8,552 research outputs found
Stability analysis of static solutions in a Josephson junction
We present all the possible solutions of a Josephson junction with bias
current and magnetic field with both inline and overlap geometry, and examine
their stability. We follow the bifurcation of new solutions as we increase the
junction length. The analytical results, in terms of elliptic functions in the
case of inline geometry, are in agreement with the numerical calculations and
explain the strong hysteretic phenomena typically seen in the calculation of
the maximum tunneling current. This suggests a different experimental approach
based on the use, instead of the external magnetic field the modulus of the
elliptic function or the related quantity the total magnetic flux to avoid
hysteretic behavior and unfold the overlapping curves.Comment: 36 pages with 17 figure
Theoretical models for classical Cepheids. VIII. Effects of helium and heavy elements abundance on the Cepheid distance scale
Previous nonlinear fundamental pulsation models for classical Cepheids with
metal content Z <= 0.02 are implemented with new computations at super-solar
metallicity (Z=0.03, 0.04) and selected choices of the helium-to-metal
enrichment ratio DeltaY/Delta Z. On this basis, we show that the location into
the HR diagram of the Cepheid instability strip is dependent on both metal and
helium abundance, moving towards higher effective temperatures with decreasing
the metal content (at fixed Y) or with increasing the helium content (at fixed
Z). The contributions of helium and metals to the predicted Period-Luminosity
and Period-Luminosity-Color relations are discussed, as well as the
implications on the Cepheid distance scale. Based on these new results, we
finally show that the empirical metallicity correction suggested by Cepheid
observations in two fields of the galaxy M101 may be accounted for, provided
that the adopted helium-to-metal enrichment ratio is reasonably high (Delta
Y/Delta Z ~ 3.5).Comment: 23 pages, including 6 postscript figures, accepted for publication on
Ap
Wetting of gradient fields: pathwise estimates
We consider the wetting transition in the framework of an effective interface
model of gradient type, in dimension 2 and higher. We prove pathwise estimates
showing that the interface is localized in the whole thermodynamically-defined
partial wetting regime considered in earlier works. Moreover, we study how the
interface delocalizes as the wetting transition is approached. Our main tool is
reflection positivity in the form of the chessboard estimate.Comment: Some typos removed after proofreading. Version to be published in
PTR
Triggering the Formation of Halo Globular Clusters with Galaxy Outflows
We investigate the interactions of high-redshift galaxy outflows with
low-mass virialized (Tvir < 10,000K) clouds of primordial composition. While
atomic cooling allows star formation in larger primordial objects, such
"minihalos" are generally unable to form stars by themselves. However, the
large population of high-redshift starburst galaxies may have induced
widespread star formation in these objects, via shocks that caused intense
cooling both through nonequilibrium H2 formation and metal-line emission. Using
a simple analytic model, we show that the resulting star clusters naturally
reproduce three key features of the observed population of halo globular
clusters (GCs). First, the 10,000 K maximum virial temperature corresponds to
the ~ 10^6 solar mass upper limit on the stellar mass of GCs. Secondly, the
momentum imparted in such interactions is sufficient to strip the gas from its
associated dark matter halo, explaining why GCs do not reside in dark matter
potential wells. Finally, the mixing of ejected metals into the primordial gas
is able to explain the ~ 0.1 dex homogeneity of stellar metallicities within a
given GC, while at the same time allowing for a large spread in metallicity
between different clusters. To study this possibility in detail, we use a
simple 1D numerical model of turbulence transport to simulate mixing in
cloud-outflow interactions. We find that as the shock shears across the side of
the cloud, Kelvin-Helmholtz instabilities arise, which cause mixing of enriched
material into > 20% of the cloud. Such estimates ignore the likely presence of
large-scale vortices, however, which would further enhance turbulence
generation. Thus quantitative mixing predictions must await more detailed
numerical studies.Comment: 21 pages, 11 figures, Apj in pres
Theoretical Models for Classical Cepheids: IV. Mean Magnitudes and Colors and the Evaluation of Distance, Reddening and Metallicity
We discuss the metallicity effect on the theoretical visual and near-infrared
PL and PLC relations of classical Cepheids, as based on nonlinear, nonlocal and
time--dependent convective pulsating models at varying chemical composition. In
view of the two usual methods of averaging (magnitude-weighted and
intensity-weighted) observed magnitudes and colors over the full pulsation
cycle, we briefly discuss the differences between static and mean quantities.
