4,526 research outputs found
Infrared observations of possible protostars
Energy distribution measurements on infrared objects located in H2 and gaseous nebulae region
Optical and infrared spectrophotometry of 18 Markarian galaxies
Slit spectra, spectrophotometric scans and infrared broad band observations are presented. Eight of the program galaxies can be classified as Seyfert galaxies. Arguments are given that thermal, nonthermal and stellar radiation components were present. One group of Seyfert galaxies was characterized both by the presence of a high density region of gas and by a continuum dominated by nonthermal radiation. The continua of the remaining program Seyferts, which did not have a high density region of gas, were dominated by thermal radiation from dust and a stellar continuum. Ten of the galaxies, which are not Seyfert galaxies, are shown to be examples of extragalactic H 2 regions
Two-micron spectrophotometry of the galaxy NGC 253
A very strong Brackett-gamma hydrogen emission line, and the 2.3 micron CO stellar absorption feature were measured in NGC 253. The presence and strength of the CO feature indicates that late type giant stars produce most of the 2.2 micron continuum emission, while the rate of ionization implied by strength of the Brackett-gamma line indicates that much, perhaps all, of the luminosity detected at far infrared wavelengths originates from a large number of OB stars. As compared to the corresponding region of the Galaxy, the number of massive young stars in the central 200 pc of NGC 253 is thirty times greater, but the total mass of stars is roughly the same
Clean and As-covered zinc-blende GaN (001) surfaces: Novel surface structures and surfactant behavior
We have investigated clean and As-covered zinc-blende GaN (001) surfaces,
employing first-principles total-energy calculations. For clean GaN surfaces
our results reveal a novel surface structure very different from the
well-established dimer structures commonly observed on polar III-V (001)
surfaces: The energetically most stable surface is achieved by a Peierls
distortion of the truncated (1x1) surface rather than through addition or
removal of atoms. This surface exhibits a (1x4) reconstruction consisting of
linear Ga tetramers. Furthermore, we find that a submonolayer of arsenic
significantly lowers the surface energy indicating that As may be a good
surfactant. Analyzing surface energies and band structures we identify the
mechanisms which govern these unusual structures and discuss how they might
affect growth properties.Comment: 4 pages, 3 figures, to be published in Appears in Phys. Rev. Lett.
(in print). Other related publications can be found at
http://www.rz-berlin.mpg.de/th/paper.htm
Ion mass spectrometer
An ion mass spectrometer is described which detects and indicates the characteristics of ions received over a wide angle, and which indicates the mass to charge ratio, the energy, and the direction of each detected ion. The spectrometer includes a magnetic analyzer having a sector magnet that passes ions received over a wide angle, and an electrostatic analyzer positioned to receive ions passing through the magnetic analyzer. The electrostatic analyzer includes a two dimensional ion sensor at one wall of the analyzer chamber, that senses not only the lengthwise position of the detected ion to indicate its mass to charge ratio, but also detects the ion position along the width of the chamber to indicate the direction in which the ion was traveling
Novel Reconstruction mechanisms: A comparison between group-III-nitrides and "traditional" III-V-semiconductors
We have studied the driving forces governing reconstructions on polar GaN
surfaces employing first-principles total-energy calculations. Our results
reveal properties not observed for other semiconductors, as for example a
strong tendency to stabilize Ga-rich surfaces. This mechanism is shown to have
important consequences on various surface properties: Novel and hitherto
unexpected structures are stable, surfaces may become metallic although GaN is
a wide-bandgap semiconductor, and the surface energy is significantly higher
than for other semiconductors. We explain these features in terms of the small
lattice constant of GaN and the unique bond strength of nitrogen molecules.Comment: 13 pages, 5 figure
Dirichlet Boundary Value Problems of the Ernst Equation
We demonstrate how the solution to an exterior Dirichlet boundary value
problem of the axisymmetric, stationary Einstein equations can be found in
terms of generalized solutions of the Backlund type. The proof that this
generalization procedure is valid is given, which also proves conjectures about
earlier representations of the gravitational field corresponding to rotating
disks of dust in terms of Backlund type solutions.Comment: 22 pages, to appear in Phys. Rev. D, Correction of a misprint in
equation (4
Differentially rotating disks of dust: Arbitrary rotation law
In this paper, solutions to the Ernst equation are investigated that depend
on two real analytic functions defined on the interval [0,1]. These solutions
are introduced by a suitable limiting process of Backlund transformations
applied to seed solutions of the Weyl class. It turns out that this class of
solutions contains the general relativistic gravitational field of an arbitrary
differentially rotating disk of dust, for which a continuous transition to some
Newtonian disk exists. It will be shown how for given boundary conditions (i.
e. proper surface mass density or angular velocity of the disk) the
gravitational field can be approximated in terms of the above solutions.
Furthermore, particular examples will be discussed, including disks with a
realistic profile for the angular velocity and more exotic disks possessing two
spatially separated ergoregions.Comment: 23 pages, 3 figures, submitted to 'General Relativity and
Gravitation
Membrane Protein Spatial Organization in MDCK Cell Cultures
Studies on the organization of membrane proteins such as GPCRs in live cells help to draw important conclusions regarding cellular communication, and have become a significant topic in drug therapy development. Observing proteins in MDCK cells, which are able to form three-dimensional cultures, provides an intermediate step to better understanding the organization and behavior of these proteins in a tissue-like culture. In this study, fluorescent stains were used on fixed cells to show the morphological differences between MDCK cells in monolayer cultures and three-dimensional cultures, which form hollow, spherical cysts of tightly packed cells. Monolayer cells were transfected to view the spatial organization of the GPCR protein, opsin, in live MDCK cells, and compared to the non-GPCR protein, plexin. Images from the transfected cells for both opsin and plexin show successful localization of the proteins to the cellular membrane in a relatively even dispersion, with no obvious sorting to any particular domain of the cell. This understanding of membrane protein organization in monolayer MDCK cultures provides a strong basis for future experiments to be performed on three-dimensional, tissue-like cultures of these cells
The influence of short range forces on melting along grain boundaries
We investigate a model which couples diffusional melting and nanoscale
structural forces via a combined nano-mesoscale description. Specifically, we
obtain analytic and numerical solutions for melting processes at grain
boundaries influenced by structural disjoining forces in the experimentally
relevant regime of small deviations from the melting temperature. Though
spatially limited to the close vicinity of the tip of the propagating melt
finger, the influence of the disjoining forces is remarkable and leads to a
strong modification of the penetration velocity. The problem is represented in
terms of a sharp interface model to capture the wide range of relevant length
scales, predicting the growth velocity and the length scale describing the
pattern, depending on temperature, grain boundary energy, strength and length
scale of the exponential decay of the disjoining potential. Close to
equilibrium the short-range effects near the triple junctions can be expressed
through a contact angle renormalisation in a mesoscale formulation. For higher
driving forces strong deviations are found, leading to a significantly higher
melting velocity than predicted from a purely mesoscopic description.Comment: 10 page
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