2,700,314 research outputs found
The Multi-engine ASP Solver ME-ASP: Progress Report
MEASP is a multi-engine solver for ground ASP programs. It exploits algorithm
selection techniques based on classification to select one among a set of
out-of-the-box heterogeneous ASP solvers used as black-box engines. In this
paper we report on (i) a new optimized implementation of MEASP; and (ii) an
attempt of applying algorithm selection to non-ground programs. An experimental
analysis reported in the paper shows that (i) the new implementation of \measp
is substantially faster than the previous version; and (ii) the multi-engine
recipe can be applied to the evaluation of non-ground programs with some
benefits
QCD sum rules at finite density in the large-N_c limit: The coupling of the rho-nucleon system to the D_{13}(1520)
QCD sum rules are studied for the vector-isovector current at finite baryon
density in the limit of large number of colors N_c. For the condensate side it
is shown that in this limit the four-quark condensate factorizes also for the
finite density case. At the hadronic side the medium dependence is expressed in
terms of the current-nucleon forward scattering amplitude. Generalizing vector
meson dominance we allow for a direct coupling of the current to the nucleon as
well as a coupling via the rho meson. We discuss the N_c dependence of (a)
modifications of the pion cloud of the rho meson, (b) mixing with other mesons
(in particular a_1 and omega) and (c) resonance-hole excitations R N^{-1}. We
show that only the last effect survives in the large-N_c limit. Saturating the
sum rules with a simple hadronic ansatz which allows for the excitation of the
D_{13}(1520) we determine the coupling of the latter to the rho-nucleon and the
photon-nucleon system. These couplings are hard to determine from vacuum
physics alone.Comment: 13 pages, 2 figure
Pre-main-sequence stars in the young open cluster NGC 1893: II. Evidence for triggered massive star formation
(Abridged) The open cluster NGC 1893, illuminating the HII region IC410,
contains a moderately large population of O-type stars and is one of the
youngest clusters observable in the optical range. We have probed the stellar
population of NGC 1893 in an attempt to determine its size and extent. We
classify a large sample of cluster members with new intermediate resolution
spectroscopy. We use H-alpha slitless spectroscopy of the field to search for
emission line objects, identifying 18 emission-line PMS stars. We then combine
existing optical photometry with 2MASS JHKs photometry to detect stars with
infrared excesses, finding close to 20 more PMS candidates. While almost all
stars earlier than B2 indicate standard reddening, all later cluster members
show strong deviations from a standard reddening law, which we interpret in
terms of infrared excess emission. Emission-line stars and IR-excess objects
show the same spatial distribution, concentrating around two localised areas,
the immediate vicinity of the pennant nebulae Sim 129 and Sim 130 and the area
close to the cluster core where the rim of the molecular cloud associated with
IC 410 is illuminated by the nearby O-type stars. In and around the emission
nebula Sim 130 we find three Herbig Be stars with spectral types in the B1-4
range and several other fainter emission-line stars. We obtain a complete
census of B-type stars by combining Stroemgren, Johnson and 2MASS photometry
and find a deficit of intermediate mass stars compared to massive stars. We
observe a relatively extended halo of massive stars surrounding the cluster
without an accompanying population of intermediate-mass stars. The overall
picture of star formation in NGC 1893 suggests a very complex process.Comment: Acepted for publication in Astronomy & Astrophysics, 11 figures, uses
new aa macro. For a version with full resolution figures (Fig. 7 and 8 in
colour) use http://www.dfists.ua.es/~ignacio/download/aa6654.pd
The main sequence of three red supergiant clusters
Massive clusters in our Galaxy are an ideal testbed to investigate the properties and evolution of high-mass stars. They provide statistically significant samples of massive stars of uniform ages. To accurately determine the intrinsic physical properties of these stars, we need to establish the distances, ages and reddening of the clusters. One avenue to achieve this is the identification and characterization of the main-sequence (MS) members of red supergiant (RSG) rich clusters. Here, we utilize publicly available data from the UKIDSS Galactic Plane Survey. We show that point spread function photometry in conjunction with standard photometric decontamination techniques allows us to identify the most likely MSmembers in the 10-20 Myr old clusters RSGC 1-3. We confirm the previous detection of the MS in RSGC 2 and provide the first MS detection in RSGC 1 and RSGC 3. There are in excess of 100 stars with more than 8Mâ?? identified in each cluster. These MS members are concentrated towards the spectroscopically confirmed RSG stars. We utilize the J-K colours of the bright MS stars to determine the K-band extinction towards the clusters. The differential reddening is three times as large in the youngest cluster RSGC 1 as compared to the two older clusters RSGC 2 and RSGC 3. Spectroscopic follow-up of the cluster MS stars should lead to more precise distance and age estimates for these clusters as well as the determination of the stellar mass function in these high-mass environments. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society
Star formation in young star cluster NGC 1893
We present a comprehensive multi-wavelength study of the star-forming region
NGC 1893 to explore the effects of massive stars on low-mass star formation.
