781 research outputs found
Modelling of the circulation in the Northwestern Mediterranean Sea with the Princeton Ocean Model
International audienceThe Princeton Ocean Model ? POM (Blumberg and Mellor, 1987) has been implemented in the Northwestern Mediterranean nested (in one-way off-line mode) to a general circulation model of the Mediterranean Sea ? OGCM (Pinardi and Masetti, 2000; Demirov and Pinardi, 2002) in order to investigate if this model configuration is capable of reproducing the major features of the circulation as known from observations and to improve what has been made by previous numerical modeling works. According to the model results, the large-scale cyclonic circulation in the northern part of the Northwestern Mediterranean is, at least in the upper layers, less coherent in winter and spring than in summer and autumn. Furthermore, there is evidence that the mesoscale structure (eddies and meanders) is, during all year, a significant dynamic characteristic in this region of the Mediterranean Sea. Finally, concerning the circulation in the lower layers has been confirmed that the Levantine Intermediate Water and the Western Mediterranean Deep Water follow essentially a cyclonic path during all year
CCD Washington photometry of four poorly studied open clusters in the two inner quadrants of the galactic plane
Complementing our Washington photometric studies on Galactic open clusters
(OCs), we now focus on four poorly studied OCs located in the first and fourth
Galactic quadrants, namely BH 84, NGC 5381, BH 211 and Czernik 37. We have
obtained CCD photometry in the Washington system and passbands down
to 18.5 magnitudes for these four clusters. Their positions and
sizes were determined using the stellar density radial profiles. We derived
reddening, distance, age and metallicity of the clusters from extracted
color-magnitude diagrams (CMDs), using theoretical isochrones
computed for the Washington system. There are no previous photometric data in
the optical band for BH 84, NGC 5381 and BH 211. The CMDs of the observed
clusters show relatively well defined main sequences, except for Czernik 37,
wherein significant differential reddening seems to be present. The red giant
clump is clearly seen only in BH 211. For this cluster, we estimated the age in
(1000) Myr, assuming a metallicity of = 0.019. BH 84 was
found to be much older than it was previously believed, while NGC 5381 happened
to be much younger than previously reported. The heliocentric distances to
these clusters are found to range between 1.4 and 3.4 kpc. BH 84 appears to be
located at the solar galactocentric distance, while NGC 5381, BH 211 and
Czernik 37 are situated inside the solar ring.Comment: 30 pages, 9 figures, 10 table
Spectral evolution of star clusters in the Large Magellanic Cloud: I. Blue concentrated clusters in the age range 40-300 Myr
Integrated spectroscopy of a sample of 17 blue concentrated Large Magellanic
Cloud (LMC) clusters is presented and its spectral evolution studied. The
spectra span the range ~3600-6800A with a resolution of ~14A FWHM, being used
to determine cluster ages and, in connection with their spatial distribution,
to explore the LMC structure and cluster formation history. Cluster reddening
values were estimated by interpolation, using the available extinction maps. We
used two methods to derive cluster ages: (i) template matching, in which line
strengths and continuum distribution of the cluster spectra were compared and
matched to those of template clusters with known astrophysical properties, and
(ii) equivalent width (EW) method, in which new age/metallicity calibrations
were used together with diagnostic diagrams involving the sum of EWs of
selected spectral lines (KCaII, G band (CH), MgI, Hdelta, Hgamma and Hbeta).
The derived cluster ages range from 40Myr (NGC2130 and SL237) to 300Myr
(NGC1932 and SL709), a good agreement between the results of the two methods
being obtained. Combining the present sample with additional ones indicates
that cluster deprojected distances from the LMC center are related to age in
the sense that inner clusters tend to be younger. Spectral libraries of star
clusters are useful datasets for spectral classifications and extraction of
parameter information for target star clusters and galaxies. The present
cluster sample complements previous ones, in an effort to gather a spectral
library with several clusters per age bin.Comment: 13 pages, 22 figures. Accepted for publication in A&
A tunable single photon quantum router
We propose an efficient single-photon router comprising two resonator
waveguide channels coupled by several sequential cavities with embedded
three-level atoms. We show that the system can operate as a perfect four-way
single-photon switch. We also demonstrate that an incident single-photon
propagating in one of the waveguides can be routed into one or the other output
channels; such routing can be controlled by the external classical
electromagnetic field driving the atoms. We argue that, under appropriate
conditions, the efficiency of such routing can be close to 100\% within a broad
operational bandwidth, suggesting various applications in photonics.Comment: 8 pages, 8 figure
Mutation and SARS-CoV-2 strain competition under vaccination in a modified SIR model
The crisis caused by the COVID-19 outbreak around the globe raised an
increasing concern about the ongoing emergence of variants of SARS-CoV-2 that
may evade the immune response provided by vaccines. New variants appear due to
mutation, and as the cases accumulate, the probability of the emergence of a
variant of concern increases. In this article, we propose a modified SIR model
with waning immunity that captures the competition of two strain classes of an
infectious disease under the effect of vaccination with a highly contagious and
deadly strain class emerging from a prior strain due to mutation. When these
strains compete for a limited supply of susceptible individuals, changes in the
efficiency of vaccines may affect the behaviour of the disease in a non-trivial
way, resulting in complex outcomes. We characterise the parameter space
including intrinsic parameters of the disease, and using the vaccine
efficiencies as control variables. We find different types of transcritical
bifurcations between endemic fixed points and a disease-free equilibrium and
identify a region of strain competition where the two strain classes coexist
during a transient period. We show that a strain can be extinguished either due
to strain competition or vaccination, and we obtain the critical values of the
efficiency of vaccines to eradicate the disease. Numerical studies using
parameters estimated from publicly reported data agree with our theoretical
results. Our mathematical model could be a tool to assess quantitatively the
vaccination policies of competing and emerging strains using the dynamics in
epidemics of infectious diseases.Comment: 28 pages, 8 figure
A super lithium-rich red-clump star in the open cluster Trumpler 5
Context. The existence of lithium-rich low-mass red giant stars still
represents a challenge for stellar evolution models. Stellar clusters are
privileged environments for this kind of investigation. Aims. To investigate
the chemical abundance pattern of the old open cluster Trumpler\,5, we observed
a sample of four red-clump stars with high-resolution optical spectrographs.
