^-=4.756(28), R_{chi}=1.723(13), Q_c=0.326(3) and Q_2=1.201(10). We find
excellent agreement of the data with the parametric representation of the
asymptotic equation of state as found by field theory methods. The comparison
of the susceptibility data to the corresponding scaling function shows a
marginal difference in the symmetric phase, which can be explained by the
slightly different value for R_{chi} used in the parametrization. The shape of
the correlation-length-scaling function is similar to the one of the
susceptibility, as expected from earlier parametrizations. The peak positions
of the two scaling functions are coinciding within the error bars.Comment: 27 pages, 14 Ps-figures, Latex2e, 10 pages added, including the
scaling function of the correlation length, to appear in Nucl. Phys.
Study of extremely reddened AGB stars in the Galactic bulge
Context. Extremely reddened AGB stars lose mass at high rates of >10^-5
Msun/yr. This is the very last stage of AGB evolution, in which stars in the
mass range 2.0--4.0 Msun (for solar metallicity) should have been converted to
C stars already. The extremely reddened AGB stars in the Galactic bulge are
however predominantly O-rich, implying that they might be either low-mass stars
or stars at the upper end of the AGB mass range. Aims. To determine the mass
range of the most reddened AGB stars in the Galactic bulge. Methods. Using
Virtual Observatory tools, we constructed spectral energy distributions of a
sample of 37 evolved stars in the Galactic bulge with extremely red IRAS
colours. We fitted DUSTY models to the observational data to infer the
bolometric fluxes. Applying individual corrections for interstellar extinction
and adopting a common distance, we determined luminosities and mass-loss rates,
and inferred the progenitor mass range from comparisons with AGB evolutionary
models. Results. The observed spectral energy distributions are consistent with
a classification as reddened AGB stars, except for two stars, which are
proto-planetary nebula candidates. For the AGB stars, we found luminosities in
the range 3000--30,000 Lsun and mass-loss rates 10^-5--3x10^-4 Msun/yr. The
corresponding mass range is 1.1--6.0 Msun assuming solar metallicity.
Conclusions. Contrary to the predictions of the evolutionary models, the
luminosity distribution is continuous, with many O-rich AGB stars in the mass
range in which they should have been converted into C stars already. We suspect
that bulge AGB stars have higher than solar metallicity and therefore may avoid
the conversion to C-rich. The presence of low-mass stars in the sample shows
that their termination of the AGB evolution also occurs during a final phase of
very high mass-loss rate, leading to optically thick circumstellar shells
Non-perturbative determination of anisotropy coefficients in lattice gauge theories
We propose a new non-perturbative method to compute derivatives of gauge
coupling constants with respect to anisotropic lattice spacings (anisotropy
coefficients), which are required in an evaluation of thermodynamic quantities
from numerical simulations on the lattice. Our method is based on a precise
measurement of the finite temperature deconfining transition curve in the
lattice coupling parameter space extended to anisotropic lattices by applying
the spectral density method. We test the method for the cases of SU(2) and
SU(3) gauge theories at the deconfining transition point on lattices with the
lattice size in the time direction -- 6. In both cases, there is a
clear discrepancy between our results and perturbative values. A longstanding
problem, when one uses the perturbative anisotropy coefficients, is a
non-vanishing pressure gap at the deconfining transition point in the SU(3)
gauge theory. Using our non-perturbative anisotropy coefficients, we find that
this problem is completely resolved: we obtain and
on and 6 lattices, respectively.Comment: 24pages,7figures,5table
Correlation lengths and scaling functions in the three-dimensional O(4) model
We investigate numerically the transverse and longitudinal correlation
lengths of the three-dimensional O(4) model as a function of the external field
H. From our data we calculate the scaling function of the transverse
correlation length, and that of the longitudinal correlation length for T>T_c.
