2,976 research outputs found
Exact-Diagonalization Studies of Inelastic Light Scattering in Self-Assembled Quantum Dots
We report exact diagonalization studies of inelastic light scattering in
few-electron quantum dots under the strong confinement regime characteristic of
self-assembled dots. We apply the orthodox (second-order) theory for scattering
due to electronic excitations, leaving for the future the consideration of
higher-order effects in the formalism (phonons, for example), which seem
relevant in the theoretical description of available experiments. Our numerical
results stress the dominance of monopole peaks in Raman spectra and the
breakdown of selection rules in open-shell dots. The dependence of these
spectra on the number of electrons in the dot and the incident photon energy is
explicitly shown. Qualitative comparisons are made with recent experimental
results.Comment: 11 pages, 11 figure
Instability of wormholes supported by a ghost scalar field. I. Linear stability analysis
We examine the linear stability of static, spherically symmetric wormhole
solutions of Einstein's field equations coupled to a massless ghost scalar
field. These solutions are parametrized by the areal radius of their throat and
the product of the masses at their asymptotically flat ends. We prove that all
these solutions are unstable with respect to linear fluctuations and possess
precisely one unstable, exponentially in time growing mode. The associated time
scale is shown to be of the order of the wormhole throat divided by the speed
of light. The nonlinear evolution is analyzed in a subsequent article.Comment: 12 pages, 1 figure, minor changes, to appear in Classical and Quantum
Gravit
Pneumomediastinum as a complication of emphysematous cholecystitis: Case report
<p>Abstract</p> <p>Background</p> <p>Emphysematous cholecystitis is a variant of acute cholecystitis which is generally caused by gas-forming organisms. Emphysematous cholecystitis may cause gas spreading within the subcutaneous tissue, peritoneal cavity and retroperitoneum.</p> <p>Case presentation</p> <p>We present a case of emphysematous cholecystitis in a middle-aged diabetic patient who, postoperatively, presented edema in both flanks and left chest crepitation on palpation, associated with hemodynamic worsening. Computed tomography scan of the chest and abdomen revealed a large pneumomediastinum, pneumoretroperitoneum, gas in subcutaneous tissue and flank abscesses. In both blood and surgical wound exudate cultures, <it>Escherichia coli </it>was found.</p> <p>Conclusion</p> <p>Emphysematous cholecystitis should be considered as a possible cause of pneumomediastinum.</p
Fitting LINER nuclei within the AGN family: A matter of obscuration?
In this paper we study the nuclear obscuration of galaxies hosting LINERs
based on their X-ray and optical emission. They show column densities (N_H) at
soft energies (0.5-2 keV) mostly related to the diffuse emission around the
AGN, showing a correlation with the optical extinction. N_H at hard energies
(2-10 keV) seem to be much higher than what would be expected from the optical
extinction. They might be associated to the inner regions of the AGN, buried at
optical wavelengths. The main result of this paper is that around 50% of our
LINER sample shows signatures of Compton-thickness (CT) according to the most
common tracers: the X-ray spectral index, F(2-10 keV)/F([OIII]) ratio and
EW(FeKa). However, the EWs of CT LINERs are significantly lower than in CT Sy,
suggesting that the 2-10 keV emission is dominated by electron scattering of
the otherwise invisible AGN, or by emission from shocked gas associated to star
formation rather than by reflection from the inner wall of the torus. However,
no clear relation seems to exist between galaxies with optical dust lanes and
X-ray classified CT objects. This may suggest that CT sources should be related
to absorbing material located at the very inner regions of the AGN, maybe in
the putative dusty torus. Larger M_BH and lower Eddington ratios than Sy
galaxies have been found. This effect can be better attributed to LINER nuclei
being hosted by earlier morphological types than Sy nuclei. However, it has to
be noted that, once a proper correction to the X-ray luminosity is applied,
LINERs show Eddington ratios overlapping those of Sy 2. We speculate with a
possible scenario for LINER nuclei: an inner obscuring matter similar to that
of Sy 2, and an external obscuring matter responsible for the optical
extinction. CT sources appear to be more common among LINERs than Sy.Comment: Accepted for publication in Ap
The nature of nuclear Halpha emission in LINERs
To get insight in the nature of the ionized gas in the nuclear region of
