4,156 research outputs found
On a new theoretical framework for RR Lyrae stars I: the metallicity dependence
We present new nonlinear, time-dependent convective hydrodynamical models of
RR Lyrae stars computed assuming a constant helium-to-metal enrichment ratio
and a broad range in metal abundances (Z=0.0001--0.02). The stellar masses and
luminosities adopted to construct the pulsation models were fixed according to
detailed central He burning Horizontal Branch evolutionary models. The
pulsation models cover a broad range in stellar luminosity and effective
temperatures and the modal stability is investigated for both fundamental and
first overtones. We predict the topology of the instability strip as a function
of the metal content and new analytical relations for the edges of the
instability strip in the observational plane. Moreover, a new analytical
relation to constrain the pulsation mass of double pulsators as a function of
the period ratio and the metal content is provided. We derive new
Period-Radius-Metallicity relations for fundamental and first-overtone
pulsators. They agree quite well with similar empirical and theoretical
relations in the literature. From the predicted bolometric light curves,
transformed into optical (UBVRI) and near-infrared (JHK) bands, we compute the
intensity-averaged mean magnitudes along the entire pulsation cycle and, in
turn, new and homogenous metal-dependent (RIJHK) Period-Luminosity relations.
Moreover, we compute new dual and triple band optical, optical--NIR and NIR
Period-Wesenheit-Metallicity relations. Interestingly, we find that the optical
Period-W(V,B-V) is independent of the metal content and that the accuracy of
individual distances is a balance between the adopted diagnostics and the
precision of photometric and spectroscopic datasets.Comment: 51 pages, 20 figures, 9 tables, accepted for publication on Ap
Ruptured aortic arch aneurysm: transposition of aortic arch branches after insertion of thoracic endovascular stent with extra-anatomic brain perfusion
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Small-angle neutron scattering (SANS) characterization of 13.5 Cr oxide dispersion strengthened ferritic steel for fusion applications
Small-angle neutron scattering (SANS) has been utilized for micro-structural investigation on laboratory heats of oxide dispersion strengthened (ODS) 13.5 Cr wt % ferritic steel, with 0.3 wt% YO and with variable Ti and W contents. The results show that increasing the Ti content from 0.2 to 0.4 wt% a distribution of nano-clusters develops, tentatively identified as YTiO, with average radii as small as 6.5 Å and volume fractions increasing from 0.021 to 0.032. The measured SANS cross-sections show also the growth of much larger defects, possibly Cr oxides. Furthermore, the ratio of magnetic to nuclear SANS components shows that the defect composition varies both with their size and with the Ti and the W content. These results are in qualitative agreement with transmission electron microscopy (TEM) observations, showing a striking influence of Ti addition on particle size refinement. However, while TEM is limited in statistics and minimum observable size of the Ti-rich nano-clusters, the defect distributions obtained by these SANS measurements provide complementary information useful for morphological characterization of the micro-structure in the investigated material
The Stationary Phase Method for a Wave Packet in a Semiconductor Layered System. The applicability of the method
Using the formal analysis made by Bohm in his book, {\em "Quantum theory"},
Dover Publications Inc. New York (1979), to calculate approximately the phase
time for a transmitted and the reflected wave packets through a potential
barrier, we calculate the phase time for a semiconductor system formed by
different mesoscopic layers. The transmitted and the reflected wave packets are
analyzed and the applicability of this procedure, based on the stationary phase
of a wave packet, is considered in different conditions. For the applicability
of the stationary phase method an expression is obtained in the case of the
transmitted wave depending only on the derivatives of the phase, up to third
order. This condition indicates whether the parameters of the system allow to
define the wave packet by its leading term. The case of a multiple barrier
systems is shown as an illustration of the results. This formalism includes the
use of the Transfer Matrix to describe the central stratum, whether it is
formed by one layer (the single barrier case), or two barriers and an inner
well (the DBRT system), but one can assume that this stratum can be comprise of
any number or any kind of semiconductor layers.Comment: 15 pages, 4 figures although figure 4 has 5 graph
Estimation of vegetation cover resilience from satellite time series
Resilience is a fundamental concept for understanding vegetation as a dynamic component of the climate system. It expresses the ability of ecosystems to tolerate disturbances and to recover their initial state. Recovery times are basic parameters of the vegetation's response to forcing and, therefore, are essential for describing realistic vegetation within dynamical models. Healthy vegetation tends to rapidly recover from shock and to persist in growth and expansion. On the contrary, climatic and anthropic stress can reduce resilience thus favouring persistent decrease in vegetation activity. <br><br> In order to characterize resilience, we analyzed the time series 1982–2003 of 8 km GIMMS AVHRR-NDVI maps of the Italian territory. Persistence probability of negative and positive trends was estimated according to the vegetation cover class, altitude, and climate. Generally, mean recovery times from negative trends were shorter than those estimated for positive trends, as expected for vegetation of healthy status. Some signatures of inefficient resilience were found in high-level mountainous areas and in the Mediterranean sub-tropical ones. This analysis was refined by aggregating pixels according to phenology. This multitemporal clustering synthesized information on vegetation cover, climate, and orography rather well. The consequent persistence estimations confirmed and detailed hints obtained from the previous analyses. Under the same climatic regime, different vegetation resilience levels were found. In particular, within the Mediterranean sub-tropical climate, clustering was able to identify features with different persistence levels in areas that are liable to different levels of anthropic pressure. Moreover, it was capable of enhancing reduced vegetation resilience also in the southern areas under Warm Temperate sub-continental climate. The general consistency of the obtained results showed that, with the help of suited analysis methodologies, 8 km AVHRR-NDVI data could be useful for capturing details on vegetation cover activity at local scale even in complex territories such as that of the Italian peninsula
