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Session 91 - Structure of Molecular Clouds.
Oral session, Wednesday, January 15
This dissertation presents an observational study of the magnetic fields, kinematics, and structures of small galactic molecular clouds known as starless Bok globules (SBGs).
Fourteen SBGs were polarimetrically imaged at optical wavelengths to characterize the structures of their embedded magnetic fields. More than 1000 stars (75 per SBG) were measured to 0.1% precision, as faint as apparent visual magnitude 17. SBGs show evidence of predominantly uniform fields, though strong coupling of the gas to embedded magnetic fields is unlikely. Non-uniform components of magnetic fields may contain 20% of total magnetic energy densities, and the magnetic field directions decorrelate on angular sizes of a few arcminutes.
To characterize SBG gas kinematics and density profiles, SBGs were mapped with very high spectral resolution in the lowest energy-state rotation lines of the carbon monoxide (CO) molecule and its isotopes at more than 100 positions per cloud. Most SBGs are in approximate solid- body rotation and are more centrally condensed than other small molecular clouds. Their angular velocities are distributed in two groups: rapid rotators and slow rotators. Rotational kinetic energy in SBGs is not a significant source of support against gravity and external pressure. Most SBGs are too cold to be thermally supported, but non-thermal kinetic energies marginally exceed gravitational potentials. Analyses indicate that half are near virial equilibrium and half are expected to contract.
Core regions of SBGs were observed in the lines of ammonia (NH_3) and cyanoacetylene (HC_3N), which are sensitive to moderately high gas densities. No emission was detected, limiting central densities to less than 10^4 cm^-3, lower than in dark star-forming cloud cores, but similar to values seen in high galactic latitude clouds.
Key structural and kinematic parameters of SBGs are tested for correlations with magnetic field parameters. SBG rotation axis directions are well-correlated with cloud magnetic field directions, implying magnetic braking is no longer efficient at redistributing SBG angular momentum.
SBGs are found to be strongly condensed clouds exhibiting clumpy substructures and rotational motions well correlated with the presence of embedded magnetic fields. A substantial fraction of these clouds may be the sites of future star formation.
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