3,740 research outputs found
Comparative radiosensitivity of Medfly cells and embryos.
This research is dealing with the effect of 6O Co gamma radiation on cultured "In vitro" cells and on embryos at different developmental stages, of Ceratitis capitata Wiedemann. The parameters ana1yzed for both the cells and the embryos were growrh, survival and mortality rates. The immediate and late effects of irradiation were also studied at the level of egg hatching, larval life cycle, emergence of adults and their fertility. A particular result that became evident in the cormparison of the radiosensitivity was that the ceIls "in vitro" had a greater radioresistance than the very young embryos by a factor of 3, but the ceIls had less resistance by a factor of 8.5 when compared with the developed embryos. In general, with an increase in dose there was an increase in damage; even at 1200 rad a prolonged arrest was found in the growth of the cell population, and with 2400 rad it was found in the developmenr of 24 h old embryos. Confronting embryos of different ages, it was noted that the same quantitative effect was obtained with doses proportional to the age of the embryos: the same mortality effect in 30 min old embryos irradiated with 300 rad as in those 24 h old treated with 4800 rad was observed. The results obtained are presented and discussed.Peer reviewe
Dualities Compositeness and Spacetime Structure of 4d Extreme Stringy Black Holes
We study the BPS black hole solutions of the (truncated) action for heterotic
string theory compactified on a six-torus. The O(3,Z) duality symmetry of the
theory, together with the bound state interpretation of extreme black holes, is
used to generate the whole spectrum of the solutions. The corresponding
spacetime structures, written in terms of the string metric, are analyzed in
detail. In particular, we show that only the elementary solutions present naked
singularities. The bound states have either null singularities (electric
solutions) or are regular (magnetic or dyonic solutions) with near-horizon
geometries given by the product of two 2d spaces of constant curvature. The
behavior of some of these solutions as supersymmetric attractors is discussed.
We also show that our approach is very useful to understand some of the
puzzling features of charged black hole solutions in string theory.Comment: 17 pages, LaTex, no figure
Galaxy Morphological Segregation in Clusters: Local vs. Global Conditions
We study the relative fraction of galaxy morphological types in clusters, as
a function of the projected local galaxy density and different global
parameters: cluster projected gas density, cluster projected total mass density
, and reduced clustercentric distance. Since local and global densities are
correlated, we have considered different tests to search for the parameters to
which segregation show the strongest dependence. Also, we have explored the
results of our analysis applied to the central regions of the clusters and
their outskirts. We consider a sample of clusters of galaxies with temperature
estimates to derive the projected mass density profile and the 500 density
contrast radius () using the NFW model and the scaling relation
respectively. The X-ray surface brightness profiles are used to obtain the
projected gas density assuming the hydrostatic equilibrium model. Our results
suggest that the morphological segregation in clusters is controlled by the
local galaxy density in the outskirts. On the other hand, the global projected
mass density, shows the strongest correlation with the fraction of
morphological types in the central high density region, with a marginal
dependence on the local galaxy density.Comment: 10 pages, 8 figures, Accepted AJ (February 2001 issue
On the use of scaling relations for the Tolman test
The use of relations between structural parameters of early type galaxies to
perform the Tolman test is reconsidered. Scaling relations such as the FP or
the Kormendy relation, require the transformation from angular to metric sizes,
to compare the relation at different z values. This transformation depends on
the assumed world model: galaxies of a given angular size, at a given z, are
larger (in kpc) in a non-expanding universe than in an expanding one.
Furthermore, the luminosities of galaxies are expected to evolve with z in an
expanding model. These effects are shown to conspire to reduce the difference
between the predicted SB change with redshift in the expanding and non
expanding cases. We find that the predictions for the visible photometric bands
of the expanding models with passive luminosity evolution are very similar to
those of the static model till z about 1, and therefore, the test cannot
distinguish between the two world models. Recent good quality data are
consistent with the predictions from both models. In the K-band, where the
expected (model) luminosity evolutionary corrections are smaller, the
differences between the xpanding and static models amount to about 0.4 (0.8)
magnitudes at z = 0.4 (1). It is shown that, due to that small difference
between the predictions in the covered z-range, and to the paucity and
uncertainties of the relevant SB photometry, the existing K-band data is not
adequate to distinguish between the different world metrics, and cannot be yet
used to discard the static case. It is pointed out that the scaling relations
could still be used to rule out the non-evolving case if it could be shown that
the coefficients change with the redshift.Comment: Latex, 15 pages with 2 figures. To be published in ApJ Letter
A Light Supersymmetric Axion in an Anomalous Abelian Extension of the Standard Model
We present a supersymmetric extension of the Standard Model (USSM-A) with an
anomalous U(1) and Stueckelberg axions for anomaly cancellation, generalizing
similar non-supersymmetric constructions. The model, built by a bottom-up
approach, is expected to capture the low-energy supersymmetric description of
axionic symmetries in theories with gauged anomalous abelian interactions,
previously explored in the non-supersymmetric case for scenarios with
intersecting branes. The choice of a USSM-like superpotential, with one extra
singlet superfield and an extra abelian symmetry, allows a physical axion-like
particle in the spectrum. We describe some general features of this
construction and in particular the modification of the dark-matter sector which
involves both the axion and several neutralinos with an axino component. The
axion is expected to be very light in the absence of phases in the
superpotential but could acquire a mass which can also be in the few GeV range
or larger. In particular, the gauging of the anomalous symmetry allows
independent mass/coupling interaction to the gauge fields of this particle, a
feature which is absent in traditional (invisible) axion models. We comment on
the general implications of our study for the signature of moduli from string
theory due to the presence of these anomalous symmetries.Comment: 46 pages, 28 figures. Revised version, accepted for a publication on
Phys.Rev.
The AdS/CFT correspondence in two dimensions
We review recent progress in understanding the anti-de Sitter/conformal field
theory correspondence in the context of two-dimensional dilaton gravity theory.Comment: Contribution to the Proceedings of the Euroconference on "Brane New
World and Noncommutative Geometry", Turin, October 200
On the energy-shell contributions of the three-particle~-~ three-hole excitations
The response functions for the extended second and third random phase
approximation are compared. A second order perturbation calculation shows that
the first-order amplitude for the direct excitation from the ground
state cancels with those that are engendered by the - coupling. As
a consequence nonvanishing effects to the response involve off
energy shell renormalization only. On shell processes are absent.Comment: 12 pages text (LaTex) and 1 figure included, to be published in Phys.
Rev.
Successful Coronary Stent Retrieval From a Pedal Artery
The purpose of this article is to report complications
from a coronary drug-eluting stent lost in the
peripheral circulation. We report the case of successful
retrieval of a sirolimus coronary stent from a pedal artery in
a young patient who underwent coronary angiography for
previous anterior myocardial infarction. Recognition of
stent embolization requires adequate removal of the device
to avoid unwelcome clinical sequelae.
Keywords Coronary stent Microsnare technique
Peripheral embolization Stent retrieva
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