601 research outputs found
Wavefunction tomography of topological dimer chains with long-range couplings
The ability to tailor with a high accuracy the inter-site connectivity in a
lattice is a crucial tool for realizing novel topological phases of matter.
Here, we report the experimental realization of photonic dimer chains with
long-range hopping terms of arbitrary strength and phase, providing a rich
generalization of the celebrated Su-Schrieffer-Heeger model. Our experiment is
based on a synthetic dimension scheme involving the frequency modes of an
optical fiber loop platform. This setup provides direct access to both the band
dispersion and the geometry of the Bloch wavefunctions throughout the entire
Brillouin zone allowing us to extract the winding number for any possible
configuration. Finally, we highlight a topological phase transition solely
driven by a time-reversal-breaking synthetic gauge field associated with the
phase of the long-range hopping, providing a route for engineering topological
bands in photonic lattices belonging to the AIII symmetry class
A Period and a Prediction for the Of?p Spectrum Alternator HD 191612
The observational picture of the enigmatic O-type spectrum variable HD191612
has been sharpened substantially. A symmetrical, low-amplitude light curve with
a period near 540 d has recently been reported from Hipparcos photometry. This
period satisfies all of the spectroscopy since at least 1982, including
extensive new observations during 2003 and 2004, and it has predicted the next
transition during September--October 2004. Measurements of the H alpha
equivalent width reveal a sharp emission peak in the phase diagram, in contrast
to the apparently sinusoidal light curve. The He II absorption-line strength is
essentially constant, while He I varies strongly, possibly filled in by
emission in the O6 state, thus producing the apparent spectral-type variations.
The O8 state appears to be the "normal" one. Two intermediate O7 observations
have been obtained, which fall at the expected phases, but these are the only
modern observations of the transitions so far. The period is too long for
rotation or pulsation; although there is no direct evidence as yet for a
companion, a model in which tidally induced oscillations drive an enhanced wind
near periastron of an eccentric orbit appears promising. Further observations
during the now predictable transitions may provide a critical test. Ultraviolet
and X-ray observations during both states will likely also prove illuminating.Comment: 7 pages, 3 figures, 1 table; scheduled for the 2004 December 10 issue
of ApJL, Vol. 617, No. 1. ApJ
Theory of extraordinary optical transmission through subwavelength hole arrays
We present a fully three-dimensional theoretical study of the extraordinary
transmission of light through subwavelength hole arrays in optically thick
metal films. Good agreement is obtained with experimental data. An analytical
minimal model is also developed, which conclusively shows that the enhancement
of transmission is due to tunneling through surface plasmons formed on each
metal-dielectric interfaces. Different regimes of tunneling (resonant through a
''surface plasmon molecule", or sequential through two isolated surface
plasmons) are found depending on the geometrical parameters defining the
system.Comment: 4 pages, 4 figure
Orbital and physical properties of the sigma Ori Aa, Ab, B triple system
We provide a complete characterization of the astrophysical properties of the sigma Ori Aa, Ab, B hierarchical triple system and an improved set of orbital parameters for the highly eccentric sigma Ori Aa, Ab spectroscopic binary. We compiled a spectroscopic data set comprising 90 high-resolution spectra covering a total time span of 1963 days. We applied the Lehman-Filhes method for a detailed orbital analysis of the radial velocity curves and performed a combined quantitative spectroscopic analysis of the sigma Ori Aa, Ab, B system by means of the stellar atmosphere code FASTWIND. We used our own plus other available information on photometry and distance to the system for measuring the radii, luminosities, and spectroscopic masses of the three components. We also inferred evolutionary masses and stellar ages using the Bayesian code BONNSAI. The orbital analysis of the new radial velocity curves led to a very accurate orbital solution of the sigma Ori Aa, Ab pair. We provided indirect arguments indicating that sigma Ori B is a fast-rotating early B dwarf. The FASTWIND+BONNSAI analysis showed that the Aa, Ab pair contains the hottest and most massive components of the triple system while sigma Ori B is a bit cooler and less massive. The derived stellar ages of the inner pair are intriguingly younger than the one widely accepted for the sigma Orionis cluster, at 3 +/- 1 Ma. The outcome of this study will be of key importance for a precise determination of the distance to the s Orionis cluster, the interpretation of the strong X-ray emission detected for sigma Ori Aa, Ab, B, and the investigation of the formation and evolution of multiple massive stellar systems and substellar objects
The cluster population of the irregular galaxy NGC 4449 as seen by the Hubble Advanced Camera for Surveys
We present a study of the star cluster population in the starburst irregular
galaxy NGC 4449 based on B, V, I, and Ha images taken with the Advanced Camera
for Surveys on the Hubble Space Telescope. We derive the cluster properties
such as size, ellipticity, and total magnitudes. Cluster ages and masses are
derived fitting the observed spectral energy distributions with different
population synthesis models. Our analysis is strongly affected by the
age-metallicity degeneracy; however, if we assume a metallicity of ~1/4 solar,
as derived from spectroscopy of HII regions, we find that the clusters have
ages distributed quite continuously over a Hubble time, and they have masses
from ~10^3 M_sun up to ~2 x 10^6 M_sun, assuming a Salpeters' IMF down to 0.1
M_sun. Young clusters are preferentially located in regions of young star
formation, while old clusters are distributed over the whole NGC 4449 field of
view, like the old stars (although we notice that some old clusters follow
linear structures, possibly a reflection of past satellite accretion). The high
SF activity in NGC 4449 is confirmed by its specific frequency of young massive
clusters, higher than the average value found in nearby spirals and in the LMC
(but lower than in other starburst dwarfs such as NGC 1705 and NGC 1569), and
by the flat slope of the cluster luminosity function (dN(L_V)\propto L_V^{-1.5}
dL for clusters younger than 1 Gyr). We use the upper envelope of the cluster
log(mass) versus log(age) distribution to quantify cluster disruption, and do
not find evidence for the high (90%) long-term infant mortality found by some
studies. For the red clusters, we find correlations between size, ellipticity,
luminosity and mass: brighter and more massive clusters tend to be more
compact, and brighter clusters tend to be also more elliptical.Comment: Accepted for publication on AJ, one data point changed in Fig. 1
Synthetic High-Resolution Line Spectra of Star-Forming Galaxies Below 1200A
We have generated a set of far-ultraviolet stellar libraries using spectra of
OB and Wolf-Rayet stars in the Galaxy and the Large and Small Magellanic Cloud.
