17,097 research outputs found
Stellar populations in the Carina region: The Galactic plane at l = 291
Previous studies of the Carina region have revealed its complexity and
richness as well as a significant number of early-type stars. In many cases,
these studies only concentrated on the central region or were not homogeneous.
This latter aspect, in particular, is crucial because very different ages and
distances for key clusters have been claimed in recent years. The aim of this
work is to study in detail an area of the Galactic plane in Carina. We analyze
the properties of different stellar populations and focus on a sample of open
clusters and their population of YSOs and highly reddened early stars. We also
studied the stellar mass distribution in these clusters and the possible
scenario of their formation. Finally, we outline the Galactic spiral structure
in this direction. We obtained photometric data for six young open clusters
located in Carina at l = 291, and their adjacent stellar fields, which we
complemented with spectroscopic observations of a few selected targets. We also
culled additional information from the literature. Our results provide more
reliable estimates of distances, color excesses, masses, and ages of the
stellar populations in this direction. We estimate the basic parameters of the
studied clusters and find that they identify two overdensities of young stellar
populations. We find evidence of PMS populations inside them, with an apparent
coeval stellar formation in the most conspicuous clusters. We also discuss
apparent age and distance gradients in the direction NW-SE. We study the mass
distributions of several clusters in the region. They consistently show a
canonical IMF slope. We discover and characterise an abnormally reddened
massive stellar population. Spectroscopic observations of ten stars of this
latter population show that all selected targets were massive OB stars. Their
location is consistent with the position of the Car-Sag spiral arm.Comment: 15 pages, 13 figure
First clear evidence of quantum chaos in the bound states of an atomic nucleus
We study the spectral fluctuations of the Pb nucleus using the
complete experimental spectrum of 151 states up to excitation energies of
MeV recently identified at the Maier-Leibnitz-Laboratorium at Garching,
Germany. For natural parity states the results are very close to the
predictions of Random Matrix Theory (RMT) for the nearest-neighbor spacing
distribution. A quantitative estimate of the agreement is given by the Brody
parameter , which takes the value for regular systems and
for chaotic systems. We obtain which
is, to our knowledge, the closest value to chaos ever observed in experimental
bound states of nuclei. By contrast, the results for unnatural parity states
are far from RMT behavior. We interpret these results as a consequence of the
strength of the residual interaction in Pb, which, according to
experimental data, is much stronger for natural than for unnatural parity
states. In addition our results show that chaotic and non-chaotic nuclear
states coexist in the same energy region of the spectrum.Comment: 9 pages, 1 figur
Coulomb blockade without potential barriers
We study transport through a strongly correlated quantum dot and show that
Coulomb blockade can appear even in the presence of perfect contacts. This
conclusion arises from numerical calculations of the conductance for a
microscopic model of spinless fermions in an interacting chain connected to
each lead via a completely open channel. The dependence of the conductance on
the gate voltage shows well defined Coulomb blockade peaks which are sharpened
as the interaction strength is increased. Our numerics is based on the
embedding method and the DMRG algorithm. We explain the emergence of Coulomb
blockade with perfect contacts by a reduction of the effective coupling matrix
elements between many-body states corresponding to successive particle numbers
in the interacting region. A perturbative approach, valid in the strong
interaction limit, yields an analytic expression for the interaction-induced
suppression of the conductance in the Coulomb blockade regime.Comment: Fixed problems with eps figure
Optimization of soliton ratchets in inhomogeneous sine-Gordon systems
Unidirectional motion of solitons can take place, although the applied force
has zero average in time, when the spatial symmetry is broken by introducing a
potential , which consists of periodically repeated cells with each cell
containing an asymmetric array of strongly localized inhomogeneities at
positions . A collective coordinate approach shows that the positions,
heights and widths of the inhomogeneities (in that order) are the crucial
parameters so as to obtain an optimal effective potential that yields
a maximal average soliton velocity. essentially exhibits two
features: double peaks consisting of a positive and a negative peak, and long
flat regions between the double peaks. Such a potential can be obtained by
choosing inhomogeneities with opposite signs (e.g., microresistors and
microshorts in the case of long Josephson junctions) that are positioned close
to each other, while the distance between each peak pair is rather large. These
results of the collective variables theory are confirmed by full simulations
for the inhomogeneous sine-Gordon system
Late time tails of the massive vector field in a black hole background
We investigate the late-time behavior of the massive vector field in the
background of the Schwarzschild and Schwarzschild-de Sitter black holes. For
Schwarzschild black hole, at intermediately late times the massive vector field
is represented by three functions with different decay law , ,
, while at asymptotically late times
the decay law is universal, and does not
depend on the multipole number . Together with previous study of massive
scalar and Dirac fields where the same asymptotically late-time decay law was
found, it means, that the asymptotically late-time decay law \emph{does not depend} also \emph{on the spin} of the field under
consideration. For Schwarzschild-de Sitter black holes it is observed two
