Date of Award
Doctor of Philosophy (PhD)
Physics and Astronomy
Dr. Ralph E. Pudritz
We develop a model of self-gravitating, pressure truncated, filamentary molecular clouds with a rather general helical magnetic field topology. By comparing with existing observational data, our analysis suggests that the mass per unit length of many filamentary clouds is significantly reduced by the effects of external pressure, and that toroidal fields play a significant role in squeezing clouds. We show that there is an upper limit to the mass per unit length allowed for equilibrium, whose value depends on the strength and character of the magnetic field threading the filament. Clouds that are below this critical mass per unit length are always stable against radial gravitational collapse. Our theoretical models involve 3 parameters; two to describe the mass loading of the poloidal and toroidal fields, and a third to describe the radial concentration of the filament. We find that many of our models with helical fields are in good agreement with the observed ∼r-2 radial density structure of filamentary clouds. Unmagnetized filaments and models with purely poloidal magnetic fields result in steep density gradients that are not allowed by the observations. We consider the stability of our models against axisymmetric modes of fragmentation. Many of our models fragment gravitationally, although some are subject to MHD-driven "sausage" modes of instability. Our main result is that the toroidal magnetic field helps to stabilize long wavelength gravitational instabilities, but short wavelength MHD "sausage" instabilities result when the toroidal field is sufficiently strong. Many of our models lie in a physical regime where the growth rates of gravitational and MHD instabilities are at a minimum. We then go on to develop a model of the helically magnetized cores that might originate from finite segments of our filament models. Only modest toroidal fields are required to produce prolate cores, with mean projected axis ratios in the range 0.3-1. Thus, many of our models are in good agreement with the observed shapes of cores (Myers et al 1991, Ryden 1996), which find axis ratios distributed about a mean value in the range 0.5-0.6. We show that the Bonnor-Ebert critical mass is reduced by about 20%, as a result of the helical field in our models.
Fiege, Jason D., "Filamentary molecular clouds and their prolate cores" (1999). Open Access Dissertations and Theses. Paper 1852.