The spectra were collected with the Far Ultraviolet Spectroscopic Explorer and
cover a wavelength range from 1003.1 to 1182.7A at a resolution of 0.127A. The
libraries extend from the earliest O- to late-O and early-B stars for the
Magellanic Cloud and Galactic libraries, respectively. Attention is paid to the
complex blending of stellar and interstellar lines, which can be significant,
especially in models using Galactic stars. The most severe contamination is due
to molecular hydrogen. Using a simple model for the H line strength, we
were able to remove the molecular hydrogen lines in a subset of Magellanic
Cloud stars. Variations of the photospheric and wind features of CIII 1176, OVI
1032, 1038, PV 1118, 1128, and SIV 1063, 1073, 1074 are discussed as a function
of temperature and luminosity class. The spectral libraries were implemented
into the LavalSB and Starburst99 packages and used to compute a standard set of
synthetic spectra of star-forming galaxies. Representative spectra are
presented for various initial mass functions and star formation histories. The
valid parameter space is confined to the youngest ages of less than 10 Myr for
an instantaneous burst, prior to the age when incompleteness of spectral types
in the libraries sets in. For a continuous burst at solar metallicity, the
parameter space is not limited. The suite of models is useful for interpreting
the restframe far-ultraviolet in local and high-redshift galaxies.Comment: 33 pages including 13 figures, accepted for publication in Ap
A randomized double-blind control study of early intra-coronary autologous bone marrow cell infusion in acute myocardial infarction: the REGENERATE-AMI clinical trial
Clinical trials suggest that intracoronary delivery of autologous bone marrow-derived cells (BMCs) 1–7 days post-acute myocardial infarction (AMI) may improve left ventricular (LV) function. Earlier time points have not been evaluated. We sought to determine the effect of intracoronary autologous BMC on LV function when delivered within 24 h of successful reperfusion therapy.
Methods and results
A multi-centre phase II randomized, double-blind, and placebo-controlled trial. One hundred patients with anterior AMI and significant regional wall motion abnormality were randomized to receive either intracoronary infusion of BMC or placebo (1:1) within 24 h of successful primary percutaneous intervention (PPCI). The primary endpoint was the change in left ventricular ejection fraction (LVEF) between baseline and 1 year as determined by advanced cardiac imaging. At 1 year, although LVEF increased compared with baseline in both groups, the between-group difference favouring BMC was small (2.2%; 95% confidence interval, CI: −0.5 to 5.0; P = 0.10). However, there was a significantly greater myocardial salvage index in the BMC-treated group compared with placebo (0.1%; 95% CI: 0.0–0.20; P = 0.048). Major adverse events were rare in both treatment groups.
Conclusion
The early infusion of intracoronary BMC following PPCI for patients with AMI and regional wall motion abnormality leads to a small non-significant improvement in LVEF when compared with placebo; however, it may play an important role in infarct remodelling and myocardial salvage.UK Stem Cells Foundation, the Heart Cells Foundation, and Barts and the London Charity. Funding to pay the Open Access publication charges for this article was provided by the Barts Cardiovascular Biomedical Research Unit (CVBRU)
O stars with weak winds: the Galactic case
We study the stellar and wind properties of a sample of Galactic O dwarfs to
track the conditions under which weak winds (i.e mass loss rates lower than ~
1e-8 Msol/yr) appear. The sample is composed of low and high luminosity dwarfs
including Vz stars and stars known to display qualitatively weak winds.
Atmosphere models including non-LTE treatment, spherical expansion and line
blanketing are computed with the code CMFGEN. Both UV and Ha lines are used to
derive wind properties while optical H and He lines give the stellar
parameters. Mass loss rates of all stars are found to be lower than expected
from the hydrodynamical predictions of Vink et al. (2001). For stars with log
L/Lsol > 5.2, the reduction is by less than a factor 5 and is mainly due to the
inclusion of clumping in the models. For stars with log L/Lsol < 5.2 the
reduction can be as high as a factor 100. The inclusion of X-ray emission in
models with low density is crucial to derive accurate mass loss rates from UV
lines. The modified wind momentum - luminosity relation shows a significant
change of slope around this transition luminosity. Terminal velocities of low
luminosity stars are also found to be low. The physical reason for such weak
winds is still not clear although the finding of weak winds in Galactic stars
excludes the role of a reduced metallicity. X-rays, through the change in the
ionisation structure they imply, may be at the origin of a reduction of the
radiative acceleration, leading to lower mass loss rates. A better
understanding of the origin of X-rays is of crucial importance for the study of
the physics of weak winds.Comment: 31 pages, 42 figures. A&A accepted. A version of the paper with full
resolution figures is available at
http://www.mpe.mpg.de/~martins/publications.htm
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