Dissertation: The Role of Nonideal Magnetohydrodynamic Effects, Gravitational Instability, and Episodic Accretion in Star-Formation

(Cassiopeia – Autumn 2022)


by Dr. Indrani Das
Thesis defended on July 25, 2022
Department of Physics and Astronomy, Western University
Thesis advisor: Prof. Shantanu Basu

Link to the Electronic thesis: https://ir.lib.uwo.ca/etd/8728/

Abstract
My dissertation focuses on the effect of magnetic fields on disk and core evolution during star-formation. We investigate the fragmentation scales of gravitational instability of a rotationally-supported self-gravitating protostellar disk using linear perturbation analysis in the presence of two nonideal magnetohydrodynamic (MHD) effects: Ohmic dissipation and ambipolar diffusion. Our results show that molecular clouds exhibit a preferred lengthscale for collapse that depends on mass-to-flux ratio, magnetic diffusivities, and the Toomre-Q parameter. In addition, the influence of the magnetic field on the preferred mass for collapse leads to a modified threshold for the fragmentation mass, as opposed to a Jeans mass, that might lead to giant planet formation in the early embedded phase. Furthermore, we apply the nonideal MHD threshold for fragmentation scales to fit the data of prestellar core lifetimes and as well as the number of enclosed cores formed in a clump, as found with the observations of Herschel and Submillimeter Array (SMA), respectively. Our results show that the trend found in the observed lifetime and fragmentation mass cannot be explained in a purely hydrodynamic scenario. Our best-fit model exhibits B ~ n0.43, which signifies a means to indirectly infer the effect of the ambipolar diffusion on mildly supercritical dense regions of molecular clouds. We also develop a semi-analytic formalism of episodic mass accretion (therefore episodic luminosity) from a disk to star, which provides a good match to the observed luminosity distribution of protostars. In contrast, neither a constant nor a time-dependent but smoothly varying mass accretion rate is able to do so. Our analytic work provides insight into global MHD simulations of protoplanetary disks that we carry out using the FEOSAD numerical code. Our numerical results demonstrate the long-term evolution of disks, including the formation and evolution of clumps, and especially the episodic nature of accretion, which might explain the origin of observed knots in the molecular jet outflows.

Keywords: ISM: clouds – magnetic fields – magnetohydrodynamics (MHD) – stars: formation, gravitational collapse, disk evolution: episodic accretion, young stellar objects (YSOs)

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