13,641 research outputs found
Geometric properties of generalized symmetric spaces
It is shown that four-dimensional generalized symmetric spaces can be
naturally equipped with some additional structures defined by means of their
curvature operators. As an application, those structures are used to
characterize generalized symmetric spaces
Turbulent dissipation in the interstellar medium: implications for galaxy formation and evolution
We study turbulent dissipation in the ISM and explore some implications for
galaxy formation and evolution using 2D MHD numerical simulations of
compressible fluids. The turbulent kinetic energy E_k is injected by stellar
sources formed self-consistently in the simulation. In the ISM-like fluid,
regimes of both forced and decaying turbulence coexist. In the active turbulent
regions (forced regime), E_k is dissipated locally and efficiently. In the
decaying regime (far from input sources), E_k(t) decays ~(1+t)^-0.8. The
residual turbulent motions may propagate distances of the order of the observed
disk height, suggesting that turbulence may be the responsible of vertical
support and star formation self-regulation at the disk level, but not at the
level of the whole cosmological halo, as would be required in some models of
galaxy formation.Comment: 3 pages, uses rmaa.cls, to appear in "Astrophysical Plasmas: Codes,
Models, and Observations", Eds. J. Franco, J. Arthur, N. Brickhouse,
Rev.Mex.AA Conf. Series. An error in the calculation of the dissipation time,
t_d, was corrected (it referred to the dissipation in the forced regions).
Now we report the global t_d, which is the relevant quantity for the whole
flo
Applying a one-dimensional PDR model to the Taurus molecular cloud and its atomic envelope
In this contribution, we test our previously published one-dimensional PDR
model for deriving total hydrogen volume densities from HI column density
measurements in extragalactic regions by applying it to the Taurus molecular
cloud, where its predictions can be compared to available data. Also, we make
the first direct detailed comparison of our model to CO(1-0) and far-infrared
emission. Using an incident UV flux G0 of 4.25 ({\chi} = 5) throughout the main
body of the cloud, we derive total hydrogen volume densities of \approx 430
cm-3, consistent with the extensive literature available on Taurus. The
distribution of the volume densities shows a log-normal shape with a hint of a
power-law shape on the high density end. We convert our volume densities to H2
column densities assuming a cloud depth of 5 parsec and compare these column
densities to observed CO emission. We find a slope equivalent to a CO
conversion factor relation that is on the low end of reported values for this
factor in the literature (0.9 x 1020 cm-2 (K km s-1)-1), although this value is
directly proportional to our assumed value of G0 as well as the cloud depth. We
seem to under-predict the total hydrogen gas as compared to 100 {\mu}m dust
emission, which we speculate may be caused by a higher actual G0 incident on
the Taurus cloud than is generally assumed.Comment: 14 pages, 14 figures. Accepted for publication in MNRA
Strong Field Gravitational Lensing by a Kerr Black Hole
We consider a Kerr black hole acting as a gravitational deflector within the
geometrical optics, and point source approximations. The Kerr black hole
gravitational lens geometry consisting of an observer and a source located far
away and placed at arbitrary inclinations with respect to the black hole's
equatorial plane is studied in the strong field regime. For this geometry the
null geodesics equations of our interest can go around the black hole several
times before reaching the observer. Such photon trajectories are written in
terms of the angular positions in the observer's sky and therefore become "lens
equations". As a consequence, we found for any image a simple classification
scheme based in two integers numbers: the number of turning points in the polar
coordinate , and the number of windings around the black hole's
rotation axis. As an application, and to make contact with the literature, we
consider a supermassive Kerr black hole at the Galactic center as a
gravitational deflector. In this case, we show that our proposed computational
scheme works successfully by computing the positions and magnifications of the
relativistic images for different source-observer geometries. In fact, it is
shown that our general procedure and results for the positions and
magnifications of the images off the black hole's equatorial plane, reduce and
agree with well known cases found in the literature.Comment: ReVTeX, 19 pages, 10 figures and 7 tables. Final version with some
grammatical changes to abstract and text (equations left unchanged).
Submitted to PR
Radiodetection of neutrino interactions in ice
We study the Cherenkov radiopulses emitted in PeV and EeV neutrino
interactions in ice. We discuss how the rich radiation pattern in the 100 MHz
to 1 GHz frequency range, in principle allows the measurement of shower
elongation produced in neutrino interactions opening up the possibility of
flavor recognition identifying charged current electron neutrino interactions.Comment: 4 pages, 2 ps figures, to be published in the proceedings of 26th
International Cosmic Ray Conference (Salt Lake City, 1999
Inflationary Cosmology: From Theory to Observations
The main aim of this paper is to provide a qualitative introduction to the
cosmic inflation and its relationship with current cosmological observations.
The inflationary model solves many of the fundamental problems that challenge
the Standard Big Bang cosmology i.e. Flatness, Horizon and Monopole problem,
and additionally provides an explanation for the initial conditions observed
throughout the Large-Scale Structure of the Universe, such as galaxies. In this
review we describe the general solutions carry out by a single scalar field.
Then with the use of current surveys, we show the constraints imposed on the
inflationary parameters which allow us to make the connection
between theoretical and observational cosmology. In this way, with the latest
results, it is possible to choose or at least to constrain the right
inflationary model, parameterised by a single scalar field potential .Comment: Lecture Notes in Cosmology. 37 pages, 15 figures, 2 table
Homogeneous affine surfaces: affine Killing vector fields and Gradient Ricci solitons
The homogeneous affine surfaces have been classified by Opozda. They may be
grouped into 3 families, which are not disjoint. The connections which arise as
the Levi-Civita connection of a surface with a metric of constant Gauss
curvature form one family; there are, however, two other families. For a
surface in one of these other two families, we examine the Lie algebra of
affine Killing vector fields and we give a complete classification of the
homogeneous affine gradient Ricci solitons. The rank of the Ricci tensor plays
a central role in our analysis
Conformal geometry of non-reductive four-dimensional homogeneous spaces
We classify non-reductive four-dimensional homogeneous conformally Einstein
manifolds.Comment: New version correcting some inaccuracies in the original pape
Half conformally flat gradient Ricci almost solitons
The local structure of half conformally flat gradient Ricci almost solitons
is investigated, showing that they are locally conformally flat in a
neighborhood of any point where the gradient of the potential function is
non-null. In opposition, if the gradient of the potential function is null,
then the soliton is a steady traceless -Einstein soliton and is
realized on the cotangent bundle of an affine surface
Locally conformally flat Lorentzian quasi-Einstein manifolds
We show that locally conformally flat quasi-Einstein manifolds are globally
conformally equivalent to a space form or locally isometric to a -wave or a
warped product
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