We show that the behavior of the synthetic mean magnitudes and colors fully
reproduces the observed trend of Galactic Cepheids, supporting the validity of
the model predictions. In the second part of the paper we show how the estimate
of the mean reddening and true distance modulus of a galaxy from Cepheid VK
photometry depend on the adopted metal content, in the sense that larger
metallicities drive the host galaxy to lower extinctions and distances.
Conversely, self-consistent estimates of the Cepheid mean reddening, distance
and metallicity may be derived if three-filter data are taken into account. By
applying the theoretical PL and PLC relations to available BVK data of Cepheids
in the Magellanic Clouds we eventually obtain Z \sim 0.008, E(B-V) \sim 0.02
mag, DM \sim 18.63 mag for LMC and Z \sim 0.004, E(B-V) \sim 0.01 mag., DM \sim
19.16 mag. for SMC. The discrepancy between such reddenings and the current
values based on BVI data is briefly discussed.Comment: 16 pages, 11 postscript figures, accepted for publication on Ap
Vortex polarity switching by a spin--polarized current
The spin-transfer effect is investigated for the vortex state of a magnetic
nanodot. A spin current is shown to act similarly to an effective magnetic
field perpendicular to the nanodot. Then a vortex with magnetization (polarity)
parallel to the current polarization is energetically favorable. Following a
simple energy analysis and using direct spin--lattice simulations, we predict
the polarity switching of a vortex. For magnetic storage devices, an electric
current is more effective to switch the polarity of a vortex in a nanodot than
the magnetic field
Raman solitons in transient SRS
We report the observation of Raman solitons on numerical simulations of
transient stimulated Raman scattering (TSRS) with small group velocity
dispersion. The theory proceeds with the inverse scattering transform (IST) for
initial-boundary value problems and it is shown that the explicit theoretical
solution obtained by IST for a semi-infinite medium fits strikingly well the
numerical solution for a finite medium. We understand this from the rapid
decrease of the medium dynamical variable (the potential of the scattering
theory). The spectral transform reflection coefficient can be computed directly
from the values of the input and output fields and this allows to see the
generation of the Raman solitons from the numerical solution. We confirm the
presence of these nonlinear modes in the medium dynamical variable by the use
of a discrete spectral analysis.Comment: LaTex file, to appear in Inverse Problem
A delta Scuti distance to the Large Magellanic Cloud
We present results from a well studied delta Scuti star discovered in the
LMC. The absolute magnitude of the variable was determined from the PL relation
for Galactic delta Scuti stars and from the theoretical modeling of the
observed B,V,I light curves. The two methods give distance moduli for the LMC
of 18.46+-0.19 and 18.48+-0.15, respectively, for a consistent value of the
stellar reddening of E(B-V)=0.08+-0.02. We have also analyzed 24 delta Scuti
candidates discovered in the OGLE II survey of the LMC, and 7 variables
identified in the open cluster LW 55 and in the galaxy disk by Kaluzny et al.
(2003, 2006). We find that the LMC delta Scuti stars define a PL relation whose
slope is very similar to that defined by the Galactic delta Scuti variables,
and yield a distance modulus for the LMC of 18.50+-0.22 mag. We compare the
results obtained from the delta Scuti variables with those derived from the LMC
RR Lyrae stars and Cepheids. Within the observational uncertainties, the three
groups of pulsating stars yield very similar distance moduli. These moduli are
all consistent with the "long" astronomical distance scale for the Large
Magellanic Cloud.Comment: Accepted for publication on A
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