AbstractsAstronomy & Space Science

We have studied the effects of poloidal magnetic fields on two classes of outflows via time-dependent, numerical magnetohydrodynamic simulations. Correspondingly, this thesis is divided into two parts: a study of winds from isolated stars with dipole magnetic fields; and a study on outflows from pre-main sequence stars, surrounded by and interacting with accretion disks.We begin with a study of the effects of stellar dipole magnetic field on winds from isolated stars. Our simulations indicate that a wind from the surface of a non-rotating AGB star, if sufficiently ionized, may be channeled into an outflowing disk by a dipolar field of only a few Gauss. This type of wind may be partly responsible for shaping planetary nebulae. If the star rotates, the additional magnetic pressure, associated with the azimuthal component of the field (generated by rotation), has the effect of enhancing the outflowing disk and simultaneously produces a jet. We show that the resulting quadrupolar density pattern (disk plus jet) in the wind may explain observations of reflected starlight in proto-planetary nebulae.Next, we carry out a parameter study of an episodic magnetospheric inflation (EMI) model for launching outflows from young stellar objects (YSO's), first proposed by Hayashi et al. (1996) and Goodson et al. (1997). The basic mechanism produces an intermittent, collimated jet and an uncollimated, wide-angle wind and partially explains the mass loss and intermittent accretion of YSO's. We find that the EMI mechanism is robust, and the system is self-regulating (i.e., the outflow properties depend relatively weakly on the parameters we varied). Also, the addition of a weak vertical field initially threading the accretion disk has no effect on the EMI mechanism. However, we demonstrate that the weak vertical field can collimate the entire flow (including the component that is initially launched with a wide opening angle) into a physically broader and more powerful jet than produced by the central launching mechanism alone.