375 research outputs found
Large Scale B-Field in Stationary Accretion Disks
We reconsider the problem of the formation of a large-scale magnetic field in
the accretion disks around black holes. In contrast with previous work we take
into account the nonuniform vertical structure of the disk. The high electrical
conductivity of the outer layers of the disk prevents the outward diffusion of
the magnetic field. This implies a stationary state with a strong magnetic
field in the inner parts of the accretion disk close to the black hole.Comment: 5 pages, 2 figure
Outflows and Jets from Collapsing Magnetized Cloud Cores
Star formation is usually accompanied by outflow phenomena. There is strong
evidence that these outflows and jets are launched from the protostellar disk
by magneto-rotational processes. Here, we report on our three dimensional,
adaptive mesh, magneto-hydrodynamic simulations of collapsing, rotating,
magnetized Bonnor-Ebert-Spheres whose properties are taken directly from
observations. In contrast to the pure hydro case where no outflows are seen,
our present simulations show an outflow from the protodisk surface at ~ AU and
a jet at ~ 0.07 AU after a strong toroidal magnetic field build up. The large
scale outflow, which extends up to ~ AU at the end of our simulation, is driven
by toroidal magnetic pressure (spring), whereas the jet is powered by
magneto-centrifugal force (fling). At the final stage of our simulation these
winds are still confined within two respective shock fronts. Furthermore, we
find that the jet-wind and the disk-anchored magnetic field extracts a
considerable amount of angular momentum from the protostellar disk. The initial
spin of our cloud core was chosen high enough to produce a binary system. We
indeed find a close binary system (separation ~ 3 R_sol) which results from the
fragmentation of an earlier formed ring structure. The magnetic field strength
in these protostars reaches ~ 3 kG and becomes about 3 G at 1 AU from the
center in agreement with recent observational results.Comment: revised version, accepted for publication in ApJ, a higher resolution
version of this paper can be downloaded at
http://www.physics.mcmaster.ca/~banerjee/outflows.pd
Vertical Structure of Stationary Accretion Disks with a Large-Scale Magnetic Field
In earlier works we pointed out that the disk's surface layers are
non-turbulent and thus highly conducting (or non-diffusive) because the
hydrodynamic and/or magnetorotational (MRI) instabilities are suppressed high
in the disk where the magnetic and radiation pressures are larger than the
plasma thermal pressure. Here, we calculate the vertical profiles of the {\it
stationary} accretion flows (with radial and azimuthal components), and the
profiles of the large-scale, magnetic field taking into account the turbulent
viscosity and diffusivity and the fact that the turbulence vanishes at the
surface of the disk.
Also, here we require that the radial accretion speed be zero at the disk's
surface and we assume that the ratio of the turbulent viscosity to the
turbulent magnetic diffusivity is of order unity. Thus at the disk's surface
there are three boundary conditions. As a result, for a fixed dimensionless
viscosity -value, we find that there is a definite relation between the
ratio of the accretion power going into magnetic disk winds to the
viscous power dissipation and the midplane plasma-, which is the ratio
of the plasma to magnetic pressure in the disk. For a specific disk model with
of order unity we find that the critical value required for a
stationary solution is , where the disk's
half thickness. For weaker magnetic fields, , we argue that
the poloidal field will advect outward while for it will
advect inward. Alternatively, if the disk wind is negligible (), there are stationary solutions with .Comment: 5 pages, 3 figure
Aligning spinning black holes and accretion discs
We consider the alignment torque between a spinning black hole and an
accretion disc whose angular momenta are misaligned. This situation must hold
initially in almost all gas accretion events on to supermassive black holes,
and may occur in binaries where the black hole receives a natal supernova kick.
We show that the torque always acts to align the hole's spin with the total
angular momentum without changing its magnitude. The torque acts dissipatively
on the disc, reducing its angular momentum, and aligning it with the hole if
and only if the angle theta between the angular momenta J_d of the disc and J_h
of the hole satisfies the inequality cos theta > -J_d / 2 J_h. If this
condition fails, which requires both theta > pi/2 and J_d < 2 J_h, the disc
counteraligns.Comment: MNRAS, in pres
Influence of the Magnetic Coupling Process on the Advection Dominated Accretion Flows around Black Holes
A large-scale closed magnetic field can transfer angular momentum and energy
between a black hole (BH) and its surrounding accretion flow. We investigate
the effects of this magnetic coupling (MC) process on the dynamics of a hot
accretion flow (e.g., an advection dominated accretion flow, hereafter ADAF).
The energy and angular momentum fluxes transported by the magnetic field are
derived by an equivalent circuit approach. For a rapidly rotating BH, it is
found that the radial velocity and the electron temperature of the accretion
flow decrease, whereas the ion temperature and the surface density increase.
The significance of the MC effects depends on the value of the viscous
parameter \alpha. The effects are obvious for \alpha=0.3 but nearly ignorable
for \alpha=0.1. For a BH with specific angular momentum, a_*=0.9, and
\alpha=0.3, we find that for reasonable parameters the radiative efficiency of
a hot accretion flow can be increased by about 30%.Comment: 21 pages, 7 figures. Changed after the referee's suggestions.
