20,396 research outputs found
Kaluza-Klein electrically charged black branes in M-theory
We present a class of Kaluza-Klein electrically charged black p-brane
solutions of ten-dimensional, type IIA superstring theory. Uplifting to eleven
dimensions these solutions are studied in the context of M-theory. They can be
interpreted either as a p+1 extended object trapped around the eleventh
dimension along which momentum is flowing or as a boost of the following
backgrounds: the Schwarzschild black (p+1)-brane or the product of the
(10-p)-dimensional Euclidean Schwarzschild manifold with the (p+1)-dimensional
Minkowski spacetime.Comment: 16 pages, uses latex and epsf macro, figures include
Relativistic J-matrix method
The relativistic version of the J-matrix method for a scattering problem on
the potential vanishing faster than the Coulomb one is formulated. As in the
non-relativistic case it leads to a finite algebraic eigenvalue problem. The
derived expression for the tangent of phase shift is simply related to the
non-relativistic case formula and gives the latter as a limit case. It is due
to the fact that the used basis set satisfies the ``kinetic balance
condition''.Comment: 21 pages, RevTeX, accepted for publication in Phys. Rev.
3-D Photoionization Structure and Distances of Planetary Nebulae II. Menzel 1
We present the results of a spatio-kinematic study of the planetary nebula
Menzel 1 using spectro-photometric mapping and a 3-D photoionization code. We
create several 2-D emission line images from our long-slit spectra, and use
these to derive the line fluxes for 15 lines, the Halpha/Hbeta extinction map,
and the [SII] line ratio density map of the nebula. We use our photoionization
code constrained by these data to derive the three-dimensional nebular
structure and ionizing star parameters of Menzel 1 by simultaneously fitting
the integrated line intensities, the density map, and the observed morphologies
in several lines, as well as the velocity structure. Using theoretical
evolutionary tracks of intermediate and low mass stars, we derive a mass for
the central star of 0.63+-0.05 Msolar. We also derive a distance of 1050+_150
pc to Menzel 1.Comment: To be published in ApJ of 10th February 2005. 12 figure
Abelian Duality
We show that on three-dimensional Riemannian manifolds without boundaries and
with trivial first real de Rham cohomology group (and in no other dimensions)
scalar field theory and Maxwell theory are equivalent: the ratio of the
partition functions is given by the Ray-Singer torsion of the manifold. On the
level of interaction with external currents, the equivalence persists provided
there is a fixed relation between the charges and the currents.Comment: 11 pages, LaTeX, no figures, a reference added, submitted to Phys.
Rev.
Type IIB instanton as a wave in twelve dimensions
0-brane of type IIA string theory can be interpreted as a dimensional
reduction of a gravitational wave in 11 dimensions. We observe that a similar
interpretation applies also to the D-instanton background of type IIB theory:
it can be viewed as a reduction (along one spatial and one time-like direction)
of a wave in a 12-dimensional theory. The instanton charge is thus related to a
linear momentum in 12 dimensions. This suggests that the instanton should play
as important role in type IIB theory as the 0-brane is supposed to play in type
IIA theory.Comment: 7 pages, harvmac (minor corrections and a reference added
On the Luminosities and Temperatures of Extended X-ray Emission from Planetary Nebulae
We examine mechanisms that may explain the luminosities and relatively low
temperatures of extended X-ray emission in planetary nebulae. By building a
simple flow structure for the wind from the central star during the proto, and
early, planetary nebulae phase, we estimate the temperature of the X-ray
emitting gas and its total X-ray luminosity. We conclude that in order to
account for the X-ray temperature and luminosity, both the evolution of the
wind from the central star and the adiabatic cooling of the post-shocked wind's
material must be considered. The X-ray emitting gas results mainly from shocked
wind segments that were expelled during the early planetary nebulae phase, when
the wind speed was moderate. Alternatively, the X-ray emitting gas may result
from a collimated fast wind blown by a companion to the central star. Heat
conduction and mixing between hot and cool regions are likely to occur in some
cases and may determine the detailed X-ray morphology of a nebula, but are not
required to explain the basic properties of the X-ray emitting gas.Comment: ApJ, submitted; 16 page
High-velocity collimated outflows in planetary nebulae: NGC 6337, He 2-186, and K 4-47
We have obtained narrow-band images and high-resolution spectra of the
planetary nebulae NGC 6337, He 2-186, and K 4-47, with the aim of investigating
the relation between their main morphological components and several
low-ionization features present in these nebulae. The data suggest that NGC
6337 is a bipolar PN seen almost pole on, with polar velocities higher than 200
