5,992 research outputs found
Non-Reciprocal MEMS Periodic Structure
In recent years, active periodic structures with in-time modulated parameters have drawn ever-increasing attention due to their peculiar (and sometimes exotic) wave propagation properties. Although many experimental works have shown the efficacy of time-modulation strategies, the benchmarks proposed until now have been mostly proof-of-concept demonstrators, with little attention to the feasibility of the solution for practical purposes. In this work, we propose a micro electro-mechanical system (MEMS) periodic structure with modulated electromechanical stiffness featuring non-reciprocal band-gaps that are frequency bands where elastic waves are allowed to travel only in one direction. To this aim, we derive a simplified analytical lumped-parameter model, which is then verified through numerical simulations of both the lumped-parameter system and the high-fidelity multiphysics finite element model including electrostatic effects. We envision that this system, which can easily be manufactured through standard MEMS production processes, may be used as a directional filter in MEMS devices such as insulators and circulators
Interface pinning and slow ordering kinetics on infinitely ramified fractal structures
We investigate the time dependent Ginzburg-Landau (TDGL) equation for a non
conserved order parameter on an infinitely ramified (deterministic) fractal
lattice employing two alternative methods: the auxiliary field approach and a
numerical method of integration of the equations of evolution. In the first
case the domain size evolves with time as , where is
the anomalous random walk exponent associated with the fractal and differs from
the normal value 2, which characterizes all Euclidean lattices. Such a power
law growth is identical to the one observed in the study of the spherical model
on the same lattice, but fails to describe the asymptotic behavior of the
numerical solutions of the TDGL equation for a scalar order parameter. In fact,
the simulations performed on a two dimensional Sierpinski Carpet indicate that,
after an initial stage dominated by a curvature reduction mechanism \`a la
Allen-Cahn, the system enters in a regime where the domain walls between
competing phases are pinned by lattice defects.
The lack of translational invariance determines a rough free energy
landscape, the existence of many metastable minima and the suppression of the
marginally stable modes, which in translationally invariant systems lead to
power law growth and self similar patterns. On fractal structures as the
temperature vanishes the evolution is frozen, since only thermally activated
processes can sustain the growth of pinned domains.Comment: 16 pages+14 figure
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
Uncovering the kiloparsec-scale stellar ring of NGC5128
We reveal the stellar light emerging from the kiloparsec-scale, ring-like
structure of the NGC5128 (Centaurus A) galaxy in unprecedented detail. We use
arcsecond-scale resolution near infrared images to create a "dust-free" view of
the central region of the galaxy, which we then use to quantify the shape of
the revealed structure. At the resolution of the data, the structure contains
several hundreds of discreet, point-like or slightly elongated sources. Typical
extinction corrected surface brightness of the structure is K_S = 16.5
mag/arcsec^2, and we estimate the total near infrared luminosity of the
structure to be M = -21 mag. We use diffraction limited (FWHM resolution of ~
0.1", or 1.6 pc) near infrared data taken with the NACO instrument on VLT to
show that the structure decomposes into thousands of separate, mostly
point-like sources. According to the tentative photometry, the most luminous
sources have M_K = -12 mag, naming them red supergiants or relatively low-mass
star clusters. We also discuss the large-scale geometry implied by the
reddening signatures of dust in our near infrared images.Comment: 5 pages, 4 figures, accepted for publication in A&A Letters. A
version with high resolution images can be downloaded from
http://www.helsinki.fi/~jtkainul/CenALette
Jet driven motions in the Narrow Line region of NGC1068
We have obtained HST FOC f/48 long-slit spectroscopy of the inner 4" of the
Narrow Line Region of NGC 1068 between 3500-5400A with a spectral resolution of
1.78A/pixel. At a spatial scale of 0.0287" per pixel these data provide an
order of magnitude improvement in resolution over previous ground based spectra
and allow us to trace the interaction between the radio jet and the gas in the
NLR. Our results show that, within +/-0.5" of the radio-jet the emission lines
are kinematically disturbed and split into two components whose velocity
separation is 1500 km/sec. The filaments associated with the radio lobe also
show a redshifted kinematic disturbance of the order of 300 km/sec which
probably is a consequence of the expansion of the radio plasma.
Furthermore, the material enveloping the radio-jet is in a much higher
ionization state than that of the surrounding NLR gas. The highest excitation
is coincident with the jet axis where emission in the coronal line of [FeVII]
3769A is detected and the HeII 4686A is strong but where [OII] 3727A is
depressed. This large localized increase in ionization on the jet axis is
accompanied by the presence of an excess continuum. Because the electron
density is substantially larger in the jet compared to the surrounding NLR,
these results can only be explained if there is a more intense ionizing
continuum associated with the jet. This can be accomplished in a variety of
ways which include an intrinsically anisotropic nuclear radiation field, a
reduced gas covering factor or the presence of a local ionization source.
The morphology, kinematics and, possibly, the ionization structure of the NLR
in the vicinity of the jet of NGC 1068 are a direct consequence of the
interaction with the radio outflow.Comment: 11 pages, 5 figures, ApJ Letters in pres
Classical Cepheid Pulsation Models: IX. New Input Physics
We constructed several sequences of classical Cepheid envelope models at
solar chemical composition () to investigate the dependence of
the pulsation properties predicted by linear and nonlinear hydrodynamical
models on input physics. To study the dependence on the equation of state (EOS)
we performed several numerical experiments by using the simplified analytical
EOS originally developed by Stellingwerf and the recent analytical EOS
developed by Irwin. Current findings suggest that the pulsation amplitudes as
well as the topology of the instability strip marginally depend on the adopted
EOS.
We also investigated the dependence of observables predicted by theoretical
models on the mass-luminosity (ML) relation and on the spatial resolution
across the Hydrogen and the Helium partial ionization regions. We found that
nonlinear models are marginally affected by these physical and numerical
assumptions. In particular, the difference between new and old models in the
location as well as in the temperature width of the instability strip is on
average smaller than 200 K. However, the spatial resolution somehow affects the
pulsation properties. The new fine models predict a period at the center of the
Hertzsprung Progression (9.84 days) that reasonably agree with
empirical data based on light curves ( days;
\citealt{mbm92}) and on radial velocity curves ( days;
\citealt{mall00}), and improve previous predictions by Bono, Castellani, and
Marconi (2000, hereinafter BCM00).Comment: 35 pages, 7 figures. Accepted for publication in the Astrophysical
Journa
The supermassive black hole of M87 and the kinematics of its associated gaseous disk
We have obtained long-slit observations of the circumnuclear region of M87 at three different locations, with a spatial sampling of 0.028" using the Faint Object Camera f/48 spectrograph on board HST. These data allow us to determine the rotation curve of the inner ~1" of the ionized gas disk in [OII]3727 to a distance as close as 0.07" (~5pc) to the dynamic center, thereby significantly improving on both the spatial resolution and coverage of previous FOS observations. We have modeled the kinematics of the gas under the assumption of the existence of both a central black hole and an extended central mass distribution, taking into account the effects of the instrumental PSF, the intrinsic luminosity distribution of the line, and the finite size of the slit. We find that the central mass must be concentrated within a sphere whose maximum radius is 0.05" (~3.5pc) and show that both the observed rotation curve and line profiles are consistent with a thin--disk in keplerian motion. We conclude that the most likely explanation for the observed motions is the presence of a supermassive black hole and derive a value of M_{BH} = (3.2+/-0.9) 10^9 M_{sun} for its mass
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
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