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Doctor of Philosophy




Basu, Shantanu


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 $B\propto n^{0.43}$, 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 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.

Summary for Lay Audience

Stars are the essential links between galaxies and planetary systems. The present-day solar system is thought to be created about 4.5 billion years ago from the solar nebula- a giant cloud of dust and gas in interstellar space (i.e., the space between the stars in a galaxy). During the transformation from a gaseous nebula into a star-disk system there are many fundamental physical mechanisms such as gravity, thermal pressure, and the magnetic fields that have to come into action together. Their role in star formation is a matter of ongoing study. To form a star, enough matter has to pile up to the center of a gas cloud such that no other force can prevent the gravitational collapse. In recent times, cutting-edge observations have revealed new horizons to studying star formation at high resolution and even beyond the confines of our Galaxy. In this thesis, different evolutionary epochs of star formation have been studied, starting from the collapse of a gas cloud to the birth of a star-disk system where the planets may also form. Our results show that the gravitational fragmentation scales include the effects beyond that set by thermal pressure alone. These findings signify the indirect imprints of the magnetic fields on the star-forming clouds that are consistent with observations. Furthermore, we develop a model that explains the basic mechanisms of how the matter falls onto the central star through a series of luminous eruptive outbursts. Finally, we show that magnetic field can further drive the rapid accumulation of matter from the inner disk onto the center, thereby expediting star formation.

Creative Commons License

Creative Commons Attribution 4.0 License
This work is licensed under a Creative Commons Attribution 4.0 License.