km/s. The bright inner ring of the nebula is interpreted to be the "equatorial"
density enhancement. It contains a number of low-ionization knots and outward
tails that we ascribe to dynamical instabilities leading to fragmentation of
the ring or transient density enhancements due to the interaction of the
ionization front with previous density fluctuations in the ISM. The lobes show
a pronounced point-symmetric morphology and two peculiar low-ionization
filaments whose nature remains unclear. The most notable characteristic of He
2-186 is the presence of two high-velocity (higher than 135 km/s) knots from
which an S-shaped lane of emission departs toward the central star. K 4-47 is
composed of a compact core and two high-velocity, low-ionization blobs. We
interpret the substantial broadening of line emission from the blobs as a
signature of bow shocks, and using the modeling of Hartigan, Raymond, & Hartman
(1987), we derive a shock velocity of 150 km/s and a mild inclination of the
outflow on the plane of the sky. We discuss possible scenarios for the
formation of these nebulae and their low-ionization features. In particular,
the morphology of K 4-47 hardly fits into any of the usually adopted mass-loss
geometries for single AGB stars. Finally, we discuss the possibility that
point-symmetric morphologies in the lobes of NGC 6337 and the knots of He 2-186
are the result of precessing outflows from the central stars.Comment: 16 pages plus 7 figures, ApJ accepted. Also available at
http://www.iac.es/publicaciones/preprints.htm
The "Twin Jet" Planetary Nebula M2-9
We present a model for the structure, temporal behavior, and evolutionary
status of the bipolar nebula M2-9. According to this model the system consists
of an AGB or post-AGB star and a hot white dwarf companion, with an orbital
period of about 120 years. The white dwarf has undergone a symbiotic nova
eruption about 1200 years ago, followed by a supersoft x-ray source phase. The
positional shift of the bright knots in the inner nebular lobes is explained in
terms of a revolving ionizing source. We show that the interaction between the
slow, AGB star's wind, and a collimated fast wind from the white dwarf clears a
path for the ionizing radiation in one direction, while the radiation is
attenuated in others. This results in the mirror-symmetric (as opposed to the
more common point-symmetric) shift in the knots. We show that M2-9 provides an
important evolutionary link among planetary nebulae with binary central stars,
symbiotic systems, and supersoft x-ray sources.Comment: 13 pages + 2 figures. Submitted to Ap
Intrinsic hole localization mechanism in magnetic semiconductors
The interplay between clustering and exchange coupling in magnetic
semiconductors for the prototype (Ga_{1-x},Mn_x)As with manganese
concentrations x of 1/16 and 1/32 in the interesting experimental range is
investigated. For x ~ 6 %, when all possible arrangements of two atoms within a
large supercell are considered, the clustering of Mn atoms at nearest-neighbour
Ga sites is energetically preferred. As shown by spin density analysis, this
minimum energy configuration localizes further one hole and reduces the
effective charge carrier concentration. Also the exchange coupling constant
increases to a value corresponding to lower Mn concentrations with decreasing
inter Mn distance.Comment: Accepted for publication in Journal of Physics: Condensed Matte
Star formation and figure rotation in the early-type galaxy NGC2974
We present Galaxy Evolution Explorer (GALEX) far (FUV) and near (NUV)
ultraviolet imaging of the nearby early-type galaxy NGC2974, along with
complementary ground-based optical imaging. In the ultraviolet, the galaxy
reveals a central spheroid-like component and a newly discovered complete outer
ring of radius 6.2kpc, with suggestions of another partial ring at an even
larger radius. Blue FUV-NUV and UV-optical colours are observed in the centre
of the galaxy and from the outer ring outward, suggesting young stellar
populations (< 1Gyr) and recent star formation in both locations. This is
supported by a simple stellar population model which assumes two bursts of star
formation, allowing us to constrain the age, mass fraction and surface mass
density of the young component pixel by pixel. Overall, the mass fraction of
the young component appears to be just under 1per cent (lower limit,
uncorrected for dust extinction). The additional presence of a nuclear and an
inner ring (radii 1.4 and 2.9kpc, respectively), as traced by [OIII] emission,
suggests ring formation through resonances. All three rings are consistent with
a single pattern speed of km/s/kpc, typical of S0 galaxies and only
marginally slower than expected for a fast bar if traced by a small observed
surface brightness plateau. This thus suggests that star formation and
morphological evolution in NGC2974 at the present epoch are primarily driven by
a rotating asymmetry (probably a large-scale bar), despite the standard
classification of NGC2974 as an E4 elliptical.Comment: 13 pages, 10 figures, Changed content, Accepted for publication in
MNRA
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