3,376 research outputs found
An Investigation into the Geometry of Seyfert Galaxies
We present a new method for the statistical investigation into the
distributions of the angle beta between the radio axis and the normal to the
galactic disk for a sample of Seyfert galaxies. We discuss how further
observations of the sample galaxies can strengthen the conclusions. Our data
are consistent with the hypothesis that AGN jets are oriented randomly in
space, independent of the position of the plane of the galaxy. By making the
simple assumption that the Standard Model of AGN holds, with a universal
opening angle of the thick torus of phi_c, we demonstrate a statistical method
to obtain an estimate of phi_c. Our data are not consistent with the
simple-minded idea that Seyfert 1s and Seyfert 2s are differentiated solely by
whether or not our line of sight lies within some fixed angle of the jet axis.
Our result is significant on the 2 sigma level and can thus be considered only
suggestive, not conclusive. A complete sample of Seyfert galaxies selected on
an isotropic property is required to obtain a conclusive result.Comment: 13 pages, Tex, 5 Postscript figures. Accepted Ap
The observable effects of tidally induced warps in protostellar discs
We consider the response of a protostellar disc to a tidally induced warp and
the resultant changes in the spectral energy distribution (SED). We argue that
for typical protostellar disc parameters the warp is communicated through the
disc in a wave-like fashion. We find that the main effects of the warp tend to
be at large radii (greater than 30 AU) and, for sufficiently small viscosity,
can be quite long-lived. This can result in non-uniform illumination of the
disc at these radii and can induce significant changes to the SED at
wavelengths greater than 100 microns.Comment: 7 pages, 9 figures. Accepted by MNRA
Radiation-Driven Warping: The Origin of Warps and Precession in Accretion Disks
A geometrically thin, optically thick, warped accretion disk with a central
source of luminosity is subject to non-axisymmetric forces due to radiation
pressure; the resulting torque acts to modify the warp. In a recent paper,
\cite{pri96} used a local analysis to show that initially planar accretion
disks are unstable to warping driven by radiation torque. Here we extend this
work with a global analysis of the stable and unstable modes. We confirm
Pringle's conclusion that thin centrally-illuminated accretion disks are
generically unstable to warping via this mechanism; we discuss the
time-evolution and likely steady-state of such systems and show specifically
that this mechanism can explain the warping of the disk of water masers in NGC
4258 and the 164-day precession period of the accretion disk in SS 433.
Radiation-driven warping and precession provides a robust mechanism for
producing warped, precessing accretion disks in active galactic nuclei and
X-ray binary systems.Comment: 16 pages, latex, 3 figure
Radiation induced warping of protostellar accretion disks
We examine the consequences of radiatively driven warping of accretion disks
surrounding pre-main-sequence stars. These disks are stable against warping if
the luminosity arises from a steady accretion flow, but are unstable at late
times when the intrinsic luminosity of the star overwhelms that provided by the
disk. Warps can be excited for stars with luminosities of around 10 solar
luminosities or greater, with larger and more severe warps in the more luminous
systems. A twisted inner disk may lead to high extinction towards stars often
viewed through their disks. After the disk at all radii becomes optically thin,
the warp decays gradually on the local viscous timescale, which is likely to be
long. We suggest that radiation induced warping may account for the origin of
the warped dust disk seen in Beta Pictoris, if the star is only around 10-20
Myr old, and could lead to non-coplanar planetary systems around higher mass
stars.Comment: 12 pages, including 3 figures. ApJ Letters, in pres
Variability of black hole accretion discs: The cool, thermal disc component
We extend the model of King et al. (2004) for variability in black hole
accretion discs, by taking proper account of the thermal properties of the
disc. Because the degree of variability in the King et al. (2004) model depends
sensitively on the ratio of disc thickness to radius, H/R, it is important to
follow the time-dependence of the local disc structure as the variability
proceeds. In common with previous authors, we develop a one-zone model for the
local disc structure. We agree that radial heat advection plays an important
role in determining the inner disc structure, and also find limit-cycle
behaviour. When the stochastic magnetic dynamo model of King et al. (2004) is
added to these models, we find similar variability behaviour to before.
