789 research outputs found
Can Nonlinear Hydromagnetic Waves Support a Self-Gravitating Cloud?
Using self-consistent magnetohydrodynamic (MHD) simulations, we explore the
hypothesis that nonlinear MHD waves dominate the internal dynamics of galactic
molecular clouds. We employ an isothermal equation of state and allow for
self-gravity. We adopt ``slab-symmetry,'' which permits motions
and fields perpendicular to the mean field, but permits gradients
only parallel to the mean field. The Alfv\'en speed exceeds the sound
speed by a factor . We simulate the free decay of a spectrum of
Alfv\'en waves, with and without self-gravity. We also perform simulations with
and without self-gravity that include small-scale stochastic forcing.
Our major results are as follows: (1) We confirm that fluctuating transverse
fields inhibit the mean-field collapse of clouds when the energy in Alfv\'en-
like disturbances remains comparable to the cloud's gravitational binding
energy. (2) We characterize the turbulent energy spectrum and density structure
in magnetically-dominated clouds. The spectra evolve to approximately
with ,
i.e. approximately consistent with a ``linewidth-size'' relation . The simulations show large density contrasts, with high
density regions confined in part by the fluctuating magnetic fields. (3) We
evaluate the input power required to offset dissipation through shocks, as a
function of , the velocity dispersion , and the scale
of the forcing. In equilibrium, the volume dissipation rate is
, for a cloud of
linear size and density . (4) Somewhat speculatively, we apply our
results to a ``typical'' molecular cloud. The mechanical power input requiredComment: Accepted for publication in Ap.J. 47 pages, 13 postscript figures.
Report also available at http://cfa-www.harvard.edu/~gammie/MHD.p
Numerical Models of Accretion Disks
I review recent numerical studies of accretion disks, focusing on
measurements of the turbulent shear stress, or , in the shearing box
model. I conclude with a list of astronomically relevant open questions that
can be settled via future numerical experiments.Comment: 9 pages, LaTeX, aipproc.sty, invited review at "Accretion Processes
in Astrophysical Systems: Some Like It Hot", eds. S. Holt and T. Kallma
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