LINERs we have performed a study of HST Halpha imaging of 32 LINERs. The main
conclusion from this analysis is that for the large majority of LINERs (84%) an
unresolved nuclear source has been identified as well as extended emission with
equivalent sizes ranging from few tens till about hundredths of parsecs. Their
morphologies appear not to be homogeneous being basically grouped into three
classes:nuclear outflow candidates (42%), core-halo morphologies (25%) and
nuclear spiral disks (14%). Clumpy structures reminiscent of young stellar
clusters are not a common property on LINERs. The remaining 5 galaxies are too
dusty to allow a clear view of the ionized gas distribution. A size-luminosity
relation has been found between the equivalent radius of the Halpha emission
and the (2-10 keV) X-ray luminosities. Both ionised gas morphologies and the
size-luminosity relation are indistinguishable from those of low luminosity
Seyferts, suggesting the same origin for the NLR of LINERs and Seyferts. Also a
relation between soft X-rays and ionized gas has been suggested for the first
time in LINERs. From multiwavelength data, only 4 out of the 32 LINERs have no
evidences on an AGN nature of theirnuclear sources from multiwavelength data,
but extremely obscured AGNs cannot be discarded out given the Compton thick
signatures of their X-ray emission. For the confirmed AGN LINERs, their Halpha
imaging favour core-halo and outflow morphologies (65% of the cases). Finally,
their calculated Eddington ratios show that our LINER sources radiate at
sub-Eddington regime, with core-halo systems having on average larger Eddington
ratios than outflow candidates.Comment: Accepted for publication in Astronomy and Astrophysic
A Lorentz Invariant Pairing Mechanism: Relativistic Cooper Pairs
We study a Lorentz invariant pairing mechanism that arises when two
relativistic spin-1/2 fermions are subjected to a Dirac string coupling. In the
weak coupling regime, we find remarkable analogies between this relativistic
bound system and the well known superconducting Cooper pair. As the coupling
strength is raised, quenched phonons become unfrozen and dynamically contribute
to the gluing mechanism, which translates into novel features of this
relativistic superconducting pair.Comment: Revtex4 file, color figures with less resolution to comply with arxiv
restriction
Soft X-ray spectral variations of the narrow line Seyfert l galaxy Markarian 766
The X-ray variability of the narrow-line Seyfert 1 galaxy Markarian 766 is studied using nine ROSAT PSPC data sets. The spectrum is well described by a power law combined with a blackbody (kT ∼ 70 eV) soft excess. Examination of flux ratio changes and variability amplitude in three X-ray bands shows that the power-law component varies continuously on time-scales of ∼ 5000 s and is steeper when it is brighter. In contrast, variability of the soft excess is not detected. Spectral modelling of 31 spectra from different observations and at a range of count rates is also consistent with a picture in which the power law is steeper when it is brighter, and in which the soft-excess component does not vary. The power-law variability can be explained if the power law is produced by variable thermal or non-thermal Comptonization of soft photons. This behaviour is similar to that of Galactic black hole candidates in the low state. The X-ray and multiwavelength properties of Markarian 766 are shown to be very similar to those of other narrow-line Seyfert 1s. This may mean that the rapid X-ray variability seen in other narrow-line Seyfert 1s may also not originate in their strong soft-excess components
Apex scavengers from different European populations converge at threatened savannah landscapes
Over millennia, human intervention has transformed European habitats mainly through extensive livestock grazing. “Dehesas/Montados” are an Iberian savannah-like ecosystem dominated by oak-trees, bushes and grass species that are subject to agricultural and extensive livestock uses. They are a good example of how large-scale, low intensive transformations can maintain high biodiversity levels as well as socio-economic and cultural values. However, the role that these human-modified habitats can play for individuals or species living beyond their borders is unknown. Here, using a dataset of 106 adult GPS-tagged Eurasian griffon vultures (Gyps fulvus) monitored over seven years, we show how individuals breeding in western European populations from Northern, Central, and Southern Spain, and Southern France made long-range forays (LRFs) of up to 800 km to converge in the threatened Iberian “dehesas” to forage. There, extensive livestock and wild ungulates provide large amounts of carcasses, which are available to scavengers from traditional exploitations and rewilding processes. Our results highlight that maintaining Iberian “dehesas” is critical not only for local biodiversity but also for long-term conservation and the ecosystem services provided by avian scavengers across the continent