We are now better placed to put physical constraints on model parameters. In
particular, we find that in order to be consistent with the low degree of
variability seen in the thermal disc component of black hole binaries, we need
to limit the energy density of the poloidal field that can be produced by local
dynamo cells in the disc to less than a few percent of the energy density of
the dynamo field within the disc itself.Comment: 18 pages, 17 figures, accepted by MNRA
Effects of Radiation Forces on the Frequency of Gravitomagnetic Precession Near Neutron Stars
Gravitomagnetic precession near neutron stars and black holes has received
much recent attention, particularly as a possible explanation of 15--60 Hz
quasi-periodic brightness oscillations (QPOs) from accreting neutron stars in
low-mass X-ray binaries, and of somewhat higher-frequency QPOs from accreting
stellar-mass black holes. Previous analyses of this phenomenon have either
ignored radiation forces or assumed for simplicity that the radiation field is
isotropic, and in particular that there is no variation of the radiation field
with angular distance from the rotational equatorial plane of the compact
object. However, in most realistic accretion geometries (e.g., those in which
the accretion proceeds via a geometrically thin disk) the radiation field
depends on latitude. Here we show that in this case radiation forces typically
have an important, even dominant, effect on the precession frequency of test
particles in orbits that are tilted with respect to the star's rotational
equator. Indeed, we find that even for accretion luminosities only a few
percent of the Eddington critical luminosity, the precession frequency near a
neutron star can be changed by factors of up to . Radiation forces
must therefore be included in analyses of precession frequencies near compact
objects, in such varied contexts as low-frequency QPOs, warp modes of disks,
and trapped oscillation modes. We discuss specifically the impact of radiation
forces on models of low-frequency QPOs involving gravitomagnetic precession,
and show that such models are rendered much less plausible by the effects of
radiation forces.Comment: 15 pages LaTeX including three figures, submitted to Ap
The Exciting Lives of Giant Molecular Clouds
We present a detailed study of the evolution of GMCs in a galactic disc
simulation. We follow individual GMCs (defined in our simulations by a total
column density criterion), including their level of star formation, from their
formation to dispersal. We find the evolution of GMCs is highly complex. GMCs
often form from a combination of smaller clouds and ambient ISM, and similarly
disperse by splitting into a number of smaller clouds and ambient ISM. However
some clouds emerge as the result of the disruption of a more massive GMC,
rather than from the assembly of smaller clouds. Likewise in some cases, clouds
accrete onto more massive clouds rather than disperse. Because of the
difficulty of determining a precursor or successor of a given GMC, determining
GMC histories and lifetimes is highly non-trivial. Using a definition relating
to the continuous evolution of a cloud, we obtain lifetimes typically of 4-25
Myr for >10^5 M GMCs, over which time the star formation efficiency
is about 1 %. We also relate the lifetime of GMCs to their crossing time. We
find that the crossing time is a reasonable measure of the actual lifetime of
the cloud, although there is considerable scatter. The scatter is found to be
unavoidable because of the complex and varied shapes and dynamics of the
clouds. We study cloud dispersal in detail and find both stellar feedback and
shear contribute to cloud disruption. We also demonstrate that GMCs do not
behave as ridge clouds, rather massive spiral arm GMCs evolve into smaller
clouds in inter-arm spurs.Comment: 15 pages, 16 figures, accepted for publication in MNRA
Fuelling Active Galactic Nuclei
We suggest that most nearby active galactic nuclei are fed by a series of
small--scale, randomly--oriented accretion events. Outside a certain radius
these events promote rapid star formation, while within it they fuel the
supermassive black hole. We show that the events have a characteristic time
evolution. This picture agrees with several observational facts. The expected
luminosity function is broadly in agreement with that observed for
moderate--mass black holes. The spin of the black hole is low, and aligns with
the inner disc in each individual feeding event. This implies radio jets
aligned with the axis of the obscuring torus, and uncorrelated with the
large--scale structure of the host galaxy. The ring of young stars observed
about the Galactic Centre are close to where our picture predicts that star
formation should occur.Comment: MNRAS, in pres
Magnetospherically-trapped dust and a possible model for the unusual transits at WD 1145+017
The rapidly evolving dust and gas extinction observed towards WD 1145+017 has
opened a real-time window onto the mechanisms for destruction-accretion of
planetary bodies onto white dwarf stars, and has served to underline the
importance of considering the dynamics of dust particles around such objects.
Here it is argued that the interaction between (charged) dust grains and the
stellar magnetic field is an important ingredient in understanding the physical
distribution of infrared emitting particles in the vicinity of such white
dwarfs. These ideas are used to suggest a possible model for WD 1145+017 in
which the unusual transit shapes are caused by opaque clouds of dust trapped in
the stellar magnetosphere. The model can account for the observed transit
periodicities if the stellar rotation is near 4.5 h, as the clouds of trapped
dust are then located near or within the co-rotation radius. The model requires
the surface magnetic field to be at least around some tens of kG. In contrast
to the eccentric orbits expected for large planetesimals undergoing tidal
disintegration, the orbits of magnetospherically-trapped dust clouds are
essentially circular, consistent with the observations.Comment: 5 pages, accepted to MNRAS Letter
The alignment of disk and black hole spins in active galactic nuclei
The inner parts of an accretion disk around a spinning black hole are forced
to align with the spin of the hole by the Bardeen-Petterson effect. Assuming
that any jet produced by such a system is aligned with the angular momentum of
either the hole or the inner disk, this can, in principle provide a mechanism
for producing steady jets in AGN whose direction is independent of the angular
momentum of the accreted material. However, the torque which aligns the inner
disk with the hole, also, by Newton's third law, tends to align the spin of the
hole with the outer accretion disk. In this letter, we calculate this alignment
timescale for a black hole powering an AGN, and show that it is relatively
short. This timescale is typically much less than the derived ages for jets in
radio loud AGN, and implies that the jet directions are not in general
controlled by the spin of the black hole. We speculate that the jet directions
are most likely controlled either by the angular momentum of the accreted
material or by the gravitational potential of the host galaxy.Comment: 4 pages, LateX file, accepted for publication in ApJ Letter
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