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The Formation of Giant Molecular Clouds

This thesis considers theoretical issues connected with the formation of giant molecular clouds. Chapter 1 reviews observations of interstellar gas clouds of massM > 10 M ̄. It critically reviews theories of cloud formation and discusses what constraints observations place on these theories. Chapter 2 considers the linear stability and responsiveness of a local model of a mixed star and gas disk. It is found that (1) the stability properties of the mixed star and gas model are similar to those of the two fluid model studied by other workers (2) the presence of the gas makes it possible for a density wave to cross the Lindblad resonance (3) the corotation amplifier can be either inhibited or enhanced (over that of the stars or gas in isolation) in the mixed system. Applications of the results to the galaxy and to young galactic disks are discussed. Chapter 3 considers the nonlinear development of gravitational instability in a local model of a galaxy disk. An unstable, initially smooth isothermal disk is simulated using the smoothed-particle hydrodynamics code TREESPH of Hernquist & Katz. The simulations produce objects with mass similar to that of giant molecular clouds in the solar neighborhood on a timescale of ∼ < 10 yr. The evolution of an initially azimuthal magnetic field is followed in the limit that the field is weak. Collapse occurs perpendicular to the field lines, so that the magnetic pressure ∝ ρ. 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 fields 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.