12,470 research outputs found
A triangular thin shell finite element: Linear analysis
The formulation of the linear stiffness matrix for a doubly-curved triangular thin shell element, using a modified potential energy principle, is described. The strain energy component of the potential energy is expressed in terms of displacements and displacement gradients by use of consistent Koiter strain-displacement equations. The element inplane and normal displacement fields are approximated by complete cubic polynomials. The interelement displacement admissibility conditions are met in the global representation by imposition of constraint conditions on the interelement boundaries; the constraints represent the modification of the potential energy. Errors due to the nonzero strains under rigid body motion are shown to be of small importance for practical grid refinements through performance of extensive comparison analyses
Solar Flare X-ray Source Motion as a Response to Electron Spectral Hardening
Context: Solar flare hard X-rays (HXRs) are thought to be produced by
nonthermal coronal electrons stopping in the chromosphere, or remaining trapped
in the corona. The collisional thick target model (CTTM) predicts that sources
produced by harder power-law injection spectra should appear further down the
legs or footpoints of a flare loop. Therefore, hardening of the injected
power-law electron spectrum during flare onset should be concurrent with a
descending hard X-ray source.
Aims: To test this implication of the CTTM by comparing its predicted HXR
source locations with those derived from observations of a solar flare which
exhibits a nonthermally-dominated spectrum before the peak in HXRs, known as an
early impulsive event.
Methods: HXR images and spectra of an early impulsive C-class flare were
obtained using the Ramaty High-Energy Solar Spectroscopic Imager (RHESSI).
Images were reconstructed to produce HXR source height evolutions for three
energy bands. Spatially-integrated spectral analysis was performed to isolate
nonthermal emission, and to determine the power-law index of the electron
injection spectrum. The observed height-time evolutions were then fit with
CTTM-based simulated heights for each energy.
Results: A good match between model and observed source heights was reached,
requiring a density model that agreed well with previous studies of flare loop
densities.
Conclusions: The CTTM has been used to produce a descent of model HXR source
heights that compares well with observations of this event. Based on this
interpretation, downward motion of nonthermal sources should indeed occur in
any flare where there is spectral hardening in the electron distribution during
a flare. However, this would often be masked by thermal emission associated
with flare plasma pre-heating.Comment: 8 pages, 5 figure
X-raying the Winds of Luminous Active Galaxies
We briefly describe some recent observational results, mainly at X-ray
wavelengths, on the winds of luminous active galactic nuclei (AGNs). These
winds likely play a significant role in galaxy feedback. Topics covered include
(1) Relations between X-ray and UV absorption in Broad Absorption Line (BAL)
and mini-BAL quasars; (2) X-ray absorption in radio-loud BAL quasars; and (3)
Evidence for relativistic iron K BALs in the X-ray spectra of a few bright
quasars. We also mention some key outstanding problems and prospects for future
advances; e.g., with the International X-ray Observatory (IXO).Comment: 7 pages, 3 figures, to appear in proceedings of the conference "The
Monster's Fiery Breath: Feedback in Galaxies, Groups, and Clusters", June
2009, Madison, Wisconsi
Shear buckling of square perforated plates
The behavior of thin square perforated plates under the action of uniform shear deformation is studied experimentally and analytically using finite element analysis. Elastic Shear buckling strength is established as a function of the diameter of a round, centrally located hole in the plate. Post buckling behavior and the behavior of perforated plates with various ring stiffeners are also studied experimentally
The Exceptionally Soft X-ray Spectrum of the Low-mass Starburst Galaxy NGC 1705
NGC 1705 is one of the optically brightest and best studied dwarf galaxies.
It appears to be in the late stage of a major starburst and contains a young
super star cluster. Type II supernovae are therefore likely to have been a
major effect in the recent evolution of this galaxy and are likely to have
produced a superbubble whose affects on the low-density ambient interstellar
medium can be ideally studied. ROSAT PSPC observations of this galaxy reveal
two striking blobs of X-ray emission embedded in \Ha loops which can be
interpreted as both sides of the upper plumes of the same superbubble. These
sources are a surprise. They are much softer than those observed from other
starburst dwarf galaxies, and are so soft that they should have been blocked if
the observed Galactic HI column density were uniformly distributed across NGC
1705 or if the sources were embedded in the HI disk of NGC 1705. In addition,
the total X-ray luminosity in the ROSAT energy band of 1.2x10^{38} erg s^{-1}
is low in comparison to similar objects. We discuss possible models for the two
X-ray peaks in NGC 1705 and find that the sources most likely originate from
relatively cool gas of one single superbubble in NGC 1705. The implications of
the exceptional softness of these sources are addressed in terms of intrinsic
properties of NGC 1705 and the nature of the foreground Galactic absorption.Comment: 7 pages, 2 ps-figures, LATEX-file; accepted for publication in
ApJ.Letter
Seeking large-scale magnetic fields in a pure-disk dwarf galaxy NGC 2976
It is still unknown how magnetic field-generation mechanisms could operate in
low-mass dwarf galaxies. Here, we present a detailed study of a nearby
pure-disk dwarf galaxy NGC 2976. Unlike previously observed dwarf objects, this
galaxy possesses a clearly defined disk. For the purpose of our studies, we
performed deep multi-frequency polarimetric observations of NGC 2976 with the
VLA and Effelsberg radio telescopes. Additionally, we supplement them with
re-imaged data from the WSRT-SINGS survey. The magnetic field morphology
discovered in NGC 2976 consists of a southern polarized ridge. This structure
does not seem to be due to just a pure large-scale dynamo process (possibly
cosmic-ray driven) at work in this object, as indicated by the RM data and
dynamo number calculations. Instead, the field of NGC 2976 is modified by past
gravitational interactions and possibly also by ram pressure inside the M 81
galaxy group environment. The estimates of total (7 muG) and ordered (3 muG)
magnetic field strengths, as well as degree of field order (0.46), which is
similar to those observed in spirals, suggest that tidally generated magnetized
gas flows can further enhance dynamo action in the object. NGC 2976 is
apparently a good candidate for the efficient magnetization of its
neighbourhood. It is able to provide an ordered (perhaps also regular) magnetic
field into the intergalactic space up to a distance of about 5 kpc. Tidal
interactions (and possibly also ram pressure) can lead to the formation of
unusual magnetic field morphologies (like polarized ridges) in galaxies out of
the star-forming disks, which do not follow any observed component of the
interstellar medium (ISM), as observed in NGC 2976. These galaxies are able to
provide ordered magnetic fields far out of their main disks.Comment: 16 page
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