Numerical Simulations of Mass Outflows from Star  Regions
Abstract
The development, testing, and application of a two dimensional radiation magnetohydrodynamics (RMHD) code is described in detail. Well developed Eulerian hydrodynamic algorithms are used, but are implemented in a new covariant formalism which facilitates simulations in any orthogonal coordinate system. The recently developed constrained transport (CT) algorithm is implemented for the numerical evolution of the components of a magnetic field for MHD simulations. This algorithm guarantees the numerical evolved field components will satisfy the divergence free constraint at all times. It is found, however, that in its original form the CT algorithm is unable to propagate all modes of the MHD wave families (in particular shear Alfven waves) stably. Therefore, an extension to the CT algorithm is developed using the method of characteristics (MOC), and it is demonstrated that the resulting hybrid CT/MOC method does provide for the stable evolution of all modes of MHD wave families. A two dimensional full transport algorithm to evolve the radiation moment equations is developed and tested for RHD simulations. The moment equations are closed with the tensor variable Eddington factor whose components are computed from angular quadratures of the specific intensity which, in turn, is computed from a formal solution of the two dimensional transfer equation using the method of short characteristics. This algorithm for multidimensional RHD differs significantly from more commonly used methods based on the diffusion approximation. Particular care is taken to test all the algorithms used in the code extensively. Detailed descriptions of the setup and results for each test problem used for the HD, MHD, and RHD algorithms is provided. The code is then applied to two important problems regarding mass outflows from star forming regions. In the first application, the possibility that a steady, isotropic, nonmagnetic protostellar wind can be collimated into a bipolar outflow by magnetic stresses in the ambient medium is considered. The time evolution of the cavity evacuated by the protostellar wind is studied in detail, and it is found that for parameters typical of molecular clouds, such "bubbles" are able to reproduce the observed characteristics of bipolar outflows. In the second application, the time evolution of a magnetised accretion disk surrounding a protostar is computed to discover if magnetic stresses in the disk can drive a wind with the observed properties of protostellar mass outflows. We find that strong, collimated outflows can be generated, but that the details of the flow depend intimately on the assumed initial conditions of the problem. These latter simulations provide the basis for future numerical studies of the generation of magnetic winds from accretion disks.
 Publication:

Ph.D. Thesis
 Pub Date:
 1990
 Bibcode:
 1990PhDT........16S
 Keywords:

 HYDRODYNAMICS;
 Physics: Astronomy and Astrophysics, Physics: Fluid and Plasma, Physics: Radiation;
 Algorithms;
 Computer Programs;
 Computerized Simulation;
 Magnetohydrodynamic Waves;
 Method Of Characteristics;
 Star Formation;
 Stellar Mass;
 Formalism;
 Magnetic Fields;
 Stellar Magnetic Fields;
 Stellar Winds;
 Wave Propagation;
 Astrophysics