Using near-infrared colours, slitless spectroscopy and narrow-band
photometry in the cluster region we have identified candidate young stellar
objects (YSOs) distributed in a pattern from the cluster to one of the nearby
nebulae Sim 129. The colour-magnitude diagram of the YSOs indicates
that majority of these objects have ages between 1 to 5 Myr. The spread in the
ages of the YSOs may indicate a non-coeval star formation in the cluster. The
slope of the KLF for the cluster is estimated to be , which agrees
well with the average value () reported for young clusters. For the
entire observed mass range the value of the slope of
the initial mass function, ', comes out to be , which is
in agreement with the Salpeter value of -1.35 in the solar neighborhood.
However, the value of ' for PMS phase stars (mass range ) is found to be which is shallower than the
value () obtained for MS stars having mass range indicating a break in the slope of the mass function at . Estimated ' values indicate an effect of mass segregation
for main-sequence stars, in the sense that massive stars are preferentially
located towards the cluster center. The estimated dynamical evolution time is
found to be greater than the age of the cluster, therefore the observed mass
segregation in the cluster may be the imprint of the star formation process.
There is evidence for triggered star formation in the region, which seems to
govern initial morphology of the cluster.Comment: Accepted for the publication in MNRAS, 21 pages, 26 figures, 10
table
Similar self-organizing scale-invariant properties characterize early cancer invasion and long range species spread
Occupancy of new habitats through dispersion is a central process in nature.
In particular, long range dispersal is involved in the spread of species and
epidemics, although it has not been previously related with cancer invasion, a
process that involves spread to new tissues. We show that the early spread of
cancer cells is similar to the species individuals spread and that both
processes are represented by a common spatio-temporal signature, characterized
by a particular fractal geometry of the boundaries of patches generated, and a
power law-scaled, disrupted patch size distribution. We show that both
properties are a direct result of long-distance dispersal, and that they
reflect homologous ecological processes of population self-organization. Our
results are significant for processes involving long-range dispersal like
biological invasions, epidemics and cancer metastasis.Comment: 21 pages, 2 figure
Finite width effects in \phi radiative decays
The decay widths \phi \to \gamma f_0(980) and \phi \to \gamma a_0(980) are
calculated taking into account the finite widths of the scalar resonances
f_0(980) and a_0(980). The latter are shown to be essential in order to obtain
meaningful results. Simultaneously we also study the decays \phi\to \gamma
\pi^0\pi^0 and \gamma \pi^0\eta where a good reproduction of the recent
experimental data is obtained, pointing out the necessity of a \phi\gamma
K^0\bar{K}^0 contact vertex. The calculated decay rates to \gamma f_0(980) and
\gamma a_0(980) are in good agreement with the experimental ones without
invoking isospin breaking in the couplings of the f_0(980) and a_0(980)
resonances to the K^+ K^- and K^0 \bar{K}^0 channels, at odds with recent
proposals. The derived formula for calculating these \phi radiative decay
widths can be also applied in their own experimental analyses in order to
obtain more precise results.Comment: 20 pages, 6 figures. One reference is corrected. Version to be
published in Nuclear Physics
Effect of spironolactone and potassium canrenoate on cytosolic and nuclear androgen and estrogen receptors of rat liver
