Presentation #231.02D in the session Star Formation.
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 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 provides 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 disk to star, which provides a good match to the observed luminosity distribution of YSOs, whereas 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, and I will also introduce some numerical results that demonstrate the long-term evolution of disks, including the formation and evolution of clumps and the episodic nature of accretion.