Accepted for publication in the Astrophysical Journa
The Excitation, Propagation and Dissipation of Waves in Accretion Discs: The Non-linear Axisymmetric Case
We analyse the non-linear propagation and dissipation of axisymmetric waves
in accretion discs using the ZEUS-2D hydrodynamics code. The waves are
numerically resolved in the vertical and radial directions. Both vertically
isothermal and thermally stratified accretion discs are considered. The waves
are generated by means of resonant forcing and several forms of forcing are
considered. Compressional motions are taken to be locally adiabatic (). Prior to non-linear dissipation, the numerical results are in excellent
agreement with the linear theory of wave channelling in predicting the types of
modes that are excited, the energy flux by carried by each mode, and the
vertical wave energy distribution as a function of radius. In all cases, waves
are excited that propagate on both sides of the resonance (inwards and
outwards). For vertically isothermal discs, non-linear dissipation occurs
primarily through shocks that result from the classical steepening of acoustic
waves. For discs that are substantially thermally stratified, wave channelling
is the primary mechanism for shock generation. Wave channelling boosts the Mach
number of the wave by vertically confining the wave to a small cool region at
the base of the disc atmosphere. In general, outwardly propagating waves with
Mach numbers near resonance {\cal M}_{\rm r} \ga 0.01 undergo shocks within a
distance of order the resonance radius.Comment: 28 pages, 21 figures - 8 as GIF, 13 embedded postscript, Accepted for
publication in MNRAS. Full postscript version available from
http://www.astro.ex.ac.uk/people/mbat
Forced oscillations in a hydrodynamical accretion disk and QPOs
This is the second of a series of papers aimed to look for an explanation on
the generation of high frequency quasi-periodic oscillations (QPOs) in
accretion disks around neutron star, black hole, and white dwarf binaries. The
model is inspired by the general idea of a resonance mechanism in the accretion
disk oscillations as was already pointed out by Abramowicz & Klu{\'z}niak
(\cite{Abramowicz2001}). In a first paper (P\'etri \cite{Petri2005a}, paper I),
we showed that a rotating misaligned magnetic field of a neutron star gives
rise to some resonances close to the inner edge of the accretion disk. In this
second paper, we suggest that this process does also exist for an asymmetry in
the gravitational potential of the compact object. We prove that the same
physics applies, at least in the linear stage of the response to the
disturbance in the system. This kind of asymmetry is well suited for neutron
stars or white dwarfs possessing an inhomogeneous interior allowing for a
deviation from a perfectly spherically symmetric gravitational field. We show
by a linear analysis that the disk initially in a cylindrically symmetric
stationary state is subject to three kinds of resonances: a corotation
resonance, a Lindblad resonance due to a driven force and a parametric sonance.
The highest kHz QPOs are then interpreted as the orbital frequency of the disk
at locations where the response to the resonances are maximal. It is also found
that strong gravity is not required to excite the resonances.Comment: Accepte
Variability in black hole accretion discs
Observations of accreting systems often show significant variability (10-20
percent of accretion luminosity) on timescales much longer than expected for
the disc regions releasing most of the luminosity. We propose an explicit
physical model for disc variability, consistent with Lyubarskii's (1997)
general scheme for solving this problem. We suggest that local dynamo processes
can affect the evolution of an accretion disc by driving angular momentum loss
in the form of an outflow (a wind or jet). We model the dynamo as a small-scale
stochastic phenomenon, operating on roughly the local dynamical timescale. We
argue that large-scale outflow can only occur when the small-scale random
processes in neighbouring disc annuli give rise by chance to a coherent
large-scale magnetic field. This occurs on much longer timescales, and causes a
bright large-amplitude flare as a wide range of disc radii evolve in a coherent
fashion. Most of the time, dynamo action instead produces small-amplitude
flickering. We reproduce power spectra similar to those observed, including a
1/f power spectrum below a break frequency given by the magnetic alignment
timescale at the inner disc edge. However the relation between the black hole
mass and the value of the break frequency is less straightforward than often
assumed in the literature. The effect of an outer disc edge is to flatten the
spectrum below the magnetic alignment frequency there. We also find a
correlation between the variability amplitude and luminosity, similar to that
found in some AGN.Comment: 13 pages, 8 figures; MNRAS accepte
A study of the interacting binary V 393 Scorpii
We present high resolution J-band spectroscopy of V 393 Sco obtained with the
CRIRES at the ESO Paranal Observatory along with a discussion of archival IUE
spectra and published broad band magnitudes. The best fit to the spectral
energy distribution outside eclipse gives = 19000 500 for the
gainer, = 7250 300 for the donor, = 0.13 0.02
mag. and a distance of = 523 60 pc, although circumstellar material
was not considered in the fit. We argue that V 393 Sco is not a member of the
open cluster M7. The shape of the He I 1083 nm line shows orbital modulations
that can be interpreted in terms of an optically thick pseudo-photosphere
mimicking a hot B-type star and relatively large equatorial mass loss through
the Lagrangian L3 point during long cycle minimum. IUE spectra show several
(usually asymmetric) absorption lines from highly ionized metals and a narrow
L emission core on a broad absorption profile. The overall behavior of
these lines suggests the existence of a wind at intermediate latitudes. From
the analysis of the radial velocities we find = 0.24 0.02
and a mass function of = 4.76 0.24 M. Our observations favor
equatorial mass loss rather than high latitude outflows as the cause for the
long variability.Comment: 13 pages, 14 figures, 7 tables. Accepted for publication in MNRAS,
main journa
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