2,750 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 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
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
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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