Other
Scientific paper
Aug 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992phdt.........5g&link_type=abstract
Ph.D. Thesis Princeton Univ., NJ.
Other
5
Evolution, Interstellar Gas, Molecular Clouds, Rotation, Corotation, Formation, Gravitational Effects, Hydrodynamics, Nonlinearity, Stability, Torque, Two Dimensional Models, Vorticity
Scientific paper
Chapter 1 reviews the observational status of gravitationally bound interstellar gas clouds of mass M greater than 104 solar mass. It critically reviews theories of cloud formation and discusses observational constraints. Chapter 2 considers the linear stability and responsiveness of a local, two-dimensional model of a mixed star and gas disk. We find that: (1) The combined system is always less stable than either the gas or stars in isolation. (2) The presence of the gas makes it possible for a density wave to cross the Lindblad resonance. (3) The gain of the corotation amplifier can be either raised or lowered over the gain of the corotation amplifer in the stars or gas alone. Chapter 3 considers the nonlinear development of gravitational instability in a local model of a galaxy disk. Simulations of an unstable, isothermal disk are carried out using the smoothed-particle hydrodynamics code TREESPH of Hernquist and Katz. The simulations produce objects that have a mass similar to that of giant molecular clouds in the solar neighborhood on a timescale of approximately less than 107 yr. The evolution of an initially azimuthal field is followed in the limit that the field is weak. Collapse always occurs perpendicular to the field lines, so that the magnetic pressure varies as rho2. Chapter 4 considers the rotation of giant molecular clouds. We calculate the expected rotation frequency in several models of cloud formation. Gravitational instability produces rapid rotation, and this can be understood in terms of the conservation of potential vorticity. Magnetic field torque the cloud in a retrograde sense and can de-spin a cloud in a fraction of an epicyclic time. The rotation of giant molecular clouds is unlikely to provide a strong constraint on cloud formation theories until the magnetic field can be self-consistently included.
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