2,150 research outputs found
Curvature and Acoustic Instabilities in Rotating Fluid Disks
The stability of a rotating fluid disk to the formation of spiral arms is
studied in the tightwinding approximation in the linear regime. The dispersion
relation for spirals that was derived by Bertin et al. is shown to contain a
new, acoustic instability beyond the Lindblad resonances that depends only on
pressure and rotation. In this regime, pressure and gravity exchange roles as
drivers and inhibitors of spiral wave structures. Other instabilities that are
enhanced by pressure are also found in the general dispersion relation by
including higher order terms in the small parameter 1/kr for wavenumber k and
radius r. These instabilities are present even for large values of Toomre's
parameter Q. Unstable growth rates are determined in four cases: a
self-gravitating disk with a flat rotation curve, a self-gravitating disk with
solid body rotation, a non-self-gravitating disk with solid body rotation, and
a non-self-gravitating disk with Keplerian rotation. The most important
application appears to be as a source of spiral structure, possibly leading to
accretion in non-self-gravitating disks, such as some galactic nuclear disks,
disks around black holes, and proto-planetary disks. All of these examples have
short orbital times so the unstable growth time can be small.Comment: 30 pages, 5 figures, scheduled for ApJ 520, August 1, 199
Symmetric eikonal model for projectile-electron excitation and loss in relativistic ion-atom collisions
At impact energies GeV/u the projectile-electron
excitation and loss occurring in collisions between highly charged ions and
neutral atoms is already strongly influenced by the presence of atomic
electrons. In order to treat these processes in collisions with heavy atoms we
generalize the symmetric eikonal model, used earlier for considerations of
electron transitions in ion-atom collisions within the scope of a three-body
Coulomb problem. We show that at asymptotically high collision energies this
model leads to an exact transition amplitude and is very well suited to
describe the projectile-electron excitation and loss at energies above a few
GeV/u. In particular, by considering a number of examples we demonstrate
advantages of this model over the first Born approximation at impact energies
--30 GeV/u, which are of special interest for atomic physics
experiments at the future GSI facilities.Comment: 14 pages, 5 figure
Hemi-Synthesis and Anti-Oomycete Activity of Analogues of Isocordoin
Indexación: Web of Science; Scopus.An efficient synthesis of a series of 4-oxyalkyl-isocordoin analogues (2–8) is reported for the first time. Their structures were confirmed by1H-NMR,13C-NMR, and HRMS. Their anti-oomycete activity was evaluated by mycelium and spores inhibition assay against two selected pathogenic oomycetes strains: Saprolegnia parasitica and Saprolegnia australis. The entire series of isocordoin derivatives (except compound 7) showed high inhibitory activity against these oomycete strains. Among them, compound 2 exhibited strong activity, with minimum inhibitory concentration (MIC) and minimum oomyceticidal concentration (MOC) values of 50 µg/mL and 75 µg/mL, respectively. The results showed that 4-oxyalkylated analogues of isocordoin could be potential anti-oomycete agents.http://www.mdpi.com/1420-3049/22/6/96
A differential method for bounding the ground state energy
For a wide class of Hamiltonians, a novel method to obtain lower and upper
bounds for the lowest energy is presented. Unlike perturbative or variational
techniques, this method does not involve the computation of any integral (a
normalisation factor or a matrix element). It just requires the determination
of the absolute minimum and maximum in the whole configuration space of the
local energy associated with a normalisable trial function (the calculation of
the norm is not needed). After a general introduction, the method is applied to
three non-integrable systems: the asymmetric annular billiard, the many-body
spinless Coulombian problem, the hydrogen atom in a constant and uniform
magnetic field. Being more sensitive than the variational methods to any local
perturbation of the trial function, this method can used to systematically
improve the energy bounds with a local skilled analysis; an algorithm relying
on this method can therefore be constructed and an explicit example for a
one-dimensional problem is given.Comment: Accepted for publication in Journal of Physics
Chitooligosaccharides modified by maillard reaction with glucose inhibit adhesion of enteropathogenic bacteria to mucin
The HI content of the Eridanus group of galaxies
The HI content of galaxies in the Eridanus group is studied using the GMRT
observations and the HIPASS data. A significant HI deficiency up to a factor of
2-3 is observed in galaxies in the high galaxy density regions. The HI
deficiency in galaxies is observed to be directly correlated with the local
projected galaxy density, and inversely correlated with the line-of-sight
radial velocity. Furthermore, galaxies with larger optical diameters are
predominantly in the lower galaxy density regions. It is suggested that the HI
deficiency in Eridanus is due to tidal interactions. In some galaxies,
evidences of tidal interactions are seen. An important implication is that
significant evolution of galaxies can take place in the group environment. In
the hierarchical way of formation of clusters via mergers of groups, a fraction
of the observed HI deficiency in clusters could have originated in groups. The
co-existence of S0's and severely HI deficient galaxies in the Eridanus group
suggests that galaxy harassment is likely to be an effective mechanism for
transforming spirals to S0's.Comment: 21 pages; Accepted for publication in Journal of Astroph. & Astron.
March, 200
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A 15 µm selected sample of hHigh-z starbursts and AGNs
We report results from our Spitzer GO-1 program on IRS spectroscopy of a large sample of Luminous Infrared Galaxies and quasars selected from the European Large Area ISO Survey (ELAIS). The selected ELAIS sources have a wide multi-wavelength coverage, including ISOCAM, ISOPHOT, IRAC
and MIPS (from SWIRE), and optical photometry. Here we present the sample selection and results from the IRS spectroscopy
Buenas prácticas para implementación del comercio electrónico en pymes
Los avances tecnológicos en conectividad y accesibilidad a redes de información han estimulado nuevos modelos de negocio, y se ha favorecido así el tránsito de la economía tradicional a la economía electrónica. En consecuencia, el binomio empresa-tecnología tiende a ser inseparable; sin embargo, la presencia de las pymes colombianas en Internet es aún incipiente. El objetivo de este artículo es presentar un modelo innovador que sirva como instrumento para incrementar la confianza de las transacciones ejecutadas en el marco de la comercialización electrónica, orientado a las pymes colombianas y basado en metodologías y herramientas cuyos criterios de legalidad y seguridad garanticen un medio adecuado para dicha comercialización
Postural changes in women with chronic pelvic pain: a case control study
<p>Abstract</p> <p>Background</p> <p>Chronic pelvic pain (CPP) is a lower abdominal pain lasting at least 6 months, occurring continuously or intermittently and not associated exclusively with menstruation or intercourse. Although the musculoskeletal system has been found to be involved in CPP, few studies have assessed the contribution of posture in women with CPP. We aimed to determine if the frequency of postural changes was higher in women with CPP than healthy subjects.</p> <p>Methods</p> <p>A case-control study included 108 women with CPP of more than 6 months' duration (CPP group) who consecutively attended at the Hospital of the University of São Paulo and 48 healthy female volunteers (control group). Postural assessment was noninvasive and performed in the standing position, with the reference points of Kendall used as normal parameters. Factors associated with CPP were assessed by logistic regression analysis.</p> <p>Results</p> <p>Logistic regression showed that the independent factors associated with CPP were postural changes in the cervical spine (OR 4.1; 95% CI 1.6–10.7; p < 0.01) and scapulae (OR 2.9; 95% CI 1.1–7.6; p < 0.05).</p> <p>Conclusion</p> <p>Musculoskeletal changes were associated with CPP in 34% of women. These findings suggest that a more detailed assessment of women with CPP is necessary for better diagnosis and for more effective treatment.</p
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