7,290 research outputs found
SPH simulations of irradiation-driven warped accretion discs and the long periods in X-ray binaries
We present three dimensional smoothed particle hydrodynamics (SPH)
calculations of irradiation-driven warping of accretion discs. Initially
unwarped planar discs are unstable to the radiation reaction when the disc is
illuminated by a central radiation source. The disc warps and tilts and
precesses slowly in a retrograde direction; its shape continuously flexes in
response to the changing orientation of the Roche potential. We simulate ten
systems: eight X-ray binaries, one cataclysmic variable (CV), and a `generic'
low mass X-ray binary (LMXB). We adopt system parameters from observations and
tune a single parameter: our model X-ray luminosity () to reproduce the
observed or inferred super-orbital periods. Without exception, across a wide
range of parameter space, we find an astonishingly good match between the
observed and the model . We conclude irradiation-driven warping
is the mechanism underlying the long periods in X-ray binaries. Our Her X-1
simulation simultaneously reproduces the observed , the "main-" and
"short-high" X-ray states and the orbital inclination. Our simulations of SS
433 give a maximum warp angle of , a good match to the cone
traced by the jets, but this angle is reached only in the outer disc. In all
cases, the overall disc tilt is less than \degrees{13} and the maximum disc
warp is less than and or equal to \degrees{21}.Comment: 17 pages, 14 figures, shorter abstract (24 lines limit
Subset Warping: Rubber Sheeting with Cuts
Image warping, often referred to as "rubber sheeting" represents the deformation of a domain image space into a range image space. In this paper, a technique is described which extends the definition of a rubber-sheet transformation to allow a polygonal region to be warped into one or more subsets of itself, where the subsets may be multiply connected. To do this, it constructs a set of "slits" in the domain image, which correspond to discontinuities in the range image, using a technique based on generalized Voronoi diagrams. The concept of medial axis is extended to describe inner and outer medial contours of a polygon. Polygonal regions are decomposed into annular subregions, and path homotopies are introduced to describe the annular subregions. These constructions motivate the definition of a ladder, which guides the construction of grid point pairs necessary to effect the warp itself
Effect of the Magellanic Clouds on the Milky Way disk and VICE VERSA
The satellite-disk interaction provides limits on halo properties in two
ways: (1) physical arguments motivate the excitation of observable Galactic
disk structure in the presence of a massive halo, although precise limits on
halo parameters are scenario-dependent; (2) conversely, the Milky Way as a
whole has significant dynamical effect on LMC structure and this interaction
also leads to halo limits. Together, these scenarios give strong corroboration
of our current gravitational mass estimates and suggests a rapidly evolving
LMC.Comment: 12 pages, 8 Postscript figures, uses paspconf.sty. To appear in the
Third Stromlo Symposium: The Galactic Halo (ASP Conference Series), in press.
HTML version available at: http://www-astro.phast.umass.edu/~weinberg/stroml
Source to Accretion Disk Tilt
Many different system types retrogradely precess, and retrograde precession
could be from a tidal torque by the secondary on a misaligned accretion disk.
However, a source to cause and maintain disk tilt is unknown. In this work, we
show that accretion disks can tilt due to a force called lift. Lift results
from differing gas stream supersonic speeds over and under an accretion disk.
Because lift acts at the disk's center of pressure, a torque is applied around
a rotation axis passing through the disk's center of mass. The disk responds to
lift by pitching around the disk's line of nodes. If the gas stream flow ebbs,
then lift also ebbs and the disk attempts to return to its original
orientation.
To first approximation, lift does not depend on magnetic fields or radiation
sources but does depend on mass and the surface area of the disk. Also, for
disk tilt to be initiated, a minimum mass transfer rate must be exceeded. For
example, a disk around a 0.8 compact central
object requires a mass transfer rate greater than
Myr, a value well below known mass transfer
rates in Cataclysmic Variable Dwarf Novae systems that retrogradely precess and
that exhibit negative superhumps in their light curves and a value well below
mass transfer rates in protostellar forming systems
Magnetically warped discs in close binaries
We demonstrate that measurable vertical structure can be excited in the
accretion disc of a close binary system by a dipolar magnetic field centred on
the secondary star. We present the first high resolution hydrodynamic
simulations to show the initial development of a uniform warp in a tidally
truncated accretion disc. The warp precesses retrogradely with respect to the
inertial frame. The amplitude depends on the phase of the warp with respect to
the binary frame. A warped disc is the best available explanation for negative
superhumps.Comment: 11 pages, 10 figures, MNRAS accepte
Radiative transfer modelling of parsec-scale dusty warped discs
Warped discs have been found on (sub-)parsec scale in some nearby Seyfert
nuclei, identified by their maser emission. Using dust radiative transfer
simulations we explore their observational signatures in the infrared in order
to find out whether they can partly replace the molecular torus. Strong
variations of the brightness distributions are found, depending on the
orientation of the warp with respect to the line of sight. Whereas images at
short wavelengths typically show a disc-like and a point source component, the
warp itself only becomes visible at far-infrared wavelengths. A similar variety
is visible in the shapes of the spectral energy distributions. Especially for
close to edge-on views, the models show silicate feature strengths ranging from
deep absorption to strong emission for variations of the lines of sight towards
the warp. To test the applicability of our model, we use the case of the
Circinus galaxy, where infrared interferometry has revealed a highly elongated
emission component matching a warped maser disc in orientation and size. Our
model is for the first time able to present a physical explanation for the
observed dust morphology as coming from the AGN heated dust. As opposed to
available torus models, a warped disc morphology produces a variety of silicate
feature shapes for grazing lines of sight, close to an edge-on view. This could
be an attractive alternative to a claimed change of the dust composition for
the case of the nearby Seyfert 2 galaxy NGC 1068, which harbours a warped maser
disc as well.Comment: accepted by MNRA
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