420 research outputs found
Discovery of X-ray emission from the proto-stellar jet L1551 IRS5 (HH 154)
We have for the first time detected X-ray emission associated with a
proto-stellar jet, on the jet emanating from L1551 IRS5. The IRS5 proto-star is
hidden beyond a very large absorbing column density, making the direct
observation of the jet's emission possible. The observed X-ray emission is
likely associated with the shock ``working surface'', i.e. the interface
between the jet and the circumstellar medium. The X-ray luminosity emanating
from the jet is moderate, at LX ~ 3 times 10^29 erg/s, a significant fraction
of the luminosity normally associated with the coronal emission from young
stars. The spectrum of the X-ray emission is compatible with thermal emission
from a hot plasma, with T ~ 0.5 MK, fully compatible with the temperature
expected (on the basis of the jet's velocity) for the shock front produced by
the jet hitting the circumstellar medium.Comment: To appear in "Stellar Coronae in the Chandra and XMM Era", ASP
Conference Series in press, F. Favata & J. Drake ed
Static and Dynamic Anomalies in a Repulsive Spherical Ramp Liquid: Theory and Simulation
We compare theoretical and simulation results for static and dynamic
properties for a model of particles interacting via a spherically symmetric
repulsive ramp potential. The model displays anomalies similar to those found
in liquid water, namely, expansion upon cooling and an increase of diffusivity
upon compression. In particular, we calculate the phase diagram from the
simulation and successfully compare it with the phase diagram obtained using
the Rogers-Young (RY) closure for the Ornstein-Zernike equation. Both the
theoretical and the numerical calculations confirm the presence of a line of
isobaric density maxima, and lines of compressibility minima and maxima.
Indirect evidence of a liquid-liquid critical point is found. Dynamic
properties also show anomalies. Along constant temperature paths, as the
density increases, the dynamics alternates several times between slowing down
and speeding up, and we associate this behavior with the progressive
structuring and de-structuring of the liquid. Finally we confirm that mode
coupling theory successfully predicts the non-monotonic behavior of dynamics
and the presence of multiple glass phases, providing strong evidence that
structure (the only input of mode coupling theory) controls dynamics.Comment: Static and Dynamic Anomalies in a Repulsive Spherical Ramp Liquid:
Theory and Simulatio
Transitions between Inherent Structures in Water
The energy landscape approach has been useful to help understand the dynamic
properties of supercooled liquids and the connection between these properties
and thermodynamics. The analysis in numerical models of the inherent structure
(IS) trajectories -- the set of local minima visited by the liquid -- offers
the possibility of filtering out the vibrational component of the motion of the
system on the potential energy surface and thereby resolving the slow
structural component more efficiently. Here we report an analysis of an IS
trajectory for a widely-studied water model, focusing on the changes in
hydrogen bond connectivity that give rise to many IS separated by relatively
small energy barriers. We find that while the system \emph{travels} through
these IS, the structure of the bond network continuously modifies, exchanging
linear bonds for bifurcated bonds and usually reversing the exchange to return
to nearly the same initial configuration. For the 216 molecule system we
investigate, the time scale of these transitions is as small as the simulation
time scale ( fs). Hence for water, the transitions between each of
these IS is relatively small and eventual relaxation of the system occurs only
by many of these transitions. We find that during IS changes, the molecules
with the greatest displacements move in small ``clusters'' of 1-10 molecules
with displacements of nm, not unlike simpler liquids.
However, for water these clusters appear to be somewhat more branched than the
linear ``string-like'' clusters formed in a supercooled Lennar d-Jones system
found by Glotzer and her collaborators.Comment: accepted in PR
Occurrence and antimicrobial resistance of arcobacter spp. Recovered from aquatic environments
Arcobacter spp. are emerging waterborne and foodborne zoonotic pathogens responsible for gastroenteritis in humans. In this work, we evaluated the occurrence and the antimicrobial resistance profile of Arcobacter isolates recovered from different aquatic sources. Besides, we searched for Arcobacter spp. in seaweeds and the corresponding seawater samples. Bacteriological and molecular methods applied to 100 samples led to the isolation of 28 Arcobacter isolates from 27 samples. The highest prevalence was detected in rivers followed by artificial ponds, streams, well waters, and spring waters. Seaweeds contained a higher percentage of Arcobacter than the corresponding sea-water samples. The isolates were identified as Arcobacter butzleri (96.4%) and Arcobacter cryaerophilus (3.6%). All the isolates showed a multi-drug resistance profile, being resistant to at least three different classes of antibiotics. Molecular analysis of genetic determinants responsible for tetracycline resistance in nine randomly chosen isolates revealed the presence of tetO and/or tetW. This work confirms the occurrence and the continuous emergence of antibiotic-resistant Arcobacter strains in environmental samples; also, the presence of quinolone-resistant Arcobacter spp. in aquatic sources used for water supply and irrigation represents a potential risk for human health
Behavior of four main dairy pathogenic bacteria during manufacturing and ripening of pecorino siciliano cheese
Background: Consumption of raw cheese may be associated with different diseases. This study aimed to evaluate behavior of four pathogenic bacteria during manufacture and ripening of Protected Designation of Origin (PDO) Pecorino Siciliano cheese. Methods: The experimental cheese groups were inoculated with pathogenic bacteria, including Escherichia coli O157, Listeria monocytogenes, Salmonella Enteritidis, and Staphylococcus aureus. The cheese making processes were monitored from milk curdling until 3 months ripened cheeses and the levels of Lactic Acid Bacteria (LAB) and the four dairy pathogens were evaluated by plate counts. Randomly Amplified Polymorphic DNA (RAPD)-Polymerase Chain Reaction (PCR) analysis was applied to confirm that the colonies isolated during the several steps of production were the same strains added in milk. Statistical analysis was done using XLStat software. Results: The levels of mesophilic and thermophilic coccus and rod LAB in curd were comparable in both trials and reached values between 8-9 log10 Colony Forming Unit (CFU)/g in cheeses at 90 days of ripening. The four pathogenic bacteria were found in experimental curd at levels higher than those inoculated in milk and completely disappeared after 60 days of ripening. The RAPD analysis clearly demonstrated the presence of the added strain during production and confirmed the results of plate counts. Conclusion: This work showed that the production conditions of PDO Pecorino Siciliano cheese decreased growth of E. coli O157, L. monocytogenes, S. Enteritidis, and S. aureus
Photoevaporation and close encounters: how the environment around Cygnus OB2 affects the evolution of protoplanetary disks
In our Galaxy, star formation occurs in a variety of environments, with a
large fraction of stars formed in clusters hosting massive stars. OB stars have
an important feedback on the evolution of protoplanetary disks around nearby
young stars and likely on the process of planet formation occurring in them.
