2,924 research outputs found
On the stability of self-gravitating accreting flows
Analytic methods show stability of the stationary accretion of test fluids
but they are inconclusive in the case of self-gravitating stationary flows. We
investigate numerically stability of those stationary flows onto compact
objects that are transonic and rich in gas. In all studied examples solutions
appear stable. Numerical investigation suggests also that the analogy between
sonic and event horizons holds for small perturbations of compact support but
fails in the case of finite perturbations.Comment: 10 pages, accepted for publication in PR
Differential effects of Alzheimer\u27s disease and Huntington\u27s disease on the performance of mental rotation
he ability to spatially rotate a mental image was compared in patients with Alzheimer\u27s disease (AD; n = 18) and patients with Huntington\u27s disease (HD; n = 18). Compared to their respective age-matched normal control (NC) group, the speed, but not the accuracy, of mental rotation abnormally decreased with increasing angle of orientation for patients with HD. In contrast, the accuracy, but not the speed, of rotation abnormally decreased with increasing angle of orientation for patients with AD. Additional analyses showed that these unique patterns of performance were not attributable to different speed/accuracy trade-off sensitivities. This double dissociation suggests that the distinct brain regions affected in the two diseases differentially contribute to speed and accuracy of mental rotation. Specifically, the slowing exhibited by HD patients may be mediated by damage to the basal ganglia, whereas the spatial manipulation deficit of AD patients may reflect pathology in parietal and temporal lobe association cortices important for visuospatial processing. (JINS, 2005, 11, 30–39.
No news for Kerr-Schild fields
Algebraically special fields with no gravitational radiation are described.
Kerr-Schild fields, which include as a concrete case the Kinnersley photon
rocket, form an important subclass of them.Comment: 4 pages, Revtex
Laudatores Temporis Acti, or Why Cosmology is Alive and Well - A Reply to Disney
A recent criticism of cosmological methodology and achievements by Disney
(2000) is assessed. Some historical and epistemological fallacies in the said
article have been highlighted. It is shown that---both empirically and
epistemologically---modern cosmology lies on sounder foundations than it is
portrayed. A brief historical account demonstrates that this form of
unsatisfaction with cosmology has had a long tradition, and rather meagre
results in the course of the XX century.Comment: 11 pages, no figures; a criticism of astro-ph/0009020; Gen. Rel.
Grav., accepted for publicatio
Extended radio emission in BL Lac objects - I: the images
We have observed 28 sources selected from the 1Jy sample of BL Lac objects
(Stickel et al. 1991) with the Very Large Array (VLA) in A, B and D
configurations at 1.36, 1.66 and 4.85 GHz, and/or with the Westerbork Synthesis
Radio Telescope (WSRT) at 1.40 GHz. In this paper we present high sensitivity
images at arcsecond resolution of the 18 objects showing extended structure in
our images, and of another source from the FIRST (Faint Images of the Radio Sky
at Twenty-cm) survey (Becker et al. 1995). In general our high sensitivity
images reveal an amount of extended emission larger than previously reported.
In some objects the luminosity of the extended structure is comparable with
that of FR~II radio sources. A future paper will be devoted to the
interpretation of these results.Comment: 12 pages, 35 figures, to appear on A&A Supp. Ser., postscript file
with figures included available at
http://www.ira.noto.cnr.it/staff/carlo/ds1030.ps.g
Killing vectors and anisotropy
We consider an action that can generate fluids with three unequal stresses
for metrics with a spacelike Killing vector. The parameters in the action are
directly related to the stress anisotropies. The field equations following from
the action are applied to an anisotropic cosmological expansion and an
extension of the Gott-Hiscock cosmic string
Accretion onto the Companion of Eta Carinae During the Spectroscopic Event: II. X-Ray Emission Cycle
We calculate the X-ray luminosity and light curve for the stellar binary
system Eta Carinae for the entire orbital period of 5.54 years. By using a new
approach we find, as suggested before, that the collision of the winds blown by
the two stars can explain the X-ray emission and temporal behavior. Most X-ray
emission in the 2-10 \kev band results from the shocked secondary stellar
wind. The observed rise in X-ray luminosity just before minimum is due to
increase in density and subsequent decrease in radiative cooling time of the
shocked fast secondary wind. Absorption, particularly of the soft X-rays from
the primary wind, increases as the system approaches periastron and the shocks
are produced deep inside the primary wind. However, absorption can not account
for the drastic X-ray minimum. The 70 day minimum is assumed to result from the
collapse of the collision region of the two winds onto the secondary star. This
process is assumed to shut down the secondary wind, hence the main X-ray
source. We show that this assumption provides a phenomenological description of
the X-ray behavior around the minimum.Comment: The Astrophysical Journal, in pres
Solution generating theorems for perfect fluid spheres
The first static spherically symmetric perfect fluid solution with constant
density was found by Schwarzschild in 1918. Generically, perfect fluid spheres
are interesting because they are first approximations to any attempt at
building a realistic model for a general relativistic star. Over the past 90
years a confusing tangle of specific perfect fluid spheres has been discovered,
with most of these examples seemingly independent from each other. To bring
some order to this collection, we develop several new transformation theorems
that map perfect fluid spheres into perfect fluid spheres. These transformation
theorems sometimes lead to unexpected connections between previously known
perfect fluid spheres, sometimes lead to new previously unknown perfect fluid
spheres, and in general can be used to develop a systematic way of classifying
the set of all perfect fluid spheres. In addition, we develop new ``solution
generating'' theorems for the TOV, whereby any given solution can be
``deformed'' to a new solution. Because these TOV-based theorems work directly
in terms of the pressure profile and density profile it is relatively easy to
impose regularity conditions at the centre of the fluid sphere.Comment: 8 pages, no figures, to appear in the proceedings of the NEB XII
Conference (Recent Developments in Gravity), 29 June - 2 July, 2006, Napflio,
Greec
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