We show that the scaling functions do not only describe the critical behaviours
of the correlation lengths but encompass as well the predicted Goldstone
effects, in particular the H^{-1/2}-dependence of the transverse correlation
length for T<T_c. In addition, we determine the critical exponent
delta=4.824(9) and several critical amplitudes from which we derive the
universal amplitude ratios R_{chi}=1.084(18), Q_c=0.431(9), Q_2^T=4.91(8),
Q_2^L=1.265(24) and U_{xi}^c=1.99(1). The last result supports a relation
between the longitudinal and transverse correlation functions, which was
conjectured to hold below T_c but seems to be valid also at T_c.Comment: 24 pages, 13Ps-figures, Latex2e,one page added,version to appear in
Nucl. Phys. B[FS
Distances of Stars by mean of the Phase-lag Method
Variable OH/IR stars are Asymptotic Giant Branch (AGB) stars with an
optically thick circumstellar envelope that emit strong OH 1612 MHz emission.
They are commonly observed throughout the Galaxy but also in the LMC and SMC.
Hence, the precise inference of the distances of these stars will ultimately
result in better constraints on their mass range in different metallicity
environments. Through a multi-year long-term monitoring program at the Nancay
Radio telescope (NRT) and a complementary high-sensitivity mapping campaign at
the eMERLIN and JVLA to measure precisely the angular diameter of the
envelopes, we have been re-exploring distance determination through the
phase-lag method for a sample of stars, in order to refine the
poorly-constrained distances of some and infer the currently unknown distances
of others. We present here an update of this project.Comment: 4 pages, 1 figure, 2 tables, to appear in the Proceedings of the IAU
Symposium No. 336: Astrophysical Masers: Unlocking the Mysteries of the
Univers
Time Course of the Increase in the Myocardial Slow Inward Current after a Photochemically Generated Concentration Jump of Intracellular cAMP
Voltage-clamped atrial trabeculae from bullfrog hearts were exposed to membrane-permeant photolyzable o-nitrobenzyl esters of cAMP and cGMP. UV flashes produced intracellular concentration jumps of cAMP or cGMP. With the cAMP derivative, flashes resulted in an increased slow inward current (Isi), producing a broadened action potential. The Isi reached a maximum 10-30 sec after the flash and decreased over the next 60-300 sec. The first increases were observable within 150 msec; this value is an upper limit imposed by the instrumentation. Responses to flashes lasted longer at higher drug concentrations and in the presence of the phosphodiesterase inhibitor papaverine; effects of flashes developed and decreased faster at higher temperature. Although the amplitude of the Isi was increased, its waveform and voltage sensitivity were not affected. Intracellular concentration jumps of cAMP failed to affect the muscarinic K+ conductance. There were no observable effects of cGMP concentration jumps. The data confirm (i) that cAMP regulates the Isi and (ii) that the 5- to 10-sec delay between application of ß-agonists and the onset of positive inotropic effects, observed in previous studies, has been correctly ascribed to events prior to the interaction between cAMP and protein kinase
OH 12.8-0.9: A New Water-Fountain Source
We present observational evidence that the OH/IR star OH 12.8-0.9 is the
fourth in a class of objects previously dubbed "water-fountain" sources. Using
the Very Long Baseline Array, we produced the first images of the water maser
emission associated with OH 12.8-0.9. We find that the masers are located in
two compact regions with an angular separation of ~109 mas on the sky. The axis
of separation between the two maser regions is at a position angle of 1.5 deg.
East of North with the blue-shifted (-80.5 to -85.5 km/s) masers located to the
North and the red-shifted (-32.0 to -35.5 km/s) masers to the South. In
addition, we find that the blue- and red-shifted masers are distributed along
arc-like structures ~10-12 mas across oriented roughly perpendicular to the
separation axis. The morphology exhibited by the water masers is suggestive of
an axisymmetric wind with the masers tracing bow shocks formed as the wind
impacts the ambient medium. This bipolar jet-like structure is typical of the
three other confirmed water-fountain sources. When combined with the previously
observed spectral characteristics of OH 12.8-0.9, the observed spatio-kinematic
structure of the water masers provides strong evidence that OH 12.8-0.9 is
indeed a member of the water-fountain class.Comment: 12 pages, 2 figures (1 color), accepted for publication in the Ap J
Letter
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