The nearby massive association Cygnus OB2 is an outstanding laboratory to study
this feedback. It is the closest massive association to our Sun, and hosts
hundreds of massive stars and thousands of low mass members. In this paper, we
analyze the spatial variation of the disk fraction in Cygnus OB2 and we study
its correlation with the local values of Far and Extreme ultraviolet radiation
fields and the local stellar surface density. We present definitive evidence
that disks are more rapidly dissipated in the regions of the association
characterized by intense local UV field and large stellar density. In
particular, the FUV radiation dominates disks dissipation timescales in the
proximity (i.e. within 0.5 pc) of the O stars. In the rest of the association,
EUV photons potentially induce a significant mass loss from the irradiated
disks across the entire association, but the efficiency of this process is
reduced at increasing distances from the massive stars due to absorption by the
intervening intracluster material. We find that disk dissipation due to close
stellar encounters is negligible in Cygnus OB2, and likely to have affected 1%
or fewer of the stellar population. Disk dissipation is instead dominated by
photoevaporation. We also compare our results to what has been found in other
young clusters with different massive populations, concluding that massive
associations like Cygnus OB2 are potentially hostile to protoplanetary disks,
but that the environments where disks can safely evolve in planetary systems
are likely quite common in our Galaxy.Comment: Accepted for publication in ApJS as part of the special issue on the
Chandra Cygnus OB2 Legacy Projec
Effect of bond lifetime on the dynamics of a short-range attractive colloidal system
We perform molecular dynamics simulations of short-range attractive colloid
particles modeled by a narrow (3% of the hard sphere diameter) square well
potential of unit depth. We compare the dynamics of systems with the same
thermodynamics but different bond lifetimes, by adding to the square well
potential a thin barrier at the edge of the attractive well. For permanent
bonds, the relaxation time diverges as the packing fraction
approaches a threshold related to percolation, while for short-lived bonds, the
-dependence of is more typical of a glassy system. At intermediate
bond lifetimes, the -dependence of is driven by percolation at low
, but then crosses over to glassy behavior at higher . We also
study the wavevector dependence of the percolation dynamics.Comment: Revised; 9 pages, 9 figure
Heptagon-Containing Nanographene Embedded into [10]Cycloparaphenylene
We report the synthesis and characterization of a novel type of nanohoop, consisting of a cycloparaphenylene derivative incorporating a curved heptagon-containing Ï-extended polycyclic aromatic hydrocarbon (PAH) unit. We demonstrate that this new macrocycle behaves as a supramolecular receptor of curved Ï-systems such as fullerenes C60 and C70, with remarkably large binding constants (ca. 107 Mâ1), as estimated by fluorescence measurements. Nanosecond and femtosecond spectroscopic analysis show that these host-guest complexes are capable of quasi-instantaneous charge separation upon photoexcitation, due to the ultrafast charge transfer from the macrocycle to the complexed fullerene. These results demonstrate saddle-shaped PAHs with dibenzocycloheptatrienone motifs as structural components for new macrocycles displaying molecular receptor abilities and versatile photochemical responses with promising electron-donor properties in host-guest complexes
The evolution of vibrational excitations in glassy systems
The equations of the mode-coupling theory (MCT) for ideal liquid-glass
transitions are used for a discussion of the evolution of the
density-fluctuation spectra of glass-forming systems for frequencies within the
dynamical window between the band of high-frequency motion and the band of
low-frequency-structural-relaxation processes. It is shown that the strong
interaction between density fluctuations with microscopic wave length and the
arrested glass structure causes an anomalous-oscillation peak, which exhibits
the properties of the so-called boson peak. It produces an elastic modulus
which governs the hybridization of density fluctuations of mesoscopic wave
length with the boson-peak oscillations. This leads to the existence of
high-frequency sound with properties as found by X-ray-scattering spectroscopy
of glasses and glassy liquids. The results of the theory are demonstrated for a
model of the hard-sphere system. It is also derived that certain schematic MCT
models, whose spectra for the stiff-glass states can be expressed by elementary
formulas, provide reasonable approximations for the solutions of the general
MCT equations.Comment: 50 pages, 17 postscript files including 18 figures, to be published
in Phys. Rev.
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