754 research outputs found
Three-Dimensional Simulations of Magnetized Thin Accretion Disks around Black Holes: Stress in the Plunging Region
We describe three-dimensional general relativistic magnetohydrodynamic
simulations of a geometrically thin accretion disk around a non-spinning black
hole. The disk has a thickness over the radial range
. In steady state, the specific angular momentum profile of the
inflowing magnetized gas deviates by less than 2% from that of the standard
thin disk model of
Novikov & Thorne (1973). Also, the magnetic torque at the radius of the
innermost stable circular orbit (ISCO) is only of the inward flux of
angular momentum at this radius. Both results indicate that magnetic coupling
across the ISCO is relatively unimportant for geometrically thin disks.Comment: 4 pages, 4 figures, ApJL accepte
The Stability of Magnetized Rotating Plasmas with Superthermal Fields
During the last decade it has become evident that the magnetorotational
instability is at the heart of the enhanced angular momentum transport in
weakly magnetized accretion disks around neutron stars and black holes. In this
paper, we investigate the local linear stability of differentially rotating,
magnetized flows and the evolution of the magnetorotational instability beyond
the weak-field limit. We show that, when superthermal toroidal fields are
considered, the effects of both compressibility and magnetic tension forces,
which are related to the curvature of toroidal field lines, should be taken
fully into account. We demonstrate that the presence of a strong toroidal
component in the magnetic field plays a non-trivial role. When strong fields
are considered, the strength of the toroidal magnetic field not only modifies
the growth rates of the unstable modes but also determines which modes are
subject to instabilities. We find that, for rotating configurations with
Keplerian laws, the magnetorotational instability is stabilized at low
wavenumbers for toroidal Alfven speeds exceeding the geometric mean of the
sound speed and the rotational speed. We discuss the significance of our
findings for the stability of cold, magnetically dominated, rotating fluids and
argue that, for these systems, the curvature of toroidal field lines cannot be
neglected even when short wavelength perturbations are considered. We also
comment on the implications of our results for the validity of shearing box
simulations in which superthermal toroidal fields are generated.Comment: 24 pages, 12 figures. Accepted for publication in ApJ. Sections 2 and
5 substantially expanded, added Appendix A and 3 figures with respect to
previous version. Animations are available at
http://www.physics.arizona.edu/~mpessah/research
Did Fomalhaut, HR 8799, and HL Tauri Form Planets via the Gravitational Instability? Placing Limits on the Required Disk Masses
Disk fragmentation resulting from the gravitational instability has been
proposed as an efficient mechanism for forming giant planets. We use the planet
Fomalhaut b, the triple-planetary system HR 8799, and the potential protoplanet
associated with HL Tau to test the viability of this mechanism. We choose the
above systems since they harbor planets with masses and orbital characteristics
favored by the fragmentation mechanism. We do not claim that these planets must
have formed as the result of fragmentation, rather the reverse: if planets can
form from disk fragmentation, then these systems are consistent with what we
should expect to see. We use the orbital characteristics of these recently
discovered planets, along with a new technique to more accurately determine the
disk cooling times, to place both lower and upper limits on the disk surface
density--and thus mass--required to form these objects by disk fragmentation.
Our cooling times are over an order of magnitude shorter than those of Rafikov
(2005),which makes disk fragmentation more feasible for these objects. We find
that the required mass interior to the planet's orbital radius is ~0.1 Msun for
Fomalhaut b, the protoplanet orbiting HL Tau, and the outermost planet of HR
8799. The two inner planets of HR 8799 probably could not have formed in situ
by disk fragmentation.Comment: 5 pages, 1 figure, accepted for publication in ApJ
Analysis of Clumps in Molecular Cloud Models: Mass Spectrum, Shapes, Alignment and Rotation
Observations reveal concentrations of molecular line emission on the sky,
called ``clumps,'' in dense, star-forming molecular clouds. These clumps are
believed to be the eventual sites of star formation. We study the
three-dimensional analogs of clumps using a set of self-consistent,
time-dependent numerical models of molecular clouds. The models follow the
decay of initially supersonic turbulence in an isothermal, self-gravitating,
magnetized fluid. We find the following. (1) Clumps are intrinsically triaxial.