Micro-structural effects of irradiation temperature and helium content in neutron irradiated B-alloyed Eurofer97-1 steel
The micro-structural effects of different neutron irradiation temperatures and helium contents, for 16 dpa dose, have been investigated by means of small-angle neutron scattering (SANS) in B-alloyed ferritic/martensitic steel Eurofer97-1 (0.12 C, 9 Cr, 0.2 V, 1.08 W wt%, B concentrations up to 1000 ppm); due to B transmutations, fusion relevant He/dpa values are expected to be produced under neutron irradiation. SANS measurements have been carried out on a sample irradiated at 350 °C, with estimated helium content of 5600 appm, and compared to previous SANS results, obtained on two other irradiated samples of this same B-alloyed steel. These new measurements confirm that for such high helium contents the SANS cross-section increases in order of magnitude and the magnetic SANS component is strongly reduced, compared to lower helium content (400 appm). Such effects are attributed to increase in helium bubbles density and to the presence of micro-cavities, produced after dissolution of large B-carbides. The SANS data analysis procedure has been improved, also thanks to the additional information provided by the new measurements, and more accurate helium bubble size distributions have been obtained for all the investigated samples. For 5600 appm helium content, bubble volume fractions are found of 0.025 for the sample irradiated at 350 °C and of 0.041 for the previously investigated sample irradiated at 400 °C, significantly increasing with the irradiation temperature. These values are approximately one order of magnitude larger than the value of 0.003 previously found for the sample with 400 appm helium. The size distributions are compared with electron microscopy observations of these same samples. It appears that the occurrence of complex micro-structural changes in irradiated Eurofer97-1 steel should be taken in due account when considering its application under high He/dpa ratio values. Keywords: Helium effects, Neutron irradiation, Small angle neutron scattering, Electron microscop
Shedding Light on Diatom Photonics by means of Digital Holography
Diatoms are among the dominant phytoplankters in the worl's ocean, and their
external silica investments, resembling artificial photonics crystal, are
expected to play an active role in light manipulation. Digital holography
allowed studying the interaction with light of Coscinodiscus wailesii cell wall
reconstructing the light confinement inside the cell cytoplasm, condition that
is hardly accessible via standard microscopy. The full characterization of the
propagated beam, in terms of quantitative phase and intensity, removed a
long-standing ambiguity about the origin of the light. The data were discussed
in the light of living cell behavior in response to their environment
Effects of Acute Cadmium Exposure on the Pituitary Gland of Podarcis sicula
Reptiles are rarely used in studies on the possible toxic effects of heavy metals even if they are susceptible to
the accumulation of persistent pollution due also to their presence in a variety of habitats. Cadmium is a heavy metal, a
significant environmental pollutant and an endocrine disruptor. Therefore the aim of this study was to analyze the
cytotoxic effects of cadmium on the pituitary gland of the lizard Podarcis sicula after an acute exposure to this metal. The
analysis were carried out after 2, 7 and 16 days following the intraperitoneal injection of a single and massive dose of
cadmium chloride. The pituitary glands were analyzed by histological and immuhistochemical stains. Besides cadmium
accumulation in brain was measured by atomic absorption spectrometry. Cadmium concentration increased in lizard brain
lightly after 2 days and widely after 16 days. The tissue of the pituitary gland appeared slightly atrophied in a few areas
only at 7 and 16 days after treatment. Moreover an increase in intensity of immunostaining and occurrence of some
adenohypophyseal cells was revealed respect to control lizards. This evidence suggests an inhibitory effect of cadmium on
the normal hormonal secretion. Evidently an acute cadmium exposure in P. sicula involves the accumulation of this metal
in the brain but also the alteration of the normal endocrine function of the pituitary gland
The Carina Project. X. On the kinematics of old and intermediate-age stellar populations
We present new radial velocity (RV) measurements of old (horizontal branch)
and intermediate-age (red clump) stellar tracers in the Carina dwarf
spheroidal. They are based on more than 2,200 low-resolution spectra collected
with VIMOS at VLT. The targets are faint (20<V<21.5 mag), but the accuracy at
the faintest limit is <9 kms-1. These data were complemented with RV
measurements either based on spectra collected with FORS2 and FLAMES/GIRAFFE at
VLT or available in the literature. We ended up with a sample of 2748 stars and
among them 1389 are candidate Carina stars. We found that the intermediate-age
stellar component shows a well defined rotational pattern around the minor
axis. The western and the eastern side of the galaxy differ by +5 and -4 km s-1
when compared with the main RV peak. The old stellar component is characterized
by a larger RV dispersion and does not show evidence of RV pattern. We compared
the observed RV distribution with N-body simulations for a former disky dwarf
galaxy orbiting a giant MilkyWay-like galaxy (Lokas et al. 2015). We rotated
the simulated galaxy by 60 degrees with respect to the major axis, we kept the
observer on orbital plane of the dwarf and extracted a sample of stars similar
to the observed one. Observed and predicted Vrot/{\sigma} ratios across the
central regions are in remarkable agreement. This evidence indicates that
Carina was a disky dwarf galaxy that experienced several strong tidal
interactions with the Milky Way. Owing to these interactions, Carina
transformed from a disky to a prolate spheroid and the rotational velocity
transformed into random motions.Comment: 21 pages, 16 figures, 4 tables. Accepted for publication in Ap
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