3,314 research outputs found
Stability of general relativistic Miyamoto-Nagai galaxies
The stability of a recently proposed general relativistic model of galaxies
is studied in some detail. This model is a general relativistic version of the
well known Miyamoto-Nagai model that represents well a thick galactic disk. The
stability of the disk is investigated under a general first order perturbation
keeping the spacetime metric frozen (no gravitational radiation is taken into
account). We find that the stability is associated with the thickness of the
disk. We have that flat galaxies have more not-stable modes than the thick ones
i.e., flat galaxies have a tendency to form more complex structures like rings,
bars and spiral arms.Comment: 11 pages, 5 figures, accepted for publication in MNRA
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
A Luminous Companion to SGR 1806-20
We have obtained infrared spectra of the star suggested to be the counterpart
of the soft gamma-ray repeater (SGR) 1806-20. We found strong emission lines
similar to those seen in the spectra of the rare Luminous Blue Variables and
B[e] stars. A He I absorption line is also seen, from which we infer a spectral
type O9--B2. This classification, in combination with the minimum distance of
\simgt6 kpc inferred from its extinction, makes the star one of the most
luminous in the Galaxy. We infer that it is a companion to SGR 1806-20, and
suggest that the presence of a companion is somehow related to the SGR
phenomenon.Comment: 5 pages, AASTEX text+table and 2 PostScript figures (needs LaTeX
style files aaspptwo.sty, epsf.sty and rotate.sty). In case of problems,
contact [email protected]. Postscript file of complete article available
on request. (Replaced because first version had one wrong reference in it
Near Infrared Observations of a Redshift 4.92 Galaxy: Evidence for Significant Dust Absorption
Near-infrared imaging and spectroscopy have been obtained of the
gravitationally lensed galaxy at z=4.92 discovered in HST images by Franx et
al. (1997). Images at 1.2, 1.6 and 2.2 microns show the same arc morphology as
the HST images. The spectrum with resolution \lambda / \Delta\lambda ~ 70 shows
no emission lines with equivalent width stronger than 100 A in the rest frame
wavelength range 0.34 to 0.40 microns. In particular, [OII]3727 A and
[NeIII]3869 A are not seen. The energy distribution is quite blue, as expected
for a young stellar population with the observed Ly alpha flux. The spectral
energy distribution can be fit satisfactorily for such a young stellar
population when absorption by dust is included. The models imply a reddening
0.1 mag < E(B-V) < 0.4 mag. The stellar mass of the lensed galaxy lies in the
range of 2 to 16 x 10^9 Msun. This is significantly higher than estimates based
on the HST data alone. Our data imply that absorption by dust is important to
redshifts of ~5.Comment: LaTeX with ApJ journal format, 2 postscript figures, ApJL, accepte
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
We present a three-parameter family of solutions to the stationary
axisymmetric Einstein equations that describe differentially rotating disks of
dust. They have been constructed by generalizing the Neugebauer-Meinel solution
of the problem of a rigidly rotating disk of dust. The solutions correspond to
disks with angular velocities depending monotonically on the radial coordinate;
both decreasing and increasing behaviour is exhibited. In general, the
solutions are related mathematically to Jacobi's inversion problem and can be
expressed in terms of Riemann theta functions. A particularly interesting
two-parameter subfamily represents Baecklund transformations to appropriate
seed solutions of the Weyl class.Comment: 14 pages, 3 figures. To appear in "General Relativity and
Gravitation". Second version with minor correction
Space missions to comets
The broad impact of a cometary mission is assessed with particular emphasis on scientific interest in a fly-by mission to Halley's comet and a rendezvous with Tempel 2. Scientific results, speculations, and future plans are discussed
Construction and analysis of surface phase diagrams to describe segregation and dissolution behavior of Al and Ca in Mg alloys
Segregation and dissolution behavior of Mg alloyed with Ca and Al are studied by performing density functional theory calculations considering an extensive set of surface structures and compositions. Combining a McLean-like approach to describe disordered surface structures with ab initio surface science approaches for ordered surface structures we construct surface phase diagrams for these alloys. We utilize these diagrams to study segregation phenomena and chemical trends for surfaces in contact with a dry environment or with an aqueous electrolyte. We show that the presence of water dramatically impacts the stability and chemical composition of the considered metallic surfaces. We furthermore find that the two alloying elements behave qualitatively different: whereas Ca strongly segregates to the surface and becomes dissolved upon exposure of the surface to water, Al shows an anti-segregation behavior, i.e., it remains in Mg bulk. These findings provide an explanation for the experimentally observed increase/decrease in corrosion rates when alloying Mg with Al/Ca
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
The frequency of T Tauri companion stars
We present the results of a magnitude limited (K≤8.5 mag) multiplicity survey of T Tauri stars in two nearby star forming regions, Tauris-Auriga and Ophiuchus-Scorpius. Each of the 69 stars in the sample was observed at K(2.2 microns) with an infrared array camera on the Hale 5-m Telescope at Palomar Observatory and imaged using two-dimensional speckle imaging techniques. Thirty three companion stars were found of which 15 were new detections. One of the main results of this survey indicates that the binary star frequncy in the projected linear separation range 14 to 225 AU for T Tauri stars (59 ± 16%) is a factor of 3.5 greater than that of the solar-type main sequence stars (17 ±3%). Given the limited angular separation range of this survey, i.e., both the spectroscopic and wide binaries are missed, the rate at which binaries are detected suggests that most, if not all, T Tauri stars have companions. We propose that the observed overabundance of companions to the T Tauri stars relative to their older counterparts on the main sequence is an evolutionary effect; in this scheme triple and higher order T Tauri star systems, which are observed at higher frequencies than for the solar-type main sequence stars, are disrupted by close encounters with another star or system of stars
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