207 research outputs found
Equilibrium Initialization and Stability of Three-Dimensional Gas Disks
We present a new systematic way of setting up galactic gas disks based on the
assumption of detailed hydrodynamic equilibrium. To do this, we need to specify
the density distribution and the velocity field which supports the disk. We
first show that the required circular velocity has no dependence on the height
above or below the midplane so long as the gas pressure is a function of
density only. The assumption of disks being very thin enables us to decouple
the vertical structure from the radial direction. Based on that, the equation
of hydrostatic equilibrium together with the reduced Poisson equation leads to
two sets of second-order non-linear differential equation, which are easily
integrated to set-up a stable disk. We call one approach `density method' and
the other one `potential method'. Gas disks in detailed balance are especially
suitable for investigating the onset of the gravitational instability. We
revisit the question of global, axisymmetric instability using fully
three-dimensional disk simulations. The impact of disk thickness on the disk
instability and the formation of spontaneously induced spirals is studied
systematically with or without the presence of the stellar potential. In our
models, the numerical results show that the threshold value for disk
instability is shifted from unity to 0.69 for self-gravitating thick disks and
to 0.75 for combined stellar and gas thick disks. The simulations also show
that self-induced spirals occur in the correct regions and with the right
numbers as predicted by the analytic theory.Comment: 17 pages, 10 figures, accepted by MNRA
Stability of Magnetized Disks and Implications for Planet Formation
This paper considers gravitational perturbations in geometrically thin disks
with rotation curves dominated by a central object, but with substantial
contributions from magnetic pressure and tension. The treatment is general, but
the application is to the circumstellar disks that arise during the
gravitational collapse phase of star formation. We find the dispersion relation
for spiral density waves in these generalized disks and derive the stability
criterion for axisymmetric disturbances (the analog of the Toomre
parameter ) for any radial distribution of the mass-to-flux ratio
. The magnetic effects work in two opposing directions: on one hand,
magnetic tension and pressure stabilize the disk against gravitational collapse
and fragmentation; on the other hand, they also lower the rotation rate making
the disk more unstable. For disks around young stars the first effect generally
dominates, so that magnetic fields allow disks to be stable for higher surface
densities and larger total masses. These results indicate that magnetic fields
act to suppress the formation of giant planets through gravitational
instability. Finally, even if gravitational instability can form a secondary
body, it must lose an enormous amount of magnetic flux in order to become a
planet; this latter requirement represents an additional constraint for planet
formation via gravitational instability and places a lower limit on the
electrical resistivity.Comment: accepted in Ap
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