Turbulence and Mass Motion in Galactic Molecular Clouds

Citation

Lichten, Stephen Morris (1984) Turbulence and Mass Motion in Galactic Molecular Clouds. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/wbm2-9x67. https://resolver.caltech.edu/CaltechETD:etd-09092008-083043

Abstract

The main subject of this thesis is the origin and maintenance of supersonic gas motions in dense molecular clouds.

In Chapters 1 and 2, we interpret and model millimeter wavelength CO line profiles whose broad wings and self-reversals provide clues about the nature of the observed supersonic motions. Broad molecular line wings provide evidence for high-velocity bipolar gas flows powered by strong stellar winds. The self-reversed CO line profiles are due in part to radiative transfer effects in molecular clouds which, in some cases, are expanding or contracting; Monte Carlo simulations of radiative transfer, however, demonstrate that cloud fragmentation may be responsible for some observed properties of the line profiles. The detection of broad-wing sources in quiescent, dark nebulae, shows that energetic outflows are common in a wide range of molecular clouds.

In Chapter 3, our high resolution maps of molecular and atomic line emission across molecular cloud edges show that these clouds are embedded in warm H I halos which appear in emission against the galactic H I background. We also find that molecular cloud internal motions may be correlated with the H I halo temperature; however, the halo masses are much less than those of the molecular clouds and exert only a limited influence on internal cloud kinematics.

The statistical and autocorrelation analyses of Chapters 4 and 5 show that molecular clouds are clumpy and contain hundreds of turbulent fragments whose relative velocities are supersonic, but whose internal velocity dispersions are not. Shock dissipation processes, clump collisions, and gas compression have major roles in molecular cloud kinematics. Our observations indicate that turbulent energy is supplied at small scales, as opposed to the dissipationless energy transfer from large to small scales which is known to occur in incompressible fluids with high Reynolds numbers. Energetic outflows from embedded young stars may break apart, fragment, and compress portions of molecular clouds, and may be capable of originating and sustaining the supersonic turbulence. Additional observational and theoretical studies of cloud clumpiness and of the rate of energy transfer from stellar outflows to ambient molecular material, are required before this mechanism for cloud support can be evaluated.

Thesis Files