Dissertation: From large-scale molecular clouds to filaments and cores: Unveiling the role of the magnetic fields in star formation

(Cassiopeia – Winter / hivers 2018)

by Dr. Sayantan Auddy
Thesis defended on July 10, 2018
Department of Physics and Astronomy, Western University
Thesis advisor: Prof. Shantanu Basu

Abstract
I present a comprehensive study of the role of strong magnetic fields in characterizing the structure of molecular clouds. We run three-dimensional turbulent non-ideal magnetohydrodynamic simulations (with ambipolar diffusion) to see the effect of magnetic fields on the evolution of the column density probability distribution function (PDF). Our results indicate a systematic dependence of the column density PDF of molecular clouds on magnetic field strength and turbulence, with observationally distinguishable outcomes between supercritical (gravity dominated) and subcritical (magnetic field dominated) initial conditions. We find that most cases develop a direct power-law PDF, and only the subcritical clouds with turbulence are able to maintain a lognormal body of the PDF but with a power-law tail at high values. I also present a scenario for the formation of oscillatory quasi-equilibrium magnetic ribbons in turbulent subcritical molecular clouds. The synthetic observed relation between apparent width in projection versus observed column density is relatively flat, similar to observations of molecular cloud filaments, and unlike the simple expectation based on a Jeans length argument. Additionally, I develop a core field structure (CFS) method which requires spatially resolved observations of the nonthermal velocity dispersion from the Green Bank Ammonia survey (GAS) of the L1688 region of the Ophiuchus molecular cloud along with the column density map to determine magnetic field strength profile across dense cores. By applying the CFS method we find that for most cores in Ophiuchus the mass-to-flux ratio is decreasing radially outward.

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