1,138 research outputs found
Evidence of diffusive fractal aggregation of TiO2 nanoparticles by femtosecond laser ablation at ambient conditions
The specific mechanisms which leads to the formation of fractal
nanostructures by pulsed laser deposition remain elusive despite intense
research efforts, motivated mainly by the technological interest in obtaining
tailored nanostructures with simple and scalable production methods. Here we
focus on fractal nanostructures of titanium dioxide, , a strategic
material for many applications, obtained by femtosecond laser ablation at
ambient conditions. We model the fractal formation through extensive Monte
Carlo simulations based on a set of minimal assumptions: irreversible sticking
and size independent diffusion. Our model is able to reproduce the fractal
dimensions and the area distributions of the nanostructures obtained in the
experiments for different densities of the ablated material. The comparison of
theory and experiment show that such fractal aggregates are formed after
landing of the ablated material on the substrate surface by a diffusive
mechanism. Finally we discuss the role of the thermal conductivity of the
substrate and the laser fluence on the properties of the fractal
nanostructures. Our results represent an advancement towards controlling the
production of fractal nanostructures by pulsed laser deposition.Comment: 21 page
The building up of the disk galaxy M33 and the evolution of the metallicity gradient
The evolution of radial gradients of metallicity in disk galaxies and its
relation with the disk formation are not well understood. Theoretical models of
galactic chemical evolution make contrasting predictions about the time
evolution of metallicity gradients. To test chemical evolution models and trace
the star formation and accretion history of low luminosity disk galaxies we
focus on the Local Group galaxy M33. We analyze O/H and S/H abundances in
planetary nebulae, H{\sc ii} regions, and young stars, together with known
[Fe/H] abundances in the old stellar population of M33. With a theoretical
model, we follow the time evolution of gas (diffuse and condensed in clouds),
stars, and chemical abundances in the disk of M33, assuming that the galaxy is
accreting gas from an external reservoir. Our model is able to reproduce the
available observational constraints on the distribution of gas and stars in M33
and to predict the time evolution of several chemical abundances. In
particular, we find that a model characterized by a continuous infall of gas on
the disk, at a rate of yr, almost
constant with time, can also account for the relatively high rate of star
formation and for the shallow chemical gradients. Supported by a large sample
of high resolution observations for this nearby galaxy, we conclude that the
metallicity in the disk of M33 has increased with time at all radii, with a
continuous flattening of the gradient over the last Gyr.Comment: 16 pages, 11 figures, A&A accepte
Metal production in M33: space and time variations
Nearby galaxies are ideal places to study in detail metallicity gradients and
their time evolution. We consider chemical abundances of a new sample of \hii\
regions complemented with previous literature data-sets. We compare \hii\
region and PN abundances obtained with a common set of observations taken at
MMT. With an updated theoretical model, we follow the time evolution of the
baryonic components and chemical abundances in the disk of M33, assuming that
the galaxy is accreting gas from an external reservoir. Supported by a uniform
sample of nebular spectroscopic observations, we conclude that: {\em i}) the
metallicity distribution in M33 is very complex, showing a central depression
in metallicity probably due to observational bias; {\em ii}) the metallicity
gradient in the disk of M33 has a slope of -0.037 0.009 dex kpc in
the whole radial range up to 8 kpc, and -0.044 0.009 dex kpc
excluding the central kpc; {\em iii}) there is a small evolution of the slope
with time from the epoch of PN progenitor formation to the present-time.}Comment: A&A accepted, 15 Pags, 13 Figs, language correctio
Genetic variability among Blastoschizomyces capitatus isolates from different clinical sources.
Sixteen clinical isolates and nine ATCC reference strains of Blastoschizomyces capitatus were analysed genetically, examined for the cellobiose, arbutin and salicin assimilation and tested for the aspartyl-proteinase secretion. The restriction endonuclease analysis (REA) with HpaII and HinfI enzymes and the electrophoretic karyotype (EK) were investigated. Both the restriction enzymes revealed two groups (I, II) constituted by the same isolates: 17 isolates (68%) in group I and 8 (32%) in group II. The EK analysis revealed sixteen groups. These data prompts for a genetic variability of the isolates of Blastoschizomyces capitatus and their account in two distinct genetic groups as suggested by REA. This grouping was confirmed by examing the utilisation of cellobiose, arbutin and salicin. The tests for secretory aspartyl proteinase (Sap) were positive only for three isolates, suggesting a marginal role of this specific enzyme in pathogenesis for these isolates
Thermo-mechanical behavior of surface acoustic waves in ordered arrays of nanodisks studied by near infrared pump-probe diffraction experiments
The ultrafast thermal and mechanical dynamics of a two-dimensional lattice of
metallic nano-disks has been studied by near infrared pump-probe diffraction
measurements, over a temporal range spanning from 100 fs to several
nanoseconds. The experiments demonstrate that, in these systems, a
two-dimensional surface acoustic wave (2DSAW), with a wavevector given by the
reciprocal periodicity of the array, can be excited by ~120 fs Ti:sapphire
laser pulses. In order to clarify the interaction between the nanodisks and the
substrate, numerical calculations of the elastic eigenmodes and simulations of
the thermodynamics of the system are developed through finite-element analysis.
