3,028 research outputs found
On the O II ground configuration energy levels
The most accurate way to measure the energy levels for the O II 2p^3 ground
configuration has been from the forbidden lines in planetary nebulae. We
present an analysis of modern planetary nebula data that nicely constrain the
splitting within the ^2D term and the separation of this term from the ground
^4S_{3/2} level. We extend this method to H II regions using high-resolution
spectroscopy of the Orion nebula, covering all six visible transitions within
the ground configuration. These data confirm the splitting of the ^2D term
while additionally constraining the splitting of the ^2P term. The energies of
the ^2P and ^2D terms relative to the ground (^4S) term are constrained by
requiring that all six lines give the same radial velocity, consistent with
independent limits placed on the motion of the O+ gas and the planetary nebula
data.Comment: 20 pages, 3 figures. To be published in Ap
The group of causal automorphisms
The group of causal automorphisms on Minkowski space-time is given and its
structure is analyzed
Fluorescent Excitation of Spectral Lines in Planetary Nebulae
Fluorescent excitation of spectral lines is demonstrated as a function of
temperature-luminosity and the distance of the emitting region from the central
stars of planetary nebulae. The electron densities and temperatures are
determined, and the method is exemplified through a detailed analysis of
spectral observations of a high excitation PN, NGC 6741, observed by Hyung and
Aller(1997). Fluorescence should also be important in the determination of
element abundances. It is suggested that the method could be generally applied
to determine or constrain the luminosity and the region of spectral emission in
other intensively radiative sources such as novae, supernovae, and active
galactic nuclei.Comment: 5 pages, 4 figures (fig.4 in color), ApJ (in press
Pulsar Magnetosphere : A General Relativistic Treatment
A fully general relativistic description of the pulsar magnetosphere is
provided. To be more concrete, a study of the pulsar magnetosphere is performed
in the context of general relativistic magnetohydrodynamics (MHD) employing the
so-called Grad-Shafranov approach. Not surprisingly, the resulting
Grad-Shafranov equations and all the other related general relativistic MHD
equations turn out to take essentially the same structures as those for the
(rotating) black hole magnetosphere. Other different natures between the two
cases including the structure of singular surfaces of MHD flows in each
magnetosphere are essentially encoded in the different spacetime (metric)
contents. In this way, the pulsar and the black hole magnetospheres can be
described in an unified fashion. Particularly, the direction of poloidal
currents circulating in the neutron star magnetosphere turns out to be the same
as that of currents circulating in the black hole magnetosphere which, in turn,
leads to the pulsar and the black hole spin-downs via the ``magnetic braking''.Comment: 42 pages, 3 figures, Mon. Not. R. Astron. Soc.(MNRAS), in pres
Development of an ex vivo model for the study of cerebrovascular function utilizing isolated mouse olfactory artery
OBJECTIVE: Cerebral vessels, such as intracerebral perforating arterioles isolated from rat brain, have been widely used as an ex vivo model to study the cerebrovascular function associated with cerebrovascular disorders and the therapeutic effects of various pharmacological agents. These perforating arterioles, however, have demonstrated differences in the vascular architecture and reactivity compared with a larger leptomeningeal artery which has been commonly implicated in cerebrovascular disease. In this study, therefore, we developed the method for studying cerebrovascular function utilizing the olfactory artery isolated from the mouse brain. METHODS: The olfactory artery (OA) was isolated from the C57/BL6 wild-type mouse brain. After removing connective tissues, one side of the isolated vessel segment (approximately -500 ”m in length) was cannulated and the opposite end of the vessel was completely sealed while being viewed with an inverted microscope. After verifying the absence of pressure leakage, we examined the vascular reactivity to various vasoactive agents under the fixed intravascular pressure (60 mm Hg). RESULTS: We found that the isolated mouse OAs were able to constrict in response to vasoconstrictors, including KCl, phenylephrine, endothelin-1, and prostaglandin PGH(2). Moreover, this isolated vessel demonstrated vasodilation in a dose-dependent manner when vasodilatory agents, acetylcholine and bradykinin, were applied. CONCLUSION: Our findings suggest that the isolated olfactory artery would provide as a useful ex vivo model to study the molecular and cellular mechanisms of vascular function underlying cerebrovascular disorders and the direct effects of such disease-modifying pathways on cerebrovascular function utilizing pharmacological agents and genetically modified mouse models
How dsDNA breathing enhances its flexibility and instability on short length scales
We study the unexpected high flexibility of short dsDNA which recently has
been reported by a number of experiments. Via the Langevin dynamics simulation
of our Breathing DNA model, first we observe the formation of bubbles within
the duplex and also forks at the ends, with the size distributions independent
of the contour length. We find that these local denaturations at a
physiological temperature, despite their rare and transient presence, can lower
the persistence length drastically for a short DNA segment in agreement with
experiment
Robust Fast Direct Integral Equation Solver for Quasi-Periodic Scattering Problems with a Large Number of Layers
We present a new boundary integral formulation for time-harmonic wave diffraction from two-dimensional structures with many layers of arbitrary periodic shape, such as multilayer dielectric gratings in TM polarization. Our scheme is robust at all scattering parameters, unlike the conventional quasi-periodic Greenâs function method which fails whenever any of the layers approaches a Wood anomaly. We achieve this by a decomposition into near- and far-field contributions. The former uses the free-space Greenâs function in a second-kind integral equation on one period of the material interfaces and their immediate left and right neighbors; the latter uses proxy point sources and small least-squares solves (Schur complements) to represent the remaining contribution from distant copies. By using high-order discretization on interfaces (including those with corners), the number of unknowns per layer is kept small. We achieve overall linear complexity in the number of layers, by direct solution of the resulting block tridiagonal system. For device characterization we present an efficient method to sweep over multiple incident angles, and show a 25Ă speedup over solving each angle independently. We solve the scattering from a 1000-layer structure with 3 Ă 105 unknowns to 9-digit accuracy in 2.5 minutes on a desktop workstation
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