different regimes in the late-time decay of perturbations: non-oscillatory
exponential damping for small values of and oscillatory quasinormal mode
decay for high enough . Numerical and analytical results are found for these
quasinormal frequencies.Comment: one author and new material are adde
Intensification of oxidation capacity using chloroalkanes as additives in hydrodynamic and acoustic cavitation reactors
The effect of the presence and absence of the chloroalkanes, dichloromethane (CH2Cl2), chloroform (CHCl3) and carbon tetrachloride (CCl4) on the extent of oxidation of aqueous I- to I3- has been investigated in (a) a liquid whistle reactor (LWR) generating hydrodynamic cavitation and (b) an ultrasonic probe, which produces acoustic cavitation. The aim has been to examine the intensification achieved in the extent of oxidation due to the generation of additional free radicals/oxidants in the reactor as a result of the presence of chloroalkanes. It has been observed that the extent of increase in the oxidation reaction is strongly dependent on the applied pressure in the case of the LWR. Also, higher volumes of the chloroalkanes favour the intensification and the order of effectiveness is CCl4> CHCl3 > CH2Cl2. However, the results with the ultrasonic probe suggest that an optimum concentration of CH2Cl2 or CHCl3 exists beyond which there is little increase in the extent of observed intensification. For CCl4, however, no such optimum concentration was observed and the extent of increase in the rates of oxidation reaction rose with the amount of CCl4 added. Stage wise addition of the chloroalkanes was found to give marginally better results in the case of the ultrasonic probe as compared to bulk addition at the start of the run. Although CCl4 is the most effective, its toxicity and carcinogenicity may mean that CH2Cl2 and CHCl3 offer a safer viable alternative and the present work should be useful in establishing the amount of chloroalkanes required for obtaining a suitable degree of intensification
Internal mode mechanism for collective energy transport in extended systems
We study directed energy transport in homogeneous nonlinear extended systems
in the presence of homogeneous ac forces and dissipation. We show that the
mechanism responsible for unidirectional motion of topological excitations is
the coupling of their internal and translation degrees of freedom. Our results
lead to a selection rule for the existence of such motion based on resonances
that explains earlier symmetry analysis of this phenomenon. The direction of
motion is found to depend both on the initial and the relative phases of the
two harmonic drivings, even in the presence of noise.Comment: Final version, to appear in Physical Review Letter
Soliton ratchets in homogeneous nonlinear Klein-Gordon systems
We study in detail the ratchet-like dynamics of topological solitons in
homogeneous nonlinear Klein-Gordon systems driven by a bi-harmonic force. By
using a collective coordinate approach with two degrees of freedom, namely the
center of the soliton, , and its width, , we show, first, that
energy is inhomogeneously pumped into the system, generating as result a
directed motion; and, second, that the breaking of the time shift symmetry
gives rise to a resonance mechanism that takes place whenever the width
oscillates with at least one frequency of the external ac force. In addition,
we show that for the appearance of soliton ratchets, it is also necesary to
break the time-reversal symmetry. We analyze in detail the effects of
dissipation in the system, calculating the average velocity of the soliton as a
function of the ac force and the damping. We find current reversal phenomena
depending on the parameter choice and discuss the important role played by the
phases of the ac force. Our analytical calculations are confirmed by numerical
simulations of the full partial differential equations of the sine-Gordon and
systems, which are seen to exhibit the same qualitative behavior. Our
results are in agreement with recent experimental work on dissipation induced
symmetry breaking.Comment: Minor corrections, several references added, accepted for publication
in Chao
Variation of Creep Resistance in Ferritic Steels by a Heat Treatment
AbstractIn the power plants, boiler pipes and heaters, are made with ferritic steels low alloy. These steels have a microstructure with fine stable alloy carbides that impede the movement of the dislocations, however it is inevitable that during long periods of service or very critical conditions, microstructural changes occur that are responsible for the loss of material strength. In the past decades the 1Cr-0.5Mo steel was used, but it has been replaced by ferritc steels containing higher amounts of Cr and Mo, with the addition of other micro alloying elements such as niobium, titanium and vanadium to increase their mechanical strength. The objective of this work is to study the creep behavior of 1Cr-0.5Mo steel and to compare its strength when prior to service it is subjected to different heat treatments that improve its conditions of service, as that is beneficial from the economical point of view. Tensile creep tests were performed at a temperature range between 843 and 893K, and applied stresses between 131 and 205MPa in the material reception conditions comparing its behavior with others that previously has undergone different heat treatments. From experimental data the characteristic parameters were calculated such as the creep coefficient of stress and activation energy. The microstructural variation of the original material was also analyzed, after heat treatment and creep samples were characterized by optical microscopy, scanning electron microscopy and analysis by dispersive X- ray spectroscopy, to evaluate the effects of kinetics changes occurred in the precipitated phases and the presence of microstructural damage, such as nucleation, growth and coalescence of micro cavities. The microhardness of the phases present in the different samples were also measured
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