Spironolactone and potassium canrenoate are diuretics that are used widely for management of cirrhotic ascites. The administration of spironolactone frequently leads to feminization, which has been noted less frequently with the use of potassium canrenoate, a salt of the active metabolite of spironolactone. The use of these two drugs has been associated with decreases in serum testosterone levels and spironolactone with a reduction in androgen receptor (AR) activity. This decrease in AR has been cited as the cause of the antiandrogen effect of these drugs. We therefore assessed the effect of both drugs on levels of androgen and estrogen receptors (ER) in the liver, a tissue that is responsive to sex steroids. Three groups of male rats (n = 12 rats each) were studied. Group 1 (control) received vehicle only; group 2 received spironolactone (5 mg/day); group 3 received potassium canrenoate (5 mg/day). After 21 days of treatment, the animals of all groups were killed and liver tissue was assayed for nuclear and cytosolic AR and ER, and for male specific estrogen binder (MEB), an androgen-responsive protein. Both drugs drastically decreased the nuclear AR content, as compared with the control group, but only spironolactone decreased cytosolic AR. When the total hepatic content of AR is considered, a highly significant decrease is observed only in rats treated with spironolactone. This reduction in hepatic AR content suggested loss of androgen responsiveness of liver. We confirmed this by assessing levels of MEB, and found that livers from group 2 animals had no detectable MEB activity, whereas livers from both group 1 and 3 had normal MEB activity. No changes were observed in nuclear ER and cytosolic ER of group 3 as compared with group 1. Nuclear estrogen receptor decreased and cytosolic ER increased in group 2, but with no change in total ER content. These results indicate that (a) only spironolactone appears to act as an antiandrogen in liver, resulting in a decrease in both AR and male specific estrogen binder content, and (b) neither drug results in elevated hepatic ER content, although spironolactone-treated animals show an altered subcellular localization. © 1987
Measurement of Gamma(phi -> eta' gamma)/Gamma(phi -> eta gamma) and the pseudoscalar mixing angle
We have measured the radiative decays phi -> eta gamma, phi ->etaprime gamma
selecting pi+ pi- gamma gamma gamma final state in a sample of about 5 times
10^7 phi mesons produced at the Frascati phi factory DAFNE. We obtain Gamma(phi
-> etaprime gamma)/Gamma(phi -> eta gamma)=(4.70 +- 0.47 +- 0.31) times 10^-3.
From this result we derive new accurate values for the branching ratio BR(phi
->etaprime gamma) = (6.10 +- 0.61 +- 0.43) times 10^-5, and the mixing angle of
pseudoscalar mesons in the flavour basis phi_P=(41.8 +1.9 -1.6) degrees.Comment: Submitted to Phys. Lett.
Rho meson properties from combining QCD-based models
Aiming at the calculation of the properties of rho-mesons, non-perturbative
QCD-based methods are discussed concerning their potentials as well as their
short-comings. The latter are overcome by combining these techniques. The
utilized methods are (i) the chiral constituent quark model deduced from the
instanton vacuum model and large-N_c arguments, (ii) chiral perturbation theory
unitarized by the inverse amplitude method and (iii) QCD sum rules. Advantages
of the combination of these methods are especially the absence of un-physical
quark-production thresholds and parameter-free results. Already in the chiral
limit and in leading order in 1/N_c one obtains a reasonable result for the
mass of the rho-meson, namely m_rho = 790 \pm 30 MeV. Using the KSFR relation
the universality of the rho-meson coupling is recovered. The latter is found to
be g = 6.0 \pm 0.3.Comment: 16 pages, 1 figure, Revtex
- …