Thermal and Magnetorotational Instability in the Interstellar Medium
Thermal and Magnetorotational Instability in the Interstellar Medium
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Date
2005-11-09
Authors
Piontek, Robert Andrew
Advisor
Ostriker, Eve C
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Abstract
We have performed three sets of numerical experiments designed to
study turbulence in the interstellar medium (ISM) driven by the
magnetorotational instability. Our models are local, account for
galactic rotation and shear, include magnetic fields, and a cooling
function which permits two stable phases of gas in pressure
equilibrium.
The first set of simulations was performed in two dimensions, in the
radial-vertical plane. These simulations laid the groundwork for the
future 3D models to come. The numerical method for including the
cooling function, as well as conduction, was developed and
implemented. These simulations gave us a glimpse into the workings of
the MRI in the presence of a two-phase medium.
In our second set of simulations we extend our models to three
dimensions. This allowed us to study the saturated state of the MRI
in the presence of a two-phase medium. The scaling of velocity
dispersion with density was found to be steeper than that of single
phase models, so that at low densities larger turbulent amplitudes
were found. The interaction between MRI driven turbulence and the
phase structure of the gas was examined in detail. We concluded that
turbulence can drive gas into a thermally unstable state, but a two
phase model of the ISM was still a fairly good approximation.
Finally, we added vertical gravity to our third set of models. Now,
rather than specify the mean density, the vertical distribution of gas
in the simulation domain is determined self-consistently. In these
models cold dense clouds form due to TI and sink to the mid-plane.
Turbulence driven by the MRI thickens the disk compared to
non-turbulent models by as much as 100%. Turbulent amplitudes in the
cold medium are relatively low, however, as the increased
concentration of cold clouds near the mid-plane keep them relatively
isolated from the more turbulent warm medium.
Whether or not the MRI is a significant source of turbulence in the
ISM is still a question without a definitive answer, but this thesis
has made significant progress in furthering our understanding of the
behavior of the MRI in a two-phase medium.