One of them (#3416) reveals extremely strong lithium lines in its spectrum.
Methods. One-dimensional, local thermodynamic equilibrium analysis was
performed on the spectra of the observed stars. A 3D-NLTE analysis was
performed to derive the lithium abundance of star #3416. Results. Star #3416 is
super Li-rich with A(Li)=3.75\,dex. The lack of Li enrichment
(Li/Li2%), the low carbon isotopic ratio
(C/C=143), and the lack of evidence for radial velocity
variation or enhanced rotational velocity (\kms) all suggest
that lithium production has occurred in this star through the Cameron & Fowler
mechanism. Conclusions. We identified a super Li-rich core helium-burning,
red-clump star in an open cluster. Internal production is the most likely cause
of the observed enrichment. Given the expected short duration of a star's
Li-rich phase, enrichment is likely to have occurred at the red clump or in the
immediately preceding phases, namely during the He-flash at the tip of the red
giant branch (RGB) or while ascending the brightest portion of the RGB.Comment: 7 pages, 2 figures. Accepted for publication in A&A Letter
Promoting collaboration in a competitive context: school improvement networks in Chile
ABSTRACT: Chile has developed the school improvement networks (SINs) strategy to support the work of school leaders. The purpose of this paper is to analyze the functioning and effect of the SINs strategy as perceived by principals and curriculum coordinators across the country. DESIGN/METHODOLOGY/APPROACH: An adapted version of the Educational Collaborative Network Questionnaire was applied to a sample of 1,723 participants from 1,375 schools distributed in 398 networks. Descriptive, factor and sub-group statistical analyses by school performance categories and by different roles within these schools and networks are presented. FINDINGS: Results indicate that school leaders perceive SINs as an opportunity to work effectively in shared projects that can later be implemented in their own schools. Participants indicate that they can share knowledge in their networks and use it to solve problems in their own schools, which is especially relevant for secondary school leaders who work in difficult circumstances. Results suggest that it is important to facilitate greater autonomy for school leaders in their networks, especially regarding decision making about network goals and activities that are more significant to their contexts. ORIGINALITY/VALUE: This is a national study of a recent school improvement strategy, which provides evidence, from the perspective of school leaders, of its strengths and improvement areas. This study shows that despite being in a competitive context, principals and curriculum coordinators value the opportunities to learn from and with others. These results can be of value for other contexts attempting to promote school networks as a means for school and system improvement
NGC 2849 and NGC 6134: two more BOCCE open clusters
We present CCD photometry of two southern open clusters. As part of the Bologna Open Cluster Chemical Evolution project we obtained BVI and UBVI imaging for NGC 2849 and NGC 6134, respectively. By means of the synthetic colour-magnitude diagram method and using various evolutionary sets of stellar evolution tracks with various metallicities, we determined at the same time age, distance and reddening. We also determined an approximate metallicity for NGC 2849, for which the information is not available from sounder methods like high-resolution spectroscopy. NGC 2849 turned to be 0.85-1.0 Gyr old with a solar metallicity. The foreground reddening is E(B - V) = 0.28 - 0.32, and the true distance modulus (m - M) 0 = 13.8-13.9. For NGC 6134 we did not obtain fully consistent answers from the V, B - V and V, V - I photometry, an unexpected problem, since both the metallicity and the reddening are known (from high-resolution spectroscopy and the U - B, B - V two colours diagram, respectively). This may either indicate a difficulty of current models (evolutionary tracks and/or models of atmosphere) to accurately reproduce colours, or be related to differences in the metal mixture assumed by the models and those of the clusters. Assuming the spectroscopic abundance and the colour excess [E(B - V) = 0.35] from the U - B, B - V plot, we derived a best age between 0.82 and 0.95 Gyr and a distance modulus 10.5. In agreement with previous studies, the NGC 6134 colour-magnitude diagram shows also a clear main sequence gap at V ˜ 15 and B-V ˜ 0.9-1.0 that is unexplained by canonical stellar evolution models.Fil: Ahumada, Andrea Veronica. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Cordoba. Observatorio Astronomico de Cordoba. Departamento de Astrofisica Estelar; ArgentinaFil: Cignoni, M.. Universitá di Bologna. Dipartimento di Astronomia; Italia;Fil: Bragaglia, A.. INAF-Osservatorio Astronomico di Bologna; Italia;Fil: Donati, P.. Universitá di Bologna. Dipartimento di Astronomia; Italia;Fil: Tosi, M.. INAF-Osservatorio Astronomico di Bologna; Italia;Fil: Marconi, G.. European Southern Observatory (ESO); Chile
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