Assessment of sugarcane cultivars with stable reaction to Xanthomonas albilineans under mechanical inoculation conditions
Leaf scald caused by Xanthomonas albilineans in sugarcane is one of the most important diseases around the world since it causes severe losses in both agricultural and industrial yields. In Cuba, resistance to this disease is considered a major selection criterion within the breeding program. The aim of this work was to detect sugarcane cultivars with stable reaction to X. albilineans by using both additive main effects and multiplicative interaction (AMMI) and linear general models. For this, 16 cultivars planted simultaneously in 2015, 2016, and 2017 in two locations: Jovellanos (Matanzas) and Florida (Camagüey) in Cuba were mechanically inoculated with a bacterial isolate previously characterized by sequencing its DNA fragment. Disease severity was recorded in plant cane and first ratoon, and results were consistent in both analyses and revealed that L55-5 and C323-68 were the most susceptible cultivars and C1051-73 was the most resistant; however, C1051-73, C275-80, C86-12, C88-382, C89-147, My5514, and Ty86-28 were the most stable across the years and localities evaluated. Results will allow adapting the methodology for the evaluation of the reaction to leaf scald of new sugarcane cultivars.Fil: La O Hechavarría, María. Instituto de Investigaciones de la Caña de Azúcar; CubaFil: Puchades Izaguirre, Yaquelín. Instituto de Investigaciones de la Caña de Azúcar; CubaFil: Pérez Pérez, Yosel. Instituto de Investigaciones de la Caña de Azúcar; CubaFil: Michavila, Gabriela. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán. Instituto de Tecnología Agroindustrial del Noroeste Argentino. Provincia de Tucumán. Ministerio de Desarrollo Productivo. Estación Experimental Agroindustrial "Obispo Colombres" (p). Instituto de Tecnología Agroindustrial del Noroeste Argentino; ArgentinaFil: Casas González, Mario A.. Instituto de Investigaciones de la Caña de Azúcar; CubaFil: Pérez Pérez, Juana. Instituto de Investigaciones de la Caña de Azúcar; CubaFil: Carvajal Jaime, Omelio. Instituto de Investigaciones de la Caña de Azúcar; CubaFil: Montalván Delgado, Joaquín. Instituto de Investigaciones de la Caña de Azúcar; CubaFil: Peña Malavera, Andrea Natalia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán. Instituto de Tecnología Agroindustrial del Noroeste Argentino. Provincia de Tucumán. Ministerio de Desarrollo Productivo. Estación Experimental Agroindustrial "Obispo Colombres" (p). Instituto de Tecnología Agroindustrial del Noroeste Argentino; ArgentinaFil: Mesa Lopez, José. Instituto de Investigaciones de la Caña de Azúcar; CubaFil: Castagnaro, Atilio Pedro. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán. Instituto de Tecnología Agroindustrial del Noroeste Argentino. Provincia de Tucumán. Ministerio de Desarrollo Productivo. Estación Experimental Agroindustrial "Obispo Colombres" (p). Instituto de Tecnología Agroindustrial del Noroeste Argentino; ArgentinaFil: Perera, María Francisca. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán. Instituto de Tecnología Agroindustrial del Noroeste Argentino. Provincia de Tucumán. Ministerio de Desarrollo Productivo. Estación Experimental Agroindustrial "Obispo Colombres" (p). Instituto de Tecnología Agroindustrial del Noroeste Argentino; Argentin
Ages and metallicities of circumnuclear star formation regions from Gemini IFU observations
Aims: We derive the age and metallicity of circumnuclear star formation
regions (CNSFRs) located in the spiral galaxies NGC6951 and NGC1097, and
investigate the cause of the very low equivalent widths of emission lines found
for these regions.
Methods: We used optical two-dimensional spectroscopic data obtained with
Gemini GMOS-IFUs and a grid of photoionization models to derive the the
metallicities and ages of CNSFRs.
Results: We find star formation rates in the range 0.002-0.14 Msun/yr and
oxygen abundance of 12+log(O/H)~8.8 dex, similar to those of most metal-rich
nebulae located in the inner region of galactic disks.
Conclusions: We conclude that the very low emission-line equivalent widths
observed in CNSFRs are caused by the ``contamination'' of the continuum by (1)
contribution of the underlying bulge continuum combined with (2) contribution
from previous episodes of star formation at the CNSFRs.Comment: Accepted for publication in Astronomy and Astrophysic
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