84,504 research outputs found
How to Detect the Signatures of Self-Gravitating Circumstellar Discs with the Atacama Large Millimetre/sub-millimetre Array
In this paper we present simulated Atacama Large Millimetre/sub-millimetre
Array (ALMA) observations of self-gravitating circumstellar discs with
different properties in size, mass and inclination, located in four of the most
extensively studied and surveyed star-forming regions. Starting from a Smoothed
Particle Hydrodynamics (SPH) simulation and representative dust opacities, we
have initially constructed maps of the expected emission at sub-mm wavelengths
of a large sample of discs with different properties. We have then simulated
realistic observations of discs as they may appear with ALMA using the Common
Astronomy Software Application ALMA simulator. We find that, with a proper
combination of antenna configuration and integration time, the spiral structure
characteristic of self-gravitating discs is readily detectable by ALMA over a
wide range of wavelengths at distances comparable to TW Hydrae (pc), Taurus - Auriga and Ophiucus (pc) star-forming regions.
However, for discs located in Orion complex (pc) only the largest
discs in our sample (outer radius of 100 au) show a spatially resolved
structure while the smaller ones (outer radius of 25 au) are characterized by a
spiral structure that is not conclusively detectable with ALMA.Comment: 12 pages, 10 figure
Integral field unit spectroscopy of 10 early type galactic nuclei: I - Principal component analysis Tomography and nuclear activity
Most massive galaxies show emission lines that can be characterized as
LINERs. To what extent this emission is related to AGNs or to stellar processes
is still an open question. In this paper, we analysed a sample of such galaxies
to study the central region in terms of nuclear and circumnuclear emission
lines, as well as the stellar component properties. For this reason, we
selected 10 massive ( > 200 km/s) nearby (d < 31 Mpc) galaxies and
observed them with the IFU/GMOS (integral field unit/Gemini Multi-Object
Spectrograph) spectrograph on the Gemini South Telescope. The data were
analysed with principal component analysis (PCA) Tomography to assess the main
properties of the objects. Two spectral regions were analysed: a yellow region
(5100-5800 A), adequate to show the properties of the stellar component, and a
red region (6250-6800 A), adequate to analyse the gaseous component. We found
that all objects previously known to present emission lines have a central
AGN-type emitting source. They also show gaseous and stellar kinematics typical
of discs. Such discs may be co-aligned (NGC 1380 and ESO 208 G-21), in
counter-rotation (IC 1459 and NGC 7097) or misaligned (IC 5181 and NGC 4546).
We also found one object with a gaseous disc but no stellar disc (NGC 2663),
one with a stellar disc but no gaseous disc (NGC 1404), one with neither
stellar nor gaseous disc (NGC 1399) and one with probably ionization cones (NGC
3136). PCA Tomography is an efficient method for detecting both the central AGN
and gaseous and stellar discs. In the two cases (NGC 1399 and NGC 1404) in
which no lines were previously reported, we found no evidence of either nuclear
or circumnuclear emission, using PCA Tomography only.Comment: 26 pages, 17 figures, accepted for publication in MNRA
Basic properties of toroidal structures in Kerr--de Sitter backgrounds
Perfect fluid tori with uniform distribution of the specific angular momentum
orbiting the Kerr-de Sitter black holes or naked singularities are studied.
Closed equipotential surfaces corresponding to stationary toroidal discs are
allowed only in the spacetimes admitting stable circular geodesics. The last
closed surface crosses itself in the cusp(s) enabling outflow(s) of matter from
the torus due to the violation of hydrostatic equilibrium. The repulsive
cosmological constant, , implies the existence of the outer cusp
(with a stabilizing effect on the tori because of "excretion", i.e., outflow of
matter from the torus into the outer space) and the strong collimation of open
equipotential surfaces along the rotational axis. Both the effects take place
nearby the so-called static radius where the gravitational attraction is just
balanced by the cosmic repulsion. The plus-family discs (which are always
corotating in the black-hole backgrounds but can be counterrotating, even with
negative energy of the fluid elements, in some naked singularity backgrounds)
are thicker and more extended than the minus-family ones (which are always
counterrotating in all backgrounds). If the parameters of naked-singularity
spacetimes are very close to the parameters of extreme black-hole spacetimes,
the family of possible disc-like configurations includes members with two
isolated discs where the inner one is always a counterrotating accretion disc.
Mass estimates for tori with nonrelativistic adiabatic equation of state give
limits on their central mass-density, for which the approximation of test fluid
is adequate.Comment: Updated version of the Section talk at Albert Einstein Century
International Conference at Palais de l'Unesco, Paris, France, 18-23 July,
2005; to appear in the Proceedings; AIP style files included; 8 page
Optical effects related to Keplerian discs orbiting Kehagias-Sfetsos naked singularities
We demonstrate possible optical signatures of the Kehagias-Sfetsos naked
singularity spacetimes representing spherically symmetric vacuum solution of
the modified Ho\v{r}ava gravity. In such spacetimes, accretion structures
significantly different from those present in the standard black hole
spacetimes occur due to the "antigravity" effect causing existence of an
internal static sphere surrounded by Keplerian discs. We focus our attention on
the optical effects related to the Keplerian accretion discs, constructing the
optical appearance of the Keplerian discs, the spectral continuum due to their
thermal radiation, and spectral profiled lines generated in the innermost parts
of such discs. The KS naked singularity signature is strongly encoded in the
characteristics of predicted optical effects, especially in the case of the
spectral continuum and spectral lines profiled by the strong gravity of the
spacetimes, due to the region of the vanishing of the angular velocity gradient
influencing the effectivity of the viscosity mechanism. We can conclude that
optical signatures of the Kehagias-Sfetsos naked singularities can be well
distinguished from the signatures of the standard black holes
The Complete Jamming Landscape of Confined Hard Discs
An exact description of the complete jamming landscape is developed for a
system of hard discs of diameter , confined between two lines separated
by a distance . By considering all possible local
packing arrangements, the generalized ensemble partition function of jammed
states is obtained using the transfer matrix method, which allows us to
calculate the configurational entropy and the equation of state for the
packings. Exploring the relationship between structural order and packing
density, we find that the geometric frustration between local packing
environments plays an important role in determining the density distribution of
jammed states and that structural "randomness" is a non-monotonic function of
packing density. Molecular dynamics simulations show that the properties of the
equilibrium liquid are closely related to those of the landscape.Comment: 5 Pages, 4 figure
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