At this light, we unambiguously show that the observed 2DSAW velocity shift
originates from the mechanical interaction between the 2DSAWs and the
nano-disks, while the correlated 2DSAW damping is due to the energy radiation
into the substrate.Comment: 13 pages, 10 figure
Galactic chemical evolution of heavy elements: from Barium to Europium
We follow the chemical evolution of the Galaxy for elements from Ba to Eu,
using an evolutionary model suitable to reproduce a large set of Galactic
(local and non local) and extragalactic constraints. Input stellar yields for
neutron-rich nuclei have been separated into their s-process and r-process
components. The production of s-process elements in thermally pulsing
asymptotic giant branch stars of low mass proceeds from the combined operation
of two neutron sources: the dominant reaction 13C(alpha,n)16O, which releases
neutrons in radiative conditions during the interpulse phase, and the reaction
22Ne(alpha,n)25Mg, marginally activated during thermal instabilities. The
resulting s-process distribution is strongly dependent on the stellar
metallicity. For the standard model discussed in this paper, it shows a sharp
production of the Ba-peak elements around Z = Z_sun/4. Concerning the r-process
yields, we assume that the production of r-nuclei is a primary process
occurring in stars near the lowest mass limit for Type II supernova
progenitors. The r-contribution to each nucleus is computed as the difference
between its solar abundance and its s-contribution given by the Galactic
chemical evolution model at the epoch of the solar system formation. We compare
our results with spectroscopic abundances of elements from Ba to Eu at various
metallicities (mainly from F and G stars) showing that the observed trends can
be understood in the light of the present knowledge of neutron capture
nucleosynthesis. Finally, we discuss a number of emerging features that deserve
further scrutiny.Comment: 34 pages, 13 figures. accepted by Ap
EEG correlates of social interaction at distance
This study investigated EEG correlates of social interaction at distance between twenty-five pairs of participants who were not connected by any traditional channels of communication.
Each session involved the application of 128 stimulations separated by intervals of random duration ranging from 4 to 6 seconds. One of the pair received a one-second stimulation from a light signal produced by an arrangement of red LEDs, and a simultaneous 500 Hz sinusoidal audio signal of the same length. The other member of the pair sat in an isolated sound-proof room, such that any sensory interaction between the pair was impossible.
An analysis of the Event-Related Potentials associated with sensory stimulation using traditional averaging methods showed a distinct peak at approximately 300 ms, but only in the EEG activity of subjects who were directly stimulated. However, when a new algorithm was applied to the EEG activity based on the correlation between signals from all active electrodes, a weak but robust response was also detected in the EEG activity of the passive member of the pair, particularly within 9 – 10 Hz in the Alpha range. Using the Bootstrap method and the Monte Carlo emulation, this signal was found to be statistically significant
The stellar populations of spiral disks.II Measuring and modeling the radial distribution of absorption spectral indices
The radial distributions of the Mg2 and Fe5270 Lick spectral indices have
been measured to large radial distances on the disks of NGC 4303 and NGC 4535
using an imaging technique based on interference filters. These data, added to
those of NGC 4321 previously published in Paper I of this series are used to
constraint chemical (multiphase) evolutionary models for these galaxies.
Because the integrated light of a stellar disk is a time average over the
history of the galaxy weighted by the star formation rate, these constraints
complement the information on chemical gradients provided by the study of HII
regions which, by themselves, can only provide the alpha-elements abundance
accumulate over the life of the galaxy. The agreement between the observations
and the model predictions shown here lends confidence to the models which are
then used to describe the time evolution of galaxy parameters such as star
formation rates, chemical gradients, and gradients in the mean age of the
stellar population.Comment: to be published in Astrophysical Journa
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