This explains the observed deficit of clumps with a projected axis ratio near
unity, and the apparent prolateness of clumps. (2) Simulated clump axes are not
strongly aligned with the mean magnetic field within clumps, nor with the
large-scale mean fields. This is in agreement with observations. (3) The clump
mass spectrum has a high-mass slope that is consistent with the Salpeter value.
There is a low-mass break in the slope at \sim 0.5 \msun, although this may
depend on model parameters including numerical resolution. (4) The typical
specific spin angular momentum of clumps is . This is larger than the median specific angular momentum of binary
stars. Scaling arguments suggest that higher resolution simulations may soon be
able to resolve the scales at which the angular momentum of binary stars is
determined.Comment: 14 pages, 13 figures, to appear in 2003 July 20 Ap
Vortices in Thin, Compressible, Unmagnetized Disks
We consider the formation and evolution of vortices in a hydrodynamic
shearing-sheet model. The evolution is done numerically using a version of the
ZEUS code. Consistent with earlier results, an injected vorticity field evolves
into a set of long-lived vortices, each of which has a radial extent comparable
to the local scale height. But we also find that the resulting velocity field
has a positive shear stress, . This effect appears
only at high resolution. The transport, which decays with time as t^-1/2,
arises primarily because the vortices drive compressive motions. This result
suggests a possible mechanism for angular momentum transport in low-ionization
disks, with two important caveats: a mechanism must be found to inject
vorticity into the disk, and the vortices must not decay rapidly due to
three-dimensional instabilities.Comment: 8 pages, 10 figures (high resolution figures available in ApJ
electronic edition
Conformally flat black hole initial data, with one cylindrical end
We give a complete analytical proof of existence and uniqueness of
extreme-like black hole initial data for Einstein equations, which possess a
cilindrical end, analogous to extreme Kerr, extreme Reissner Nordstrom, and
extreme Bowen-York's initial data. This extends and refines a previous result
\cite{dain-gabach09} to a general case of conformally flat, maximal initial
data with angular momentum, linear momentum and matter.Comment: Minor changes and formula (21) revised according to the published
version in Class. Quantum Grav. (2010). Results unchange
The formation of the coronal flow/ADAF
We develop a new method to describe the accretion flow in the corona above a
thin disk around a black hole in vertical and radial extent. The model is based
on the same physics as the earlier one-zone model, but now modified including
inflow and outflow of mass, energy and angular momentum from and towards
neighboring zones. We determine the radially extended coronal flow for
different mass flow rates in the cool disk resulting in the truncation of the
thin disk at different distance from the black hole. Our computations show how
the accretion flow gradually changes to a pure vertically extended coronal or
advection-dominated accretion flow (ADAF). Different regimes of solutions are
discussed. For some cases wind loss causes an essential reduction of the mass
flow.Comment: 8 pages, 4 figures, accepted for publication in A&
A cool disk in the Galactic Center?
We study the possibility of a cool disk existing in the Galactic Center in
the framework of the disk-corona evaporation/condensation model. Assuming an
inactive disk, a hot corona should form above the disk since there is a
continuous supply of hot gas from stellar winds of the close-by massive stars.
Whether the cool disk can survive depends on the mass exchange between the disk
and corona. If the disk-corona interaction is dominated by evaporation and the
rate is larger than the Bondi accretion rate in the Galactic Center, the disk
will be depleted within a certain time period and no persistent disk will
exist. On the other hand, if the interaction results in hot gas steadily
condensing into the disk, an inactive cool disk might survive. For this case we
further investigate the Bremsstrahlung radiation from the hot corona and
compare it with the observed X-ray luminosity. Our model shows that, for
standard viscosity in the corona (alpha=0.3), the mass evaporation rate is much
higher than the Bondi accretion rate and the coronal density is much larger
than that inferred from Chandra observations. An inactive disk can not survive
such strong evaporation. For small viscosity (alpha<0.07) we find condensation
solutions. But detailed computations show that in this case there is too much
X-ray radiation from the corona to be in agreement with the observations.
Therefore, we conclude that there should be no thin/inactive disk presently in
the Galactic Center. However, we do not exclude that the alternative
non-radiative model of Nayakshin (2004) might instead be realized in nature.Comment: 8 pages, including 3 figures, accepted for publication in A&
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